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THE EFFECT OF ANTEROPOSTERIOR MOVEMENT OF THE
MAXILLARY DENTITION ON FACIAL HEIGHT
Arthur A. Jones, D.M.D.
A Thesis Presented to the Graduate Faculty of
Saint Louis University in Partial Fulfillment
of the Requirements for the Degree of
Master of Science in Dentistry
2015
COMMITTEE IN CHARGE OF CANDIDACY:
Professor Rolf G. Behrents,
Chairperson and Advisor
Professor Eustaquio A. Araujo
Professor Emeritus Lysle E. Johnston Jr.
Associate Clinical Professor Donald R. Oliver
i
Dedication
To my wife, Ashley, for her patience and understanding
during the many months I devoted to this project, and for
her love and support throughout this entire residency
experience.
To my parents, whom I credit for every one of my
achievements in life.
I would not be the person I am today
without their unwavering love and support.
ii
Acknowledgement
I
wish
to
express
my
sincere
gratitude
to
the
following individuals:
Dr. Behrents, for his guidance throughout my time in
residency and his time spent on this project;
Dr. Lysle E. Johnston Jr., for his assistance with the
structuring of this project and his time spent on
corrections;
Dr. Araujo, for his valuable input on the project
design and time spent reviewing corrections;
Dr. Oliver, for your incredible attention to detail
and time spent on corrections;
Dr. Kevin Walde, for providing the sample used in this
thesis; and
Dr. Heidi Israel, for her patience and assistance with
the statistics for this study.
iii
Table of Contents
List of Tables.............................................v
List of Figures...........................................vi
CHAPTER 1: INTRODUCTION....................................1
CHAPTER 2: REVIEW OF THE LITERATURE........................2
Contemporary Reasons for Extractions.............2
The Wedge Hypothesis.............................3
Vertical Changes in Untreated Subjects...........5
Vertical Changes During Treatment................7
Extractions.................................7
Cervical Pull Headgear.....................13
Intraoral Molar Distalizers................16
Distalization vs Upper Premolar Extraction.18
Summary and Statement of Thesis.................19
Literature Cited................................22
CHAPTER 3: JOURNAL ARTICLE................................26
Abstract........................................26
Introduction....................................28
Literature Review..........................28
Materials and Methods...........................32
Sample.....................................32
Cephalometric Analysis.....................34
Error Study................................40
Statistical Analysis.......................40
Results.........................................42
Discussion......................................48
Limitations................................48
Effects of Treatment.......................49
Molar Displacement.........................50
Mandibular Rotation........................51
Vertical Changes in Menton.................53
Summary and Conclusions.........................55
Literature Cited................................57
Vita Auctoris.............................................61
iv
List of Tables
Table 2.1:
Studies reporting differential vertical effects
resulting from extraction treatment...........12
Table 2.2:
Studies reporting vertical changes due to
cervical pull headgear treatment..............15
Table 2.3:
Studies analyzing vertical effects resulting
from molar distalization......................17
Table 3.1:
Comparison of T1 measurements between groups..42
Table 3.2:
Descriptive statistics for T1, T2, and T2-T1
changes for Frog group........................43
Table 3.3:
Descriptive statistics for T1, T2, and T2-T1
changes for U4s extraction group..............44
Table 3.4:
Comparison of T2 measurements between groups..45
Table 3.5:
Comparison of T2-T1 changes between groups....47
Table 3.6:
Mandibular basal bone rotation compared to the
literature....................................52
v
List of Figures
Figure 3.1:
Diagram of the Frog appliance................33
Figure 3.2:
Vertical measurements relative to SN-7°......36
Figure 3.3:
AP measurements relative to SN-7°
perpendicular................................36
Figure 3.4:
Mandibular basal bone rotation relative to
SN-7°........................................38
Figure 3.5:
Movement of U6 relative to maxillary basal
bone, and rotation of mandibular basal bone
relative to maxilla..........................39
Figure 3.6:
Movement of L6 relative to mandibular basal
bone.........................................40
vi
CHAPTER 1: INTRODUCTION
Many factors are considered in the decision to extract
or not extract in an orthodontic treatment plan.
Among
these is a belief that facial vertical dimension may be
altered by this decision.
This belief has come to be known
as the “wedge” hypothesis.
This concept encompasses supp-
osed effects resulting from distal molar movement, as well
as mesial molar movement.
Accordingly, this literature
review will attempt to clarify the nature of changes in
facial vertical measurements resulting from treatments that
cause anteroposterior (AP) molar movement within “the
wedge.”
It will begin with a brief review of contemporary
reasons for extraction and an introduction to the wedge
hypothesis.
A necessary discussion of vertical growth in
untreated subjects will lead into a progression through the
literature reporting vertical effects resulting from
extraction treatment, cervical pull headgear, and various
intraoral distalizers.
In the final section, the short
list of studies encompassing upper premolar extraction and
distalization treatment will be discussed.
1
CHAPTER 2: REVIEW OF THE LITERATURE
Contemporary Reasons For Extraction
In his article titled “To extract or not extract: A
diagnostic decision, not a marketing decision,” S. Jack
Burrow wrote: “Evidence-based treatment is certainly the
direction that modern orthodontics should be taking.
But
unfortunately much orthodontic research evidence is being
ignored because of profit considerations, ease of treatment, and marketing efforts.”1
Clinicians take many factors
into account when deciding to extract or not extract in a
given case, but true to Burrow’s statement, many of the
current trends in nonextraction treatment seem to be the
result of marketing efforts focused on increasing profits
and not necessarily doing what is best for the patient.
Numerous personal beliefs and biases unique to each
clinician also likely play a role this decision.
Some of
these beliefs are based on sound evidence, whereas others
may simply be based on personal opinions or anecdotal evidence.
It is necessary to examine these beliefs to deter-
mine what warrants consideration in the decision to extract
or not extract.
A 1996 study conducted by Baumrind and coworkers2
offers a somewhat current view of orthodontists’ reasons
for tooth extraction.
The records of 72 subjects were
2
examined and treatment planned by a panel of five experienced clinicians.
Reasons for their decision to extract or
not extract in each case were recorded and ranked for importance.
Crowding was cited as the most important factor
favoring extractions, followed closely by incisor protrusion, desire to improve profile, severity of Class II or
large anteroposterior (AP) discrepancy, and stability.
One
of these five factors was considered as the main reason for
extraction in 84% of cases.
These five factors can be viewed as interconnected
entities in orthodontic treatment planning, and the benefits of extraction treatment for their correction is well
documented in the literature.
However, another factor,
“closing the bite,” was cited as a reason for extraction in
7% of the cases in the study.
For many clinicians, the so-
called “wedge effect” is responsible for this bite closure
following extractions.
The Wedge Hypothesis
The wedge hypothesis is based on an idea that the
anatomical relationship of the maxilla to the mandible is
wedged apart to a greater or lesser extent by anteroposterior (AP) movement of the interposed dentition.
Basic
geometry would suggest that anterior movement of the
3
dentition would result in retraction of the wedge, and
conversely, posterior movement would cause a greater
insertion of the wedge.
Although the term “wedge” was not used, Schudy is
often referenced as one of the first to describe a disproportionate effect of molar extrusion on anterior facial
height and overbite.
