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Long-term dental stability of cases with an uncorrected anterior tooth-size discrepancy

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Long-term dental stability of cases with an uncorrected anterior tooth-size discrepancy

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Content LONG-TERM DENTAL STABILITY OF CASES WITH AN UNCORRECTED
ANTERIOR TOOTH-SIZE DISCREPANCY

by
Elaine N. Chow

A Thesis Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(CRANIOFACIAL BIOLOGY)

May 2011

Copyright 2011 Elaine N. Chow

Dedication

To Wallace L. Chow, a person who as my father and best friend, has shown me
that achieving goals in life can and should be done with integrity and hard work,
but always with a positive attitude. He has been there for me without fail and I
couldn’t have picked a better person as a mentor for my life.
ii

Acknowledgements
Dr Peter Sinclair
Dr Anne-Marie Bollen
Dr Robert Keim
Dr Tom Houlihan
Dr Glenn Sameshima
Dr Michael Paine
Ellen Grady
USC Orthodontic class of 2011
iii

Table of Contents
Dedication ii
Acknowledgements iii
List of Tables vi
List of Figures vii
Abstract viii
Chapter 1: Introduction 1
Proposed study 3
Chapter 2: Review of Literature 4
Tooth Size Discrepancy 4
Long-term Changes in Stability 6
Arch length 7
Transverse 7
Extractions 8
Interarch dimensions 9
Methods for Case Assessment 9

Chapter 3: Hypotheses 12
Chapter 4: Materials and Methods 14
Sample 14
Methods 14
Intercanine width 15
Intermolar width 15
Arch length 15
Overbite 17
Overjet 17
Irregularity Index 17
ABO – OGS measurements 18
Statistics 22

Chapter 5: Results 23
Tooth-sizes 23
Dental Arch Dimensions 23
Intercanine width 23
Intermolar width 23
iv

Total arch length 24
Overbite 24
Overjet 25
Irregularity Index 25
ABO - OGS Measurements 29
Alignment 29
Marginal ridges 29
Buccolingual inclination 29
Occlusal contact 30
Occlusal relationship 30
Overjet 30
Interproximal spaces 30
ABO scores 31
Chapter 6: Discussion 34
Arch Dimensions 34
ABO – OGS measurements 37

Chapter 7: Limitations and Future Directions 40
Limitations 40
Future Directions 41
Chapter 8: Conclusions 42
References 44

v

List of Tables
Table 1: Average tooth sizes for the Control and Anterior tooth-size 26
discrepancy groups
Table 2: Dental arch dimensions: Control vs. Anterior tooth-size 27
discrepancy
Table 3: Dental arch dimensions: Changes over time 28
Table 4: ABO – OGS measurements: Control vs. Anterior 32
tooth-size discrepancy

Table 5: ABO – OGS measurements: Changes over time 33
vi

List of Figures
Figure 1: ABO measure gauge 10
Figure 2: Intercanine and Intermolar width 15
Figure 3: Arch length 15
Figure 4: Little’s Irregularity Index 18
Figure 5: Alignment 19
Figure 6: Marginal ridges 19
Figure 7: Buccolingual inclination 20
Figure 8: Occlusal contacts 20
Figure 9: Occlusal relationship 21
Figure 10: Overjet 21
Figure 11: Interproximal contacts 22
vii

Abstract
Background: Little et al performed a long-term stability study of cases with four
premolar extractions 10 and 20 years post-retention. It was found that less than
30% have a satisfactory anterior occlusion, regardless of how well the case
finished. About 30% of the general population has an anterior tooth-size
discrepancy, yet no stability studies have been conducted on these types of
cases.

Purpose: This study aimed to assess the long-term stability of cases with an
untreated anterior tooth-size discrepancy.

Methods: We evaluated the existing cases from the original long-term stability
study and separated them into two groups: no anterior tooth-size discrepancy
and anterior tooth-size discrepancy. The included cases were measured with
digital calipers accurate to 0.01 mm for intercanine width, intermolar width, arch
length, overjet, overbite, the Irregularity Index, and seven ABO cast grading
criteria at 3 time points: pre-treatment (T1), post-treatment (T2), and post-
retention (T3). The measurements were averaged and compared within
themselves and between groups.

Results: Cases with anterior tooth-size discrepancy at T2 had worse occlusal
relationship, higher Irregularity Index, and the alignment was not as good as
viii

ix

those without an anterior tooth-size discrepancy. Most of the differences between
the two groups disappeared by T3. The exception was the occlusal relationship
and occlusal contact measurements, which stayed significantly worse for the
anterior tooth-size discrepancy group.

Conclusion: According to the occlusal relationship measurement, it appears more
difficult to fit the teeth together ideally in those with an anterior tooth-size
discrepancy, and this does not self-correct over time. In most other factors, an
anterior tooth-size discrepancy does not greatly affect long-term stability.
Chapter 1: Introduction
A tooth size discrepancy between the maxillary and mandibular arches is a
problem in achieving the perfect occlusion for some patients. Bolton described a
tooth size discrepancy as an anterior ratio (canine to canine) outside of 77.2%
and an overall ratio (from first molar to first molar) outside of 99.3%
1
.
Malocclusions due to an excess of mandibular tooth size discrepancies can
include a reduced overjet, possibly placing the patient in an edge-to-edge
relationship; decreased overbite, and difficulty achieving an Angle Class I molar
and canine relationship. A tooth size excess in the maxillary arch could also
cause difficulty in achieving an Angle Class I molar and canine relationship, as
well as excess overjet. According to Crosby and Alexander
2
, 22.9% of the
general population has an anterior tooth-size discrepancy, whereas Freeman et
al
3
found 30.6% of their study sample had a significant anterior tooth-size
discrepancy. With such high percentages of anterior tooth-size discrepancies
and the potential malocclusion issues they present, how orthodontists deal with
this issue and then maintain it is of high interest.

