Download THE CORRELATION BETWEEN THE LOWER INCISOR ANGLE

THE CORRELATION BETWEEN THE LOWER INCISOR ANGLE AND
STABILITY
Binh N. Tran, D.D.S.
An Abstract Presented to the Faculty of the Graduate School
of Saint Louis University in Partial Fulfillment
of the Requirements for the Degree of
Master of Science in Dentistry
2007
ABSTRACT
Lateral cephalograms and study casts of 62 patients
with Class I and II malocclusions were evaluated to
determine if any relationships exist between the change in
lower incisal to mandibular plane angle and occlusal
stability one year after treatment.
A statistically
significant but weak correlation (r=0.328, p<0.01) was
found between the changes in the lower incisal angle and
irregularity index one year posttreatment.
However no
significant difference in incisor irregularity was found
when the sample was broken down into subgroups of cases
finished with <4° change and cases with ≥4° change in lower
incisor angle.
In addition, no difference was observed
whether the cases where finished with lower incisor angle
with Tweed’s recommended range (85°-93°) or not.
1
THE CORRELATION BETWEEN THE LOWER INCISOR ANGLE AND
STABILITY
Binh N. Tran, D.D.S.
A Thesis Presented to the Faculty of the Graduate School
of Saint Louis University in Partial Fulfillment
of the Requirements for the Degree of
Master of Science in Dentistry
2007
COMMITTEE IN CHARGE OF CANDIDACY:
Associate Professor Ki Beom Kim
Chairperson and Advisor
Assistant Professor Maria Atique
Assistant Professor Gus Sotiropoulus
DEDICATIONS
To my wonderful family:
My beautiful wife, Michele
My curious son, Maximus
My precious daughter, Audrey
Thank you for your being part of my life and giving me
the motivation to finish this project.
ii
ACKNOWLEDGEMENTS
I would like to acknowledge the following individuals:
Dr. Ki Beom Kim for chairing my thesis committee. You
have been a great mentor. Thank you for your guidance and
your time in the development and writing of this thesis.
Dr. Gus Sotiropoulos for serving on my thesis
committee.
You have been a great teacher.
Thank you for
your time in the development and writing of this thesis.
It has been a pleasure to know you and to work with you.
Dr. Maria Atique for serving on my thesis committee.
It has been a privilege to work with you and I appreciate
your guidance throughout the research process.
TABLE OF CONTENTS
iii
List of Tables............................................v
List of Figures..........................................vi
CHAPTER 1:
INTRODUCTION..................................1
CHAPTER 2:
REVIEW OF THE LITERATURE
Irregularity Index (II).......................4
Normal Development............................5
Pretreatment Factors..........................6
Extraction and Nonextraction..................6
Late Mandibular Growth.......................10
Third Molars.................................10
Transeptal Fibers............................10
Early Versus Late Treatment..................12
Tooth Dimension..............................13
References...................................17
CHAPTER 3:
JOURNAL ARTICLE
Abstract.....................................24
Introduction.................................25
Materials and Methods........................32
Selection Criteria........................32
Cepahalometric Analysis...................33
Model Analysis............................33
Statistical Analysis......................34
Results......................................35
Discussion...................................38
Comparisons with Previous Studies.........38
Limitations...............................42
Conclusions..................................42
Literature Cited.............................44
Vita Auctoris............................................48
iv
LIST OF TABLES
Table 1:
A Review of the Literature...................31
Table 2:
Analysis of Error............................35
Table 3:
Mean ± SD of the Sample and Subgroups........36
Table 4:
Comparison between Groups within Tweed Norms
(85°-93°) and Other IMPAs.....................37
Table 5:
Comparison between Groups with Incisal Changes
< and ≥ 4°...................................37
v
LIST OF FIGURES
Figure 1: Irregularity Index...........................34
Figure 2: Correlation between the Lower Incisal Angle and
Irregularity Index...........................40
vi
CHAPTER I: INTRODUCTION
The four objectives that many orthodontists strive
to attain are health, function, aesthetics, and stability
of the occlusion.
Of these four objectives, stability is
the most difficult to maintain.
There are many protocols
or techniques,1 both old and new, that today’s orthodontist
can choose to achieve a stable result.
However, the
efficacy of the chosen plan of treatment remains
questionable because of the lack of agreement in the
literature regarding a stable occlusion.2
Before orthodontics was a specialty, Norman
Kingsley proposed that a good occlusion is the most
important factor in determining a stable occlusion.1
Charles H. Tweed, prescribed to the idea that the
mandibular dental arch, acting as a template for the
maxillary arch, plays a key role in stability.
Based on
seventeen years of “factual” clinical evidence gathered
from his practice, Tweed interpreted Edward H. Angle’s
definition of “the line of occlusion” as laying the teeth
in a functional, mechanical balanced axial inclination over
alveolar bone, especially the mandibular incisor.3
Furthermore, Tweed supports this claim by noting that
approximately 20% of all his cases that he considered
1
successful had an incisal to mandibular plane angle ranging
about 8° from 85° to 93°.
This range of lower incisal angle
is also corroborated by Allan Brodie4 and Holly Broadbent5
who published papers on dentofacial pattern of untreated
children with an average lower incisor angle of
87.9º, respectively.
88.3º and
Historically, the lower incisal angle
has been considered a major factor in the treatment
objectives and is synonymous with the Tweed philosophy of
treatment.
Another major concept on stability is the
Equilibrium Theory that proposes the dentition is kept in a
balanced position by its environment (i.e. perioral
muscles, tongue, transeptal ligaments, etc.).
