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A comparison of premolar extraction rates in single-phase versus two-phase treatment of Class II malocclusions

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A comparison of premolar extraction rates in single-phase versus two-phase treatment of Class II malocclusions

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A COMPARISON OF PREMOLAR EXTRACTION RATES IN SINGLE-PHASE
VERSUS TWO-PHASE TREATMENT OF CLASS II MALOCCLUSIONS
by
DanThanh Hoang Nguyen Fields

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

May 2010

Copyright 2010 DanThanh Hoang Nguyen Fields
ii

Dedication
To my husband, Michael Fields, for standing by me through dental school and
orthodontic residency, even when the stress did not bring out the best in me.
To my son, Asher Fields, for sparing Mommy-time so that I could have thesis-time.
And to my parents, Hoang and Thanh Nguyen, and sister, HaThanh Nguyen, for their
unfailing love and support of my seeming endless academic endeavors.
iii

Acknowledgements
A special thank you to:
Dr. Glenn Sameshima
Dr. Nile Sorenson
iv

Table of Contents

Dedication ........................................................................................................................................ ii
Acknowledgements ......................................................................................................................... iii
List of Tables .................................................................................................................................. vi
List of Figures ................................................................................................................................ vii
Abstract ......................................................................................................................................... viii
Chapter 1 – Introduction .................................................................................................................. 1
The Debate ................................................................................................................... 5
Recent Research ........................................................................................................... 6
Chapter 3 – Hypotheses ................................................................................................................. 11
Hypotheses ................................................................................................................. 11
Secondary Hypotheses ............................................................................................... 12
Chapter 4 – Materials and Methods ............................................................................................... 13
Subject Screening and Inclusion Criteria .................................................................. 13
Two-Phase Treatment Group – Case Selection ......................................................... 14
Single-Phase Treatment Group – Case Selection ...................................................... 15
Patient Sample, Distribution and Premolar Extraction Rate ..................................... 15
Cephalometric Tracings and Measurements .............................................................. 16
Institutional Review Board Approval ........................................................................ 17
Chapter 5 – Results ........................................................................................................................ 18
Premolar Extraction Rate ........................................................................................... 18
Cephalometric Measurements ................................................................................... 18
Molar Severity ........................................................................................................... 24
Headgear .................................................................................................................... 24
Treatment Time ......................................................................................................... 25
Age and Gender ......................................................................................................... 27
Premolar Extraction Rate ........................................................................................... 28
Cephalometric Measures ........................................................................................... 30
Molar Severity ........................................................................................................... 32
Headgear .................................................................................................................... 32
Treatment Time ......................................................................................................... 33
Age and Gender ......................................................................................................... 34
Chapter 7 – Limitations ................................................................................................................. 35

v

Chapter 8 – Conclusions ................................................................................................................ 36
Secondary Conclusions .............................................................................................. 37
Bibliography .................................................................................................................................. 38

vi

List of Tables
Table 1: Comparison of Inclusion Criteria for NIH Studies 9
Table 2: Extraction Frequency In Single- and Two-Phase Treatment Groups 15
Table 3: Male versus Female Frequency 15
Table 4: Frequency of Headgear Use in Treatment 16
Table 5: Cephalometric Measurements 16
Table 6: Extraction Rates in Single- versus Two-Phase Treatment Groups (Chi
Squared Contingency Table)
18
Table 7: Comparison of Cephalometric Measures at Initial, Middle, and Final Time
Points for Two-Phase Treatment Group
20
Table 8: Comparison of Cephalometric Measures at Initial and Final Time Points for
Single-Phase Treatment Group
20
Table 9: Comparison of Change in Cephalometric Measurements Between Single
and Two-Phase Treatment Groups
21
Table 10: Comparison of Cephalometric Measures at Initial and Final Time Points in
Extraction Cases
21
Table 11: Comparison of Cephalometric Measures at Initial and Final Time Points in
Non-Extraction Cases
22
Table 12: Comparison of Cephalometric Measurements at Initial and Final Time
Points Between Single- and Two-Phase Treatment Groups
22
Table 13: Definition of Molar Severity Scores 24
Table 14: Change in Molar Severity Score in the Two-Phase Group 24
Table 15: Change in ANB With and Without Headgear 24
Table 16: Total Treatment time in Single- versus Two-Phase Treatment Groups 25
Table 17: Total Treatment Time in Extraction versus Non-Extraction Cases 26
Table 18: Total Treatment Time in Male versus Female Subjects 26
Table 19: Sex Specific Mean Ages in Single- versus Two-Phase Treatment Groups 27
vii

List of Figures
Figure 1: Design of University of Florida Trial 7
Figure 2: Design of University of North Carolina Trial 8
Figure 3: Criteria for Selecting Patients 14
Figure 4: Initial ANB Angles 23
Figure 5: Final ANB Angles 23
Figure 6: Total Treatment Time in Single- versus Two-Phase Treatment Groups 25
Figure 7: Total Treatment Time During Adolescence in Single- versus Two-Phase
Treatment Groups
26

viii

Abstract
Introduction: The pros and cons of two-phase treatment of Class II malocclusions have
been examined in general. This study specifically evaluates both the ability of growth
modification to prevent the need for premolar extractions and the resulting cephalometric
outcomes.
Methods: The treatment of 98 patients in a Southern California private practice was
evaluated. Records were examined to compare premolar extraction rates and final
cephalometric numbers in single- and two-phase cases.
Results: In the single-phase group, 49.2% were treated with premolar extractions and
only 22.9% of the two-phase group required premolar extractions – a significant
difference (p = 0.01). Final cephalometric measurements showed no significant
difference between the two treatment modalities.
Conclusion: Early orthodontic intervention of Class II malocclusions allowed for reduced
rates of premolar extraction without compromising the cephalometric outcomes.