Currently, there are various ways of approaching an anterior tooth-size
discrepancy. If there is a maxillary deficiency, possibly due to small or peg
laterals, orthodontists may plan to bond or place veneers on the laterals, but this
commits the patient to lifelong maintenance and replacement of the restorations
4-
8
. Another approach is to use interproximal reduction on the lower anterior teeth
1
if there is a 2-3mm discrepancy
7-9
. A larger discrepancy where interproximal
reduction is not enough can use a one lower incisor extraction
7-8, 10-11
. Finally, if
the patient does not want to commit themselves to lifelong maintenance,
extractions are too big of a change, and the mandibular anterior teeth are too
narrow for interproximal reduction, the orthodontist may take up room in the
maxillary arch by placing beauty bends
7-8
. Although orthodontists use these
various methods, there is no study showing the stability of treating these high
numbers of patients that have an anterior tooth-size discrepancy.

Orthodontists prevaricate on what factors will cause a more stable orthodontic
outcome, but a study conducted by Little et al
13-14
showed there is no predictive
factor for stability, such as Angle class, length of retention, age of patient at start
of treatment, gender, nor Irregularity Index of pre or post treatment. All that could
be “predicted” is that whether there was orthodontic treatment or not, arch width
and length decreased over time. This study also showed that premolar
extractions over time give low stability: 10 years post-retention, less than 30% of
patients showed satisfactory anterior occlusion, and at 20 years post-retention
only 10% could be considered satisfactory. A study done by Riedel et al
11

showed that a one or two lower incisor extraction pattern is more stable than a
premolar extraction, but should be used only in specific cases, due to causes
other issues such as increased overjet. A single lower incisor extraction had
71% acceptable results after 10 years, and the two incisor extraction cases were
2
stable in 44% of the cases. It was presumed that none of the previous studies
had used circumferential supracrestal fiberotomies, as these cases were treated
before this technique became popularized. These studies looked at long-term
stability in extraction cases, but no one has researched the stability of anterior
tooth-size discrepancies, despite it affecting such a large portion of the
population.

Proposed study:
Using records from the University of Washington, we will measure casts and
place them in 2 groups: Anterior tooth-size discrepancy and the control with no
anterior tooth-size discrepancy. The casts will be measured with digital calipers
to the nearest 0.01 mm at three time points: pre-treatment, post-treatment, and at
least 10 years post-retention. Their intercanine width, intermolar width, overjet,
overbite, arch length, Irregularity Index, and ABO scores will be recorded and
evaluated for differences in stability.
3
Chapter 2: Review of Literature
Tooth Size Discrepancy:
The relative size of maxillary and mandibular teeth and their relationship to each
other has been of interest to many pioneer researchers
1, 9, 15
. The most
commonly applied research came from Bolton
1
when he measured the
mesiodistal dimensions of teeth and compared the summed results between
arches. For excellent occlusion, he calculated that the overall and anterior ratios
between upper and lower teeth should be 91.3% +/- 0.26 (1.91SD) and 77.2% +/-
0.22 (1.65 SD), respectively. It is now thought that Bolton’s standard deviations
may be too small to be clinically significant
16-18
. Othman and Harradine
18

suggest expressing anterior tooth-size discrepancies in millimeters rather than a
ratio, and finds that 2 mm is clinically significant. Recently, Endo et al
16

supported Othman and Harradine’s findings that 2 mm is clinically significant, but
alongside a ratio greater than 2 SD.

Crosby et al
2
found 22.9% of the general population had an anterior tooth-size
discrepancy greater than 2 SD. The high percentage of patients with this type of
tooth-size discrepancy is further reinforced by Freeman et al
3
, who found 30.6%
of their patient pool outside of 2 SD. Many studies have been conducted in
various ethnic populations that continue to support Crosby et al’s
2
findings, such
as: Endo et al
19
who studied a Japanese population (21.6%); Uysal et al
20

studied a Turkish population (21.3%); Paredes et al
21
studied a Spanish
4
population (21%); Wedrychowska-Szulc
24
studied a Polish population (31.2%);
Araujo et al
22
studied a predominantly Brazilian population (22.7%); and Bernabe
et al
23
studied a Peruvian population (20.5%). These ethnic specific studies also
found no significant difference of anterior tooth-size discrepancy between
sexes
19-23
.

The prevalence of an anterior tooth-size discrepancy in malocclusions has been
studied and overall been found to be unrelated to a specific Angle classification.
Class I; Class II, division 1; and Class II, division 2 malocclusions did not have a
greater number of patients with an anterior tooth-size discrepancy in any group
according to Crosby and Alexander
2
. Uysal et al
20
also found no difference in
percentage of patients with an anterior tooth-size discrepancy in each
malocclusion group (Class I; Class II, division 1; Class II, division 2; Class III).
However, he did find this condition occurred more frequently in patients with
malocclusions than in a control group of untreated normal occlusion subjects.
Although these two studies found no relationship between an anterior tooth-size
discrepancy and Angle classification, research by Araujo et al
22
found a higher
percentage of anterior tooth-size discrepancy in patients with Class I and Class
III malocclusion. Also, the severity of the anterior tooth-size discrepancy was
found to be greater in Class III malocclusions than for Class I or II patients. This
finding is supported by Wedrychowska-Szulc et al
24
who found more anterior
tooth size discrepancies in males with Class III and Class I malocclusions.
5
Crosby and Alexander
2
did not include Class III patients in their study, which
could be the reason no statistical difference was found between malocclusion
groups in that study.

Long-term Changes in Stability:
Orthodontists want to know what factors will make their cases stable throughout
life. Understanding what happens to the untreated dentition over time is
important to learning how various treatment mechanics may affect long-term
stability.

