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Comparison of cortical bone thickness between second premolars and first molars in the maxilla and mandible in four ethnic groups
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Comparison of cortical bone thickness between second premolars and first molars in the maxilla and mandible in four ethnic groups
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Content
COMPARISON OF CORTICAL BONE THICKNESS BETWEEN SECOND
PREMOLARS AND FIRST MOLARS IN THE MAXILLA AND MANDIBLE IN
FOUR ETHNIC GROUPS
by
Sage Monroe Humphries
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 2007
Copyright 2007 Sage Monroe Humphries
ii
Dedication
This work is dedicated to my mother and my fiancé. Without their love and support I
would not be where I am today.
iii
Acknowledgements
Dr. Glenn T. Sameshima: I would like to thank my research advisor and thesis committee
chair. I am especially grateful for his guidance and the time and effort he devoted to this
project. His statistical prowess has proven to be invaluable.
iv
Table of Contents
Dedication ii
Acknowledgements iii
List of Tables vi
List of Figures vii
Abstract ix
Chapter One: Introduction 1
Chapter Two: Review of the Literature 5
The Maxilla and the Mandible 5
The Alveolar Process 7
Cortical Bone Thickness 7
Evolution and Cortical Bone Thickness 9
Cone Beam Computerized Tomography 11
Implants in Orthodontics 16
Overview 16
Site Evaluation 21
Clinical Applications 24
Race and Ethnicity 27
Chapter Three: Hypotheses 29
Research Hypotheses 29
Secondary Hypotheses 29
Null Hypotheses 30
Secondary Null Hypotheses 30
Chapter Four: Materials and Methods 31
Chapter Five: Results 37
Patient Demographics 37
Alveolar Height 39
Maxilla and Mandible 43
Right and Left 44
Ethnic Groups 45
Gender 47
Gender Differences within Ethnic Groups 50
Chapter Six: Discussion 54
Alveolar Height 54
Maxilla and Mandible 56
Right and Left 57
v
Ethnic Groups 58
Gender 61
Gender Differences within Ethnic Groups 61
Clinical Implications 61
Study Limitations 63
Future Studies 65
Chapter Seven: Conclusions 66
References 68
vi
List of Tables
Table 1: Summary of intraclass correlation coefficients for each site 36
Table 2: Ethnic and gender composition of the sample population 37
Table 3: Descriptive statistics for CBT at each measurement site 40
Table 4: Summary of the mean difference between measuring sites in
the mandible 42
Table 5: Summary of the mean difference between measuring sites in
the maxilla 42
Table 6: Summary of the mean difference in maxillary and mandibular CBT 43
Table 7: Summary of the paired mean difference in CBT between the right
and left sides of the maxilla and the mandible 44
Table 8: Summary of the mean difference in CBT at the LL6 site 45
Table 9: Summary of the mean difference in CBT at the LL9 site 46
Table 10: Summary of the mean difference in CBT at the UL3 site 46
Table 11: Summary of the mean difference in CBT at the UL6 site 47
Table 12: Summary of the mean CBT for females and males at each site 49
Table 13: Summary of the mean CBT difference between males and
females at each site 50
Table 14: Summary of the mean CBT for Africa-American females and
African-American males at each site 51
Table 15: Summary of CBT between the maxillary second premolar and
first molar of 23 Korean cadavers 55
vii
List of Figures
Figure 1: Cross-section of a mandibular molar and mandible 8
Figure 2: Diagrammatic representation of the image capture technique
of CBCT devices 12
Figure 3: Newtom QR-DVT 3G 13
Figure 4: Example of a mini-implant temporary anchorage device 18
Figure 5: Intraoral example of three mini-implants 20
Figure 6: Report example 33
Figure 7: Transaxial images with reference planes 34
Figure 8: Histogram demonstrating the distribution of ethnic
groups and gender within each ethnic group 38
Figure 9: Histogram demonstrating the distribution of females
and males within the complete sample 38
Figure 10: Histogram demonstrating the distribution of age with the
complete sample 39
Figure 11: Histogram of the mean CBT for each site surveyed in
the mandible 41
Figure 12: Histogram of the mean CBT for each site surveyed in
the maxilla 41
Figure 13: Histogram of the mean CBT for each gender group at each
mandibular measuring site 48
Figure 14: Histogram of the mean CBT for each gender group at each
maxillary measuring site 48
Figure 15: Histogram comparing female and male CBT within the
Asian sample 52
Figure 16: Histogram comparing female and male CBT within the
African-American sample 52
Figure 17: Histogram comparing female and male CBT within the
Caucasian sample 53
viii
Figure 18: Histogram comparing female and male CBT within the
Hispanic sample 53
Figure 19: Histogram demonstrating the mean CBT for each ethnic group
at each mandibular measuring site 60
Figure 20: Histogram demonstrating the mean CBT for each ethnic group
at each maxillary measuring site 60
Figure 21: Factors effecting measurements at U9 and U12 64
ix
Abstract
The objective of this study was to compare cortical bone thickness (CBT)
between gender groups, ethnic groups, the maxilla and mandible, and the right and left
sides of each jaw. Cone beam computerized tomography volumetric data of 157 patients
was used to generate transaxial slices between the second premolar and first molar in the
maxilla and mandible. CBT was measured at 3, 6, 9, and 12mm from the crest of the
alveolar ridge in each dental quadrant. Mandibular CBT (
!
x = 1.84mm) was significantly
greater than maxillary CBT (
!
x = 1.33mm) and CBT in the right side of each jaw was
greater than the left side (
!
x difference 0.05- 0.1mm). For the majority of sites surveyed
there was no significant difference in CBT between ethnic groups. CBT was greater in
males than females at all sites but was only significant at UL3 and UL6.
1
Chapter One: Introduction
The study and classification of bones and teeth have had a tremendous impact
upon many aspects of historic and modern science. Much of the knowledge we have of
hominid evolution is derived from the study of the mineralized remains of the past. Also,
the study of bones contributes to the ever-growing knowledge base of the health sciences.
It is this information that facilitates advancement of modern health care.
The analysis of cortical bone thickness is of importance to anthropologists and
dentists alike. With this information it is possible to extrapolate general population habits.
Anthropologists have been able to use information related to cortical bone thickness (CBT)
to characterize the biomechanical forces placed upon the jaws. By analyzing torsional
properties anthropologists have been able to make inferences about dietary adaptations and
extrapolate population habits (Chen X. and Chen H., 1998). For example, it has been
proposed that the robusticity of the Australopithecine mandibular corpus represents a
structural response to counter stresses associated with torsion. This may reflect repetitive
loads of an unusually severe magnitude, ultimately providing insight into dietary habits of
the Australopithecine (Daegling and Grine, 1991).
Material properties and their variations in individual bone organs are
important for understanding bone adaptation and quality at a tissue level and are essential
for accurate mechanical models. Additionally, without knowledge of material properties
the interpretation of finite element models is complicated (Schwartz-Dabney, Dechow,
2003). Previous mandibular finite element models have incorrectly assumed cortical and
cancellous bone to be isotropic, homogenous, and linearly elastic (Meijer et al. 1993;
Schwartz-Dabney and Dechow 2003). The knowledge of variations in CBT in the jaws will
contribute to the development of more accurate mechanical models.
2
Cortical bone thickness is paramount for the dentist or oral maxillofacial
surgeon planning to extract a tooth. A thick and dense buccal cortical plate may indicate
the need for surgical exodontia. The extraction of teeth requires expansion of the
buccocortical plate and if the cortical bone is thick and dense root fracture may occur.
Older patients tend to have denser bone, which is less likely, in comparison to the elastic
bone characteristic of younger patients, to adequately expand during tooth luxation
(Peterson et al, 1998).
The thickness of the buccal bone plate, the bone labial to the tooth root, is
critical for accurate and successful endodontic apical surgery. The anatomical distance
between the mandibular first molar apices and the buccal bone plate have been reported to
be between 4.18 mm and 7.35 mm (Frankle et al, 1990). Surgical access is extremely
difficult when the buccal bone plate is thick. Even with appropriate access proper
instrumentation and obturation is very difficult and the likelihood of failure increases.
Apical surgery involving the palatal root of the maxillary molars may have similar
problems due to the anatomic distance between the root apex and the buccal plate. When
accessibility is limited due to a thick buccal bone plate a lingual approach may be
appropriate. However, this approach is restricted due to adjacent anatomical structures and
a confined workspace. The thickness of the buccal bone plate, including the CBT, is
critical for proper planning of apical surgery (Jin et al, 2005).
Orthodontic anchorage, the resistance to undesirable tooth movements, is
one of the most critical concerns in orthodontics. In recent years mini-implants have been
used as an alternative to traditional anchorage modalities. Mini-implants provide a
compliance independent means for absolute skeletal anchorage.
3
Mini-implants do not osseointegrate like traditional endosseous implants. The
retention and stability of the mini-implant is derived from mechanical interdigitation
between the cortical bone and the mini-implant interface (Huja et al, 2005; Miyawaki et al,
2003; Struckhoff et al, 2006). Therefore, sites with thick and dense cortical bone are the
most favorable sites to place mini-implants (Kyung et al, 2003). The maxilla is composed
of relatively thin cortices connected with fine trabeculae, whereas the mandible is
composed of thick cortical bone connected with coarse trabeculae (Atkinson, 1964). The
holding strength of mini-implants has been shown to be greatest in the posterior mandible
where CBT is the greatest (Huja et al, 2005; Struckhoff et al, 2006). Subjects with a high
mandibular plane angle and large gonial angle have increased incidence of mini-implant
failure in the posterior mandible (Miyawaki et al, 2003). These subjects have thin cortical
bone in the posterior mandible (Kasai et al, 1996; Masumoto et al, 2001; Tsunori et al,
1998) that is associated with weak masticatory musculature. Several studies have found a
direct correlation between cortical bone thickness and pull-out strength (Huja et al, 2005;
Struckhoff et al, 2006). One study reported a holding strength of 122 N in 1mm of cortical
bone and 174 N in 2mm of cortical bone (Struckhoff et al, 2006).
There are few studies that report the CBT in the maxilla and the mandible.
Carano et al. (2005), reported a CBT between 2mm and 3mm in both jaws. Kruger et al.
(1986), reported a CBT of 3.1 mm to 3.2 mm in the mandibular molar region. Schwartz-
Dabney and Dechow (2003) reported upon CBT throughout the human dentate mandible
but did not survey posterior interradicular sites specifically.
The interradicular space between the maxillary and mandibular second
premolar and first molar is one of the most common sites for mini-implant placement.
CBT in these regions has not been adequately studied and to date, no comparisons between
4
various ethnic groups have been made. The goal of this study is to analyze the CBT at
various heights from the alveolar ridge between the second premolar and first molar in
each jaw and compare CBT among gender groups, 4 different ethnic populations, and
between the maxilla and mandible.