1

Chapter 1 – Introduction
Long has orthodontics, and the dental profession at large, been an intricate
amalgamation of science and artistic vision. Long, also, has been the debate regarding
the timing of Class II treatment. Early intervention has fallen in and out of favor without
a consensus as to its merits, so treatment timing ultimately becomes a decision made by
each practitioner based upon instinct, creative touch, and individual experience. The
issue has yet to be laid to rest and continues to be one of great relevance due to the
prevalence of Class II malocclusions: In 1970, Horowitz reported Class II malocclusions
in 1 out of every 4 children between the ages of 10 and 12
1
. The problem of prominent
maxillary incisors is at the forefront of dental issues in the public consciousness, and
being a profession driven by function and esthetics, reaching a concrete understanding of
how to most effectively and efficiently treat Class II malocclusions is paramount.
The etiology of Class II malocclusion often involves skeletal disproportion
resulting in an apical base discrepancy in addition to the dental presentation.
Disproportions may be due to excess in the maxilla, deficiency in the mandible, or both.
Because mandibular growth is unpredictable in nature, the treatment of patients with
disharmonies attributable to the mandible is especially challenging. The mandible in
Class II patients tends toward horizontal deficiency, which is of particular difficulty due
to its tendency to also rotate downward and backward.
2, 3, 4, 5
Involvement of the
skeleton allows for growth modification during preadolescent years: therefore, the
decision when to treat rests in the orthodontist’s hands.
There are three general approaches to the treatment of Class II malocclusions:
2

1. Two-phase treatment calls for treatment in two phases beginning with early
intervention during the preadolescent years with limited goals, including
correction to Class I molar, decreasing overbite and overjet, incisor alignment
and growth modification through orthopedic appliances. Finishing and
detailing occur during the second phase of the two-phase treatment modality.
2. Single-phase treatment comprehensively addresses the problem list during the
adolescent years. Due to diminished growth potential and the decreased
ability to manipulate growth, skeletal disharmonies are camouflaged through
dental compensations.
3. Orthognathic surgery is the most invasive approach to achieve ideal positions
of the maxilla and mandible once cessation of growth of the craniofacial
complex has been verified.
Those who frown upon two-phase treatment believe that the gains made during
the first phase of treatment are lost by the end of the second, making those treated with
two phases skeletally and dentally indistinguishable from those patients whose treatment
was completed in a single phase. For these orthodontists, increased financial burden on
the patient and extended treatment time are reasons not to attempt growth modification
in preadolescence.
6-15

Orthodontists whose treatment philosophy incorporates growth modification
techniques cite benefits in addition to skeletal correction: reduced risk of injury to
prominent upper incisors
16-18
, prevention dysfunctions
19-21
, psychosocial advantages
during formative years
22- 30
, and facilitation of a less complicated second phase of
treatment, including a reduced need for permanent tooth extractions.
6
However, to date,
3

statistical significance in support of the claim for reduced extraction rates is lacking.
This study seeks to provide such evidence in order to better equip practitioners with the
confidence with which to make treatment decisions.
In 1987, the World Health Organization recognized the need for an in-depth study
of the timing and outcomes of Class II treatment and the United States National Institutes
of Health (NIH) funded prospective randomized clinical trials in response. Their
findings, favoring single-phase treatment, have facilitated a resurgence of active debate in
orthodontic circles and academic literature.
7, 9, 11, 12
The investigators in these studies
found no significant difference in overjet, final ANB, or peer assessment rating (PAR)
scores between the single- and two-phase treatment groups, suggesting that early
orthodontic intervention is no more effective in preadolescence as it is in adolescence.
31
Significant findings included increased treatment time and monetary investment for
patients.
Differences in extraction rates that bordered statistical significance were also
reported in these studies. This raises the question as to whether or not growth
modification can reduce the need for premolar extractions. If a causal relationship could
be established, the benefit of a full complement of permanent dentition might motivate
orthodontists to incorporate two-phase treatment of Class II malocclusions into their
treatment philosophies. In addition, PAR scores, which only evaluate upper and lower
incisor alignment in relationship to one another, were used as a measure of quality of
treatment outcomes. Surely, cephalometric measurements are a superior indicator of
skeletal and dental relationships.
4

This study tests the hypothesis that growth modification provided by two-phase
treatment of Class II malocclusions benefits patients by reducing the extraction rate in
comparison to treatment in a single phase. Additionally, the investigators hypothesize
that cephalometric outcomes in two-phase treatment are more ideal than those in single-
phase. 900 consecutively treated cases of a private practice in Southern California were
evaluated, 98 of which met all inclusion criteria. Complete orthodontic records were
analyzed and final outcomes were compared between single- and two-phase treatment
groups.
The purposes of this study are to:
1. Compare premolar extraction rates between single- and two-phase treatment of
Class II malocclusions.
2. Compare final cephalometric outcomes between single- and two-phase treatment
of Class II malocclusions.
5

Chapter 2 – Literature Review
The Debate
Throughout the history of orthodontics, usage of early intervention with growth
modification techniques in two-phase treatment modalities has gone hot and cold within
the orthodontic community. Depending upon the variable climate of the academic
literature, practitioners might prefer to treat Class II malocclusions with either single- or
two-phase protocols. Recently, a number of clinical trials have bolstered support for
single-phase treatment during adolescent years, citing it as a more effective and efficient
approach.
10-13, 15, 89-92
These studies claim that, at best, two-phase treatment is equivalent
in outcome to single-phase treatment. Even so, staunch advocates of two-phase treatment
continue to defend the benefit of establishing Class I molar relationships during an early
first phase and a simplified the second phase of treatment, during which the occlusion is
fine-tuned.
The concept of two-phase treatment is based upon the ability to modify a patient’s
skeletal growth using appliances such as functional appliances or headgears. Clinicians
who incorporate growth modification into their treatment philosophies have observed that
the best orthopedically-obtained Class II corrections have been achieved during the early
years of active growth.
16, 33, 34
The mechanism by which this occurs, however, is yet to be
completely understood. Glass and Bridges have suggested that less mineralized bones
allow for ease of deformation, while Reitan posits the more cellular structure of sutures
and ligaments as being responsible for growing tissues to be more responsive to
orthopedic forces.
35-37
Furthermore, animal studies investigating growth of the
craniofacial complex support early growth modification, indicating that, although the
6

tissues in both adults and younger animals are amenable to change via orthopedic forces,
the magnitude and rate of change are higher in younger animals.
38-42

Regardless of the specific appliance used to garner skeletal growth modification,
research has shown the results to be extremely variable and due to a combination of
factors. It has been shown in supplementary studies that differences in technique are not
responsible for this variability.
74,76,80-81
The corrective changes that have been achieved
have been attributed to a blend of true orthopedic change – maxillary retraction or
mandibular growth – and dentvoalveolar compensations when using functional
appliances and headgear. Current orthodontic understanding of both functional
appliance
43-60
and headgear treatment
61-79
is that most of the orthopedic effect is on the
maxilla and that a substantial amount of the Class II correction is dentoalveolar.
Additional consideration has been given to the natural growth potential of the mandible
to grow in a favorable forward direction: the successes attributed to growth modification
techniques may very well be simply an expression of the individual’s genetics.
82-83