His papers in the 1960s focused on
the delicate balance of condylar growth and molar eruption
as the critical factor controlling vertical and horizontal
chin position.3 According to Schudy, when treating hyperdivergent patients, “we must not contribute toward a greater
open bite by moving the molars occlusally and/or distally,”
and concerning hypodivergent patients, “maxillary molars
should be moved distally as much as possible.”4
He also
strongly favored extraction treatment in hyperdivergent
phenotypes and nonextraction in hypodivergent phenotypes.
Other prominent authors5 have agreed with this philosophy.
These seminal papers laid the groundwork for a belief
that changes in vertical dimension could be dictated by the
decision to extract or not extract.
With the birth of a
plausible hypothesis, the supposed wedge effect from AP
molar movement went seemingly unquestioned.
Any appliance
that moved molars distally was immediately deemed to
4
increase facial height, while extraction was automatically
deemed to decrease facial height.
Vertical Changes in Untreated Subjects
Before exploring the effect of orthodontic treatment
on vertical dimension, it is necessary to have an understanding of the normal changes in facial height due to growth
alone.
Among the changes in the facial dimensions from
childhood into adulthood, changes in the vertical dimension
are perhaps the most significant.
The rate of vertical
growth peaks during the adolescent growth spurt, but
continues at a lower level throughout adulthood.6
Maxillary vertical changes result mainly from sutural
lowering along with an increase in height of the alveolar
processes.
Osseous modeling tends to be resorptive on the
nasal floor and depository on the palatal surface, also
contributing to the lowering of the palatal plane.
The
mandibular condyle is the chief site of vertical growth in
the mandible, with the height of the ramus expected to
increase 1 mm to 2 mm each year from childhood to
adolescence.7
Significant rotational changes are also occurring in
each jaw at the same time.
Björk and Skieller’s8 implant
studies in the 1960s showed that overall mandibular changes
5
result from rotation of basal bone relative to the cranial
base, along with concurrent remodeling of the exterior
surfaces.
This distinction between basal, or core rotation, in
contrast with apparent rotation of external surfaces is an
important concept when evaluating changes.
Although, on
average, from childhood through adolescence the mandibular
basal bone rotates 15 degrees forward, backward and
compensatory external remodeling of around 11-12 degrees
results in an apparent decrease of the mandibular plane
(MP) of only 2 to 4 degrees.
Björk and Skieller’s9,10
implant studies also showed that the maxillary basal bone
undergoes significant rotation during growth as well, but
compensatory remodeling tends to equal the internal rotation.
This balance usually results in maintenance of the
palatal plane angle relative to the cranial base.
Compens-
atory tooth eruption serving to maintain occlusal contacts
also occurs during this vertical and rotational growth.
Deviations from the normal internal or external
remodeling patterns in either jaw can result in abnormal
vertical facial proportionality.
Although somewhat rare,
backward rotation of the mandibular basal bone in conjunction with backwards surface remodeling is said to result in
6
a hyperdivergent phenotype with increased vertical
proportions.7
A longitudinal study published in 1974 by Riolo et
al.11 at the University of Michigan followed numerous
cephalometric measures from six to sixteen years of age
from an untreated sample of 47 males and 36 females.
Lin-
ear measures of total facial height (TFH) and lower anterior facial height (LAFH) consisted of the distances from
nasion to menton (Na-Me), and anterior nasal spine to menton (ANS-Me), respectively.
In males, over the entire age
range, mean yearly increases of TFH and LAFH were 3.0 mm,
and 1.8 mm, respectively.
In females, over the entire age
range, mean yearly increases of TFH and LAFH were 1.8 mm,
and 0.8 mm, respectively.
Vertical Changes During Treatment
Extractions
The reported effects of extractions on facial height
in the literature are mixed, with no solid evidence to support a consistent effect.
In 1990, Garlington and Logan12
studied the effect of enucleation of mandibular second
premolars in hyperdivergent subjects.
Using Björk and
Skieller’s9,10 method of structural superimposition, they
observed a forward rotation of the mandible in 17 of the 23
7
subjects.
When the LAFH measurements were compared to
hyperdivergent norms described by Isaacson and co-workers13
the treatment had resulted in a statistically significant
decrease in LAFH.
In an attempt to control for growth as a confounding
factor, Chua et al.14 used data from the Michigan growth
standards to convert pre-treatment and post-treatment LAFH
measurements to standardized Z scores for 174 Class I and
Class II subjects.
Changes in Z scores for LAFH were
tested for significant differences between a four premolar
extraction group and a nonextraction group.
The nonextr-
action group exhibited a significant increase in LAFH,
while the extraction group exhibited no significant change.
The mechanics used in the nonextraction group were poorly
defined, however, and actual molar movements were not
measured in either group.
Kocadereli15 compared two groups of 40 Class I subjects
treated either nonextraction or with four premolar extractions.
The upper first molar (U6) movement relative to PTV
was also measured.
Following extraction, the U6 came for-
ward 3 mm more than in the nonextraction group.
Interest-
ingly, even with this significant difference in anterior
molar movement, no vertical differences were found to be
statistically significant between the groups.
8
A consensus in orthodontics is that more anchorage
loss can be expected with second premolar extraction when
compared to first premolar extraction, suggesting more of
an effect on vertical dimension.
Kim et al.16 compared four
first premolar extraction cases to four second premolar
extraction cases as a test of the theory that greater anchorage loss in the second premolar extraction cases should
have a more significant effect on vertical dimension.
Importantly, arch length discrepancies were matched between
the groups to allow for a meaningful comparison of molar
movements, and no extraoral anchorage, elastics, expansion
devices, or transpalatal arches were used during treatment.
No significant between-groups differences were found when
the linear changes in facial vertical dimension were compared.
There were significant differences in the maxilloma-
ndibular angle (MMA) and SN to palatal plane angle (SN-PP),
but these differences amounted to roughly 1°.
In clinical
practice, changes of this magnitude can be considered
insignificant, and perhaps cannot be distinguished from
routine tracing error.
If vertical changes due to the wedge effect following
extractions are difficult to demonstrate in growing patients, studying adults with minimal growth potential provides an opportunity to elucidate such an effect.
9
Ramesh
and others17 examined 45 adult subjects with SN-GoGn measures of >32.
The subjects were treated with upper and
lower first premolar extractions.
They found no signif-
icant change in vertical facial dimensions.
The upper and
lower molars came forward 2.3 mm, and 2.2 mm, respectively.
The upper molar also extruded 2.2 mm; however, a movement
that may have negated any potential vertical effects
resulting from anterior molar movement.
With the exception of Garlington and Logan,12 several
of the previous studies suffer from the inadequacy of relying on surface landmarks to detect mandibular rotational
The studies using Björk and Skieller’s9,10 method
changes.
of structural superimposition may paint a clearer picture
of the actual changes occurring in each jaw during
treatment.
A recent study from 2013 conducted by Bayirli et al.18
used this method to compare four premolar extraction treatment to a matched group of untreated controls.
Maxillary
and mandibular core rotations relative to the cranial base
were measured.
There was essentially no rotation of the
mandibular core relative to the cranial base in the treatment group, whereas in the untreated controls roughly 1° of
forward rotation was observed.
In 2015, Charalampakis19
conducted a similar comparison of four first premolar
10
extractions, four second premolar extractions, and nonextraction treatment by way of structural superimposition and
a cartesian grid.