In theory,
altering the oral environment will cause a change in the
occlusion; therefore to create a stable occlusion, teeth
should be place in a position balanced by the forces around
it.
Many authors tried to explain this theory by citing
clinical cases of muscular abnormalities causing
malocclusion due to an imbalance of forces on the
dentition.6-8
However, there is a lack of direct evidence
to support this theory.9
In orthodontics, it is desirable to keep the
treatment results as stable as possible for the lifetime of
the patient. Crowding of the lower incisors can be
2
corrected by a few general methods:
extract teeth to gain
space, procline the incisors, perform interproximal
reduction, or expansion.
Each of these treatment options
can affect the lower incisor angle cephalometrically and
potentially alter any equilibrium that may have existed
before.
The purpose of this retrospective study is to
determine whether changing the lower incisor angle during
treatment affects the stability of the occlusion after
orthodontic treatment.
3
CHAPTER 2: REVIEW OF THE LITERATURE
Irregularity Index
There are many factors that may play a role in
stability.
To test these factors, there must be a method
to record stability.
Because a change from good occlusion
to malocclusion is usually expressed early as mandibular
incisor crowding, a change in lower incisor crowding over
time is often used to measure stability.10-13
In 1975,
Robert Little proposed and tested a reliable and valid
method of scoring the lower incisor crowding called the
irregularity index, which relies on a digital caliper to
measure the total displacement of the anatomic contact
points of the lower anterior teeth.14
This method is often
used to compare the changes in crowding from posttreatment
to postretention.10-12,15
By comparing the change in
irregularity index between groups with different treatments
rendered, an investigator can compare the relative
difference in stability.
4
Normal Development of the Occlusion
Before we can begin to understand the stability of
a corrected occlusion, it is prudent to examine the trends
and normal development of untreated occlusions.
Sinclair
and Little13 evaluated dental casts of 65 untreated normal
occlusions at three different dental stages (mixeddentition, early permanent dentition, and adult dentition)
and reported the following findings:
(1) arch length
(Figure 1) decrease from mixed dentition into early
adulthood, (2) intercanine and intermolar width is
relatively stable with statistically significant decrease
occurring in females from 13 to 20 years, (3) overjet and
overbite increased from 9 to 13 years and then decrease
from 13 to 20 years, resulting in minimal net change, and
(4) incisor irregularity increase from 13 to 20 years,
slightly more irregularity for females.
consistent with previous research.16,17
These findings are
Richardson et al.18
and Bishara et al.19 evaluating crowding of the lower
incisors in untreated occlusion after age 18 found that
crowding continues into the third and fourth decade of
life, respectively.
A difference is that the late adult
crowding is minimal, often less than 1 mm over 10 years.18,19
5
Pretreatment Factors
Many studies20-22 have attempted to evaluate possible
factors associated with a stable occlusion.
In 1981,
Little et al.15 did a retrospective study using 65 first
premolar extraction cases which had undergone routine
edgewise orthodontic therapy followed by retention and at
least 10 years removal of all retention devices.
No
significance differences in irregularity index at the
pretreatment or postretention period were found between
Angle malocclusion classes, sexes, ages, and arch width
change at the canines or molars.15
All factors had a
correlation coefficient less than r=0.38.15
Arch width
measured across the mandibular canine teeth typically
reduces posttreatment whether the case was expanded during
treatment or not.11
Little also observed that cases with
minimal pretreatment crowding was worse at postretention,
while severe initial crowding showed improvements in
postretention.
Extraction versus Nonextraction
As a continued study of Little’s report, Shield and
colleagues23 assessed the records of 54 first-premolar
6
extraction cases treated with traditional edgewise therapy
and were at least 10 years out into postretention, where
patients stopped using any forms of retainer.
The purpose
of this study was to appraise the cephalometric records
taken at pretreatment (T1), posttreatment (T2), and
postretention (T3) for any variables that may be correlated
with stability and mandibular anterior alignment.
All
cases had a posttreatment irregularity index of less than
3.0 mm.
Treatment (T1-T2) and postretention (T2-T3)
changes in incisor axial inclinations had slight
correlation coefficients for ı to NA° (r<-0.55, p≤0.01) and
ī to NPg° (r=-0.58, p≤0.01).
The data suggested a slight
tendency for incisor inclination to return toward the
pretreatment value during the postretention period.
Furthermore, they found no significant association between
any specific pre- and postreatment cephalometrics
parameters such as incisor position and skeletal
development with long-term mandibular irregularity.
However, Shield suggests, “It is conceivable that there
exists an envelope in which tooth movement may be
accomplished without significant inclination relapse.”
23
Due to the selection bias of his sample, Shield is not able
to compare the various incisor angulations changes and its
association with postretention irregularity.
7
Similar to Shield’s findings, a long-term
postretention (at least 10 years) study24 from Baylor
University on Class I premolar extraction treatment using
the Tweed edgewise treatment philosophy found low
correlation (r≤±0.57) between postretention irregularity
index and any of the following parameters:
intermolar
widths, pretreatment mandibular incisor inclination,
incisor proclination during treatment, and posttreatment
irregularity.
width.
No correlation was reported for intercanine
The cases in the Baylor study had postretention
relapse comparable to that reported in the study of
untreated normal occlusions by previous authors.13
However
in this study, using their described Tweed treatment
philosophy, the results of the study indicated satisfactory
long-term stability (irregularity index <3.5 mm) in 80% of
the treated patients as compared to Little’s15 finding of
less than 30% of his cases, which included Class II and I
malocclusions.
