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Cirtual 3D placement of temporary orthodontic anchorage implants
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Cirtual 3D placement of temporary orthodontic anchorage implants
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Content
VIRTUAL 3D PLACEMENT OF TEMPORARY ORTHODONTIC ANCHORAGE
IMPLANTS
by
Craig Cheung
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 2009
Copyright 2009 Craig Cheung
ii
Table of Contents
List of Tables iv
List of Figures v
Abstract vi
Chapter 1: Introduction 1
Chapter 2: Review of Literature 4
Temporary Orthodontic Mini Screw 4
Mini Screw History 4
Mini Screw Materials 6
Mini Screw Design 6
Mini Screw Stability 9
Orthodontic Mini Screw Loading 10
Site Selection 11
Mini Screw Failure 13
Clinical Uses 14
Complications 16
CBCT 18
History 18
How Cone Beam Computed Tomography (CBCT) works 21
Image Resolution: Pixel and Voxel Size 23
Temporary Orthodontic Mini Screws and CBCT 24
CBCT Radiation Dosage 24
CBCT Accuracy 25
Limitations of CBCT 26
DICOM 27
Chapter 3: Assumptions 30
Chapter 4: Limitations 31
Chapter 5: Delimitations 32
Chapter 6: Materials and Methods 33
Sample Description 33
Measurement Methods 33
CT Conditions 33
Methodology 34
Reliability of the measurements 37
iii
Chapter 7: Results / Data Analysis 41
Chapter 8: Discussion 48
Conclusion 54
References 55
iv
List of Tables
Table 1: Sample 41
Table 2: Sample Age 41
Table 3: Female Measurements 41
Table 4: Male Measurements 42
Table 5: Average of Male and Female Measurements 42
Table 6: Differences in the Average Lengths
Between Male and Females 43
Table 7: Pearson Correlation Coefficients 43
Table 8: Average Distances by Age Group 44
Table 9: Pearson Correlation Coefficients
for Distances and Age Groups 45
v
List of Figures
Figure 1: Linkow’s use of blade implants 4
Figure 2: Creekmore and Eklund’s use of bone implants 5
Figure 3: Orthodontic mini screw design and features 7
Figure 4: Wire splint for mini screw implantation 13
Figure 5: Mini screw implants for posterior intrusion 15
Figure 6: Mini screw implants for class II correction 15
Figure 7: Mini screw implants for class III correction 16
Figure 8: CT vs. CBCT 20
Figure 9: CT vs. CBCT RAW DATA 20
Figure 10: Voxel size 22
Figure 11: Image Compression 29
Figure 12: Insertion site for virtual mini screw on photograph 35
Figure 13: Cast measurement for triangulation 35
Figure 14: Virtual mini screw and triangulation of insertion
site on 3D volume 37
Figure 15: Virtual mini screw inserted, distances measured 38
Figure 16: Axial view of virtual mini screw 39
Figure 17: Lack of resolution of lingual cortical plate 40
vi
Abstract
The purpose of this study is to calculate the 3D position of a virtual mini screw using
CBCT imaging. Patient records and DICOM images (n=92) were obtained from USC
Graduate Orthodontics. With the aid of patient photographs and casts, a virtual mini
screw implant was implanted between the first and second premolar on the 3D image.
3D positions from anatomical landmarks were measured using InVivo software. Results
showed that the average mini screw to second bicuspid root distance is 0.83 ± .24 mm,
and the mini screw to first molar root distance is 0.85 ± .22 mm. The average implant to
the molar root apex distance is 8.6 ± 2.1 mm. Due to the lack of resolution of the 3D
images, the horizontal measurements (lingual cortical plate to mini screw tip) could not
be measured. Not all 3D coordinates could be measured with the given 3D data, but
useful clinical data can be derived from the results.
1
Chapter 1: Introduction
Temporary mini screws have revolutionized the way orthodontist approach and treatment
plan tooth movements. Newton’s third law of motion, “For every action there is an equal
and opposite reaction”, also applies to orthodontics. The goal of an orthodontist is to
achieve desired tooth movements without producing undesirable side effects.
Traditionally, anchorage is reinforced by increasing the number of teeth, by using the oral
musculature, or by using extraoral appliances. Also, to protect anchorage loss,
orthodontic manufactures have focused and researched on improving a frictionless
appliance system to reduce the amount of undesirable tooth movements. Mini screws can
act as an absolute anchor, and the side effects of orthodontic tooth movement can be
completely avoided. In 1970, Linkow was one of the first clinicians to propose the use of
the blade implant as anchorage for class II elastics.