No significant differences in vertical
changes were found between the groups.
Although a clinically significant wedge effect should
be immediately evident at the completion of treatment, data
is conflicting regarding changes in facial height in longterm post-treatment comparisons of subjects treated with or
without extractions.
Interestingly, in a study of 63 bord-
erline subjects with a mean post-treatment follow up of
14.5 years, Paquette and coworkers20 found that the overall
changes in LAFH during the observation period were significantly less in subjects treated with extractions than those
treated without extractions; however, the changes during
treatment alone were not significantly different.
A simi-
lar study of 62 extraction and nonextraction subjects by
Luppanapornlarp and Johnston,21 with a mean post-treatment
follow up time of 15.3 years, showed no significant differrences in vertical changes during treatment or in the posttreatment period.
Table 2.1 summarizes the literature reporting
differential vertical effects between extraction treatment
and nonextraction treatment.
11
Table 2.1. Studies reporting differential vertical effects
resulting from extraction treatment.
Author
Year
Sample(s)
Garlington &
Logan12
1990
23 U4 L5
Paquette et
al.20
1992
30 non-ext
Differential
Vertical
Effect?
Yes
Yes
33 U4 L4
Luppanapornlarp
& Johnston21
1993
29 non-ext
No
33 extraction*
Chua et al.14
1993
89 non-ext
Yes
89 extraction*
Kocadereli15
1999
40 non-ext
No
40 U4 L4
Kim et al.16
2005
27 U4 L4
No
27 U5 L5
Kumari & Fida22
2010
41 non-ext
No
40 U4 L4
Ramesh et al.17
2012
45 U4 L4
Bayirli et al.18
2013
36 untreated
No
Yes
36 U4 L4
Charalampakis19
2015
67 non-ext
No
61 U4 L4
63 U5 L5
non-ext = non-extraction, U4 = upper first premolars, L4 =
lower first premolars, U5 = upper second premolars, L5 = lower
second premolars, *extraction pattern not specified
12
In summary, few studies have been able to document a
significant effect of extraction treatment on vertical
dimension or mandibular rotation.
While the previously
mentioned studies of extraction treatment focus on the
effects of relative anterior movement of the dentition, the
belief that molar distalization “into the wedge” would lead
to increased facial height was also embedded into the minds
of many.
Cervical Pull Headgear
If moving molars distally and occlusally caused opening of the mandibular plane, the Kloehn type cervical pull
headgear was seen as the perfect appliance for such an
effect.
In a large sample of Class II subjects treated in
the mixed dentition with partial fixed appliances and headgear, Baumrind et al.23 found that the rate of increase in
LAFH was 1.5 times that of untreated controls.
The authors
suggested that this treatment modality should be avoided
when an increase in LAFH is undesirable.
However, in an
even larger study of 200 cases, Boecler et al.24 found that
mean differences in vertical changes are negligible among
groups treated with cervical headgear, combination headgear, or no extraoral force at all.
Despite limited support for an effect on facial
height, the cervical pull headgear controversy still cont-
13
inued to resurface.
A prospective randomized clinical
trial in 1998 by Ghafari et al.25 comparing cervical headgear to the functional regulator provided some interesting
data relative to the wedge hypothesis.
In the headgear
group the mandibular plane rotated backwards 1° relative to
the functional regulator group, a difference not considered
clinically significant.
The interesting finding, however,
was that the palatal plane to mandibular plane angle (PPMP) decreased even with this small amount of backward mandibular rotation.
This finding is precisely opposite the
expected increase of this angle in a true wedge effect
Other recent studies26,27
resulting from distalization.
also fail to find significant differences in facial height
following the use of cervical pull headgear.
Once again, using the structural method of superimposition would provide a better picture of the actual changes occurring during treatment.
Haralabakis28 used this
method to measure differences in actual mandibular rotation
between high and low angle patients treated with cervical
pull headgear.
Interestingly, both high and low angle
groups still showed a marked counterclockwise rotation of
the mandible relative to SN, with a difference between them
of only .86°.
This result represented another finding that
counters the expected wedge effect.
14
The study by Baumrind et al.23 in 1981 seemed to offer
some solid evidence supporting a wedge effect following the
use of cervical pull headgear, but it was followed by less
convincing evidence in the years after.
Table 2.2
summarizes studies reporting changes in facial height or
mandibular rotation due to treatment with cervical pull
headgear.
Table 2.2 Studies reporting vertical changes due to
cervical pull headgear treatment.
Author
Year
Baumrind et
al.23
1981
Sample(s)
74 Cervical
Differential
Vertical
Effect?
Yes
50 Untreated
Boecler et al.24
1989
89 Cervical
No
33 No Headgear
Ghafari et al.25
1998
35 Headgear
No
28 Functional
Regulator
Alvanos26
2004
30 High angle
No
30 Low angle
Haralabakis &
Sifakakis28
2004
31 High angle
No
29 Low angle
Freitas et al.27
2008
25 Cervical
16 Untreated
15
No
Intraoral Molar Distalizing Appliances
Numerous intraoral distalizing appliances have been
developed over the years.
Almost universally, some variat-
ion of a Nance button with wire attachments bonded to premolars serves as the anchorage unit, with active components
ranging from nickel titanium coil springs, to larger diameter spring extensions embedded in the acrylic button.
Given the similarity of effects of headgear and intraoral distalizers on molar position,29 both appliances would
be expected to have similar effects on vertical dimension.
Of importance is the fact that the majority of studies on
intraoral distalizers are conducted immediately after the
distalization phase, not after the completion of treatment.
Many of these studies30-32 attribute significant changes in
lower facial height or mandibular plane angle to driving
molars distally “into the wedge,” although no comparisons
were made to untreated controls.
When data is examined after completion of comprehensive treatment any significant differences in vertical
dimension might be lost.
Table 2.3 displays the data from
studies of intraoral distalizers reporting changes in LAFH
or MP after the completion of treatment with full fixed
appliances.
16
Table 2.3 Studies analyzing vertical effects resulting from
molar distalization.
Author(s)
Chiu et al.33
Sample
Appliance
N=32
Pendulum
Molar
Movement
(mm)
-0.5
N=22
Pendulum
N=32
N=43
ΔMP
(°)
ΔLAFH
(mm)
0.9
4.4
0.1
0.4
3.2
Distal Jet
0.5
0.1
3.9
Bone
-1.9
0.3
..
2005
Angelieri et
al.34 2006
Chiu et al.33
2005
Hourfar et al.35
2014
anchored
Frog
The changes shown in table 2.3 following comprehensive
treatment are unimpressive if one assumes an average treatment time of 2.5 years.
Extrapolation of mean yearly incr-
eases in LAFH of 1-2 mm from the Michigan growth standards11
provides relative values in untreated controls.
Reported
changes in MP are also very similar to those of untreated
controls.
Also important to note in table 2.1, are the
modest overall changes in molar position after comprehensive treatment.
Anchorage loss after distalization is
frequently observed in the next phase of treatment, leaving
17
final molar positions very similar to the pre-treatment
position.34
Importantly, none of the previously mentioned
studies used structural superimposition to measure changes
in mandibular core rotation, all relied on changes in
surface landmarks.
Distalization vs. Upper Premolar Extraction
All of the previously mentioned studies present a gap
in the literature when testing the wedge hypothesis.