The authors24 suggested that minimal lower
incisal angle change influenced their success.
An
alternative reason for their findings discrepancy is that
the Baylor study had an average of 1.8 mm of posttreatment
irregularity and Little may have had a greater average,
which he did not report, but can be inferred from a
8
continuation of his study by Shield and colleagues23 who
reported cases of postreatment irregularity up to 2.9 mm.
In a study of 30 Class I malocclusions treated
nonextraction, Weinberg and Sadowsky,25 based on strength of
the correlation (p<0.05), determined that pretreatment
lower incisor crowding is moderately associated with
increased posttreatment arch length (r=-0.68) and slightly
associated with arch depth (r=-0.55), interfirst-premolar
width (r=-0.45), intermolar width (r=-0.45), and
intersecond-premolar width (r=-0.36).
The correlations
from this study suggest that pretreatment crowding is
resolved primarily by increasing arch length via proclining
the lower incisor or expanding the arch.
Comparing nonextraction and extraction treatment
effects on long-term stability is intuitively better tested
by comparing similar pretreatment malocclusion.
Paquette,
Beattie, and Johnston26 did a long-term comparison study of
nonextraction and premolar extraction edgewise therapy in
“borderline” Class II patients and reported no difference
in posttreatment irregularity.
This conclusion is also
corroborated by other studies10,27 comparing lower incisor
irregularity between extraction and nonextraction cases.
9
Post-Puberty Mandibular Growth
Paquette and associates26 also found a moderate
correlation (r=-0.75, p<0.01) between the difference in
growth between the jaws and lower incisor movement
posttreatment.
The authors26 argue that relapse/settling of
the occlusion is due to growth after treatment and that any
changes made during treatments have little effect on
destabilization of the occlusion as seen on an untreated
normal occlusion.13
It is evident that the dentition
changes with growth18,19 and that facial growth continues
throughout a person’s life.19,28
Therefore, it is possible
that the moderate correlation of differential jaw growth
and dental changes in posttreatment are coincidental events
with no relationship.
Third Molars
The effects of third molars on late lower arch
crowding continue to be controversial.
Richardson29
concluded that during the teenage years, pressure from the
back of the third molar plays some role in the cause of
late mandibular incisor crowding.
Several studies30-32 were
unable to demonstrate that third molars exert pressure on
10
the teeth mesial to them.
Zachrisson33 in a recent review
of the literature concluded that the current evidence and
any influence that the third molar may have on lower
incisor crowding does not justify the trauma that the
patient must endure from extracting third molars.
Transseptal Fibers
A histological study on transseptal fibers and
relapse following bodily retraction of teeth on eight
Macaca rhesus monkeys showed that teeth with transseptal
fibers removed before moving into an extraction space had
virtually 0 mm of relapse (returning to their original
position) while the control with intact transseptal fibers
showed about 50 percent relapse in the first 12 hours.34
To apply this theory of periodontal fiber influence on
orthodontic relapse in humans, a circumferential
supracrestal fiberotomy (CSF) technique was developed by
Edwards35 to surgically transect the transseptal and the
epithelial tissue surrounding the tooth.
A long-term
prospective evaluation of CSF in alleviating orthodontic
relapse was later done by Edwards36 showed that the CSF
procedure significantly reduced relapse and was more
effective in rotational than labiolingual relapse.
11
The
study reports a mean mandibular irregularity of 3.12 mm for
the control group (n=22) and 2.01 mm of irregularity for
the CSF group (n=26) over a period of approximately 8 to 11
years postretention.
Early Versus Late Treatment
One of the reasons for early treatment as suggested
by Lee and Dugoni37,38 is that early mixed dentition
treatment using leeway space may improve lower incisor
stability.
In a study of nonextraction cases, Dugoni and
associates39 propose that early treatment of anterior
crowding with a passive lingual arch allow teeth to self
correct and supracrestal fibers could reorganize around the
incisors aligned position.
In a study of early (mixed
dentition) versus late treatment (permanent dentition)of
crowded extraction cases, Haruki and Little40 found a
significant mandibular irregularity index difference at
postretention between the groups.
In that study, all the
patients had first premolar extraction with the difference
being the time of premolar extraction either in the mixed
dentition or permanent dentition.
The early treatment
group’s lower anterior teeth were aligned using fixed
appliances.
No serial extraction cases were included in
12
the study.
In contrast, when Little and associates41,42
compare the incisor irregularities between the early
treatment with serial extraction and late treatment with
permanent teeth extraction, they found no difference in
postretention irregularity index.
In these studies41,42 the
patients who had serial extraction, their teeth were
allowed to undergo physiologic drift and then treated with
fixed appliance later during comprehensive treatment.
In
summary, early treatment appears to be only significant if
the lower anterior teeth are aligned before comprehensive
treatment; perhaps this early alignment gives the
supracrestal fibers time to reorganize around the straight
teeth before comprehensive treatment.
Tooth Dimension
Studying tooth size and dimension began as early as
the 1800s with GV Black43, who established a data base of
tooth size means and Ballard44 in 1944 who found that 90% of
his subjects had right-left tooth width asymmetry.
Then in
1958, Bolton45 performed a tooth size study on fifty-five
cases with excellent occlusions.
The mesiodistal tooth
width was measured on all the teeth on each cast from first
molar to contralateral first molar.
13
The sum of the widths
of the twelve mandibular teeth were divided by the sum of
the widths of the twelve maxillary teeth and then
multiplied by 100 to arrive at an overall ratio.