5
Chapter Two: Review of the Literature
The Maxilla and the Mandible
The study of bones is an age-old fascination of man. Much of what we know
about hominid evolution is derived from the study of bones and teeth recovered from the
earth (Graber et al, 2005). The skull, the most complex osseous structure in the body is
composed of the neurocranium, the part of the skull containing the brain, and
viscerocranium, the portion of the skull derived from the branchial arches (Thomas, 1997).
The two jaws, the maxilla and the mandible, make up approximately 25 percent of the skull
and comprise the majority of the viscerocranium (Berkovitz et al, 1999).
The majority of the mandible and maxilla is comprised of lamellar bone, a
highly organized, and well-mineralized connective tissue. Generally speaking, bone is
composed of 67 percent inorganic matrix and 33 percent organic matrix. The mineral
phase, hydroxyapetite, is secreted by osteoblasts as needle-like crystallites and undergoes
secondary mineralization, crystal growth, to form mature plate-like crystallites (Berkowitz
et al, 1999; Graber et al, 2005). Approximately 90 percent of the organic component of
bone is Type I collagen. The remainder is comprised of osteocalcin, sialoproteins,
osteonectin, phosphoproteins, and bone specific protein (Berkovitz et al, 1999; Ten Cate,
1998).
All bone can be characterized by the presence of a dense outer sheet of
compact bone and inner supporting network of trabecular bone. And, all bone develops
into either a dense (compact) cortical bone structure or a trabecular (cancellous) bone
structure (Cadet et al, 2003). Mature bone whether trabecular or compact is histologically
identical and consists of lamella or microscopic thin plates (Ten Cate, 1998). In the
developing pig mandible cortical bone develops in the corpus of the mandible by the
6
coalescence of trabecular elements (Mulder et al, 2005). In cortical bone there is less space
between trabeculations and the trabeculation thickness is greater than cancellous bone
(Mulder et al, 2005).
The maxilla consists of a body and the alveolar, frontal, palatine, and
zygomatic processes. The maxilla begins to develop intramembranously in utero at the 8
th
week. Intramembranous bone formation involves the development of bone directly within
a soft connective tissue matrix without the presence of a cartilaginous precursor. The
maxilla grows downward and forward through a remodeling process (Enlow and Hans,
1998). In general bone is deposited along the posterior superior portion of the maxilla but,
depending on the location, the remodeling processes may oppose each other or produce an
additive effect. For example, bone is resorbed along the floor of the nasal cavity and
deposited on the oral surface of the palate while the anterior part of the alveolar process is
resorbed (Proffitt et al, 2000).
The mandible consists of a body, ramus, symphysis, condyle, and the alveolar
and coronoid processes. In contrast to the maxilla, the mandible exhibits features of both
intramembranous and endochondral bone growth. During the 7
th
week in utero an
ossification site forms adjacent to Meckel’s cartilage. Intramembranous bone formation
radiates from this site. At approximately the 12th week of development, secondary
cartilages develop within the mandible (Berkovitz et al, 1999). These secondary cartilages
serve as a scaffold for endochondral bone growth.
The growth of the mandible, like the maxilla, occurs in a downward and
forward direction. The translation of the mandible occurs through bone deposition along
the posterior border of the ramus, condyle, and coronoid process (Enlow and Hans, 1996).
7
The Alveolar Process
The alveolar process consists of buccal and lingual cortical plates, joined by
interdental and interradicular septa, bony sockets to support the dentition, and an inner
supporting network of bone. The anatomy of the alveolar socket is determined by the
morphology of the roots of the teeth that they house, and the functional demands placed
upon them. The alveolar process is distinguished from the body of the mandible or maxilla
by an arbitrary boundary at the level of the root apices (Berkovitz et al, 1999).
The form of the alveolus is dependent upon the functional demands placed
upon it. It is well documented that under tension bone deposition is stimulated, while under
pressure bone resorption is stimulated. This is the physiologic mechanism of orthodontic
tooth movement. When an orthodontic force is placed upon a tooth, there is osteoclastic
activity and bone resorption on the pressure side and osteoblastic activity and bone
deposition on the tension side.
Cortical Bone Thickness
Molar bite forces have been measured between 300-400 kPa (Hansdottir and
Bakke, 2003). The functional loads are equal and opposite but the maxilla transfers stress
to the cranium, whereas the mandible must sustain the entire load. For this reason the
mandible is stronger and stiffer than the maxilla (Graber et al, 2005). The maxilla is
composed of relatively thin cortices connected with fine trabeculae, whereas the mandible
is composed of thick cortical bone connected with coarse oriented trabeculae (Atkinson,
1964) (Figure 1). This anatomical feature helps the mandible resist torsional loading
(Graber et al, 2005).
8
Figure 1. Cross-section of a mandibular molar and mandible. This image demonstrates the
directional axes and bony relationships of the tooth and the alveolus and, the relative
thickness of the buccal and lingual cortical plate.
It has been reported that there is a relationship between morphological features
that relate to masticatory function and facial type and CBT in the molar region of the
mandible (Masumoto et al, 2001; Tsunori et al, 1998). The CBT is greater in subjects with
small gonial and mandibular plane angles and large posterior facial height (Kasai et al,
1996; Masumoto et al, 2001; Tsunori et al, 1998). There exists a positive correlation
between CBT and cortical bone density (Sato et al, 2005). It has been shown that bone
remodeling in the alveolar bone is stimulated by the flexure produced by mechanical
stimulation from mastication through the periodontal membrane and suggested that CBT
and density may increase as masticatory function develops (Sato et al, 2005). The highest
bite forces are found in subjects with a low gonial angle, flat mandibular plane angle, large
posterior facial height, and small anterior facial height (Ingervall and Thilander, 1974).
The increased masticatory loading common in low gonial and mandibular plane angle
subjects appears to result in an adaptive increase in CBT and density.
9
There is a relationship between the position of the mandibular molars and
CBT (Sato et al, 2005). The bone inclination in the mandibular molar region of low angle
subjects is more lingual than in high angle subjects (Tsunori et al, 1998). The molars that
receive buccally directed masticatory force are supported by thick lingually inclined buccal
cortical bone (Tsunori et al, 1998).
With the recent use of mini-implants for orthodontic anchorage, the CBT in
the region between the second premolar and first molar is of interest to the dentist placing
implant anchors. One report indicates that the CBT in the mandibular molar area is
between 3.1 and 3.2 mm (Kruger et al, 1986). Carano et al (2005) reported an average
thickness between 2mm and 3mm in the molar region of both jaws. A study on 10
Caucasian cadavers by Schwartz-Dabney and Dechow (2003) found that the thickness of
the cortical bone in the molar region ranges between 1.4- 2.8 mm. They found that CBT
did not differ by gender or age although a small sample size may compromise the statistical
significance of these findings.
Evolution and Cortical Bone Thickness
Anthropologists have used mandibular torsional properties to make inferences
about primate and hominid dietary adaptations and most of the methods used are based
upon assumptions related to periodontal and alveolar properties (Chen X. and Chen H.,
1998). A comprehensive knowledge of the material properties and their variations is
essential to properly understand bone adaptation and quality at a tissue level. It is also
essential for the development of accurate mechanical models. The human dentate mandible
demonstrates unique regional variation in direction of maximum stiffness, elastic
properties, cortical density and cortical thickness. Properties of alveolar bone play an
10
important role in determining the strain field of the mandible. Chen and Chen (1998)
suggested that primate mandibles behave like a closed-section under torsion under the
limiting condition that the alveolar bone stiffness is more than half of the value of cortical
bone; alveolar bone can then be modeled as cortical bone with minimal loss of accuracy.
In addition, Chen suggests that the minimum cortical thickness should be considered for
the torsional strength (Chen X. and Chen H., 1998). As the information about the material
properties of the mandible becomes more complete it allows for the development of better
mechanical models (Schwartz-Dabney and Dechow, 2003).
Through the study of the cortical bone in the jaws, anthropologists have been
able to expand upon the early hominid knowledge base. Early hominids such as
Australopithecus africanus and Paranthropus robustus have increased mandibular
robustness (expressed as the ratio of mandibular corpus breadth to corpus height) in
comparison to modern apes and Homo sapiens. In order to understand the precise manner
in which mandible robustness increased in australopithecines, Daegling and Grine (1991),
examined the mandibles of hominids, extant great apes, and modern humans via
computerized tomography (CT). They found that the mechanical properties of early
hominid mandibles differ from living hominoids in terms of cross-sectional shape,
although the relative amount of compact bone (compact bone area/total subperiosteal area)
utilized is similar in the two groups (Daegling and Grine, 1991). Still, the mandibular
corpus of A. africanus is characterized by economic use of cortical bone (Daegling and
Grine, 1991; Schwartz and Conroy 1996). Accordingly, one may infer that the CBT at any
given point will likely vary between early hominids and living hominoids. Additionally
the cortical bone along the buccal aspect of the mandibular corpus is thicker than the
lingual (Daegling and Hotzman, 2003). This may be explained by superposed sources of
11
bone strain producing large strain gradients between buccal and lingual aspects of the
mandibular corpus and that these gradients may be associated with local variations in bone
mass (Daegling and Hotzman, 2003).
Schwartz and Conroy (1996) found that Otavipithecus namibiensis has cortical
bone distribution that differs from extant hominoids and early hominids. They also found
that the mechanical design of the posterior portion of the mandibular corpus is highly
resistant to torsional strain and that Otavipithecus and A. africanus have mandibles that are
designed to resist increased masticatory loads with relatively less cortical bone area.
Cone Beam Computerized Tomography
Cone beam computerized tomography (CBCT) was introduced in the 1990s as
an evolutionary process resulting from the demand for three-dimensional (3D) information
obtained by conventional computerized tomography (CT) scans (Kau et al, 2005). CBCT
provides an innovative means of image scanning and volumetric reconstruction of CT data
(Lascala et al, 2004). CBCT provides 3D data within the working range of dental
radiography and has been employed more routinely in recent years (Danforth, 2003;
Walker et al, 2005).
Computerized tomography has been considered the gold standard of 3D
imaging of dental structures for many years (Schmuth et al, 1992). Nevertheless, there are
a number of limitations to the application of CT in dentistry such as, degradation of image
quality due to artifacts from metallic dental restorations, extended examination time, and
the incapacity of CT to reproduce thin image sections (Holberg et al, 2005; Misch, 1990,
Schwarz et al, 1987) Additionally, these machines require considerable space, are very
expensive, and deliver a higher radiation exposure than conventional radiographic units.
12
Figure 2. Diagrammatic representation of the image capture technique of CBCT devices
(Modified from Mr. Arun Singh, Imaging Sciences, Hatfield PA, USA).
Cone beam computerized tomography acquisitional systems have been
designed to counter some of the limitations of CT (Halazonetis, 2005). The object of
interest is captured as the radiation source falls onto a two-dimensional detector, which
allows a single rotation of the radiation source to capture an entire region of interest (Kau
et al, 2005) (Figure 2). The cone beam produces a more focused beam and considerably
less scatter radiation than fan-shaped CT devices therefore increasing the X-ray utilization
13
and reducing X-ray tube capacity required for volumetric scanning (Mah et al, 2003;
Sukovic, 2003). These component innovations allow CBCT to be smaller and less
expensive while reducing the radiation exposure.