Comp-
arisons of different extraction patterns, or different
distalization appliances alone explore only that respective
aspect of the wedge hypothesis.
Comparison of upper first
premolar extraction treatment with distalization, however,
simultaneously explores both aspects of the wedge hypothesis and could elucidate any differences in vertical dimension due to AP movement of the dentition.
Comparisons of
this nature are relatively scarce in the literature.
In
2009, de Almeida-Pedrin et al.36 compared the pendulum appliance to cervical headgear, and maxillary premolar extractions.
The study failed to find any significant differ-
ences between the changes in SN-GoGn or LAFH in any of the
groups.
Two recent master’s theses37,38 compared groups
treated with upper premolar extraction to the distal jet
appliance.
When the changes in SN-GoGn were compared bet-
ween the groups, no statistically significant differences
18
were found.
A significant weakness is that none of these
studies employed structural superimposition to evaluate
mandibular rotation.
Summary and Statement of Thesis
If a clinician is faced with the dilemma of choosing
between molar distalization or and extraction of upper
first premolars, is it valid to base the decision on the
belief that the two treatments lead to different vertical
effects?
Despite attempts to clarify changes in vertical dimension resulting from nonextraction or extraction treatment,
the literature thus far is inconsistent with regard to a
clinically significant wedge effect.
Evidence thus far
does not support a decrease in vertical dimension resulting
from anterior movement of posterior teeth during extraction
treatment, although some studies do show differential changes between nonextraction treatment and extraction treatment.
Support for an increase in vertical dimension or
mandibular plane angle following molar distalization “into
the wedge” is also inconsistent.
Some studies with data
immediately following distalization report an effect on
vertical dimension, with overall treatment changes
19
appearing very similar to those due to normal vertical
growth alone.
A weakness in all of the previously mentioned studies
is the fact that the focus is on just one aspect of the
wedge hypothesis, such as studying varying amounts of distalization or mesial molar displacement alone.
A study
encompassing both aspects of the wedge hypothesis will
provide a clearer picture of differences if the effect can
be demonstrated.
Starting with similar pre-treatment cond-
itions, one treatment should lead to relative distal movement of the upper first molar, and the other should lead to
relative anterior movement of the upper first molar.
This
study will explore the possibilities of increasing or
decreasing vertical dimension as a result of the treatment
decision.
The lack of studies comparing rotational changes with
structural superimposition also presents a significant gap
in the literature when distalization is compared to upper
premolar extraction treatment.
Structural superimposition
will be used to obtain valid measures of actual mandibular
basal bone rotation relative to the maxilla and SN-7°.
Superimposition will also provide a clear picture of molar
movements relative to each respective jaw.
Vertical and
horizontal reference planes will also be established to
20
allow measurement of components of change in various
surface landmarks.
21
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22
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2013;144:682-90.
19. Charalampakis V. Effects of extraction vs nonextraction
orthodontic treatment on vertical dental and skeletal
measurements. Master’s Thesis, Saint Louis University,
2015.
20. Paquette DE, Beattie JR, Johnston LE, Jr. A long-term
comparison of nonextraction and premolar extraction
edgewise therapy in "borderline" Class II patients. Am
J Orthod Dentofac Orthop. 1992;102:1-14.
21. Luppanapornlarp S, Johnston LE, Jr. The effects of
premolar-extraction: a long-term comparison of outcomes
in "clear-cut" extraction and nonextraction Class II
patients. Angle Orthod. 1993;63:257-72.
23
22. Kumari M, Fida M. Vertical facial and dental arch
dimensional changes in extraction vs. non-extraction
orthodontic treatment. J Coll Physicians Surg Pak.
2010;20:17-21.
23. Baumrind S, Korn EL, Molthen R, West EE. Changes in
facial dimensions associated with the use of forces to
retract the maxilla. Am J Orthod. 1981;80:17-30.
24. Boecler PR, Riolo ML, Keeling SD, TenHave TR. Skeletal
changes associated with extraoral appliance therapy: an
evaluation of 200 consecutively treated cases. Angle
Orthod. 1989;59:263-70.
25. Ghafari J, Shofer FS, Jacobsson-Hunt U, Markowitz DL,
Laster LL. Headgear versus function regulator in the
early treatment of Class II, Division 1 malocclusion: a
randomized clinical trial. Am J Orthod Dentofac Orthop.
1998;113:51-61.
26. Alvanos A. A study of the effect of rapid maxillary
expansion and cervical pull headgear on patients having
Class II malocclusion with low or high mandibular plane
angles. Master’s Thesis, Saint Louis University, 2004.
27. Freitas MR, Lima DV, Freitas KMS, Janson G, Henriques
JFC. Cephalometric evaluation of Class II malocclusion
treatment with cervical headgear and mandibular fixed
appliances. Eur J Orthod. 2008;30:477-82.
28. Haralabakis NB, Sifakakis IB. The effect of cervical
headgear on patients with high or low mandibular plane
angles and the "myth" of posterior mandibular rotation.
Am J Orthod Dentofac Orthop. 2004;126:310-17.
29. Taner TU, Yukay F, Pehlivanoglu M, Çakırer B. A
comparative analysis of maxillary tooth movement
produced by cervical headgear and pend-x appliance.
Angle Orthod. 2003;73:686-91.
30. Chaques-Asensi J, Kalra V. Effects of the pendulum
appliance on the dentofacial complex. J Clin Orthod.
2001;35:254–7.
24
31. Ghosh J, Nanda RS. Evaluation of an intraoral maxillary
molar distalization technique. Am J Orthod Dentofac
Orthop. 1996;110:639–46.
32. Kircelli BH, Pektas ZO, Kircelli C. Maxillary molar
distalization with a bone-anchored pendulum appliance.
Angle Orthod. 2006;76:650-9.
33. Chiu PP, McNamara Jr JA , Franchi L. A comparison of
two intraoral molar distalization appliances: distal
jet versus pendulum. Am J Orthod Dentofac Orthop.
2005;128:353–65.
34. Angelieri F, Almeida RR, Almeida MR, Fuziy A.
Dentoalveolar and skeletal changes associated with the
pendulum appliance followed by fixed orthodontic
treatment. Am J Orthod Dentofac Orthop. 2006;129:520-7.
35. Hourfar J, Ludwig B, Kanavakis G.. An active, skeletally
anchored transpalatal appliance for derotation,
distalization and vertical control of maxillary first
molars. J Orthod 2014;41:S24-32.
36. De Almeida-Pedrin RR, Henriques JF, de Almeida RR, de
Almeida MR, McNamara JA Jr. Effects of the pendulum
appliance, cervical headgear, and 2 premolar
extractions followed by fixed appliances in patients
with Class II malocclusion. Am J Orthod Dentofac
Orthop. 2009;136:833-42.
37. Fotakido E. Comparison of soft tissue and dentoalveolar
effects of the tad-supported distal jet versus upper
premolar extractions. Master’s Thesis, Saint Louis
University, 2015.
38. Kaplan NL. Comparison of effects of the distal Jet
appliance, upper premolar extraction and headgear in
patients with Class II malocclusion. Master’s Thesis,
Saint Louis University, 2011.
25
CHAPTER 3: JOURNAL ARTICLE
Abstract
Purpose:
This investigation seeks to determine if antero-
posterior movement of the maxillary dentition during
orthodontic treatment affects facial vertical changes by
nature of a “wedge effect.”