The same
method was used to set up an anterior ratio, consisting of
the sum of the widths of the anterior six teeth.
Bolton
found the overall ratio average to be 91.3±0.26 and the
anterior ratio average to be 77.2±0.22.
This tooth size
data was found to be closely related to that published by
both Black and Ballard.
Disharmonies in tooth size can be
corrected by extraction of a tooth or teeth, placement of
overcontoured restorations, or removal of tooth structure
by interproximal reduction.
There are some orthodontists who believe that tooth
dimension is related to stability.
Peck and Peck46 studied
the lower incisal dimensions of 45 white female subjects
with untreated perfect mandibular alignment and 70 subjects
of comparable age and not selected as part of the perfect
mandibular incisor group.
They found that the mean
mesiodistal (MD) crown diameters for the mandibular central
and lateral incisors in the perfect-alignment group were
smaller than the control population and the mean
faciolingual (FL) crown diameters for the mandibular
central and lateral incisors were larger in the perfect
alignment sample than in the control population sample.
14
Based on these findings, Peck and Peck47 introduced an index
(MD/FL x 100) to define well proportioned teeth.
The MD/FL
ratios for the central and lateral lower incisors are
88.4±4.3 and 90.4±4.8, respectively.
To correct any ratio
in excess of 100, the authors suggest interproximal
reduction as a method to bring the lower incisors closer to
ideal.
The authors acknowledge that there are other
factors that may influence lower incisor alignment;
therefore it would not be strange to find crowded
proportional sized incisors.
To test Peck and Peck’s finding, Gilmore and
Little48 did a retrospective postretention study, evaluating
the 164 cases and found weak (r<0.60) to no correlation
between the MD width or MD/FL ratio of the lower incisors
and the lower incisor irregularities.
These results were
also corroborated by Shah and associates49 from the United
Kingdom, who tried to improve measurements for the MD and
FL widths by cross-sectioning the incisors on study cast
and correlating the MD/FL ratio with the irregularity
index.
Other studies50,51 have failed to show any moderate
correlation (r>0.70) between incisor morphology and incisor
irregularity.
One study by Rhee and Nahm52 found a moderate
correlation coefficient of 0.77 between the ratio of the MD
width at the incisal and cervical areas of the lower
15
incisors and incisor irregularity.
The authors suggest
that the area of contact between adjacent incisors is
relevant to stability.
However, it is important to note
that the subjects for this study were Asians and that the
other studies were Caucasians; the difference in race may
play a role in the influence of the incisor anatomy and
thus, result in different study outcomes.
16
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18
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19
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20
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21
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22
THE CORRELATION BETWEEN THE LOWER INCISOR ANGLE AND
STABILITY
Binh N. Tran, D.D.S.
Gus Sotiropolous, D.D.S., M.S.
Maria Atique. D.D.S.
Ki Beom Kim, D.D.S., M.S.D., Ph.D.
An Abstract Presented to the Faculty of the Graduate School
of Saint Louis University in Partial Fulfillment
of the Requirements for the Degree of
Master of Science in Dentistry
2007
CHAPTER 3: JOURNAL ARTICLE
Abstract
Lateral cephalograms and study casts of 62 patients
with Class I and II malocclusions were evaluated to
determine if any relationships exist between the change in
lower incisal to mandibular plane angle and occlusal
stability one year after treatment.
A statistically
significant but weak correlation (r=0.328, p<0.01) was
found between the changes in the lower incisal angle and
irregularity index one year posttreatment.
However no
significant difference in incisor irregularity was found
when the sample was broken down into subgroups of cases
finished with <4° change and cases with ≥4° change in lower
incisor angle.
In addition, no difference was observed
whether the cases where finished with lower incisor angle
with Tweed’s recommended range (85°-93°) or not.
23
Introduction
The four objectives that many orthodontists strive
to attain are health, function, aesthetics, and stability.
Of these four objectives, stability is the most difficult
to maintain.
There are many protocols or techniques,1 both
old and new, that today’s orthodontist can choose to
achieve a stable result.
However, the efficacy of the
chosen plan of treatment remains questionable because of
the lack of agreement in the literature regarding a stable
occlusion.2
In orthodontics, it is desirable to keep the
treatment results as stable as possible for the lifetime of
the patient. Crowding of the lower incisors can be
corrected by a few general methods that:
extract teeth to
gain space, procline the incisors, perform interproximal
enamel reduction, or expansion.
Each of these treatment
options can affect the lower incisor angle
cephalometrically and potentially alter any equilibrium
that may have existed before.
The purpose of this
retrospective study was to determine whether changing the
lower incisor angle during treatment affects the stability
of the occlusion after treatment.
24
There are many factors that may play a role in
stability.
To test these factors, there must be a method
to record stability.
Because a change from good occlusion
to malocclusion is usually expressed early as mandibular
incisor crowding, a change in lower incisor crowding over
time is often used to measure stability.3-6
In 1975,
Robert Little proposed and tested a reliable and valid
method of scoring the lower incisor crowding called the
irregularity index, which relies on a digital caliper to
measure the total displacement of the anatomic contact
points of the lower anterior teeth.7
This method is often
used to compare the changes in crowding from posttreatment
to postretention.3-5,8
By comparing the change in
irregularity index between groups with different treatments
rendered, an investigator can compare the stability gain
after treatment.
Before we can begin to understand the stability of
a corrected occlusion, it is prudent to examine the trends
and normal development of untreated occlusions.