There are four major CBCT acquisitional systems in clinical use. These
include the NewTom QR-DVT 3G (Quantitative Radiology, srl, Verona, Italy), the I-CAT
(Imaging Sciences International, Hatfield, USA), CB MercuRay (Hitachi Medical
Corporation, Tokyo, Japan), and 3D Accuitomo (J Moria Mfg Corp, Kyoto, Japan). These
machines differ in size, tube current, tube voltage, scan time, field of view, voxel size,
reconstruction time, and price (Kau et al, 2005).
Figure 3. NewTom QR-DVT 3G (Quantitative Radiology, srl, Verona, Italy).
14
The NewTom 3G (Figure 3) operates similarly to CT. The patient is placed in
the supine position and the head and neck are scanned. CBCT can provide 3D images with
up to 4 times less radiation than a conventional CT (Schulz et al, 2004). An effective
absorbed radiation dose from an imaging session of approximately 75 seconds with the
NewTom QR-DVT is 50.2!Sv (Mah et al, 2003). This is approximately 1.4 % or 5 days of
the annual per capita background radiation dose of 3600!Sv in the USA (Ludlow et al,
2003). This is considerably less than conventional CT which results in an effective
radiation dose of 17.6!Sv to 656.9!Sv for a maxillary examination and 124.9!Sv to
528.4!Sv for a mandibular examination depending upon the volume of the jaw and the
operational settings on the CT (Mah et al, 2003). The effective dose of a panoramic
examination is 2.9!Sv to 9.6 !Sv and for a full-mouth series is 33!Sv to 84 !Sv depending
on the operational setting such as kVp, mA, film speed, and collimation. For reference,
each passenger on a single flight from Paris to Tokyo has an effective radiation dose of 139
!Sv (Bottollier-Depois et al, 2003). Ultimately, the effective dose of CBCT is
significantly smaller than conventional CT and is within the range of traditional dental
imaging modalities (Lascala et al, 2004).
At present there are a number of clinical applications of CBCT being reported
(Sukovic, 2003). Currently CBCT is being used in oral diagnoses, surgery, and to locate
incidental oral abnormalities (Kau et al, 2005). Three-dimensional volumetric imaging is
being used routinely in 3D airway analysis and for visualization of temporomandibular
joint morphology. CBCT data can very helpful for the imaging of the mandibular canal
prior to surgical removal of mandibular 3
rd
molars (Heurich et al, 2002). For the
implantologist conventional radiographs often do not supply the necessary diagnostic
information regarding alveolar bone quality and height (Maher, 1991). CBCT overcomes
15
many of the limitations of conventional radiography and can be used to accurately assess
bone dimensions, bone quality and alveolar height (Kau et al, 2005). Implant planning is
currently one of the most common uses for CBCT (Hatcher et al, 2003; Holberg et al,
2005).
There are a multitude of clinical applications of CBCT in orthodontics. With
CBCT technology a single scan can offer an orthodontist the diagnostic information of
conventional panoramic films, lateral cephalograms, occlusal films, a TMJ series as well as
views unobtainable with conventional radiography such as axial and 3D views (Kau et al,
2005). The 3D orientation of impacted canines is very important for the surgical and
orthodontic management of impacted canines. The prevalence of maxillary canine
impaction has been reported to be between 1 and 3 percent, 80% of which are located
palatally and 20% of which are located bucally (Kau et al, 2005; Stewart et al, 2001).
Cone beam volumetric imaging provides invaluable information about impacted canines
and assists in the treatment and understanding of these cases surgically and orthodontically
(Walker et al, 2005).
Recently, researchers have studied the geometric and linear accuracy of the
digital volume tomograms obtained with the NewTom QR-DVT 9000 (Quantitative
Radiology, srl, Verona, Italy), the predecessor to the NewTom QR-DVT 3G. Marmulla et
al (2005), found that the volume produced by the NewTom 9000 has geometric distortion
below the resolution power of the tomograms. These results indicate that the images
presented by the NewTom 9000 are geometrically correct and, from a geometric
standpoint, are suitable for 3D diagnosis and treatment planning (Marmulla et al, 2005).
A study by Lascala et al, in 2004, sought out to evaluate the accuracy of linear
measurements obtained in CBCT images generated by the NewTom 9000. The results
16
showed that actual measurements were always larger than those obtained from the CBCT
images, but these differences were only significant for structures outside of the
dentomaxillofacial area, therefore validating the accuracy of linear measurements on
CBCT images (Lascala et al, 2004).
Ultimately, the accuracy and the low radiation dose of the NewTom make
CBCT a useful technique for imaging of the dentomaxillofacial complex. Researchers
around the world are currently utilizing CBCT technology. As databases at imaging
facilities grow these data become a great source of information for dental research.
Implants in Orthodontics
Overview
Orthodontic anchorage, the resistance to undesirable tooth movement, is one
of the most critical elements in edgewise orthodontic treatment. Traditional anchorage
modalities include headgears, lower lingual holding arches, Nance appliances, and
transpalatal arches (TPAs). Each of these methods is not without its limitations. Success
of headgear is completely compliance dependent and it is often not worn due to esthetic
and social concerns (Egolf et al, 1990). The literature regarding the effectiveness of lower
lingual holding arches, TPAs, and Nance appliances are confounding. Some studies have
shown that TPAs increase resistance to anchorage loss while others have found the TPA
does nothing to impede the forward movement of the maxillary molars (Bobak et al, 1997).
Studies have shown that there is anchorage loss associated with the use of the Nance
appliance and a high incidence of inflammation of the palatal tissue under the acrylic
button.
17
In recent years skeletal anchorage, involving the implantation of traditional
endosseous osseointegrated implants, miniplates, miniscrews, and microscrews have been
used as an alternative to conventional orthodontic anchorage methods (Favero et al, 2002;
Roberts et al, 1994; Shellhart et al, 1996). Skeletal anchorage provides absolute
anchorage and minimizes the compliance necessary for extraoral anchorage devices.
Linkow first suggested the use of endosseous osseointegrated implants, such
as those used for prosthetic rehabilitation, for anchorage in 1970 (Linkow, 1970). It was
not until nearly 15 years later that Roberts et al. reported the use of osseointegrated,
implants in orthodontic anchorage in rabbits (Roberts et al, 1984). In the late 1980s
implants began to be used for anchorage and then as permanent abutments for tooth
replacement (Carano et al, 2005). Although osseointegrated implants have their place as
anchors in multidisciplinary treatment, they are not routinely used today due to large size,
limited number of implant sites and indications, difficult placement, long waiting period,
difficulty of removal and high cost (Liou et al, 2004).
Mini-implants or miniscrews (Figure 4) have recently been introduced as a
simpler alternative to endosseous implants in orthodontics (Costa, et al, 1998; Kanomi,
1997). The advantages of mini-implants include smaller size, greater number of implant
sites and indications, shorter treatment time, simpler surgical placement and orthodontic
connection, minimal waiting period, no need for laboratory work, easier removal after
treatment, and lower cost (Carano et al, 2005; Liou et al, 2004).
18
Figure 4. Example of a mini-implant temporary anchorage device. The Mondeal mini-
implant (left) is 7mm in length and 2mm in diameter. The Orlus mini-implant (right) is
6mm in length and 1.8mm in diameter.
Creekmore and Eklund, in 1983, suggested that a small screw could withstand
a constant force of sufficient magnitude and duration to reposition the entire anterior
maxillary dentition without becoming loose, painful, infected, or pathologic but, it was not
until 1997 that Kanomi described the first mini-implant designed for orthodontic use. To
date, there is no universally accepted name for the implanted devices. The term
“temporary anchorage device” (TAD) refers to all variations of implants, onplants, screws,
and pins that are placed specifically for orthodontic use (Mah et al, 2005). Many refer to
the devices as miniscrews due to their shape and design, but to avoid negative
connotations, the use of terms such as “pin” or “implant” has been suggested (Mah et al,
2005).
Mini-implants typically range from 1mm to 2mm in diameter and 4mm to
12mm in length and are typically made of either medical grade 4 or 5 titanium, although
19
stainless steel has been used (Carano et al, 2005; Mah et al, 2005; Schnelle et al, 2004).
The small size makes it possible to place mini-implants into interradicular spaces. The
maximum load is specific to each kind of fixture but generally speaking the maximum load
is proportional to the diameter and the length of the fixture and the quality and quantity of
bone (Favero et al, 2002). To date, a conical self-tapping tapered screw with a polished
surface is most commonly used. The polished surface helps to minimize gingival irritation
and inflammation and osseointegration, therefore facilitating easy removal (Mah et al,
2005). There are a variety of head designs commercially available including button-like
cross head, bracket-like hex head, holes, and hooks. The site of placement is dependent
upon the biomechanical requirements of the case at hand and the quality and quantity of
bone available (Favero et al, 2002). Placement sites reported in humans include, buccal
and lingual interradicular spaces, retromolar, and extraction sites in the maxilla and the
mandible. Other sites include the anterior nasal spine, palate, and infrazygomatic crest of
the maxilla and the mandibular symphysis. The most commonly utilized site is between
the roots of the maxillary and mandibular second premolars and molars (Figure 5), the
interradicular site between the maxillary later incisors and canines, and the midsaggital and
paramedial region of the palate.
20
Figure 5. Intraoral example of three mini-implants. The mini-implants are placed in the
interradicular space between the maxillary right canine and first premolar, the maxillary
first premolar and second premolar and the mandibular right second premolar and first
molar.
There is variation in placement methodologies. There are many orthodontists
that place mini-implants themselves while others refer the patient to a dentist, periodontist,
or oral surgeon (Mah et al, 2005). Most clinicians place mini-implants under a local
infiltration of a dental anesthetic (Lin and Liou, 2003; Kanomi, 1997; Park et al, 2001)
whereas others report using only topical anesthetic. Many advocate the use of self-tapping
screws (Mah et al, 2005) while other clinicians prefer to prepare a pilot hole prior to
placement (Kanomi, 1997). There are clinicians that reflect a mucoperiosteal flap
(Giancotti et al, 2003; Kanomi, 1997) to place mini-implants and yet many do not
(Giancotti et al, 2003; Lin and Liou, 2003).
There is controversy as to when mini-implants can be loaded with orthodontic
forces. Some authors report that the worst time to load mini-implants is two weeks after
placement due to remodeling (Mah et al, 2005), others recommend loading mini-implants
21
two weeks after placement (Lin and Liou, 2003; Park et al, 2001), while other authors
advocate immediate loading (Costa et al, 1998; Giancotti et al, 2003; Gray and Smith,
2000; Melsen and Verna, 1999). A study in beagle dogs found that the bone supporting
monocortical screws is sufficient to withstand immediate loading and support tooth-
moving forces (Huja et al, 2005). Another study found that immediate loading is possible
if the applied force is less than 2N (Miyawaki et al, 2003). Primary stability, or mechanical
retention, results from the mechanical interdigitation of the implant anchor and the cortical
bone (Miyawaki et al, 2003). It has been suggested that fixtures can be loaded immediately
because the primary stability of the mini-implant is sufficient to sustain regular orthodontic
loading (Costa et al, 1998; Melsen and Verna, 1999).
