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Contact Mechanics and Three-Dimensional Alignment of Normal Dog Elbows

Permanent Link: http://ufdc.ufl.edu/UFE0043866/00001

Material Information

Title: Contact Mechanics and Three-Dimensional Alignment of Normal Dog Elbows
Physical Description: 1 online resource (103 p.)
Language: english
Creator: Cuddy, Laura C
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2011

Subjects

Subjects / Keywords: biomechanics -- coronoid -- dog -- elbow -- osteotomy
Veterinary Medicine -- Dissertations, Academic -- UF
Genre: Veterinary Medical Sciences thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Fragmented medial coronoid process of the ulna is the most common cause of forelimb lameness in dogs, and often leads to debilitating elbow osteoarthritis. Using a cadaveric model, we investigated the biomechanics of normal dog elbows, and determined that changes in elbow flexion angle and forearm rotation increase pressures across the elbow joint. We subsequently investigated a novel surgical procedure, proximal ulnar rotational osteotomy, designed to unload the medial compartment of the elbow. We determined that this osteotomy shifted load from the medial to the lateral compartment by altering the weight-bearing axis through the limb. Future investigation includes development of a custom bone plate and testing in clinical cases.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Laura C Cuddy.
Thesis: Thesis (M.S.)--University of Florida, 2011.
Local: Adviser: Lewis, Daniel D.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2014-12-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2011
System ID: UFE0043866:00001

Permanent Link: http://ufdc.ufl.edu/UFE0043866/00001

Material Information

Title: Contact Mechanics and Three-Dimensional Alignment of Normal Dog Elbows
Physical Description: 1 online resource (103 p.)
Language: english
Creator: Cuddy, Laura C
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2011

Subjects

Subjects / Keywords: biomechanics -- coronoid -- dog -- elbow -- osteotomy
Veterinary Medicine -- Dissertations, Academic -- UF
Genre: Veterinary Medical Sciences thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Fragmented medial coronoid process of the ulna is the most common cause of forelimb lameness in dogs, and often leads to debilitating elbow osteoarthritis. Using a cadaveric model, we investigated the biomechanics of normal dog elbows, and determined that changes in elbow flexion angle and forearm rotation increase pressures across the elbow joint. We subsequently investigated a novel surgical procedure, proximal ulnar rotational osteotomy, designed to unload the medial compartment of the elbow. We determined that this osteotomy shifted load from the medial to the lateral compartment by altering the weight-bearing axis through the limb. Future investigation includes development of a custom bone plate and testing in clinical cases.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Laura C Cuddy.
Thesis: Thesis (M.S.)--University of Florida, 2011.
Local: Adviser: Lewis, Daniel D.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2014-12-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2011
System ID: UFE0043866:00001


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1 CONTACT MECHANICS AND THREE DIMENSIONAL ALIGNMENT OF NORMAL DOG ELBOWS By LAURA C. CUDDY A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2011

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2 2011 Laura Cuddy

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3 To my family

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4 ACKNOWLEDGMENTS To the UF Surgery Service, my surrogate family, I am eternally grateful for the opportunity to train in one of the best residency programs worldwide. I hope I make you proud. To my mentors, Dr. Dan Lewis and Dr. Antonio Pozzi for their intellectual su pport, dedication and hard work. To Dr. Bryan Conrad and Dr. Scott Banks for the many long and frustrating hours spent analyzing kinematic data. To Dr. MaryBeth Horodyski for her consistent and cheerful guidance with statistical analysis. To my fellow Irishman Dr. Noel Fitzpatrick for his intellectual and financial contributions, without which this study would not have been possible. To the UF College of Veterinary Medicine Graduate Office for their financial support and guidance To my resident mates, past and present, thank you for all your support and guidance during our journey To my family, for their continuing support and encouragement.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................. 4 LIST OF TABLES ............................................................................................................ 7 LIST OF FIGURES .......................................................................................................... 8 LIST OF ABBREVIATIONS ........................................................................................... 10 ABSTRACT ................................................................................................................... 11 C HAPTER 1 INTRODUCTION .................................................................................................... 13 History of Elbow Research ...................................................................................... 13 Elbow ...................................................................................................................... 14 Anatomy ........................................................................................................... 14 Ultrast ructure and Composition of the Normal MCP ......................................... 15 Medial Coronoid Pathology ..................................................................................... 17 Epidemiology .................................................................................................... 17 Etiopathogenesis .............................................................................................. 19 Genetics ..................................................................................................... 19 Abnormal endochondral ossification .......................................................... 20 Abnormal bone struc ture ............................................................................ 20 Abnormal elbow biomechanics .................................................................. 21 Histopathology .................................................................................................. 25 Diagnosis .......................................................................................................... 28 Treatment for Medial Coronoid Disease ................................................................. 29 Conservative management ............................................................................... 29 Removal of pathologic cartilage and bone ........................................................ 31 Arthroscopy ................................................................................................ 31 Sub total coronoidectomy .......................................................................... 32 Biceps Ulnar Release Procedure ..................................................................... 33 Corrective Osteotomies .................................................................................... 34 Ulnar osteotomy ......................................................................................... 34 Humeral osteotomy .................................................................................... 35 Elbow Denervation ........................................................................................... 37 Salvage Procedures ......................................................................................... 37 Medial compartment resurfacing ................................................................ 37 Total elbow arthroplasty ............................................................................. 38 2 CONTACT MECHANICS AND 3D ALIGNMENT OF NO RMAL DO G ELBOWS .... 39 Background ............................................................................................................. 39

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6 Materials and Methods ............................................................................................ 41 Specimen Preparation ...................................................................................... 41 Testing Protocol ............................................................................................... 46 Testing Sequence ............................................................................................ 47 Data Analysis ................................................................................................... 47 Statistical Analysis ............................................................................................ 50 Results .................................................................................................................... 50 Contact Mechanics ........................................................................................... 50 Anatomic Location of Peak Contact Pressure .................................................. 54 3D Alignment .................................................................................................... 55 Humeroradial articulation ........................................................................... 55 Humeroulnar articulation ............................................................................ 56 Proximal radioulnar articulation .................................................................. 57 Discussion .............................................................................................................. 64 3 EFFECT OF PROXIMAL U LNAR ROTATIONAL OSTE OTO MY ON CONTACT MECHANICS AND 3D ALI GNMENT OF NORMAL DOG ELBOWS ....................... 68 Background ............................................................................................................. 68 Materials and Methods ............................................................................................ 70 Spe cimen Preparation ...................................................................................... 70 Testing Protocol ............................................................................................... 70 Testing Sequence ............................................................................................ 70 Data Analysis ................................................................................................... 72 Statistical Analysis ............................................................................................ 74 Results .................................................................................................................... 74 Contact Mechanics ........................................................................................... 74 3D Alignment .................................................................................................... 76 Humeroradial articulation ........................................................................... 76 Humeroulnar articulation ............................................................................ 76 Proximal radioulnar articulation .................................................................. 76 Proximal ulnadistal ulna ............................................................................ 80 Discussion .............................................................................................................. 81 4 CONCLUSION ........................................................................................................ 86 LIST OF REFERENCES ............................................................................................... 91 BIOGRAPHICAL SKETCH .......................................................................................... 103

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7 LIST OF TABLES Table page 1 1 Modified Outerbridge Scoring System for arthroscopic evaluation of cartilage pathology ............................................................................................................ 31 2 1 Contact mechanical data acquired from the medial and lateral elbow compartments at each of the nine elbow poses tested ....................................... 52 2 2 Static 3D alignment of the humeroradial articulation for the nine elbow poses tested .................................................................................................................. 58 2 3 Static 3D alignment of the humeroulnar articulation for the nine elbow poses tested .................................................................................................................. 60 2 4 Static 3D alignment of the proximal radioulnar articulation for the nine elbow poses tested ....................................................................................................... 62 3 1 Contact mechanical parameters obtained from the medial and lateral elbow compartments preand post osteotomy for each of the nine elbow poses tested .................................................................................................................. 75 3 2 Static 3D alignment of the humeroradial articulation preand post osteotomy for the nine elbow poses tested .......................................................................... 77 3 3 Static 3D alignment of the humeroulnar articulation preand post osteotomy for the nine elbow poses tested .......................................................................... 78 3 4 Static 3D alignment of the proximal radioulnar articulation preand post osteotomy for the nine elbow poses tested ........................................................ 79 3 5 3D orientation of the distal ulna relative to the proximal ulna following proximal ulnar rotational osteotomy .................................................................... 81

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8 LIST OF FIGURES Figure page 1 1 Anatomy of the normal elbow ............................................................................. 14 1 2 Cranial view of a normal left forelimb demonstrating the mechanical axis (red) and anatomic axis (black) .......................................................................... 15 1 3 Primary and secondary trabecular orientation in the MCP ................................. 16 1 4 Normal MCP subchondral bone densities in juvenile and geriatric dogs ............ 17 1 5 FMCP configurations: tip and radial incisure ...................................................... 18 1 6 Characteristic stance associated with MCD ....................................................... 19 1 7 Proximal radioulnar step incongruency ............................................................... 22 1 8 Physiologic joint incongruency ............................................................................ 23 1 9 Elliptical ulnar trochlear notch ............................................................................. 24 1 10 Biceps brachii/brachialis common tendon of insertion and the MCP .................. 25 1 11 Subchondr al microcrack formation in FMCP ...................................................... 26 1 12 Regional bone mineral density in dogs with FMCP ............................................ 27 1 13 Axial CT slice demonstrating a fragment in the apex of the MCP ....................... 28 1 14 Standard arthroscopic portals for the dog elbow and arthroscopic image demonstrating a cartilage flap at the apex of the MCP ....................................... 32 1 15 Biceps ulnar release procedure .......................................................................... 34 1 16 Pfeil ulnar osteotomy .......................................................................................... 35 1 17 Canine unicompartmental elbow resurfacing ...................................................... 38 2 1 Contact areas (A) and force distribution (B) in the normal elbow ........................ 40 2 2 Limb loaded in the custom jig in the materials testing machine .......................... 45 2 3 I scan sensor and representative pressure map obtained .................................. 47 2 4 Objective contact mechanical outcome measurements ...................................... 48 2 5 Joint co ordinate system ..................................................................................... 49

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9 2 6 Displacement of the MCP and corresponding superimposed contact maps in neutral, pronation and supination ....................................................................... 51 2 7 PCP location ....................................................................................................... 54 3 1 Location and orientation of the osteotomy .......................................................... 72 3 2 A. Plate luting and stabilization and B. caudal displacement of the apex of the MCP ................................................................................................................... 72 3 3 Contact parameters and 3D alignment A. preand B. post osteotomy ............... 73 3 4 3D alignment of the proximal radioulnar joint post osteotomy ............................ 80

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10 LIST OF ABBREVIATION S CA Contact area CP Mean contact pressure CT Computed tomography EI Elbow incongruency FMCP Fragmented medial coronoid process MCD Medial compartment disease MCP Medial coronoid process MRI Magnetic resonance imaging OCD Osteochondritis dissecans PCP Peak contact pressure UAP Ununited anconeal process 2D Two dimensional 3D Three dimensional

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11 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science CONTACT MECHANICS AND THREE DIMENSIONAL ALIGNMENT OF NORMAL DOG ELBOWS By Laura Cuddy December 2011 Chair: Daniel D. Lewis Major: Veterinary Medical Sciences Fragmented medial cor onoid process (FMCP) is the most common cause of forelimb lameness in dogs FMCP refers to a spectrum of pathology ranging from cartilage malacia, fibrillation or fissuring, to subchondral bone erosion or eburnation affecting the medial coronoid process (MCP) of the elbow of dogs. The exact cause of F M CP has not yet been elucidated, but histopathological studies have suggested that repetitive exces sive loading may lead to microfracture formation in the trabecular subchondral bone of the craniodistal apex or the radial incisure of the MCP Arthroscopic assessment and debridement of pathologic cartilage and subchondral bone is considered the preferred treatment by many surgeons due to the decreased morbidity and increased magnification compared with arthrotomy. A djunctive treatments to unload the MCP have been investigated, although none have gained widespread popularity. Irrespective of the method of treatment elected, osteoarthritis of the elbow inevitably progresses, and the prognosis for complete return to normal function is guarded. There is a paucity of studies investigating the biomechanics of normal dog elbows. Contact areas have been assessed using joint casting techniques and more recently

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12 tactile array pressure sensors have been employed to estimate joint forces transmitted across the proximal radioulnar joint. These techniques, however, provide mostly qualitative data and were performed at one static elbow pose. Kinematic studies of the forelimb of dogs at a walk and a trot have been investigated in two and three dimensions although kinematics of each the humeroradial, humeroulnar and proximal radioulnar joint during different elbow poses has not yet been investigated. Chapter 2 describes the effect of three elbow flexion angles spanning the range reported during the stance phase at a walk, and antebrachial rotation angles on the contact mechanics and 3D alignment of normal dog elbows. We found that altering elbow flexion and antebrachial rotation angles significantly altered the contact mechanics and alignment of normal dog elbows. This study provided a model for future investigation of pathologic states, as well as surgical procedures advoc ated for the treatment of FMCP. Chapter 3 describes the effect of a novel surgical procedure, proximal ulnar rotational osteotomy on the magnitude and distribution of contact pressures on the articular surface of the medial and lateral elbow compartments and on 3D limb alignment in the normal dog elbow. We found that this procedure rotates the apex of the MCP caudal and abaxial to the radial head and decreases contact pres sures in the medial compartment, likely through alteration in the mechanical axis through the humeroradial articulation. This procedure may therefore unload the medial compartment of the elbow and ameliorate pain in dogs with medial compartment disease

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13 CHAPTER 1 INTRODUCTION History of Elbow Research Until the mid 1970s, most cases of osteoarthrosis of the elbow joint in the dog were considered to be primary in nature, with secondary osteoarthritis occurring due t o UAP.1 F M CP of the ulna along with OCD of the humeral trochlea, was first identified in dogs in 1974, and was initially classified as ununited coronoid proces s.2,3 The term ununited implied that nonunion resulted from failure of normal ossification of a physis, however, as the MCP is not a s econdary center of ossification this was determined to be a misnomer and replaced with fragmented by Olsson in 1976.4 6 I t was at first believed that F MCP and OCD w ere both man ifestations of osteochondrosis, the retention of cartilage due to a disturbance in the process of endochondral ossification, with the subchondral bone only secondarily affected.1,68 It was proposed that the thickened cartilage originating from osteochondrosis of the MCP easily ossified due to its vascular supply from the annular ligament forming the osteochondral fragment commonly identified.1 H istopathologic studies subsequently substantiated that F M CP is not a manifestation of osteochondrosi s, but is instead most likely a result of supraphysiologic loading, potentially of abnormal subchondral bone, cumulating microfracture formation in fatigue failure of the trabecular subchondral bone of the craniodistal apex or the radial incisure of the MC P .9 11 Despite extensive research over the past three decades, the exact etiopathogenesis of FCP has yet to be elucidated and remains controversial

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14 Elbow Anatomy The elbow of the dog is a compound joint consisting of the humeroulnar, humeroradial and proximal radioulnar articulations.12,13 The elbow is most commonly described as a hinge joint, with motion primarily occurring in a sagittal plane as the smooth, concave trochlear notch of the ulna glides along the humeral trochlea. The radial head and lateral coronoid process of the ulna articulate with the capitulum of the humerus, whereas the MCP of the ulna articulates with the trochlea of the humerus. The ulna has a concave surface between the medial and lateral coronoid processes the radial notch in which the radial head pivots to allow pronation and supination of the antebrachium .12 The annular ligament attaches to the apices of the medial and lateral coronoid processes encircling but not attached to the enclosed radial head to enable motion during antebrachial rotation.12 The elbow may therefore be referred to more correctly as a trochoginglymus joint, although these hinge and pivot motions are considered to occur independently.13,14 Figure 11. Anatomy of the normal elbow15

