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RELATIONSHIP OF MESIODISTAL TOOTH SIZE TO EXTRACTION RATE AND
POST-TREATMENT CHANGES IN THE CLASS II DIVISION 1 MALOCCLUSION
JUDDSON ROBERTS REED
A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTERS OF SCIENCE
UNIVERSITY OF FLORIDA
Juddson Roberts Reed
TABLE OF CONTENTS
L IS T O F T A B L E S .......... .... .... .... .. .. ......... ................................................ .. iv
LIST OF FIGURES .............. ... ................................ ......... ..v
ABSTRACT .............. .................. .......... .............. vi
1 IN TR O D U C TIO N ......................................................................... .... .. ........
2 MATERIAL AND METHODS................................................... ............... 3
S am p le .......................................................................... . 3
M eth od s ........................................................................... . 3
D ata A naly sis ............................................... 4
3 R E SU L T S ................................................................ .6
4 D ISC U SSIO N ..................................................... 12
5 C O N C L U SIO N S ................................................................16
L IST O F R E FE R E N C E S ............................................................................... 17
B IO G R A PH IC A L SK E T C H ........................................................................................ 20
LIST OF TABLES
3-1. M ean tooth w idth (values in m m .).................................................... ....................7
3-2. Tooth width and initial PAR by sex ........................................ ....................... 7
3-3. Percent of subjects with extractions by treatment group, sex, and DC7 molar
c la ss sev e rity ..............................................................................................................9
LIST OF FIGURES
3-1. Mean combined 2-2 tooth width by sex for extraction and nonextraction subjects .10
3-2. Change from DCF in mean PAR score and components for extraction and
nonextraction groups ...................................................... ................ ........... 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
RELATIONSHIP OF MESIODISTAL TOOTH SIZE TO EXTRACTION RATE AND
POST-TREATMENT CHANGES IN THE CLASS II DIVISION 1 MALOCCLUSION
Juddson Roberts Reed
Chair: Timothy T. Wheeler
Major Department: Orthodontics
Mesiodistal tooth size has been implicated in untreated crowding and post-
treatment alignment change. The purpose of this study was to determine if tooth size is
related to malocclusion severity, extraction frequency, and post-treatment change in a
group of class II division 1 subjects treated as part of a 2-phase randomized controlled
clinical trial at the University of Florida. Included subjects were those who began the
study and had initial Peer Assessment Rating (PAR) scores (n=257) and who completed
the study and had final PAR scores (n=204). Tooth width was measured on dental casts
at the completion of treatment or at the time point closest to completion of treatment, and
extracted teeth were noted from the casts. It was shown that males had significantly
larger teeth. Initial PAR and molar class severity did not differ by sex, but tooth size was
significantly correlated in males with initial overjet (R=0.23 [upper 2-2], 0.16 [combined
2-2]) and PAR score (R=0.28 [upper 2-2], 0.18 [combined 2-2]). Tooth size had
significant correlations with lower anterior alignment for females (R=0.20 [upper 2-2],
0.27 [lower 2-2], 0.23 [combined 2-2]). Tooth size was not related to initial molar class
severity. More males were treated with second phase extractions with the majority of
extractions done in the upper arch. Females only showed significant differences in tooth
size when comparing extraction and non-extraction subjects, suggesting that females
received extractions for excess tooth mass or protrusion, and males received extractions
for classification purposes. There were no differences in post-treatment PAR score
change between extraction and non-extraction subjects.
Tooth size, along with available arch length and desired ultimate tooth position, is
part of the important space analysis during orthodontic diagnosis and treatment planning.
Tooth size has been studied throughout the history of orthodontics to explain pre- and
post-treatment crowding, differences in classification, and even for extraction guidelines.
