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Slow maxillary expansion for the treatment of unilateral crossbite in pre-adolescents: a long-term retrospective study of the changes in arch dimension

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Slow maxillary expansion for the treatment of unilateral crossbite in pre-adolescents: a long-term retrospective study of the changes in arch dimension

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Content SLOW MAXILLARY EXPANSION FOR THE TREATMENT OF UNILATERAL
CROSSBITE IN PRE-ADOLESCENTS: A LONG-TERM RETROSPECTIVE STUDY
OF THE CHANGES IN ARCH DIMENSION

by

Christian Alexander Wong

________________________________________________________________

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

May 2008

Copyright 2008 Christian Alexander Wong

ii
DEDICATION

To my Family and Friends:
Benjamin Wong
Brenda Wong
Melanie Johnston
Todd Johnston
Olivia Johnston
Bethina Abrahams
Adrian Ching
Peter Buckley

iii
ACKNOWLEDGEMENTS

A special thank you to:

Dr. Peter Sinclair

Dr. David Kennedy

Dr. Robert Keim

Dr. Reyes Enciso

Dr. Holly Moon

Ellen Grady

Michelle Berry

USC Graduate Orthodontic Clinical Faculty

USC Graduate Orthodontic Staff

USC Graduate Co-Residents

iv
TABLE OF CONTENTS

Dedication ii
Acknowledgements iii
List of Tables v
List of Figures vi
Abstract vii
Chapter 1: Introduction 1
Chapter 2: Review of Literature 4
Chapter 3: Hypotheses 18
Chapter 4: Materials and Methods 22
Chapter 5: Results 29
Chapter 6: Discussion 39
Chapter 7: Assumptions 45
Chapter 8: Limitations 46
Chapter 9: Summary 47
Chapter 10: Conclusions 48
Bibliography 49

v
LIST OF TABLES

Table 1 - Comparison of Experimental Groups at T1 29

Table 2 - Comparison of Pooled Test Groups (TG) 30
and the Control Group (CG) at T1

Table 3 - Comparison of Experimental Groups at T2-T1 31

Table 4 - Comparison of Experimental Groups at T2 31

Table 5 - Comparison of the Changes T2-T1 between 32
the Treated (TG) and Control (CG) Group

Table 6 - Comparison of Pooled Test Groups (TG) and 33
the Control Group (CG) at T2

Table 7 - Comparison of Experimental Groups at T2-T3 34

Table 8 - Comparison of the Changes T2-T3 between 35
the Treated (TG) and Control (CG) Group

Table 9 - Comparison of Experimental Groups at T1-T3 36

Table 10 - Comparison of the Changes T3-T1 between 36
the Treated (TG) and Control (CG) Group

Table 11 - Comparison of Experimental Groups at T3 37

Table 12 - Comparison of Pooled Test Groups (TG) 38
and the Control Group (CG) at T3

vi
LIST OF FIGURES

Figure 1 – Early Expander by Angell 11
Figure 2 – The Haas Expander 12
Figure 3 – The Hyrax Expander 13
Figure 4 – The Quad Helix Expander 13
Figure 5 – Skeletal Effects of Maxillary Expansion 15
Figure 6 – Centroid 22
Figure 7 – Arch Perimeter 27
Figure 8 – Arch Length 27
Figure 9 – Measurement of Molar Angulation 28

vii
ABSTRACT

The purpose of this study was to evaluate the treatment changes and post-treatment
stability in dental arch dimensions in unilateral crossbite patients treated by SME while
comparing the Haas, Hyrax, and Quad Helix appliances. Serial dental casts of 110
patients were evaluated at 3 time points: pre-expansion (T1), post-expansion (T2), and
approximately 4 years later, before fixed appliance therapy (T3). All measurements were
compared with published growth norms. Treatment by SME resulted in similarly
favorable amounts of expansion by all three expanders in all measurements for both
arches. Maxillary intercanine and intermolar width increased by 4.5 mm and 3.5 mm,
respectively, from T1 to T3. Maxillary arch circumference also increased by 1 mm.
Mandibular width did not change. At 4 years post-expansion, 97% of intercanine
expansion and 80% of intermolar expansion was maintained.

1
CHAPTER 1: INTRODUCTION

Posterior crossbite (PXB) exists when there is an abnormal buccal-lingual relationship
between opposing molars and premolars in centric occlusion, often due to a constricted
maxilla.
1
The prevalence of PXB ranges from 8% to 23% in primary and mixed
dentition
2-4
; in most cases, the malocclusion is bilateral (in centric relation), and the child
will frequently shift the mandible to one side resulting in a unilateral presentation (90%)
in centric occlusion.
4-6

If left untreated, posterior crossbites could lead to facial asymmetries and irregular
condylar growth patterns in adults.
7,8
Additional studies have even suggested a possible
link with PXBs and temporomandibular joint (TMJ) symptoms.
9,10

A number of early treatments for posterior crossbite have been used, with expansion of
the maxilla the most common. The primary goal of such treatment is to widen the
constricted palate; however, additional benefits can be gained, such as increased space in
the dental arches to alleviate crowding, and increased space in the nasal passages to
alleviate breathing difficulties. For these reasons, PXB is usually an indication for early
orthodontic treatment.

The most effective time for maxillary expansion is during the pre-pubertal or pubertal
stage, before interdigitation of the mid-palatal suture and other circummaxillary sutures
have occurred.
11,12
After puberty, greater forces are required to separate these sutures;
2
eventually, surgical intervention is necessary to “unlock” the sutures and widen the
palate.
11,13

Rapid maxillary expansion (RME) is the most common treatment modality used to widen
the palate in growing patients when transverse deficiency exists. RME utilizes a
maxillary appliance with an embedded jackscrew turned twice daily producing at
separation of 0.5 mm (1 turn = 0.25 mm), while delivering a cumulative force of 100 N
(20 pounds) across the mid-palatal suture.
1,11
In theory, RPE occurs rapidly enough so
that primarily skeletal separation of the palate occurs, with little or no dental expansion or
tipping. However, some studies have shown that the force levels used in RME result in
micro-fractures of the mid-palatal suture, microtrauma to the temporomandibular joint,
significant dental relapse, and external root resorption.
14-17

In light of these complications, slow maxillary expansion (SME) has been advocated by
some investigators since it is believed to result in more healthy physiological
response.
11,18-20
SME involves only 5N to 20N (1 to 4 pounds) of force, resulting from 1
turn per second day for the Hyrax and Haas appliances, and one molar width of
expansion for the Quad helix appliance.
21

Animal studies have suggested that SME is effective at maintaining sutural integrity
during expansion and at producing a more stable result than RME.
19,22
Clinical studies
have also supported this, although these studies were done using small sample sizes in the
3
short-term.
13,20,23
For this reason, long term clinical studies with greater samples are
necessary to validate its use.

The changes in arch dimensions due to expansion have also been investigated. Most
studies, however, suffer from problems such as small sample sizes, bias, confounding
variables, lack of method analysis, lack of long-term data, blinding in measurements,
deficient statistical methods, and a lack of controls. In his meta-analysis review,
Lagravere
24,25
searched the literature and found that only 4 articles met his inclusion
criteria: three involving RME and one involving SME. Although he saw significant
increases in expansion with RME, all studies involved the use of fixed appliances post
expansion. Therefore, the effects of pure expansion alone on arch dimensions could not
be determined.

For this reason, there is a need for a clinical SME study with a large sample size to
evaluate the effects of expansion alone, without supplementary appliances, while taking
growth, age, gender and appliance design into consideration. Here, we plan to conduct a
retrospective clinical study to evaluate short and long-term effects of SME on arch
dimensions in primary or early mixed dentition patients exhibiting a unilateral posterior
crossbite.