Sinclair
and Little6 evaluated dental casts of 65 untreated normal
occlusions at three different dental stages (mixeddentition, early permanent dentition, and adult dentition)
and reported the following findings:
(1) arch length
(Figure 1) decrease from mixed dentition into early
25
adulthood, (2) intercanine and intermolar width is
relatively stable with statistically significant decrease
occurring in females from 13 to 20 years, (3) overjet and
overbite increased from 9 to 13 years and then decrease
from 13 to 20 years, resulting in minimal net change, and
(4) incisor irregularity increase from 13 to 20 years,
slightly more irregularity for females.
consistent with previous research.9,10
These findings are
Richardson and
Gormley11 and Bishara et al.12 evaluating crowding of the
lower incisors in untreated occlusion after age 18 found
that crowding continues into the third and fourth decade of
life, respectively.
A difference is that the late adult
crowding is minimal, often less than 1 mm over 10 years.11,12
Many studies13-15 have attempted to evaluate possible
factors associated with a stable occlusion.
In 1981,
Little et al.8 did a retrospective study using 65 first
premolar extraction cases which had undergone routine
edgewise orthodontic therapy followed by retention and at
least 10 years removal of all retention devices.
No
significance differences in irregularity index at the
pretreatment or postretention period were found between
Angle malocclusion classes, sexes, ages, and arch width
change at the canines or molars.8
All factors had a
correlation coefficient less than r=0.38.8
26
Arch width
measured across the intercanine teeth typically reduces
posttreatment whether the case was expanded during
treatment or not.13
Little also observed that cases with
minimal pretreatment crowding was worse at postretention,
while severe initial crowding showed improvements in
postretention.
As a continued study of Little’s report, Shield and
colleagues16 assessed the records of 54 first-premolar
extraction cases treated with traditional edgewise therapy
and were at least 10 years out into postretention, where
patients stopped using any form of retainer.
The purpose
of this study was to appraise the cephalometric records
taken at pretreatment (T1), posttreatment (T2), and
postretention (T3) for any variables that may be correlated
with stability and mandibular anterior alignment.
All
cases had a posttreatment irregularity index of less than
3.0 mm.
Treatment (T1-T2) and postretention (T2-T3)
changes in incisor axial inclinations had slight
correlation coefficients for ı to NA° (r<-0.55, p≤0.01) and
ī to NPg° (r=-0.58, p≤0.01).
The data suggested a slight
tendency for incisor inclination to return toward the
pretreatment value during the postretention period.
Furthermore, they found no significant association between
any specific pre- and postreatment cephalometrics
27
parameters such as incisor position and skeletal
development with long-term mandibular irregularity.
However, Shield and colleagues suggest, “It is conceivable
that there exists an envelope in which tooth movement may
be accomplished without significant inclination relapse.”
16
Due to the selection bias of their sample, Shield et al.
was not able to compare the various incisor angulations
changes and its association with postretention
irregularity.
In a study of 30 Class I malocclusions treated
nonextraction, Weinberg and Sadowsky,17 based on strength of
the correlation (p<0.05), determined that pretreatment
lower incisor crowding is moderately associated with
increased posttreatment arch length (r=-0.68) and slightly
associated with arch depth (r=-0.55), interfirst-premolar
width (r=-0.45), intermolar width (r=-0.45), and
intersecond-premolar width (r =-0.36).
The correlations
from this study suggest that pretreatment crowding is
resolved primarily by increasing arch length via proclining
the lower incisor or expanding the arch.
Comparing nonextraction and extraction treatment
effects on long-term stability is intuitively better tested
by comparing similar pretreatment malocclusion.
Paquette,
Beattie, and Johnston18 did a long-term comparison study of
28
nonextraction and premolar extraction edgewise therapy in
“borderline” Class II cases and reported no difference in
posttreatment irregularity.
This conclusion is also
corroborated by other studies3,19 comparing lower incisor
irregularity between extraction and nonextraction cases.
For a complete review of the pertinent literature, see
Table 1.
29
Table 1. A Review of the Literature.
Authors
Uhde et al20
(n=72)
Artun et al3
(n=78)
Little et al8
(n=61)
Glenn et al21
(n=28)
Little et al22
(n=30)
McReynolds et al.