Site Evaluation
It is accepted that mini-implants do not completely osseointegrate (Costa et al,
1998; Kanomi, 1997) resulting in stability during treatment but still allowing for easy
removal at the commencement of treatment (Schnelle et al, 2004). The amount of bone
contact at insertion influences the primary stability of the fixture (Huja et al, 2005) and the
thickness and density of the cortical bone are important for the retention and stability of the
screw. Accordingly, sites with thick and dense cortical bone are most favorable (Kyung et
al, 2003). The maxilla and mandible differ in the quality and thickness of cortical bone.
Therefore stability and success can be expected to differ between the maxilla and the
mandible (Huja et al, 2005).
A high mandibular plane angle has been associated with an increased
incidence of mini-implant failure (Miyawaki et al, 2003). Subjects with a high mandibular
plane angle have thinner cortical bone in the mandibular first molar region than subjects
22
with a moderate to low mandibular plane angle (Masumoto et al, 2001; Tsunori et al,
1998). Thus, it appears that mechanical interdigitation between the mini-implant and the
cortical bone is critical for the retention and stability of the fixture (Miyawaki et al, 2003).
For this reason it has been suggested that cortical bone contour and thickness be examined
by CBCT prior to implantation.
Several studies have utilized self-tapping screws in the maxilla and mandible
of beagle dogs to analyze the biomechanical stability of mini-implants. Huja et al (2005)
used pull-out strength to evaluate anchorage value and the primary stability of the implant
anchor at the time of implantation. Significant differences in pull-out strength and CBT in
different locations in the maxilla and mandible were reported. Greater pull-out strength and
CBT was found in the posterior region of the jaws. They found a correlation between CBT
and pull-out strength of the self-tapping monocortical screws (Huja et al, 2005). Another
study (Struckhoff et al, 2006), found that pull-out strength was greater in the posterior
mandible sites than the maxillary anterior and palatal sites. Pull-out strength was
correlated with cortical bone thickness and there was no significant difference in pull-out
strength at the time of implantation and six months afterward. This study offers an
estimate of holding power of mini-implants for each millimeter of CBT. The authors
report a static holding power of 122 N for 1mm of cortical bone purchase and 174 N for
2mm of cortical bone purchase (Struckhoff, 2006).
When placing mini-implants interradicularly, there must be at least 3mm to
4mm of what is referred to as bone stock between roots (Schnelle et al, 2004). Fixtures
commonly used in clinical practice typically range from 1.2 mm to 2mm in diameter
(Costa et al, 1998; Mah et al, 2005) and it is necessary that there is at least 1mm of bone
between the fixture and the periodontal ligament (Schnelle et al, 2004). It is important that,
23
whenever possible, mini-implants are placed in attached tissue. This minimizes likelihood
of mini-implant failure due to inflammation of peri-implant tissue (Miyawaki et al, 2003;
Kyung et al, 2003). Three mm of bone stock or more is primarily available mesial to the
maxillary and mandibular first molars and distal to the mandibular first molars (Schnelle et
al, 2004). A study found that the greatest amount of bone stock in the maxilla is located
palatally between the maxillary second premolar and first molar (Carano et al, 2004). At
most interradicular sites, adequate bone stock is available halfway down the root length
(Schnelle et al, 2004), which is typically covered by unattached mucosa (Lang, Loe, 1972).
This indicates that in most cases one cannot predictably implant fixtures in bone covered
with attached gingiva. There is an increase in sites with at least 3mm of bone stock after
orthodontic treatment. These findings indicate that there may be an increase in
interradicular sites available for placement after orthodontic treatment and initial root
aligning (Schnelle et al, 2004).
Mini-implants provide stable anchorage but do not remain absolutely
stationary throughout periods of orthodontic loading. A study found that the screw head
moved forward significantly (0.4mm) under orthodontic loading. The tipping and
extrusion of the screw tail and body were not statistically significant (Liou et al, 2004).
This information should be kept in mind when planning the placement site of an implant
anchor.
Although cortical bone thickness is an important factor in determining the
success of a mini-implant (Kim et al, 2006), it is not the only factor. As previously
mentioned, bone density is likely to play an important role. Other factors effecting mini-
implant retention and stability include bone hardness, bone elasticity, trabecular orientation
and soft tissue thickness.
24
Tissue thickness has an impact on the success of mini-implant therapy since
screws must pass through the soft tissue. In cases with thin keratinized tissue there is often
less inflammation. In a study on 23 Korean cadavers the mean soft-tissue thickness
between the maxillary second premolar and first molar was 1.43 to 1.77mm (Kim et al,
2006).
The density and elastic moduli of the mandible ranges from 1747.0 to 2024.0
mg/cm
3
and from 10 to 29.9 respectively (Schwartz-Dabney and Dechow, 2003). The bone
density between the second premolar and first molar ranges from 1913.3 to 1968.6 mg/cm
3
(Schwartz-Dabney and Dechow, 2003). Ultimately, these factors have not been adequately
studied in the maxilla and mandible in relation to orthodontic mini-implants.
Clinical Applications
There are many ways in which skeletal anchorage is being applied to
orthodontic treatment. One common use is for maximum anchorage treatment of skeletal
Class II malocclusions and bimaxillary protrusion (Park et al, 2001). A typical mechanical
approach in this type of treatment entails the placement of the mini-implant in the
interradicular space between the second premolar and first molar. Retraction is achieved
via a NiTi spring placed between the mini-implant and a hook distal to the lateral incisors.
Skeletal anchorage is being used to effectively distalize molars. In recent
years maxillary molar distalization appliances such as the Distal Jet, Pendulum appliance,
or the Wilson appliance, have been used in the treatment of Class II malocclusions.
Studies have shown that, despite the use of palatal coverage to resist anchorage loss, there
is considerable anterior anchorage loss with the use of these appliances (Bolla et al, 2002).
25
The Distal Jet has been used with mini-implants to prevent anterior anchorage loss (Carano
et al, 2005).
Another clinical application of mini-implants is dental intrusion (Chang et al,
2004; Park et al, 2004), one of the most difficult tooth movements. Molar intrusion is
allowing the successful treatment of severe anterior open bites previously thought to be
only correctable through surgical posterior maxillary impaction (Park et al, 2004). At
present, no data exist regarding the stability of molar intrusion. Mini-implants are also
being used for incisor intrusion and treatment of vertical maxillary excess (Carano et al,
2005).
Mini-implants are being used routinely to upright molars (Park et al, 2004).
Molars tipped mesially into an extraction space, impacted 2
nd
molars and severe buccal or
lingual tipping are being successfully treated using mini-implant anchorage. Other uses
include molar mesialization (Kyung et al, 2003), correction of canted occlusal plane,
alignment of dental midlines (Carano et al, 2005), extrusion of impacted canines (Park et
al, 2004), and space closure. More obscure uses include intermaxillary anchorage
(elastics) (Carano et al, 2005), space closure with vacuum-formed splints and mini-implant
anchorage (Park et al, 2005), and intermaxillary fixation in lingual-orthodontic surgical
patients (Paik et al, 2002).
The most common complication with mini-implants in orthodontic treatment
is soft tissue irritation (Costa et al, 1998). Velo, in 2005, reported that only 2 of 543
samples displayed inflammation around the implant anchor (Velo, 2005). High mandibular
plane angle, inflammation of the peri-implant tissue, and fixture diameter less than 1.2mm
are risk factors for mini-implant mobility and failure (Miyawaki et al, 2003). Three-
dimensional (3D) bone and mini-implant finite element models suggest that the maximum
26
stresses are located around the neck of fixture, in the marginal bone (Gallas et al, 2005). It
is important that the cortical bone around the neck of the anchor implant is preserved
clinically in order to prevent failure.
Damage to the root structure is of concern to the clinician utilizing skeletal
anchorage. If a self-tapping screw encounters a root during manual implantation it is
immediately evident that a root has been contacted and any damage will be minimal
(Melsen and Verna, 2005). The use of low-speed drills may limit the clinician’s ability to
detect contact with a root due to the lack of tactile sensation (Melsen and Verna, 2005). To
avoid damage to root structure it is advised that screws be placed at an angle of 30˚ to 40˚
to the long axes of the teeth (Lee et al, 2001; Park et al, 2001). This minimizes the chance
of inadvertently damaging a root and also increases the span of mechanical interdigitation
between the mini-implant and cortical bone. This will ultimately enhance retention and
stability of the mini-implant.
Adjacent structures, such as nerves or blood vessels could potentially be
damaged during fixture implantation. With careful planning and presurgical radiographs
(CBCT, periapical or panoramic), this can be avoided. Although orthodontic forces are not
great enough to fracture a mini-implant, the torsional force associated with placement or
removal may occasionally result in fracture. This is more likely in subjects with thick and
dense cortical bone (Carano et al, 2005). Fracture is an additional reason that the use of
screws less than 1.2mm in diameter is not recommended (Mah et al, 2005). As with any
surgical procedure, there is always a risk for swelling and infection. Fortunately, there are
no major post surgical complications that have been reported to date (Carano et al, 2005).
27
Race and Ethnicity
Race and ethnicity are used within biomedical research to gain a better
understanding of human biology, specifically the relative underlying genetic and
environmental factors. Additionally, race and ethnicity are used as a scientific basis from
which to develop health policy (Bhopal and Donaldson, 1998). While race and ethnicity
represent separate but overlapping concepts, they are often used synonymously in
biomedical literature. Race was once thought of as a biological construct, which indicates
distinct genetically different populations (Chaturvedi and McKeigue, 1994). This notion is
no longer accepted and researchers have begun to define race as a social construct based
upon phenotypic genetic expression (Freeman, 1998). It appears that health outcomes are
derived from an interaction between race and social-environmental risk factors (Ford,
2002).
The concept of ethnicity is replacing the concept of race (Bhopal and
Donaldson, 1998). Ethnicity refers to a shared culture which may or may not include
shared origin, shared psychological characteristics and attitudes, shared language, religion
and cultural traditions. Ethnic boundaries are not well defined and are dynamic over time
(Ford et al, 2002).
According to Kaplan and Bennett (2003), there are three challenges to
researchers, clinicians, and policy makers when writing about race and ethnicity. They
include, accounting for the limitations of race/ethnicity data; distinguishing between
race/ethnicity as a risk factor or as a risk marker; and finding a way to write about
race/ethnicity that does not stigmatize a we/they dichotomy between health professionals
and populations of color (Kaplan and Bennett, 2003). Of particular interest is the
limitation of grouping people on the basis of race/ethnicity. Each category such as
28
“black,” Hispanic,” or “Asian,” does not have a uniform definition and individuals within
each group are heterogeneous. In studies using race or ethnicity data it is important that the
authors describe the way in which individuals were assigned to racial or ethnic category
(Kaplan and Bennett, 2003).
