Download Julie Rambo, D.M.D. of Saint Louis University in Partial Fulfillment

THE LONG-TERM STABILITY OF INTERPROXIMAL
REDUCTION IN MANDIBULAR INCISORS
Julie Rambo, D.M.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
THE LONG-TERM STABILITY OF INTERPROXIMAL
REDUCTION IN MANDIBULAR INCISORS
Julie Rambo, D.M.D.
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
Abstract
Stability is one of the most important goals in orthodontic treatment; however, the
factors producing the greatest stability are still largely unknown. The purpose of this
study is to determine if interproximal reduction increases long-term stability of the
mandibular incisors in Class I and II malocclusions. The study sample consisted of
patients of both sexes with Angle Class I or II malocclusions who received nonextraction treatment under the care of one private practitioner. The pretreatment, posttreatment, and post-retention lateral cephalograms and dental casts were examined in
fifty-five cases. The sample was divided into two groups, one consisting of 26 patients
having had 2 mm or more interproximal reduction and the other, control group,
consisting of 29 patients with no interproximal reduction. All patients had a fixed lower
lingual retainer placed at the time of removal of orthodontic appliances, which remained
in place an average of 3.48 years. The post-retention records were taken an average of
8.7 years after removal of the fixed lower lingual retainer. Changes between the time
intervals were evaluated with paired t tests to determine statistical significance. The
mean relapse of mandibular anterior crowding for the control group (no IPR) was
20.23%, and for the sample group (IPR) it was 18.45%. For both the control and sample
groups, the IMPA values remained fairly constant throughout treatment and retention.
Interproximal enamel reduction does not appear to influence the long-term stability of
mandibular incisors.
1
COMMITTEE IN CHARGE OF CANDIDACY:
Assistant Professor Donald R. Oliver,
Chairperson and Advisor
Assistant Clinical Professor J. Clifton Alexander
Assistant Professor Maria I. Atique
i
Acknowledgements
I would like to express my appreciation and gratitude to the following individuals:
Dr. Don Oliver, for his continual support, guidance, and expertise in the
development and mentorship of my thesis.
Dr. Maria Atique, for her encouragement and constructive input in the
composition of my thesis.
Dr. Cliff Alexander, for his support and guidance in the construction of my thesis.
Dr. Wick Alexander, for allowing me to collect my research data from his vast
collection of archived patient records.
Dr. Heidi Israel, for all of her help and time in the development and formulation
of my data statistics.
ii
Dedication
This thesis is dedicated to my husband, Scott, for his endless love, patience and
support throughout my orthodontic residency; and to my parents, Frank and Carol, who
taught me the importance of education in life and always encouraged me to pursue my
dreams.
iii
Table of Contents
List of Tables .......................................................................................................................v
List of Figures .................................................................................................................... vi
CHAPTER 1: INTRODUCTION ........................................................................................1
CHAPTER 2: REVIEW OF THE LITERATURE
Stability ........................................................................................................3
Attempts to Improve Stability in Treatment ................................................6
Individual Tooth Shape Deviations .............................................................8
Inter-Arch Tooth Size Discrepancies.........................................................10
Enamel Reduction......................................................................................12
The Effects of Interproximal Enamel Reduction .......................................15
Methods of Measuring Crowding ..............................................................19
Summary ....................................................................................................20
References..................................................................................................22
CHAPTER 3: JOURNAL ARTICLE
Abstract ......................................................................................................27
Introduction................................................................................................28
Materials and Methods...............................................................................30
Research design .............................................................................30
Dental Cast Measurements ............................................................32
Cephalometric Measurements........................................................34
Data Organization, Reduction, and Analysis.................................34
Results ........................................................................................................36
Dental Cast Measurements ............................................................36
Cephalometric Measurements........................................................36
Discussion ..................................................................................................39
Dental Cast Analysis......................................................................39
Cephalometric Analysis .................................................................43
Comparison of Present Study with Other Selected Studies ...........44
Conclusions................................................................................................47
Literature Cited ..........................................................................................48
Vita Auctoris......................................................................................................................50
iv
List of Tables
Table 1: Characteristics of Control and Sample Groups ...................................................31
Table 2: Classification of Control and Sample Groups based on Initial Irregularity ........31
Table 3: Dental Cast Measurement Statistics for Group A (Initial Mild Irregularity
<3.5 mm)..............................................................................................................36
Table 4: Dental Cast Measurement Statistics for Group A (Initial Moderate to Severe
Irregularity >3.5 mm) ..........................................................................................37
Table 5: Dental Cast Measurement Statistics for Pooled Groups......................................37
Table 6: Cephalometric Measurement Statistics for Group A (Initial Mild Irregularity
<3.5 mm)..............................................................................................................38
Table 7: Cephalometric Measurement Statistics for Group B (Initial Moderate to
Severe Irregularity >3.5 mm)...............................................................................38
Table 8: Cephalometric Measurement Statistics for Pooled Groups .................................38
Table 9: Irregularity Index and Percent Relapse for Pooled Groups .................................40
Table 10: Irregularity Index and Percent Relapse for Subdivided Groups A and B..........40
Table 11: Comparison of Present Study with Other Selected Studies...............................45
v
List of Figures
Figure 1: Little’s Irregularity Index ...................................................................................33
Figure 2: Intercanine width................................................................................................33
vi
Chapter 1: Introduction
The goal of modern orthodontics is to create the best balance among occlusal
relationships, dental and facial esthetics, stability of the result and long-term
maintenance, and restoration of the dentition. Achieving all these ideals, however, is
quite difficult in many patients. Excess tooth structure often prevents correct alignment
of the teeth within the arch. This results in crowding of the dentition due to a tooth sizearch length discrepancy (TSALD), which is considered the most common form of
malocclusion treated by orthodontists.1 According to the National Health and Nutrition
Estimates Survey (NHANES III), 45.5% of children aged 8 to 11 and 66% of adults do
not have well-aligned mandibular incisors.
When insufficient space exists in the mandibular arch, there are several methods
to relieve the crowding. One method is transverse arch expansion, and another technique
is advancement/proclination of the anterior teeth. Unfortunately, both methods have been
found to be unstable long-term in many situations. Other options to resolve crowding
include distalization of the teeth in the arch or reduction of tooth structure, the latter of
which can be accomplished through extraction or interproximal enamel reduction. The
non-extraction approach increases the space available by expanding arch widths and
increasing arch lengths, while the extraction and enamel reduction approaches create
space by removing tooth mass. The amount of crowding, as well as the facial profile,
often determine the appropriate treatment strategy.
If the crowding falls in the moderate range of 3-7mm, enamel reduction is an
attractive option to create space. Interproximal reduction is frequently performed on the
mandibular anterior dentition during orthodontic treatment to correct tooth-size
1
discrepancies and arch length deficiencies. In addition, it has been argued that by
broadening the interproximal contacts, the stability of the arch increases. The abrasion
experienced by Stone Age man lead to flatter interdental contacts, which increased
stability of the arch and reduced problems of incisal crowding.2 Stability is one of the
most important goals in orthodontic treatment; however, the factors producing the
greatest stability are still largely unknown. More recent studies have found narrow
intercanine width, high initial incisor irregularity, and thinner mandibular cortices to be
significant predictors of post-retention relapse.3-5 Orthodontic relapse occurs often, and
mandibular incisor crowding is one of the aspects of treatment that relapses most.
Mandibular incisor irregularity may alter occlusal stability, including overbite and
overjet, as well as maxillary incisor alignment. This demonstrates the importance of
stable, well-aligned lower incisors in orthodontics.
Since mandibular anterior crowding is one of the main areas affected by
orthodontic relapse, it would be helpful to find methods to prevent this occurrence.
Although much research has been done on mandibular anterior crowding, no published
studies focused solely on the effect of interproximal reduction on the long-term stability
of lower incisors. The purpose of this study is to determine if interproximal reduction
increases long-term stability of the mandibular incisors in Class I and II malocclusions.
2
Chapter 2: Review of the Literature
Stability
The Princeton Dictionary defines stability as the quality of being free from change
or variation. In orthodontics, stability is defined as “maintaining the dentition as it is at
the end of treatment.” One goal of orthodontic treatment is to achieve long-term stability
of the occlusion. 6 Angle7 believed that orthodontic correction will remain stable if the
teeth are aligned into a normal occlusion and provided with adequate retention.
However, long-term observation after treatment often displays a high rate of relapse
following retention. One of the most difficult aspects of the orthodontic treatment
process is maintaining the treatment result.8 Maturational changes, along with posttreatment tooth alterations, work against long-term stability. Many studies have
examined the alterations in the occlusion and dental relationships after orthodontic
treatment. Uhde, Sadowsky, and BeGole9 found that “dental relationships tended to
return toward their pretreatment values after treatment, while still retaining a major part
of the correction.”