Materials and Methods:
A
sample of 28 Class II subjects treated with maxillary first
premolar extraction (U4s) were compared to a sample of 28
Class II subjects treated with the SMD (Frog) molar
distalizer using a retrospective study design.
The pre-
treatment and post-treatment lateral cephalograms for each
subject were hand traced.
Measurements of landmark
positions were made from established vertical and
horizontal reference planes.
Structural superimposition
was also used to measure AP and vertical molar movements
within each jaw, as well as actual mandibular core rotation
relative to the cranial base and maxilla.
Results:
When
measured from maxillary superimposition, upper premolar
extraction treatment resulted in a mean mesial movement of
the U6 of 3.31 mm, and distalization with the Frog resulted
in a mean distal movement of the U6 of -0.79 mm.
The U6
extruded 1 mm more in the U4s group than the Frog group.
Despite the statistically significantly different changes
in molar position, independent t-tests revealed no
26
significant differences in facial vertical measurements or
mandibular rotation between the two groups.
Small forward
rotation of mandibular basal bone was noted in both
treatment groups.
Conclusions:
Differences in molar
extrusion weaken the validity of the test, but likely
represent an inherent barrier to achieving a clinically
significant wedge effect.
This study does not support the
idea that anteroposterior movement of molars within “the
wedge” causes changes in facial vertical dimension.
27
Introduction
Many factors are considered in the decision to treat
with or without extractions.
For many clinicians, a belief
in the so-called “wedge hypothesis” is an appealing concept
that may tip the scales toward extraction or nonextraction.
The wedge like anatomical relationship of the maxilla and
mandible in the lateral view forms the basis for this hypothesis.
Basic geometry would suggest that AP movement of
the dentition within this dental configuration should alter
lower anterior facial height (LAFH) through hinging of the
mandible.
This hypothesis most often surfaces when consid-
ering extractions in hopes of controlling, or decreasing
vertical dimension, although it is also appealing when trying to increase facial height through molar distalization
in hypodivergent patients.
Literature Review
Among the changes occurring in the facial structures
during childhood and adolescence, those in the vertical
dimension are some of the most significant.
These changes
are largely the result of a balance between sutural growth
of the maxilla and growth of the mandibular condyle.1
Comp-
lex rotational changes, masked to some extent by external
remodeling also occur in each jaw during this time.2-4
28
The
usual pattern of mandibular rotation is forward, or counterclockwise, in relation to the maxilla and cranial base,
but compensatory remodeling of the external processes can
significantly alter the appearance of the changes.
Alth-
ough the rate of change is greatest in childhood and adolescence, vertical facial growth continues at a lower rate
throughout life with slight differences in directional
tendencies existing between males and females.5
The extent that orthodontic treatment can alter these
vertical changes has long been debated.
An ability to ach-
ieve predictable and stable alterations in vertical dimension or mandibular rotation through nonsurgical treatment
modalities would be an invaluable skill for orthodontists,
as mandibular rotation impacts facial esthetics by changing
the vertical and horizontal position of the chin.
AP occl-
usal relationships are also directly affected by this
rotation.
Schudy is often referenced as one of the first to describe a disproportionate effect of molar movement on incisor relationships.
In the 1960s, Schudy6,7 suggested avoi-
ding occlusal or distal movement of molars in open bite
cases, and encouraged it in hypodivergent patients.
Other
prominent authors8 have agreed with this concept, recommending extractions in high angle cases and nonextraction
29
treatment in low angle cases.
These influential papers
laid the groundwork for a belief that vertical dimension
can be altered by the decision whether or not to extract.
Some studies9,10 suggest that vertical dimension is
increased with nonextraction treatment relative to extraction treatment.
Others11-14 fail to find significant diff-
erences in vertical changes over the course of the two
treatments.
Although a wedge effect should be evident
immediately post-treatment, studies of long-term differences after extraction and nonextraction therapy are inconclusive as well.
One study15 found that in the overall
observation period changes in LAFH were significantly less
in subjects treated with extractions, while another16 showed
no significant differences in vertical changes during a
similar period of observation.
By the nature of a wedge effect, virtually any appliance causing distal molar movement was automatically
assumed to cause “clockwise” rotation of the mandible.
The
Kloehn type headgear, with its posterior and inferior direction of pull, was one of the first modalities to be implicated in causing a wedge effect.
Early studies17,18 reported
significant increases in vertical dimension with cervical
traction, but more recent studies19-22 fail to support this
effect.
30
The majority of studies on intraoral distalizing
appliances focus on data immediately after the distalization phase,23-25 attributing increases in LAFH and mandibular
plane (MP) to distalization “into the wedge.”
However,
studies reporting changes after comprehensive treatment
with fixed appliances26-28 show overall vertical changes that
are comparable to those due to normal growth alone.
Studies in Class II subjects comparing molar distalization to upper premolar extraction treatment are relativeely scarce in the literature, however, they provide an opportunity to contrast the supposed effects resulting from
anterior or posterior movement of the dentition.
The eff-
ectiveness of modern intraoral distalizing appliances now
allows for nonextraction treatment of AP discrepancies that
at one time almost certainly required extractions.
This
option often creates a dilemma when cases fall into a gray
area where distalization and extraction of upper premolars
are competing treatment options.
For some clinicians, dev-
iations in LAFH or MP may tip the scales to favor one or
the other.
One study29 compared the changes in SN-GoGn and LAFH
among groups of Class II subjects treated with headgear,
distal jet, and upper first premolar extraction and found
no significant differences between the groups.
31
Two similar
investigations30,31 reporting only SN-GoGn as a vertical
indicator also failed to find significant differences between the two treatments.
Importantly, none of these stu-
dies used structural superimposition to determine the
actual rotation of the mandibular basal bone relative to
the cranial base or maxilla.
The intent of this study was to compare molar distalization treatment to upper first premolar extraction treatment, with the goal of elucidating differential effects on
facial vertical dimension due to AP movement of the maxillary dentition.
To the author’s knowledge, few studies
have used structural superimposition to compare upper
premolar extraction treatment to molar distalization.
The
null hypothesis of the study was that there are no
differences in facial vertical changes or mandibular
rotation between Class II subjects treated with molar
distalization or extraction of upper first premolars.
Materials and Methods
Sample
The data for this retrospective study were obtained
from 112 lateral cephalograms from 56 subjects divided into
2 groups based on treatment.
For the molar distalization
group, the tooth-borne Simplified Molar Distalizer (SMD),
32
or “Frog”, was selected somewhat arbitrarily from the
numerous available appliance designs.
Developed by Walde,32
this appliance shares the same Nance button and bonded occlusal rests as other distalizers, but incorporates a sagittally oriented jackscrew connected to a transpalatal arch
as the active element.
A diagram of the Frog appliance can
be seen below in figure 3.1.
Figure 3.1. Diagram of the Frog appliance (photo used by
permission of Dynaflex)
The 28 subjects for the SMD (Frog group) treatment
group were obtained from the private office of a single
clinician.
The 28 subjects in the upper first premolar
extraction (U4s group) treatment group were obtained from
the archives of Saint Louis University Center for Advanced
Dental Education.
33
Subjects were initially selected based on the following inclusion criteria:
1. adolescent 2. Caucasian
3. bilateral Class II molar relationship 4. no expansion
devices 5. no congenitally missing teeth, and 6. diagnostic
quality pre (T1) and post (T2) treatment lateral cephalograms.