In a robust study examining four premolar extraction cases, Little et al
13-14
stated
that stability could not be predicted by any factor, such as gender, Angle class,
length of retention, quality of finish, and other characteristics. Additionally, he
stated that over a 10 year post-retention period, less than 30% of cases had
satisfactory alignment, and in 20 years, only 10% were satisfactory. Riedel et
al
11
found greater stability of cases over a 10 year post-retention period in
patients with one (71% acceptable) or two (44% acceptable) lower incisor
extractions. As with previous studies
12, 25-33, 36
, there was a post-retention
decrease in arch length and intercanine width. The intermolar width decreased
for the one-incisor extraction group, but had no change in two-incisor extraction
group
11
. Supporting these findings was the study by Bishara et al
33
in which
overbite and overjet were both increased 14.8 months post-retention.
6
Previous studies did not find a predictive factor for stability
12-14, 35-36
. More
recently, Artun et al
32
and especially Ormiston et al
34
, claim there are predictive
factors associated with decreased stability. These factors include gender (males
are 4 times as likely to be unstable), continued growth, higher Irregularity Index,
higher pre-treatment PAR score, Angle classification (Class II is 2 times more
unstable), a short initial arch length, and narrow intercanine width.

Arch length
Without orthodontic treatment, arch length decreases from the primary to
permanent dentition
4, 12, 25, 26, 35, 39
. The greatest increase of growth in both
arches is during the first two years of life, peaking around eight years of age in
the mandibular arch and thirteen years old for the maxillary arch
27
. After
reaching peak arch length, both the maxillary and mandibular arch lengths
decrease significantly throughout adulthood, with or without treatment
27, 29-30, 35-36,
39-41, 43
.

Transverse
The greatest intercanine width is seen during preteen years, with females
showing more degradation during teen years, and then remaining relatively
stable
12, 25-26, 28
. Decreases have been found in the mandibular intermolar width
over time for both sexes
26
, as well as females only
25
. Conversely, another study
found no significant change in mandibular intermolar width over time
31
.
7
It would seem that expanding the arches would counteract the decrease of
intercanine width. However, Sinclair et al
25
found intercanine width decreased
three times as quickly in treated malocclusions, Artun et al
32
and Erdinc et al
31

stated increasing the width is associated with relapse, and Little
12
said the only
way to prevent the collapse is lifetime retention
42
. Even if the arches are not
expanded, a decrease in intercanine width still occurs
33, 43
.

Extractions
For many years, there was a belief that early extraction to relieve crowding would
result in long term stability. This unproven theory was rejected by Little et al
12, 37

while comparing early versus late extraction of the four first premolars over a ten
year post-retention study. McReynolds
36
also showed there was no difference
between early and late extractions of second premolars and their long term
stability of arch length, width, and Incisor Irregularity.

As the current trend is to attempt treating more cases non-extraction when
possible, it would be interesting to know if one particular treatment modality
beneficially influences long-term stability. According to Erdinc et al
31
, there is no
difference in stability between extraction and non-extraction cases from T2 to T3.
According to these studies, extractions, whether first or second premolars, early
or late, or non-extraction provide no influence on stability over time

8
Interarch relationships
In untreated occlusions, overjet is generally stable
25, 38, 39, 41
. Overbite in
untreated occlusions was found to decrease by Sinclair et al
25
, but Lee
38
and
Harris
39
found this dimension to be stable. Studying treated cases of patients in
retention for an average of only 14.8 months, Bishara
33
found an increase in
overbite and overjet. This finding is supported by many other studies
31, 32, 34
.
Harris
39
studied 60 patients from age 20 to 55 years and found little change in
molar relationships. Lee
38
also found molar relationships in untreated cases to
be relatively stable over time.

Methods for Case Assessment:
The ABO assesses finished cases using an Objective Grading System (OGS)
which became established in 1999 after a series of four field tests over five
years
44-45
. After implementing this new system, the highest passing rate in years
was produced (89%)
45
. This is speculated to be due to the ability for candidates
to score their own cases, along with the standard measuring gauge, before
presenting them to the Board. When scoring cases, the Objective Grading
System looks at eight criteria: alignment, marginal ridges, buccolingual
inclination, occlusal relationships, occlusal contacts, overjet, interproximal
contacts, and root angulation
44, 46
. The directors of the American Board of
Orthodontics have validated the exam after repeated comparisons of objective
and subjective systems. Reliability of the Objective Grading System is being
9
ensured by the use of a precise measuring instrument, as well as training and
calibration for the scorers to establish a confidence interval to account for
interrater variability.
Figure 1. ABO measuring gauge
44

The Irregularity Index developed by Little
47
quantifies the severity of mandibular
anterior irregularity. Five measurements are taken from the anatomical contact
points instead of the clinical contact points, from the distal of the lateral to the
distal of the contralateral lateral. Reliability and validity were measured and
deemed satisfactory.

Orthodontists often believe the quality of their finished case enhances stability,
despite studies that state all cases degrade over time
13-14, 32, 49
. Fernandes
48

looked at long term stability utilizing the PAR and ABO - OGS, and found no
correlation between quality of finish and stability. Other researchers found a
better quality finish was obtained through a greater treatment change, and these
cases had more relapse
46, 49
. Although this finding alone supports Fernandes
48
,
these studies went on further to state that although there was more relapse in the
10
cases with better finishes, when evaluated at T3, they still had better occlusal
relationships than the poorly finished cases
46, 49
.
11
Chapter 3: Hypotheses
H1: Cases with untreated anterior tooth-size discrepancies will have significantly
less overjet at 10 years post-retention than cases with no anterior tooth-size
discrepancy.

H2: Cases with untreated anterior tooth-size discrepancies will have significantly
less overbite at 10 years post-retention than cases with no anterior tooth-size
discrepancy.

H3: Cases with untreated anterior tooth-size discrepancies will have a
significantly worse ABO score at 10 years post-retention than cases with no
anterior tooth-size discrepancy.

H4: Cases with untreated anterior tooth-size discrepancies will have less
mandibular intercanine width at 10 years post-retention than cases with no
anterior tooth-size discrepancy.