4
CHAPTER 2: REVIEW OF THE LITERATURE

Development and Etiology of Posterior Crossbites

Posterior crossbite (PXB) is one of the most frequently observed malocclusions of the
deciduous and mixed dentition periods. It is defined as an abnormal buccal-lingual
relation between opposing molars, premolars or both in centric occlusion.
26
Posterior
crossbites can be bilateral or unilateral. A bilateral crossbite exists when this abnormal
relation exists on both sides of the arch; a unilateral crossbite exists when this abnormal
relation exists on one side of the arch.

The reported incidence of all types of posterior crossbites combined ranges from 8-23%
of the population.
3,27-30
The wide variation may be explained by the type of appliance
used, the length the follow-up period, different populations, and the author’s definition
for successful correction of crossbite.

The most common form of posterior crossbite is a unilateral presentation with a
functional shift of the mandible toward the crossbite side - this occurs in 80-97% of the
posterior crossbite cases.
3,28,30
Here, the patient will habitually shift the mandible to one
side, referred to as a functional shift, and the relation presents as a unilateral discrepancy
in centric occlusion (CO), but with a symmetric maxilla and a bilateral presentation in
centric relation (CR).

5
The prevalence of unilateral PXBs with a functional mandibular shift is 8.4% at the
deciduous dentition stage and to 7.2% at the mixed dentition stage.
3
This decrease
suggests that a spontaneous self-correction in the deciduous dentition can occur, although
there are conflicting results concerning the degree of self-correction. Most studies have
shown that the frequency of spontaneous self correction rages from 0-9%.
3,30
At the same
time, PXBs can occur spontaneously from the primary to mixed dentition at a rate of
7%.
30

The etiology of posterior crossbite can be attributed to one or a combination of dental,
skeletal, and neuromuscular components. Most commonly, a small maxilla to mandible
width ratio exists, arising from either genetic or environmental factors. For example,
upper airway obstruction due to allergic rhinitis or hypertrophied adenoids can result in
mouth breathing, which been correlated with PXB.
31
Additional causes include finger or
soother sucking habits, swallowing patterns, infant intubations, and craniofacial
abnormalities such as cleft palate.
31-33

Treatment Strategies and Long-term Studies

If left untreated, PXB can lead to facial asymmetries, and the deviated closure pattern
could interfere with condylar growth and development. Additionally, it may predispose
towards mandibular dysfunction.
9,34
Evidence has also shown that early correction of
PXB may help prevent signs and symptoms of temporomandibular disorder (TMD).
2,9

6
Therefore, early forms of treatment most often aim to eliminate the crossbite and restore
the interarch relationship to allow for normal skeletal and dental development. A second
advantage of early treatment is the increase in maxillary arch perimeter, thereby
providing additional space for unerupted teeth and reducing future dental crowding. This
may even negate the need for future tooth extractions during orthodontic treatment.

One of the most important factors affecting the success of expansion is the age of the
patient. The most suitable age for maxillary expansion is the pubertal or prepubertal
period, before interdigitation of the circummaxillary sutures are complete.
35
After
puberty, excessive forces are required with limited success, and in many cases, surgical
intervention is required to “unlock” the circummaxillary rigidity. Another factor to
consider is the gender of the patient, as females reach skeletal maturity before males, and
therefore should be treated at an earlier age.

Early treatment of PXB generally involves grinding the primary canine or expanding the
maxillary arch, or both. A review by Petren and Bondemark
36
concluded that primary
canine grinding is only effective in children younger than 5 years old, and is only
effective in 20% of those with mild transverse deficiencies. This is supported by Proffitt
1

who recommends that selective grinding of primary canines in primary or mixed
dentition is useful at treating intercanine deficiency, not inter-molar deficiency, and only
in younger patients. In the Cochrane meta-analysis, Harrison and Ashby
12
further stated
that “the removal of premature contacts in the primary teeth is effective in preventing a
posterior crossbite from being perpetuated to the mixed dentition and permanent teeth.”
7
They concluded that when grinding alone was insufficient, a removable maxillary
expander was necessary in preventing the persistence of crossbite in the permanent
dentition.

Expansion in growing individuals through removable or permanent appliances can be
accomplished in two ways: Rapid Maxillary Expansion (RME) and Slow Maxillary
Expansion (SME). With either method, the goal is to produce maximum separation of the
midpalatal suture, with minimal dental tipping, since sutural opening is much more stable
that tooth tipping.
11

Currently, the more popular modality of PXB treatment is RME. This technique employs
a jackscrew embedded into a fixed appliance; when turned, it transfers a cumulative force
of 100N (20 pounds) from the appliance to the teeth, and then to the supporting
structures.
11,37
The rate of expansion generally ranges from 0.2 mm to 0.5 mm per day
over a period of 1-3 weeks, depending on the amount of expansion needed to correct the
crossbite. In many cases, this rate of expansion is determined by the amount of
discomfort that is tolerated by the patient. Once expanded, the appliance is left in the
mouth for 4-6 months to allow for reorganization and remodeling of the sutural
connective and skeletal tissues, promoting sutural stability of the expanded arch.

RME has been the preferred expansion moldality since the speed and force levels at
which rapid expansion takes place is believed to only affect the underlying skeletal
structures (the palate) with little dental movement (tooth tipping). Because of this, it was
8
originally believed to be more stable.
38-41
However, clinical and histological studies have
shown that relapse between the palatal shelves occurs almost immediately, and the teeth
do move in the buccal direction relative to the collapsing palate as they are held in place
by the expander. The net treatment effect is approximately equal skeletal and dental
expansion.
1
In addition, microtrauma of the temporomandibular (TMJ) joint,
microfractures at the mid-palatal suture and external root resorption are observed in RME
treatment.
14-17
Furthermore, there is some evidence that RME dramatically affects the
surrounding skeletal structures. Iseri
42
evaluated the resistance to RME generated by the
surrounding structures using the finite element method on a modeled human skull. The
highest stress levels were observed at the sphenoid and zygomatic bones but not in the
mid-palatal suture.

To eliminate these disadvantages and obtain increased physiological tissue reaction, slow
maxillary expansion (SME) has been advocated.
13,23,43
It appears that 1 millimeter per
week is the maximum rate at which the tissue of the midpalatal suture can adapt so that
tearing and hemorrhaging are minimized compared with RME protocols.
13
To produce
expansion at this rate, 10 to 20 N (2 to 4 pounds) of force appear optimal, depending on
the age of the patient.
1,13

In rats and rabbits, Storey
22
showed that slow expansion (0.5 mm to 1 mm per week)
maintained the sutural integrity during expansion, and that relapse potential was
significantly less than that of RME. He hypothesized that “slow palatal expansion with
continued growth of bony serrations within the suture provides the best form of retention
9
with the least potential for relapse.” Furthermore, two additional studies observed less
tipping of abutment teeth and greater sutural stability in primates who underwent SME
than those who underwent RME.
19,44

Clinical studies have also supported the advantages of SME. Mossaz-Joelson
20

demonstrated in 10 patients that after 12 weeks of post-(slow)expansion with either a
banded or bonded expander, SME had the same amount of skeletal versus dental
movements to that of RME, but with a lower relapse tendency. A study by Hicks
13

involving 5 patients also reported similar results. Although the clinical studies seem
promising, greater sample sizes are needed before any conclusions can be made.