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15 Normal, fullthickness, cartilage free areas are present on the ulnar trochlear notch and on the humeral condyle.1618 Initial studies purported that the lateral and medial coronoid processes constituted only 2025% of the total articular and weight bearing surface of the proximal radioulnar articulation, and that the remaining 7570% was contributed by the radial head.19 However, more recent studies have suggested that force transmission through the radial head and MCP is more evenly distributed between the medial and lateral compart ments .20,21 This may be attributable to the inherent varus alignment of normal dog elbows.22 Figure 12 Cranial view of a normal left forelimb demonstrating the mechanical axis (red) and anatomic axis (black) Ultrast ructure and Composition of the N ormal MCP The MCP is not a separate center of ossification, and growth is considered to occur by interstitial growth of the cartilaginous anlage.23,24 Endochondral ossification of the

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16 cartilaginous anlage of the MCP starting at the apex and the junction with the trochlear notch, gradually covers the MCP with cortical bone between 818 weeks after birth.25 The trabecular structure in the MCP displays early formation at the caudomedial edge and distinct alignment towards the humeral side at an unusually early age compared to other bones, likely an adaptation to significant mechanical loading at an early age according to Wolffs law .25,26 The orientation of the trabeculae indicate that the MCP is loaded primarily perpendicular to the humeroulnar surface during normal weight bearing with secondary orientation towards the annular ligamen t due to tensile stress .26 The alignment of the trabecular bone in the radial notch is not consistent with compressive loading occurring across the proximal radioulnar joint.26 The overlying articular cartilage is formed early in development and is not renewed, compared with subchondral bone which demonstrates regular turnover.27 Figure 13. Primary and secondary trabecular orientation in the MCP26 The histologic appearance of normal MCPs is consistent with normal hyaline cartilage and bone, with subjective articular cartilage thinning occurring with age.28 CT

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17 osteoabsorptiometry of normal dog elbows has demonstrated increases in the subchondral bone density with increasing age, with highest subchondral bone densities at the MCP and the ce ntral aspect of the humeral trochlea.20 Increased subchondral bone density is correlated to previous load history, and it can be therefore inferred that higher force transmission occurs through the MCP than the radial head, in direct contradiction to the conventional belief of the radial head bearing the majority of load.19 The increased force transmission may be due to the lower surface area of the medial coronoid process, or a direct reflection of physiologic humeroulnar incongrency.29 Figure 14 Normal MCP subchondral bone densities in juvenile and geriatric dogs20 Medial Coronoid Pathology Epidemiology Elbow dysplasia is an umbrella term grouping four developmental disease processes of the elbow joints of dogs: Fragmented Medial Coronoid Process ( F M CP ) Ununited Anconeal Process ( UAP ) Osteochondritis Dissecans ( OCD) of the humeral trochlea and elbow incongruency (EI) FMCP is the most commo n of the se conditions, encompassing a s pectrum of patholo gy ranging from cartilage malacia, fibrillation or fissuring to subchondral bone erosion, eburnation or fracture a ffecting the MCP of the

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18 ulna.30 Fragmentation of the MCP may either occur at the apex of the MCP or in the region of the radial incisure at the lateral aspec t of the MCP .22 Figure 15 FMCP configurations: tip and radial incisure22 FMCP is most commonly recognized in growing, large and giant breed dogs between four to nine months of age.1,31 Bernese Mountai n dogs, Labrador retrievers, Golden retrievers and Rottweilers are breeds that are commonly affected with Bernese Mountain Dogs 140 times and Rottweilers 36 times more likely to develop FMCP than comparable mixedbreed dogs .32 Males are 1.6 times more likely to be affected than females, although this may be a reflection of differential rate of growth.33 Clinical signs associated with MCD include a stiff or a stilted forelimb gait, most obvious when first rising or after prolonged rest or vigorous exercise.1,34 Bilateral involvement is common, in which case lameness may be difficult to detect.1 A n ecdotally, dogs affected with FMC P often stand with the elbow adducted, carpus abducted and the antebrachium supinated in a presumed attempt to shift weight away from the affected medial compartment to the lateral compartment of the elbow joint, although this theory has not been substantiated.1,35

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19 Figure 16 Characteristic stance associated with MCD Photograph courtesy of Antonio Pozzi. Etiopathogenesis While the etiopathogenesis of other components of elbow dysplasia such as UAP or OCD of the humeral trochlea are well understood, there is no uniform agreement regarding the etiopathogenesis of FMCP. Abnormal endochondral ossification of the MCP, abnormal bone structure or abnormal biomechanics of the elbow joint have been proposed as potential etiologies.1,6,11,16,17 Genetics FMCP wa s identified as an inherited trait in the early 1990s .3639 The genes responsible have yet to be identified and phenotypically normal animals can produce offspring affected with FMCP, making identi fication of parent animals with low risk of transmission difficult.39,40 In a study of Bernese Mountain Dogs in The Netherlands, the associa ted common ancestors of FMCP and EI were different, indicating that these may

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20 be genetically separate diseases in these populations.39 Despite ex tensive breeding programs, the incidence of FMCP c an only be reduced and not eliminated.41 Further research including identification of genetic markers, such as bi allelic single nucle otide polymorphisms in dogs with FMCP, is required to further reduce the incidence.33 Abnormal endochondral ossification Early researchers suggested that FMCP w as a form of osteochondrosis, a disturbance in endochondral ossification, supported by the c linical observation that FMCP and OCD are commonly identified in the same elbow and that chondromalacia in the region of the MCP is commonly identified.2,3,30,42 However, histopathological studies of FMCP s have demonstrated that these lesions are nonhealing fractures, likely a result of repetitive excessive loading cumulating in fatigue failure and microfracture formation originating in the trabecular subchondral bone of the craniodistal tip or the r adial incisure of the MCP.9,10 Furthermore, diffuse microcracks have been identified in the subchondral bone of the MCP, extending beyond the grossly diseased bone.9 Abnormal bone structure Subtrochlear sclerosis is one of the first radiographic signs of FMCP in dogs.43 Dogs with FMCP have a higher proportion of trabecular bone in the MCP, an adaptive response to increased loading consistent with Wolffs law.26 However CT osteoabsorptiometric studies have identified that subchondral bone density also increases with increasing age in normal dogs.20 Increased stiffness may make this region less deformable, making it prone to fatigue fracture as well as transmitting increased force to the overlying cartilage.44 It has also been suggested that the ability of the subchondral bone in the region of the MCP to repair microdamage may be abnormal, resulting in accumulation of microdamage and subsequent fatigue fracture.9

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21 Abnormal elbow biomechanics Currently abnormal elbow bi omechanics, specifically humeroulnar incongruency or conflict resulting in supraphysiologic loading of the MCP is considered the most likely etiology for FMCP.45 Incongruency refers to the malalignment of articular surfaces.46 The main current biomechanical th eories proposed to cause humeroulnar conflict and resultant supraphysiologic loading of the MCP include static elbow incongruency (radio ulnar length disparity), dynamic elbow incongruency (radio uln ar longitudinal incongruence), ulnar trochlear notch geometric incongruency primary rotational instability of the radius and ulna relative to the distal humerus a nd musculotendinous mismatch.16,17,4548 Some authors have implicated tensile forces originating from the annular ligament resulting in an avulsion of the MCP, although this theory has not been substantiated.26 Static or dynamic proximal r adioulnar step incongruency Disparate or asynchronous growth of the paired radius and ulna may occur during skeletal development, resulting in a short radius. This phenomenon has been identified in puppies in which the ulna was t ransiently as much as 3 mm longer than the radius at 4 6 months of age .17 A radiographic study revealed that Swiss mountain dogs wit h FMCP had a proportionally shorter radius compared with the ulna a longer proximal ulna and an increased distance between the trochlear notch and proximal radius compared with unaffected dogs of the same breed.49 Experimental shortening of the radius increases contact area of the MCP in cadaveric limbs and FMCP has been induced by traumatic premature closure of the distal radial physis.50,51 It is, however, uncommon to diagnose radioulnar incongruency at the time of diagnosis of FMCP and it is postulated that this may th erefore be a temporary state,

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22 only occurring at certain joint positions or during certain phases of development and inapparent at the time of diagnosis of FMCP. The maturing intraosse o us ligament has been postulated to begin to resist longitudinal movement between the radius and ulna around 6 months of age, and in an unpublished longitudinal radiographic study, incongruency through the proximal radioulnar joint could be induced up to five months of age by manual manipulation.15,52 Furthermore, le sions associated with FMCP appeared at a time when the MCP was elevated more proximal to the radial head, and following fragmentation a period of longitudinal growth of the radius reestablished normal congruency of the proximal radioulnar joint.52 CT studies have demonstrated increased humeroradial joint space and significant radioulnar incongruency in dogs with MCD compared with normal control dogs .53 3D image rendering of CT images has been found to be 82% sensitive and 100% specific for detecting radioulnar step lesions induced in cadaveric specimens.54 Arthroscopic estimation has been described to be 86% sensitive and 100% specific for positive radiouln ar incongruency (short radius)55 and has a higher diagnostic value compared with both radiography and CT.56 Figure 17 P roximal radioulnar step incongruency46

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23 Incongruency between humeral trochlea and ulnar trochlear notch The trochlear notch of the elbow has a small concave incongruence, which allows for more equal stress distribution under high load and ensures better nutrition of the articular cartilage.46,57 Such physiologic incongruence h as also been noted in humans but is not thought to be involved in the devel opment of FMCP in dogs .46,57 Figure 18 Physi ologic joint incongruency46,57 The trochlear notc h reaches its maximum diameter at the apex of the MCP.47 Trochlear notch dysplasia as a cause for FMCP was first proposed in 1986.16,17 In radiographic and cadaveric studies, a more elliptical trochlear notch with decreased diameter at the apex of the MCP and at the anconeal process was identified in dogs a f fec ted with elbow arthrosis and in medium and largebreed dogs compared with normal dog s.16,17 It was subsequently postulated that the abnormally small curvature of the trochlear notch would lead to overloading of the underdeveloped MCP in immature dogs, resulting in fragmentation.16,17 However, if the trochlear notch is more elliptical, the humeral condyle would exert pressure on both the MCP and the anconeal process and it is uncommon to identify concurrent UAP and F M CP in the same joint.58 Subsequent studies have not identified a significant difference in the radius of curvature

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24 of the ulnar trochlear notch between breeds predisposed to developing FMCP and those rarely affected.47,59 Figure 19 Elliptical ulnar trochlear notch46 Primary rotational instability The articular circumference of the radial head is greater than the corresponding circumference of the radial notch of the ul na, allowing axial rotation during pronation and supination of the antebrachium. FMCP occurring in the region of the radial incisure, may be related to primary rotational instability occurring during pronation or supination, resulting in lateral shear and excessive loading of the MCP between the radial head and the medial humeral condyle.33,45 Musculotendinous mismatch. This refers to a disparity between the muscle tensions generated during pronation or supination of the antebrachi um relative to the humerus. Supraphysiologic loading of the MCP purportedly occurs during maximal flexion of the elbow as a result of the forces generated by the medially located combined tendon of insertion of the biceps brachii and brachialis muscles.45,60 These forces cause supination of the antebrachium and a resultant shearing force between the radial head and the radial incisure at the level of the proximal radioulnar joint.45 Supination of the antebrachium actually results in a pivoting of the radial head externally

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25 with minimal motion of the ulna. The pull of the biceps brachii and brachialis tendon of insertion would instead draw the MCP cranially and shear it against the radial head, reflecting what occurs during pronation at the joint surface and consistent with trabecular orientation demonstrated in lesions associated wi th the radial incisure.6 1 Figure 110. B iceps brachii/brachialis common tendon of insertion and the MCP60 Histopathology Although FMCP was initially considered to occur due to cartilage retention related to osteochondrosis a landmark study comparing humeral condylar OCD flaps and FMCPs removed from elbows at surgery using light and transmission electron microscopy substantiated a defect in endochondral ossificat ion associated with the OCD flaps .10 I n contrast pathology of FMCP lesions was more consistent with subchondral bone fracture and fibrous repair.10 However, a more recent case study identi fied incidental histologic lesions consistent with OCD in a case of FMCP in an asymptomatic 20week old Golden Retriever.62 To substantiate the theory that FMCP is due to subchondral bone fracture rather than ost eochondrosis, a more recent

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26 histomorphometric study of F MCPs excised at surgery revealed diffuse damage, osteocyte loss and increased porosity in the subchondral bone of the FMCP, particularly in the radial incisure, the region of the MCP that commonly fra gments.9 FMCP specimens demonstrated higher fatigue microdamage compared with normal MCPs, with fatigue microdamage positively correlating with the severity of FMCP.9 It was concluded that fatigue fracture of the subchondral trabecular bone is therefore important in the pathogenesis of FMCP .9 FMCPs have a wide variety of histological and immunohistochemical characteristics compared with normal MCPs, with lower collagen type X in young dogs with FMCP.28 Cartilage necrosis has been induced in the stifles of pigs by selectively impairing the cartilage canal blood supply.63 Investigati on into the anatomic sites of cartilage canals in young golden retriever dogs has not revealed any association between their location and those in which FMCP occurs commonly.64 Figure 111. Subchondral microcrack formation in FMCP9

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27 A recent microCT study noted that trabecular orientation on the MCP was significantly different between dogs with no evidence of FMCP (contr ol) and dogs with transverse tip fragments involving the radial incisure on horizontal plane imaging and on sagittal plane imaging, although there was no difference between controls and dogs with transverse tip fragments solely.61 It is therefore unlikely that a single unified theory can explain medial coronoid pathology occurring at the apex of MCP which is consistent with acute overloading and that occurring at the radial incisure which is more indicative or chronic abnormal torsional loading.61,65 Bone mineral density patterns reveal that the MCP is eccentrically loaded in dogs with and without FMCP with 50% higher bone mineral density at the abaxial half versus the axial half of the MCP .66 T his suggests that load transmission occurs primarily across the abaxial surface of the MCP, and may result in microcrack form ation and fragmentation at the junction between these two regions.66 In dogs with FMCP, bone mineral density was actually highest in the centroabaxial portion of the MCP than near the apex as would be expect ed if supraphysiologic loading of the apex was responsible for asso ciated pathology.66 Figure 112. Regional bone mineral density in dogs with FMCP66

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28 Diagnosis Radiography is used as a routine screening procedure for elbow disease. Findings associated with MCD are usually related to secondary osteoarthritic changes, and include osteophyte formation on the proximal anconeus and proximal radius, as well as subchondral sclerosis in the region of the semi lunar notch and medial coronoid process.6769 Radiographic assessment of osteophytosis of the elbow correlates poorly with arthroscopic findings and MCD is often diagnosed in dogs with few or no radiographic changes .70 The gold standard for diagnosis of pathology referable to the medial coronoid process is a combination of CT and arthroscopi c examination. CT allows evaluation of subchondral bone surfaces and elbow congruency, but does not yield information on status of the overlying cartilage.69 Arthroscopy on the other hand is very useful to assess surface cartilage, but cannot identify isolated subchondral bone lesions that are not associated with surface cartilage lesions. These two modalities are therefore considered complimentary for the diagnosis of MCD.71 Figure 113. Axial CT slice demonstrating a fragment in the apex of the MCP71