2
Linkow placed bilateral blade
implants in the mandible, to which he used elastics from the upper teeth to the lower
implants. In 1983, Creekmore and Eklund used a bone screw below the anterior nasal
spine for intrusion of the maxillary teeth. Since the wide spread acceptance of mini
screw implants as orthodontic anchorage, there have been numerous reports of their
clinical efficacy in providing anchorage against unwanted orthodontic forces.
3, 4
Mini
screw implants have been used in a variety of tooth movements such as: protraction,
retraction, intrusion and extrusion.
Mini screw implants can also be used in treating a Class II malocclusion patient.
Retrusive mandibles are often the cause of a class II malocclusion, but a protrusive
2
maxilla can also be the cause. Several conventional options are available to treat a class
II malocclusion with a protrusive maxilla. Treatment consists of extractions, headgear,
class II elastics, and/or functional appliances. Extraction of 1
st
premolars is a viable
treatment, but it does not leave the patient in an ideal occlusion, with the 1
st
molar in a
class II relationship. Headgear has proved to be an effective tool, but it requires patient
compliance. Class II elastics and functional appliances also have proved to be successful
in treatment of a class II malocclusion, but it also requires patient compliance and causes
the lower incisors to flare labially.
By placing mini screw implants between the second bicuspid and the first molar,
maxillary arch distalization can be achieved without side effects or patient compliance,
thus correcting a class II malocclusion. Advantages of using mini screw implants as
orthodontic anchorage include ease of application, minimal patient compliance, and the
ability to immediately load. Since mini screw implants do not osseointegrate, the
surgical procedure for inserting or removing the mini screw is straightforward, with
minimal complications. Studies have proved the efficiency of maxillary arch distaliztion
with mini screws, but have never addressed the amount of distalization before the root
contacts the mini implant.
With the advent of the cone beam computed tomography (CBCT) and 3D software, the
oral facial complex can accurately be reproduced on to a computer screen, and unlike a
2D image, distortion, magnification and superimpositions can be completely avoided.
3
The purpose of this study is to calculate the 3 dimensional position of a mini screw
implant relative to adjacent teeth. With this information, clinicians can accurately
treatment plan mini screw implants, by estimating the amount of maxillary arch
distalization or protraction before the roots contact the mini screw.
Chapter 2: Review of Literature
Temporary Orthodontic Mini Screw
Mini Screw History
The use of mini screw implants for orthodontic anchorage is not a novel idea. In 1945,
the concept of using a steel pin attachment to the mandible was intended not only to
move teeth, but to correct for a skeletal class II malocclusion by “exerting pull on the
mandible”.
5
In 1970, Linkow was the first clinician to propose use a blade implant as
anchorage for a skeletal class II correction (figure 1).
2
To correct the malocclusion,
Linkow used elastics from the blade implant to the maxillary arch.
Figure 1) Linkow’s use of blade implants in the mandibular arch for
orthodontic anchorage in skeletal class II correction
In 1983, Creekmore and Eklund placed a bone screw below the anterior nasal spine to
intrude maxillary incisors (figure 2).
6
4
Figure 2) Creekmore and Eklund used a bone screw at the anterior nasal spine to
intrude maxillary incisors
In 1998, Block and Hoffman designed a thin titanium palatal onplant as a skeletal
anchorage device for orthodontics. The design consisted of a textured and coated
hydroxyapatite surface on one side, and a threaded hole on the opposite side for the
abutment. After a healing period and osseointegration of approximately 12 weeks,
orthodontic forces were loaded on the abutment.
7
Another study involved placing
vitallium (Dentsply) screws in beagle dogs. This study concluded the that “immediate
MSI (Mini Screw Implant) loading with light forces (25 and 50 g) can be accomplished
5
6
with high rates of success, producing clinically relevant amounts of tooth movement that
are not influenced by the amount of force or the location at which they are applied”.
8
Mini Screw Materials
Currently, titanium has become the standard for mini screw implants. Titanium is
absolutely inert in the human body, immune to attack from bodily fluids, compatible with
bone growth, strong and flexible. Under stress and fatigue, titanium is completely
resistant to the body’s environment due to its protective oxide layer, which forms
naturally in the presence of oxygen. The oxide layer film also gives titanium its unique
soft grey appearance. This layer is highly adherent and insoluble, preventing any
reaction from the body’s tissues. Titanium is lighter than stainless steel, and yet it has a
yield strength twice that of stainless steel. This gives titanium the highest strength to
weight ratio of any metal suited for dental use. Also, titanium’s modulus of elasticity and
coefficient of thermal expansion are similar to bone, reducing the potential of implant
failure.