H5: Cases with untreated anterior tooth-size discrepancies will have an
increased Irregularity Index at 10 year post-retention than cases with no anterior
tooth-size discrepancy.

12
H6: Cases with untreated anterior tooth-size discrepancies will have no
significant difference of mandibular intermolar width at 10 years post-retention
than cases with no anterior tooth-size discrepancy.
13
Chapter 4: Materials and Methods
Sample:
Cases were selected from the records housed at the University of Washington,
Department of Orthodontics, which consists of patients treated by residents, as
well as private practices. These cases were coded and had no identifying
markers on them. To be included, the records must have had a complete set of
casts at pre-treatment (T1), post-treatment (T2), and at least 10 years post-
retention (T3).

The sample collected was narrowed down from 200 potential cases, to 64 Class I
non-extraction and four premolar extraction cases. All cases were treated with
edgewise mechanics and had varied lengths of retention. The cases were
divided into two groups: the control consisted of 35 cases with less than 1mm
tooth-size discrepancy; the experimental group consisted of 29 cases with a
significant anterior tooth-size discrepancy of 2 mm or greater.

Methods:
The measurements were obtained by a single calibrated examiner. All cast
measurements were taken at two separate time points and evaluated with an
Interclass Correlation Coefficient test to assess for intrarater reliability. The
results for all variables were greater than 0.80 and the methods were deemed
reliable. The following measurements were made at all three time points, except
14
the ABO-OGS which was recorded at T2 and T3 only. The ABO-OGS grading in
this study did not include second molars.

Intercanine width
The intercanine width is the distance from cusp tip to cusp tip of the mandibular
canines, or estimated cusp tip if there was attrition, using 0.01 mm digital calipers
(Figure 2). If the permanent mandibular canines were not fully erupted at T1, this
measurement was excluded.

Intermolar width
The intermolar width is the distance from mesiobuccal cusp tip to mesiobuccal
cusp tip of the mandibular first molars, or their estimated cusp tips if there was
attrition using 0.01 digital calipers (Figure 2).

Arch length
The arch length is the summed distances from the mesial of the mandibular
central incisor to the mesial of the mandibular first molars on right and left sides,
using 0.01 mm digital calipers (Figure 3).

15

Figure 2. Intercanine and Intermolar width

Figure 3. Arch length
16
Overbite
Overbite is the amount of lower incisor tooth coverage from the incisal edge of
the mandibular incisor to the incisal edge of the maxillary incisor, always
measuring the left central incisors. A straight edge was used to mark where the
incisal edge of the maxillary central incisor covered the lower central incisor, and
a 1 mm marked UNC periodontal probe was used to measure the length of
overbite.

Overjet
Overjet is the distance from the labial surface of the mandibular incisor to the
labial surface of the maxillary incisor, always measuring the left central incisor.
This was measured with a millimeter ruler.

Irregularity Index
The Irregularity Index is the summed distance of the amount of displaced
contacts points from the mesial of the mandibular canine to the mesial of the
contralateral mandibular canine. If the permanent mandibular canines were not
fully erupted at T1, this contact point was excluded while the contact points
between incisors were taken.
17
Figure 4. Little’s Irregularity Index
47

ABO – OGS measurements
ABO – OGS measurements were taken using the ABO Objective Grading
System for casts. The official ABO – OGS measurements include all teeth, from
second molar to second molar, but we only measured from first molar to first
molar. The measurements taken in this study were alignment, marginal ridges,
buccolingual inclination, occlusal contacts, occlusal relationship, overjet, and
interproximal contacts.
18
5a

5b 5c
Figure 5. Alignment: 5a: Ideal alignment. 5b: 1 point. 5c: 2 points.
44

6a

6b 6c
Figure 6. Marginal ridges: 6a – Ideal. 6b – 1 point. 6c – 2 points.
44

19
7a

7b 7c
Figure 7. Buccolingual inclination: 7a – Ideal. 7b – 1 point. 7c – 2 points.
44

8a

8b 8c
Figure 8. Occlusal contacts: 8a – Ideal. 8b – 1 point. 8c – 2 points.
44

20
9a

9b 9c
Figure 9. Occlusal relationship: 9a – Ideal. 9b – 1 point. 9c – 2 points.
44

10a

10b 10c
Figure 10. Overjet: 10a – Ideal. 10b – 1 point. 10c – 2 points.
44

21
11a

11b 11c
Figure 11. Interproximal contacts: 11a – Ideal. 11b – 1 point. 11c – 2
points.
44

Statistics:
Initially, independent-samples T tests were conducted to compare the control to
anterior tooth-size discrepancy group means for all variables at all time points
(T1, T2, and T3).
Next, paired-samples T tests were conducted to compare the means for each
variable at T1 versus T2. Since T1 was not measured for the ABO variable
scores, the ABO variables were not included in T1-T2 comparisons. Likewise, all
variables were compared between T2 and T3. All variables except the ABO
scores were also compared between T1 and T3.

22
Chapter 5: Results
Tooth-sizes:
There were no significant differences found between the averaged mesiodistal
widths of any teeth between the control and the group with an anterior tooth-size
discrepancy (Table 1).

Dental Arch Dimensions (Table 2, 3):
Intercanine width
Pretreatment, both groups started with similar intercanine distances (control =
25.50, experimental = 25.23) and were both significantly expanded (control =
+1.21, experimental = +1.22, p < .05) during treatment. However, post-retention,
they both significantly relapsed (control = -2.08, experimental = -1.36, p < .05) to
a smaller intercanine width than their initial measurement (control = 24.62,
experimental = 25.09).