A few studies have shown that larger teeth are associated with crowding.1'2
However other studies found no association between tooth size and crowding3 while
others will claim that tooth size and arch dimension have equal contributions.4 Many
authors5-10 have found males to have larger teeth than females, and ethnic tooth size
differences have been shown.9 The size of the crowns of teeth have been shown to be
linked to heredity through twin studies."1
Skeletal and dental classification have been shown to influence tooth size and
Bolton's discrepancies, class III subjects having greater differences between maxillary
and mandibular tooth size than class I and class II, which did not differ.9'12-14
Incisor shape as defined as the ratio between mesiodistal and faciolingual
dimensions15 and contact point to cervical area mesiodistal width16 are the major
determinants in lower anterior crowding of untreated arches. However, other studies
have failed to show correlation between incisor shape and crowding and have suggested
that mesiodistal width alone correlates to crowding. 17,1
Gilmore and Little19 examined the relationship between mandibular incisor
dimensions and arch alignment in 164 class I and II cases 10 years after completion of
treatment. They found a weak positive correlation between incisor dimension and
crowding. Glenn et al.20 evaluated 28 nonextraction treated cases 8 years post-retention
and found no association between mesiodistal or faciolingual incisor dimension and
pretreatment or post-treatment incisor crowding.
Multiple scoring systems have been utilized to assign quantitative descriptors to
treatment results such as Little's irregularity index21 and the PAR index.22 Birkeland et
al.23 found that pre-treatment PAR score was a good predictor for post-treatment PAR
score and long term outcome. Pavlow24 found that post-treatment PAR score and PAR
score change is not related to Phase I treatment type in a group of class II subjects treated
during a prospective clinical trial.
Given the contradiction in the literature about tooth size and crowding, we sought
to examine if this relationship existed for class II subjects. In addition, we wanted to
determine if sexual dimorphism in tooth size existed in our cohort. Finally, we wanted to
evaluate whether tooth size related to extraction rate and PAR changes during retention.
MATERIAL AND METHODS
This study included a sample of individuals with Class II malocclusion who
participated in a prospective randomized controlled trial at the University of Florida. The
design of this study was published by Keeling et al.25 in 1998.
Tooth measurements were done on stone casts with digital calipers by one
examiner [JRR]. The mesiodistal width was measured and recorded at the contact point
for maxillary and mandibular incisors, canines, and premolars, bilaterally. Stone models
have been validated for accurate measurement by Gilmore and Little,19 and bilateral
measurements are advocated by Ballard26 due to his documentation of right-left tooth size
discrepancies. A digital caliper was oriented parallel to the occlusal plane of the teeth
and the vestibular surface of the model as per the method standardized by Moorrees and
Measurements were done on final models. In the case of extraction treatment, the
size of any extracted tooth was taken from models at an appropriate previous time point if
available; the teeth that were extracted were noted for use in analysis. If the final models
were not of good diagnostic value (fractured or distorted) measurements were done on
the next available retention models. Of the 312 subjects who began the study's first
phase, 257 had tooth measurements and initial PAR scores available for analysis. For
subjects who may not have completed the study, models at the latest time point available
were measured. Dental casts of diagnostic value were available for 204 of the 208
subjects who completed the study.
The measurements were summed for each subject as summed upper and lower 2nd
premolar to 2nd premolar (5-5) and summed upper and lower lateral and central incisors
(2-2) for data analysis.
Initial molar class (mild, bilateral one-half cusp; moderate, 1 side three-fourths
cusp; severe, 1 side full cusp) was used to grade the severity of the class II
Peer assessment rating (PAR) scores were used to assess post-treatment stability.
The American weighted, raw unweighted, and component (upper and lower alignment,
overjet, and overbite) scores were used as stability variables. To quantify post-treatment
stability, the difference between PAR scores and components at DCF and DCR was
calculated as ([PAR score change] = [PAR score @ DCR1,2,3...] [PAR score @
To determine examiner reliability, tooth width of ten randomly chosen subjects'
casts was measured twice two weeks apart. For each tooth measured, the difference
between the two measurement time points was averaged among the ten subjects. Mean
difference ranged from 0.10mm for the lower right canine to 0.20 for upper right lateral
Chi-square test was used to compare the number of individuals with and without
extractions by phase I treatment group, sex, and initial molar class severity.
Combined tooth width between male and females and between extraction and
non-extraction subjects was compared with a 2 sample t-test.
Wilcoxon rank sum was used to compare combined tooth size and PAR score at
DC1 by sex. It was also used to compare PAR score components and PAR score
component change between extraction and non-extraction subjects.
Pearson correlation coefficients were used to examine the correlation between
tooth size and DC 1 PAR score and components.
ANOVA was used to determine significant differences between DC1 molar class
severity groups due to tooth size.