Mini Screw Design
A typical mini screw implant has three basic components: a core, a helix (the thread), and
a head (figure 3).
9, 10
Each component is crucial to the function of the mini screw. The
head of the screw provides a means for applying twisting torque to the core and thread,
and also acts as a point of orthodontic force application. Most mini screw implants are of
a male type head design. This provides an articulation point to the screwdriver and offers
control during implantation. The core forms the support of the screw and is wrapped in a
helical thread. The cross-sectional area of the core is extremely important to the strength
of the mini screw, because the core diameter ultimately determines the torsional strength.
9, 10
The greater the core diameter, the lower the incidence of screw fracture during the
implantation procedure. The shank extends from head to the beginning of the threads.
The pitch is the distance between the threads on the screw. The lead of the screw is the
distance the screw will advance with each 360 degree turn. In a single threaded screw,
the pitch will equal the lead.
Core
Head
Helix
(thread)
Lateral Cutting
Groove
No Thread
Neck
Increasing Core
Diameter
Figure 3) Temporary orthodontic mini screw implant design features
Of the characteristics of a mini screw implant, the length has a minor effect on
distribution of stress, and the thread design and the diameter have the most significant
effect. According to finite-element analysis, implants that extend 4.0 mm in bone, and
7
8
implants that extend 6.0 mm in bone show a negligible difference in stress distribution.
11
However, clinically, mini screw implants extending 4.0 mm into bone showed
unsatisfactory success rates. The 4.0 mm mini screw showed a high rate of failure, due to
the insufficient length to clinically engage cortical bone. Therefore, at least 5.0 mm of
screw length is needed to account for the tissue thickness and to effectively engage bone.
A mini screw length of 5.0 mm or more has not shown any increase in load distribution,
unless it is used for bicortical anchorage.
12
Mini screw diameter has a significant effect
on stress distribution in bone. The larger the diameter of the mini screw allows for a more
favorable stress distribution. According to 3D finite element model analysis, a 1.4 mm
diameter implant that is placed in cortical bone (1.2 mm thick) can tolerate 150g of
orthodontic force, while a 1.8 mm diameter implant can tolerate 350g of orthodontic
force.
11
To increase the success rate of a mini screw implantation, features have been added.
These features include: a threadless coronal section of the screw; a tapered core from
apical to coronal; a lateral cutting groove; and a sandblasted or acid etched surface. The
threadless cylindrical neck helps to control tissue overgrowth, and aids in the soft tissue
adaptation. The tapered core increases stability, by condensing bone as the mini screw is
inserted into cortical bone. Mechanical retention of the mini screw to bone can increase
by sandblasting or acid etching the surface of the mini screw. A lateral cutting grove
helps to prevent the concentration of stress, which could lead to mini screw fracture
(figure 3). In 2008, Kim showed that microgrooves on the screws surface could have
9
some effects on the arrangement of gingival connective tissue fibers, and could positively
affect soft tissue and bone tissue adaptation around the mini screw implant.
13
The majority of orthodontic mini screw implants are either self-tapping, or self-drilling.
Self-tapping screws have a fluted leading edge and require a pre-drilling procedure. Self-
drilling screws have a corkscrew like tip and pre-drilling is not required.
14
Most common mini screws have diameters ranging from 1.2 to 2 mm. A tapered mini
screw that has an initial diameter of 1.5 mm will have a decreased diameter at the tip to
1.2–1.1 mm. The difference between the initial diameter and the tip is approximately
0.3–0.4 mm.
15
The length of the mini screw can vary from 4 mm to 12 mm.
Mini Screw Stability
With the insertion of the mini screw implant, surgical soft tissue and bone trauma is
inevitable. Damage to the cells and matrix initiates the healing process of bone and soft
tissue. The healing process occurs in three distinct phases: inflammation phase,
reparative phase, and remodeling phase.
16
Ideally the healing process would result in a
broad interface between the implant and the tissue. The stability of the mini screw
implant comes from the characteristics of the interface. An osseous interface is much
more desirable than a fibrous interface.
17
Fibrous tissue formation at the bone-implant
interface is one of the most important risk factors of mini screw failure. For a mini screw
to be stable and successful, it is detrimental to have sufficient support from bone rather
than tissue. Optimal bone repair requires a sufficient amount of reparative cells and
10
nutrients. Therefore, local circulation is extremely crucial, and bone repair will not begin
unless local circulation has been established.