Compliance is of major concern when dealing with treatment using functional
appliances and headgear. Several studies have reported higher levels of cooperation
among preadolescent patients
84-86
while other studies have not found this correlation.
87-88
Although the jury is still out regarding the connection between age and compliance,
clinicians who employ growth modification claim to experience increased cooperation
among their younger patients.
Recent Research
In 1987, the United States National Institutes of Health funded prospective
randomized clinical trials to investigate optimal timing of treatment of Class II
7

malocclusions. The most cited of these independent trials are those conducted by the
University of Florida and University of North Carolina. Investigators at these
universities sought to establish whether growth modification could be accomplished
during early treatment and whether this modification would affect subsequent treatment
and final outcomes.
10-13, 15, 89-92
Each study had a control group which was treated in a
single phase and an experimental group treated in two phases. The experimental design
of the Univeristy of Florida and University of North Carolina are shown in Figure 1 and
Figure 2, respectively.
Figure 1: Design of University of Florida Trial

Class II Molars
(min bilateral ½ step)
Randomization
Observation Bionator Headgear/Biteplane
Phase 1 Treatment
(24 months)
Dr. “A” Dr. “B”
Dr. “C” Dr. “D”
Phase 2 Treatment
(Patient sees all doctors)
Phase 1 Phase 2
8

Figure 2: Design of University of North Carolina Trial

As shown in Table 1, the inclusion criteria varied between the University of
Florida and University of North Carolina studies. At the University of Florida, molar
classification was used to select subjects while the University of North Carolina used
overjet. During Phase 1 of the two-phase treatment at both universities, patients were
either treated with a bionator functional appliance or headgear. At the University of
Florida, however, anterior biteplanes were used to disclude the occlusion posteriorly in
combination with the high-pull or cervical headgear based upon the patients’ mandibular
plane angle. The University of North Carolina did not use anterior biteplanes and chose a
combi-headgear appliance.

9

Table 1: Comparison of Inclusion Criteria for NIH Studies
University of Florida University of North
Carolina
Overjet > 0 mm ≥ 7 mm
Oberbite > 0 mm ---
Class II Molar Severity Bilateral ≥ half-step
If one side < half-
step, other side >
half-step
---
Dentition Fully erupted permanent first
molars
≤ 3 permanent cuspids or
bicuspids
All permanent incisors and
first molars erupted
All permanent teeth
(excluding third molars)
developing per panoramic
radiographic evidence
Cephalometric Criteria --- ---
Number of Treating
Orthodontists
4 for Phase 1 1 for Phase 2

Treatment plans for each subject were established using different approaches at
the University of Florida and North Carolina. Florida used a “consensus” treatment plan
from records sent to randomly selected orthodontists. Alternatively, North Carolina
investigators allowed one of four participating orthodontists to determine the subjects’
treatment plans.
Collection of data in the two trials also followed different protocols. The
University of Florida trial examined the Phase 1 outcomes after establishment of Class I
molar classification or if the Phase 1 treatment reached 24 months in duration –
whichever came first. The University of North Carolina trial examined the same
outcomes after 15 months of Phase 1 treatment.
Results from the prospective randomized clinical trials at the University of
Florida and North Carolina both stated that functional appliances and headgear are
10

effective for growth modification – based upon severity of first molar classification –
during preadolescence in order to achieve statistically significant differences between the
control (observation) group and the experimental group. However, they found that at the
completion of treatment, all subjects were indistinguishable in Peer Assessment Rating
(PAR) scores, ANB, extraction rates, and length of treatment in adolescence. Wheeler
from the University of Florida went as far as to say, “Although there are certainly other
reasons to consider Phase 1 treatment for a Class II malocclusion, an improved skeletal or
dental outcome is not one.”
15
This statement has stirred the waters in the orthodontic
community and presently the hope is that additional data on two-phase treatment
outcomes will soon provide unbiased evidence upon which clinicians can base their
treatment decisions.
Lastly, a Cochrane Review of two-phase orthodontic treatment, titled
“Orthodontic treatment for prominent upper front teeth in children, was completed in
2007. Based upon a thorough search of the literature, the conclusion was that providing
orthodontic treatment for children with Class II malocclusions in two phases does not
have any advantages over a single-phase protocol. The authors, did acknowledge that
early treatment was beneficial in appropriately selected patients: those with arch length
discrepancies, functional crossbites, potential for accidental fracture of upper incisors,
and those who struggle with self-image.
31

11

Chapter 3 – Hypotheses
Hypotheses
1. There is a statistically significant difference in premolar extraction rates between
the single-and two-phase treatment groups.
2. There is a statistically significant difference in cephalometric measurements
between single- and two-phase treatment groups:
a. Single-Phase Treatment Group
i. Initial versus final cephalometric measurements
b. Two-Phase Treatment Group
i. Initial versus mid-treatment cephalometric measurements
ii. Initial versus final cephalometric measurements
iii. End-of-Phase 1 versus final cephalometric measurements
3. There is a statistically significant difference in cephalometric measurements when
controlling for extractions:
a. Single- versus two-phase treatment extraction patients
b. Single-versus two-phase treatment non-extraction patients
4. There is a statistically significant decrease in molar severity for two-phase
treatment – as compared between initial molar severity and molar severity at the
start of Phase 2.
5. There is a statistically significant difference in total treatment time and time spent
in fixed appliances between single- and two-phase treatment groups.
12

6. There is a statistically significant correlation between headgear treatment in
single-phase cases and greater improvement in ANB and fewer premolar
extractions.
Secondary Hypotheses
1. There is a statistically significant difference between length of treatment and
treatment modality, with non-extraction cases completing orthodontic treatment
faster than extraction cases.
2. There is a statistically significant difference between length of treatment and
gender, with females completing orthodontic treatment faster than males.
3. There is a statistically significant difference between patient age at start of
treatment, with females starting two-phase treatment earlier than males.