29
Chapter Three: Hypotheses
Research Hypotheses
1) There is a significant difference in the CBT measured between the second
premolar and first molar between African-American, Asian, Caucasian, and
Hispanic populations.
2) There is a significant difference in the CBT measured between the second
premolar and first molar between female and male populations.
3) There is a significant difference in CBT between the mandible and maxilla.
Secondary Research Hypotheses
1) There is no significant difference in CBT between left and right side of the same
jaw.
2) There is a significant difference in CBT measured between the second premolar
and first molar based upon distance from the crest of the alveolar ridge.
30
Null Hypotheses
Null Hypotheses
1) There is not a significant difference in the CBT measured between the second
premolar and first molar between African-American, Asian, Caucasian, and
Hispanic populations.
2) There is not a significant difference in the CBT measured between the second
premolar and first molar between female and male populations.
3) There is not a significant difference in CBT between the mandible and maxilla.
Secondary Null Hypotheses
1) There is a significant difference in CBT between left and right side of the same
jaw.
2) There is not a significant difference in CBT measured between the second
premolar and first molar based upon distance from the crest of the alveolar ridge.
31
Chapter Four: Materials and Methods
The primary data source was CBCT (cone beam computerized tomography)
volumetric data from the archives of the Redmond Imaging Center at the University of
Southern California School of Dentistry (USCSD). The study included 155 patients
(between the ages of 16-35) each from four ethnic groups (Asian, African-American,
Caucasian, Hispanic). Patient ethnicity was determined from a personal data sheet filled
out at the beginning of orthodontic treatment in the Department of Advanced Orthodontics
at the USCSD. The assumptions and inclusion and exclusion criteria are as follows:
Assumptions
1) No subjects had an undiagnosed metabolic bone disease.
Inclusion criteria:
1) Presence of second premolars and first molars in all dental quadrants.
2) Verification of ethnicity through photographs or patient information documents.
3) Age from 16 to 35.
4) High quality CBCT volumetric data.
Exclusion criteria:
1) Radiographic evidence of pathology within the maxilla or mandible.
2) Patients with known craniofacial anomalies.
3) Patients with missing first molar.
4) Patients with missing second premolar for non-orthodontic reason.
5) Patients with retained deciduous second molar.
32
6) Poor quality CBCT volumetric data with indistinct cortical borders, artifacts, or
blurring.
CBCT data were acquired using NewTom QR-DVT 3G (Quantitative
Radiology, srl, Verona, Italy). Scans were obtained with a 12” sensor providing a
reconstruction volume of 110 X 150 mm. The device acquires 360 images at 1 image per
angular degree intervals and imaging time is approximately 75 seconds. The image
resolution was 512 X 512 pixels and 12 bits per pixel. The reconstruction matrix voxel
was 0.25 X 0.25 X 0.3 mm (Walker et al, 2005).
Once a patient was selected they were assigned an anonymous
identification number and age, ethnicity, and gender were recorded into a spreadsheet.
Software supplied by Quantitative Radiology was used to produce a secondary
reconstruction of the volumetric data. Transaxial slices (1mm) were generated between the
second premolar and first molar in each quadrant of the maxilla and mandible. Each
transaxial slice was oriented perpendicular to the buccal plate of the alveolar ridge. The
transaxial images were inserted into a report template with ruler bars and then exported
using a PEERNET.DRV ePro 5.0 [216X280mm] print driver (Peernet Inc., Ontario,
Canada) to ensure magnification fidelity (Figure 6). Each report was subsequently stored
on a password locked external hard drive.
33
Figure 6. Report example. An example of the report exported from the Newtom 3G. The
report consists of 4 transaxial slices located between the second premolar and first molar in
each dental quadrant. The report also contains a mandibular occlusal image depicting the
location of the mandibular slices.
Linear measurements of the CBT were conducted on each transaxial image
using Photoshop CS2 (Adobe Systems Inc., San Jose, CA). CBT, defined anatomically as
the thickness from the periosteum to the cortical-trabecular interface (Schwartz-Dabney,
Dechow, 2003), was measured in each jaw at 3, 6, 9, and 12 mm increments from the crest
of the alveolar ridge along the long axis of the alveolar process. Reference planes were
created at each 3 mm intervals starting at the crest of the alveolar ridge using Adobe
Photoshop CS2 (Figure 7).
34
Figure 7. Transaxial images with reference planes. Each dental quadrant is shown with
the appropriate reference planes. Reference planes were placed at 3mm, 6mm, 9mm, and
12mm from the crest of the alveolar ridge oriented along the long axis of the ridge.
Data were collected and recorded in a spreadsheet using Excel 2004 for Mac
(Microsoft Corporation, Redmond, WA). Descriptive statistics, including mean, range,
mode, and standard deviation were obtained for each measurement site. The data was
35
subsequently imported into Statistical Package for the Social Sciences 14.0 (SPSS Inc.,
Chicago, Il) for the remaining statistical analysis. The right and left sides of each jaw and
the maxilla and mandible were compared using paired sample t-tests. The Mean CBT in
each quadrant at various distances from the alveolar ridge were compared via repeated
measures analysis of variance (RMANOVA). CBT and ethnicity were compared using a
univariate analysis of variance (ANOVA) with ethnicity as the fixed factor and
measurement location as the dependent variable. Bonferonni and LSD post hoc tests were
conducted to determine which non-null scenarios were likely to be true. Male and female
CBT was compared via independent samples t-tests.
The protocol for this study was submitted to the University of Southern
California, Health Sciences Campus, Institutional Review Board (IRB) for the review and
approval prior to the initiation of data collection. The study protocol was approved and an
approved proposal number, HS-06-00150, was assigned.
Internal validity (intra-rater reliability) was determined by re-measuring 15
randomly selected subjects 4 weeks after the initial measurement. Identical procedure and
standards were used during each measuring session. Mean CBT was compared via paired
samples t-tests for first and second measurements at each location. An intraclass
correlation coefficient was calculated for each measurement location to verify reliability.
The intraclass single measures and average measures correlation coefficients ranged from
0.85 to 0.97 and 0.92 to 0.99, respectively (Table 1).
36
Table 1. Summary of intraclass correlation coefficients for each site.
Measurement
Site
Single
Measures
Average
Measures
Measurement
Site
Single
Measures
Average
Measures
LR3mm 0.973 0.987 UR3mm 0.908 0.952
LR6mm 0.969 0.984 UR6mm 0.928 0.962
LR9mm 0.973 0.986 UR9mm 0.846 0.922
LR12mm 0.972 0.986 UR12mm 0.926 0.962
LL3mm 0.881 0.937 UL3mm 0.865 0.927
LL6mm 0.93 0.963 UL6mm 0.917 0.957
LL9mm 0.947 0.973 UL9mm 0.937 0.968
LL12mm 0.915 0.956 UL12mm 0.943 0.971
37
Chapter Five: Results
Patient Demographics
The sample consisted of 154 consecutively scanned patients from four ethnic
groups (Asian, African-American, Caucasian, Hispanic) (Table 2). The mean sample size
for each ethnic group was 38.5 subjects. The African-American sample (N=18) was less
than the mean while the Hispanic sample (N=51) was greater (Table 2 and Figure 8). The
gender distribution was almost equal among all patients and within ethnic groups.
Females (N=80) comprised 51.9% of the sample while males (N=74) accounted for the
remaining 48.1% (Table 2 and Figure 9). The age range was 16 to 35.5 years of age at the
time of the scan. The mean age was skewed to the left with a mean of 24 years of age
(Figure 10).
Table 2. Ethnic and gender composition of the sample population.
Female % Female Male % Male Total
Asian 24 57.1 18 42.9 42
African-
American
9 50 9 50 18
Caucasian 21 48.8 22 51.2 43
Hispanic 26 51 25 49 51
Total
Sample
80 51.9 74 48.1 154
38
Figure 8. Histogram demonstrating the distribution of ethnic groups and gender within
each ethnic group.
Figure 9. Histogram demonstrating the distribution of females and males within the
complete sample.
39
Figure 10. Histogram demonstrating the distribution of age within the complete sample.
Cortical Bone Thickness
Alveolar Height
The mean CBT (measured from the external cortical plate to the internal cortical
plate) varied at each site measured (Table 3 and Figures 11 and 12). The CBT was site
dependent. Within any given dental quadrant there was a significant (p<0.01) increase in
CBT as the distance from the crest of the alveolar ridge increased except for the two most
superior (closest to the alveolar crest) measurements in each quadrant of the maxilla
(between UR 3mm and UR6mm and between UL3 mm and UL 6mm) (Table 4 and 5,
Figure 11 and 12).
40
Table 3. Descriptive Statistics for CBT at each measurement site.
Mean Std Deviation Min Max
LR 3mm 1.70 0.28 1.1 2.5
LR 6mm 1.80 0.28 0.9 2.5
LR 9mm 1.96 0.33 0.9 2.9
LR 12mm 2.07 0.35 0.9 2.9
LL 3mm 1.60 0.29 0.9 2.9
LL 6mm 1.74 0.31 0.8 2.8
LL 9mm 1.87 0.33 0.9 2.8
LL 12mm 2.01 0.37 1.0 3.2
UR 3mm 1.32 0.20 0.8 1.9
UR 6mm 1.32 0.18 0.8 1.9
UR 9mm 1.37 0.20 0.8 2.5
UR 12mm 1.48 0.38 0.9 4.6
UL 3mm 1.23 0.21 0.8 2.0
UL 6mm 1.24 0.19 0.7 1.7
UL 9mm 1.32 0.28 0.8 3.4
UL 12mm 1.38 0.23 0.8 2.2
41
Figure 11. Histogram of mean CBT for each site surveyed in the mandible.
Figure 12. Histogram of the mean CBT for each site surveyed in the maxilla.
* Indicates lack of significant difference from 3mm measurement in the same dental quadrant.
42
Table 4. Summary of the mean difference between measuring sites in the mandible.
(I)
Factor1
(J)
Factor1
Mean
Difference
(I-J)
Std. Error
Sig.
(I)
Factor1
(J)
Factor1
Mean
Difference
(I-J)
Std. Error
Sig.
LR3 LR6 -0.10 0.02 <0.01 LL3 LL6 -0.13 0.02 <0.01
LR9 -0.26 0.03 <0.01 LL9 -0.27 0.02 <0.01
LR12 -0.37 0.03 <0.01 LL12 -0.41 0.03 <0.01
LR6 LR3 0.10 0.02 <0.01 LL6 LL3 0.13 0.02 <0.01
LR9 -0.16 0.02 <0.01 LL9 -0.14 0.02 0.00
LR12 -0.27 0.02 <0.01 LL12 -0.28 0.03 <0.01
LR9 LR3 0.26 0.03 <0.01 LL9 LL3 0.27 0.02 <0.01
LR6 0.16 0.02 <0.01 LL6 0.14 0.02 <0.01
LR12 -0.11 0.02 <0.01 LL12 -0.14 0.02 <0.01
LR12 LR3 0.37 0.03 <0.01 LL12 LL3 0.41 0.03 <0.01
LR6 0.27 0.02 <0.01 LL6 0.28 0.03 <0.01
LR9 0.11 0.02 <0.01 LL9 0.14 0.02 <0.01
Table 5. Summary of the mean difference between measuring sites in the maxilla.