There were 257 study participants who had available tooth width data and initial
PAR. There were more males (n=156) than females (n=101), but there were nearly equal
numbers in each of the three phase I groups (bionator n=93; observation n=78; headgear
n=86). The majority of the subjects had a high initial molar class severity (n= 18), with
less in the low (n=62) and mild (n=86) categories. The majority of the subjects were
White (n=238), with less Black (n=4), other (n=2) and Hispanic (n=13) subjects.
Table 3-1 shows the mean tooth width for this sample. The smallest tooth was the
mandibular central incisor (5.28mm) and the largest was the maxillary central incisor
(8.69mm). The greatest standard deviation was noted for the maxillary left lateral
Table 3-2 depicts the mean 2-2 tooth size and DC1 PAR scores by sex. Males had
significantly greater combined 2-2 tooth width for the maxillary and mandibular arches
independently and together. However, there were no significant differences by sex for
weighted PAR score at DC1. For females, the correlation coefficient between tooth size
and DC1 PAR score and components were significant between upper 2-2 (R=0.20), lower
2-2 (R=0.27), and combined 2-2 (R=0.23) width for lower anterior alignment only at
p<0.05. For males there was no significant correlation between tooth size and lower
incisor alignment. However, males showed significant correlations for the following
combinations: weighted PAR and upper 2-2 (R=0.28) and combined 2-2 (R=0.18); raw
par and upper 2-2 (R=0.22); and overjet and upper 2-2 (R=0.23) and combined 2-2
(R=0.16), all at p<0.05. There were no other significant correlations between tooth size
and PAR scores and components (data not shown).
Table 3-1. Mean tooth width (values in mm.)
Tooth Number N Mean Std Dev Minimum Maximum
UR5 247 6.65 0.50 5.34 9.77
UR4 248 6.90 0.44 5.42 8.86
UR3 241 7.82 0.47 6.54 9.31
UR2 255 6.63 0.54 5.16 8.31
UR1 257 8.65 0.55 6.36 10.05
UL1 257 8.69 0.56 7.20 10.85
UL2 257 6.67 0.59 5.09 8.89
UL3 240 7.79 0.48 6.57 9.05
UL4 248 6.91 0.42 5.79 8.71
UL5 245 6.65 0.64 5.43 10.93
LL5 240 7.16 0.53 5.84 9.09
LL4 250 7.03 0.47 5.78 8.87
LL3 248 6.79 0.42 5.71 8.87
LL2 257 5.84 0.42 4.90 7.09
LL1 257 5.29 0.34 4.25 6.13
LR1 256 5.28 0.39 4.33 8.89
LR2 257 5.84 0.36 4.97 6.90
LR3 248 6.74 0.45 5.27 8.25
LR4 251 6.98 0.51 5.25 8.69
LR5 243 7.07 0.51 5.74 8.68
Tooth number is given in Palmer notation
left; LR lower right)
(UR, upper right; UL, uppe
r left; LL, lower
Table 3-2. Tooth width and initial PAR by sex
Upper 2-2, mm. 30.93 0.16
Lower 2-2, mm. 22.47 0.10
Combined 2-2, mm. 53.41 0.23
2 sample t-test used
There were no significant tooth size differences between DC 1 molar class severity
groups for males and females (data not shown).
Table 3-3 shows subjects treated with extractions in any arch who completed the
class II study and had casts available for measurement (n=204). Note that the values for
upper arch extractions represent any subjects treated with upper extractions including
those who may have been treated with lower extractions (this is true for the lower
extraction data as well). Although there were no significant sex differences between the
number of subjects treated by nonextraction and extraction, more males were treated with
extractions, and the difference was nearly significant (p=0.053 in both arches, p=0.0710
in the upper arch). There was a significantly greater number of subjects in the phase I
observation group treated by extractions in any (upper or lower) arch and the upper arch
alone, but not for subjects treated with lower arch extractions. A similar pattern of
significance was seen when subjects were grouped by DC7 molar class severity.
Figure 3-1 shows the combined 5-5 and 2-2 tooth width by sex and extraction for
class II study participants who finished the second phase of treatment and for whom
values for all 20 anterior teeth were available (n=169). Males had significantly larger
teeth than females (p < .001). Subjects treated with extractions had more combined tooth
width than those treated without extractions, though the difference is significant only for
females (p < .05).