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29 Although useful for examining soft tissue structures around th e elbow joint, u ltrasonography is of limited diagnostic value for diagnosis of MCD, with poor agreement between ultrasonographic and surgical findings.72 Bo ne scintigraphy has been purported as a useful diagnostic tool for MCD in dogs, especially where some ambiguity remains regarding the source of lameness after standard diagnostic techniques.73,74 MRI has been utilized to characterize the anatomy of normal dog elbows and showed good correlation with surgical findings .75,76 MRI is more useful to define the soft tissue structures around the elbow, and its use in the diagnosis of FMCP is limited due to the small size of the dog elbow the complex articulations and the thin cartilage surface.77 Magnetic resonance arthrography using gadolinium DTPA, has been recommended for imaging the canine elbow .78 Delayed gadolinium enhanced MRI of cartilage has recently been investigated as a preoperative tool to predict outcome in dogs with FMCP.79 The decrease in glycosaminoglycans associated with cartilage d amage should result in concentration of gadolinium on T1weighted images. Biomarkers have been investigated as an additional diagnostic tool for elbow osteoarthritis.80,81 Collagenase generated cleavage neoepitope of type I I collagen is increased in dogs with MCD compared with dogs with unaffected elbows, and there is moderate correlation between the amount and the severity of cartilage lesions.81 Decreased serum tissue inhibitor of metalloproteinases 1 has been detected in dogs with osteoarthritis.80 These tests have not been implemented clinic ally. Treatment for Medial Coronoid Disease Conservative management Conservative management consists of weight management, consistent, nonconcussi ve exercise, and medications to treat inflammation and pain.35 Medical

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30 management results in a more rapid return to weight bearing than surgical intervention, and in the longer term similar degrees of osteoarthritis have been noted in both groups.82,83 Althoug h weight loss is one of th e tenets of treatment for osteoarthritis, a longitudinal study investigating the effect of feeding on the development of elbow osteoarthritis also found a significant benefit to lifelong feeding of a diet 25% less dense in calories compared with a control diet.84 Despite widespread use, electrostimulated a cupuncture has not been demonstrated to have any effect in objective or subjective outcome measures in dogs with elbow osteoarthritis secondary to elbow dysplasia.85 Intra articular administration of various substances has been reported to ameliorate pain and mitigate progression of osteoarthritis of the elbow; however none of these subst ances have gained widespread clinical acceptance Intra articular a utologous adiposederived mesenchymal stem cells have been advocated for the treatment of elbow osteoarthri tis, however definitive randomized, controlled, doubleblinded studies are required to definitively demonstrate their effectiveness.86 Intra articular autologous conditioned plasma appears superior to intraarticular cortisone and hyal uronic acid in a prospective, ran domized, doubleblinded study .87 A lthough canine platelets cannot be concentrated to the same degree as in humans or horses, increased concentration of growth factors may result in anti inflammatory, anti degradative and analgesic effects.87 Intra articular botulinum toxin type A shows promise as any agent to reduce pain and lameness associated with osteoarthritis, although further investigation with objective outcome measurement is warranted.88 Analgesia is attributed to localized muscular paralysis and inhibition of release of neurotransmitters than facilitate nociception.8 8

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31 Removal of pathologic cartilage and bone Surgical intervention is often advised for dogs up to 24 months of age with clinical or radiographic signs. Dogs with significant osteoarthritic changes are usually considered poor surgical candidates and conservative treatment is recommended.34 Gross pathology of the cartilage of the MCP is commonly characterized arthroscopically using the Modi fied Outerbridge Scoring System and may range from cartilage malacia, fibrillation or fissuring to subchondral bone erosion or eburnation.89 Table 11. Modified Outerbridge Scoring System for arthroscopic evaluation of cartilage pathology89 Modified Outerbridge Score Gross Cartilage Quality 0 Normal 1 Chondromalacia 2 Partial thickness fibrillation 3 Deep fibrillation 4 Full thickness cartilage loss 5 Subchondral bone eburnation Erosive cartilage lesions, often referred to as kissing lesions, frequently develop on the articulating surface of the humeral trochlea in association with MCP pathology. Kissing lesions are much more likely to occur when there are displaced fragments of the medial coronoid process rather than in dogs with non displaced fissures Arthroscopy Arthroscopic examination of the elbow joint of the dog was first reported in 1993 and, in combination with CT is considered the gold standard for diagnosis of medial coronoid pathology due to the complementary nature of these modalities .30,71 Arthroscopy has been demonstrated to be superior to both arthrotomy and medical management for the treatment of medial coronoid disease due to increased magnification and capacity for more thorough examination of the elbow joint .90 The

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32 establishment of arthroscopic portals for examination of elbow is considered safe as long as standard portals are employed.91 Figure 114. Standard arthroscopic portals for the dog elbow and arthroscopic image demonstrating a cartilage flap at the apex of the MCP92 Subtotal c oronoidectomy Removal of a larger portion of the MCP o f the ulna, regardless of the extent of gross pathology, has been advocated as an alternative to focal treatment or osteotomy of pathologic cartilage or bone.93 The rationale for this treatment stems from histopathologic studies which have demonstrated microscopic subchondral bone pathology extending further than that grossly visible to encompass the entire cranial portion of the MCP, which may be a source of residual pain and inflammation.9 Subjective lameness scoring has indicated a favorable outcome following this procedure, although a randomized, blinded prospective clinical trial with objective outcome measures is required to further substantiate the purported benefits compared with focal treatment.93

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33 Bic eps Ulnar Release P rocedure A large portion of the combined tendon of insertion of the biceps brachii and brachialis muscles inserts on the abaxial portion of th e MCP with the remainder attaching to the proximal radius.60 It has been proposed that contraction of this biceps brachii brachialis myotendinous unit may rotate the MCP against the radial head, resulting in supraphysiologic loading and subsequent fatigue fracture of the MCP in the region of the radial incisure.45,60 Some authors therefore advocate a tenotomy of the distal insertion of the combined tendon of insertion onto the ridge, immediately caudal to the abaxial part of the MCP .22,45 Dogs are considered candidates for this procedure when rotational instability with excessive force of supination is s uspected as a cause for medial coronoid disease, or when a focal subchondral lesion in the region of the radial incisure is present.45 Biceps ulnar release procedure, therefore, is considered primarily in juvenile dogs with clinical forelimb lameness attributable to the elbow but with minimal arthroscopic or CT changes to mitigate progression of disease instead of dogs with endstage coronoid disease.45 A study of 39 dogs with 49 biceps ulnar release procedures alone or in combination with SCO demonstrated significant im provement in lameness based on visual analogue scores performed by the owners, and no significant difference was noted between operated limbs and the contralateral normal limb on force plate analysis.94 More recently, arthroscopic assisted bicep ulnar release procedure with a custom designed guarded cutting knife has been described with good subjective outcome .22 However, there are as yet no biomechanical studies or randomized, controlled clinical studies to substantiate these findings.

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34 Figure 115. Biceps ulnar release procedure45 Corrective O steotomies Ulnar osteotomy Static or d ynamic proximal ulnar osteotomy. Ulnar osteotomy has resulted in radiographic resolution of elbow osteoarthritis in a case report in one dog following bilateral FMCP removal and unilateral ulnar osteotomy.95 The main complication associated with fixation of osteotomized segments is implant failure. Dynamic proximal uln ar osteotomy was first described in 1998 to displace the MCP in a caudomedial direction and reduce force transmission through the medial compartment.96 A proximal ulnar osteotomy obliqued in two planes ( caudoproximal to craniodistal and proximolateral to distomedial ) has since been recommended to reduce the incidence of varus associated with an osteotomy obliqued solely in the sagittal plane.45 Pfeil proximal ulnar osteotomy. A transverse ulnar osteotomy has been designed to realign the mechanical axis of the limb and unload the medial compartment of the elbow by re aligning the mechanical axis of the forelimb, similar to sliding humeral

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35 osteotomy and high tibial wedge osteotomy in people. This procedure has been utilized clin ically with initial promising results although randomized, controlled clinical trial with objective outcome measures (Vezzoni, personal communication) Figure 116. Pfeil ulnar osteotomy Image courtesy of Aldo Vezzoni. Distal ulnar osteotomy Distal ulnar osteotomy is less complex to perform and theoretically associated with lower morbidity due to excessive translation of the proximal segment. However, distal ulnar ostectomy does not restore normal proximal radioulnar joint congruenc y, most likely because of the interrosseous ligament preventing proximal movement of the proximal ulnar segment although exvivo studies have demonstrated no difference in proximal displacement of the ulna with release of the interosseous muscle.50,97 Humeral osteotomy Sliding humeral osteotomy. S liding humeral osteotomy is a surgical procedure designed to unload the medial compartment of the elbow through realignment of the

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36 mechanical axis of the limb.98,99 The elbow inherently has a degree of varus angulati on, and therefore the medial compartment experiences significant compressive load.98,100 A mid diaphyseal transverse osteotomy is performed and the proximal segment is translated lateral to t h e distal segment and stabilized with a custom, locking, stepped plate applied to the medial aspect of the humerus .101 By translating the distal segment medially, alignment is restored between the mechanical axis of the limb and the center of the elbow.98,99 E xvivo biomechanical studies have demonstrated that contact area and pressures transmitted across the humeroulnar joint decrease following slidi ng humeral osteotomy.98,99 A case seri es of 49 dogs with 59 limbs in which SHO had been performed with or without focal treatment of the diseased MCP revealed a subjective decrease in lameness and pain score s three and six months post operatively although no control group or objective outcome measures were available.101 Histopathology performed in one dog 17 months post operativ ely revealed resurfacing of the humeral trochlea with fibrocartilage.101 Major complicat ion rates associated with this procedure, primarily implant failure and humeral fracture were initially unacceptable, although more recent generations of the procedure have been associated with a reduced severe complication rate in the region of 5% which is deemed clinically acceptable.101 Wedge humeral osteotomy. In biomechanical studies, a 10 opening medi al wedge osteotomy reduce d contact area in the medial elbow compartment, but did not change contact pressures across the humeroulnar and humeroradial joints .98,99 This procedure has not been utilized clinically Rotational humeral osteotomy. An ex vivo cadaveric study investigating a transverse osteotomy of the distal humerus with 15 external rotation noted lateral shifts

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37 in peak pressure location and center of pressure, although no significant differences in contact area, peak or mean contact pressure occurred.102 Further investigation of this procedure is required before clinical implementation. Elbo w D enervation Denervation of the sensory supply to the joint capsule of dog elbow s via combined medial and lateral approaches may be a feasible treatment option to reduce pain and potentially mitigate the progression of elbow osteoarthritis in dogs.103 R esearch in cadaveric specimens and in four clinically normal dogs demonstrated appropriate remo val or nerve tissue with no reported adverse affects to forelimb function or cutaneous sensation.103 This procedure will likely only benefit dogs in which soft tissue structures, such as the joint capsule or synovium, are significant contributors to pain, as the subchondral bone itself is a significant source of pain in osteoarthritis.103,104 This procedure has not yet been implemented clinically. Salvage P rocedures Medial compartment resurfacing The Canine Uni compartmental E lbow system (CUE) (Arthrex, Inc., Naples, FL) has been developed to enable r e surfacing of the me dial compartment of the elbow as a salvage technique to reduce pain associated with eburnation of the subchondral bone of the medial compartment. The technique is considered reasonably low morbidity and less invasive than total elbow arthroplasty as luxation of the elbow is not required and minimal bone stock is removed. A cobalt chrome humeral implant with a bone ingrowth surface (Biosync) is implanted into the humeral trochlea, with a corresponding ultra heavy molecular weight polyethylene cylinder implanted in the opposing medial coronoid process. P reliminary results in clinical cases are promis ing with 80% of cases

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38 returning to acceptable or full limb function with an ave rage six month recovery period.105 Figure 1 17. Canine unicompartmental elbow resurfacing105 Total elbow arthroplasty Total elbow replacement or arthroplasty is warranted when endstage disease of both medial and lateral elbow compartments is present. The system m ost commonly used currently is the TATE Elbow system (BioM edtrix LLC., Boonton, NJ ). When compared to the earl ier IOWA system, the TATE Elbow offers several advancements. The TATE Elbow is considered more of a resurfacing implant and as such requires a less invasive approach, reducing the incidence of post operative luxation and fracture and theoretically hastening post operative recovery. The IOWA system is a cemented system and may undergo aseptic loosening whereas the TATE Elbow is cementless with a porous surface that relies on bony ingrowth for stability and therefore should have a lower risk of aseptic loos ening, and a longer lifespan. Complications associated with total elbow arthroplasty include fracture of the humerus, ulna or medial epicondyle, infection and implant failure and may be catastrophic, potentially necessitating amputation.

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39 CHAPTER 2 CONTACT M ECHANICS AND 3D ALIG NMENT OF NORMAL DOG ELBOWS Background 3D kinematic analysis in humans has revealed that supination and pronation of the antebrachium is complex and may significantly alter joint contact pressures at the elbow.106,107 In vivo 2D and 3D kinematic studies and inverse dynamics analysis of nor mal and diseased forelimbs of dogs at a walk and trot have been reported.108111 These modalities address movement of the elbow only in the sagittal plane, and cannot assess motion between the three individual articulations of t he elbow during motor activity. Previous cadaveric studies have evaluated relative ar ticular contact areas in the humeroulnar and humeroradial articulations using joint casting techniques in axially loaded normal elbows and following surgical procedures designed to unload the medial compartment of the elbow.50,98,112 Solid casting techniques allow quantification of contact area, but may underestimate or fail to adequately evaluate some contact regions and cannot assess joint contact pressures. Trans articular force maps of normal elbows and elbows following sliding and wedge humeral osteotomies have been generated using tactile array pressure sensors; however, the exact contact area, peak and mean contact pressure and peak pressure location could not be obtained from the data supplied by these sensors.21,99 In addition, these studies have been performed only at one static pose, with the elbow at 135 flexion and with the antebrachium in a neutral position.21,50,98,99,112

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40 Figure 21. C ontact areas (A)112 and force distribution (B) in the normal elbow21 The most salient limitation of all previous studies investigating elbow biomechanics in dogs is that the 3D joint alignment and contact mechanics have not been evaluated simultaneously. An integrated approach to investigation of joint pathology has been advocated in human orthopedic research and a similar approach may help to elucidate the etiopathogenesis of elbow dysplasia in dogs.113 Some authors have implicated proximal radioulnar joint instability or incongruency in the pathogenesis of FMCP .45,60 Understanding the effects of antebrachial rotation and elbow flexion and extension on elbow contact mechanics and alignment would aid our understanding of normal elbow function and develop a methodology to study the biomechanical basis of elbow dysplasia. Surgical techniques involving the elbow could subsequently be evaluated using a model that allows acquisition of both 3D alignment and contact mechanical parameters. The objectives of this cadaveric study were to: 1) evaluate the effect of antebrachial rotation (neutral, 16 pronation and 28 supination) and elbow flexion ang le (135, 115 and 155) on humeroulnar and humeroradial contact area and pressures and

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41 2) simultaneously determine the 3D alignment of each the humeroradial, humeroulnar and radioulnar joints at each elbow pose. We hypothesized that 1) in pronation mean ( CP ) and peak contact pressure (PCP) would increase in the medial compartment and decrease in the lateral compartment, with a cranial shift of anatomic location of PCP in the medial com partment, and 2) in supination CP and PCP would decrease in the medial com partment and increase in the lateral compartment, with a caudal shift of the anatomic location of PCP in the medial compartment. We further hypothesized that contact pressures would increase in flexion and decrease in extension. Minimal axial rotation of t he ulna was expected during pronation and supination of the antebrachium, with the majority of motion occurring due to gliding of the radial head relative to the radial incisure of the ulna. We also hypothesized that in pronation the apex of MCP would rotate internally and translate proximally relative to the center of the radial head, and conversely in supination the apex of MCP would rotate externally and translate distally relative to the center of the radial head. Materials and Methods Specimen Preparat ion Thoracic limbs (n=18) were collected by disarticulation at the glenohumeral joint in 18 adult dogs (mean 27 4 kg body weight) that were euthanized for reasons unrelated to this study. An equal number of unpaired right (n=9) and left (n=9) thoracic li mbs were selected for this study. Orthogonal radiographic projections were obtained of each elbow and specimens were excluded if there was any evidence of elbow pathology. The specimens were wrapped in saline (0.9% NaCl) solution soaked towels, sealed in d ouble plastic bags and stored at 20C.