18
Osteoporosis, osteopenia,
hyperthyroidism and radiation therapy may affect the healing process of bone.
Ultimately, the density of cortical bone is the most important factor to the primary
stability of the mini screw implant.
19
Orthodontic Mini Screw Loading
After implantation, the implant can be immediately loaded or loaded after a healing
period. Both loading methods have shown equal success rates with an orthodontic force
of 250g. In 2008, Garfunkle studied the success rates of immediate loaded (within 1
week), delayed loaded (between 3 -5 weeks), and unloaded (never loaded) mini screw
implants. Using a mixed-model analysis, Garfunkle found that there was no statistically
significant difference between the success rates of immediately loaded mini screws
(80.0%) and delayed loaded mini screws (80.95%), but the success rate for loaded mini
screws (80.49%) was significantly higher than that of unloaded mini screws (60.98%).
The higher success rate of the loaded mini screw implants could be due to the minimized
micro motions around the loaded mini screws, thereby decreasing the likelihood of peri-
implant bone resorption or fibrous encapsulation. Neither the timing of force application,
nor the force itself precipitated failure of the mini screw. Clinical orthodontic forces can
be applied immediately to mini screws with high clinical success rates.
20
11
Site Selection
Site selection is critical and requires careful consideration of the hard and soft tissues,
accessibility, patient comfort, and biomechanical needs. There are two main
considerations that govern site selection for the placement of mini-screws:
21
1. The site of placement dictated by the quality and quantity of suitable bone with
particular reference to the interdental root spaces.
2. The site of anchorage dictated by the malocclusion
In 2006, Poggio studied the “safe zones” of mini screw placement. In the maxilla, the
greatest amount of mesiodistal bone was on the palatal side, between the second premolar
and the first molar. The least amount of bone was in the tuberosity. The greatest
thickness of bone in the buccolingual dimension was between the first and second molars,
whereas the least was found in the tuberosity. In the mandible, the greatest amount of
mesiodistal dimension was between first and second premolar. The least amount of bone
was between the first premolar and the canine. In the buccolingual dimension, the
greatest thickness was between first and second molars. The least amount of bone was
between first premolar and the canine.
22
Mini screw implants used as maxillary molar anchorage have been generally placed in
the inter-alveolar septum between the upper first molar and second premolar from the
buccal side. Buccal implantation may cause complications such as, root damage or
perforation of the maxillary sinus. In 2004, Ishii et al. studied anatomical variations of
dried skulls. On average, Ishii found that the maxillary sinus is located approximately 10
12
mm apical from the alveolar crest. Ishii also concluded that the area 6-8 mm deep from
the crest of the alveolar ridge in the tooth root apical direction is the safest and most
secure position for implantation of mini screw implants.
23
Depending on the patient’s level of attached gingiva, implanting a mini screw at the
safest site could be in the alveolar tissue. Mini screws placed in nonkeratinized alveolar
tissues have greater failure rates than those in attached tissues.
24
The movable,
nonkeratinized alveolar mucosa can be easily irritated. Soft tissue inflammation around
the mini screw is directly associated with increased mobility.
25
Additionally, mini
screws placed in regions of thick keratinized tissue, such as the palatal slope, are less
likely to obtain adequate bony stability. Thin keratinized tissue, seen in the dentoalveolar
or midpalatal region, is ideal for mini screw placement.
26
In 1976, Ainamo showed that the anatomical width of attached gingiva does not differ
between sexes, but that it increases significantly with age. The study concluded that the
mucogingival junction remains at a genetically predetermined location while the teeth
move in an occlusal direction through adult life. In the absence of concurrent retraction
of the gingival margin, this results in an increase of the width of attached gingiva with
advancing age.
27
Once a desired implantation site has been decided, a wire stent can be fabricated to aid in
placement. Accurate placement can be difficult, especially when the roots are in close
proximity. Stents are easy to fabricate, inexpensive, and can be used with a variety of
mini screws. Its precise positioning of interradicular mini screws helps prevent trauma to
anatomical structures and thus reduce mini screw failure rates. The wire guide consists
of two parts: a positioning gauge, attached to the tooth distal to the mini screw placement
site, and a directional guide, attached to the tooth mesial to the mini screw. Prior to
placement, a PA radiograph is taken with the stent. (figure 4)
28
Figure 4) Wire guide for mini screw implantation
Mini Screw Failure
The factors below are currently regarded as possible reasons for implant loss.