13

Chapter 4 – Materials and Methods
Subject Screening and Inclusion Criteria
The records examined for this retrospective study belonged to patients treated in a
private practice of an American Board of Orthodontics (ABO) Diplomate and Angle
Society member in Yorba Linda, CA. 900 consecutively treated cases were screened for
those presenting with Class II malocclusions and that were subsequently treated with
either one or two phases immediately upon diagnosis regardless of their age at
presentation.
Screening was conducted in two rounds with the goal of obtaining subjects who
were both skeletally and dentally Class II at the start of treatment. During the first
screening, patient models from February 1998 to October 2004 were examined for Class
II malocclusions with a minimum of bilateral quarter-step or unilateral half-step Class II
molar classification in patients under the age of 16 years and 0 months. Patients who
presented with abnormally contoured restorations were excluded from the study. This
initial screening resulted in the selection of 316 patients to undergo the second round of
screening under more stringent criteria.
The second set of inclusion criteria required that patients have a full permanent
dentition with the exception of possibly missing third molars, a minimum of bilateral
half-step of unilateral three-quarter-step Class II molar classification, positive overbite,
positive overjet, and complete orthodontic records, including initial and final radiographs
photographs, and models. Furthermore, those with ankylosed teeth and surgical
treatment plans were not included. There was no limit for mandibular plane angles
14

(MPA) or number of erupted permanent teeth. At the end of both rounds of screening, 98
cases were selected for this study.
Figure 3: Criteria for Selecting Patients

Two-Phase Treatment Group – Case Selection
Cases included in the two-phase treatment group were required to have a
minimum of bilateral half-step or unilateral three-quarter-step Class II molar
classification, positive overbite, and positive overjet. No minimum amount of overjet
was established in order to include Class II Division 2 malocclusions. In addition, these
patients were not to have missing teeth other than third molars, ankylosed teeth, or
surgical treatment plans.
In the practice in which this study was conducted, the first phase of treatment
employs a headgear for growth modification for a maximum of 12-13 months, whether or
not Class I molars were achieved. Upon completion of Phase 1 treatment, patients were
given retainers, space maintainers, or no appliance at the discretion of the orthodontist.
Consecutively Treated
Cases
Full Records
¼ Cusp Class II Molars
Positive Overbite
Positive Overjet
Class II Molars
(min bilateral ½ step)
No Missing/Ankylosed
Teeth
No Surgery Required
900 patients
98 patients
15

Patients were placed on recall until the start of Phase 2 treatment with fully bonded
appliances. All patients included in the two-phase treatment group completed both
phases of treatment.
Single-Phase Treatment Group – Case Selection
Criteria for selection of subjects in the single-phase treatment group were the
same as those in the two-phase treatment group. These patients either presented for
consultation during adolescence, and thus were not candidates for growth modification,
or opted to be treated in a single phase.
Patient Sample, Distribution and Premolar Extraction Rate
Of the 98 patients selected for this study, 63 underwent single-phase treatment
and 35 completed treatment in two phases. Among the single-phase treatment group, 31
(49.2%) required premolar extractions. Eight (22.9%) of the two-phase treatment group
were treated with extraction of premolars.
Table 2: Extraction Frequency In Single- and Two-Phase Treatment Groups
Total Extraction Non-Extraction
Single-Phase 63 31 (49.2%) 32 (50.8%)
Two-Phase 35 8 (22.9%) 27 (77.1%)
Total 98 39 59

Table 3: Male versus Female Frequency
Total Single-Phase Two-Phase
Male 36 24 12
Female 62 39 23
Total 98 63 35

16

Table 4: Frequency of Headgear Use in Treatment
Total Single-Phase Two-Phase
Headgear 50 16 34
No Headgear 48 47 1
Total 98 63 35

Cephalometric Tracings and Measurements
Initial, progress, and final lateral cephalometric radiographs were scanned with an
Epson Expression (Epson, US) at 150 dpi resolution. Each was imported into and traced
with Dolphin Imaging 10.5 Premium software (Dolphin Imaging Company, US). All
cephalometric measurements and statistics were calculated using these digitized tracings.
Eleven cephalometric measurements were made for tracing in order to evaluate angular,
linear, and proportional change (see Table 5). Two tracings were recorded for
single-phase patients – initial and final – while three tracings were recorded for
two-phase patients – initial, progress, and final.
Table 5: Cephalometric Measurements
A-P Maxilla SNA (
o
)
A-P Mandible SNB (
o
)
Maxilla to Mandible ANB (
o
)
Lower Incisor L1-NB (
o
)
L1-NB (mm)
IMPA (L1-MP) (
o
)
Vertical (Skeletal) FMA (MP-FH) (
o
)
Dental Relationship Interincisal Angle (U1-L1) (
o
)
Upper Incisor U1-NA (
o
)
U1-NA (mm)
Chin Projection Pog-NB (mm)

Statistical Methods
The Pearson chi squared test was used to evaluate the differences in extraction
rate between single and two-phase treated patients. Due to the small sample size of
two-phase extraction patients, the Wilcoxon signed rank test was used to compare the
17

change in a continuous variable for a given group of patient (e.g. change in ANB for a
single-phase patients) and the Mann-Whitney-Wilcoxon U-test was used to compare
differences of a continuous variable between different groups (e.g. difference in ANB
between single and two-phase patients); this allowed performing meaningful comparisons
of the means without needing to assume a normal distribution. Means, standard
deviations, and p-values were reported. All statistical analyses were conducted by the
investigators. Significance was set at p ≤ 0.05 for the Pearson chi squared test and
p < 0.01 for all other statistical tests.
Institutional Review Board Approval
The proposal to undertake this investigation was approved by the Institutional
Review Board (IRB) on February 24, 2009. USC UP-09-00029
18

Chapter 5 – Results
Premolar Extraction Rate
The difference in extraction rates between single- and two-phase treatment groups
was found to be statistically significant, as shown in Table 6 below, with a p-value of
0.01 using the Pearson chi squared test. The extraction rate for the single-phase treatment
group, 49.2% is approximately twice that of the two-phase treatment group, 22.9%.
Table 6: Extraction Rates in Single- versus Two-Phase Treatment Groups (Chi
Squared Contingency Table)
Extract NonExtract Total p
Single-phase 31 32 63 0.01
Two-phase 8 27 35
Total 39 59 98

Cephalometric Measurements
Statistical significance was established for the change in value for most of the
cephalometric measurements (the difference between the initial and final cephalometric
values for the measurements taken) for both the single- and two-phase treatment groups
(see p-values for each measurement in Table 7 and Table 8 below). No significant
change was observed in SNB angle for either group, nor in U1NA or FMA for the single-
phase treatment group.