Many articles have been published with respect to the changes in arch dimensions due to
maxillary expansion. Results, however, have been contradictory due to the variability
with regard to sample size, age range, speed of expansion (RME vs. SME), amount of
expansion achieved, and retention methods used in these studies.
18,25,38,41,45-51

Recently, Lagravere
47
reviewed 164 such articles in his meta-analysis, and only four met
his inclusion criteria.
25
Most articles, including two previous meta-analyses and one
systematic review, were excluded due to the absence of a control group in their samples
to factor out normal growth changes. It has been demonstrated that during normal
growth, both maxillary and mandibular transpalatal widths increase, from the early mixed
dentition to the permanent dentition, and arch length decreases in young adults.
52-54

10
Lagravere’s analysis of the four articles (including 3 RME and 1 SME study) suggested
that clinically significant long-term maxillary molar width increases (3.7-4.8 mm) could
be achieved. He also found a significant overall gain in maxillary (6 mm) and mandibular
(4.5 mm) arch perimeter in adolescents treated with RME and edgewise appliances.
He noted that one problem with these studies, however, is that they all utilized fixed
edgewise appliances (braces) after expansion. Therefore, it is difficult to discriminate
between the arch dimension changes due to the expander or by the fixed appliance.
Lagravere concludes his article by stating that “the results remain inconclusive and do not
add new knowledge on the long-term effect of [expansion] because they are based on
very limited studies of second-level evidence.”

A second article by Lagravere
21
reviewed the skeletal and dental changes with fixed SME
treatment. Of the 8 articles that met his inclusion criteria, most used either the quad helix
or the minne expander. The minne expander uses a coil spring, while the quad helix uses
wire bending with helixes to exert the expansion force. His concluded that “none of [the
articles] included a control group that did not receive treatment to factor out changes in
the dental arch and skeletal structures associated with normal growth. Even though SME
treatments involve an average treatment time of three to four months, normal growth
patterns can produce some dental and skeletal changes during that time” Therefore, the
studies should be “reviewed with caution. “

11
History of Maxillary Expansion for Crossbite Correction

Opening the midpalatal suture to gain maxillary
arch width was first introduced by Emerson
Angell in 1860.
55
He used a jackscrew with
contra-rotating heads as illustrated below.

Figure 1: Early Expander by Angell

At first, there was much skepticism with the technique. His opponents believed that was
either anatomically impossible or too dangerous to be used. Others, including Edward
Angle, remained indifferent to the procedure, and this was responsible for its
discontinued use at the turn of the century within the United States. Angle and other
influential men believed that they could gain all the benefits of palatal expansion by
conventional expansion of the buccal teeth without the possible risks involved with such
a drastic procedure. Europeans, on the other hand, continued to use the procedure from
the early to mid-1900’s and beyond.

In 1956, Korkhaus re-introduced the concept of palatal expansion to North America. His
cephalometric tracings of patients treated with maxillary expansion sparked the interest in
Andrew J. Haas, an orthodontic resident at the time. Haas’ graduate thesis examining
palatal expansion in the pig allowed him to make several observations: (1) the procedure
is apparently pain free, (2) the mid-palatal suture offered very little resistance to
12
spreading, with suture openings of 15 mm in two weeks time, (3) the mandibular teeth,
without treatment, uprighted or expanded probably in response to altered forces of
occlusion and change in muscle balance, and (4) intermolar width was increased, up to 7
mm. Andrew Haas then went on to develop the Haas appliance, which is described
below. Additional expanders were since then developed and maxillary expansion has
become a common of treatment modality in Orthodontics.
56

Types of Expanders

The Haas Expander
The Haas expander was first introduced by Andrew Haas in the 1960s.
56
It is a tissue-
borne fixed split acrylic maxillary expander soldered to the first molar and canine bands.
A jackscrew allows for separation of the two acrylic pads, which transmits the forces
directly to the palatal soft tissue and the skeletal bone. A rigid appliance designed in this
manner was believed to result in maximum orthopedic effects, and minimal tooth tipping.
One disadvantage of this expander is that the acrylic plates can cause irritation during
expansion if food is caught between the acrylic and the palate.

Figure 2: The Haas Expander
13
The Hyrax Expander
The Hyrax design was introduced by Biederman in 1968 with the goal of producing
effective palatal expansion with minimal palatal irritation.
57
Unlike the Haas appliance,
the Hyrax does not contain acrylic pads and therefore does not come in contact with soft
tissue. It is a rigid appliance, soldered to bands bonded to the maxillary first molars and
first premolars. Expansion is achieved with a jackscrew.

Figure 3: The Hyrax Appliance

Quad Helix

Figure 4: The Quad Helix Appliance
14
The quad helix is an effective appliance used to expand the maxilla in children in the
deciduous or early-mixed dentition. Unlike the Haas and Hyrax appliances, the Quad
helix does not employ a jackscrew and therefore does not rely on patient compliance to
expand the arch. Once cemented, it provides a continuous, slow form of palatal
expansion. It also does not contain acrylic, and therefore has limited soft tissue irritation.

The appliance is commonly made of .038 inch wire and soldered to bands cemented to
either the maxillary first permanent molars or the deciduous second molars, depending on
the age of the patient. The initial expansion activation is placed in the appliance prior to
cementation, and the net effect is expansion of the buccal segments and a rotation of the
banded tooth.

Chaconas and Caputo suggest an initial 8 mm of expansion prior to cementation.
Additional adjustments can be made with the appliance cemented in place, all resulting in
a continuous force.
58

Physiological Effects of Expansion

During expansion, the maxilla separates in a wedge-shaped manner in both the vertical
and the anteroposterior planes; the base of the pyramid located at the oral side in the
vertical plane and anteriorly along the anteroposterior plane. The reason for the wedge-
shaped opening in the anteroposterior plane is due to the pterygomaxillary connection
that binds the sphenoid bone to the maxillary bones. In addition, the maxilla moves in a
15
forward and downward direction, likely due to the disposition of the maxillocranial
sutures
59
, and rotates in a clockwise manner, with the center of A-P rotation located
between the lateral and the medial pterygoid plates.
60,61
The displacement and rotation in
position of the maxilla causes a downward and backward rotation of the mandible, which
increases the vertical dimension of the lower face and decreases the effective length of
the mandible.

Figure 5: Skeletal Effects of Maxillary
Expansion. A: Triangle pattern of expansion
pattern from the frontal plane. B: Occlusal view
of maxillary expansion. The midpalatal suture
opening occurs with greater separation
anteriorly.

16

Haas also suggested that the alveolar processes bend and move laterally while the
midpalate swings inferiorly, resulting in an enlarged nasal cavity and a widened palate.
Pavlin and Vukenevic showed that in the frontal plane, the fulcrum of rotation of each
half of the maxilla passes through the lower part of the nasal process and the inferior
orbital rim.
62

The distant structures of the craniofacial skeleton – the zygomatic bone, temporal bone,
nasion, and frontal bone – are also affected by transverse orthopedic forces. Along the
sutures, Jafari
61
et al demonstrated maximum stresses in the internasal, nasofrontal, and
nasomaxillary sutures, with considerably lower stresses in the other articulations.

The main resistance to expansion is not the mid-palatal suture itself, but rather the
surrounding structures with which the maxilla articulates, particularly the sphenoid and
the zygomatic bones. The pterygoid plates are connected to the horizontal plates of the
palatine bones, and since the pterygoid plates can bend only to a limited extent with
pressure, the sphenoid limits the ability of the palatine bones to separate at the midsagittal
plane.
63

Additional changes can be seen dentally. As the suture opens, a central diastema forms.
The crowns of the central incisors initially converge while the roots diverge. The roots
will then converge back to their original axial inclincations (see figure below). This
process takes approximately 4 months.
59

17
Purpose of the Study

To determine the changes in maxillary arch perimeter, arch depth, arch width, and molar
angulation due to slow maxillary expansion (SME) of constricted maxillary arches in pre-
adolescent patients exhibiting a unilateral posterior crossbite and a functional mandibular
shift. We will then measure the effect of maxillary expansion on the mandibular arch in
terms of the same four criteria: arch perimeter, arch depth, arch width, and molar
angulation. Thirdly, we will evaluate the stability of the expansion in both arches through
the permanent dentition. Finally, we will compare these changes to those that occur
during normal growth, so that the pure effects of expansion can be determined.