(I)
Factor1
(J)
Factor1
Mean
Difference
(I-J)
Std. Error
Sig.
(I)
Factor1
(J)
Factor1
Mean
Difference
(I-J)
Std. Error
Sig.
UR3 UR6 0.00 0.01 0.71 UL3 UL6 -0.01 0.01 0.25
UR9 -0.06 0.01 <0.01 UL9 -0.10 0.02 <0.01
UR12 -0.19 0.03 <0.01 UL12 -0.16 0.02 <0.01
UR6 UR3 0.00 0.01 0.71 UL6 UL3 0.01 0.01 0.25
UR9 -0.06 0.01 <0.01 UL9 -0.09 0.02 <0.01
UR12 -0.18 0.03 <0.01 UL12 -0.15 0.02 <0.01
UR9 UR3 0.06 0.01 <0.01 UL9 UL3 0.10 0.02 <0.01
UR6 0.06 0.01 <0.01 UL6 0.09 0.02 <0.01
UR12 -0.12 0.03 <0.01 UL12 -0.06 0.02 0.01
43
Table 5, Continued.
UR12 UR3 0.19 0.03 <0.01 UL12 UL3 0.16 0.02 <0.01
UR6 0.18 0.03 <0.01 UL6 0.15 0.02 <0.01
UR9 0.12 0.03 <0.01 UL9 0.06 0.02 0.01
Maxilla and Mandible
The statistical data indicate there is a significant difference in CBT between the
maxilla and the mandible. The overall mean CBT was 1.84mm in the mandible and was
1.33mm in the maxilla. The mean difference of each pair (maxillary and mandibular
measurement in the same quadrant and with distance from alveolar crest) ranged from
0.38mm to 0.62mm (Table 6). The mean difference accounted for 29% to 47% of the
average maxillary CBT and 21% to 34% of the average mandibular CBT.
Table 6. Summary of the mean difference in maxillary and mandibular CBT.
Pair
Paired Difference
Mean
Standard
Deviation
Standard
Error Mean
Sig
LR3mm - UR3mm 0.382 0.285 0.03 <0.01
LR6mm - UR6mm 0.482 0.292 0.03 <0.01
LR9mm - UR9mm 0.590 0.290 0.03 <0.01
LR12mm - UR12mm 0.580 0.385 0.03 <0.01
LL3mm - UL3mm 0.373 0.286 0.02 <0.01
LL6mm - UL6mm 0.501 0.315 0.03 <0.01
LL9mm - UL9mm 0.551 0.365 0.03 <0.01
LL12mm - UL12mm 0.618 0.362 0.03 <0.01
44
Right and Left
The data indicated a significant difference in CBT between the right and left sides
of each jaw. The mean CBT was greater on the right side than on the left side for each
pairing (right and left sides of the maxilla or mandible at the same distance from the
alveolar crest) and the mean difference ranged from 0.05mm to 0.11mm (Table 7). The
mean mandibular CBT on the right side was 1.88mm, 0.07mm greater than the left side
(
!
x = 1.81mm). The maxillary CBT on the right side was 1.37mm, 0.08mm greater than the
left side (
!
x =1.29mm). These differences account for 4% to 8% of the mean maxillary
CBT and 3% to 6% of the mean mandibular CBT.
Table 7. Summary of the paired mean difference in CBT between the right and left sides of
the maxilla and the mandible.
Pair
Paired Difference
Mean
Standard
Deviation
Standard
Error Mean
Sig
LR3mm – LL3mm 0.094 0.293 0.02 <0.01
LR6mm – LL6mm 0.060 0.248 0.02 0.01
LR9mm – LL9mm 0.086 0.266 0.02 <0.01
LR12mm – LL12mm 0.065 0.238 0.02 <0.01
UR3mm - UL3mm 0.082 0.148 0.01 <0.01
UR6mm - UL6mm 0.077 0.149 0.01 <0.01
UR9mm - UL9mm 0.046 0.208 0.02 0.01
UR12mm - UL12mm 0.105 0.350 0.03 <0.01
45
Ethnic Groups
In 4 of the 16 surveyed sites there was a significant difference in CBT among
specific ethnic groupings. The sites in which significant differences were observed were
the mandibular left quadrant at the 6mm (LL6) and 9mm (LL9) sites and in the maxillary
left quadrant at the 3mm (UL3) and 6mm (UL6) sites (Table 8). At LL6 CBT was
significantly greater in the African-American (AA) group than in the Asian (A) and
Hispanic (H) groups by 0.22mm and 0.05mm, respectively. The mean CBT in the
Caucasian (C) group was 0.15mm greater than the A group (p<0.05). There were no other
significant differences in CBT among ethnic groups at LL6.
Table 8. Summary of the mean difference in CBT at the LL6 site.
Ethnicity (I) Ethnicity (J) Mean Difference (I-J) Std. Error Sig
A AA -0.22* 0.09 0.01
C -0.15* 0.07 0.04
H -0.05 0.07 0.42
AA A 0.22* 0.09 0.01
C 0.08 0.09 0.38
H 0.17* 0.08 0.05
C A 0.15* 0.07 0.04
AA -0.08 0.09 0.38
H 0.09 0.06 0.16
H A 0.05 0.07 0.42
AA -0.17* 0.08 0.05
C -0.09 0.06 0.16
* The mean difference is significant at the 0.05 level.
At LL9 the African-American CBT was significantly greater than all other ethnic
groups (Table 9). The mean CBT difference between AA and A, C, and H was 0.25mm,
0.22mm, and 0.23mm respectively. At UL3 the Hispanic and Caucasian groups were
significantly greater than the Asian group by 0.11mm and 0.10mm respectively (Table 10).
46
At UL6 the Hispanic group had thicker cortical bone than the Asian and Caucasian groups
by 0.11mm and 0.08mm respectively (Table 11).
Table 9. Summary of the mean difference in CBT at the LL9 site.
Ethnicity (I) Ethnicity (J) Mean Difference (I-J) Std. Error Sig
A AA -0.25* 0.09 0.01
C -0.03 0.07 0.64
H -0.02 0.07 0.82
AA A 0.25* 0.09 0.01
C 0.21* 0.09 0.02
H 0.23* 0.09 0.01
C A 0.03 0.07 0.64
AA -0.21* 0.09 0.02
H 0.02 0.07 0.79
H A 0.02 0.07 0.82
AA -0.23* 0.09 0.01
C -0.02 0.07 0.79
* The mean difference is significant at the 0.05 level.
Table 10. Summary of the mean difference in CBT at the UL3 site.
Ethnicity (I) Ethnicity (J) Mean Difference (I-J) Std. Error Sig
A AA -0.01 0.06 0.89
C -0.10* 0.05 0.03
H -0.11* 0.04 0.01
AA A 0.01 0.06 0.89
C -0.09 0.06 0.12
H -0.10 0.06 0.07
C A 0.10* 0.05 0.03
AA 0.09 0.06 0.12
H -0.01 0.04 0.73
H A 0.11* 0.04 0.01
AA 0.10 0.06 0.07
C 0.01 0.04 0.73
* The mean difference is significant at the 0.05 level.
47
Table 11. Summary of the mean difference in CBT at the UL6 site.
Ethnicity (I) Ethnicity (J) Mean Difference (I-J) Std. Error Sig
A AA -0.03 0.05 0.59
C -0.04 0.04 0.37
H -0.11* 0.04 <0.01
AA A 0.03 0.05 0.59
C -0.01 0.05 0.87
H -0.08 0.05 0.10
C A 0.04 0.04 0.37
AA 0.01 0.05 0.87
H -0.08* 0.04 0.05
H A 0.11* 0.04 <0.01
AA 0.08 0.05 0.10
C 0.08* 0.04 0.05
* The mean difference is significant at the 0.05 level.
It should be noted that ANOVA and the LSD post hoc indicated there was a
significant difference in 11 of 96 (9%) comparisons between CBT and ethnicity. The
Bonferonni test indicated significant difference for only 2 of 96 (2%) CBT comparisons
analyzed. The significant differences were located at the LL9 and UL6 sites. At LL9 the
African-American group was significantly greater than the Asian group (
!
x difference =
0.25mm) and at UL6 the Hispanic group was significantly greater than the Asian group (
!
x
difference = 0.11mm).
Gender
The data indicated that there is no statistical difference in CBT between gender
groups (within all ethnic groups) at all measurement sites except UL3 and UL6. Although
not statistically significant, CBT was greater in males at every measurement site (Figure
13, Figure 14). Mean CBT for each gender at each site and mean differences are shown in
Tables 12 and 13.
48
Figure 13. Histogram of the mean CBT for each gender group at each mandibular
measuring site.
Figure 14. Histogram of the mean CBT for each gender group at each maxillary measuring
site.
49
Table 12. Summary of mean CBT for females and males at each site.
Measurement Site Sex N Mean Std. Deviation
LR3mm F 75 1.66 0.29
M 70 1.74 0.27
LR6mm F 74 1.76 0.27
M 71 1.83 0.30
LR9mm F 76 1.94 0.33
M 72 1.97 0.33
LR12mm F 76 2.05 0.36
M 73 2.09 0.34
LL3mm F 77 1.58 0.26
M 72 1.62 0.32
LL6mm F 77 1.71 0.29
M 71 1.77 0.34
LL9mm F 77 1.86 0.31
M 73 1.89 0.34
LL12mm F 76 1.98 0.35
M 73 2.04 0.39
UR3mm F 74 1.30 0.20
M 62 1.34 0.19
UR6mm F 75 1.31 0.21
M 62 1.34 0.14
UR9mm F 75 1.36 0.17
M 64 1.39 0.23
UR12mm F 72 1.46 0.29
M 64 1.51 0.45
UL3mm F 78 1.19 0.21
M 66 1.27 0.19
UL6mm F 79 1.21 0.20
M 67 1.27 0.16
UL9mm F 77 1.31 0.25
M 66 1.34 0.31
UL12mm F 74 1.37 0.24
M 63 1.40 0.21
50
Table 13. Summary of the mean CBT difference between females and males at each site.