19

Table 9 shows there were no significant differences in the change in cephalometric
measurements between single-phase and two-phase treatment groups, except between the
changes for single-phase and the changes for the first treatment portion of the two-phase
group. No statistically significant difference was observed for cephalometric
measurements between single- and two-phase groups for non-extraction cases, as shown
in Table 10 and
20

Table 11.
Table 7: Comparison of Cephalometric Measures at Initial, Middle, and Final Time
Points for Two-Phase Treatment Group

Initial Middle p
1
Final p
2
p
3
SNA 83.35 ± 5.67 82.53 ± 5.64 0.11 80.87 ± 4.64 <0.01 <0.01
SNB 77.00 ± 4.30 75.96 ± 12.48 0.08 77.78 ± 3.38 0.77 0.10
ANB 6.35 ± 2.59 4.67 ± 2.82 <0.01 3.10 ± 2.47 <0.01 <0.01
U1NAmm 3.60 ± 2.60 4.88 ± 2.42 <0.01 5.05 ± 2.69 0.90 0.01
U1NAdeg 23.03 ± 8.50 24.71 ± 6.54 0.30 27.22 ± 5.53 0.05 <0.01
L1NBmm 4.69 ± 2.08 5.23 ± 2.11 0.01 6.12 ± 2.15 <0.01 <0.01
L1NBdeg 23.99 ± 6.24 23.86 ± 7.28 0.90 29.02 ± 6.45 <0.01 <0.01
PogNBmm 1.37 ± 1.72 1.56 ± 1.75 0.33 2.94 ± 2.01 <0.01 <0.01
IntAng 126.87 ± 10.90 126.92 ± 10.16 0.90 119.73 ± 8.15 <0.01 <0.01
FMA 29.98 ± 4.91 31.50 ± 4.73 <0.01 31.66 ± 5.14 0.95 0.01
IMPA 92.05 ± 17.06 92.12 ± 6.58 0.11 97.58 ± 6.54 <0.01 <0.01
p-values computed using Wilcoxon signed rank test
p
1
: initial to middle
p
2
: middle to final
p
3
: initial to final

Table 8: Comparison of Cephalometric Measures at Initial and Final Time Points for
Single-Phase Treatment Group

Initial Final p
SNA 82.96 ± 3.73 80.63 ± 3.87 <0.01
SNB 78.24 ± 3.38 77.98 ± 3.77 0.26
ANB 4.71 ± 1.70 2.68 ± 2.11 <0.01
U1NAmm 3.92 ± 2.63 4.45 ± 2.88 0.13
U1NAdeg 20.41 ± 7.28 25.98 ± 7.39 <0.01
L1NBmm 4.94 ± 2.04 5.60 ± 1.87 <0.01
L1NBdeg 24.75 ± 6.13 27.49 ± 5.25 <0.01
PogNBmm 1.45 ± 1.73 2.90 ± 1.91 <0.01
IntAng 130.14 ± 10.73 123.79 ± 8.43 <0.01
FMA 29.16 ± 5.90 30.13 ± 6.57 0.10
IMPA 93.67 ± 6.45 96.18 ± 6.04 <0.01
p-value computed using Wilcoxon signed rank test

21

Table 9: Comparison of Change in Cephalometric Measurements Between Single and
Two-Phase Treatment Groups

p
1
p
2
p
3

SNA 0.91 0.19 <0.01
SNB 0.06 0.80 0.03
ANB 0.03 0.20 0.16
U1NAmm 0.30 0.39 0.27
U1NAdeg 0.15 0.02 <0.01
L1NBmm 0.07 0.45 0.72
L1NBdeg 0.25 0.07 0.01
PogNBmm 0.93 0.79 <0.01
IntAng 0.65 0.89 <0.01
FMA 0.20 0.40 0.22
IMPA 0.32 0.04 <0.01
p-values computed using Mann-Whitney-Wilcoxon U-test
p
1
: Compare single-phase initial-final to two-phase initial-final
p
2
: Compare single-phase initial-final to two-phase middle-final
p
3
: Compare single-phase initial-final to two-phase initial-middle
Table 10: Comparison of Cephalometric Measures at Initial and Final Time Points in
Extraction Cases

Single-Phase Two-Phase

Initial Final Initial Final p
SNA 83.35 ± 3.57 81.82 ± 3.80 82.86 ± 5.28 80.57 ± 4.07 0.47
SNB 79.02 ± 3.42 79.50 ± 3.94 76.57 ± 4.45 77.68 ± 3.46 0.23
ANB 4.30 ± 1.78 2.43 ± 2.24 6.29 ± 2.22 2.89 ± 1.99 0.04
U1NAmm 3.83 ± 2.52 4.78 ± 2.95 3.60 ± 2.29 5.19 ± 2.35 0.52
U1NAdeg 19.93 ± 6.91 27.52 ± 7.77 23.19 ± 8.44 27.73 ± 5.67 0.04
L1NBmm 4.47 ± 1.85 5.44 ± 1.72 4.41 ± 2.08 6.12 ± 2.24 0.11
L1NBdeg 24.29 ± 5.88 28.36 ± 5.08 23.30 ± 6.23 29.45 ± 6.75 0.31
PogNBmm 1.62 ± 1.48 2.63 ± 1.77 1.68 ± 1.58 2.76 ± 1.89 0.86
IntAng 131.48 ± 10.60 121.68 ± 8.61 126.64 ± 11.20 119.51 ± 8.49 0.46
FMA 26.68 ± 5.41 27.43 ± 6.57 29.60 ± 4.99 31.36 ± 4.92 0.16
IMPA 94.47 ± 6.83 98.01 ± 5.51 91.84 ± 19.19 97.92 ± 6.98 0.51
p-value computed using Mann-Whitney-Wilcoxon U-test

22

Table 11: Comparison of Cephalometric Measures at Initial and Final Time Points in
Non-Extraction Cases