18
CHAPTER 3: HYPOTHESIS

1. How does the increase in maxillary arch perimeter, arch depth, and arch width
compare between the Hyrax, Haas, and Quadhelix appliances when used to expand
constricted maxillas of pre-adolescents?

Ho: There are no differences between the increases in maxillary arch perimeter,
depth, and width due to the Hyrax, Haas, and Quadhelix appliances when used to
expand constricted maxillas of pre-adolescents.
H
1
: The Haas appliance will show the greatest change in maxillary arch perimeter,
depth, and width when compared to the Hyrax and Quadhelix appliances, when
used to expand constricted maxillas of pre-adolescents.

2. How does the increase in maxillary molar angulation compare between the Hyrax,
Haas, and Quadhelix expanders when used to treat constricted maxillas of pre-
adolescents?

Ho: There are no differences between the increases in maxillary molar angulation
due to the Hyrax, Haas, and Quadhelix appliances when used to expand
constricted maxillas of pre-adolescents.
H
1
: The Haas will show the greatest increase in maxillary molar angulation when
compared to the Hyrax and Quadhelix appliances, when used to expand
constricted maxillas of pre-adolescents.
19

3. How stable is the increase in maxillary arch perimeter, depth, and width when
treated with the Hyrax, Haas, and Quadhelix appliances at post-expansion in the
permanent dentition?

Ho: The increases in arch perimeter, depth, and width due to the Hyrax, Haas, and
Quadhelix appliances are equally stable when used to expand constricted maxillas
of pre-adolescents.
H
1
: The changes in arch perimeter, depth, and width are more stable when using
the Haas appliance than the Hyrax or Quadhelix appliances when used to expand
constricted maxillas of pre-adolescents.

4. How does the increase in mandibular arch perimeter, arch depth, and arch width
compare when using the Hyrax, Haas, and Quadhelix appliances to expand
constricted maxillas of pre-adolescents?

Ho: There are no differences between the increases in mandibular arch perimeter,
depth, and width due to the Hyrax, Haas, and Quadhelix appliances when used to
expand constricted maxillas of pre-adolescents.
H
1
: The Haas will show the greatest change in mandibular arch perimeter, depth,
and width when compared to the Hyrax and Quadhelix appliances, when used to
expand constricted maxillas of pre-adolescents.

20
5. How does the increase in mandibular molar angulation compare between the
Hyrax, Haas, and Quadhelix expanders when used to treat constricted maxillas of pre-
adolescents?

Ho: There are no differences between the increases in mandibular molar
angulation due to the Hyrax, Haas, and Quadhelix appliances when used to
expand constricted maxillas of pre-adolescents.
H
1
: The Haas will show the greatest increase in mandibular molar angulation
when compared to the Hyrax and Quadhelix appliances, when used to expand
constricted maxillas of pre-adolescents.

6. How stable is the increase in mandibular arch perimeter, depth, and width and
molar angulation when treated with the Hyrax, Haas, and Quadhelix appliances at
post-expansion in the permanent dentition?

Ho: The increases in mandibular arch perimeter, depth, and width and molar
angulation due to the Hyrax, Haas, and Quadhelix appliances are equally stable
when used to expand constricted maxillas of pre-adolescents.
H
1
: The changes in mandibular arch perimeter, depth, and width and molar
angulation are more stable when using the Haas appliance than the Hyrax or
Quadhelix appliances when used to expand constricted maxillas of pre-
adolescents.

21
Specific Aims:

1. To study the increases in maxillary arch perimeter, arch depth, arch width,
and molar angulation due to slow maxillary expansion (SME) in the early
mixed dentition using the Haas, Hyrax, and Quad Helix appliances in
patients with a unilateral posterior crossbite and a functional mandibular
shift.
2. To study the clinical stability of SME in the maxillary arch in the early
and late mixed dentition in terms of arch perimeter, arch depth, arch
width, and molar angulation gain when using the Haas, Hyrax, and Quad
Helix appliances to treat patients exhibiting a unilateral posterior crossbite.
3. To compare maxillary arch perimeter, depth, and width, and molar
angulation in patients in our sample before and after SME with published
growth norms (patients without posterior crossbite) at a minimum of three
time intervals: T1 (initial records), T2 (after expansion and once the
expander is removed), and T3 (at least 1-year post expansion).
4. To determine the response of the mandibular arch in terms of the same 4
criteria (arch perimeter, depth, and width, and molar angulation) due to
expansion of the opposing constricted maxilla. Here, pre- and post-SME
measurements will be taken.
5. To compare the mandibular arch measurements (arch perimeter, depth,
and width and molar angulation) pre- and post-SME with published
(Michigan) growth norms.
22
CHAPTER 4: METHODS AND MATERIALS

Definitions

T1: time point before the start of treatment
T2: time point immediately after expansion was complete
T3: time point at least 2 years after expansion was complete and before fixed
appliances (braces) are delivered
SME: slow maxillary expansion. (1 turn every two days)
PXB: posterior crossbite. In this study, PXB is defined as at least 2 consecutive
posterior teeth in crossbite, including edge to edge, but excluding the simple
crossbite in centric occlusion (crossbite due to arch length deficiency, ectopic
eruption).
TG: Treatment Group
CG: Control Group
Mx: Maxilla or Maxillary
Mn: Mandible or Mandibular

Figure 6: Centroid

23
Centroid: The determined center of each tooth, established by first finding the
midpoint (A) of a line connecting the mesial and distal landmarks. A second
midpoint (B) was constructed midway between the buccal and lingual landmarks
of the tooth. The centroid (C) is located at the midpoint between points A and B.
(Figure 6)
Maxillary and mandibular intermolar width: Determined by measuring the
intercentroid distance (mm) between the permanent first molars to a tenth of a
millimeter.
Maxillary and mandibular intercanine width: Determined by the measuring the
intercusp tip distance (mm) between the permanent canines.
Arch Length: The anterior-posterior length of the maxilla and mandible,
respectively, determined by measuring the length of a perpendicular line
constructed from a contact point between the mesial contact points of central
incisors to line connecting contact points between second premolars and first
permanent molars or the second primary molars and first permanent molars.
Arch Circumference: Determined by constructing a line from the mesial contact
point of the first permanent molar through the mesial and distal contact points of
the six anterior teeth to the mesial contact point of the opposite permanent molar.
Control Group: University of Michigan Elementary and Secondary School
Growth Study.
Molar Angulation: Angle between the right and left permanent first molars. An
increase in angulation represents buccal crown tipping. A decrease in angulation
represents lingual crown tipping.
24
Subjects

All patients were treated in a private orthodontic practice of one practitioner (DBK) in
Vancouver, B.C., Canada between January 1981 and June 2007. A hand search was
performed with all records to identify unilateral posterior crossbite cases that had T1, T2,
and T3 records. A total of 330 cases were found. The 330 cases were then screened for
inclusion and exclusion criteria

Inclusion Criteria:
1. Patients exhibit a unilateral posterior crossbite
2. Patients exhibit a functional mandibular shift
3. Patients are no older than 11 years at T1
4. T1, T2, and T3 models present
5. Expansion was attempted with the Haas, Hyrax, or Quad Helix appliance
6. Progress notes present at each time point
7. Permanent first molars present
8. T2 to T3 at least 2 years apart
9. No history of surgical or other treatment that would affect the SME effects
during the expansion period
10. A unilateral crossbite at present at T1, corrected at T2, and remained
corrected at T3

25
Exclusion Criteria:
1. Missing T1, T2 or T3 models
2. Treated with more than one type of expander
3. Patients with craniofacial anomalies (e.g. cleft palate)
4. Less than 2 years between T2 and T3
5. Patients with severe mandibular prognathism or excessive mandibular
asymmetric growth

Using these criteria, 110 cases were selected: 56 Haas cases, 26 Hyrax cases, 28 Quad
helix cases. All patients were treated with slow maxillary expansion alone – no
additional fixed appliances or retainers were used.