The subjects in the 2 groups were selected to
match pre-treatment age and sex as closely as possible.
Each group consisted of 14 males and 14 females, with a
mean starting age of 11 years and 9 months in the Frog
group, and 12 years and 3 months in the U4s group.
The
mean treatment duration in each group was 2 years and
11 months.
Cephalometric Analysis
Pre-treatment (T1) and post-treatment (T2) digital
lateral cephalograms for each subject in the Frog group
were printed on high quality paper and hand-traced on acetate paper.
For the U4s group, pre-treatment and post-
treatment lateral cephalograms were hand-traced on acetate
paper.
Magnification values for all cephalograms were
known, allowing for adjustment of all measurements to zero
magnification.
The first part of the analysis involved the construction of vertical and horizontal reference planes for each
tracing.
The horizontal reference plane, used for vertical
34
measurements, was established at the level of nasion using
the averaged sella-nasion line minus seven degrees (SN-7°).
The vertical reference plane, used for AP measurements, was
established perpendicular to SN-7° at the anterior-most
point on the wall of sella turcica.
Reference planes were
first constructed on the pre-treatment radiograph, then
transferred to the post-treatment radiograph after cranial
base superimposition based on stable structures decribed by
Björk and Skieller.2
Five vertical landmarks (posterior
nasal spine [PNS], anterior nasal spine [ANS], upper molar
cusp tip [UMT], lower molar cusp tip [LMT], and menton
[Me]) were identified for vertical linear measurements.
Four landmarks (subspinale [A], upper molar mesial contact
[UMC], lower molar mesial contact [LMC], and supramentale
[B]) were identified for AP measurements relative to the
SN-7° perpendicular.
Vertical and AP measurements were
made to various landmarks as shown in figures 3.2 and 3.3.
35
90°
7°
N
S
ANS
PNS
UMT
LMT
Me
Figure 3.2. Vertical measurements relative to SN-7°.
90°
N
S
A
UMC
LMC
B
Figure 3.3. AP measurements relative to SN-7°
perpendicular.
36
7°
The palatal plane was established using a line through
the landmarks ANS and PNS.
The mandibular plane (MP) was
drawn as a tangent to the lower border of the mandible
through Me.
The angular relationship formed by the palatal
plane and mandibular plane formed the palatal plane to
mandibular plane angle (PP-MP°).
The mandibular plane
angle was measured relative to the SN-7° reference line.
Structural superimposition as described by Björk and
Skieller2-4 was also used to measure AP and vertical
movements of molars relative to each respective jaw, as
well as actual mandibular basal bone rotation relative to
the cranial base and maxilla.
Superimposing on stable
structures in the cranial base allows for measurement of
the angular change in mandibular fiducial lines, revealing
the basal bone rotation (Δ Mand fiducial).
Figure 3.4
illustrates the measurement of mandibular basal bone
rotation relative to the cranial base.
37
Pre-treatment
Post-treatment
Δ Mand. fiducial
Figure 3.4. Mandibular basal bone rotation relative to
cranial base.
The maxillary superimposition was used to measure the
angular change in mandibular fiducial lines relative to the
maxilla (ΔMand/Max fiducial).
Forward, or counterclockwise
rotations were assigned a negative value, and backward, or
clockwise rotations were assigned positive values.
The landmarks UMT and LMT were used in the measurement
of vertical displacements of the upper first molar (U6) and
lower first molar (L6) to each respective jaw.
Vertical
displacements of the U6 and L6 were measured perpendicular
to the palatal and mandibular planes, respectively.
Extru-
sive displacements were represented by positive values,
while intrusive displacements were represented by negative
38
values.
The landmarks UMC and LMC were used for the
measurement of AP displacements of the U6 and L6 relative
to each respective jaw.
AP molar movements were measured
parallel to the mean functional occlusal plane (MFOP), as
described by Johnston.33
Anterior, or mesial molar displac-
ements, were represented by positive values, while posterior, or distal displacements, were represented by negative
values.
Figures 3.5 and 3.6 illustrate these measurements
made by way of the structural superimposition technique.
U6 V
U6 AP
Pre-treatment
Post-treatment
Δ Mand./Max.
fiducial
Figure 3.5. Movement of U6 relative to maxillary basal
bone, and rotation of mandibular basal bone relative to
maxilla.
39
L6 AP
L6 V
Pre-treatment
Post-treatment
Figure 3.6. Movement of L6 relative to mandibular basal
bone.
Error Study
Ten cephalograms were randomly selected for retracing
for a reliability analysis based on Cronbach’s alpha.
The
lowest alpha value obtained was .92 for U6 vertical displacement, indicating a high degree of reliability.
Statistical Analysis
All statistical analysis was performed with the
Statistical Package for the Social Science (IBM SPSS,
Version 2.0, Armonk, NY).
Descriptive data were calculated
for T1 and T2 measurements in each group, and then compared
at each time point with independent samples t-tests to
detect T1 and T2 differences.
Mean changes during treat-
40
ment (T2-T1) within each group were compared using paired
t-tests.
Independent samples t-tests were used to detect
between group differences in mean treatment changes (T2T1).
A significance level of p <.05 was set for all
statistical analyses.
41
Results
Descriptive statistics for T1 measurements are shown
in table 3.2.
At T1, statistically significant differences
were found in the AP position of A and overjet (OJ).
In
the U4s group, the mean T1 position of A point was 2.56 mm
anteriorly displaced relative to the Frog group.
The mean
OJ in the Frog group was 3.19 mm, in contrast to 6.95 mm in
the U4s group.
Table 3.1. Comparison of T1 measurements between groups.
Measure
Frog
U4s
P
Mean
SD
Mean
SD
OJ-Overjet (mm)
3.19
1.80
6.95
2.41
<.001
MP (°)
26.59
5.40
24.53
5.54
.166
PP-MP (°)
25.05
4.92
24.70
5.54
.800
V-ANS (mm)
49.46
3.89
48.03
2.76
.118
V-PNS (mm)
47.84
2.90
47.85
2.52
.992
V-Me (mm)
105.07
6.64
105.10
4.90
.984
V-UMT (mm)
67.44
4.03
67.86
3.28
.668
V-LMT (mm)
67.27
3.97
67.31
3.23
.972
AP-A (mm)
58.57
4.53
61.13
4.70
.043
AP-UMC (mm)
35.09
4.90
36.78
4.82
.197
AP-LMC (mm)
33.18
4.80
33.69
4.68
.687
AP-B (mm)
50.16
5.91
51.77
6.07
.317
Descriptive statistics for T1 and T2 measurements,
along with the results of paired t-tests for T1 and T2
42
measurements within each group are shown in tables 3.2 and
3.3.
Table 3.2. Descriptive statistics for T1, T2, and T2-T1
changes for Frog group.