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42 The limbs were thawed to room temperature the day prior to testing. Tissues were kept moist during testing by intermittently spraying the specimens with room temperature isotonic saline (0.9% NaCl). Soft tissue structures proximal to the elbow including skin, fascia, muscle and neurovascular tissue were excised, excluding the origins of the antebrachial muscles and the medial and lateral collateral ligaments on the medial and lateral epicondyles The extensor car pi radialis, common digital extensor, and supinator muscle bellies were excised to expose the cranial aspect of the elbow and radial diaphysis. The anconeus muscle, anconeal ligament and the joint capsule were completely excised. The annular ligament was preserved. Fiduciary arrays were implanted in the humerus, radius and ulna to determine the 3D, static pose of the elbow during testing. Three 25 mm long M3 nylon phillips machine screws (McMaster Carr Supply Company, Cleveland, OH) were implanted in both the humerus and the radius forming the fiduciary array. Fiduciary arrays custom made with 3.175 mm diameter fiberglass rod (McMaster Carr Supply Company, Cleveland, OH) we re implanted into each the proximal ulna, distal ulna and the third metacarpal bone. 0.0625 mm diameter stainless steel (type 302) balls (McMaster Carr Supply Company, Cleveland, OH) were adhered with tape to the center of each screw head and into three depressions drilled in each fiberglass array as markers for CT id entification. A CT scan was performed with the specimen in a supine position for identification of the metal ball markers and defined anatomic landmarks ( Aquilion 8 slice MultiDetector Row Com puted Tomography Unit, Toshiba, Tustin, CA. ). Images were reconstructed by use of bone and soft tissue algorithms with a slice thickness of 1 mm. Specimens were excluded if there was evidence of joint pathology on CT examination. Following CT, the

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43 stainles s steel ball markers were carefully removed from the screw heads and fiberglass arrays to minimize displacement of the fiduciary arrays. A 4.0 mm diameter hole was drilled transversely through the humeral condyle, from the medial to the lateral humeral epicondyle. A medial epicondylar osteotomy, circumscribing the transcondylar hole, was performed using an oscillating bone saw, while preserving the origin of the medial collateral ligament and the antebrachial flexor musculotendinous units on the medial epi condyle. The medial epicondyle was reflected distally, which allowed sensors to be placed in both the medial and lateral elbow compartments. The joint was inspected grossly following the osteotomy and elbows were subsequently excluded if there was evidence of cartilage damage in the medial aspect of the humeral condyle subsequent to the osteotomy or gross evidence of elbow pathology including cartilage malacia, fibrillation or eburnation, subchondral bone exposure or fragmentation in the region of the medial coronoid process, ununited anconeal process or cartilage lesions on the humeral trochlea or osteoarthritis. One strip of a custom I scan sensor (Tekscan, Inc. Boston, MA) was placed in medial compartment and the other in the lateral compartment of the el bow. The sensors were secured cranially by placing a 2.7 mm cortical bone screw (Synthes, Inc., West Chester, PA) through the reinforced peripheral tabs devoid of sensing elements into the proximal radial diaphysis. The medial epicondyle was anatomically r epositioned and secured by insertion of a 3.8 mm transcondylar screw through the hole previously drilled. A nut placed on the screw was used to compress the osteotomy. Braided steel cable (1.6 mm diameter) was passed through a 2.5 mm diameter hole drilled transversely through the caudal aspect of the olecranon process and a

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44 crimp (McMasterCarr Supply Company, Cleveland, OH) was used to secure the two ends of the cable to form a loop 20 mm in diameter. A hole was drilled from caudodistal to cranioproximal c entrally through the proximal humerus. The hole was initiated at the insertion of the teres major muscle and emerged distal to the insertion of the teres minor muscle. A 4.8 mm diameter steel J hook (McMaster Carr Supply Company, Cleveland, OH) was placed through this hole and secured with a hex nut. A turnbuckle link (Stanley Hardware, New Britain, CT) was attached between the hook in the proximal humerus and the cable loop in the caudal olecranon. This mechanism simulated overall pull of the triceps muscl es and allowed for alteration of the length of t he simulated triceps mechanism. The limbs were loaded in a material testing machine (MTS 858 MiniBionix, MTS, Eden Prairie, MN) and secured to a custom jig proximally by passing a 6 mm diameter and 76 mm overall length adjustable clevis pin (McMasterCarr Supply Company, Cleveland, OH) from the jig transversely through a hole drilled through the humeral head at a level just distal to the lesser tubercle and securing with a cotter pin (Figure 2 1) The distal part of the jig consisted of a 66 mm internal diameter, 3.2 mm thick D2 ring (IMEX TM Veterinary Inc. Longview, TX) secured 50 mm proximal to a wooden base with 100 mm long, 6 mm diameter threaded rods (IMEX TM Veterinary Inc. Longview, TX). The paw was pl aced through the D2 ring. Pronation and supination of the antebrachium was performed by rotating a 2.8 mm diameter centrally threaded full splintage pin ( IMEX TM Veterinary Inc. Longview, TX) placed transversely through the metacarpal bones about the circu mference of the D2 ring The position of the pin was maintained using 6.3 mm Titanium hybrid rods with 6 mm thread mounted to the D2 ring with 6 mm Hex nuts

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45 ( IMEX TM Veterinary Inc. Longview, TX). The base was covered in coarse sandpaper with a digit block at the cranial aspect of the nails The paw was not rigidly fixed to the base and the metacarpal pad was skewered with a 1.6 mm Kirschner wire protruding 5 mm above the surface of the base to prevent excessive cranial transl ation of the paw during loading Figure 2 2 Limb loaded in the custom jig in the materials testing machine

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46 Testing Protocol T he turnbuckle was adjusted to maintain an initial elbow flexion angle of 135 5 corresponding to the mean elbow flexion angle during the stance phase of trotting in healthy Greyhounds and walking and trotting in mixedbreed dogs .108,111,114, 115 An axial load of 50 N was applied with the abduction/adduction component of the jig unconstrained. With real time monitoring of the contact patterns, the jig was locked in a position that maintained approximately 50:50 medial to lateral force distri bution. A probe was used to apply gentle pressure to the sensing elements overlying the most cranial aspec t of the radial head and the apex of the medial coronoid process, creating points of reference to enable future determination of relative anatomic loc ations of peak contact pressure on the contact map. A static axial load of 200 N was applied by the material testing machine, corresponding to the peak vertical ground reaction force of dogs at a walk (5.0 7.5 N/kg).116 The elbow flexion angle was measured during loading with one arm of the goniometer aligned along the shaft of the humerus to the center of the humeral head, and the other arm along the radial diaphysis to the ulnar styloid to ensure that elbow flexion remained within 5 of the desired angle. Instantaneous intraarticular contact area and pressure measurements were acquired after maintaining peak force for 5 seco nds using the I scan system (Tekscan Inc., Boston, MA), consisting of a custom designed, plastic laminated, thinfilm (0.1 mm) electronic pressure sensor, sensor handle, and data acquisition and analysis software. The sensor consists of two 31 x 12 mm sens ing pads with 0.01 MPa sensitivity and a measurement r ange of 0.530 MPa. Each new sensor was conditioned and calibrated according to the manufacturers guidelines immediately before testing of the specimen. W ith the specimen loaded at 200 N, the 3D static pose of the fiduciary markers on each

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47 the humerus, radius, ulna and third metacarpal were digitized using a Microscribe 3DX digitizing arm (Immersion Corp., San Jose, CA) possessing an accuracy of 0.23 mm.117 Figure 23. I scan sensor and representative pressure map obtained Testing Sequence Each specimen was tested with the antebrachium in a neutral position, in 16 pronation and in 28 supination, approximating the reported midpoint of the linear region of load deformat ion curves generated from antebrachii rotated to failur e.118 These values corresponded with the subjective normal degrees of pronation and supination obtained during pilot studies. This testing protocol was repeated with the elbow in 115 and 155 of flexion 5 by adjusting the turnbuckle, approximati ng the extremes of reported angles of the elbow during weight bearing at a walk. Data Analysis The digital pressure sensing data acquisition software was used to generate a contact map. The parameters measure d using the I scan system included the CA, CP, PCP and anatomic location of PCP in each compartment. CA was defined as the area

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48 in contact between the capitulum of the humeral condyle and the radial head in the lateral compartment and between the trochlea of the humeral condyle and the medial coronoid process of the ulna in the medial compartment. The CP was defined as the average of the pressures across each contact area. The PCP was defined as the highest pressure measured in each contact area. The relati ve anatomic location of PCP for each pose was defined as the distance from the cranial tip of the radial head in the late ral compartment and from the apex of the MCP in the medial compartment caud al to the peak pressure sensel. Figure 24 O bjective con tact mechanical outcome measurements Joint co ordinate systems have been well established in human kinematic studes in order to ensure repeatability of reference points for each joint.119,120 Such coordinate systemics have previously been described for the dog.121 The locations of the stainless steel balls attached to the fiduciary arrays and the defined anatomic landmarks for the humerus (center of the humeral head, medial and lateral epicondyles), radius (center of radial head, radial and ulnar styloid processes) and ulna (apex of MCP apex of the

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49 olecranon, radial and ulna styloid processes) were identified on the CT scans using ImageJ software (ImageJ 1.42q, NIH, USA). Therefore the orientation of the anatomic landmarks relative to the fiduciary arrays with respect to the CT volume was known. Rotations (flexion/extension, varus/valgus inter nal/external) and translations (cranial caudal, medial lateral, proximal distal) of th e center of the radial head relative to the origin of the humerus (a point midway on a line between the medial and lateral epicondyles), the apex of the MCP relative to t he humeral origin and of the center of the radial head to the apex of the MCP were c alculated using body fixed axes. Calculations were performed using a custom written computer program (Matlab, T he MathWorks Inc., Natick, MA). Figure 25. Joint co ordinate system

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50 Statistical A nalysis Separate 1way repeated measures ANOVAs were used to evaluate differences in dependent variables of contact mechanics (CA, CP, PCP and anatomic location of PCP in the medial and lateral compartments) at the three elb ow flexion angles (115, 135 and 155) and the 3 elbow positions (neutral, supination, pronation). Separate 1 way repeated measures ANOVA s was used to evaluate differences in rotation ( internal external rotation varus valgus flexion extension) and translation ( cranial caudal proximal distal and medial lateral ) for each the humeroradial, humeroulnar and radioulnar joints for each of the nine elbow poses Where significant differences were detected post hoc pairwise comparisons with a Bonferroni adjustm ent were calculated. All statistical analyses were performed using SPSS statistical software (version 17.0; SPSS Inc, Chicago, IL) with the level of significance for all statistical tests set at Results Contact Mechanics CA was significantly greater in the lateral (58 64% total CA) compartment than the medial compartment at all flexion angles and antebrachial rotations (Table 21, Figure 2 2). There was no significant difference in contact pressure distribution between medial and lateral compartments at any pose. At all three flexion angles, pronation decreased CA in both medial and lateral compartments, with CA ranging from 7792% and 9096% of neutral, respectivel y. No consistent alteration in CP was noted in either compartment. Pronation sig nificantly increased PCP in the lateral compartment (110% of neutral) at 155. At all three elbow flexion angles, supination decreased CA in both medial and lateral compartments, with CA ranging from 8185% and 9091% of neutral respectively.

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51 Supination si gnificantly increased PCP in the lateral compartment at all flexion angles (ranging from 106122% of neutral) and in the medial compartment at 155 (115% of neutral). When the elbow was flexed to 115, CA decreased 14% in the medial compartm ent at a neutral rotation, and CP and PCP increased in both medial and lateral compartments. When the elbow was extended to 155, CA, CP and PCP decreased in the medial and lateral compartments. Figure 26. Displacement of the MCP and corresponding superimposed contact maps in neutral, pronat ion and supinat i on

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52 Table 21. Contact mechanical data acquired from the medial and lateral elbow compartments at each of the nine elbow poses tested 135 115 155 Neutral (1) Pronation (2) Supination (3) Neutral (4) Pronation (5) Supination (6) Neutral (7) Pron ation (8) Supination (9) Contact area (mm 2 ) Total Medial Lateral 209.2 31.3 87.0 13.9 122.2 24.6 179.5 27.8 67.3 12.7 P1-2 = 0.001 112.2 24.4 P1-2 = 0.013 181.2 22.9 70.8 14.9 P1-3 0.001 P2-3 = 1.0 110.3 16.0 P1-3 = 0.007 P2-3 = 1.0 198.2 22.0 74.7 14.5 P1-4 = 0.002 123.5 17.5 P1-4 = 1.0 187.2 25.7 69.3 15.4 P4 -5 = 0.407 P2 -5 = 1.0 117.9 24.2 P4 -5 = 0.369 P2 -5 = 0.157 175.7 20.2 63.3 15.4 P4-6 0.001 P5-6 = 0.41 P3-6 = 0.07 112.4 14.3 P4-6 = 0.03 P5-6 = 0.66 P3-6 = 1.0 184.7 24.5 74.9 15.8 P1-7 = 0.023 P4-7 = 1.0 109.8 18.2 P1-7 = 0.02 P4-7 = 0.28 164.6 32.1 63.1 20.4 P7 -8 = 0.122 P2 -8 = 0.768 P5 -8 = 0.325 101.5 22.5 P7 -8 = 0.048 P2 -8 = 0.056 P5 -8 = 0.002 158.6 23.6 62.4 13.2 P7 -9 = 0.03 P8 -9 = 1.0 P3 -9 = 0.08 P6 -9 = 1.0 98.2 17.9 P7 -9 =0.004 P8 -9 = 1.0 P3 -9 = 0.01 P 6 9 = 0.06 Peak pressure magnitude (MPa) Medial Lateral 8.4 1.9 8.2 1.7 9.1 2.3 P1-2 = 0.098 7.9 1.6 P1-2 = 1.0 8.8 2.1 P1-3 = 0.33 P2-3 = 1.0 8.7 1.6 P1-3 = 0.005 P2-3 = 0.01 9.0 2.3 P1-4 = 0.016 8.0 1.5 P1-4 = 1.0 9.5 2.5 P4 -5 = 0.286 P2 -5 =0.04 8.8 1.7 P4 -5 = 0.008 P2 -5 = 0.014 8.7 1.9 P4-6 = 0.48 P5-6 = 0.07 P3-6= 1.0 8.7 2.0 P4-6 = 0.004 P5-6 = 1.0 P3-6 = 1.0 7.4 1.7 P1-7 0.001 P4-7 0.001 6.6 1.4 P1-7 0.001 P4-7 0.001 7.3 2.7 P7 -8 = 1.0 P2 -8 = 0.001 P5 -8 0.001 7.0 1.4 P7 -8 = 0.223 P2 -8 0.001 P5 -8 0.001 8.5 1.8 P7 -9 0.001 P8 -9 = 0.07 P3 -9 = 0.34 P6 -9 = 0.95 8.1 1.7 P7 -9 0.001 P8 -9 = 0.001 P3 -9 = 0.007 P 6 9 = 0.04