As listed above, PXB is defined as at least 2 consecutive posterior teeth in crossbite,
including edge to edge, but excluding simple crossbites in centric occlusion, such as
those due to arch length deficiency or ectopic eruption.

The expanders were arbitrarily chosen, not based on the severity of the PXB. However,
the quad helix was used if rotated permanent first molars were present. The Haas was
used if an anterior openbite was present so a tongue crib could be added to the appliance
to correct the discrepancy.

The treatment protocol involved one turn (0.25mm) every 2 days for the Haas or Hyrax
appliance and one activation every 4 to 6 weeks for Quad Helix. The appliances were
26
over expanded approximately 1 mm each side. The appliance was left intra-orally
(without activation) for retention for 3 months. When the appliance was removed, a new
set of study models in centric occlusion (CO) was taken (T2). A third set of records
(study models, radiographs) was taken at least 2 years post-expansion (T3).

Measurements

Maxillary and mandibular models at T1, T2, and T3, placed face-down, were
photocopied onto 8” x 11” white paper. A clear ruler was included in all photocopies to
allow for measurement calibration. A digital caliper, to the nearest 1/10 of a millimeter,
was used to measure the photocopied image of the ruler to ensure that no image
magnification error existed. The image to model ratio was 1:1. In addition, 10 cast
models and their photocopied images were measured for self-calibration four times, twice
at the start of measurements, and twice after all the measurements were taken. The before
and after calibration measurements were the same.

Predicted changes of each criteria due to normal growth was calculated via a quadratic
interpolation using published norms from the Michigan growth study to match with
gender and chronological age of study subjects (years and months)
64
.

Molar angulation was measured directly on the models, using a protractor as described
below. No published norms exist for this variable.

27
Measurement of Variables

Arch Perimeter was determined by constructing a line form the mesial contact points
of one molar through the mesial and distal contact points of the six anterior teeth to
the mesial contact point of the opposite molar (see below).

Figure 7: Arch Perimeter Figure 8: Arch Length

Arch Length was determined by measuring the length of a perpendicular line
constructed from the contact point between the mesial contact points of the central
incisors to a line connecting the most distal point of the first molars (Figure 8).

Intermolar width is the intercentroid distance (mm) between maxillary and
mandibular permanent first molars and canines, using the method described in
Standards of Human Occlusal Development, Moyers 1976.

Intermolar angulation:. The angulation of the maxillary and mandibular first
permanent molars was determined by measuring the angle of intersection of lines
passing through the mesial buccal and mesial lingual cusps. A protractor was used
28
and measured to the closest 0.5 degree. Angulation of less than 180
o
indicated that the
lingual crown torque; values above 180
o
implied buccal crown torque.

Figure 9: Measurement of Molar Angulation

Statistical Analysis

All tests between the three expander groups were performed with the one-way ANOVA
with the Dunnett T3 post-hoc test. These include between-group tests at T1, T2, T3, and
for the changes from T1-T2, T2-T3, and T1-T3. The paired t-test was used to compare
the TG from one time point to another (e.g., T1 vs. T2, T2 vs. T3, and T1 vs. T3). When
comparing the TG to the CG, the one-sample t-test was used. These include tests at T1,
T2, T3, and for the changes from T1-T2, T2-T3, and T1-T3.

29
CHAPTER 5: RESULTS:

T1 – Comparison of Pre-treatment Arch Forms

Both the maxillary and mandibular dental arches showed no significant differences
(p<0.001) between expander groups at T1 (Table 1). Therefore, no selection bias existed
at the beginning of the study.

Table 1: Comparison of Experimental Groups at T1

Haas Hyrax Quad Helix Sig
Measure (mm) Mean SD Mean SD Mean SD

Mx Arch Circumference +75.44 3.90 +74.52 4.06 +75.31 2.95 ns
Mx Intercanine Width +28.58 2.57 +28.64 2.30 +29.13 2.37 ns
Mx Intermolar Width +41.26 2.48 +41.79 2.43 +42.00 2.35 ns
Mn Arch Length +38.38 1.81 +38.00 2.38 +38.58 1.86 ns
Mn Molar Angulation +204.80 8.26 +201.85 7.16 +202.50 7.32 ns

Mn Arch Circumference +69.86 3.58 +70.23 4.19 +69.42 3.51 ns
Mn Intercanine Width +26.13 2.13 +26.30 1.95 +26.25 1.85 ns
Mn Intermolar Width +41.17 2.58 +41.76 2.71 +41.68 2.09 ns
Mn Arch Length +35.04 2.56 +35.54 1.98 +35.16 1.91 ns
Mn Molar Angulation +157.06 10.15 +160.90 7.36 +159.86 9.24 ns

*** p<0.001, **p<0.01, *p<0.05, ns-not significant

When compared to the control group (CG), maxillary arches of the treatment group (TG)
were mildly but significantly narrower by approximately 1 mm (p<0.001) for both the
canines and molars (Table 2). Maxillary arch circumference was also shorter by
approximately 4.5 mm (p<0.001) while maxillary arch length was approximately equal to
that of the CG.

30
Table 2: Comparison of Pooled Test Groups (TG) and the Control Group (CG) at T1

Mean
TG CG Difference
Measurement Mean SD Mean (CG-TG) Sig

Mx Arch Circumference +75.19 3.7 +79.85 +4.66 ***
Mx Intercanine Width +28.32 2.45 +29.36 +1.04 ***
Mx Intermolar Width +41.58 2.44 +42.75 +1.17 ***
Mx Arch Length +38.34 1.97 +37.91 -0.43 *
Mx Molar Angulation +203.5 7.82

Mn Arch Circumference +69.84 3.69 +70.25 +0.41 ns
Mn Intercanine Width +26.19 1.98 +24.13 -2.05 ***
Mn Intermolar Width +41.45 2.49 +40.63 -0.82 **
Mn Arch Length +35.19 2.27 +37.08 +1.89 ***
Mn Molar Angulation +158.7 9.4

*** p<0.001, **p<0.01, *p<0.05, ns-not significant

For the mandibular arch, the TG arches were narrower by approximately 2 mm (p<0.01)
between the canines but were similar to the CG between the molars (p=ns). Arch
circumference was similar, but TG arch length was close to 2 mm shorter than the CG.

T1-T2 – Evaluation of Treatment Effects

We observed no significant differences between the three expanders for all measured
variables of both arches from T1-T2: all expanders performed equally well (Table 3).
Consequently, no significant differences were found at T2 between the expanders for all
variables (Table 4). The most clinically relevant findings pertain to arch width and
circumference (p<0.001) (Table 5). Active expansion resulted in intercanine and
intermolar increases of 4.5 mm (P<0.001) and increases of approximately 3.5 mm in arch
31
circumference. Maxillary arch length remained stable while 4 degrees of buccal crown
tipping took place between the molars.