The change in mean PAR scores and components from final to recall time points is
shown in Figure 3-2. The subjects represented are those who completed the study and
who had models scored by the PAR index. The number of subjects that were recalled
decreased as the interval from treatment completion increased. Also, not all subjects
were recalled at every year for retention. Therefore, the number of subjects not only
decreases in the later retention time points, but the subject pool is different at each
retention time point. There were no significant differences between extraction and
nonextraction groups at any data collection point for change in mean weighted or raw
PAR, upper anterior alignment, and overjet. The nonextraction group had a significantly
greater mean lower anterior alignment score change at DCR6. The nonextraction group
also had a significantly greater mean overbite score change at DCR3.
Table 3-3. Percent of subjects with extractions by treatment group, sex, and DC7 molar
Percent (n) Upper arch Lower arch Both arches p
(n=204) 81% (166) 19% (38) 5%(11) 19% (38)
Male 77% (94) 23% (28) 7% (8) 23% (28) n.s.
Female 88% (72) 12% (10) 4% (3) 12% (10) n.s.
Phase I treatment
Bionator 89% (59) 11% (7) 3% (2) 11% (7) < 0.05
Observation 72% (48) 28% (19) 9% (6) 28% (19) < 0.05
Headgear 83% (59) 17% (12) 4% (3) 17% (12) < 0.05
Initial molar class severity
High 74% (73) 26% (26) 4% (4) 26% (26) < 0.05
Low 90% (43) 10% (5) 2% (1) 10% (5) < 0.05
Mild 88% (50) 12% (7) 11% (6) 12% (7) < 0.05
Extraction Upper Arch = any subject with extractions in upper arch.
Extraction Lower Arch
Extraction Both Arches
Chi-square test used.
any subject with extractions in the lower arch.
=any subject with extractions in either arch.
5s 00 N Male Female
57 0oo Nonextraction 75 62
E 560 Extraction 25 7
56 00 All Subjects 100 69
51 00oo All Patients
Figure 3-1. Mean combined 2-2 tooth width by sex for extraction and nonextraction
subjects (with standard error).
2 sample t-test used. *Significant difference male v. female p < .001. tSignificant
difference (extraction v. nonextraction) p < .05.
2b. Change in raw PAR score from DCF
2d. Change in lower anterior alignment from DCF
* ~ I
R1 R2 R3 R4 R5 R6 R7 R8
N (Nonext) 121 96 63 46 41 40 21 14
N (Ext) 29 16 11 7 9 10 2 2
Figure 3-2. Change from DCF in mean PAR score and components for extraction and
nonextraction groups (with standard error). 2a, weighted PAR; 2b, raw PAR; 2c, upper
anterior alignment; 2d, lower anterior alignment; 2e, overjet; 2f, overbite.
Wilcoxon rank sum test used. *Significant (extraction v. nonextraction) at p < .05.
2a Change in weighted PAR score from DCF
2c Change in antenor upper alignment from F
2e change n overjet from DCF
2f Change in overbite rom DCF
Previous retrospective studies examined tooth size differences by comparing
crowded and non-crowded dentitions.1'2 This study, while retrospective, utilizes a subject
pool treated as part of a prospective randomized controlled clinical trial.25 Phase I
treatment was determined by randomization, while phase II treatment plan was
determined by using a collaborative treatment plan by sending phase II records to
multiple practitioners across the country.29 It should be noted that the extraction
frequency represented here is a result of model analysis and represents actual treatment
rather than the collaborative treatment plan.
Our investigation of tooth size in Class II subjects had a similar outcome as
previously reported studies5-10 with respect to sex differences. Our findings showed that
males had approximately 3.4 mm greater tooth mass for the 20 anterior teeth and 1.3 to
1.5mm greater tooth mass for the 8 anterior teeth. Molar class severity and DC 1 PAR
score did not differ by sex, suggesting that tooth size may not influence these measures.
We were not able to confirm the findings of Lavelle9 in regards to ethnic differences in
tooth size due to the predominately white sample in this study.