Single-Phase Two-Phase

Initial Final Initial Final p
SNA 82.56 ± 3.91 79.39 ± 3.60 84.99 ± 6.96 81.91 ± 6.44 0.85
SNB 77.44 ± 3.19 76.42 ± 2.89 78.44 ± 3.64 78.11 ± 3.29 0.59
ANB 5.13 ± 1.54 2.95 ± 1.96 6.58 ± 3.76 3.82 ± 3.75 0.71
U1NAmm 4.01 ± 2.77 4.10 ± 2.81 3.60 ± 3.64 4.56 ± 3.77 0.61
U1NAdeg 20.91 ± 7.72 24.39 ± 6.72 22.52 ± 9.27 25.47 ± 4.95 0.71
L1NBmm 5.42 ± 2.15 5.76 ± 2.03 5.62 ± 1.88 6.12 ± 1.93 0.94
L1NBdeg 25.23 ± 6.44 26.59 ± 5.34 26.30 ± 6.11 27.55 ± 5.42 0.66
PogNBmm 1.27 ± 1.97 3.18 ± 2.02 .31 ± 1.86 3.56 ± 2.41 0.10
IntAng 128.76 ± 10.85 125.96 ± 7.80 127.64 ± 10.49 120.47 ± 7.35 0.34
FMA 31.73 ± 5.32 32.91 ± 5.38 31.26 ± 4.71 32.70 ± 6.09 0.83
IMPA 92.84 ± 6.03 94.29 ± 6.06 92.74 ± 6.71 96.43 ± 4.99 0.59
p-value computed using Mann-Whitney-Wilcoxon U-test

As shown in Table 12, there is a statistically significant difference between the
initial ANB for single- and two-phase treatment groups. There was no significant
difference for any of the other initial or final cephalometric measurements between the
intial and final treatment groups.
Table 12: Comparison of Cephalometric Measurements at Initial and Final Time
Points Between Single- and Two-Phase Treatment Groups

Initial

Final
SNA 0.87 0.35
SNB 0.05 0.84
ANB <0.01 0.22
U1NAmm 0.54 0.35
U1NAdeg 0.06 0.43
L1NBmm 0.54 0.16
L1NBdeg 0.76 0.31
PogNBmm 0.70 0.96
IntAng 0.16 0.02
FMA 0.67 0.29
IMPA 0.78 0.31
p-values computed using Mann-Whitney-Wilcoxon U-test
23

There did not appear to be any statistically significant difference between the
initial or final ANB angle values for either the single- or two-phase treatment groups as
depicted in the box plots below (see Figure 4 and Figure 5).
Figure 4: Initial ANB Angles

Figure 5: Final ANB Angles

Single, Non-Extraction Single, Extraction Two, Non-Extraction Two, Extraction
-5
0
5
10
15
Initial ANB, [deg]
Single, Non-Extraction Single, Extraction Two, Non-Extraction Two, Extraction
-5
0
5
10
15
Final ANB, [deg]
24

Molar Severity
A statistically significant difference between the initial and end-of-Phase 1 molar
severity score (as defined in Table 13) was observed, as shown in Table 14.
Table 13: Definition of Molar Severity Scores
Molar Class Severity Score
Class I 0.00
Quarter-Step Class II 0.25
Half-Step Class II 0.50
Full-Step Class I 1.00

Table 14: Change in Molar Severity Score in the Two-Phase Group

Initial Phase 2 Start Difference p
Score .69 ± .27 .11 ± .26 .59 ± .32 <0.01
p-value computed using Wilcoxon signed rank test
Headgear
As shown in Table 15, there is a statistically significant difference between final
and initial ANB for both the headgear and no-headgear cases. There is no significant
difference in the change in ANB between the headgear and no-headgear cases. Only 4
out of 16 (25.0%) of those subjects treated with headgear required premolar extractions
while over half of the no-headgear cases (27 of 47) had extractions (57.4%).
Table 15: Change in ANB With and Without Headgear

Headgear No Headgear

Initial Final p
a
Initial Final p
a

p
b

ANB 4.31 ± 1.80 2.61 ± 1.80 <0.01 4.84 ± 1.67 2.71 ± 2.22 <0.01

0.22
Extraction
Rate
25.0% (4 of 16) 57.4% (27 of 47)

p
a
: p-value computed using Wilcoxon signed rank test
p
b
: p-value computed using Mann-Whitney-Wilcoxon U-test

25

Treatment Time
As shown in Table 16, there is a statistically significant difference between the
total treatment time between the single- and two-phase patient groups. Table 16 also
shows the two-phase treatment group spent less time in fully-bonded fixed appliances
during their adolescent years. Bar charts depicting the two patient groups for both the
total treatment time and treatment time during adolescence are shown in Figure 6 and
Figure 7, respectively.
Table 16: Total Treatment time in Single- versus Two-Phase Treatment Groups
Single-Phase Two-Phase p
Treatment during
adolescence
30.76 ± 6.24 23.46 ± 4.66 <0.01
Total 30.76 ± 6.24 36.12 ± 4.90 <0.01
p-value computed using Mann-Whitney-Wilcoxon U-test

Figure 6: Total Treatment Time in Single- versus Two-Phase Treatment Groups

Single-Phase Two-Phase
0
5
10
15
20
25
30
35
40
45
50
Total Treatment Time, [months]
26

Figure 7: Total Treatment Time During Adolescence in Single- versus Two-Phase
Treatment Groups

A statistically significant difference was not found between the total treatment
times of extraction and non-extraction patients, as shown in Table 17.
Table 17: Total Treatment Time in Extraction versus Non-Extraction Cases
Extract Non-Extract p
Total Time 32.37 ± 6.66 33.13 ± 5.83 0.45
p-value computed using Mann-Whitney-Wilcoxon U-test
Overall, there was no statistical significance in treatment time when comparing
males and females, as shown in Table 18.
Table 18: Total Treatment Time in Male versus Female Subjects

Single-Phase Two-Phase
Male 29.92 ± 5.61 34.67 ± 5.04
Female 31.28 ± 6.61 36.88 ± 4.76
p 0.54 0.20
p-value computed using Mann-Whitney-Wilcoxon U-test
Single-Phase Two-Phase
0
5
10
15
20
25
30
35
40
45
50
Phase 2 Treatment Time, [months]
27

Age and Gender
As shown in Table 19, there is a significant difference in ages between single-
phase and two-phase groups for both males and females. No statistically significant
difference was found in age of presentation between males and females for either
treatment group.
Table 19: Sex Specific Mean Ages in Single- versus Two-Phase Treatment Groups