Orthodontic relapse, a return to pretreatment conditions, occurs in a high
percentage of treated cases. Thilander10 states that “the typical type of orthodontic
relapse is well documented and includes crowding or spacing of teeth, return to increased
overbite and overjet, and instability of Angle Class II and III corrections.” The
University of Washington studies on stability and relapse have suggested that arch length
reduces following orthodontic treatment as it does in untreated normal occlusions. These
studies also noted that arch width across the mandibular canine teeth reduces after
treatment, and that mandibular anterior crowding continues into the third and fourth
3
decades of life and likely beyond. Relapse is notable and unpredictable, especially in
mandibular incisor alignment.11
Mandibular anterior crowding is often one of the features of the occlusion that
relapses most after orthodontic treatment. Years of research have failed to identify a
specific reason for late mandibular incisor crowding; however, it is considered a normal
part of the aging process. Sinclair and Little12 found that without orthodontic treatment,
approximately two-thirds of adolescents with good alignment and “normal” occlusions
will develop incisor irregularity by early adulthood. Unfortunately, orthodontic treatment
and retention do not protect against later crowding. According to the American
Association of Orthodontists,13 “once retainer wear stops, orthodontic patients often
demonstrate changes in incisor alignment similar to those seen in untreated individuals.”
In contrast, some research indicates that length of retention may affect the degree of
relapse. A study by Sadowsky, Schneider, BeGole, and Tahir14 showed that the
mandibular anterior teeth demonstrated relatively good alignment post-retention;
however, they felt that this may be due to a prolonged mandibular retention period with
an average of 8.4 years. At the time of removal of the fixed lingual retainer, light
interproximal stripping was done to flatten the contact points between the incisors. This
also may have contributed to the better long-term results. In the past, many orthodontists
assumed that once growth ended, the changes in occlusion would be minimal to
negligible; thus, retention would no longer be necessary. Little, Riedel, and Årtun15
discovered that this assumption is not true. Their study indicated that crowding continues
to increase during the 10- to 20-year post-retention phase, but to a lesser degree than from
the end of retention to 10 years post-retention. They concluded that “the only way to
4
ensure continued satisfactory alignment post treatment probably is by use of fixed or
removable retention for life.” Additionally Behrents16 has shown significant skeletal
changes from ages 17 to 83. This study demonstrated that facial growth, soft tissue
growth, and rotation of the mandible continues during adulthood; thus, indicating a need
for retention throughout life.
Many orthodontists believe that the alignment of the mandibular arch serves as a
template around which the upper arch develops and functions.17 Proper positioning of the
mandibular incisors is considered by many to be the most important goal in achieving
good results in orthodontic treatment. Orthodontists carefully evaluate the position of the
mandibular incisors before and throughout treatment, in an attempt to achieve stable
results.18 Multiple variables affect the stability of the lower incisors. Sanin and Savara19
stated that “the self-correction, the development, the increase, or the decrease of
crowding of the mandibular incisors is, in part, a function of the relationship among tooth
size, arch width, axial inclination of the teeth, and direction of mandibular growth.”
Much orthodontic research focuses on how and to what extent these variables affect
mandibular incisor stability.
Many believe that expansion of the dental arches will fail because molar width
and canine width are biologically predetermined at a certain distance that should not be
altered.20 Some argue that patient age can affect relapse, with corrections taking place
during periods of growth being less likely to relapse.21 Others contend that increasing the
length of retention will increase the post-retention stability.14 Tweed22 believed that
incisors must be upright over basal bone to remain stable. Another argument is that posttreatment growth of the mandible in a forward or downward direction will cause
5
crowding because the lower incisors will be restricted by the upper incisors or the lower
lip, which will lead to distal displacement and irregularity of the mandibular incisors.23
Third molars have also been blamed for crowding due to the eruption force of the teeth
against the rest of the dentition, positioned mesially to them.24 Relapse of treated
malocclusions may also be influenced by periodontal fibers,25 oral habits,26
musculature,27 occlusal functioning,28 tooth size discrepancies,29 and the normal decrease
in arch dimensions with maturation.30 Kahl-Nieke, Fischbach, and Schwarze31 confirmed
that pretreatment variables including increased incisor width, severe crowding and incisor
irregularity, arch length deficiency, constricted arches, and deep bites are associated in
the process of post-retention crowding and incisor irregularity. They also found that the
amount of expansion of the intermolar width and increased arch length were factors in
mandibular incisor relapse. No cephalometric data have proven helpful in establishing a
prognosis regarding relapse of the mandibular anterior segment.32
Attempts to Improve Stability in Treatment
Controversy still exists in the orthodontic community concerning whether
nonextraction or extraction treatment produces better long-term stability. In the early
1900s, Angle promoted nonextraction treatment, suggesting that extraction arrested facial
development and expression.7 On the other hand, Case33 advocated extractions in
orthodontic treatment, especially in situations with deficient apical bases. The degree of
mandibular arch crowding, as well as the facial profile, frequently determine the
treatment strategy in crowded malocclusions. Weinberg and Sadowsky34 suggest that
resolution of crowding can be achieved by “distal movement of the posterior teeth,
advancement of the anterior teeth, and expanding the arch transversely.” Space can also
6
be created by extracting teeth or interproximal reduction.35 The relapse patterns of
nonextraction and extraction therapy are similar; however, different conclusions have
been found in various studies.36 Glenn, Sinclair, and Alexander36 found that
postretention changes in nonextraction cases were slightly greater than that seen in
untreated individuals, but they were less than that found in some extraction cases. Twothirds of the patients treated with first-premolar extractions had unsatisfactory lower
anterior alignment after retention according to Little, Wallen, and Riedel;30 however,
Rossouw, Preston, and Lombard37 argue that clinically acceptable stability can be
achieved with no significant differences between extraction and nonextraction treatments.
Although there is no clear answer if extraction or nonextraction treatment
provides better long-term stability, there are several theories on how to improve stability.
One hypothesis is that if lower incisors are placed upright over basal bone, they are more
likely to remain in good alignment.22 Another theory is that the less teeth have to move,
the more stable they will be due to the memory of the supracrestal fibers. The relapse of
rotated teeth is primarily due to the displaced supraalveolar connective tissue fibers. In
1970, Edwards25 described a surgical technique, now referred to as circumferential
supracrestal fiberotomy (CSF) to eliminate rotational relapse. This procedure is now
used by many orthodontists; however, it cannot stabilize the entire mandibular anterior
segment, only individual rotations, since incisors retain the tendency to upright over time.
In a long-term study, Edwards38 found the CSF procedure to be more effective in
reducing rotational relapse than labiolingual relapse. This can be partially explained by
the idea that relapse in a labiolingual direction is more complex and multifactored, due to
muscle balance, root parallelism, and occlusal guidance, than strictly rotational relapse.
7
Boese39 suggested combining circumferential supracrestal fiberotomies with
reproximation of the lower anterior teeth to reduce crowding relapse. He found that
patients who had undergone both procedures demonstrated great stability. Another
thought is that tooth shape and tooth size discrepancies may affect relapse.
Individual Tooth Shape Deviations
Tooth shape can affect the alignment of the dentition. Well-aligned mandibular
incisors often possess distinctive dimensional characteristics. These teeth are
significantly smaller mesiodistally and larger faciolingually, when compared to average
tooth dimensions. Orthodontists must consider tooth shape and size when diagnosing
patients. Odontometry, the science of measuring the size and proportion of teeth, can
help the orthodontist make the correct treatment choice. The maximum mesiodistal
diameter of a tooth is routinely measured during spatial analysis of malocclusions.
Analysis of the mesiodistal tooth size allows for assessment of arch length discrepancies
and tooth size compatibility between the maxillary and mandibular arches. The
faciolingual crown diameter, however, is often overlooked in orthodontic diagnosis.
Physical anthropology, on the other hand, incorporates both faciolingual and mesiodistal
dimensions in tooth size indices. Since both dimensions appear to be related to
mandibular incisor alignment, Peck and Peck40 developed an index for orthodontic tooth
size analysis incorporating both of the dimensions. The index divides the mesiodistal
diameter in millimeters by the faciolingual diameter in millimeters and multiplies that
number by 100. The index is a numerical expression of the mandibular incisor crown
shape viewed from the incisal edge. Their findings indicate that lower incisors with
mesiodistal/ faciolingual indices significantly lower than the normal population are
8
conducive to good alignment. These teeth tend to have flatter mesial and distal surfaces,
which are less susceptible to contact slippage; thus, improving the alignment. Peck and
Peck40 suggest that “patients whose mandibular incisors have mesiodistal/faciolingual
indices above the desired ranges may well be candidates for the removal of some mesial
and/or distal tooth substance in conjunction with orthodontic therapy.” Tooth
reproximation is recommended to correct tooth shape deviations.40
Kuftinec18 tested Peck and Peck’s concept and found that the
mesiodistal/faciolingual indices predicted stability of the alignment of mandibular
incisors; however, there were individual cases which did not follow the rule. Factors
other than these indices, such as the amount of crowding at the beginning of treatment,
the Bolton tooth correlation index, and the interincisal relationship, influence the stability
of mandibular incisors. Kuftinec does not recommend orthodontists reapproximate
incisors with large indices without considering other diagnostic criteria.