Table 3: Comparison of Experimental Groups at T2-T1

Haas Hyrax Quad Helix Sig
Measure (mm) Mean SE Mean SE Mean SE

Mx Arch Circumference +3.49 0.69 +3.76 1.18 +2.53 0.80 ns
Mx Intercanine Width +4.67 0.43 +4.51 0.69 +4.66 0.62 ns
Mx Intermolar Width +4.36 0.60 +4.43 0.83 +4.40 0.74 ns
Mx Arch Length +0.42 0.38 +0.52 0.61 -0.09 0.47 ns
Mx Molar Angulation +4.03 1.63 +5.02 2.13 +2.93 2.38 ns

Mn Arch Circumference -0.75 1.35 +0.17 1.18 -0.29 0.90 ns
Mn Intercanine Width -0.27 0.36 +0.16 0.55 -0.40 0.49 ns
Mn Intermolar Width +0.53 1.05 -0.74 1.23 +0.68 0.58 ns
Mn Arch Length -0.46 0.43 -0.49 0.56 -1.23 0.59 ns
Mn Molar Angulation +3.37 1.69 +4.08 2.26 +3.91 2.31 ns

*** p<0.001, **p<0.01, *p<0.05, ns-not significant

Table 4: Comparison of Experimental Groups at T2

Haas Hyrax Quad Helix Sig
Measure (mm) Mean SD Mean SD Mean SD

Mx Arch Circumference +78.93 3.90 +78.28 4.44 +77.84 3.07 ns
Mx Intercanine Width +33.25 2.19 +33.15 2.65 +33.79 2.29 ns
Mx Intermolar Width +45.62 3.76 +46.22 3.45 +46.40 3.15 ns
Mn Arch Length +38.79 2.11 +38.51 2.00 +38.49 1.64 ns
Mn Molar Angulation +208.99 8.96 +206.87 8.18 +205.43 10.28 ns

Mn Arch Circumference +69.10 3.38 +70.40 4.36 +69.13 3.20 ns
Mn Intercanine Width +25.86 1.69 +26.38 1.99 +25.85 1.84 ns
Mn Intermolar Width +41.04 5.57 +41.02 6.60 +42.36 2.21 ns
Mn Arch Length +34.58 1.91 +35.04 2.03 +33.94 2.45 ns
Mn Molar Angulation +160.60 8.20 +164.98 8.90 +163.77 8.03 ns

*** p<0.001, **p<0.01, *p<0.05, ns-not significant

32
*** p<0.001, **p<0.01, *p<0.05, ns-not significant

All changes for the CG accounting for normal growth was minimal between T1-T2
(mean increases between 0.5 to 1 mm) (Table 5). As a result, the T2 arch maxillary width
for the TG was 2.5 to 3.5 mm wider than CG (Table 6). Maxillary arch circumference
remained shorter by 2.5 mm (P<0.001) while arch length remained the same when
compared to the CG.

Table 5: Comparison of the Changes T2-T1 between the Pooled Treated (TG) and Control (CG) Group

TG (T2-T1) CG (T2-T1)
Measurement Mean SE Mean SE Sig

Mx Arch Circumference +3.27 0.52 +1.14 1.53 ***
Mx Intercanine Width +4.56 0.32 +0.57 0.66 ***
Mx Intermolar Width +4.32 0.40 +0.70 0.68 ***
Mx Arch Length +0.32 0.28 +0.56 2.76 ns
Mx Molar Angulation +4.00 1.08

Mn Arch Circumference -0.40 0.64 +0.42 1.29 ***
Mn Intercanine Width -0.19 0.26 +0.45 0.42 ***
Mn Intermolar Width +0.27 0.56 +0.23 0.44 ns
Mn Arch Length -0.65 0.30 +0.56 2.80 ***
Mn Molar Angulation +3.76 1.12

33
Table 6: Comparison of Pooled Test Groups (TG) and the Control Group (CG) at T2

Mean
TG CG Difference
Measurement Mean SD Mean (CG-TG) Sig

Mx Arch Circumference +78.50 3.62 +80.99 -2.49 ***
Mx Intercanine Width +33.37 2.32 +29.95 +3.41 ***
Mx Intermolar Width +45.96 3.52 +43.45 +2.51 ***
Mx Arch Length +38.66 1.95 +38.47 +0.19 ns
Mx Molar Angulation +207.5 9.17

Mn Arch Circumference +69.42 3.6 +70.67 -1.25 ***
Mn Intercanine Width +25.98 1.8 +24.59 +1.39 ***
Mn Intermolar Width +41.70 3.8 +40.86 +0.84 *
Mn Arch Length +34.52 2.11 +35.54 -1.02 ***
Mn Molar Angulation +163.4 8.47

*** p<0.001, **p<0.01, *p<0.05, ns-not significant

No significant changes took place for all mandibular measurements for both the TG and
the CG from T1-T2, although 4 mm of buccal tipping occurred between the molars
(p=ns) in the treatment group. Consequently, the relationship for all variables between the
TG and CG are similar to that seen at T1.

T3-T2 – Evaluation of Post-treatment Changes

There were no significant differences between the expander groups for all measured
variables of both arches from T2 to T3 (Table 7). The most noteworthy observation was
that the maxillary arches were stable for both intercanine and intermolar width (0 to 0.5
mm) – there were no significant differences between the T2 and T3 width values (Table
7). Maxillary arch circumference reduced by 2 mm (p<0.001) while 9 degrees of
uprighting occurred at the molars.
34

Table 7: Comparison of Experimental Groups at T2-T3

Haas Hyrax Quad Helix Sig
Measure (mm) Mean SE Mean SE Mean SE

Mx Arch Circumference -2.63 0.67 -1.36 1.42 -2.17 0.94 ns
Mx Intercanine Width -0.18 0.41 -0.05 0.76 -0.10 0.61 ns
Mx Intermolar Width -0.46 0.66 -0.59 0.90 -1.33 0.74 ns
Mx Arch Length -0.96 0.32 -0.90 0.65 -1.25 0.53 ns
Mx Molar Angulation -9.21 1.51 -8.73 2.03 -10.23 2.21 ns

Mn Arch Circumference -3.78 2.22 -5.03 1.77 -5.26 1.53 ns
Mn Intercanine Width -0.28 0.29 -0.46 0.56 +0.35 0.68 ns
Mn Intermolar Width +0.51 0.81 +1.09 1.41 -0.53 0.67 ns
Mn Arch Length -1.49 0.41 -1.91 0.62 -1.67 0.60 ns
Mn Molar Angulation -0.17 1.56 +0.27 2.29 -1.05 2.20 ns
*** p<0.001, **p<0.01, *p<0.05, ns-not significant

Significant reductions were seen for mandibular arch circumference (-4.5 mm, p<0.001)
and arch length (-1.5 mm, p<0.001) whereas arch width and molar angulations remained
stable (Table 8).

35
*** p<0.001, **p<0.01, *p<0.05, ns-not significant

T1 to T3 – Evaluation of Overall Treatment Changes

When considering the overall treatment, patients in all three groups responded equally
well (Table 9). Notable changes over this 5-year period include a significant increase in
maxillary arch width by 1 to 2 mm (p<0.001) and a reduction of mandibular arch
circumference and arch length (-2 to -3 mm, p<0.001) (Table 10). The lower arch width
remained stable throughout treatment. The maxillary molars tipped to the lingual by 5
degrees (p<0.001) while mandibular molars tipped to the buccal by 3.5 degrees.

Table 8: Comparison of the Changes T2-T3 between the Pooled Treated (TG) and Control (CG) Group

TG (T3-T2) CG (T3-T2)
Measurement Mean SE Mean SE Sig

Mx Arch Circumference -2.18 0.53 -0.62 1.53 ***
Mx Intercanine Width -0.09 0.32 +1.90 0.73 ***
Mx Intermolar Width -0.67 0.40 +1.70 0.75 ***
Mx Arch Length -1.01 0.28 -0.34 2.98 ***
Mx Molar Angulation -9.35 1.07 ***

Mn Arch Circumference -4.74 0.70 -2.17 1.54 ***
Mn Intercanine Width -0.15 0.26 +0.33 0.49 *
Mn Intermolar Width +0.40 0.55 +1.17 0.45 **
Mn Arch Length -1.62 0.30 -0.85 2.87 ***
Mn Molar Angulation -0.29 1.13

36
Table 9: Comparison of Experimental Groups at T1-T3

Haas Hyrax Quad Helix Sig
Measure (mm) Mean SE Mean SE Mean SE

Mx Arch Circumference +0.87 0.71 +2.40 1.38 +0.36 0.93 ns
Mx Intercanine Width +4.50 0.45 +4.47 0.72 +4.56 0.62 ns
Mx Intermolar Width +3.90 0.54 +3.84 0.76 +3.06 0.63 ns
Mx Arch Length -0.53 0.38 -0.38 0.70 -1.34 0.57 ns
Mx Molar Angulation -5.18 1.45 -3.71 1.88 -7.30 1.75 ns