Single-Phase Two-Phase p
1
Male 12.37 ± 1.33 10.00 ± 1.27 <0.01
Female 12.25 ± 1.45 8.78 ± 1.24 <0.01
p
2
0.80 0.02

p-values computed using Mann-Whitney-Wilcoxon U-test
p
1
: Age between treatment modalities for a single gender
p
2
: Age between genders for a single treatment modality

28

Chapter 6 – Discussion
Briefly, this investigation was conducted in the private practice of an American
Board of Orthodontics (ABO) diplomate to compare premolar extraction rates in Class II
patients who were treated in single- and two-phase treatment protocols. The primary
hypotheses stated that the two-phase treatment group’s premolar extraction was less than
that of the single-phase treatment group and that cephalometric outcomes of two-phase
treatment were more ideal than single-phase cephalometric outcomes.
To minimize clinician bias, all cases examined for our study were treated by a
single orthodontist as a part of the normal patient population under a single treatment
philosophy. A total of 98 consecutive patient cases were selected to be subjects of our
study, 63 treated in a single phase and 35 in two phases. Two-phase treatment involved
growth modification using a headgear in Phase 1 to correct to Class I molar classification.
The investigators recognized that results of previous studies based in university
clinics might differ from those conclusions that could be drawn from studies in private
practice. In a university clinic, the patients are at the mercy of school-mandated
vacations and residents’ class schedules, which influences the timing of treatment. This
study was intentionally conducted in a private practice in an effort to provide data more
relevant data to help orthodontists make evidence-based clinical decisions.
Premolar Extraction Rate
A trend seen in some existing studies – which have come short of statistical
significance – indicate that extraction rates in single-phase treatments of Class II
malocclusions were increased in comparison to two-phase approaches.
91, 93, 94
Of the 98-
subject sample, the single-phase treatment group experienced a 49.2% extraction rate (31
29

of 63) and the two-phase treatment group experienced a 22.9% extraction rate (8 of 35),
approximately a 2:1 ratio between the control and experimental groups. Analysis using
the Pearson chi squared test demonstrated statistical significance at a p-value of 0.01.
At the University of Florida, investigators found an increased extraction rate in
the single-phase treatment group compared to the two-phase treatment group.
92
The
university-based sample size was much larger, with 208 subjects completing treatment.
Of the single-phase treatment group, 20% required premolar extraction. Of the two-
phase treatment group who underwent growth modification, 12% of the
headgear/biteplane group and 8% of the bionator group required premolar extraction.
Though not statistically significant, the University of Florida found correlations between
growth modification in two-phase treatment with reduced extraction rates, which have
been confirmed in our study.
The University of North Carolina found that the difference in percentage of
subjects who needed extraction was not significant among the single-phase, bionator, and
headgear groups. From the single-phase control group, 28% required premolar
extractions, while 35% of the functional appliance and 17% of the headgear group
underwent extraction. They concluded that early treatment did not reduce the need for
extraction – whether premolars or other teeth.
10
It should be noted, however, that
surgical treatment options were discussed most frequently in the single-phase treatment
group, which may have influenced patients to opt for surgical correction, circumventing
the need for dental camouflage by extraction.
11

Establishing statistical significance for a difference between extraction rates in
single- and two-phase treatment groups is of tantamount clinical importance. If previous
30

studies have concluded that there are no differences in dental outcomes at the end of
Phase 2 treatment and those treated in one phase
10,15,95
, the investigators of this study can
now claim that the benefit of a full permanent dentition without surgical correction is a
significant difference in dental outcome.
Cephalometric Measures
The Peer Assessment Rating (PAR) scoring system was used for evaluation of
dental outcomes in previous studies at the University of Florida and University of North
Carolina. It was originally created as an objective approach to assessing the severity of
malocclusions and quality of outcomes in regards to rotations and occlusion. However,
as Tulloch of the University of North Carolina admits, “It is possible for two patients to
have the same PAR score with different relationships of tooth to basal bone.”
11
Therefore, assessing the relationship of the teeth to the alveolus using cephalometric
measurements is an important task before concluding that single- and two-phase
treatment protocols are equal in outcome. For this reason, cephalometric values were
used to assess the quality of skeletal and dental outcomes.
The most obvious indicator of skeletal Class II malocclusion is a large ANB
angle. Skeletal growth modification during Phase 1 of treatment in the two-phase
treatment group allowed for correction of skeletal Class II malocclusions to near-Class I
classification (see Table 7 for values). By the end of treatment in both the single- and
two-phase treatment groups, subjects were treated to a solidly Class I skeletal
classification. There was no statistically significant difference in ANB correction
between the single- and two-phase treatment groups. Consistent with the University of
Florida and University of North Carolina studies, a statistically significant difference in
31

ANB existed between the start of adolescent treatment (start of Phase 2 in two-phase
cases). This difference was no longer evident by the completion of comprehensive
treatment, suggesting that skeletal outcomes are equal and independent of treatment
modality. Interestingly, initial ANB angles were found to differ significantly between the
single- and two-phase treatment groups. The two-phase subjects had higher starting
ANB angles, suggesting that more challenging cases, skeletally speaking, were finished
to outcomes comparable to the easier single-phase cases with the application of growth
modification techniques. A retrospective study by O’Brien of patients at the University
of Pittsburgh, evaluated the correlations between extraction rate and treatment modality,
and PAR scores. Similar to the results obtained by O’Brien
95
, the orthodontic outcomes
of this study did not vary with the treatment modality. Specifically, both the final
cephalometric measurements and changes in cephalometric measurements of this study
were not statistically significant between the single- and two-phase treatment groups.
Lower incisor proclination is not uncommon in orthodontic treatment and over-
proclination is of concern due to lack of basal bone support. So long as the lower incisors
are stably positioned in the alveolus and all other teeth are reasonably supported by bone,
the dental outcome can be considered uncompromised. Both the single- and two-phase
treatment groups experienced an acceptable amount of lower incisor proclination, as
measured by incisor mandibular plane angle (IMPA), shown in Table 7 and Table 8.
Therefore, there is no significant difference in the quality of dental outcome. Yet, when
consideration is given to the increased premolar extraction rate in the single-phase
treatment group, it is obvious that Class II correction to a similar outcome with non-
extraction therapy could be considered superior.
32