Gilmore and Little41 also performed a study on mandibular incisor dimensions
and stability. This data showed a weak association between incisor widths or
mesiodistal/faciolingual ratio and irregular alignment over time. There was less
difference between the mean mesiodistal/faciolingual ratios of the well-aligned and
crowded groups in this sample than in the Peck and Peck sample. Only lateral incisors,
not central incisors, showed a significantly different ratio in the two groups. They found
that “the pretreatment pattern that emerges as having a tendency for greater long-term
incisor irregularity is a divergent face with wide, upright incisors.” Although incisors
with greater mesiodistal dimensions are associated with crowding, the association is weak
enough that Gilmore and Little believe reduction of the widths of incisors to fit a specific
9
range cannot be expected to produce a stable alignment. Thus, orthodontists must
consider tooth shape and width along with many other factors when deciding if enamel
reduction is necessary.
Inter-Arch Tooth Size Discrepancies
When fitting the dentition together, the orthodontist must consider the mesiodistal
width of the teeth. In the late 1800s, Black42 was one of the first researchers to explore
tooth size. He measured human teeth and set up tables of means which are used as
references today. Ballard43 studied asymmetry in tooth size in 1944 and found that 90%
of his sample had right-left discrepancies in tooth width. He believed that final stability
cannot be attained if tooth size discrepancies are left, because there will be adjustments in
arch length through rotations and slipped contacts. He encouraged careful stripping of
proximal surfaces in the anterior segments, when lack of balance existed. Since
discrepancies primarily occur in the anterior segments, Ballard argued that more
improvement can be gained by stripping here, than in the buccal segments, where
inclined plane relationships impose distinct limitations.
In 1958, Bolton performed a tooth size study on fifty-five cases where excellent
occlusions existed. The mesiodistal tooth width was measured on all the teeth on each
cast from first molar to contralateral first molar. 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.”29 The same method was used to set
up an “anterior ratio,” consisting of the sum of the widths of the mandibular anterior six
teeth divided by the sum of the widths of the maxillary anterior six teeth, and then again
multiplied by 100. Bolton found the overall ratio average to be 91.3 ± 1.91 and the
10
anterior ratio average to be 77.2 ± 1.65. This tooth size data was found to be closely
related to that published by both Black and Ballard. The ratios should be used in
orthodontic diagnosis. If a substantial deviation occurs, a diagnostic setup can confirm
the precise conditions that exist. The ratio results can give insight as to how the setup
should be approached. Disharmonies in tooth size can be corrected by extraction of a
tooth or teeth, placement of overcontoured restorations, or removal of tooth structure by
stripping.
In the finishing stages of orthodontic treatment, the correct maxillary and
mandibular tooth size relationship is critical in achieving proper occlusal interdigitation.
It has been demonstrated that a tooth size discrepancy between maxillary and mandibular
teeth may cause malocclusion.29,44,45 Norderval, Wisth and Boe46 confirmed these
findings in a study on a group of adults with ideal occlusion and a group of adults with
similar occlusion except for slight crowding in the mandibular anterior segment. It was
revealed that the crowded group had significantly greater mesiodistal widths of the four
mandibular incisor teeth and similar intercanine widths. Bolton’s index was also
significantly higher in the group with crowding.
A large percentage of patients presenting for orthodontic treatment have a tooth
size discrepancy that may influence treatment goals and results.47 It has been found that
approximately 30% of orthodontic cases present with a significant anterior six tooth
discrepancy, meaning that the Bolton ratios are greater than 2 standard deviations away
from Bolton’s means. This anterior discrepancy was almost twice as likely to be due to
mandibular excess (19.7%) than to maxillary excess (10.8%).47 Crosby and Alexander48
found that only 22.9% of patients had an anterior ratio with significant variation. They
11
also found that mandibular excess tooth size discrepancies occurred in a greater
percentage of patients (13.8%) than maxillary excess (9.2%). The mandibular excess
could lead to future post-treatment relapse in the mandibular incisor area if left untreated.
In addition, Araujo and Souki49 found that the type of malocclusion affects the prevalence
of Bolton discrepancies. This study demonstrated that individuals with Angle Class I and
Class III malocclusions show a significantly greater prevalence of tooth size
discrepancies than do individuals with Class II malocclusions. They also noted that the
mean anterior tooth size discrepancy was significantly higher for Angle Class III subjects
than for Class I and Class II subjects. Due to the frequency of tooth size discrepancies, it
appears that the Bolton analysis would be extremely beneficial to orthodontists in all
initial case workups. It would allow more efficient diagnosis of problems, more
specificity in treatment planning, and a higher success rate in achieving optimal
occlusions, overbite, and overjet. In addition to helping in the finishing of cases, it might
also increase the stability of results.
Enamel Reduction
Interproximal enamel reduction (IPR), also referred to as interdental stripping, airrotor stripping, enamel approximation or slenderizing, is a frequently applied orthodontic
technique.50 It is thought that enamel reduction can help in achieving better alignment
and occlusion of the dentition and to maintain alignment over time. Some of the
indications for interproximal reduction include: good oral hygiene, Class I arch-length
discrepancies with orthognathic profiles, minor Class II dental malocclusions, and Bolton
tooth-size discrepancies.35 Enamel reduction procedures can be used to gain space, to
12
correct tooth-size discrepancies between mandibular and maxillary teeth, and to improve
tooth shape during orthodontic treatment.
Sheridan believes that interproximal enamel reduction imitates the natural process
of interdental abrasion.51 Anthropological data reveals that interdental abrasion
correlates with diet and lifestyle, and the process was compensated by mechanisms such
as “bulking of thicker enamel on tooth surfaces prone to abrasion.”51 Although
anthropology appears to validate enamel reduction, the two processes are quite different.
The abrasion observed in primitive populations was gradual and continual, due to
abrasive diets and forceful chewing, while enamel reduction is immediate.2 In spite of
this fact, the final results are almost identical. Both alter tooth-size ratios, broaden
contact areas, and produce enamel surfaces that are better able to remineralize.
Interproximal reduction is an appealing alternative to extraction and expansion
treatments due to its ability to maintain transverse arch dimension and anterior
inclinations.35 To resolve crowding in nonextraction treatment, the arch length is often
increased, a condition which has been associated with instability and future relapse.
Interproximal stripping reduces the increase in arch length during treatment.
Approximately 50% of interproximal enamel can be safely removed.40,52 By removing
enamel, the large curvatures in the anterior tooth region that make contacts instable upon
compression, can be reduced or flattened to increase stability. In biomechanical terms,
the “interproximal convex-convex contacts represent joints which correspond to stretched
dimeric link chains and which, under the influence of compressive force, form a
mechanically unstable articular system.”53 Stone Age man had interdental contacts of
transversally concave-convex dental surfaces. According to Ihlow et al.54 “increased
13
stability of the dental arch and reduced problems of incisal crowding are to be expected if
an overlapping dimeric chain is produced morphologically in each horizontal contact by
means of slight interproximal enamel reduction.” By using this technique, it is theorized
that the modern dental arch could gain stability similar to that achieved by abrasion in
Stone Age man.
Interproximal reduction may help in preventing relapse. Studies show that 40%
to 90% of patients have unacceptable dental alignment 10 to 20 years after retention.
Arch dimensions change with age. Crowding of the mandibular incisors commonly
develops and coincides with a decrease in arch length.10 One causative factor in late
crowding is thought to be mesiodistal tooth size. Begg2 studied interproximal attrition in
Australian aborigines and believed that teeth in modern men became crowded due to
teeth that are too large for the dental arches. IPR helps to relieve this crowding.
Enamel reduction also improves anterior interocclusal relations. Discrepancies in
anterior arch length due to a decreased maxillary dental arch length can be corrected by
IPR to reduce mandibular dental arch length, promote stability, and improve gingival
conditions.55 It can provide more desirable overbite and overjet relationships, which
improves anterior function in the mutually protected occlusion.