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53 Table 21 Continued 135 115 155 Neutral (1) Pronation (2) Supination (3) Neutral (4) Pronation (5) Supination (6) Neutral (7) Pronation (8) Supination (9) Mean pressure (MPa) Medial Lateral 3.6 0.7 3.3 0.8 3.7 1.0 P1 -2 = 1.0 3.7 0.9 P1 -2 = 0.02 4.0 0.9 P1-3 = 0.02 P2-3 = 0.193 3.6 0.9 P1-3 = 0.07 P2-3 = 1.0 3.9 0.9 P1-4 = 0.006 3.7 0.7 P1-4 = 0.005 4.0 1.0 P4-5 = 1.0 P2-5 = 0.03 3.9 0.8 P4-5 = 0.092 P2-5 = 0.095 4.0 1.1 P4-6 = 1.0 P5-6 = 1.0 P3-6= 1.0 3.8 0.7 P4-6 = 0.99 P5-6 = 1.0 P3-6= 0.05 3.4 0.8 P1 -7 = 0.642 P4 -7 = 0.002 3.1 0.7 P1 -7 = 0.12 P4 -7 0.001 3.0 1.2 P7 -8 = 0.273 P2 -8 = 0.01 P5 -8 = 0.01 3.1 0.9 P7 -8 = 1.0 P2 -8 = 0.001 P5 -8 0.001 4.0 0.9 P7 -9 0.001 P8 -9 = 0.002 P3 -9 = 1.0 P6 -9 = 1.0 3.3 0.7 P7 -9 = 0.23 P8 -9 = 0.47 P3 -9 = 0.008 P 6 9 0.001 Peak pressure location (mm)* Medial Lateral 6.4 2.0 7.4 1.7 4.6 1.8 P1 -2 = 0.17 8.8 3.0 P1 -2 = 0.55 8.5 2.3 P1-3 = 0.11 P2-3 0.001 6.9 1.5 P1-3 = 1.0 P2-3 = 0.2 6.8 2.3 P1-4 = 1.0 8.1 3.7 P1-4 = 1.0 4.8 2.3 P4-5 =0.15 P2-5 = 1.0 8.3 3.4 P4-5 = 1.0 P2-5 = 1.0 8.6 2.3 P4-6 = 0.157 P5-6 = 0.07 P3-6= 1.0 7.1 3.9 P4-6 = 0.56 P5-6 = 1.0 P3-6=1.0 7.7 1.9 P1 -7 = 0.32 P4 -7 = 0.77 7.3 2.5 P1 -7 = 1.0 P4 -7 = 1.0 4.5 1.7 P7 -8 = 0.02 P2 -8 = 1.0 P5 -8 = 1.0 8.6 2.5 P7 -8 = 0.541 P2 -8 = 1.0 P5 -8 = 1.0 9.9 2.6 P7 -9 = 0.08 P8 -9 0.001 P3 -9 = 0.24 P6 -9 = 0.5 6.7 2.1 P7 -9 = 1.0 P8 -9 = 0.14 P3 -9 = 1.0 P 6 9 = 1.0 *Peak pressure location is measured in millimeters caudal to the apex of the medial coronoid process in the medial compartment and from the most cranial extent of the radial head in the lateral compartment .

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54 Anatomic Location of Peak Contact P ressure Flexion or extension of the elbow did not significant ly alter the anatomic location of PCP in the medial compartment relative to the apex of the medial coronoid process or in the lateral compartment relative to the most cranial aspect of the radial head at any antebrachial rotation (Table 21, Figure 21) At all elbow flexion angles, the anatomic location of PCP in the medial compartment was on average 4.4 mm (range 3.95.4 mm) closer to the apex of the medial coronoid process in pronation than in supination. Although the anatomic location of PCP moved on average 1.6 mm (range 1.22.2 mm) further away from the cranial aspect of the radial head in the lateral compartment during pronation and 0.7 mm (range 0.51.0 mm) closer during supination, t hese differences were not statistically significant. Figure 27. PCP locatio n

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55 3D Alignment Humeroradial a rticulation The humeral origin was defined as the midpoint of a line drawn between the medial and lateral epicondyles. At an elbow flexion angle of 135 and a neutral antebrachial rotation, the center of the radial head was loc ated 10.2 2.3 mm caudal, 3.0 1.9 mm lateral and 14.4 1.1 mm distal to the humeral origin (Table 22) The center of the radial head was externally rotated 33.6 4.2 and was 29.2 4.7 varus relative to the humeral origin. The humeroradial flexion angle was significantly more extended during pronation and more flexed during supination than the elbow flexion angle measured during mechanical testing. During pronation and supination, no significant translations in either the cranial caudal, medial lat eral or proximal distal planes occurred. During pronation, the center of the radial head rotated a mean of 5.0 (range 3.56.2) internally relative to the humeral origin and a further 4 (range 2.44.5) varus was induced compared with a neutral antebrach ial rotation. During supination, the center of the radial head rotated an average of 6.8 (range 4.87.2) externally relative to the humeral origin and varus was reduced by a mean of 9.0 (range 6.612.4). When the elbow was flexed to 115, the center of the radial head displaced 2.4 mm (range 2.12.7 mm) more c ranial and 3.4 mm (range 3.23.5 mm) more proximal on the axial plane towards the humeral origin. The center of the radial rotated 19.0 (range 17.619.6) more external relative to the humeral ori gin and varus increased by 11.0 (range 8.113.0). When the elbow was extended to 155, the center of the radial head displaced 3 mm (range 2.83.3 mm) more c audal and 2.0 mm (range 1.92.1 mm) further distal from the humeral origin. The

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56 center of the radial head rotated 13.0 (range 10.915.2) more internal and varus decreased 14.8 (range 1416.1) compared with 135 elbow flexion. Humeroulnar articulation At an elbow flexion angle of 135 and a neutral antebrachial rotation, the apex of the MCP was located 12.9 1.9 mm caudal, 6.3 2.6 mm medial and 10.4 3.0 mm distal to the origin of the humerus (Table 23). The apex of the MCP was externally rotated 32.1 5.8 and 30.5 6.2 varus relative to the humeral origin. When compared with the el bow flexion angle measured during testing, the calculated humeroulnar flexion angle was During both pronation and supination, no significant translations in either cranial caudal, medial lateral planes or proximal distal planes occurred. During pronation, the apex of the MCP rotated an average of 1.2 (range 0.71.9) internal relative to the humeral origin and 1.5 (range 1.11.9) further varus was induced. During supination, the apex of the MCP rotated an average of 1.9 (range 0.71.9) more external relative to the humeral origin and no significant alteration in varus valgus angulation occurred. When the elbow was flexed to 115, the apex of the MCP displaced 2.2 mm (range 1.82.7 mm) more caudal and 4.0 mm (range 3.5 4.8 mm) more proximal on an axial plane toward the humeral origin. The apex of the MCP was rotated 21.0 (18.623.7) more external and 10 (range 9.311.1) more varus was induced compared with an elbow flexion angle of 135. When the elbow was extended to 155, the apex of the MCP displaced 2.8 mm (range 2.82.9 mm) more cranial and 3.4 mm (range 2.93.8 mm) more distal on an axial plane away from the humeral origin. The apex of the MCP was rotated 14.0 (range 13.714.9 ) more internal and 12.8 (range 12.413.2) less varus

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57 was induced compared with an elbow flexion angle of 135 and a neutral antebrachial rotation. Proximal radioulnar articulation At an elbow flexion angle of 135 and a neutral antebrachial rotation, t he apex of the MCP was 1.5 4.5 mm caudal, 10.3 2.6 mm medial 3.9 2.2 internally rotated relative to the center of the radial head (Table 24). During pronation, the apex of the MCP moved 1.0 mm (range 0.71.2 mm) more cranial relative to the center of the radial head at 115 and 155 elbow flexion, however no other significant rotation or translation was noted. During supination, the apex of the MCP translated 2.8 mm (range 2.23.3 mm) more caudal and 1.6 mm (range 1.11.7 mm) more medial relative to the center of the radial head. No significant alteration in alignment was noted when the elbow was flexed to 115 or extended to 155.

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58 Table 22. Static 3D alignment of the humeroradial articulation for the nine elbow poses tested 135 115 155 Neutral (1) Pronation (2) Supination (3) Neutral (4) Pronation (5) Supination (6) Neutral (7) Pronation (8) Supination (9) Translations (mm) Cranial caudal Medial lateral Proximal distal 10.2 2.3 3.0 1.9 14.4 1.1 10.6 2.4 P1 -2 = 0.44 3.8 1.1 P1 -2 = 0.09 14.1 1.4 P1 -2 = 0.18 11.1 1.6 P1-3 = 0.24 P2-3 = 0.76 4.1 0.8 P1-3 = 0.248 P2-3 = 1.0 13.9 1.9 P1-3 = 0.42 P2-3 = 1.0 12.9 2.5 P1 -4 4.2 1.5 P1 -4 = 0.02 11.0 1.6 P1 -4 13.1 2.4 P4-5 = 1.0 P2-5 3.6 1.7 P4-5 = 0.06 P2-5 = 1.0 10.6 1.7 P4-5 = 0.78 P2-5 13.2 1.9 P4-6 = 0.7 P5-6 = 1.0 P3-6 4.4 1.0 P4-6 = 1.0 P5-6 = 0.09 P3-6 = 0.65 10.7 2.0 P4-6 = 0.71 P5-6 = 1.0 P3-6 7.1 2.6 P4 -7 P1 -7 3.2 1.4 P4 -7 = 0.02 P1 -7 = 1.0 16.3 1.9 P4 -7 P1 -7 7.6 2.1 P7 -8 = 0.63 P2 -8 P5 -8 2.8 1.7 P7 -8 = 0.62 P2 -8 = 0.11 P5 -8 = 0.06 16.1 1.8 P7 -8 = 1.0 P2 -8 P5 -8 8.1 1.8 P7 -9 = 0.19 P8 -9 = 0.1 P3 -9 P6 -9 3.2 1.5 P7 -9 = 1.0 P8 -9 = 0.85 P3 -9 = 0.26 P6 -9 = 0.03 16.0 1.6 P7 -9 = 0.2 P8 -9 = 1.0 P3 -9 P6 -9

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59 Table 22 Continued 135 115 155 Neutral (1) Pronation (2) Supination (3) Neutral (4) Pronation (5) Supination (6) Neutral (7) Pronation (8) Supination (9) Rotations () Flexion e xtension Internal e xternal Varus valgus 138.2 5.6 33.6 4.2 29.2 4.7 148.2 6.6 P1 -2 28.2 3.4 P1 -2 33.7 4.5 P1 -2 121.4 7.7 P1-3 P2-3 40.8 4.1 P1-3 P2-3 20.8 5.6 P1-3 P2-3 123.2 7.3 P1 -4 52.0 5.5 P1 -4 41.3 4.5 P1 -4 132.4 8.7 P4-5 P2-5 45.8 5.9 P4-5 P2-5 46.7 3.7 P4-5 P2-5 110.0 7.5 P4-6 P5-6 P3-6 60.4 4.4 P4-6 P5-6 P3-6 28.9 5.2 P4-6 P5-6 P3-6 146.1 6.9 P4 -7 P1 -7 20.8 3.6 P4 -7 P1 -7 15.2 4.9 P4 -7 P1 -7 157.1 6.8 P7 -8 P2 -8 P5 -8 17.3 3.5 P7 -8 P2 -8 P5 -8 17.6 4.7 P7 -8 P2 -8 P5 -8 125.6 5.9 P7 -9 P8 -9 P3 -9 = 0.03 P6 -9 25.6 4.3 P7 -9 P8 -9 P3 -9 P6 -9 8.6 4.1 P7 -9 P8 -9 P3 -9 P 6 9 For translational variables, positive values indicate cranial, distal and medial positions of the radius relative to the humerus. For the rotational variables, positive values indicate greater external radial rotation and varus. P values for post hoc pairw ise comparisons are given where significant dif ferences were detected by ANOVA .

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60 Table 23. Static 3D alignment of the humeroulnar articulation for the nine elbow poses tested 135 115 155 Neutral (1) Pronation (2) Supination (3) Neutral (4) Pronation (5) Supination (6) Neutral (7) Pronation (8) Supination (9) Translations (mm) Cranial caudal Medial lateral Proximal distal 12.9 1.9 6.3 2.6 10.4 3.0 12.6 1.8 P1-2 = 0.73 6.3 2.6 P1-2 =1.0 11.4 3.0 P1-2 = 0.2 13.5 2.5 P1-3 = 0.2 P2-3 = 0.04 5.0 2.4 P1-3 = 0.004 P2-3 10.2 3.5 P1-3 = 1.0 P2-3= 0.09 15.1 1.7 P1-4 5.1 2.8 P1-4 = 0.09 6.9 3.1 P1-4 15.3 2.3 P4 -5 = 1.0 P2 -5 5.5 2.7 P4 -5= 0.39 P2 -5 = 0.42 6.6 3.5 P4 -5= 0.76 P2 -5 15.3 1.8 P4-6 = 1.0 P4-5 = 1.0 P3-6 = 0.04 4.8 2.4 P4-6 = 0.35 P5-6 = 0.03 P3-6 = 1.0 6.3 3.7 P4-6 = 0.24 P5-6 = 0.62 P3-6 < 0.001 10.1 2.9 P4-7 P1-7 6.6 2.2 P4-7 =0.01 P1-7 = 0.72 14.2 2.5 P4-7 P1-7 9.8 3.3 P7 -8 = 0.86 P2 -8 = 0.003 P5 -8 7.3 2.5 P7 -8 = 0.02 P2 -8 = 0.03 P5 -8 = 0.05 14.3 3.3 P7 -8 = 1.0 P2 -8 P5 -8 10.6 2.1 P7-9 = 0.76 P8-9 = 0.45 P3-9 P6-9 6.0 2.7 P7-9 = 0.08 P8-9 0.001 P3-9 = 0.07 P6-9 = 0.04 13.6 3.5 P7-9 = 0.53 P8-9 = 0.12 P3-9 P 6 9

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61 Table 23 Continued 135 115 155 Neutral (1) Pronation (2) Supination (3) Neutral (4) Pronation (5) Supi nation (6) Neutral (7) Pronation (8) Supination (9) Rotations () Flexion e xtension Internal e xternal Varus valgus 133.5 7.7 32.1 5.8 30.5 6.2 136.3 7.7 P1-2 30.2 5.4 P1-2 = 0.04 32.4 6.7 P1-2 129.8 7.7 P1-3 P2-3 34.0 6.4 P1-3 = 0.02 P2-3 = 0.001 30.9 6.6 P1-3 = 1.0 P2-3 = 0.1 118.6 8.0 P1-4 50.7 6.3 P1-4 41.6 5.4 P1-4 120.3 9.3 P4 -5 = 0.14 P2 -5 < 0.001 50.0 6.8 P4 -5 = 1.0 P2 -5 43.0 4.8 P4 -5 = 0.09 P2 -5 114.9 7.9 P4-6 P5-6 P3-6 53.9 6.4 P4-6 P5-6 = 0.006 P3-6 40.2 5.1 P4-6 P5-6 = 0.002 P3-6 142.6 7.3 P4-7 P1-7 17.6 3.8 P4-7 P1-7 18.1 5.9 P4-7 P1-7 < 0.001 146.0 7.1 P7 -8 P2 -8 P5 -8 16.5 4.5 P7 -8 = 0.06 P2 -8 P5 -8 19.2 6.4 P7 -8 = 0.07 P2 -8 P5 -8 139.5 7.3 P7-9 P8-9 P3-9 P6-9 19.1 4.7 P7-9 = 0.02 P8-9 P3-9 0.001 P6-9 18.0 6.2 P7-9 = 1.0 P8-9 = 0.04 P3-9 P 6 9 For translational variables, positive values indicate cranial, distal and lateral positions of the proximal ulna relative to the humerus. For the rotational variables, positive values indicate greater humeroulnar flexion, external ulnar rotation and varus. P values for post hoc pairwise comparisons are given where significant dif ferences were detected by ANOVA.