Intermolar width
No significant width difference existed between groups pretreatment (control =
40.81, experimental = 41.71). However, the difference between the two groups
was statistically significant at T2 (control = 40.27 mm, experimental = 41.64 mm,
p <.05) and T3 (control = 39.10 mm, experimental = 40.81 mm, p < .05), with the
control decreasing more at both time points (-0.54 and -1.17 mm, respectively).
23
Both groups decreased width from pre-treatment to post-treatment, and then
continued to decrease through post-retention.

Total arch length
The control and experimental groups started with similar arch lengths (57.41 mm
and 58.81 mm, respectively). Both decreased a statistically significant amount
(-7.31mm and -4.61 mm, p < .05), and the control had a significantly shorter arch
length than the experimental at post-treatment (50.1 mm versus 54.17 mm, p <
.05). The control was still significantly shorter in post-retention (47.92 mm versus
52.29 mm, p < .05). There was a significant decrease in arch length from T1 to
T3 (control = -9.49 mm, experimental = -6.47 mm, p < .05).

Overbite
Pre-treatment, the patients started with excess overbite (control = 3.41 mm,
experimental = 3.22 mm) which was corrected to ideal standards during
treatment. The control group decreased a significant amount (-1.14 mm, p < .05)
during treatment. Post-retention, the overbite increased slightly for a final
measurement of 2.69 mm for control, and 2.81 mm for the anterior tooth-size
discrepancy group. There was no significance between groups.

24
25
Overjet
Both groups started with similar overjet measurements (control = 3.51 mm,
experimental = 3.60 mm). During treatment, the excess overjet was significantly
corrected (control = -1.46 mm, experimental = -1.28 mm, p < .05) to ideal
standards. After treatment, the overjet increased again to 2.54 mm for the
control, and 2.28 mm for the anterior tooth-size discrepancy group, but still within
ideal limits. No significance was found between groups, but there was a
significant decrease from pretreatment to post-retention (control = -0.97 mm,
experimental = -1.33 mm, p < .05).

Irregularity Index
Both groups started with an unacceptable Irregularity Index (control = 6.25,
experimental = 5.74) and were significantly improved (control = -5.51,
experimental = -4.38, p < .05) during treatment. The control had a significantly
better score at post-treatment than the experimental group (0.74 versus 1.36, p <
.05). After treatment, the control had a significant increase (+2.21, p < .05) in
Irregularity Index, while the experimental group had no significant change.
However, from pre-treatment to post-retention, both groups had a significant
improvement in scores (control = -0.97, experimental = -1.33, p < .05) with a final
measurement of 2.95 for the control and 2.99 for the experimental group.

Table 1. Average tooth sizes for the Control and Anterior tooth-size discrepancy oups gr
Control Anterior tooth size-discrepancy
Average
Standard
deviation Range Average
Standard
deviation Range
p
value

LL3 7.06 0.40 6.14 - 7.90 6.99 0.50 6.01 - 8.35 0.551

LL2 6.18 0.44 4.90 - 7.02 6.05 0.40 5.33 - 6.97 0.227

LL1 5.54 0.38 4.79 - 6.56 5.48 0.36 4.83 - 6.27 0.495

LR1 5.56 0.37 4.78 - 6.43 5.48 0.34 4.86 - 6.32 0.384

LR2 6.20 0.42 5.28 - 7.14 6.09 0.44 5.33 - 7.08 0.335

LR3 7.05 0.38 6.27 - 7.79 6.96 0.47 6.11 - 8.22 0.406

UR3 7.90 0.45 6.72 - 8.68 7.89 0.58 6.89 - 9.38 0.968

UR2 7.20 0.57 5.90 - 8.30 7.00 0.87 5.40 - 9.10 0.290

UR1 9.03 0.63 7.39 - 10.52 9.03 0.65 7.56 - 10.11 0.964

UL1 9.06 0.67 7.35 - 10.53 8.99 0.69 7.53 - 10.07 0.654

UL2 7.25 0.55 5.96 - 8.28 6.98 0.95 4.4 - 9.01 0.173

UL3 7.87 0.43 7.01 - 8.62 7.90 0.57 7.09 - 9.35 0.796
* significance level p < .05
26

Control Anterior tooth-size discrepancy

Time Average
Standard
deviation Range Average
Standard
deviation Range p value
Intercanine T1 25.50 2.05 20.94 - 30.47 25.23 1.83 22.08 - 28.48 0.600
T2 26.71 1.59 23.40 - 30.09 26.46 1.72 22.32 - 29.05 0.548
T3 24.62 1.95 19.71 - 27.67 25.09 1.63 20.59 - 27.77 0.307

Intermolar T1 40.81 3.48 32.1 - 47.84 41.71 2.91 35.9 - 47.26 0.274
T2 40.27 2.60 35.25 - 47.34 41.64 2.36 37.15 - 45.62 0.032*
T3 39.10 2.91 34.3 - 47.82 40.81 2.10 35.51 - 45.96 0.024*

T1 57.41 5.40 45.82 - 68.9 58.81 4.32 46.27 - 66.14 0.273 Arch
length T2 50.10 3.77 43.42 - 61.45 54.17 5.86 45.29 - 63.26 0.002*
T3 47.92 3.75 42.42 - 59.08 52.29 5.70 43.09 - 61.21 0.001*

Overjet T1 3.51 2.19 -3 - 8.5 3.60 2.54 -1 - 11 0.881
T2 2.06 0.78 0 - 3.5 2.33 0.96 1 - 5 0.218
T3 2.54 1.06 -1 - 4 2.28 0.90 1 - 4 0.288

Overbite T1 3.41 1.79 0 - 7.5 3.60 2.54 0 - 6.5 0.659
T2 2.27 1.01 0 - 5 2.45 0.88 0 - 4.5 0.463
T3 2.69 1.32 -1 - 5 2.81 1.19 1 - 6 0.696