Interproximal reduction can be achieved by means of abrasive disks, an ultrasonic
unit with aluminum hydroxide paste and reducing tips, lightning and abrasive strips, and
an electric toothbrush body incorporating mounted lightning strips.55 Abrasive strips are
best utilized for minor enamel reduction. Handpiece-mounted abrasive discs can remove
substantial amounts of enamel, thus they often require a mechanical separation.
Separated tooth surfaces allow for a more controlled reduction of enamel. Since the
14
technique is non-reversible, the amount of reduction desired should be predetermined. In
addition, “disc placement must be exact to avoid excessive enamel loss and the creation
of an abnormal contact or a ledge in the enamel.”56 The teeth and surfaces reduced, as
well as the amount reduced, should be recorded in the patient’s chart to prevent over
reduction at future appointments.
For several decades, orthodontists have discussed interproximal reduction, but airrotor stripping was first described in 1985 by Sheridan. It has gained popularity in the
years since then. Air-rotor stripping (ARS) allows clinicians to remove interproximal
enamel; however, this technique is primarily confined to the buccal quadrants, whereas
conventional IPR is often limited to the anterior segment. Sheridan56 argues that “ARS
can create substantially more space than is usually obtained by conventional
interproximal reduction procedures.” ARS procedures are thought to be able to correct
space discrepancies of up to eight millimeters, an amount often believed to require
extractions or expansion.
The Effects of Interproximal Enamel Reduction
In 1956, Hudson57 did a study on the reduction of mesiodistal diameter of
mandibular anterior teeth. He took six mandibular anterior teeth from the same patient
and set them in acrylic resin to simulate positions and tightness of contact found in the
mouth. The contact points were stripped with lightning metal strips and cloth finishing
strips. Thirty of each of the following, mandibular cuspids, lateral incisors, and central
incisors, were collected and sectioned mesiodistally to measure enamel thickness at the
contact points. Hudson found that the average thickness of enamel at the contact points
in adult mandibular incisors was 0.59 mm, in mandibular lateral incisors it was 0.66 mm,
15
and it was 0.82 mm in mandibular cuspids. There was no definite correlation between
tooth size and enamel thickness at contact points; however, overall the larger teeth had
more enamel thickness than the smaller teeth. Hudson believed that mesiodistal
reduction of teeth should be avoided, if possible, due to an increase risk of caries from
reducing the amount of protective enamel present. He thought the flattened surfaces
defeated the cleansing action of embrasures, roughened enamel would retain plaque, and
that removal of excess enamel could expose the inner portion, which contains many tufts
and spindles. If reduction is needed though, he recommended removing 0.20 mm for
each central contact, 0.25 mm for each lateral contact, and 0.30 mm for each cuspid
contact, creating approximately 3 mm of space. He also advocated separating the teeth
before reduction and polishing teeth after the removal of enamel in order to leave a
smooth surface. Tuverson55 addressed the issue of caries susceptibility and enamel
reduction by stating that the technique should be used only in patients with good oral
hygiene and low caries susceptibility. He suggested protecting the exposed enamel
surfaces with the application of topical fluoride following the reduction, just as Hudson
had advised years before.
Paskow58 contended to “give teeth sufficient room, and they will align
themselves.” He used abrasive strips and disks to gain 0.50 to 0.75 mm of space per
surface. He noted an obvious alignment within 4 weeks, and within a 3- to 6-month
period, he observed as much alignment as space permitted. He found the alignment to be
quite stable in the months and years following stripping. Clinically, the stripped lower
anterior teeth revealed no interproximal caries, and the gingiva was healthy. Radiographs
demonstrated interproximal enamel remaining to protect the dentin, no caries, and
16
interproximal bone height remaining the same. Paskow suggested interproximal
stripping immediately following band removal to prevent irregularities from occurring so
frequently. He proposed that the procedure could fall into the category of preventive
dentistry and prove to be a justifiable prophylactic measure.
Mesiodistal enamel reduction may lead to gingival improvement. Receded
interproximal gingival, often found in adults with a predisposition to periodontal disease,
leaves large interproximal spaces, often referred to as “black triangles.” Proximal enamel
reduction reduces the bell-shaped contour of the crowns, allowing the cervical areas of
adjacent teeth to move closer to one another.55 By reducing this interproximal area, the
receded gingiva can now fill in some of the space remaining and create a more normal
appearing interproximal gingival tissue.
The two main benefits of interproximal reduction are to provide broader contact
areas, thus increasing contact stability, and to increase the space available in the
mandibular anterior region. Despite the many benefits, Boese39 warns that care must be
taken, and IPR should only be used after the lower incisors are completely aligned. This
allows for a more conservative and precise approach for enamel removal. Boese also
describes a second phase of reproximation, which occurs after appliance removal, if
lower retention is not employed. Enamel reduction is performed serially over a four to
six month period following band removal. At each visit, the mandibular incisor contact
points are examined with dental floss to reveal any increases in contact point pressure. If
contact points become extremely tight or if teeth are shifting, IPR is executed. A third
phase of reproximation is usually not warranted; however, the uprighting of lower
incisors often occurs after treatment, resulting in secondary crowding. If this occurs, the
17
orthodontist must “consider the amount of stripping already performed, the amount of
enamel remaining, shape of the lower incisor, degree of overbite, and the anticipated
amount and direction of mandibular growth.”39
In 1979, Boese59 performed a study on the long term clinical results of
circumferential supracrestal fiberotomy (CSF) and reproximation on crowded mandibular
arches. The sample consisted of 40 orthodontically treated patients with no form of
retention. All patients underwent CSF and IPR to varying degrees, with the mean value
for reproximation of lower incisors being 1.69 mm. These cases demonstrated great
stability in the mandibular anterior segment 4-9 years after treatment. Boese’s findings
strongly support Peck and Peck’s belief that well-aligned mandibular incisors have a
characteristic crown shape. Since no retention was used, the lower incisors were “able to
move and post-treatment changes did occur, but incisors tended to move as a unit rather
than as individual teeth.”59 When mandibular incisor contacts are indistinct, distal forces
easily displace them. This study also looked at the long term effects of CSF and IPR on
the periodontium and revealed no significant increase in pocket depth, gingival recession,
or loss of alveolar bone height. Boese felt that the two procedures were not a guarantee
for stability, but they were extremely useful in preventing some natural changes.
Betteridge60 conducted a study on the effects of stripping and found the procedure
to be a “moderately good method of treatment with minimal adverse effects on the incisor
relationship and a trend towards improving the health of the gingivae.” She
recommended IPR only in select cases with less than 4 mm of crowding. Removing this
small amount of tooth structure has minimal effect on the incisor relationship. The study
sample demonstrated approximately a 2º retroclination of the upper and lower incisors
18
and a decrease of over 1 mm in the lower intercanine width. One year after appliance
removal, there tended to be some degree of relapse. Overall, Betteridge felt that
interdental stripping has a limited place in orthodontics for relief of lower incisor
crowding.
Interproximal stripping does not prevent relapse entirely, but it can assist in
minimizing it. In a study on post-retention relapse of mandibular anterior crowding,
Freitas et al.61 found that patients who had undergone IPR “to correct crowding and to
solve interarch tooth size discrepancies” tended to have a slightly less relapse (19.37%)
than those who did not have stripping (31.32%). Aasen and Espeland62 also found that
systematic enamel reduction helped in achieving long term stability of the lower incisors.
Their study examined 56 patients treated with IPR in the mandibular anterior region
during treatment and after removal of appliances, with no retainer use. An average of 1.9
mm of enamel was removed. From post-treatment to 3 years follow-up, the mean
increase in irregularity index score was 0.6, indicating good stability. Aasen and
Espeland even suggest that the procedure could be considered an alternative to placement
of lower retainers to maintain stability of mandibular anterior alignment.
Methods of Measuring Crowding
Mandibular anterior crowding is one of the most frequent characteristics of
malocclusion. Progressive instability often displays itself by gradual crowding of the
mandibular incisors following removal of retention devices. This mandibular incisor
irregularity often leads to maxillary crowding, deepening of the overbite, and generalized
decline of the treated case. Since the six mandibular anterior teeth are a limiting factor in
treatment and stability, Little63 developed a diagnostic index to reflect the mandibular
19
anterior condition. The Irregularity Index measures “the linear displacement of the
anatomic contact points (as distinguished from the clinical contact points) of each
mandibular incisor from the adjacent tooth anatomic contact point, the sum of these five
displacements represents the relative degree of anterior irregularity.” This measure
signifies the distance the contact points must be moved to gain anterior alignment. The
index gives orthodontists a quantitative measure of the severity of dental irregularity and
aids in diagnosis and treatment planning.