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62 Table 24. Static 3D alignment of the proximal radioulnar articulation for the nine elbow poses tested 135 115 155 Neutral (1) Pronation (2) Supination (3) Neutral (4) Pronation (5) Supinatio n (6) Neutral (7) Pronation (8) Supination (9) Translations (mm) Cranial caudal Medial lateral Proximal distal 1.5 4.5 10.3 2.6 0.3 2.3 1.5 4.3 P1 -2 = 1.0 10.5 2.2 P1 -2 = 1.0 0.9 2.9 P1 -2 = 0.62 4.3 5.0 P1-3 P2-3 8.6 4.4 P1-3 = 0.13 P2-3 = 0.18 0.4 2.2 P1-3 = 1.0 P2-3 = 0.73 2.3 4.6 P1 -4 = 0.24 10.1 2.3 P1 -4 = 1.0 0.9 2.6 P1 -4 = 0.27 0.7 4.8 P4 -5 P2 -5 = 0.3 10.2 1.8 P4 -5 = 1.0 P2 -5 = 0.97 1.1 3.1 P4 -5 = 1.0 P2 -5 = 1.0 4.5 5.1 P4 -6 P5 -6 P3 -6 = 0.82 9.0 4.3 P4 -6 =0.34 P5 -6 = 0.70 P3 -6 = 0.70 0.7 2.4 P4 -6 = 1.0 P5 -6 = 1.0 P3 -6 = 0.44 2.3 5.3 P4 -7 = 1.0 P1 -7 = 0.3 10.0 3.1 P4 -7 = 1.0 P1 -7 = 1.0 0.9 2.3 P4 -7 = 1.0 P1 -7 = 0.76 1.2 5.1 P7 -8 = 0.02 P2 -8 = 1.0 P5 -8 = 0.56 10.2 2.4 P7 -8 = 1.0 P2 -8 = 0.90 P5 -8 = 1.0 0.9 2.5 P7 -8 = 1.0 P2 -8 = 1.0 P5 -8 = 1.0 5.6 2.8 P7 -9 = 0.03 P8 -9 = 0.003 P3 -9 = 0.48 P6 -9 = 0.79 8.1 3.7 P7 -9 = 0.007 P8 -9 = 0.02 P3 -9 = 0.15 P6 -9 = 0.08 0.2 2.8 (P7 -9 = 0.98 P8 -9 = 0.91 P3 -9 = 1.0 P 6 9 = 1.0

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63 Table 24 Continued 135 115 155 Neutral (1) Pronation (2) Supination (3) Neutral (4) Pronation (5) Supination (6) Neutral (7) Pronation (8) Supination (9) Rotations () Flexion e xtension I nternal e xternal Varus valgus 2.6 4.1 3.9 2.2 3.6 5.3 9.1 2.9 P1 -2 4.2 2.5 P1 -2 4.2 5.7 P1 -2 = 0.18 12.4 6.1 P1-3 P2-3 3.8 1.7 P1-3 = 1.0 P2-3 = 1.0 3.2 6.0 P1-3 = 1.0 P2-3 = 0.24 1.9 3.0 P1 -4 = 0.94 4.4 2.2 P1 -4 = 0.26 3.3 5.9 P1 -4 = 1.0 8.4 3.1 P4 -5 P2 -5 = 0.61 4.1 2.0 P4 -5 = 0.25 P2 -5 = 1.0 4.4 5.9 P4 -5 P2 -5 = 1.0 11.7 6.6 P4 -6 P5 -6 P3 -6 = 0.81 4.2 2.0 P4 -6 = 1.0 P5 -6 = 1.0 P3 -6 = 0.44 2.7 6.0 P4 -6 = 0.28 P5 -6 = 0.001 P3 -6 = 0.82 2.1 3.3 P4 -7 = 1.0 P1 -7 = 1.0 4.1 2.3 P4 -7 = 0.34 P1 -7 = 1.0 4.1 5.7 P4 -7 = 0.17 P1 -7 = 1.0 9.5 3.2 P7 -8 P2 -8 = 1.0 P5 -8 = 0.46 3.8 2.3 P7 -8 = 0.67 P2 -8 = 0.73 P5 -8 = 0.86 5.0 5.8 P7 -8 = 0.08 P2 -8 = 0.99 P5 -8 = 0.50 16.0 3.1 P7 -9 P8 -9 P3 -9 = 0.13 P6 -9 = 0.07 3.8 2.3 P7 -9 = 1.0 P8 -9 = 1.0 P3 -9 = 1.0 P6 -9 = 0.80 3.0 5.9 P7 -9 = 0.04 P8 -9 = 0.008 P3 -9 = 0.48 P 6 9 =0.79 For translational variables, positive values indicate cranial, distal and lateral positions of the distal ulna relative to the radius. For the rotational variables, positive values indicate greater radioulnar flexion, external ulnar rotation and varus. P v alues for post hoc pairwise comparisons are given where significant dif ferences were detected by ANOVA.

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64 Discussion Our results substantiate that both antebrachial rotation and elbow flexion and extension caused alterations in elbow contact mechanical param eters and alignment In contrast to our first proposed hypothesis, both pronation and supination increased PCP and decreased CA in both medial and lateral elbow c ompartments. We suspect that this increase in PCP associated with antebrachial rotation occurs due to relative tightening of the joint as the medial and lateral collateral ligaments become progressively more taut in order to restrict motion to the sagittal plane.118 Consistent with our hypothesis, when the elbow was flexed to 115, PCP increased in the medial compartment due to proximal displace ment in the axial plane of the MCP and radial head toward the distal humerus, as well as the increased varus angulation induced with elbow flexion. Similarly when the elbow was extended to 155, PCP decreased in both compartments due to distal displacement in the axial plane of the MCP and radial head relative to the origin of the humerus and decreased varus angulation. Alterations of the mechanical axis relative to the anatomic axis at different elbow flexion angles may also contribute to the changes in PC P. This study established three distinct areas of contact in the elbow (radial head, medial coronoid process and craniolateral aspect of the anconeus) consistent with p revious humeroradial and humeroulnar joint contact area mapping performed using joint ca sting techniques, and subchondral bone densities measured with CT osteoabsorptiometry.20,112 T he articular su rface of the ulna contributed substantially to loa d transfer through the elbow, consistent with previous studies utilizing tactile array pressure sensors and with the higher subchondral bone densities not ed in the ulna compared to the radial head in dogs of all ages .20,21 Our study is the first to report t he

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65 effect of pronation and supination on the topography of the contact points in the humeroradial and humeroulnar joints Pressure mapping is an accepted method for evaluating the interaction of the articular surfaces during motion.113,122 Both in vivo and ex vivo studies have correlated change in location of contact points with abnormal kinematics and development of osteoarthritis.113 I n accordance with our hypothesis, in this static, cadaveric model the anatomic location of PCP in the medial compartment sh ifted closer to the apex of the MCP in pronation and shifted further away from the apex in supination. Interpretation of these findings with respect to a biomechanical basis for elbow dysplasia is difficult because the limbs tested in this study were normal. However, the observation that supination shifts the point of PCP away from the apex of the MCP suggests that dogs with MCD that stand with the elbow adducted and antebrachium supinated may be an attempt to unload the MCP Similar compensatory p ostural adaptations have been reported in people with unilateral knee osteoarthritis.123 Simultaneous i n vivo joint contact mapping and 3D alignment of human or dog elbows has not previously been reported. Correlations between elbow biomechanics in people with those of dogs must be interpreted with caution. Although similar anatomically, there are notable functional differences, the most obvious being the weight bearing function in dogs and the necessity for antebrachial rotation for everyday activity in people.14 The varus deviation noted in the normal dog elbows in this study is consistent with the relative pronation of the dog antebrachium, compared with humans who usually hold the antebrachium in supination, resulting in a carrying angle or valgus deviation of 1015.14 3D ki nematic studies of the superior radioulnar joint in people have revealed that during pronation and supination the radius glides around a relatively

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66 immovable ulna, although slight radial deviation of the ulna may occur during rotation with a fixed manus .14,124 A lthough both situations may occur in quadrupeds, depending on the phase of gait occu rring during the antebrachial rotation the latter combined with axial load transmission is considered the dominant pose and was simulated in this stu dy. Consistent with the results reported in people, minimal axial rotation of the ulna occurred during ant ebrachial rotation, with motion primarily occurring due to gliding of the radial head within the radial incisure and pronation caused increasing varus of the elbow, with decreasing varus in supination.124 In contrast with our hypothesis, proximal distal displacement of the radial head and apex of the medial coronoid process relative to the humeral origin or each other did not occur with pronation or supination, although this may be a reflection of the minute displacements det ec ted and the small sample size. Histopathological studies have demonstrated that medial coronoid disease likely occurs as a result of humeroulnar incongruency, resulting in repetitive excessive loading fatigue failure and microfracture formation in the tr abecular subchondral bone of the medial coronoid process .9,10 We evaluated only normal elbows and therefore cannot draw any conclusion regarding the mechanism of elbow dysplasia. However, the signi ficant effect of antebrachial rotation on contact pressures would suggest that rotational alignment of the humeroradial and humeroulnar joints may be considered an important mechanical variable in elbow biomechanics, as suggested by other authors.45,60 This is the first quantitative descriptive analysis of the ex vivo contact mechanics and 3D alignment of normal dog elbows The results of previous biomechanical studies

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67 investigating the normal dog elbow s and surgical interventions should be reinterpreted in light of the results of our study. Future investigations are w arranted to evaluate the contact mechanics and 3D alignment in patholog ic elbows, in particular elbows from dogs affected with fragmented medial coronoid process and the biomechanical effect s of surgical procedures advocated for the treatment of medial coronoid disease.

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68 CHAPTER 3 EFFECT OF PROXIMAL U LNAR ROTATIONAL OSTEOTOMY ON CONTACT MECHANICS AND 3D ALI GNMENT OF NORMAL DOG ELBOWS Background In people affected with unicompartmental knee osteoarthritis, collapse of the medial compartment may occur due to a preexisting varus deformity or progressive narrowing of the medial joint space secondary to osteoarthritis. Progressive cartilage loss and narrowing of the medial compartment joint space results in increased load transmission in the medial compartment which contributes to the degenerative process .125128 High tibial osteotomies are often performed to unload the medial com partment in people with medial gonarthrosis.129131 These osteotomies shift the mechanical axis of the limb through the lateral compartment of the knee, unloading the affected medial compartment.125,132 Cartilage resurfacing of the medial compartment has been observed in response to the improved mechanical environment.133,134 Many dogs affected with MCD develop progressive collapse of the medial joint space suggesting that a similar mechanism may occur in these dogs as in human s with unicompartmental knee osteoarthritis .135 Thus, humeral or ulnar osteotomies desig ned to unload the medial compartment have been developed to manage dogs with MCD.45,98,99,101 Sliding humeral osteotomy has been demonstrated to shift the mechanical axis of the humerus away from the medial compartment of the elbow98,99, with fibrocartilage being reported to resurface medial compartment full cartilage defects following this procedure.101 Gait adaptations have also been shown to effectively unload the medial compartment of the knee in people with medial compartment osteoarthritis.136138 Walking with a toeout gait reduces knee adduction moment, particularly in the second

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69 phase of stance, shifting the ground r eaction force more posterior and less medial, and mitigates the progression of medial knee osteoarthritis.136 Dogs with FMCP and MCD of the elbow often stand with the affected forelimb supinated, elbow adducted, carpus abducted and paw externally rotated.34 Whether this posture is involved in the pathogenesis of this condition or an adaptive mechanism to unload the medial compartment remains unclear. If this posture is an adaptive mechanism to ameliorate pain, altering the mechanical axis through the elbow by simulating this pos ture m ight unload the medial elbow compartment. Our previous study examining the joint mechanics of normal dog elbows at three different elbow flexion and antebrachial rotation angles demonstrated a significant caudal shift of peak contact pressure on the medial coronoid process during supination compared with pronation. This pressure shift could potentially unload the pathologic MCP B ased on t hese observations, w e wanted to evaluate an ulnar osteotomy procedure that could mimic supination of the antebrachium to shift the mechanical axis of the forelimb and load transmission from the medial to the lateral compartment of the elbow The purpose of the study was to investigate the effects of this proximal ulnar rotational osteotomy on the contact mechanics and 3D alignment of normal dog elbows. We hypothesized that proximal ulnar rotational osteotomy would decrease the contact pressures in the medial compartment with corresponding increases in the lateral compartment. We further hypothesized that proximal ulnar rotational osteotomy would displace the apex of the MCP caudal and distal relative to the radial head, and reduce varus angulat ion through the humeroradial articulation, with minimal changes in alignment through the humeroulnar articulation.

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70 Materials and Methods Specimen Preparation T horacic limbs (n=12) were collected by disarticulation at the glenohumeral joint in 12 adult dogs weighing 26 4 kg that were euthanatized for reasons unrelated to this study. An equal number of unpaired right (n=6 ) and left (n=6 ) thoracic limbs were selected for this study. Orthogonal radiographic projections were obtained of each elbow and specimens were excluded if there was evidence of elbow pathology. The specimens were wrapped in saline (0.9% NaCl) solution soaked towels, sealed in double plastic bags and stored at 20C. The limbs were prepared and loaded in the custom jig in the material testi ng machine exactly as described in Chapter 2. Testing Protocol A static axial load of 200 N was applied by the material testing machine.116 Instantaneous intraarticular contact area and pressure measurements were acquired after maintaining peak force for 5 seconds using the I scan system (Tekscan Inc., Boston, MA). Each sensor was conditioned and calibrated according to the manufacturers guidelines immediately before testing of the specimen. W ith the specimen loaded at 200 N, the 3D static pose of the fiduciary markers on each the humerus, radius, ulna and third metacarpal were digitized using a Microscribe 3DX digitizing ar m (Immersion Corp., San Jose, CA). Testing Sequence Each specimen was tested with the antebrachium positioned in a neutral position, in 16 pronation and in 28 supination.118 This testing protocol was repeated with the elbow in 115 and 155 of flexion 5 by adjusting the turnbuckle. Following conclusion of acquisition of biomechanical parameters in normal elbows, the transcondylar humeral

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71 screw was removed and the medial epicondyle reflected distally to allow access to the proximal radioulnar joint surface and removal and examination of the sensing strips. A caudolateral approach was made to the proximal ulna between ulnaris lateralis and the ulnar and humeral heads of the deep digital flexor muscles which were reflected from the ulnar surface using Freer periosteal elevator. An incomplete transverse osteotomy was made in the proximal ulna using an oscillating bone saw, 10 mm distal to the radial head. A custom 2.7 mm dynamic compression bone plate with a 30 central pivot was applied to the proximal ulnar segment with two 10 mm long 2.7 mm cortical bone screws. The osteotomy was completed and a Kern boneholding forceps applied to the olecranon was used to pivot the proximal ulnar segment externally around the radial head. The joint surface was inspected to eliminate gap formation at the between the radial head and the radial incisure of the ulna. The plate was loosely secured to the distal segment with an 18 mm long 2.7 mm cortical bone screw. Plate luting was performed using polymethylmethacrylate (Technovit Liquid/Powder Kit, Jorgensen Laboratories, Inc., Loveland, Co) and the distal screw tightened while the joint surface was examined. When the acrylic was dry, a 10 mm long 2.7 mm cortical bone screw w as placed in the distal segment. The sensor was recalibrated according to the manufacturers recommendations and replaced in the elbow. The specimen was loaded in the MTS machine and the testing sequence was repeated.