Measure
T1
Mean
SD
T2
Mean
SD
T2-T1
Mean
SD
MP (°)
26.59
5.40
26.21
6.17
-0.37
2.89
.498
PP-MP (°)
25.05
4.92
24.45
6.05
-0.61
3.31
.341
V-ANS (mm)
49.46
3.89
51.60
3.88
2.14
3.13
.001
V-PNS (mm)
47.84
2.90
49.87
3.73
2.03
2.32
<.001
V-Me (mm)
105.07
6.64
111.04
8.78
5.97
4.68
<.000
V-UMT (mm)
67.44
4.03
70.80
5.46
3.36
3.20
<.000
V-LMT (mm)
67.27
3.97
71.03
5.69
3.76
3.59
<.000
AP-A (mm)
58.57
4.53
60.26
5.22
1.69
3.35
.013
AP-UMC (mm)
35.09
4.90
36.13
5.38
1.04
3.44
.121
AP-LMC (mm)
33.18
4.80
37.05
5.97
3.88
3.94
<.001
AP-B (mm)
50.16
5.91
52.43
7.36
2.28
4.89
.020
U6AP (mm)
Structural Superimposition Method
..
..
..
..
-0.79 1.68
P
..
U6V (mm)
..
..
..
..
1.36
1.13
..
L6AP (mm)
..
..
..
..
1.54
1.24
..
L6V (mm)
..
..
..
..
1.93
1.59
..
ΔMand.
fiducial(°)
..
..
..
..
-1.46
3.76
..
ΔMand./Max.
fiducial
(°)
..
..
..
..
-0.89
3.33
..
43
Table 3.3. Descriptive statistics for T1, T2, and T2-T1
changes for U4s extraction group.
Measure
T1
Mean
SD
T2
Mean
SD
T2-T1
Mean
SD
MP (°)
24.54
5.54
24.48
5.94
-0.05
2.10
.894
PP-MP (°)
24.70
5.54
23.98
6.91
-0.71
2.55
.149
V-ANS (mm)
48.03
2.76
50.66
3.11
2.64
2.70
<.001
V-PNS (mm)
47.85
2.52
49.87
1.80
2.02
2.26
<.001
105.10 4.90 111.48
6.76
6.38
4.81
<.001
V-UMT (mm)
67.86
3.28
72.37
3.69
4.51
3.03
<.001
V-LMT (mm)
67.31
3.23
71.72
3.52
4.42
2.94
<.001
AP-A (mm)
61.13
4.70
60.68
5.03
-0.45
2.99
.433
AP-UMC (mm)
36.78
4.82
41.12
6.58
4.34
2.99
<.001
AP-LMC (mm)
33.69
4.68
36.33
6.22
2.64
3.24
<.001
AP-B (mm)
51.77
6.07
52.40
7.00
0.63
2.70
.230
U6AP (mm)
Structural Superimposition Method
..
..
..
..
3.31
1.67
..
V-Me (mm)
P
U6V (mm)
..
..
..
..
2.27
1.73
..
L6AP (mm)
..
..
..
..
1.51
1.26
..
L6V (mm)
..
..
..
..
2.88
2.22
..
ΔMand.
fiducial(°)
..
..
..
..
-0.48
2.80
..
ΔMand./Max.
fiducial
(°)
..
..
..
..
-2.05
3.20
..
In the Frog group, no statistically significant
changes occurred in MP° or PP-MP° during treatment.
Stati-
stically significant changes occurred in the positions of
all vertical landmarks, as well as AP landmarks, with the
exception of AP-UMC.
In the U4s group, no statistically
44
significant changes occurred in MP° or PP-MP°.
Stat-
istically significant changes occurred in the positions of
all vertical landmarks.
Among AP measurements, statis-
tically significant changes occurred in AP-UMC, AP-LMC.
No
significant changes occurred in AP-A or AP-B in the U4s
group.
A between group comparison of T2 final measurements
relative to vertical and horizontal reference planes, seen
in table 3.4, revealed a statistically significant difference only in the AP position of UMC.
Table 3.4. Comparison of T2 measurements between groups.
Measure
Frog
U4s
P
Mean
SD
Mean
SD
MP (°)
26.21
6.17
24.48
5.94
.289
PP-MP (°)
24.45
6.05
23.98
6.91
.790
V-ANS (mm)
51.60
3.88
50.66
3.11
.323
V-PNS (mm)
49.87
3.73
49.87
1.80
.992
V-Me (mm)
111.04
8.78
111.48
6.76
.834
V-UMT (mm)
70.80
5.46
72.37
3.69
.211
V-LMT (mm)
71.03
5.69
71.72
3.52
.588
AP-A (mm)
60.26
5.22
60.68
5.03
.764
AP-UMC (mm)
36.13
5.38
41.12
6.58
.003
AP-LMC (mm)
37.05
5.97
36.33
6.22
.661
AP-B (mm)
52.43
7.36
52.40
7.00
.986
45
Descriptive statistics along with the results of
independent samples t-tests of changes brought about from
growth and treatment between groups are shown in table 3.5.
Comparison of changes relative to reference planes revealed
statistically significant differences between the Frog
group and U4s group only in the AP changes of A (p<.014)
and UMC (p<.001).
T2-T1 changes measured by structural
superimposition revealed statistically significant differences between groups in the AP displacement of the U6
(p<.001).
The U6 moved distally 0.79 mm relative to the
maxilla in the Frog group, and moved mesially 3.31 mm
relative to the maxilla in the U4s group.
A statistically
significant difference in the vertical displacement of the
U6 was also noted between the groups when measured with
maxillary superimposition (p<.023).
In the Frog group, the
U6 exhibited a mean extrusion of 1.36 mm, while in the U4s
group the U6 exhibited a mean extrusion of 2.27 mm.
Struc-
tural superimposition revealed no statistically significant
differences in the rotation of the mandibular fiducial line
relative to the maxilla or SN-7°.
46
Table 3.5. Comparison of T2-T1 changes between groups.
Measure
Frog
U4s
P
Mean
SD
Mean
SD
MP (°)
-0.37
2.89
-.054
2.10
.636
PP-MP (°)
-0.61
3.31
-0.71
2.55
.893
V-ANS (mm)
2.14
3.13
2.64
2.70
.531
V-PNS (mm)
2.03
2.32
2.02
2.26
.980
V-Me (mm)
5.97
4.68
6.38
4.81
.748
V-UMT (mm)
3.36
3.20
4.51
3.03
.173
V-LMT (mm)
3.76
3.59
4.42
2.94
.458
AP-A (mm)
1.69
3.35
-0.45
2.99
.015
AP-UMC (mm)
1.04
3.44
4.34
2.99
<.001
AP-LMC (mm)
3.88
3.94
2.64
3.24
.207
AP-B (mm)
2.28
4.89
0.63
2.70
.124
Structural Superimposition Method
U6AP (mm)
-0.79
1.68
3.31
1.67
<.001
U6V (mm)
1.36
1.13
2.27
1.73
.024
L6AP (mm)
1.54
1.24
1.51
1.26
.912
L6V (mm)
1.93
1.59
2.88
2.22
.071
ΔMand.
fiducial(°)
ΔMand./Max.
fiducial (°)
-1.46
3.76
-0.48
2.80
.273
-0.89
3.33
-2.05
3.20
.189
47
Discussion
Limitations
For a valid comparison of vertical changes resulting
solely from anteroposterior displacement of the upper
molars, vertical molar displacements should be equal in
each group.
In this regard, structural superimposition
revealed a limitation of the study, with a statistically
significant difference in U6 extrusion noted between
groups.
In the U4s group, the U6 extruded 1 mm more than
in the Frog group.
It is possible that in the U4s group
the increased extrusion during mesial displacement of the
U6 negated potential vertical effects.