Norderval, Wisth, and Boe46 used another method to measure crowding. Their
technique measures the difference between the space available and the actual mesiodistal
dimension of each tooth. This method was found to have a high correlation with Little’s
Irregularity Index in a study by Puneky, Sadowsky, and BeGole.64 The correlation
coefficient between linear assessment of crowding and Little’s Irregularity Index was
0.84, which is statistically significant. The high correlation between methods indicates
that there is an expected linear trend between the linear assessment of incisor crowding
and the irregularity index.
In 1976, Betteridge65 developed another index of crowding, called the Betteridge
Index (BI). This was calculated from the ratio of the mesiodistal widths of the six
anterior teeth to the amount of space available in the labial segment. An index of 1.00
indicated ideal space balance, an index of 1.10 crowding of about 4mm, and an index
below 1.00 indicated spacing.
Summary
The literature is conflicting on the relative importance of the various factors
leading to lower incisor crowding both with and without orthodontic treatment and to the
20
solutions to mitigate it. In the area of the value of interproximal reduction, there is not a
good published study comparing two similar patient groups, one on whom interproximal
reduction has been performed in the lower incisor and canine region and the other in
which it has not. It would be important to orthodontists to know if there is a difference
between groups.
21
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23
29. Bolton A. Disharmony in tooth size and its relation to the analysis and treatment of
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30. Little RM, Wallen TR, Riedel RA. Stability and relapse of mandibular anterior
alignment-first premolar extraction cases treated by traditional edgewise orthodontics.
Am J Orthod 1981;80:349-365.
31. Kahl-Nieke B, Fischbach H, Schwarze CW. Post-retention crowding and incisor
irregularity: a long-term follow-up evaluation of stability and relapse. Br J Orthod
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32. Shields TE, Little RM, Chapko MK. Stability and relapse of mandibular anterior
alignment: a cephalometric appraisal of first-premolar-extraction cases treated by
traditional edgewise orthodontics. Am J Orthod 1985;87:27-38.
33. Case C. The question of extraction in orthodontia. Am J Orthod 1964;50:658-691.
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patients with Class I malocclusions treated nonextraction. Am J Orthod Dentofacial
Orthop 1996;110:359-364.
35. Stroud JL, English J, Buschang PH. Enamel thickness of the posterior dentition: its
implications for nonextraction treatment. Angle Orthod 1998;68:141-146.
36. Glenn G, Sinclair PM, Alexander RG. Nonextraction orthodontic therapy:
posttreatment dental and skeletal stability. Am J Orthod Dentofacial Orthop 1987;92:321328.
37. Rossouw PE, Preston CB, Lombard C. A longitudinal evaluation of extraction versus
nonextraction treatment with special reference to the posttreatment irregularity of the
lower incisors. Semin Orthod 1999;5:160-170.
38. Edwards JG. A long-term prospective evaluation of the circumferential supracrestal
fiberotomy in alleviating orthodontic relapse. Am J Orthod Dentofacial Orthop
1988;93:380-387.
39. Boese LR. Fiberotomy and reproximation without lower retention, nine years in
retrospect: part I. Angle Orthod 1980;50:88-97.
40. Peck H, Peck S. An index for assessing tooth shape deviations as applied to the
mandibular incisors. Am J Orthod 1972;61:384-401.
41. Gilmore CA, Little RM. Mandibular incisor dimensions and crowding. Am J Orthod
1984;86:493-502.
42. Black G. Descriptive anatomy of the human teeth. Philadelphia: S.S. White; 1902.
24
43. Ballard M. Asymmetry in tooth size: A factor in the etiology, diagnosis, and
treatment of malocclusion. Angle Orthod 1944;14:67-71.
44. Lundstrom A. Intermaxillary tooth width and tooth alignment in occlusion. Acta
Odontol. Scand. 1955;12:266-292.
45. Bolton A. The clinical application of tooth size analysis. Am J Orthod 1962;48:504529.
46. Norderval K, Wisth PJ, Boe OE. Mandibular anterior crowding in relation to tooth
size and craniofacial morphology. Scand J Dent Res 1975;83:267-273.
47. Freeman JE, Maskeroni AJ, Lorton L. Frequency of Bolton tooth-size discrepancies
among orthodontic patients. Am J Orthod Dentofacial Orthop 1996;110:24-27.
48. Crosby CR, Alexander RG. The occurrence of tooth size discrepancies among
different malocclusion groups. Am J Orthod Dentofacial Orthop 1989;95:457-461.
49. Araujo E, Souki M. Bolton anterior tooth size discrepancies among different
malocclusion groups. Angle Orthod 2003;73:307-313.
50. Rossouw PE, Tortorella A. Enamel reduction procedures in orthodontic treatment. J
Can Dent Assoc 2003;69:378-383.
51. Sheridan JJ. The physiologic rationale for air-rotor stripping. J Clin Orthod
1997;31:609-612.
52. Peck S, Peck H. Othodontic aspects of dental anthropology. Angle Orthod
1975;45:95-102.
53. Ihlow D, Cronau M, Kubein-Meesenburg D, Heine G, Dathe H, Hansen C et al. An
experimental method for in vivo analysis of biomechanical asymmetries of the
periodontium. J Orofac Orthop 2003;64:321-329.
54. Ihlow D, Kubein-Meesenburg D, Fanghanel J, Lohrmann B, Elsner V, Nagerl H.
Biomechanics of the dental arch and incisal crowding. J Orofac Orthop 2004;65:5-12.
55. Tuverson DL. Anterior interocclusal relations. Part I. Am J Orthod 1980;78:361-370.
56. Sheridan JJ. Air-rotor stripping. J Clin Orthod 1985;19:43-59.
57. Hudson A. A study of the effects of mesiodistal reduction of mandibular anterior
teeth. Am J Orthod 1956;42:615-624.
58. Paskow H. Self-alignment following interproximal stripping. Am J Orthod
1970;58:240-249.
25
59. Boese L. Fiberotomy and reproximation without lower retention 9 years in retrospect:
Part II. Angle Orthod 1980;50:169-178.
60. Betteridge MA. The effects of interdental stripping on the labial segments evaluated
one year out of retention. Br J Orthod 1981;8:193-197.
61. Freitas KM, de Freitas MR, Henriques JF, Pinzan A, Janson G. Postretention relapse
of mandibular anterior crowding in patients treated without mandibular premolar
extraction. Am J Orthod Dentofacial Orthop 2004;125:480-487.
62. Aasen TO, Espeland L. An approach to maintain orthodontic alignment of lower
incisors without the use of retainers. Eur J Orthod 2005;27:209-214.
63. Little RM. The irregularity index: a quantitative score of mandibular anterior
alignment. Am J Orthod 1975;68:554-563.
64. Puneky PJ, Sadowsky C, BeGole EA. Tooth morphology and lower incisor alignment
many years after orthodontic therapy. Am J Orthod 1984;86:299-305.
65. Betteridge MA. Index for measurement of lower labial segment crowding. Br J
Orthod 1976;3:113-116.
26
Chapter 3: Journal Article
Abstract
Stability is one of the most important goals in orthodontic treatment; however, the
factors producing the greatest stability are still largely unknown. The purpose of this
study is to determine if interproximal reduction increases long-term stability of the
mandibular incisors in Class I and II malocclusions. The study sample consisted of
patients of both sexes with Angle Class I or II malocclusions who received nonextraction treatment under the care of one private practitioner. The pretreatment, posttreatment, and post-retention lateral cephalograms and dental casts were examined in
fifty-five cases. The sample was divided into two groups, one consisting of 26 patients
having had 2 mm or more interproximal reduction and the other, control group,
consisting of 29 patients with no interproximal reduction. All patients had a fixed lower
lingual retainer placed at the time of removal of orthodontic appliances, which remained
in place an average of 3.48 years. The post-retention records were taken an average of
8.7 years after removal of the fixed lower lingual retainer. Changes between the time
intervals were evaluated with paired t tests to determine statistical significance. The
mean relapse of mandibular anterior crowding for the control group (no IPR) was
20.23%, and for the sample group (IPR) it was 18.45%. For both the control and sample
groups, the IMPA values remained fairly constant throughout treatment and retention.
Interproximal enamel reduction does not appear to influence the long-term stability of
mandibular incisors.
27
Introduction
One goal of orthodontic treatment is to achieve long-term stability of the
occlusion.1 Angle2 believed that orthodontic correction will remain stable if the teeth are
aligned into a normal occlusion and provided with adequate retention. However, longterm observation after treatment often displays a high rate of relapse following retention.