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72 Figure 31. L ocation and orientation of the osteotomy Figure 32. A. P late luting and stabilization and B. caudal displacement of the apex of the MCP Data Analysis The digital pressure sensing data acquisition software was used to generate a contact map and measure CA, CP and PCP in the medial, lateral and total (medial + lateral) elbow compartments. Rotations (flexion/extension, varus/valgus

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73 internal/external) and translations (cranial caudal, medial lateral, proximal distal) of th e center of the radial head relative to the origin of the humerus (a point midway on a li ne between the medial and lateral epicondyles), the apex of the MCP relative to the humeral origin and of the center of the radial head to the apex of the MCP were ca lculated using body fixed axes as previously described. Calculations were performed using a custom written computer program (Matlab, The MathWorks Inc., Natick, MA). The alignment of the humeroradial articulation is described as the displacement of the center of the radial head relative to the humeral origin, the humeroulnar articulation as th e displacement of the apex of the medial coronoid process of the ulna relative to the humeral origin, and the proximal radi oulnar articulation as the displacement of the apex of the medial coronoid process relative to the center of the radial head. Figur e 33. C ontact parameters and 3D alignment A. preand B. post osteotomy

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74 Statistical Analysis Separate 1way repeated measures ANOVAs were used to evaluate differences in dependent variables of contact mechanics (CA, CP and PCP in the medial and lateral e lbow compartments) at the three elbow flexion angles (115, 135 and 155) and the 3 elbow positions (neutral, supination, pronation) prior to and post osteotomy Separate 1 way repeated measures ANOVA s was used to evaluate differences in rotation ( internal external rotation varus valgus flexion extension) and translation ( cranial caudal proximal distal and medial lateral ) for each the humeroradial, humeroulnar and radioulnar articulations for each of the nine elbow poses prior to and post osteotomy Where significant differences were detected, post hoc pairwise comparisons with a Bonferroni adjustment were calculated. All statistical analyses were performed using SPSS statistical software (version 17.0; SPSS Inc., Chicago, IL) with the level of sig nificance for all statistical tests set at Results Contact Mechanics Following proxim al ulnar rotational osteotomy, CP decrease d a mean of 10% (range 518%) in the medial compartment and increased a mean of 17% (range 530%) in the lateral compartm ent. PCP decreased a mean of 9% (range 415%) in the medial compartment and increased a mean of 13% (range 927%) in the lateral compartment. CA did not change significantly in the medial compartment; however CA decreased a mean of 19% (range 1033%) in the lateral compartment.

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75 Table 31. Contact mechanical parameters obtained from the medial and lateral elbow compartments preand post osteotomy for each of the nine elbow poses tested 135 115 155 Neutral Pronation Supination Neutral Pronation Supination Neutral Pronation Supination Contact Pressure (MPa) Medial Pre Post 3.4 0.5 3.1 0.5 P = 0.024 3.6 0.6 3.1 0.8 P = 0.052 3.9 0.6 3.4 0.6 P = 0.243 3.8 0.6 3.5 0.6 P = 0.093 3.9 0.7 3.4 0.8 P 0.001 3.8 0.7 3.6 0.7 P = 0.006 3.3 0.6 3.1 0.7 P = 0.33 3.0 1.1 2.8 0.9 P = 0.627 3.9 0.8 3.2 0.8 P = 0.882 Lateral Pre Post 3.3 0.8 4.2 0.7 P 0.001 3.5 0.9 4.2 1.0 P = 0.009 3.5 0.7 4 0.8 P = 0.027 3.6 0.7 3.9 0.6 P = 0.014 3.8 0.7 4.3 1.0 P = 0.058 3.7 0.5 3.9 0.8 P = 0.195 3.0 0.6 3.7 0.9 P = 0.005 2.9 0.9 3.6 0.8 P = 0.006 3.2 0.7 3.8 0.8 P = 0.003 Peak Contact Pressure (MPa) Medial Pre Post 8.1 1.5 7.3 1.2 P = 0.004 8.5 1.8 7.4 1.7 P = 0.072 8.6 1.9 8.0 1.3 P = 0.125 8.6 1.8 7.9 1.6 P = 0.075 9.0 1.9 7.8 1.7 P = 0.028 8.3 1.4 8.0 1.3 P = 0.125 7.1 1.3 6.5 1.4 P = 0.116 7.0 2.7 6.5 2.1 P = 0.479 8.3 1.4 7.6 1.1 P = 0.125 Lateral Pre Post 7.9 1.4 9.7 1.7 P 0.001 7.9 1.7 8.6 1.7 P = 0.03 8.4 1.5 9.3 1.5 P = 0.002 7.8 1.4 9.1 1.6 P 0.001 8.5 1.8 9.4 1.7 P = 0.015 8.3 1.5 9.1 1.6 P = 0.013 6.3 1.0 8 1.4 P 0.001 7.0 1.4 7.7 1.5 P = 0.025 7.9 1.6 8.4 1.5 P = 0.081 Contact Area (mm 2 ) Medial Pre Post 85.2 14.8 83.8 18.8 P = 0.796 67.3 14.7 64.7 19.5 P = 0.317 68.7 15.2 78 24.2 P = 0.243 72.5 12.9 76.3 12.9 P = 0.796 70.7 13.8 62.7 14.4 P = 0.317 60.5 15.5 77.2 17.7 P = 0.006 74.6 16.8 69.2 22.4 P = 0.796 63 18.6 56.6 19.8 P = 0.317 59.6 12.3 60.3 15.5 P = 0.882 Lateral Pre Post 118.8 23.6 79.1 14.7 P 0.001 110.1 24.5 86.4 17.4 P = 0.01 110.8 18.5 92.3 22.5 P = 0.02 122.7 20.0 100 23.0 P 0.001 115.3 25.4 93.7 23.3 P = 0.017 111.7 16.4 100.7 19.1 P = 0.057 109.1 18.2 85.2 16.1 P 0.001 98.4 24.4 85.3 14.1 P = 0.033 96.6 19.3 83.2 17.6 P = 0.071

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76 3D Alignment Humeroradial articulation Following proximal ulnar rotational osteotomy, no significant alterations in alignment of the center of the radial head relative to the humeral origin occurred in the cranial caudal or medial lateral planes The center of the radial head moved a mean of 0.8 mm (range 0 1.5 mm) closer to the humeral origin, although this was only statistically significant at four of the nine elbow poses tested, and rotated externally a mean of 5 (range 1.5 7.7) relative to the humeral origin following the osteotomy. The calculated humeroradial flexion angle was significantly lower following the osteotomy at all nine elbow poses. Varus angulation of the center of the radial head relative to the humeral origin reduced a mean of 6.5 (range 3.3 10.0) followi ng the osteotomy. Humeroulnar articulation Following proximal ulnar rotational osteotomy, the apex of the medial coronoid process did not translate or rotate significantly in any plane relative to the humeral origin. There was no significant difference in humeroulnar flexion angle preand post osteotomy. Proximal radioulnar articulation Following proximal ulnar rotational osteotomy, the apex of the MCP translated a mean of 2.7 m m (range 1.7 3.4 mm) caudal and1.2 mm (range 0.4 2.0 mm) abax ial and rotate d a mean of 1.2 (range 0.2 2.1) externally relative to the center of the radial head.

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77 Table 32. Static 3D alignment of the humeroradial articulation preand post osteotomy for the nine elbow poses tested 135 115 155 Neutral Pronation Supination Neutral Pronation Supination Neutral Pronation Supination Translations (mm) Cranial caudal Pre Post Medial lateral Pre Post Proximal distal Pre Post 10.2 2.3 11.0 2.3 P = 0.159 3.0 1.9 3.2 1.2 P = 0.67 14.4 1.1 13.6 2.1 P = 0.06 10.6 2.4 10.5 2.1 P = 0.73 3.8 1.1 3.1 1.1 P = 0.026 14.1 1.4 13.7 1.8 P = 0.32 11.1 1.6 11.8 1.8 P = 0.09 4.1 0.8 3.9 1.2 P = 0.68 13.9 1.9 13.0 2.2 P = 0.04 12.9 2.5 13.0 2.4 P = 0.66 4.2 1.5 4.1 1.1 P = 0.78 11.0 1.6 10.1 1.6 P = 0.02 13.1 2.4 12.8 2.4 P = 0.26 3.6 1.7 3.1 1.5 P = 0.07 10.6 1.7 10.6 1.7 P = 0.93 13.2 1.9 13.5 2.1 P = 0.12 4.4 1.0 4.8 1.1 P = 0.18 10.7 2.0 9.4 2.0 P 7.1 2.6 7.9 1.8 P = 0.04 3.2 1.4 3.1 1.0 P = 0.83 16.3 1.9 15.8 2.3 P = 0.20 7.6 2.1 7.5 1.8 P = 0.51 2.8 1.7 2.2 2.2 P = 0.06 16.1 1.8 15.7 1.8 P = 0.18 8.1 1.8 8.8 1.5 P = 0.06 3.2 1.5 4.1 0.9 P = 0.09 16.0 1.6 15.0 1.7 P = 0.002 Rotations () Flexion extension Pre Post Axial rotation Pre Post Varus valgus Pre Post 138.2 5.6 122.4 9.7 P 33.6 4.2 40.1 7.1 P = 0.14 29.2 4.7 22.5 4.1 P = 0.004 148.2 6.6 136.2 9.6 P 28.2 3.4 33.6 5.8 P = 0.004 33.7 4.5 28.7 4.0 P = 0.01 121.4 7.7 108.9 6.5 P 40.8 4.1 45.1 6.4 P = 0.07 20.8 5.6 12.3 3.1 P 123.2 7.3 110.5 6.2 P 52.0 5.5 59.6 2.7 P 41.3 4.5 32.2 4.3 P 132.4 8.7 120.3 8.3 P 45.8 5.9 53.5 5.6 P 46.7 3.7 40.3 3.9 P = 0.001 110.0 7.5 102.2 5.8 P 60.4 4.4 65.2 3.7 P = 0.003 28.9 5.2 18.9 5.6 P 146.1 6.9 131.1 8.5 P 20.8 3.6 24.0 5.4 P = 0.007 15.2 4.9 11.4 4.9 P = 0.013 157.1 6.8 143.8 9.4 P 17.3 3.5 21.1 6.0 P = 0.003 17.6 4.7 14.3 4.7 P = 0.004 125.6 5.9 113.6 7.7 P 25.6 4.3 27.1 6.5 P = 0.15 8.6 4.1 2.8 3.1 P For translational variables, positive values indicate cranial, distal and medial positions of the radius relative to the hume rus. For the rotational variables, positive values indicate greater external radial rotation and varus. P values for post hoc pairw ise comparisons are given where significant differences were detected by ANOVA.

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78 Table 33. Static 3D alignment of the humeroulnar articulation preand post osteotomy for the nine elbow poses tested 135 115 155 Neutral Pronation Supination Neutral Pronation Supination Neutral Pronation Supination Translations (mm) Cranial caudal Pre Post Medial lateral Pre Post Proximal distal Pre Post 12.9 1.9 13.2 1.5 P = 0.36 6.3 2.6 6.4 2.9 P = 0.81 10.4 3.0 10.8 3.3 P = 0.53 12.6 1.8 12.5 2.2 P = 0.85 6.3 2.6 7.0 2.9 P = 0.22 11.4 3.0 10.6 4.5 P = 0.42 13.5 2.5 13.5 2.2 P = 0.79 5.0 2.4 5.9 2.9 P = 0.051 10.2 3.5 10.0 4.0 P = 0.71 15.1 1.7 14.7 1.4 P = 0.27 5.1 2.8 5.3 3.0 P = 0.62 6.9 3.1 6.5 3.6 P = 0.40 15.3 2.3 14.9 1.5 P = 0.53 5.5 2.7 6.4 3.0 P = 0.04 6.6 3.5 6.9 4.1 P = 0.61 15.3 1.8 14.9 1.5 P = 0.12 4.8 2.4 5.2 3.1 P = 0.37 6.3 3.7 5.8 3.6 P = 0.42 10.1 2.9 10.3 2.9 P = 0.60 6.6 2.2 7.1 2.8 P = 0.23 14.2 2.5 13.7 2.7 P = 0.40 9.8 3.3 9.6 3.3 P = 0.66 7.3 2.5 7.8 2.1 P = 0.32 14.3 3.3 14.1 2.6 P = 0.62 10.6 2.1 10.5 3.5 P = 0.96 6.0 2.7 6.4 2.7 P = 0.48 13.6 3.5 13.6 2.9 P = 0.96 Rotations () Flexion e xtension Pre Post Axial rotation Pre Post Varus valgus Pre Post 133.5 7.7 133.5 8.8 P = 0.98 32.1 5.8 33.1 7.0 P = 0.64 30.5 6.2 31.4 5.1 P = 0.53 136.3 7.7 137.0 8.7 P = 0.70 30.2 5.4 31.1 6.3 P = 0.66 32.4 6.7 32.4 4.6 P = 0.98 129.8 7.7 130.2 6.8 P = 0.82 34.0 6.4 35.1 7.4 P = 0.68 30.9 6.6 31.2 5.1 P = 0.87 118.6 8.0 119.1 8.4 P = 0.69 50.7 6.3 51.3 3.2 P = 0.68 41.6 5.4 43.1 4.6 P = 0.027 120.3 9.3 121.6 8.8 P = 0.86 50.0 6.8 49.8 5.6 P = 0.94 43.0 4.8 43.7 4.4 P = 0.16 114.9 7.9 115.4 7.9 P = 0.62 53.9 6.4 55.0 4.4 P = 0.46 40.2 5.1 41.6 4.5 P = 0.05 142.6 7.3 143.6 8.6 P = 0.26 17.6 3.8 18.4 3.7 P = 0.43 18.1 5.9 18.3 5.3 P = 0.81 146.0 7.1 147.1 9.1 P = 0.25 16.5 4.5 17.0 4.1 P = 0.59 19.2 6.4 18.3 5.4 P = 0.48 139.5 7.3 140.8 8.8 P = 0.17 19.1 4.7 19.1 3.8 P = 0.99 18.0 6.2 17.6 5.9 P = 0.76 For translational variables, positive values indicate cranial, distal and lateral positions of the proximal ulna relative to the humerus. For the rotational variables, positive values indicate greater humeroulnar flexion, external ulnar rotation and varus. P values for post hoc pairwise comparisons are given where significant dif ferences were detected by ANOVA .