Molar Severity
Growth modification in two-phase treatment of Class II malocclusions aims to
correct to a Class I molar classification. Unfortunately, Class I molar correction cannot
be assumed because the orthodontist in whose practice this study was performed limits
Phase 1 treatment to a maximum of 12-13 months, whether or not Class I molar
classification is achieved. There was greater than a half-step corrective change observed
between the initial and middle time points: Upon presentation for the start of Phase 2, a
marked reduction was noted in molar severity to less than half-cusp Class II when
compared to initial records for the two-phase treatment group (see Table 14). Because
the first phase of treatment may have ended before Class I molar correction, the molar
severity is improved but not completely corrected. The University of Florida clinical trial
considered Phase 1 treatment to a quarter-cusp Class II molar to be successful.
Therefore, by their standards, Phase 1 treatment as performed by our investigators would
be considered a great success.
Headgear
In short, this study shows that the usage of headgear, whether in single- or two-
phase treatments results in a reduced need for dental camouflage of Class II
malocclusions with premolar extractions. While there was no significant difference in
ANB change between the initial and final of all subjects, regardless of whether a
headgear was used, those who were treated with headgear were likely to complete
orthodontic treatment with a full complement of teeth (see Table 15).
33

Treatment Time
Previous studies in university settings concluded that two-phase treatment of
Class II malocclusions was less efficient than single-phase treatment modalities. In this
study, two-phase treatment took only approximately 6 months longer than single-phase
treatment. However, subjects treated in two phases spent about 7 months less time in
fully-bonded fixed appliances during their adolescent years. These values are shown in
Table 16.
These findings are in stark contrast to those reported by both the University of
Florida and the University of North Carolina. Their investigations demonstrated
significantly increased total treatment times and inconsequential reduction in adolescent
treatment time in the two-phase treatment group.
6, 11, 12, 96
Differences between the
observed treatment times between this study and at the University of Florida and
University of North Carolina could be the result of different treatment protocols: The
University of Florida allowed for a very long first phase of treatment – up to 24 months –
regardless of subject cooperation. Both universities had more than one orthodontist
supervising the residents’ treatment of patients, with the University of North Carolina
even using multiple bracket systems and archwire sequences, resulting in a large range of
treatment times. The subjects of this study, under the care of a single orthodontist, were
in Phase 1 treatment for an average of 12 months.
Two common assumptions regarding orthodontic treatment times are that
extraction treatment takes longer due to the complications inherent to space closure and
that male patients are less compliant
13
and require longer to complete treatment. Both of
these long-held tenets are not supported by this investigation: Patients who were treated
34

with extractions completed treatment nearly a month faster than non-extraction cases and
no statistically significant difference was found in treatment times between males and
females.
Age and Gender
Hand-wrist radiographs were not taken in this investigation to assess skeletal age
and peak height velocity. However, the findings of this study based upon chronological
age were similar to those of the University of Florida
97
and University of North
Carolina
11
studies.
Because males experience their peak skeletal growth two years later than females
and are less advanced dentally
98
, they were expected to present later than females in the
two-phase group and still have the potential for treatment by growth modification.
However, males and females were found to present at similar ages for treatment in both
the single- and two-phase treatment groups.
35

Chapter 7 – Limitations
Several limitations were inherent to this study due to the relatively small sample
size. The subjects, who were patients of a single private orthodontic practice in Southern
California, may not be representative of the population at large. In the same vein, the
ethnicities of the limited sample size may not be varied enough to represent the general
population. As studied by the University of California, Davis Medical Center the rate of
skeletal maturation of patients of different ethnicities varies with age and may influence
the success of growth modification during Phase 1
99
.

These limitations of the study beg
further investigation, using a larger sample.
Compliance of patients within and between the single- and two-phase treatment
groups was not measured. Whether this can be accurately measured at all is questionable,
but the importance of compliance during orthodontic treatment, particularly during
growth modification, is without question.

36

Chapter 8 – Conclusions
Based upon the findings of this investigation, the value of two-phase treatment of
Class II malocclusions cannot be denied. With minimally increased – 6 months or
approximately 16% – total treatment times and similar cephalometric outcomes, the
potential of maintaining a complete permanent dentition is a significant benefit to the
patient. Therefore, the skeletal and dental outcomes of single- and two-phase treatment
protocols cannot be equal, as concluded by previous studies, if nearly half of those treated
without growth modification lost perfectly healthy teeth in the pursuit of Class II
correction.
Conclusions
1. The premolar extraction rate is approximately double in the single-phase
treatment group in comparison to the two-phase treatment group.
2. Changes in cephalometric measurements were indistinguishable between the
single- and two-phase treatment groups.
3. Changes in cephalometric measurements were indistinguishable among those of
the single- and two-phase treatment groups who required extractions and those
who did not.
4. Growth modification using a headgear appliance was successful in achieving and
maintaining Class I molar classification in the two-phase treatment group.
5. Total treatment time was slightly increased for the two-phase treatment group, but
time in fully-bonded fixed appliances was reduced.
6. Headgear treatment in the single-phase treatment group did not affect ANB
change but did result in a reduction in premolar extractions.
37

Secondary Conclusions
1. Treatment requiring extractions do not take longer to complete.
2. Total treatment time was approximately equal between males and females.
3. Males and females presented at the same age for two-phase treatment.
38

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Abstract (if available)

Abstract Introduction: The pros and cons of two-phase treatment of Class II malocclusions have been examined in general. This study specifically evaluates both the ability of growth modification to prevent the need for premolar extractions and the resulting cephalometric outcomes.

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Asset Metadata

Creator Fields, DanThanh Hoang Nguyen (author)

Core Title A comparison of premolar extraction rates in single-phase versus two-phase treatment of Class II malocclusions

School School of Dentistry

Degree Master of Science

Degree Program Craniofacial Biology

Publication Date 04/06/2010

Defense Date 03/18/2010

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

Tag extraction rate,OAI-PMH Harvest,premolar extraction,two-phase treatment

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Sameshima, Glenn T. (committee chair), Moon, Holly (committee member), Paine, Michael (committee member)

Creator Email dtfields@gmail.com,fieldsortho@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-m2897

Unique identifier UC1123431

Identifier etd-Fields-3557 (filename),usctheses-m40 (legacy collection record id),usctheses-c127-304952 (legacy record id),usctheses-m2897 (legacy record id)

Legacy Identifier etd-Fields-3557.pdf

Dmrecord 304952

Document Type Thesis

Rights Fields, DanThanh Hoang Nguyen

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