T1 6.25 3.71 0.34 - 17.61 5.74 4.23 0 - 14.76 0.618 Irregularity
Index T2 0.74 0.73 0 - 3.13 1.36 1.52 0 - 6.53 0.050*
T3 2.95 1.90 0 - 6.87 2.99 1.89 0.32 - 7.02 0.926
Table 2. Dental arch dimensions: Control vs. Anterior tooth-size discrepancy
* significance level p < .05
27

Time
Change
Control p value
Anterior
tooth-size
discrepancy
p value
Intercanine T1 - T2 +1.21* 0.030 +1.22* 0.035
T2 - T3 -2.08* 0.000 -1.36* 0.014
T1 - T3 -0.87 0.154 -0.14 0.954

Intermolar T1 - T2 -0.54 0.756 -0.07 0.996
T2 - T3 -1.17 0.270 -0.84 0.515
T1 - T3 -1.71 0.064 -0.90 0.426

Arch length T1 - T2 -7.31* 0.000 -4.61* 0.006
T2 - T3 -2.18 0.119 -1.85 0.422
T1 - T3 -9.49* 0.000 -6.47* 0.006

Overjet T1 - T2 -1.46* 0.000 -1.28* 0.016
T2 - T3 +0.49 0.392 -0.05 0.993
T1 - T3 -0.97* 0.026 -1.33* 0.012

Overbite T1 - T2 -1.14* 0.004 -0.78 0.070
T2 - T3 +0.41 0.473 +0.36 0.551
T1 - T3 -0.73 0.102 -0.41 0.460

Irregularity Index T1 - T2 -5.51* 0.000 -4.38* 0.000
T2 - T3 +2.21* 0.001 +1.63 0.089
T1 - T3 -3.30* 0.000 -2.75* 0.002
Table 3. Dental arch dimensions: Changes over time
* significance level p < .05
28
ABO – OGS Measurements (Table 4, 5):
Alignment
The experimental group had a significantly worse alignment of teeth compared to
the control (4.86 versus 3.74, p < .05) at the end of treatment. After treatment,
both groups’ alignment became significantly worse (control = +3.03, experimental
= +2.83, p < .05), resulting in a post-treatment score of 6.77 for the control and
7.69 for the anterior tooth-size discrepancy group.

Marginal ridges
There are no significant differences between the groups at T2 (control = 2.23,
experimental = 2.72) or at T3 (control = 1.34, experimental = 1.28). During post-
retention, both groups significantly improved (control = -0.89, experimental = -
1.44, p < .05) from their post-treatment score.

Buccolingual inclination
No significance was found between groups at T2 (control = 2.86, experimental =
2.34). The control did improve over time, whereas the experimental did not
change, but not producing a significant difference between groups at T3 (control
= 2.54, experimental = 2.28).

29
Occlusal contact
The control group had similar occlusal contacts as the experimental group at
post-treatment (control = 3.71, experimental = 3.93). Over time, the control
significantly improved (-2.71, p < .05) and had a significantly better result at post-
retention (control = 1.00, experimental = 2.72, p < .05).

Occlusal relationship
Cases with an anterior tooth-size discrepancy had a worse occlusal relationship
than those without at the end of treatment (control = 3.80, experimental = 4.55).
After treatment, the control’s occlusal relationship improved and showed a
statistically significant better result than the experimental group (3.37 versus
4.69, p < .05)

Overjet
Similar overjet results were found at the end of treatment for both groups (control
= 3.11, experimental = 3.07). They also both improved after treatment, but only
the control had a significant improvement (-1.71, p < .05).

Interproximal spaces
Results at post-treatment (control = 2.43, experimental = 2.17) and post-retention
(control = 0.97, experimental = 0.31) were similar for both groups. They both had
30
31
improvement over time, but the anterior tooth-size discrepancy group improved
significantly more (-1.86, p < .05)

ABO scores
Although no significant differences were found between groups, they both were
treated well and had satisfactory scores at post-treatment (control = 21.89,
experimental = 23.66). At post-retention, their scores improved and were still
considered well treated (control = 17.40, experimental = 21.00). However, only
the control had significant improvement over time (-4.49, p < .05).
Control Anterior tooth-size discrepancy
Time Average
Standard
deviation Range Average
Standard
deviation Range p value
Alignment T2 3.74 1.98 0 - 8 4.86 2.20 0 - 10 0.036*
T3 6.77 3.52 1 - 16 7.69 2.78 1 - 13 0.258

Marginal ridges T2 2.23 1.66 0 - 8 2.72 1.98 0 - 7 0.281
T3 1.34 1.26 0 - 4 1.28 1.22 0 - 5 0.831

T2 2.86 1.87 0 - 7 2.34 1.37 0 - 5 0.224 Buccolingual
inclination T3 2.54 1.72 0 - 7 2.28 1.44 0 - 5 0.508

T2 3.71 3.79 0 - 13 3.93 4.27 0 - 13 0.830 Occlusal
contact T3 1.00 1.66 0 - 6 2.72 4.13 0 - 15 0.042*

T2 3.80 1.86 0 - 8 4.55 3.63 0 - 16 0.318 Occlusal
relationship T3 3.37 2.21 0 - 8 4.69 2.73 0 - 10 0.037*

ABO Overjet T2 3.11 2.96 0 - 14 3.07 3.36 0 - 14 0.954
T3 1.40 2.68 0 - 11 2.03 2.53 0 - 8 0.337

T2 2.43 3.18 0 - 15 2.17 2.42 0 - 10 0.723 Interproximal
spaces T3 0.97 3.27 0 - 19 0.31 0.66 0 - 2 0.289

T2 21.89 8.77 11 - 43 23.66 9.94 9 - 57 0.452 ABO-OGS
scores T3 17.40 8.91 6 - 47 21.00 8.24 9 - 43 0.101
Table 4. ABO-OGS measurements: Control vs. Anterior tooth-size discrepancy
* significance level p < .05
32

Time
Change
Control p value
Anterior
tooth-size
discrepancy
p value
Alignment T2 - T3 +3.03* 0.000 +2.83* 0.000