Measurement
Site
Equal Variances Significance
Mean
Difference
Std. Error
Difference
LR3mm Assumed 0.094 -0.08 0.05
Not assumed 0.094 -0.08 0.05
LR6mm Assumed 0.176 -0.06 0.05
Not assumed 0.177 -0.06 0.05
LR9mm Assumed 0.579 -0.03 0.05
Not assumed 0.579 -0.03 0.05
LR12mm Assumed 0.509 -0.04 0.06
Not assumed 0.509 -0.04 0.06
LL3mm Assumed 0.395 -0.04 0.05
Not assumed 0.398 -0.04 0.05
LL6mm Assumed 0.208 -0.07 0.05
Not assumed 0.211 -0.07 0.05
LL9mm Assumed 0.621 -0.03 0.05
Not assumed 0.622 -0.03 0.05
LL12mm Assumed 0.317 -0.06 0.06
Not assumed 0.318 -0.06 0.06
UR3mm Assumed 0.313 -0.03 0.03
Not assumed 0.311 -0.03 0.03
UR6mm Assumed 0.267 -0.03 0.03
Not assumed 0.250 -0.03 0.03
UR9mm Assumed 0.423 -0.03 0.03
Not assumed 0.433 -0.03 0.03
UR12mm Assumed 0.405 -0.05 0.06
Not assumed 0.417 -0.05 0.07
UL3mm Assumed 0.018* -0.08 0.03
Not assumed 0.017* -0.08 0.03
UL6mm Assumed 0.049* -0.06 0.03
Not assumed 0.045* -0.06 0.03
UL9mm Assumed 0.468 -0.03 0.05
Not assumed 0.476 -0.03 0.05
UL12mm Assumed 0.521 -0.02 0.04
Not assumed 0.516 -0.02 0.04
* Indicates statistical significance (p<0.05)
Gender Differences within Ethnic Groups
When comparing gender groups within ethnic groups the results were distinct from
the findings for the sample as a whole. For the Asian, Caucasian, and Hispanic groups
there was no significant difference in CBT between males and females at any measurement
51
site. In the African-American group there was a significant difference in CBT at UR6,
UL3, UL6, and UL9 (Table 14). Additionally, the male CBT was not greater than female
CBT at all measurements as was the case when comparing females and males among all of
the sampled ethnic groups (Figures 15-18).
Table 14. Summary of the mean CBT for African-American females and African-
American males at each site.
Measurement Site Sex N Mean Mean Difference Std. Deviation
LR3mm F 9 1.66 0.18 0.32
M 8 1.89 0.18 0.36
LR6mm F 9 1.79 0.61 0.34
M 8 1.88 0.61 0.35
LR9mm F 9 2.07 0.56 0.34
M 8 1.98 0.55 0.28
LR12mm F 9 2.20 0.88 0.35
M 9 2.18 0.88 0.25
LL3mm F 9 1.61 0.69 0.28
M 7 1.69 0.71 0.45
LL6mm F 9 1.88 0.95* 0.31
M 9 1.87 0.95* 0.41
LL9mm F 9 2.02 0.52 0.38
M 9 2.13 0.52 0.34
LL12mm F 9 2.16 0.65 0.40
M 8 2.24 0.64 0.32
UR3mm F 9 1.27 0.18 0.17
M 8 1.36 0.17 0.09
UR6mm F 9 1.20 0.02 0.17
M 8 1.39 0.02 0.11
UR9mm F 9 1.33 0.14 0.07
M 9 1.44 0.15 0.20
UR12mm F 9 1.43 0.40 0.09
M 8 1.53 0.44 0.31
UL3mm F 9 1.03 0.00** 0.20
M 8 1.33 0.00** 0.09
UL6mm F 9 1.11 0.01** 0.14
M 8 1.34 0.01** 0.16
UL9mm F 9 1.13 0.04* 0.17
M 8 1.35 0.05* 0.23
UL12mm F 9 1.28 0.31 0.16
M 7 1.37 0.33 0.20
* Indicates statistical significance (p<0.05)
** Indicates statistical significance (p<0.01)
52
Figure 15. Histogram comparing female and male CBT within the Asian sample.
Figure 16. Histogram comparing female and male CBT within the African-American
sample.
53
Figure 17. Histogram comparing female and male CBT within the Caucasian sample.
Figure 18. Histogram comparing female and male CBT within the Hispanic sample.
54
Chapter Six: Discussion
Orthodontic anchorage is one of the most critical concerns in orthodontics.
Traditional anchorage appliances (e.g. headgear, LLHA, Nance, TPA, etc.) have many
limitations. Reports indicate that these appliances enhance anchorage but they are not able
to eliminate tooth movement in the direction of the reactive force applied to the teeth
(Egolf and BeGole, 1990; Kyung et al, 2003; Bobak et al, 1997). Mini-screws provide a
compliance independent means of absolute skeletal anchorage and can prevent undesirable
tooth movement.
Orthodontic mini-implants are placed in the maxilla and mandible and do not
osseointegrate like traditional endosseous implants. Mini-implant retention and stability is
dependent upon the mechanical interdigitation between the cortical bone and implant
surface (Huja et al, 2005; Miyawaki et al, 2003; Struckhoff et al, 2006). The success of
mini-implant retention is dependent upon the thickness of the cortical bone (Kim et al,
2006).
Alveolar Height
Cortical bone thickness was site dependent and it increased as the distance from
the alveolar ridge increased except for the two most superior measurements in each
maxillary quadrant. A study of 10 human dentate mandibles reported a similar trend
(Schwartz-Dabney and Dechow 2003). This study measured CBT throughout the
mandible on 7 male (
!
x age= 64.3) and 3 female (
!
x age= 62.0) Caucasian donors. They
found that CBT in the body of the mandible increased from the alveolar crest to the inferior
border. The CBT in the first molar region in the superior, middle, and inferior third of the
body of the mandible ranged from 1.401mm to 1.86mm, 2.321mm to 2.78mm, and
55
2.781mm to 3.24mm respectively. In most instances the 3, 6, and 9mm measurements in
this study would correspond to the superior site of the Schwartz-Dabney and Dechow study
and the 12mm measurement would appear between the superior and middle site, although
it is dependent upon the overall height of the body of the mandible. These results appear to
agree with our finding of an increasing CBT apically, and a mean mandibular CBT of
1.83mm.
Some studies have not indicated any change in CBT related to vertical height from
the alveolar crest (Carano et al, 2005; Kruger et al, 1986). The trend observed in this study
contradicts a recent study, in which CBT was measured on Korean cadavers (Kim et al,
2006). In the Kim et al study, CBT was measured between the maxillary second premolar
and first molar at 2, 4, 6, 8, and 10 mm intervals from the CEJ on decalcified sections of
the maxilla in 16 men and 7 women. They found that CBT was greatest in the apical and
coronal portions (Table14).
Table 15. Summary of CBT between the maxillary second premolar and first molar of 23
Korean cadavers (Kim et al. 2006).
Measurement Site Mean Std. Dev Measurement Site Mean Std. Dev
U2mm 1.33 0.40 U8mm 1.13 0.41
U4mm 1.18 0.39 U10mm 1.17 0.40
U6mm 1.20 0.46 Total 1.20
Their data indicate a decrease in maxillary CBT as the distance from the alveolar
ridge increases with a slight increase in the final measurement (10mm). There are several
possible reasons for the discrepancy. In the Kim et al study all measurement were made
perpendicular to the cortical bone surface whereas our measurement were made
perpendicular to the long axis of the alveolar ridge. While their method allows for the
56
accurate quantification of CBT it is not as clinically relevant because mini-implants cannot
always be oriented perpendicular to the cortical bone due to concavities and convexities in
the buccal plate. Another possible explanation involves the discrepancy in sample. Our
sample was comprised of four ethnic groups between the ages of 16 and 35. The Kim et al
sample consisted of 23 Korean cadavers with a mean age of 49.5. Additionally, the power
of the Kim et al study is not ideal due to the small sample size.
Maxilla and Mandible
There was a significant difference in CBT between the maxilla (
!
x = 1.33mm) and
the mandible (
!
x = 1.83mm). These findings agree with clinical observations that cortical
bone of the mandible is thicker and denser than maxillary cortical bone (Peterson et al,
1998). The Schwartz-Dabney and Dechow (2003) study found that CBT increased with
increasing distance from the alveolar ridge and that CBT ranged from 1.401mm to 1.86mm
in the region of the majority of the mandibular measurements in this study. Kim et al
(2006) reported a mean maxillary CBT between the second premolar and first molar of
1.20mm. The slight difference in mean maxillary CBT can be explained by the difference
in measurement protocol. Their protocol involved measurement at 2mm intervals stopping
at 10mm whereas our protocol involved measurements in 4mm intervals stopping at
12mm. Without the 12mm measurement and with more measurements at the coronal
portion of the ridge, where the CBT is thinner, one would expect the mean to be less. The
mean maxillary CBT in these two studies compares favorably.
Several studies do not report a trend of increasing CBT apically along the alveolar
ridge. According to Carano et al (2005), CBT is 2-3mm in the maxilla and the mandible.
They do not make any vertical reference for this range. Kruger et al (1986), report in their
57
text that CBT in the mandibular molar region is 3.1 to 3.2mm. They do not indicate a
precise location of this thickness, indicating that cortical bone is static in the mandibular
molar region. However, differential redistribution of cortical bone is well documented and
has been proposed as an important factor influencing the mechanical properties of the
mandible (Daegling and Grine, 1991).
The difference in maxillary and mandibular CBT may account for some of the
differences in mini-implant performance in the two jaws (Umemori et al, 1999). A
difference in CBT between the maxilla and mandible makes sense on a functional level.
Thicker mandibular CBT is essential to sustain the masticatory load since the mandible
must sustain the entire functional load, whereas the maxilla can transfer a portion of the
load to the underlying cranial base.
Right and Left
There was a significant difference between CBT in the right and left sides of the
maxilla and mandible. The mean CBT in the right side was 0.075mm greater than the left
side in both jaws. There is currently no other research that has reported such a finding.
There are two possible explanations for this discrepancy. CBT has been related to
musculature. If a patient’s chewing cycle has a large transverse component it would be
expected that medial pterygoid and masseter muscles would be more active on one side,
therefore leading to an increased deposition of cortical bone. Whether this would actually
effect CBT in the region surveyed is possible, however unlikely.
Another explanation is the long capture interval of the Newtom 3G. An imaging
session is typically 75 seconds (Mah et al, 2003). It can be difficult for patients to remain
completely immobile for over a minute. It is therefore possible that there is some
58
distortion on one side due to movement late in the imaging cycle. Because the right side of
each jaw is larger than the left side of the same jaw by the same factor (0.075mm), a
distortion error is quite plausible. Linear and geometric accuracy of the Newtom has been
verified using inanimate objects. However, if this theory is correct it may indicate that the
Newtom and other CBCT capture devices are not as accurate in patients due to distortion
late in the capture cycle.
The discrepancy between the maxillary right and left and mandibular right and left
is less than 0.1mm. This error may potentially be a manifestation of human measuring
error; however, the fact that the standard deviations for measurements on each side of each
jaw are equal, human measuring error is not a very likely explanation. Ultimately, more
research is needed to verify the discrepancy in CBT between the right and left sides of each
jaw and future studies are necessary to explain the reason for any disparity. Regardless,
the side of the jaw is a factor which must be considered when treatment planning
orthodontic cases with temporary anchorage devices.