This extrusion is
in agreement with other studies13 of upper premolar extraction, and may represent an inherent barrier to causing a
clinically significant wedge effect during protraction of
the dentition.
While the sample treated with the Frog was obtained
from a single experienced clinician, the U4s sample was
obtained from a mix of several clinicians and a variety of
treatment mechanics may have been used in the cases.
This
heterogeneity in mechanics is considered a weakness in some
study designs; however, the purpose of this study was to
focus on the skeletal response to defined molar movements
and not the mechanics used to achieve them.
48
Some argue that a wedge effect only happens in hyperdivergent phenotypes with high mandibular plane angles.
In
consideration, it is important to note that the Frog group
(mean, 26.59°; SD, 5.40°) and U4s group (mean, 24.53°; SD,
5.54°) were characterized as having average mandibular
plane angles as a whole, but were distributed over a fairly
wide range.
Studies of cervical pull headgear in subjects
with high mandibular plane angles22 have, thus far, failed
to show significant differences when compared to low
angles, but perhaps future studies could compare upper
premolar extraction treatment to molar distalization in
subjects with high mandibular plane angles.
Effects of Treatment
Comparison of T2 final measurements of both groups
revealed strikingly similar positions of all landmarks,
with the only statistically significant difference being in
the AP position of UMC.
The AP positional difference of
UMC at T2 between groups amounted to 5 mm, roughly the
difference between the expected Class I molar relationship
following successful distalization, and the full step Class
II molar relationship following the extraction of upper
first premolars.
49
Given the increased protrusion and OJ at T1 in the U4s
group, the significant difference in the change of point A
relative to the Frog group was likely due to greater
anterior retraction requirements and subsequent remodeling
of cortical bone around incisor apices.34,35
A study by de
Almeida-Pedrin and coworkers29 found that A point moved 2 mm
posteriorly in the group treated with upper first premolar
extractions. The insignificant change in B point in the U4s
group was an unexpected finding, however, and may have
represented a difference in mandibular growth in this
group.
Molar Displacement
By nature of the study design, AP changes of the U6
relative to both the maxilla, and horizontal reference
planes were significantly different between the groups.
Relative to the maxillary regional superimposition, the U6
in the Frog group exhibited a mean distal displacement of
0.79 mm, while the U4s group exhibited a mesial displacement of 3.31 mm.
The distal displacement noted with the
tooth-borne frog appliance used in this study compares
favorably to other studies of tooth-borne distalizers.26,27,31
The small net anterior displacement of UMC (1.04 mm)
measured relative to the vertical reference plane in the
50
Frog group was likely due to a combination of forward
maxillary growth and a loss of anchorage after
distalization.27
The mesial molar displacement of 3.31 mm in the U4s
treatment group measured by regional superimposition
compares favorably with the findings of Luecke and Johnston,36 who reported a mean mesial displacement of 3.2 mm.
This mesial displacement of the upper first molar falls in
between that reported following the extraction of four
first premolars and four second premolars in other
studies.14,37
Mandibular Rotation
Importantly, in both treatment groups the mandibular
plane exhibited small mean forward rotations relative to
SN-7°, as well as the palatal plane.
This apparent rota-
tion of the mandibular plane compares favorably to the findings of Kaplan,31 as well as de-Almeida-Pedrin and coworkers.29
Fotakido30 found small backwards rotations in SN-
GoGn (U4s= 0.03°, Distal Jet= 0.91°), although the differences in these changes relative to those of the present
study are clinically insignificant.
These insignificant
changes of the mandibular plane suggest the decision bet-
51
ween these two treatments has little affect on mandibular
rotation.
The forward rotational pattern of the mandibular basal
bone observed in this study also seems consistent in the
literature and appears to be unaffected by different treatment modalities.
Table 3.6 compares the mandibular basal
bone rotation observed in the present study to other literature employing structural superimposition to evaluate
the effect of various treatment modalities.
Table 3.6. Mandibular basal bone rotation compared to the
literature.
Year
N
Present Study
2015
28
U4 extract
28
Distalization
13
U4 extract
-0.5
13
Untreated
-1.0
31
Cervical HG*
-1.3
29
Cervical HG**
-2.4
67
Nonext.
-0.22
61
4 extract
-0.53
63
5 extract
0.06
Meral et al.38
N. Haralabakis
& Sifakakis22
Charalampakis14
2004
2004
2015
Treatment
Mandibular
Basal
Rotation (°)
-0.48
Author
* = FMA> 28°, ** = FMA< 22°
52
-1.46
Vertical Changes in Menton
For an analysis of vertical changes in menton, age at
start, treatment duration, and method of measurement must
be taken into account.
The study by Charalampakis14 was
most similar in design to the present study, however, treatment duration was shorter and the subjects were older at
the start of treatment.
Vertical changes in Me ranged from
5.04 mm to 5.18 mm in the study by Charalampakis.
Consid-
ering the longer duration of treatment in the present
study, vertical changes in Me of 5.97 mm and 6.38 mm can be
considered comparable.
A study by Standerwick39 in 2014 provides some perspective on the vertical changes in menton observed in this
study relative to untreated controls.
Using data from the
Michigan growth standards,40 Standerwick converted the traditional measurements of LAFH and AFH into vertical vectors
relative to an SN-7° reference plane.
The yearly vertical
component of change in pogonion during puberty was 2.8 mm
in males, and 1.5 mm in females.
Averaging these values to
represent the equal distribution of males and females in
the present study, followed by multiplication of the mean
treatment time (2.9 yrs), gives an expected vertical change
of 6.2 mm relative to SN-7°.
This value is strikingly
53
similar to the changes (Frog= 5.97 mm, U4s= 6.38 mm)
observed in this study.
In consideration of these results, we were unable to
reject the null hypothesis that there are no differences in
facial vertical changes or mandibular rotation between
Class II subjects treated with molar distalization or
extraction of upper first premolars.
54
Summary and Conclusions
Differential vertical effects were not observed
between two treatments featuring different AP molar movements.
The effects on vertical dimension and mandibular
rotation were similar with molar distalization and the
extraction of maxillary first premolars.
Although the goal was to test the wedge hypothesis,
differences in molar extrusion noted between the groups
weakened the validity of the test.
However, the mechanics
used in each group are likely representative of those commonly used in clinical practice, exposing an inherent barrier to achieving a clinically significant wedge effect.
While pure anterior translation could theoretically result
in a wedge effect, it is highly unlikely that such an
effect would be clinically achievable on a consistent
basis.
In addition, any actual wedge effect is likely
masked by measurement error, dentoalveolar compensations,
and normal variations in facial growth.
This study does not lend support to the idea that
changes in vertical dimension are dictated by the decision
to distalize molars or extract upper first premolars in
Class II patients.
It appears that various orthodontic
treatments have little impact on facial growth tendencies,
55
and factors other than vertical dimension should guide the
decision to pursue extractions or molar distalization.
56
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Vita Auctoris
Arthur Alvin Jones IV was born on June 16, 1982 in
Mobile, Alabama.
He received his D.M.D degree from the
University of Alabama at Birmingham School of Dentistry in
May of 2013.
In June of 2013 he began his post-graduate
training in orthodontics at Saint Louis University, in
Saint Louis, Missouri.
He is currently a candidate for the
degree of Master of Science in Dentistry.
61