Marginal ridges T2 - T3 -0.89* 0.014 -1.44* 0.001

Buccolingual
inclination
T2 - T3 -0.32 0.466 -0.06 0.852

Occlusal contacts T2 - T3 -2.71* 0.000 -1.21 0.278

Occlusal
relationship
T2 - T3 -0.43 0.383 +0.14 0.871

ABO-OGS Overjet T2 - T3 -1.71* 0.013 -1.04 0.190

Interproximal
space
T2 - T3 -1.46 0.063 -1.86* 0.000

ABO-OGS score T2 - T3 -4.49* 0.037 -2.66 0.273
Table 5. ABO – OGS Measurements: Changes over time
* significance level p < .05
33
Chapter 6: Discussion
The hypotheses posed for this study were that at 10 years post-retention, cases
with an untreated anterior tooth-size discrepancy would have less overjet and
overbite, worse Irregularity Index, a decreased intercanine distance, no
significant difference of intermolar width, and a worse ABO-OGS score than
those without an anterior tooth-size discrepancy. Our findings in this study
rejected all of these hypotheses.

Arch Dimensions:
One of the major questions being addressed in this study was whether the
presence of an anterior tooth-size discrepancy would make it harder to obtain
good post-treatment incisor alignment, and also whether the alignment would
significantly worsen with time. Our results showed that the control group's post-
treatment Irregularity Index was significantly better than that of the experimental
group (0.74 vs. 1.36, p < .05) despite the fact that the control’s Irregularity Index
was higher at the start of treatment (6.25 vs. 5.74, p = ns). This meant that the
control group could be considered as excellently finished while the experimental
group would fall in the well finished category. Both groups showed increases in
Irregularity Index over time, with the control group’s 2.21 increase being
significantly greater than the 1.63 seen in the experimental group. As a result
both groups ended up with similar scores at the 10 year post-retention period
(Control = 2.95, Experimental = 2.99, p = ns). These findings suggest that while
34
aligning lower incisors is harder in a case with an anterior tooth-size discrepancy,
over the next decade both groups regressed towards a common mean. These
findings are similar to deFreitas, Little, and Artun
13-14, 32, 49
whom found a
significant improvement at the end of treatment, and then degradation after
treatment. It appears that both groups would benefit from long-term permanent
retention, as other forces continue to affect lower incisor alignment independent
of an anterior tooth-size discrepancy.

Attempting to finish a case with correct overjet can be a difficult task, and one of
the questions asked in this study was whether this correction would be more
difficult for cases with an anterior tooth-size discrepancy, and if this dimension
would worsen over time. Initially, both groups started with a similar excess
overjet (Control = 3.51 mm, Experimental = 3.60 mm, p = ns) and were both
corrected to a more ideal finish (Control = 2.06 mm, Experimental = 2.33 mm, p =
ns). Post-retention, there was a small but insignificant increase of overjet for the
control (+0.49 mm, p = ns) and an insignificant decrease for the anterior tooth-
size discrepancy group (-0.05 mm, p = ns). From pre-treatment to post-retention,
there was a significant decrease for both groups, but a greater decrease of
overjet for the anterior tooth-size discrepancy group (-0.97 mm vs. -1.33 mm, p <
.05). The control groups’ behavior mirrored previous studies which showed an
increase in overjet during the post-retention period
31, 33
. Interestingly, the anterior
tooth-size discrepancy group behaved more like cases with untreated
35
occlusions
25-26
which had no change in overjet post-retention. Although the
anterior tooth-size discrepancy group ended with less overjet, this was not a
significant finding. Therefore, these findings lead us to believe that it is possible
to correct either type of case to the proper overjet, and over time, there is not
enough of a change to merit clinically treating these cases any differently.

Another question addressed during this study was what occurs to the arch width
dimensions in cases with an anterior tooth-size discrepancy and do these
dimensions significantly change over time compared to controls. Interestingly,
the intercanine widths were similar pre-treatment (Control = 25.50 mm,
Experimental = 25.23 mm, p = ns) and post-retention (Control = 24.62 mm,
Experimental = 25.09 mm, p = ns), whereas the intermolar widths also were
similar pre-treatment (Control = 40.81 mm, Experimental = 41.71 mm, p = ns),
but the anterior tooth-size discrepancy group was significantly wider in post-
retention (39.10 mm vs. 40.81 mm, p < .05). The intercanine width significantly
decreased post-retention (Control = -2.08 mm, Experimental = -1.36 mm, p < .05)
for both groups, which is consistent with past literature which shows a decrease
with or without treatment, as well as with or without dental expansion
12, 25-26, 28, 31-
33, 42-43
. The outcome of intermolar width over time has been discordant in past
studies
25-26, 31
, showing either a decrease or no change. Our study shows an
insignificant decrease in intermolar width post-retention (Control = -1.17 mm,
Experimental = -0.84 mm, p = ns), but the control has a significantly smaller
36
intermolar width than the anterior tooth-size discrepancy group at T3. These
arch width findings are most likely due to the uneven distribution of the four
premolar extraction cases. The control group consisted entirely of four premolar
extractions, while the anterior tooth-size discrepancy group was made up of half
four premolar extraction, and half non-extraction cases. As the molars were
moved anteriorly to a narrower arch form to close the extraction space, the
average width for the control became smaller than the anterior tooth-size
discrepancy group due to it’s composition of purely extraction cases. The
intermolar width finding does not lead to any change in practice for cases with an
anterior tooth-size discrepancy. The significant intercanine width decrease for
both groups lends credence to Little’s
12-13
suggestion for lifetime retention for all
cases.