One of the most difficult aspects of the orthodontic treatment process is maintaining the
treatment result.3 Maturational changes, along with post-treatment tooth alterations,
work against long-term stability. Orthodontic relapse, a return to pretreatment
conditions, occurs in a high percentage of treated cases. Thilander4 states that “the
typical type of orthodontic relapse is well documented and includes crowding or spacing
of teeth, return to increased overbite and overjet, and instability of Angle Class II and III
corrections.” The University of Washington studies on stability and relapse have found
that relapse is notable and unpredictable, especially in mandibular incisor alignment.5
One of the features of the occlusion that relapses most after orthodontic treatment is
mandibular anterior crowding. Sinclair and Little6 found that without orthodontic
treatment, approximately two-thirds of adolescents with good alignment and “normal”
occlusions will develop incisor irregularity by early adulthood.
Researchers have identified several potential causative factors for post-treatment
crowding, including expansion of intercanine and intermolar width,7 flaring incisors
beyond basal bone,8 unfavorable mandibular growth,9 eruption of third molars,10 relapse
of periodontal fibers,11 and tooth size discrepancies.12 Several theories have also been
suggested to improve mandibular incisor stability, such as circumferential supracrestal
28
fiberotomies,13 interproximal enamel reduction,14 and achieving ideal mesiodistal and
faciolingual dimensions, often through the means of tooth reproximation.15
Interproximal enamel reduction is frequently performed on the mandibular
anterior dentition during orthodontic treatment to correct tooth-size discrepancies and
arch length deficiencies. In addition, it has been argued that by broadening the
interproximal contacts, the stability of the arch increases. The abrasion experienced by
Stone Age man lead to flatter interdental contacts, which increased stability of the arch
and reduced problems of incisal crowding.16 Despite the fact that stability is one of the
most important goals in orthodontic treatment, the factors producing the greatest stability
are still largely unknown. When relapse occurs, the mandibular incisor irregularity may
alter occlusal stability, including overbite and overjet, as well as maxillary incisor
alignment. This demonstrates the importance of stable, well-aligned lower incisors in
orthodontics.
The literature is conflicting on the relative importance of the various factors
leading to lower incisor crowding both with and without orthodontic treatment and to the
solutions to mitigate it. Since mandibular anterior crowding is one of the main areas
affected by orthodontic relapse, it would be helpful to find methods to prevent this
occurrence. Although much research has been done on mandibular anterior crowding,
there is no good published study comparing two similar patient groups, one on whom
interproximal reduction has been performed in the mandibular anterior region and the
other in which it has not. The purpose of this study is to determine if interproximal
reduction increases long-term stability of the mandibular incisors in Class I and II
malocclusions treated orthodontically with a non-extraction technique.
29
Materials and Methods
Research Design
The study sample consisted of patients of both sexes with Angle Class I or II
malocclusions who received non-extraction treatment under the care of Dr. R.G. “Wick”
Alexander. The age of the patients ranged between 9.67 years and 15.75 years. Fiftyfive cases with complete records before treatment (T1), at the end of treatment (T2), and
long-term (T3), a minimum of 2 years out of retention, were collected. The sample was
divided into two groups, one consisting of 26 patients (20 females and 6 males) having
had 2 mm or more interproximal reduction and the other, control group, consisting of 29
patients (20 females and 9 males) with no interproximal reduction (Table 1). All patients
had a fixed lower lingual retainer placed at the time of removal of orthodontic appliances,
which remained in place an average of 3.48 years. The post-retention records were taken
an average of 8.7 years after removal of the fixed lower lingual retainer.
Both control and sample groups were treated with 0.018-inch slot edgewise
appliances incorporating twin and single brackets. Some Tweed-based treatment
philosophy was used in the majority of the patients, along with cervical headgear and
Class II elastics. The sample group had interproximal stripping performed at the time of
the removal of a fixed lower lingual cuspid to cuspid retainer. The interproximal
reduction was executed on the mandibular anterior teeth, from canine to contralateral
canine. The mean amount of IPR performed on the anterior teeth was 2.39 mm. It is
unknown why the practitioner selected certain cases for interproximal reduction;
however, he might have selected them based on certain diagnostic characteristics, which
indicates that a selection bias might have existed in this study. The practitioner might
30
Table 1: Characteristics of Control and Sample Groups
Control (n=29)
No IPR
Sex
Females
20
Males
9
Angle Classification
Class I
12
Class II
17
Age at T1 (years)
Mean
12.21
Range
9.67-15.75
Retention time (years)
Mean
3.43
Range
1.20-6.10
Post-retention time (years)
Mean
8.15
Range
2.40-14.60
Sample (n=26)
IPR
20
6
14
12
12.70
10.25-14.5
3.54
1.80-8.40
8.18
3.00-17.90
Table 2: Classification of Control and Sample Groups based on Initial Irregularity
at Pretreatment (T1)
Control (n=29)
Sample (n=26)
No IPR
IPR
Group A (mild irregularity)
18
11
0.01-3.50 mm
Group B (moderate to
11
15
severe irregularity)
3.51 mm or greater
.
31
have felt that certain patients would benefit more from interproximal stripping than
others; thus, he chose only certain patients to undergo the technique.
The sample and control groups were divided into subgroups based on the amount
of mandibular anterior crowding present in the pretreatment records according to the
Little Irregularity Index.17 The subgroups were as follows: (1) mild irregularity
(1-3.5 mm) and (2) moderate to severe irregularity (>3.5 mm). The mild group consisted
of 11 patients from the sample and 18 patients from the control. The moderate to severe
group consisted of 15 patients from the sample and 11 from the control (Table 2).
Dental Cast Measurements
Pretreatment, post-treatment, and post-retention lateral cephalograms and dental casts
were examined. All dental casts were measured by one operator (JR) using a digital
caliper calibrated to the nearest 0.01 mm. The following dental cast variables were
obtained for each set of casts:
(i)
Irregularity index- As stated by Little,17 “the linear displacement of the
anatomic contact points of each mandibular incisor from the adjacent tooth
anatomic point, the sum of these five displacements representing the relative
degree of anterior irregularity” (Figure 1).18
(ii)
Mandibular intercanine width- The distance between the right and left
mandibular canine cusp tips or estimated cusp tips if wear facets are present
(Figure 2).19
(iii)
Overbite- Mean overlap of upper to lower central incisors.
32
Figure 1: Little’s Irregularity Index
Irregularity = A + B + C + D + E
(adapted from Little, Wallen and Riedel)18
Figure 2: Intercanine width
A = Intercanine width
(adapted from Glenn, Sinclair and Alexander)19
33
(iv)
Overjet- The distance parallel to the occlusal plane from the incisal edges of
the most labial maxillary central incisor to the most labial mandibular central
incisor.
(v)
Anterior Bolton ratio- The ratio of the sum of the mesiodistal widths of the six
mandibular anterior teeth, from canine to canine, divided by the sum of the
mesiodistal widths of the six maxillary anterior teeth. The norm value for this
ratio is 77%.
Cephalometric Measurements
The following cephalometric angular measurements were traced by one operator (JR):
(i)
IMPA- lower incisor to mandibular plane
(ii)
SN-MP – sella nasion plane to mandibular plane. This measurement was only
traced at T1 to ensure that the samples had similar growth patterns.
Data Organization, Reduction, and Analysis
To ensure accuracy and reduce examiner bias, a series of 10 dental casts and 10
radiographs were randomly selected and remeasured on a separate occasion. The
repeated measurements were used to calculate the intraclass correlation coefficient.20 If
the intraclass correlation coefficient (ICC) value is greater than or equal to 0.80, the test
is considered to demonstrate good reliability for the repeated measures, with a perfect
reliability score equaling 1.00. The ICC value for the dental cast re-measured data
equaled 0.993. This value indicated a high reliability for the dental cast measurements.
The ICC value for the cephalometric double-determination was 0.995, a value that also
indicated high reliability for the cephalometric variables.
34
Statistical analysis included descriptive statistics using the means and standard
deviations for all dental and cephalometric variables at each stage of treatment:
pretreatment (T1), post-treatment (T2), and post-retention (T3). Changes between the
time intervals were evaluated with paired t tests to determine statistical significance at a
level of P < 0.05. Treatment changes in irregularity index were calculated by subtracting
the pretreatment index from the post-treatment index (II.T2-II.T1), and the amount of
relapse was calculated by subtracting the post-treatment index from the post-retention
index (II.T3-II.T2). To calculate the percentage of relapse, treatment changes in the
index were considered a 100% correction. The percentage of relapse was calculated by
correlating the relapse amount in relation to the amount that corresponded to the 100%
correction, or the amount of treatment change.
35
Results
Dental Cast Measurements
Descriptive statistics for the dental cast measurements of each subgroup are
presented in Table 3 and Table 4. Results for the complete control and sample groups are
shown in Table 5.
Cephalometric Measurements
Descriptive statistics were also used to compare the cephalometric measures.