23 (n=46)
Haruki et al24
(n=83)
Erdinc et al19
(n=98)
Paquette et al18
(n=63)
Boley et al25
(n=32)
Shields et al16
(n=54)
Peck & Peck26
(n=115)
Sample
Cl I & II
Ext & Nonext
Cl II
Ext & Nonext
Cl I & II
Ext
Cl I & II
Nonext
Cl I & II
Ext & Serial Ext
CL I & II
Ext & Serial Ext
Early vs. Late Tx
Bicuspid Ext
Cl I & II
Ext & Nonext
Cl II, Borderline
Ext & Nonext
Cl I
Ext
Cl I & II
Ext
Untreated Female
Occlusions
Shah et al27
(n=50)
Untreated Occlusions
(Male subgroups had
NS value)
Rhee et al28
(n=69)
Puneky et al29
(n=77)
Freitas et al30
(n=56)
Edwards31
(n=194)
Driscoll et al32
(n=87)
Untreated Occlusions
Cl I & II
Ext & Nonext
Cl I & II
Ext
Cl I & II
Ext & Nonext
Cl I & II
Untreated Vs Ext
Variable 1
Variable 2
Statistical Value
Tx ∆ L3-3
PostTx ∆ L3-3
r = -0.38*
Tx ∆ L3-3
PostRet II
r = 0.314**
Tx ∆ L3-3
PostRet AL
PostRet II
PostRet II
PreTx AL
PostRet AL
r = 0.24
r = 0.52**
r = 0.83, sample small
no sig. value
Serial Group (n=30);
PostRet II
Serial Group (n=14);
PostRet II
Early Tx Group
(n=36); PostRet II
Ext Group (n=49);
PostRet II
Ext PostTx ∆ II
ABCH
Matched Ext Group
(n=30); PostRet II
Ext Group (n=32);
PostRet II
Late Tx Group (n=47);
PostRet II
Nonext Group (n=49);
PostRet II
Nonext PostTx ∆ II
L1 Movement
Tx ∆L1/MP
PostRet II
r = 0.43*
Tx ∆ L1 to NPg
L2 MD/FL
Perfect L2-2 MD & FL
widths
L1 MD1/MD2 Female
(n=25)
L2 MD1/MD2
Female (n=25)
U & L Incisor width
ratio at C & I
PostRet ∆ L1 to NPg
PostRet II
Nonperfect L2-2 MD
& FL widths
r = -0.58**
r = 0.31**
Compared means, NS
Compared means, NS
Compared means**
Compared means, NS
Compared means, NS
r = -0.75**
Compared means**
II
r = 0.52**
II
r = 0.55**
II
r > 0.74**
L1 & L2 MD/FL
Long-term PostTx II
r < 0.22
L1 & L2 MD/FL
PostTx ∆ II
r < 0.13
Control Group (n=93);
PostRet II
Untreated Group;
∆ II
CSF Tx Group
(n=101); PostRet II
Ext Treated Group; ∆
II
Compared means**
Compared means, NS
Key: Cl=Class; Ext=extraction; Nonext=nonextraction; Tx=Treatment; PostTx=posttreatment; PostRet=postretention;
∆=difference; * P<0.05; ** P<0.01; NS=not significant; L=lower; U=upper; II=incisor irregularity; AL=arch length;
ABCH=apical base change/ difference in mandibular growth relative to the maxilla; MD/FL=mesiodistal width and
faciolingual width ratio; MD1=mesiodistal width at the incisal; MD2 mesiodistal width at the gingival; C & I=cervical and
incisal; CSF=circumferential supracrestal fiberotomy
30
Materials and Methods
Sample
This retrospective study consisted of diagnostic
records from 62 cases, from the Saint Louis University
archive, with Class I (n=38) or Class II (n=24)
malocclusions and posttreatment irregularity index of no
more than 0.5 mm. A high quality posttreatment index was
required to allow detection of any minor incisor changes
that may occur after treatment.
Records are from
pretreatment, posttreatment, and one year posttreatment.
Models were required to have all their lower six anterior
teeth.
Cases with Class III malocclusion, mutilated
dentition, craniofacial anomalies, fixed lower retainers,
dental restorations on the first anterior teeth, or a
history of orthognathic surgery were excluded from the
study.
The sample consisted of a mixture of females (n=40)
and males (n=22), all within the age range from 9 to 18
years old at the time of treatment.
Treatment modalities
included all forms such as extraction, nonextraction, Tipedge mechanics, and Tweed mechanics.
31
Cephalometric Analysis
Cephalometric records for each patient were
evaluated at pretreatment (T1), posttreatment (T2), and one
year posttreatment (T3).
Each lateral cephalometric
radiograph was traced by a single operator with a number 2
lead pencil on acetate paper.
The lower incisal angle was
measured from a line through the long axis of the most
anterior incisor on the radiograph and a mandibular plane
(line) drawn from menton to an anatomical gonion.
The
anatomical gonion is a point on the mandibular angle
estimated by a bisecting line of the angle constructed from
the posterior ramus and mandibular inferior border.
The
incisor to mandibular plane angle is designated as IMPA.
Model Analysis
Models from pretreatment (T1), posttreatment (T2),
and 1 year posttreatment (T3) were measured with an
electronic caliper calibrated to the nearest hundredth by a
single operator.
Irregularity index (II) measurements
recorded, as described by Little7, a summation of the
displaced contact points between the lower six anterior
teeth (Figure 1).
32
Figure 1. Irregularity index (II) defined as
the summed displacement of adjacent
anatomic contact points of the mandibular
anterior teeth.
Statistical Analysis
The analysis of error was performed by measuring 10
records 1 week apart by the same.
The intraclass
correlation coefficient for this single operator measuring
the irregularity index was 0.9920 and for the lower incisor
to mandibular plane angle was 0.969 (Table 2).
Means and
standard deviations were calculated for all variables.
Independent t-tests were used to compare means between
subgroups.
Significance was determined at p<0.05.
Pearson
correlation was used to calculate correlation coefficient
between two variables.
33
Table 2. Analysis of Error
IMPA
IMPA'
Irregularity
Irregularity'
89
86
0.4
0
89
91.5
0.8
0.8
100.5
99.5
0
0
100
100
0.8
0.8
85
84
0.2
0.3
93
90
1.3
1.2
83
82.5
0.4
0.78
94
97
0.4
0.4
99.7
99.7
1.4
1.62
100
99
3.53
4.02
ICCIMPA
0.979*
ICCII
0.987*
*p < 0.001;
ICC= intraclass correlation
Results
Our sample consisted of 62 cases with a mean
posttreatment incisor irregularity of 0.17±0.19 mm,
age=12.64±1.72 years, treatment time=2.16±0.58 years.
Other averages are displayed on Table 3. The pretreatment
IMPA range was 72.4° to 112° and the posttreatment IMPA
range was 81.5°-111°.