Ethnic Groups
There was a significant difference in CBT in 4 of the 16 sites surveyed (LL6, LL9,
UL3, and UL6) or 9% of comparisons made. The four sites with ethnic differences in the
maxilla have a mean difference ranging from 0.08 to 0.11mm. The other 5 significant
ethnic differences have a mean difference ranging from 0.17 to 0.25mm. The Bonferonni
post hoc test indicated a significant difference in only 2 of the 96 ethnic comparisons.
Although the data indicated that there was not a significant difference in CBT
among ethnic groups at the majority of sites surveyed several trends can be observed. CBT
was greatest in the AA group at all mandibular sites except LL3 (Figure 19). The C group
59
had the second greatest CBT in all quadrants except LL3 where C CBT was greater than
AA CBT. CBT in the A and H groups was less than the CBT in the AA and C groups for
all mandibular measurements. Trends in maxillary CBT are not as distinct as those in the
mandible. The greatest maxillary CBT was in the H (6/8) and C (2/8) groups exclusively
(Figure 20). The C group had the second greatest maxillary CBT at each site except one
when not the greatest overall. Aside from these weak trends the hierarchy of maxillary
CBT is less clearly defined in the maxilla than in the mandible. This may be due to the
fact that the maxillary cortical bone is thinner resulting in less variance.
Significant differences in CBT between ethnic groups were expected at more of the
surveyed sites. Ethnic differences in tooth root length and crown size and shape are well
documented (Bishara et al, 1989; Harris and Rathbun, 1991; Lavelle, 1972; Reinheimer,
2005). There is anecdotal evidence that certain ethnic groups have thicker and denser
cortical bone. As previously mentioned, we suspect that there is distortion in the
volumetric data obtained late in the imaging cycle, which may present as increased CBT on
the right side of each jaw. If this is true, the distortion may have masked ethnic differences
in the right side of the mandible and maxilla. Because the results indicate that there are
ethnic differences at some sites, we conclude that ethnicity must be taken into account
when treatment planning orthodontic cases in which mini-implants will be used.
60
Figure 19. Histogram demonstrating the mean CBT for each ethnic group at each
mandibular measuring site.
Figure 20. Histogram demonstrating the mean CBT for each ethnic group at each maxillary
measuring site.
61
Gender
CBT was greater in males than in females at each site surveyed, although only
significant at UL3 and UL6. At UL3 the mean difference was 0.08mm and at UL6 the
mean difference was 0.06mm. Two of the four sites exhibiting significant differences in
CBT between ethnic groups were also located at UL3 and UL6. The Bonferonni post hoc
test indicated no significant difference between ethnic groups at UL3.
The Schwartz-Dabney and Dechow (2003) study found that CBT did not differ by
gender group. Another study utilizing CT scans of 66 Koreans (33 male, 33 female), found
that maxillary and mandibular teeth showed thicker bone plates in men than women, but
was not statistically significant (Jin et al, 2005).
Gender Differences within Ethnic Groups
Within the AA group there was a significant difference in CBT in half of the
maxillary sites (UR6, UL3, UL6, UL9). There was not a significant difference in CBT
between gender groups within any of the other ethnic groups. Again, UL3 and UL6 are
sites of significant difference. The AA group was compromised of 9 males and 9 females.
The validity of these findings is questionable due to the small sample size.
Clinical Implications
Cortical bone thickness is one of several important factors determining the success
of mini-implants used for orthodontic treatment. An understanding of factors effecting
CBT is important so that the clinician can place mini-implants in a site which limits
implant failure and offers the mechanical vantage required for the desired tooth movement.
Ideally, to limit implant failure, it is important that mini-implants are placed in an area with
62
thick and dense cortical bone, while remaining in attached keratinized tissue, so as to
minimize inflammation and infection.
At present there is much debate as to what implant placement protocol is best.
Some clinicians maintain that mini-implants should be placed utilizing a pilot hole in the
cortical plate, while others believe that self-tapping implants are more than adequate. It
has been suggested that a pilot hole be used in cases where the cortical plate is greater than
2mm thick (Melsen, 2005). An understanding of factors related to CBT may assist the
clinician in determining the necessity of a pilot hole for mini-implant placement.
When placing mini-implants between the second premolars and first molars, we
recommend placing the implant at the apical limit of the attached gingiva. This should limit
mini-implant failure because it ensures that the fixture is placed in the thickest cortical
bone while remaining in attached tissue. Placing the implant in thicker cortical bone will
enhance the mechanical retention of the fixture. Placing the implant in attached tissue will
limit post-operative peri-implant inflammation, the leading cause of mini-implant failure.
The difference in maxillary and mandibular CBT has important clinical
implications. Because CBT between the second premolar and first molar in the maxilla
(
!
x = 1.33mm) was thinner than in the mandible (
!
x = 1.84mm) a larger diameter mini-
implant should be used in the maxilla than in the mandible. With a larger fixture diameter,
the interdigitation between the cortical bone and the implant surface will occur over a
greater surface area, thereby enhancing retention and stability of the implant anchor.
There was a significant difference in CBT between the right and left sides of the
maxilla and the mandible. There was an overall trend that CBT was greater in males than
in females at all sites measured. There were four sites where there was a significant
63
difference in CBT between ethnic groups. Therefore, the side of the jaw, gender, and
ethnicity should all be considered when treatment planning mini-implants in orthodontics.
Study Limitations
The ideal method to measure CBT in the maxilla and mandible is in a cadaver
sample. A large study of this nature is often impractical and it would be difficult to obtain
an adequate sample size from which true conclusions can be drawn.
It is suspected that the stature and the size of the maxilla and mandible may have
an effect on CBT. In this study it was not possible to account for the size of the body, the
maxilla, and/or the mandible because the database did not contain that information. At
present, there are no studies that indicate that stature or the size of the maxilla and
mandible has an effect on CBT between second premolars and fist molars. Numerous
studies have shown that there is not a correlation between stature and tooth size (Filipsson
and Goldson 1963; Garn et al, 1958). Several studies have shown that there is no
difference in CBT between gender groups. If stature has an effect on CBT a difference in
CBT between gender groups would be expected. The effect of stature on CBT is in need
of further investigation.
A limitation of this study is the small sample and the unevenness of the groups,
especially the AA group. In addition, each subject was in the database due to some sort of
maxillofacial discrepancy. Ideally norms for CBT should be derived from a population
void of maxillofacial pathology, skeletal imbalance, or esthetic disharmony.
The maxillary CBT values were artificially increased at greater distances from the
alveolar ridge. The reason is that the cortical bone surface began to deviate from the long
axis of the ridge, thus producing a measurement site that traversed the cortical plate at an
64
angle rather than perpendicular to the cortical bone surface. Also, due to the close
proximity of the maxillary sinus, the cortical borders were not as easily identified at U9
and U12 sites. Often the cortical borders in the CBCT images would appear diffuse and
indistinct at the 12mm measurement site.
Other limitations include distortion due to patient movement and measuring error.
Cortical borders are often indistinct and require a certain amount of extrapolation by the
measurer. This may artificially increase or decrease CBT values.
Figure 21. Factors effecting measurements at U9 and U12. Both images illustrate the
artificial increase in CBT at U12 as the cortical plate deviates from the long axis of the
alveolar process. The circled area (left) depicts the convergence of the cortical plate and
the inferior border of the maxillary sinus. This can make it difficult to accurately define the
cortical borders.
65
Future Studies
With the increasing use of mini-implants in orthodontics, there is a need to further
investigate the bone at various placement sites. This information can help clinicians select
the most appropriate site for placement, with regard to bone and soft tissue, while
balancing the mechanical demands of the orthodontic treatment.
This study examined CBT at only one of many mini-implant sites. Future studies
are needed in order to characterize trends in CBT at other locations. In addition, more
studies are needed to evaluate the effect of bone density, hardness, and elasticity on fixture
stability and retention. Bone density, hardness, and elasticity and the changes of each
within any given placement site are poorly documented at this time.
66
Chapter Seven: Conclusions
1) Cortical bone thickness CBT increases towards the apical regions of the alveolar
process.
2) There is a significant difference in CBT between the maxilla and the mandible.
a. The mean maxillary CBT between the maxillary second premolar and
maxillary first molar was 1.33mm.
b. The mean CBT between the mandibular second premolar and first molar
was 1.84mm.
3) An unexpected finding of this study was that cortical bone thickness was
consistently significantly greater on the right side than the left side in both jaws.
4) Mandibular cortical bone is thicker in African-American patients, followed by
Caucasian patients, and Hispanic and Asian patients at all mandibular sites except
LL3, where Caucasian CBT is thicker than African-American CBT. The
difference in CBT is only statistically significant at LL6 and LL9 between the
African-American and Caucasian groups and the H and A groups.
a. LL6: African-American > Caucasian >> Hispanic > Asian
b. LL9: African-American > Caucasian >> Hispanic >Asian
5) There is a significant difference in maxillary CBT at UL3 and UL6. The
difference is statistically significant between the Hispanic and Asian groups and
the Caucasian and Asian groups at UL3 and between the Hispanic and Asian
groups and the Hispanic and Caucasian groups at UL6.
a. UL3: Hispanic > African-American > Caucasian > Asian
b. UL6: Hispanic > Caucasian > African-American > Asian
67
6) Cortical bone thickness is greater in males than in females but the difference is
only statistically significant at UL3 and UL6.
Because there are significant differences in CBT between the maxilla and the
mandible and the right and left sides of each jaw, we conclude that the jaw (maxilla or
mandible) and side of the jaw (right or left) must be taken into account when treatment
planning orthodontic cases utilizing mini-implants. In addition, because the results of this
study indicated there are sites with significant differences in CBT between gender groups
and ethnic groups, we conclude that ethnicity and gender must be taken into account for
treatment planning temporary anchorage devices.
68
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Abstract (if available)
Abstract
The objective of this study was to compare cortical bone thickness (CBT) between gender groups, ethnic groups, the maxilla and mandible, and the right and left sides of each jaw. CBCT volumetric data was used to generate transaxial slices between the second premolar and first molar and CBT was measured 3, 6, 9, and 12mm from the crest of the alveolar ridge in each dental quadrant. Mandibular CBT (mean = 1.84mm) was significantly greater than maxillary CBT (mean = 1.33mm) and CBT in the right side of each jaw was greater than the left side. There was a significant difference in CBT between ethnic groups at 4 of 16 surveyed sites. CBT was greater in males than females at all sites but was only significant at UL3 and UL6.
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Humphries, Sage Monroe
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Core Title
Comparison of cortical bone thickness between second premolars and first molars in the maxilla and mandible in four ethnic groups
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Master of Science
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Craniofacial Biology
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2007-05
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absolute anchorage,cortical bone thickness,ethnicity and cortical bone thickness,mini-implant,miniscrew,OAI-PMH Harvest,temporary anchorage device
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