They are presented in Table 6 and Table 7. The SN-MP angle was used to ensure that the
sample subjects were similar and not extreme in nature; therefore, it was only measured
at pretreatment. The values ranged from 20° to 40.5°, with an average of 29.5°. Results
for the entire control and sample groups are shown in Table 8.
Table 3: Dental Cast Measurement Statistics for Group A (Initial Mild Irregularity
<3.5 mm)
Variables
T1
Control
N=18
T2
T3
Interproximally Stripped Sample
N=11
T1
T2
T3
Irregularity Index
2.12±0.86
0.84±0.62*
1.31±0.63*
Intercanine width
25.04±1.64
25.51±1.29
24.80±1.18*
Overbite
3.97±1.68
2.08±0.57*
3.10±0.84*
4.54±3.14
2.95±2.07
2.42±1.21
Overjet
4.14±1.78
1.51±0.56*
1.97±0.68*
3.36±1.85
1.56±0.42*
1.95±0.82
81.04±2.29
NA
Anterior Bolton
“Ratio”
77.8±2.32
NA
NA
T1= pretreatment; T2= post-treatment; T3= post-retention.
*Statistically significant (p < 0.05).
36
2.44±0.77
0.77±0.40* 1.48±0.69*
26.00±0.96 26.28±0.78* 26.46±1.22
75.80±2.19*
Table 4: Dental Cast Measurement Statistics for Group B (Initial Moderate to
Severe Irregularity >3.5 mm)
Variables
T1
Irregularity Index
5.64±0.98
Intercanine width
25.46±2.44
Control
N=11
T2
0.94±0.63*
T3
1.54±0.90
26.10±0.91 25.46±1.18*
Interproximally Stripped Sample
N=15
T1
T2
T3
7.03±2.93
0.97±0.44*
1.57±1.55
25.34±2.29 26.42±1.18 25.89±1.23*
Overbite
4.01±1.15
1.89±0.69*
2.50±0.89
4.42±1.08
2.27±0.62* 3.01±1.06*
Overjet
5.57±2.80
1.50±0.41*
2.15±0.92*
3.80±1.11
2.02±0.61* 2.47±0.55*
Anterior Bolton
“Ratio”
80.14±6.04
NA
NA
T1= pretreatment; T2= post-treatment; T3= post-retention.
*Statistically significant (p < 0.05).
79.80±3.05
NA
74.81±2.87*
Table 5: Dental Cast Measurement Statistics for Pooled Groups
Variables
T1
Control
N=29
T2
T3
Interproximally Stripped Sample
N=24**
T1
T2
T3
Irregularity Index
3.45±1.95
0.88±0.62*
1.40±0.74*
4.46±2.35
0.83±0.36* 1.50±0.92*
Intercanine width
25.20±1.95
25.73±1.18
25.05±1.20*
Overbite
3.98±1.48
2.00±0.61*
2.87±0.89*
4.47±2.15
2.56±1.43* 2.76±1.14
Overjet
4.69±2.28
1.50±0.50*
2.04±0.77*
3.61±1.45
1.82±0.57* 2.25±0.71*
25.62±1.84 26.36±1.04* 26.13±1.23
Anterior Bolton
“Ratio”
78.74±4.27
NA
NA
80.32±2.77
NA
T1= pretreatment; T2= post-treatment; T3= post-retention.
*Statistically significant (p < 0.05).
**Two subjects were dropped for the irregularity index values due to extreme values.
37
75.23±2.60*
Table 6: Cephalometric Measurement Statistics for Group A (Initial Mild Crowding
<3.5 mm)
Variables
T1
Control
N=18
T2
T3
IMPA
96.92±5.34 97.42±4.86 97.50±4.97
T1= pretreatment; T2= post-treatment; T3= post-retention.
*Statistically significant (p < 0.05).
Interproximally Stripped Sample
N=11
T1
T2
T3
97.81±6.97
98.50±6.44
98.73±6.08
Table 7: Cephalometric Measurement Statistics for Group A (Initial Moderate to
Severe Crowding >3.5 mm)
Variables
T1
Control
N=11
T2
T3
IMPA
95.09±7.76 95.77±8.22 95.95±7.68
T1= pretreatment; T2= post-treatment; T3= post-retention.
*Statistically significant (p < 0.05).
Interproximally Stripped Sample
N=15
T1
T2
T3
93.33±6.24
96.67±5.13* 96.82±5.44
Table 8: Cephalometric Measurement Statistics for Pooled Groups
Variables
T1
Control
N=29
T2
T3
IMPA
96.48±6.25 97.07±6.13 97.21±5.94
T1= pretreatment; T2= post-treatment; T3= post-retention.
*Statistically significant (p < 0.05).
38
Interproximally Stripped Sample
N=26
T1
T2
T3
95.38±6.91
97.28±5.73* 97.66±5.69
Discussion
Dental Cast Analysis
Irregularity index
When analyzing the data for irregularity, it was noted that two subjects in the
sample group (IPR) had extreme values of irregularity. Due to the extremity of values,
these two subjects were dropped from statistical analysis for irregularity index; thus, the
sample size is reduced to 24 for the IPR group for the irregularity index values. The
control group had no outliers, so all 29 subjects remained in the analysis.
In this study, both control and sample groups demonstrated a significant reduction
in mandibular incisor irregularity after treatment; however, both groups displayed an
increase in irregularity post-retention. The mean percentage of relapse for the control
group (no IPR) was 20.23% and for the sample group (IPR), it was 18.45% (Table 9).
Although the control group had a larger percentage of relapse post-retention, it was not
statistically significant.
The post-retention irregularity increased 0.52mm for the control group, and
0.67mm for the sample group. Both of these amounts are smaller than the results of
previous studies. Little, Wallen, and Riedel18 found an increase of 2.9 mm in irregularity
between post-treatment and post-retention, while Freitas et al.21 found an increase of 1.08
mm after retention. When the control and sample groups were subdivided into groups,
using the criteria established by Little,17 according to the amount of irregularity present at
pretreatment (T1), Group A (initial mild irregularity) demonstrated a mean of 36.72%
relapse for the control group, while the sample group had a mean percentage of relapse of
42.51% (Table 10). The slightly higher percentages of relapse are due to the fact that the
39
Table 9: Irregularity Index (II) at each stage of treatment, changes in irregularity
during treatment (II.T2-II.T3), amount of relapse (II.T3-II.T2), and percent relapse
for pooled groups
Variables
Control
N=29
Interproximally Stripped Sample
N=24*
Irregularity Index T1
3.45
4.46
Irregularity Index T2
0.88
0.83
Irregularity Index T3
1.40
1.50
II.T2-II.T1
-2.57
-3.63
II.T3-II.T2
0.52
0.67
Percent relapse
20.23
18.45
T1= pretreatment; T2= post-treatment; T3= post-retention.
*Two subjects were dropped from the sample due to extreme values for irregularity index.
Table 10: Irregularity Index (II) at each stage of treatment, changes in irregularity
during treatment (II.T2-II.T3), amount of relapse (II.T3-II.T2), and percent relapse
for subdivided groups A and B
Variables
Control
<3.5 mm
>3.5 mm
Irregularity
Irregularity
N = 18
N = 11
Interproximally Stripped Sample
<3.5 mm
>3.5 mm
Irregularity
Irregularity
N = 11
N = 15
Irregularity Index T1
2.12
5.64
2.44
7.03
Irregularity Index T2
0.84
0.94
0.77
0.97
Irregularity Index T3
1.31
1.54
1.48
1.57
II.T2-II.T1
-1.28
-4.70
-1.67
-6.06
II.T3-II.T2
0.47
0.60
0.71
0.60
Percent relapse
36.72
12.76
42.51
9.90
T1= pretreatment; T2= post-treatment; T3= post-retention.
Group A = Initial mild crowding <3.5 mm, Group B = Initial moderate to severe crowding >3.5 mm.
40
initial irregularity was mild, thus even small amounts of relapse create high percentages
of relapse. In Group B (initial moderate to severe irregularity), the mean percentage of
relapse was 12.76% for the control group, and only 9.90% for the sample group;
however, this difference was not statistically significant.
Overall, both groups demonstrated a statistically significant net decrease in
irregularity from pretreatment to post-retention. The control group reduced their
irregularity by 2.05 mm, and the sample group’s irregularity decreased by 2.96 mm.