Mn Arch Circumference -5.13 0.86 -2.13 1.75 -5.55 1.53 ns
Mn Intercanine Width -0.56 0.33 -0.31 0.55 -0.05 0.68 ns
Mn Intermolar Width +1.04 0.82 +0.35 1.23 +0.16 0.66 ns
Mn Arch Length -1.95 0.48 -2.40 0.61 -2.89 0.52 ns
Mn Molar Angulation +3.37 1.74 +4.34 2.08 +2.85 2.36 ns
*** p<0.001, **p<0.01, *p<0.05, ns-not significant

*** p<0.001, **p<0.01, *p<0.05, ns-not significant

Table 10: Comparison of the Changes T3-T1 between the Pooled Treated (TG) and Control (CG) Group

TG (T3-T1) CG (T3-T1)
Measurement Mean SE Mean SE Sig

Mx Arch Circumference +1.09 0.52 +0.53 1.62 ns
Mx Intercanine Width +4.47 0.32 +2.47 0.69 ***
Mx Intermolar Width +3.65 0.40 +2.39 0.61 ***
Mx Arch Length -0.69 0.28 +0.22 2.83 ***
Mx Molar Angulation -5.36 0.99

Mn Arch Circumference -5.14 0.64 -1.76 1.23 ***
Mn Intercanine Width -0.35 0.25 +0.79 0.39 ***
Mn Intermolar Width +0.66 0.56 +1.40 0.34 ***
Mn Arch Length -2.26 0.30 -0.39 2.75 ***
Mn Molar Angulation +3.46 1.20

37
T3 – Comparison of Final Arch Forms

The transverse measurements for the maxillary arch were similar between TGs and the
norms (CG) with the exception of maxillary intercanine width, which was greater by 4
mm (Tables 11 & 12). Arch circumference and mandibular arch length was shorter in the
TG than the CG, whereas the two groups were similar in mandibular arch length.

Table 11: Comparison of Experimental Groups at T3

Haas Hyrax Quad Helix Sig
Measure (mm) Mean SD Mean SD Mean SD

Mx Arch Circumference +76.30 3.65 +76.85 5.76 +75.66 3.94 ns
Mx Intercanine Width +33.08 2.08 +33.10 2.86 +33.70 2.23 ns
Mx Intermolar Width +45.17 3.16 +45.63 3.03 +45.07 2.37 ns
Mn Arch Length +37.85 2.14 +37.61 2.66 +37.24 2.36 ns
Mn Molar Angulation +199.63 6.97 +198.13 6.36 +195.20 5.67 ns

Mn Arch Circumference +65.33 3.11 +66.26 4.35 +63.87 7.31 ns
Mn Intercanine Width +25.59 1.30 +26.03 1.94 +26.19 3.03 ns
Mn Intermolar Width +41.55 2.49 +42.11 2.83 +41.83 2.82 ns
Mn Arch Length +33.09 2.44 +33.13 2.42 +32.27 2.03 ns
Mn Molar Angulation +160.43 8.27 +165.25 7.64 +162.71 8.44 ns

*** p<0.001, **p<0.01, *p<0.05, ns-not significant

38

Table 12: Comparison of Pooled Test Groups (TG) and the Control Group (CG) at T3

Mean
TG CG Difference
Measurement Mean SD Mean (CG-TG) Sig

Mx Arch Circumference +76.27 4.28 +80.38 -4.10 ***
Mx Intercanine Width +33.24 2.32 +29.27 +3.97 ***
Mx Intermolar Width +45.25 2.92 +45.15 +0.10 ns
Mx Arch Length +37.64 2.32 +38.13 -0.49 *
Mx Molar Angulation +198.2 6.72

Mn Arch Circumference +65.44 3.69 +68.49 -3.04 ***
Mn Intercanine Width +25.81 2.01 +24.79 +1.02 ***
Mn Intermolar Width +41.76 2.64 +42.03 -0.27 ns
Mn Arch Length +32.89 2.35 +36.69 -3.80 ***
Mn Molar Angulation +162.2 8.33

*** p<0.001, **p<0.01, *p<0.05, ns-not significant

39
CHAPTER 6: DISCUSSION

The goal of this longitudinal retrospective investigation was to assess the changes in
dental dimensions of a sample of mixed-dentition patients who were treated with SME
using the Haas, Hyrax, or the Quad Helix appliances. A distinctive feature of this study
was that fixed appliances or retainers were not placed after SME therapy. This is one of
the few studies that examine the pure effects of expansion alone on arch width, arch
circumference, arch length, and molar angulation for both the upper and lower arches.

Pre-treatment Comparisons (T1)

At T1, the subjects in the three expander groups were similar (p=ns) with respect to the
five measured dimensions for both arches. This was expected since the subjects (N=110)
were from the same sample and were randomly assigned to the expander groups.
Consequently, the effects of the expanders throughout treatment could be measured
against one another with no initial selection bias within the treatment groups (TG).

Mild but significant differences (p<0.001) compared to the control group (CG) were seen
when comparing maxillary arch width. All TG subjects at T1 had a constricted maxillary
arch of approximately 1 mm between the canines (28.32 vs. 29.36 mm) and molars
(41.58 vs. 42.75 mm) when compared to the CG. This was not surprising since all the
patients selected for this study exhibited a unilateral posterior crossbite with a functional
mandibular shift. It does, however, demonstrate that the TG patients at T1 were narrower
40
than the norm (CG). Treatment was aimed at increasing the transverse dimension in order
to correct the crossbite.

We also observed that the TG’s maxillary arch circumference was shorter than the CG by
approximately 3.5 mm (75.19 vs. 79.85 mm, p<0.001)). This was expected since a
reduced arch circumference is associated with narrow arches. As the maxillary is
expanded during treatment, arch circumference increases and space is gained.
38,41,48

Active Expansion: T1-T2 Changes

All three expanders produced similar amounts of change (p=ns) for all measurements in
both arches. As a result, we determined that the expanders were equally effective at
correcting unilateral crossbites in preadolescent children. In the Cochrane review meta-
analysis, Harrison and Ashby
12
compared the effectiveness of bonded vs. banded
expanders, and the Quad Helix vs. removable expanders during SME. They drew similar
conclusions stating that these expanders were equally effective. Like our TG, the patients
in these studies were less than 9 years old when expansion treatment took place. At this
age, the midpalatal suture in preadolescents has little interdigitation, and any appliance
that delivers sufficient force is able to separate the palatal shelves.
1,11

When examining arch width, we observed a maxillary intercanine width increase of 4.56
mm (p<0.001) and an intermolar width increase of 4.32 (p<0.001) during the active
expansion phase. These results are comparable to that reported by Bell et al.
18
and
41
Boysen et al.
65
Both authors used similar methodologies to this study, treating 8-year olds
with SME while using the Quad Helix appliance. Bell reported an increase of intercanine
and intermolar width of 4.4 mm and 4.8 mm, respectively,. Boysen reported intercanine
and intermolar width increases of 5.27 and 5.61 mm, respectively. These findings
confirm that our expansion is similar to the literature when SME is performed on children
in the early mixed dentition.