ABO-OGS Measurements:
The most interesting and clinically relevant finding was seen in the ABO-OGS
occlusal relationship scores. Although there was no significant difference
between the groups at T2 (Control = 3.80, Experimental = 4.55, p = ns), the
control has a small improvement (-0.43, p = ns) while the anterior tooth-size
discrepancy group has a slight degradation (+0.14, p = ns), leading to a
significantly better post-retention score for the control (3.37 vs. 4.69, p < .05).
The lack of a significant change during the post-retention period agrees with Nett
et al
46
that also used the ABO-OGS to score the change from post-treatment to
37
post-retention, as well as other research that has found little change in molar
relationships over time
38-39
. Although there is no significant change over time,
our findings seem to allude that although it may be possible to finish cases with
an anterior tooth-size discrepancy well, it is more difficult to fit the teeth together
ideally and this is does not improve over time like the control does. For a more
ideal occlusal relationship, it may be advantageous to correct an anterior tooth-
size discrepancy to allow a more ideal, long-term result.

Another interesting finding was that the ABO-OGS occlusal contact
measurement for the control significantly improved (-2.71, p < .05) and had a
significantly better score at post-retention than the anterior tooth-size
discrepancy group (1.00 vs. 2.72, p < .05). This finding is consistent with Nett et
al’s
46
findings of an improved ABO-OGS occlusal contact score during post-
retention. Although both groups finished treatment with similar scores (Control =
3.71, Experimental = 3.93, p = ns), the cases without an anterior tooth-size
discrepancy were able to settle better over time, producing an even better
occlusion.

Whether an anterior tooth-size discrepancy existed or not, the ABO-OGS overall
score indicates the cases were able to finish with a good result at the end of
treatment (Control = 21.89, Experimental = 23.66, p = ns). However, only the
control significantly improved post-retention (-4.49, p < .05), mostly due to the
38
occlusal contact and occlusal relationship findings. The significant positive
changes of the overall ABO-OGS score for the control group after treatment are
in agreement with past research
46
. It appears that although a case with an
anterior tooth-size discrepancy can finish well, it does not improve over time as
well as those without an anterior tooth-size discrepancy.

39
Chapter 7: Limitations and Future Directions
Limitations:
1. There was an unequal retention period for cases in this study.

2. The control group consisted entirely of four premolar extraction cases,
while the anterior tooth-size discrepancy group was made up of half four
premolar extraction and half non-extraction cases.

3. The anterior tooth-size discrepancy group consisted of half maxillary
excess and half mandibular excess cases, rather than equal numbers of
the control, maxillary excess, and mandibular excess.

4. The eighth and final ABO-OGS scoring parameter was not included in this
study: root angulations. The inclusion of this criterion most likely would
have worsened the overall ABO-OGS score, and we are unable to
ascertain if there was a significant difference between the control and
anterior tooth-size discrepancy groups.

5. Second molars were not included in the ABO-OGS scoring in this study,
although it is for the official ABO-OGS. Our final scores were therefore
not indicative of the outcome of the entire occlusion.
40
6. We did not account for the difficulty of the cases pre-treatment with the
ABO Discrepancy Index score. We may therefore have not been
evaluating cases with equal difficulty.

7. Several of the cases were treated with banding of all teeth, which altered
the true ABO-OGS interproximal spaces score at post-treatment.

8. We did not account for whether the spaces were anterior or posterior, and
if they were due to band spaces, or necessary spaces.

9. No maxillary arch dimensions were accounted for.

Future Directions:
A study involving equal cases of a control, maxillary excess, and mandibular
excess would certainly be interesting to discover if the arch dimensions and
ABO-OGS scoring outcomes would be altered. Also, to have a more accurate
measurement of the final treatment and post-retention outcome, second molars
and the panoramic radiograph should be included.
41
Chapter 8: Conclusions
1. There is no great difference in long-term stability between cases with and
without an anterior tooth-size discrepancy. All of our hypotheses were
rejected.

2. The Irregularity Index has no difference between groups at 10 years post-
retention, despite the control starting with more crowding and having a
significantly better post-treatment score. It appears that both groups
regressed towards a common mean.

3. There are no significant differences of overjet and overbite measurements
at 10 years post-retention between cases with and without an anterior
tooth-size discrepancy. Therefore, cases with an anterior tooth-size
discrepancy do not need to be treated differently than those without.

4. The ABO-OGS occlusal relationship, which measures Angle
classifications of the buccal segments, has no significant differences at
post-treatment. The control has better settling, leading to a significantly
better score at post-retention.

5. The ABO-OGS occlusal contact measurement had no difference at post-
treatment, but settled better during the post-retention period and produced
42
a better result for cases without an anterior tooth-size discrepancy at post-
retention.
43
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Abstract (if available)

Abstract Background: Little et al performed a long-term stability study of cases with four premolar extractions 10 and 20 years post-retention. It was found that less than 30% have a satisfactory anterior occlusion, regardless of how well the case finished. About 30% of the general population has an anterior tooth-size discrepancy, yet no stability studies have been conducted on these types of cases.

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Asset Metadata

Creator Chow, Elaine Nicole (author)

Core Title Long-term dental stability of cases with an uncorrected anterior tooth-size discrepancy

School School of Dentistry

Degree Master of Science

Degree Program Craniofacial Biology

Degree Conferral Date 2011-05

Publication Date 04/05/2011

Defense Date 03/22/2011

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag Bolton Tooth-size discrepancy,long-term stability,OAI-PMH Harvest

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Keim, Robert G. (committee chair), Moon, Holly (committee member), Paine, Michael (committee member), Sinclair, Peter (committee member)

Creator Email echow79@hotmail.com,enchow@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-m3714

Unique identifier UC195621

Identifier etd-Chow-4475 (filename),usctheses-m40 (legacy collection record id),usctheses-c127-441664 (legacy record id),usctheses-m3714 (legacy record id)

Legacy Identifier etd-Chow-4475.pdf

Dmrecord 441664

Document Type Thesis

Rights Chow, Elaine Nicole

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Repository Name Libraries, University of Southern California

Repository Location Los Angeles, California

Repository Email cisadmin@lib.usc.edu