Both groups ended up with minimal irregularity according to Little.17
Intercanine width
In the study, both the control and sample groups showed a small increase in
intercanine width at the end of treatment, but they also demonstrated a decrease after
treatment. Only the sample (IPR) group’s intercanine width demonstrated a statistically
significant increase at post-treatment; however, this group’s post-retention width
remained slightly larger than it’s initial value. The statistically significant increase in
intercanine width in the sample group was likely due the fact that this group started out
with a higher irregularity index, and in order to accommodate this irregularity, the canine
width was expanded. The control (no IPR) group’s post-retention mean intercanine
width showed a statistically significant decrease from post-treatment, to a value slightly
lower than it’s initial value. This data supports the findings of other research which
argues that increases in intercanine width are often unstable, and that long-term changes
result in relapse toward the original dimensions.18,19,22
41
Overbite
The mean overbite decreased significantly for both groups during the period of
active treatment. Then the control group showed a significant increase in overbite during
the post-retention period. The sample group demonstrated a small, but not statistically
significant increase in overbite following treatment. Compared to the pretreatment
values, there was a significant net reduction in overbite several years out of retention for
both groups of patients.
Overjet
During treatment, overjet decreased significantly in both control and sample
groups. Both groups then showed a statistically significant increase in overjet during
retention. But overall there remained significant decreases in net overjet between
pretreatment and post-retention measurements in both groups.
Anterior Bolton ratio
The mean anterior Bolton ratio for the control group was 78.74%, which is 1.54%
higher than the accepted normal value. However, it is within the standard deviation of
the accepted normal value of 77.2% with a standard deviation of 1.65.12 This indicates
that the anterior teeth were highly balanced in size in this group of patients, thus
confirming the diagnosis that no interproximal reduction was needed. The mean anterior
Bolton ratio for the sample group at pretreatment was 80.32%, which is more than one
standard deviation away from the accepted normal value. This value might have been
one of the reasons the practitioner decided to do interproximal reduction in this group of
patients, indicating selection bias.
42
The interproximally stripped sample group showed a significant change in the
anterior Bolton ratio. The mean anterior Bolton analysis showed a 5.09% decrease from
the T1 value of 80.32% to the mean T3 value of 75.23%. When comparing these values
to the normal value of 77.2%, the results of this study illustrate a change from a slight
mandibular anterior excess to a very slight maxillary anterior tooth-size excess.
Nonetheless, the final Bolton ratio was closer to ideal than the initial ratio.
Cephalometric Analysis
The SN-MP angles recorded at the start of treatment showed that the sample of
patients in this study were similar in relation to direction of growth. The mean SN-MP
angle for the control group was 29.31° and for the sample group it was 29.71°,
demonstrating a high degree of similarity. Both values are also close to the normal value
of 32°.23
For both the control and sample groups, the IMPA values remained fairly constant
throughout treatment and retention. There was an increase in mean IMPA during
treatment for both groups; however, only the stripped group showed a statistically
significant increase of 1.9°. Since the intercanine width remained fairly constant, the
mandibular incisors might have advanced forward slightly more in the stripped group
than in the control group due to the slightly higher initial irregularity in the stripped
group. Both groups also demonstrated a small, but insignificant increase in IMPA during
the post-retention period, similar to the findings of Sadowsky et al.24 This is different
from the findings in most literature, which support the idea that incisors often relapse
toward their initial position after non-extraction treatment.25,26 Although the IMPA
increased slightly at T3, the differences were minimal. The negligible change from post-
43
treatment to post-retention indicates that the incisor positions were stable for both groups
of patients.
Comparison of Present Study with Other Selected Studies
Most studies on the long-term stability of mandibular incisors of orthodontically
treated patients have reported that over half of the initial mandibular incisor irregularity
reoccurs after treatment,18,19,27 while some studies reported less than 50% relapse at postretention.14,21,22,28 Few long-term studies have examined dentitions treated with
interproximal enamel reduction. Boese14 observed forty patients with crowded
mandibular arches which were orthodontically treated by extraction of premolars but
never retained. All patients had circumferential supracrestal fiberotomies and
interproximal enamel reduction performed. The sample had a mean irregularity index of
9.18 at pretreatment, and this value decreased to 0.62 at 4-9 years post-treatment. Boese
concluded that increased stability can be achieved through interproximal reduction and
fiberotomies, even without retention. Another study by Sadowsky et al.24 found good
alignment of the mandibular incisors at the post-retention phase in a sample of 22 patients
treated nonextraction with prolonged retention. The initial average irregularity was
5.2 mm. Light interproximal enamel reduction was done at the time of removal of the
mandibular fixed lingual retainer. The post-retention records were taken at least 5 years
after the removal of the lingual retainer, and the mean irregularity was 2.4 mm. They
concluded that the prolonged retention may play an important role in maintaining
mandibular incisor stability. Freitas et al.21 observed forty patients treated nonextraction,
who had fixed lower retention for an average of 2 years. Sixteen of the forty patients
received interproximal reduction to correct crowding and interarch size discrepancies.
44
The patients presented with a mean irregularity index of 5.46 and were found to have a
mean irregularity index of 1.95 at 5 years post-retention. The present study found similar
results when compared to the studies performed by Sadowsky et al.24 and Freitas et al.21;
however, the results were not as favorable as those found by Boese.14 The mandibular
anterior alignment in the control and sample groups both showed remarkable stability
when compared to the samples in other studies at post-retention (Table 11).
Table 11: Comparison of Present Study with Other Selected Studies
Mean Irregularity Index (mm)
Authors
Pretreatment
Post-treatment
Post-retention
Boese14 **
Little et al.18
Sadowsky et al.24 *
Glenn et al.19
Little et al.27
Freitas et al.21 *
Present
No IPR
IPR*
9.2
7.3
5.2
2.9
7.4
5.5
NA
1.7
1.0
1.0
1.7
0.9
0.6
4.6
2.4
2.2
5.3
2.0
3.5
4.5
0.9
0.8
1.4
1.5
*Interproximal enamel reduction performed on some or all patients.
**Interproximal enamel reduction and circumferential fiberotomies performed on all patients.
The long-term effects of interproximal enamel reduction warrants further
investigation. A limitation of this study is that selection bias might have existed. The
increases in intercanine width and IMPA at the end of treatment in the sample (IPR)
group could have been reasons why interproximal reduction was executed in this group.
The slightly higher irregularity index and anterior Bolton ratio in this group at
pretreatment might have been additional diagnostic characteristics considered when
45
deciding to do interproximal stripping. It would be ideal to eliminate selection bias in
future studies. Also, a larger number of patients should be selected to allow the control
and sample groups to be further divided into larger groups by amount of initial
irregularity, so that significant results can be produced when comparing the subgroups.
From the present findings, it appears that interproximal reduction produces no significant
added benefit in reducing the amount of irregularity that occurs during post-retention. On
the other hand, since the patients in the sample group initially had a higher irregularity
index, it might have slightly reduced the amount of relapse experienced if reduction had
not been performed.
Although many studies exist on the long-term stability of mandibular incisors, no
treatment has been found to prevent relapse. Late mandibular crowding is a natural
occurrence that cannot be avoided without permanent retention.3 The irregularity indices
at post-retention in the present study indicate that irregularity tends to recur in both nonstripped and stripped groups. The control group (no stripping) and the sample group, that
underwent interproximal reduction, each had 18-20% relapse of initial crowding.
46
Conclusions
The purpose of the present study was to determine if interproximal reduction
increases long-term stability of the mandibular incisors in Class I and II malocclusions.
A sample group of 26 orthodontically treated patients having had 2 mm or more of
interproximal reduction in the mandibular anterior region were compared to a control
group of 29 orthodontically treated patients having had no interproximal reduction. The
control and sample groups were evaluated long-term and the following conclusions were
reached:
1. Mandibular incisor relapse occurred in both the control and sample groups,
but the post-retention irregularity index remained minimal according to
Little’s classifications.
2. In both the control and sample groups, the IMPA values remained fairly
constant throughout treatment and retention, indicating stability of the
mandibular incisors.
3. Interproximal enamel reduction does not appear to alter the long-term stability
of mandibular incisors.
47
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49
Vita Auctoris
Julie Jefferson Rambo was born on November 22, 1978, in Lexington, Kentucky.
She graduated as valedictorian from Tates Creek High School in 1996. She then attended
Transylvania University and graduated summa cum laude, receiving a Bachelor of Arts
degree in Biology in May 2000. Julie entered the University of Kentucky College of
Dentistry in the fall of 2000, and graduated with highest honors, receiving her Doctor of
Dental Medicine degree in the spring of 2004. She was accepted into the Saint Louis
University Orthodontic residency program during her last year of dental school and
started the program in the summer of 2004. Julie married Scott Alexander Vinton on
June 18, 2005. Dr. Rambo is currently a third year orthodontic resident and will graduate
with a Master of Science degree in Dentistry in January 2007. She and her husband plan
to relocate to their home state of Kentucky, where she will join a private practice in
Lexington, Kentucky.
50