We observed a maxillary arch circumference increase of 3.27 mm (p<0.001) during our
active expansion period. Since we observed a intercanine width increase of 4.56 mm and
an intermolar increase of 4.32, we obtained an intercanine width:arch circumference ratio
of 0.72 and an intermolar width:arch circumference ratio of 0.76. The ratio is calculated
by dividing intercanine or intermolar width by arch circumference. Our intermolar:arch
circumference value is larger than that of Adkins
38
who reported a ratio molar:arch
circumference ratio of 0.54. A possible explanation for this is the Adkins used RME,
treated an older age group (mean age between 11 years and 6 months to 17 years) and
took T2 measurements only 4 months after expansion was initiated. Our TG was younger
(age 8) and T2 measurements were taken 1 year after expansion was initiated. As a result,
growth in our sample played a larger role, especially since approximately 0.5 mm of
transverse growth is expected from 8 to 9 years of age. Correcting for growth, we obtain
an intercanine:arch circumference ratio of 0.53 and an intermolar:arch circumference
ratio of 0.59, values closer to that seen in Adkin’s study.
38

42
Intercanine and Intermolar Width Stability (T2-T3 Changes)

We were able to maintain 98% of maxillary intercanine expansion (i.e., 4.47 out of 4.56
mm) and 80% of intermolar expansion over the 4-year period from T2 to T3 (i.e., 3.65
out of 4.32 mm), without the use of retainers or fixed appliances.. Unfortunately, it is
difficult to compare these findings with the literature since most other studies use RME
followed by fixed appliances and retention after the active expansion phase. Boysen’s
SME study was the only study similar to ours in this respect. He maintained 87% of
intercanine (i.e., 4.11 out of 4.70 mm) and 73% of intermolar width (i.e. 4.09 out of 5.59
mm) after the post-retention period. Interestingly, this was only 3 months after the
retention period, much shorter than the 4 years of post-expansion experienced by our TG.
Our high percentage of stability could be due to growth, since 1.5 to 2 mm of width
increase was expected over the 4-year period from ages 9 to 13. However, Schiffman’s
50

meta-analysis reported that in patients treated at an average age of 10.8 years with RME
or SME, 60% of interarch expansion is expected to remain after 5 years, much lower than
what was seen with our TG over the 4 year period. Therefore, the earlier age at which our
TG underwent expansion likely also contributed to our higher percentage of stability, an
argument consistent with previous reports.
66

Arch Perimeter Changes (T2 to T3)

From T2 to T3, significant decreases in arch perimeter when compared to the control for
both the maxillary (-2.18 vs. -0.62 mm, p<0.001) and mandibular arches (-4.74 vs. -2.17
43
mm, p<0.001) were observed. There are two possible reasons for this. First, the control
group was comprised of people of Northern European decent, whereas our sample was
comprised of a mixed Canadian population. Vancouver is a multiethnic population with a
high incidence of Asians. Lavelle
67,68
has been previously shown that Caucasians have
shorter teeth in the mesial-distal dimension than Asians or African Americans. Since our
sample of mixed ethnicities had larger teeth, there was a greater loss of leeway space,
resulting in a greater decrease in arch circumference and arch length when compared to
the Caucasian norm. A second reason could be that arch length and arch circumference
have decreased over the last 50 years (since the control study was performed) due to a
reduction in the mesial-distal widths of teeth in general. Warren and Bishara
69,70
stated
that tooth sizes are larger in contemporary children than they were 50 years ago (1960s).
This could explain in part why our TG experienced a larger decrease in arch
circumference and arch length. In another article, Warren and Bishara
69
state that arch
lengths are shorter in contemporary children, which would explain why both our arch
circumference and arch length measurements were less than the controls throughout the
entire treatment.

Molar Angulation

We observed buccal crown tipping of approximately 4 degrees for both arches during the
expansion phase (T1 to T2). This was similar to the findings by McNamara et al
41
who
saw an increases of 4 to 5 degrees in molar buccal tipping, after RME and fixed
44
appliances. Urdinc et al
71
found 10 degrees of buccal crown tipping when examing
posterior-anterior cephalometric films after SME.

From T2 to T3, however, McNamara et al
41
showed little if any changes in molar
angulation (< 1 degrees) for both arches. We obtained maxillary lingual crown tipping of
approximately 9 degrees (p<0.001) whereas mandibular molar angulation did not change
(p=ns). A possible reason for this is that no appliances were placed in our study to hold
and stabilize the molars after expansion. McNamara et al
41
used fixed appliances and
removable retainers after expansion.

45
CHAPTER 7: ASSUMPTIONS

1. The patient pool was representative of a population of mixed ethnicities.
2. All patients were growing based on age and clinical judgment.
3. Measurements were accurate and reproducible.
4. Digital calipers were accurate to 0.1 mm.
5. Measurements between the photocopies and model casts are similar.

46
CHAPTER 8: LIMITATIONS

1. The Michigan normal growth sample included individuals of Northern European
decent, while our sample consists of individuals of different ethnicities.
2. There are no randomized PXB untreated controls (negative controls) in this study.
3. The dental and skeletal effects of expansion cannot be differentiated.
4. Age, ethnicity, and amount of expansion varied between the three treatment
groups (Haas, Hyrax, and Quadhelix).
5. Measurements were taken on photocopies of dental casts

47
CHAPTER 9: SUMMARY

1. Pretreatment maxillary and mandibular dental arches showed no significant
differences between expander groups. No selection bias existed at the beginning
of the study

2. The Haas, Hyrax, and Quad Helix performed equally well during the active
expansion phase of treatment

3. SME is effective at treating unilateral posterior crossbites

4. Maxillary Intercanine and intermolar width increased by approximately 4.5 mm
during the active expansion phase of treatment

5. Maxillary Arch circumference increased by approximately 3.5 mm during the
active expansion phase of treatment

6. All treatment groups showed the same amount of relapse from age 9 to 13 for all
measured variables

7. 98% of intercanine stability was maintained from ages 9 to 13

8. 73% of intermolar stability was maintained from ages 9 to 13

9. Overall (from ages 8 to 13), approximately 4.5 mm of intercanine width was
gained from treatment

10. Overall (from ages 8 to 13), approximately 3.5 mm of intermolar width was
gained

11. Mandibular arch width and arch length did not change throughout the study
period

12. Mandibular arch circumference decreased by approximately 5 mm throughout
treatment

13. Maxillary molars tipped to the lingual by 5 degrees during the study period (ages
8 to 13)

14. Mandibular molars tipped to the buccal by 3.5 degrees during the study period
(ages 8 to 13)

48
CHAPTER 10: CONCLUSIONS

The Haas, Hyrax, and Quad Helix appliances perform equally well when treating
unilateral crossbite by SME in pre-adolescent children. By the end of the 4-year post-
expansion period, 98% of intercanine and 78% of intermolar stability was maintained
with no fixed appliances or retainers. In the mandibular arch, the molars did not expand
bodily but showed 4 degrees of passive molar uprighting during the expansion period; the
molars then remained stable during the post expansion period. This study demonstrates
that correction of unilateral crossbite by SME is effective in preadolescents and maintains
a high degree of stability in the long term.

49
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Asset Metadata

Creator Wong, Christian Alexander (author)

Core Title Slow maxillary expansion for the treatment of unilateral crossbite in pre-adolescents: a long-term retrospective study of the changes in arch dimension

School School of Dentistry

Degree Master of Science

Degree Program Craniofacial Biology

Degree Conferral Date 2008-05

Publication Date 03/25/2010

Defense Date 03/27/2008

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

Tag expander,Haas,hyrax,OAI-PMH Harvest,posterior crossbite,quad helix,slow maxillary expansion,SME,unilateral crossbite

Language English

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

Creator Email wongca@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-m1046

Unique identifier UC1276678

Identifier etd-Wong-20080325 (filename),usctheses-m40 (legacy collection record id),usctheses-c127-23125 (legacy record id),usctheses-m1046 (legacy record id)

Legacy Identifier etd-Wong-20080325.pdf

Dmrecord 23125

Document Type Thesis

Rights Wong, Christian Alexander

Type texts

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

Repository Name Libraries, University of Southern California

Repository Location Los Angeles, California

Repository Email cisadmin@lib.usc.edu