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79 Table 3 4 Static 3D alignment of the proximal radioulnar articulation preand post osteotomy for the nine elbow poses tested 135 115 155 Neutral Pronation Supination Neutral Pronation Supination Neutral Pronation Supination Translations (mm) Cranial caudal Pre Post Medial lateral Pre Post Proximal distal Pre Post 1.6 4.5 5.0 4.4 P 10.3 2.6 8.6 3.9 P = 0.048 0.3 2.3 0.5 2.4 P = 0.72 1.5 4.3 3.2 4.4 P = 0.024 10.5 2.2 10.1 3.6 P = 0.58 0.9 2.9 0.5 2.8 P = 0.40 4.3 5.0 7.7 3.5 P = 0.001 8.6 4.4 6.8 5.0 P = 0.002 0.4 2.2 0.1 2.4 P = 0.50 2.3 4.6 4.9 4.2 P = 0.001 10.1 2.3 8.9 3.7 P = 0.06 0.9 2.6 0.8 2.5 P = 0.69 0.7 4.8 2.9 4.9 P = 0.001 10.2 1.8 9.7 3.3 P = 0.54 1.1 3.1 0.9 2.6 P = 0.75 4.5 5.1 7.4 3.5 P = 0.007 9.0 4.3 7.1 5.4 P = 0.005 0.7 2.4 0.6 2.5 P = 0.82 2.3 5.3 5.2 4.7 P = 0.001 10.0 3.1 8.9 4.8 P = 0.17 0.9 2.3 0.6 2.3 P = 0.59 1.2 5.1 3.5 4.7 P = 0.005 10.2 2.4 9.6 2.9 P = 0.17 0.9 2.5 0.9 2.5 P = 0.95 5.6 2.8 8.7 2.8 P 8.1 3.7 6.1 4.7 P = 0.001 0.2 2.8 1.0 2.1 P = 0.22 Rotations () Flexion e xtension Pre Post Axial rotation Pre Post Varus valgus Pre Post 2.6 4.1 13.0 6.2 P 3.9 2.2 2.1 2.8 P = 0.052 3.6 5.3 1.1 4.8 P = 0.013 9.1 2.9 2.6 5.0 P 4.2 2.5 2.1 3.1 P = 0.009 4.2 5.7 2.7 5.6 P = 0.24 12.4 6.1 27.1 4.7 P 3.8 1.7 2.3 2.6 P = 0.08 3.2 6.0 1.2 5.0 P = 0.06 1.9 3.0 12.8 4.9 P 4.4 2.2 3.2 2.8 P = 0.054 3.3 5.9 0.6 5.0 P = 0.03 8.4 3.1 3.4 4.4 P 4.1 2.0 2.9 3.1 P = 0.04 4.4 5.9 1.4 5.1 P = 0.012 11.7 6.6 25.5 6.4 P 4.2 2.0 3.3 3.0 P = 0.18 2.7 6.0 0.5 5.1 P = 0.046 2.1 3.3 13.7 5.8 P 0.001 4.1 2.3 2.7 2.4 P = 0.07 4.1 5.7 1.9 4.2 P = 0.06 9.5 3.2 4.2 4.8 P 3.8 2.3 3.0 2.9 P = 0.25 5.0 5.8 2.6 4.2 P = 0.04 16.0 3.1 30.0 4.5 P 3.8 2.3 3.6 2.4 P = 0.83 3.0 5.9 2.0 4.8 P = 0.43 For translational variables, positive values indicate cranial, distal and lateral positions of the distal ulna relative to the radius. For rotational variables, positive values indicate greater radioulnar flexion, external ulnar rotation and varus. P values for post hoc pairwise comparisons are given where significant dif ferences were detected by ANOVA.

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80 Figure 34 3D alignment of the proximal radioulnar joint post osteotomy Proximal ulna distal ulna Following proximal ulnar rotational osteotomy, the distal ulnar segment translated a mean of 1.3 2.5 mm cranial and 0.8 1.6 mm axial, with no translation in the proximal distal plane relative to the proximal ulnar segment. The distal segment was flexed a mean of 15.7 1.4 in the sagittal plane, rotated 2.0 3.8 internally and was 1.5 3.8 less varus relative to the proximal ulnar segment post osteotomy.

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81 Table 35. 3D orientation of the distal ulna relative to the proximal ulna following proximal ulnar rotational osteotomy 135 Neutral Translations (mm) Cranial caudal Pre Post Medial lateral Pre Post Proximal distal Pre Post 1.4 5.7 2.7 4.9 P = 0.547 2.6 4.3 1.8 4.6 P = 0.1 2.2 5.8 2.2 7.3 P = 0.998 Rotations () Flexion Extension Pre Post Axial rotation Pre Post Varus valgus Pre Post 0.1 2.3 15.5 2.5 P 1.4 3.3 0.6 4.5 P = 0.226 2.7 5.3 1.2 4.8 P = 0.197 For translational variables, positive values indicate cranial, distal and lateral positions of the proximal ulna relative to the radius. For rotational variables, positive values indicate greater radioulnar flexion, external ulnar rotation and varus. P val ues for post hoc pairwise comparisons are given where significant differences were detected by ANOVA. Discussion In this cadaveric biomechanical study, proximal ulnar rotational osteotomy shifted contact pressures from the medial to the lateral elbow compartment, and externally rotated and displaced the apex of the MCP caudally and abaxially relative to the radial head. Although the alignment of the humeroulnar articulation was within normal parameter s, we found that the varus angulation of the normal humeroradial articulation was reduced following the osteotomy. These changes would appear to shift load

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82 transmission through the elbow from medial to the lateral compartment, supporting our hypothesis. The concept of altering the mechanical axis to redist ribute articular force through the joint was introduced in 1958 when the high tibial osteotomy was first described.130 Since then, this technique has gained widespread use, particularly in younger, more active patients.129 Overall, high tibial osteotomies are an effective procedure to improve knee function and reduce pain, although there is no specific evidence that this procedure is superior to conservative management.139 Its use, therefore, still remains controversial and the results are reported to deteriorate over time.131,140 Following proximal ulnar rotational osteotomy, the magnitude of humeroradial varus reduction was comparable to that following medial opening wedge high tibial osteotomy in humans, which has been reported to reduce maximum varus from 13.5 to 5.4.141 We were able to produce alterations in contact mechanics with minimal alterations in elbow alignment, and the 3D alignment of the elbow post osteotomy was similar to that observed in normal elbows positioned in supination, suggesting that elbow incongruency is not induced. The proximal distal displacement of the apex of the MCP relative to the center of the radial head following the osteotomy was negligible. In both humans and dogs, the articulation between the anconeal process and the olecranon fossa is the primary stabilizer of the elbow joint in rotation, allowing minimal alteration in the alignment of the humeroulnar joint.14,118 As the anconeal process locks in the olecranon fossa of the distal humerus, external rotation of the proximal ulna reorients the distal antebrachium laterally with external rotation centered through the humeroradial articulation. This alteration in the mechanical axis of the limb redistributes

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83 the load from the medial compartment to the lateral compartment during weight bearing, as demonstrated by the shift in CP and PCP from medial to lateral elbow compartments. The shift in mechanica l axis is further substantiated by the gross observation that the paw was further lateralized and externally rotated following the osteotomy. A toeout gait has been shown to decrease the adductor moment and lower medial compartment pressure in people with knee osteoarthritis.136 The effects of our osteotomy are therefore most likely attributable to alteration in the mechanical axis of the humeroradial articulation and decreased adductor moment of the elbow joint, rather than alteration of congruency at the pr oximal radioulnar articulation. The r eduction in medial compartment CP and PCP of 10% and 9% respectively obtained following proximal ulnar rotational osteotomy in this study is lower than that measured in human cadaveric knees following medial opening wedg e high tibial osteotomy, where CP decreased 21% and PCP decreased 17% in the medial compartment with concurrent medial collateral transection.125 Following the reduction in mechanical load, cartilage regeneration in the previ ously damaged medial compartment has been reported following both high tibial wedge osteotomy and sliding humeral osteotomy.101,133,134 Given the shift in load from medial to lateral compartments secondary to alteration of the mechanical axis of the forelimb in this study, we would expect proximal ulnar rotational osteotomy to demonstrate similar effects. The effect of the increased pressure shift to the lateral compartment is as yet unknown. It has been shown that healthy cartilage has the capacity to withstand and thicken in response to increased load in the medial compartment of the knee in people, in contrast to osteoarthritic cartilage which thins in response.113 Even in dogs with end-

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84 stage MCD the lateral compartment is often grossly without pathologic change, and the increased press ure in the lateral compartment may be tolerated if within physiologic range.113 Secondlook arthroscopic examination of the medial and lateral elbow compartments in dogs 12 months following sliding humeral osteotomy demonstrated cartilage resurfacing of previously exp osed subchondral bone on the humeral condyle, with no visible disease in the lateral compartment.101 Using delayed gadolinium enhanced magnetic resonance imaging methods, no significant change in the cartilage thickness in the lateral compartment was noted over a two year study period following high tibial osteotomy in people.134 As normal cart ilage can adapt to increases in pressure without initiating degeneration, the proximal ulnar rotational osteotomy may be most beneficial in dogs with early medial coronoid or medial compartment disease instead of cases in which endstage medial compartment disease is present.113 Furthermore, the ideal candidate for high tibial osteotomy is considered to be young (< 60 years of age) with increasing age identified as a negative prognostic factor.131,142,143 Knee adduction moment is considered by many to be highly correlated with force transmission through the medial compartment of the knee in people, although a more recent study utilizing a force transducer embedded in a total knee arthroplasty implant disput es this assumption.144,145 While high tibial osteotomy resul ts in normalization of several dynamic knee function parameters, including knee adduction moment, the effect of proximal ulnar rotational osteotomy on the gait of dogs is unknown.141 Conversely, simple g ait modification, such as walking with medialized knees, toe out gait or walking poles, or using lateral wedge shoe insoles, are attractive, noninvasive interventions to reduce knee adduction moment and consequently overall medial contact force in

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85 people.137,138 It must be considered that inducing gait changes may actually produce adaptive muscle co contraction that could actually increase the medial compartment load in vivo .146

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86 CHAPTER 4 CONCLUSION The purpose of the first part of the study was to address the fundamental biomechanics of normal dog elbows and establish a model for future investigation of pathologic states and surgical procedures to address elbow pathology. Previous studies have examined forelimb kinematics in two dimensions, whereas others have investigated contact mechanics of normal dog elbows at one static elbow pose.21,110,112 However, simultaneously evaluation of 3D kinematics and contact mechanics of normal dog el bows had not previously been performed. We found that th e biomechanics of normal dog elbows are complex and alter significantly with changes in antebrachial and elbow flexion angle; c hanges in antebrachial rotation significantly increased contact pressures and decreased contact areas in both medial and lateral elbow compartments and contact pressures were increased with increasing flexion of the elbow joint, with corresponding decreases noted in extension. We concluded that s tudies evaluating disease processes and interventional surgical procedures to treat pathology referable to the elbow should therefore evaluate the elbow in several different poses. The second part of this study investigated a novel surgical procedure using the refined thoracic limb model. Dogs with MCD stand with a characteristic stance with the elbow adducted, carpus abducted and paw externally rotated in a presumed attempt to shift load transmission away from the diseased medial compartment. Similarly hum ans walking with a toe out gait or with their knees medialized have been found to unload the medial compartment of the knee and mitigate the progression of medial compartmental osteoarthritis. While gait adaptation can be performed in humans to unload the medial compartment of the knee, this is impractical in dogs and therefore we came up with the

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87 concept of a proximal ul nar rotational osteotomy to externally rotate and abduct the forelimb distal to the elbow unloading the medial compartment. We found that p roximal ulnar rotational osteotomy shifts contact pressure from the medial to the lateral elbow compartment by decreasing varus angulation through the humeroradial articulation and displacing the MCP caudally and abaxially Proximal ulnar rotational ost eotomy may be performed in both dogs with early medial coronoid dis ease in which there is potential for cartilage resurfacing of the medial compartment, as well as those with endstage MCD in which unloading of eburnated subchondral bone may ameliorate ass ociated pain. The effect of pressure shifting to the lateral c ompartment is unknown, however t he cartilage of the lateral compartment is often grossly normal when examined arthroscopically, even in dogs with endstage MCD and normal cartilage has been shown to adapt favorably to increased pressure transmission. Proximal ulnar rotational osteotomy was designed to simulate supination in the normal forelimb, and the calculated 3D alignment of the humeroradial and humeroulnar joints post osteotomy matched closely that of preosteotomy supination. However, during supination in normal elbows preosteotomy contact pressures increased in both medial and lateral compartments, whereas post osteotomy, contact pressures decreased in the medial and increased in the lateral compartment. We suspect that the uniform bi compartmental increase in pressure observed in the normal joint in supination may occur due to tensioning of the collateral ligaments rather than a change in joint alignment. A similar phenomenon was report ed in a biomechanical analysis of high tibial osteotomy, where over tensioning and constraint of the medial collateral ligament was found to increase load transmission through the medial compartment.125

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88 Increased tension in both the medial and lateral coll ateral ligaments resulting from torqueing of the preosteotomy elbow during pronation and supination may have artificially elevated contact pressures in both medial and lateral compartments.125 Post osteotomy, tension on the collateral ligaments was not induced as the supination was achieved by uncoupling the radius from the ulna rather than by torqueing the elbow. Elbow biomechanics are complex and difficult to replicate ex vivo In this study, we adapted a testing model previously described112 which is a gross simplification of the dogs thoracic limb S imulation of physiologic loading conditions through cadaveric forelimbs is difficult as mu sculotendi n o us forces can not be simulated accurately. Our model enabled us to determine the motion of the antebrach ium relative to the humerus in all 6 degrees of freedom. Therefore we believe that elbow biomechanics were sufficiently replicated to allow relevant s tudy of the effects of elbow flexion and antebrachial rotation on humeroradial and humeroulnar contact mechanics and 3D alignment The medial epicondylar osteotomy allowed us to insert the sensors into each of the elb ow compartments while maintaining the i ntegrity of the medial collateral ligament and the origins of the flexor s of the antebrachium. The joint capsule may assist in load transmission, and was significantly disrupted to enable sensor placement, although t he anconeal process and lateral collater al ligament, the primary stabilizers of the elbow during pronation and supination respectively, were preserved.6367 Many of the normal in vivo muscular forces involved in weight bearing were not simulated in our model. The biceps musculotendinous unit has been considered a n important stabilizer of the elbow during stance phase, h owever inverse dy namic analysis of the forelimb at a walk illustrates a large extensor moment at the elbow throughout stance until just

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89 before termination when a small flexor moment occurs.60,147 Therefore the triceps muscle group which was simulated in this model would appear to be the dominant muscular group supporting the elbow throughout stance. E volution of a more complex cadaveric thoracic limb model incorporating muscular forces or in vivo studies may be beneficial for future investigations. Our analysis was restricted to flexion angles spanning the range reported during the stance phase of walking and antebrachial rotation angles approximating the reported midpoint of the linear region of loaddeformation curves generated from antebrachii rotated to failure.108111,118 These antebrachial rotational angles were consistent with measurements obtained during pilot studies of rotations at which pronation and supination occurs without undue strain on the elbow.118 The norma l degree of pronation and supination in the dog at a walk and a trot has not yet been reported and these angles may not reflect antebrachial rotation incurred while walking in a straight line and may be more representative of complex activ ities during stance, including turning with the forelimb planted The limitations of the digital pressure sensing system and 3D joint kinematic analysis have been thoroughly discussed.148152 The I Scan sensors have improved accuracy and repeatability compared with Fuji Film pressure sensitive film.153 The accuracy of the digitizing arm has been validated and demonstrates high precision.117 The inter marker distance averaged 1%, indicating minimal discrepancies in actual accuracy and precision of marker position measurements. The difference noted between elbow flexion angles measured by goniometry during testing and our kinematic analysis is most likely attributable to operator error.

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90 Cadaver studies are considered level V evidence in evidence based medicine criteria, and therefore we cannot make any clinical recommendations based on our results.154 However these results can be used as a basis for further investigation, including in vivo 3D kinematic analysis of normal dog elbows and in dogs with MCD to try to e lucidate the biomechanical factors associated with MCD Future investigation of the proximal rotational ulnar osteotomy requires development of custom locking plate and in vivo clinical analysis with objective kinematic and force plate analysis measures.

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103 BIOGRAPHICAL SKETCH Laura C. Cuddy was born in Cork, Ireland. She is the youngest of three daughters. She attended St. John the Baptist National School and later Midleton College where she completed her Leaving Certificate in 2003. She attended University College Dublin, Ireland where she earned a degree of Bachelor of Veterinary Medicine in June 2008. She completed a rotating internship at the University of Florida College of Veterinary Medicine in June 2009, and remained at the University of Florida College of Veterinary M edicine to complete a combined m aster s degree program in Small Animal Clinical Sciences and a res idency in Small Animal Surgery.