Significant difference of the means
was observed between the Class I and Class II malocclusion
in pretreatment (T1) IMPA and the treatment change (T2-T1)
in IMPA.
Significant difference in mean posttreatment
34
irregularity index was observed between male and female
patients, 0.13 mm and 0.25 mm, respectively.
Table 3. Mean ± SD of the Sample and Subgroups.
♀ (n=40)
Measurement Cl I (n=38)
Cl II (n=24)
T1 II
3.78±2.55
4.17±2.67
3.91±2.85
T2 II
0.16±0.18
0.18±0.20
0.13±0.17*
T3 II
0.68±0.91
0.74±0.55
0.75±0.89
T1 IMPA
91.51±6.87*
95.35±6.43*
93.21±7.70
T2 IMPA
93.91±7.47
97.45±6.81
95.57±7.78
T2-T1 IMPA
5.63±4.02*
3.05±3.56*
4.88±4.50
*Significance, P<0.05
♂ (n=22)
3.97±2.08
0.25±0.19*
0.62±0.56
92.61±5.32
94.75±6.71
4.18±3.01
Overall
3.93±2.59
0.17±0.19
0.70±0.79
93.00±6.91
95.28±7.37
4.63±4.02
Correlation analysis reveal significant correlation
(r=0.328, p<0.01) between the lower incisor changes during
treatment (T2-T1) and lower incisor irregularity one year
after treatment (T3-T2).
To test Dr. Tweed’s philosophy33
of a stable lower incisal angle between the ranges of 85° to
93°, t-test analysis was used compared incisor irregularity
of cases (n=22) completed within Tweed’s range and those
cases (n=40) that were out of the range. A statistic
comparison between the two groups is shown on Table 4.
No
significant differences were found between the cases with
T2 IMPAs within Tweed’s range and cases that did not.
In
addition, to maintain approximately equal sample size and
compare incisor irregularity means with the change in lower
incisor angle, the cases were divided into a group (n=30)
with ≥ 4° change to the lower incisor angle during treatment
35
and a group (n=32) with < 4° change in lower incisor angle.
A statistical analysis of the groups can be seen on Table
5.
No significant difference was observed between the mean
lower incisor irregularities of the groups before treatment
and one year after treatment.
Table 4. Comparison between Groups within Tweed Norms (85°-93°) and Other IMPAs
T1 II
Subgroups
Tweed
N
22
Mean
3.71
Other
40
4.05
Std. Deviation Std. Error Mean
2.52
0.54
2.65
0.42
Tweed
22
0.13
0.17
Other
40
0.19
0.20
T3 II
Tweed
22
0.55
0.51
Other
40
0.79
0.90
T1 IMPA
Tweed
22
89.73
5.03
Other
40
94.80
7.19
T2 IMPA*
Tweed
22
89.89
2.66
Other
40
98.24
7.46
T2-T1 IMPA
Tweed
22
3.66
2.72
Other
40
5.17
4.53
Significant difference (p<0.01) are noted with an (*)
0.04
0.03
0.11
0.14
1.07
1.14
0.57
1.18
0.58
0.72
T2 II
Table 5. Comparison between Groups with Incisal Changes < and ≥ 4°
Subgroups
N
Mean
Std. Deviation Std. Error Mean
T1 II
32
4.38
2.57
< 4°
30
3.45
2.55
≥ 4°
T2 II
32
0.17
0.19
< 4°
30
0.18
0.18
≥ 4°
T3 II
32
0.66
0.55
< 4°
30
0.75
0.99
≥ 4°
T1 IMPA
32
93.95
7.10
< 4°
30
91.98
6.67
≥ 4°
T2 IMPA
32
95.07
7.07
< 4°
30
95.50
7.79
≥ 4°
T2-T1 IMPA*
32
1.74
1.05
< 4°
30
7.72
3.71
≥ 4°
Significant difference (p<0.01) are noted with an (*)
36
0.45
0.47
0.03
0.03
0.10
0.18
1.26
1.22
1.25
1.42
0.19
0.68
Discussion
Weinberg and Sadowsky17 concluded from their study
that the resolution of crowding is achieved by generalized
expansion of the buccal segments and advancing the lower
incisors. Previous studies3,16,25,34 have suggested that
proclined incisors tend to relapse after treatment.
Proclining the lower incisors can also effectively increase
arch length.
Previous studies8,19,23 have also shown that
arch length decreases over time following orthodontic
treatment.
Together these data support the concept that
changing the lower incisal angle may have some
consequential effect on crowding of the mandibular
incisors.
However, the current study suggests that
changing the lower incisor angulation contributes to
postreatment lower incisor crowding, but this relationship
is very variable.
As indicated by the literature,
stability is multifactoral2,3,11,15,16,30,35; therefore
variability is expected.
Schulhof et al.36 (1977) did a study comparing the
change in lower incisal edge in relation to a perpendicular
to the Frankfort horizontal and the absolute arch
discrepancy.
From a total sample of 78, they found no
statistical difference between the change in lower incisor
37
position and stability.
The study compared 3 groups: group
I (n=23) with lower incisors moved forward 1 mm or more,
group II (n=26) with
lower incisors finished between 2 mm
lingual and 1 mm labial of their original position, and
group III (n=29) with the lower incisors retracted 2 mm or
more.
Schulhof’s data is consistent with our findings.
We
found no difference in instability whether the lower
incisor angle is changed ≥4° or <4° from its original
position.