There were differences between males and females in regards to the pattern of
correlation between initial PAR components and tooth size. Anterior tooth size in males
significantly correlated with overjet and weighted PAR. The significant correlation in
females was with lower anterior alignment. The suggestion is that larger teeth in class II
children is manifested differently for boys than girls, such that boys show their arch
length discrepancy with overjet, and girls with lower crowding. The correlation between
lower incisor size and lower anterior alignment (R=0.27, females) is similar to the
correlation between incisor size and Little's irregularity index reported for treated and
untreated cases by Smith et al.18 and to that reported by Gilmore and Little19 for post-
retention cases. However, correlations in the range or R=0.30 are of questionable clinical
While there were no significant differences in the number of males and females
requiring second phase extractions, the trend was that more males (28) than females (10)
had extractions, suggesting that the greater tooth mass may be related to more males
being treated with extractions.
Comparing nonextraction and extraction subjects showed that subjects treated with
extractions had significantly greater mean tooth mass for the 5-5 and 2-2 measurements.
This comparison alone would suggest that subjects with larger teeth had more crowding
and that our findings agree with those of Doris et al.1 and Norderval et al.2 However,
only 11 of the 38 extraction subjects had extractions in both arches. With the majority of
extractions being done in the upper arch, the suggestion is that the majority of extractions
were done for classification and maxillary anterior retraction rather than for crowding.
Furthermore, extraction frequency also showed significant differences with respect
to DC7 molar class severity, with 26.3% of severe class II subjects having extractions and
11.4% of low and mild class IIs having extractions. This is also suggestive that phase II
extraction decision is heavily influenced by the overall requirement for attaining a
desirable finishing classification.
The influence of phase I treatment on the frequency of extractions was significant.
Those with bionator or headgear phase I treatment had significantly fewer extractions
(13.9%) than the phase I observation group (28.4%). The suggestion is that phase I
treatment cuts the likelihood of having extractions during phase II treatment in half. A
similar study done at the University of North Carolina30 found an extraction rate of
approximately 17% in the observation group, 15% in the headgear group, and 38% in the
bionator group. They noted that the difference in extraction rate approached but was not
The extraction rates for this study do not reflect the previously reported consensus
treatment plans29 as our extraction frequency was determined by looking at post-
treatment casts rather than the treatment plan. Therefore, these rates, while showing
interesting trends, should be interpreted with caution as they represent the treatment
preferences of a few practitioners.
There was a significantly greater difference in tooth size between extraction groups
for females, but not for males. With more males overall requiring extractions, less
difference in tooth size between extraction and nonextraction subjects is due to a majority
of subjects having extractions for classification purposes rather than for crowding.
Post DCF PAR score and component change did not show any trends in statistical
significance when comparing extraction and nonextraction groups. A possible
explanation for this is that there is no difference in relapse between extraction and
nonextraction subjects. However, different methods of retention were used for these
subjects, including removable Hawleys and fixed retention. One might feel that fixed
retention would prevent alignment changes. Efforts are being made to group these
subjects by retention type to analyze relapse. However, this could be difficult due to the
comparison of PAR values of subjects with fixed retention to those with Hawley retainers
due to the variable compliance with Hawley retainers. Additionally, the impact of third
molar status has not been examined.
In summary, males had significantly larger teeth than females. Initial PAR score
and molar classification did not differ between males and females. For males, initial
weighted PAR score and overjet correlated with upper and combined upper and lower
anterior tooth size, while in females showed correlation between lower anterior alignment
and all measures of anterior tooth size. Tooth size was not related to initial molar class
During the second phase of treatment, there was a trend for more male subjects
being treated with extractions during the second phase of treatment. The majority of
extractions were done in the upper arch, with very few subjects having extractions in the
lower arch. The difference in tooth width for females treated with extractions compared
to females treated without extractions was much greater than the same difference for
males. There were significant differences in the number of subjects receiving extractions
when grouped by phase I treatment group and by DC7 molar class severity, with the
observation group and the severe class IIs having more extractions. There were few
significant differences between extraction groups with respect to PAR and component
scores at the end of treatment and throughout the retention period
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Juddson Reed received his BA in chemistry from Wake Forest University in 1999.
He received a DMD in 2003 and a Certificate in Orthodontics and Master of Science in
dental sciences in 2006 from the University of Florida College of Dentistry.