In a recent article by Janson and associates37, it
was showed by statistical analysis that a compromised upper
bicuspids extraction treatment for a severe Class II
malocclusion with a large overjet is not any less stable
than 4 bicuspids extraction.
The authors concluded that
proclining the lower incisors does not make the treatment
less stable.
Unfortunately, the authors show no
statistics to recommend how much proclination is still
acceptable; the present study shows no statistical
significant difference between the incisor irregularities
of cases with a change in the lower incisor angle ≥4° and
<4°.
This implies that the stability is the same whether
the lower incisal angle is proclined more than 4° or not.
However, as illustrated on the scattergram (Figure 2) of
38
the lower incisal angle change and irregularity index, we
observe greater variability in irregularity when lower
incisor angle is changed more than 4° during treatment.
4.5
Irregularity Index (mm)
4
3.5
3
2.5
2
1.5
1
0.5
0
0
2
4
6
8
10
12
14
16
18
IMPA Change (degrees)
Figure 2. Correlation Between the Absolute Change in Lower Incisal Angle and
Irregularity Index at 1 Year Posttreatment. n=62; r=0.328; p<0.05
The data of this study is also consistent with a
study38 that evaluated proclined incisors on adults with
surgically treated mandibular prognathism; the author found
no difference in irregularity index between a group (n=29)
with more than 10° proclined lower incisors and a group
(n=33) with minimal change in incisal inclination.
In
discussion, the author38 states that prerequisites to
significantly proclining the incisors are:
(1) the
skeletal malrelationship is corrected surgically and (2)
39
inclinations of the teeth are within the range of
established norms.
We tested the Tweed norms and found no
statistical difference in crowding between the group with
posttreatment IMPA within Tweed norms (85° to 93°) and the
group outside of Tweed norms.
Our incisors were
significantly tipped outside of the Tweed norms without any
surgical correction.
This suggests that the lower incisor
normal range might be larger than what Tweed33 suggested or
placing the lower incisor out of Tweed’s range does not
significantly affect posttreatment stability.
Lenz and associates39 examined the change in incisal
position during treatment of 55 patients and its relation
to postretention PAR scores.
They found no significant
relation with the PAR score to any of the incisal
measurements (interincisal angle, L1-APo, L1-GoGn, and U1PP).
In contrast to the current study, we found a
significant (p<0.01) correlation coefficient (r=0.328)
between the IMPA and one year posttreatment irregularity
index.
The difference in findings suggests that the lower
incisal angle may not have any or little influence in the
stability of the posterior occlusion, which is accounted
for in the PAR score.
Limitations
40
One limitation of this study is that the
irregularity index is not representative of a malocclusion
where the incisors may have normal contacts interproximally
but are aligned in a zigzag pattern.
Anterior teeth with
perfect contacts but aligned in zigzag pattern would
technically record as 0.0 mm for its irregularity index.
Another limitation is that the posttreatment time evaluated
was relatively short and the posttreatment irregularity
does not truly reflect the actual instability of the
malocclusion.
There was no way to control for the amount
of time that each patient wore their retainers.
However
because the groups where randomly selected with regards to
retainer wear time, this factor will statistically cancel
out any differences that it may have on our results.
Conclusion
To summarize, it can be concluded that:
•
Changing the lower incisial angle is significantly
related to postreatment lower incisal crowding,
r=0.328 (P<0.01).
•
There is no statistically significant difference in
stability whether the lower incisor changes were
beyond 4 degrees or not, but more variability in
41
posttreatment crowding was observed when the lower
incisor was changed beyond 4 degrees.
•
Cases finished within Tweed’s lower incisal range (85°93°) did not have more stable results than cases that
were outside this range.
42
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Vita Auctoris
Binh Tran was born on February 2, 1975 in Vietnam.
migrated to the United States in 1980.
He
He attended the
University of California, Los Angeles (UCLA) from 19931998.
While at UCLA, he participated in a study abroad
program in Beijing, China for the summer and winter
quarters of 1996.
After graduating from UCLA with a B.S.
in Physiology, he spent a year teaching at Roosevelt Middle
School in Oakland, California at the same time continued to
do research studies which he began as an undergraduate at
UCLA in 1997.
He met his wife, Michele Ma, in April of
1997 and they married in December 16, 2000.
He entered
dental school at the University of Southern California in
2000 and receive his D.D.S. degree in 2004.
During dental
school, the following papers were published:
1. Yee, H.F., Melton, A.C., and Tran, B.N. Rho A/Rho-Associated Kinase Mediates
Fibroblast Contractile Force Generation. Biochemical and Biophysical Research
Communications, Feb. 2001: Vol 280, 1340-1345.
2. Saab, S, Tam SP, Tran BN, Melton, AC, Tangkijvanich, P, Wong HC, Yee, Jr HF,
Myosin Mediates Contractile Force Generation by Hepatic Stellate Cells in Response
to Endothelin-1, Journal of Biomedical Science, 2002;9:607-612.
3. *Kernochan, L, *Tran, B, Tangkijvanich, P, Tam, SP, and Yee, Jr HF, Endothelin-1
Stimulates Human Colonic Myofibroblast Contraction and Migration, GUT, January
2002, Vol 50, No 1, 65-70.
*These authors contributed equally to this study
Following graduation from dental school, he began his
studies at Saint Louis University, in pursuit of a Master’s
Degree from the Orthodontics program.
Following graduation
on January 2007, he plans to move with his wife and two
48
children, Maximus and Audrey, to northern California to
begin work as an orthodontist.
49