29
1. Application of excessive forces on the mini-implant
2. Peri-implantitis, when inserted in the unattached mucosa
3. Insufficient primary stability
4. Bone damage at insertion (bone compression, bone overheating)
5. Cortical bone is thinner than 0.5 mm and low density of trabeular bone
30
13
14
In 2007, Chen found no significant differences in failure rates among the mini screw
implants for the following variables: gender, type of malocclusion, local or full-arch
treatment, implantation on the buccal or lingual side, length of the screw, loading pattern,
or the duration of the healing phase.
31
In 2005, Lee found that in patients 15 years and
older, 5-10% of implants may loosen, and in patients younger than 15 years, about 10-
20% of implants may loosen.
32
Clinical Uses
Published clinical case reports show that temporary anchorage devices have been
successfully used in a variety of malocclusions. Mini screw implants have been
successfully applied in: correction of open bites by posterior intrusion
33
(figure 5), class
II correction (figure 6), class III correction
34
(figure 7), or in any treatment that requires
maximum anchorage control. Mini screws can be used in two ways, either directly or
indirectly. Direct anchorage utilizes forces that originate from the actual implant. This
means that the orthodontic force is applied directly from the implant to a tooth or arch
wire. Indirect anchorage utilizes an implant to stabilize dental units, which in turn, serve
as the anchor units. This implies that the implant is used to stabilize teeth to which the
force is applied, thereby rendering those teeth as indirect absolute anchors.
Figure 5) The use of mini screw implants to intrude posterior teeth to close an
open bite.
Figure 6) Temporary orthodontic mini screw implant used in maxillary distaliztion
of the right buccal segment.
15
Figure 7) Temporary orthodontic mini screw implant used in correction of a class
III malocclusion.
Complications
Complications may arise with the use of mini screw implants. During the implantation
procedure, an incorrect site or angulation could cause damage to adjacent roots. Since
the tooth is much denser than the surrounding bone, the orthodontist can detect adjacent
root damage by sensing the increased resistance during insertion. Also, when the mini
screw implant comes in contact with the surrounding PDL, the patient would feel extreme
pain and discomfort.
Mini screw loosening is the most common complication. This could be related to the
bone quality of the site, excessive torque during placement, or approximation to the root
surface. Initially, if the mini screw does become loose, the orthodontist can try to drive
the mini screw further into bone to gain an increased primary stability. If that fails, the
screw should be taken out and a larger diameter screw could be used.
16
17
Soft tissue could overgrow the mini screw implant, and engulf the mini screw head. This
can be generally avoided by placing the head of the screw through attached gingiva and
not through the unattached alveolar mucosa.
Mini screw implant fracture is a rare complication upon implantation. If excessive torque
is used on the mini screw during placement, the mini screw tip may fracture in bone. An
attempt should be made to remove the remaining fragments.
Anatomic structures can be damaged during placement of a mini screw. If an implant is
placed too apically in the area of the upper premolars and molars, the maxillary sinus
could be perforated. To avoid perforating the sinus, the mini screw should be placed in
attached gingiva. The sinus floor is deepest in the first molar region. Penetration of the
sinus membrane has been well documented. Small perforations (< 2mm) of the maxillary
sinus heal by themselves without complications.
35-37
If the maxillary sinus has been
perforated, the small diameter of the mini screw does not warrant its immediate removal.
The inferior alveolar, mental foramen and greater palatine nerves could be accidentally
damaged, if an implant is placed haphazardly. Most minor nerve injuries not involving
complete tears are transient, with full correction in 6 months. Long-standing sensory
aberrations might require pharmacotherapy (corticosteroids), microneurosurgery,
grafting, or laser therapy.
38
18
CBCT
History
Since Broadbent introduced cephalometric radiology to orthodontics in 1931, imaging
has been a crucial adjunct to the diagnosis and treatment planning of an orthodontic
patient. These cephalometric radiographs relied on orthodontists to derive angular and
linear measurements from anatomical landmarks. Current radiographs, such as
panoramic radiographs and lateral cephalograms, are limited to a 2 dimensional (2D)
projection. These 2D diagnostic radiographic images are inherently subject to
magnification, distortion, superimposition, and misrepresentation of structures. Unlike
2D radiographs, cephalometric radiographs constructed from CBCT scans can use the
right or left half of the skull, thereby overcoming the superimposition of the ramus, body,
molar and condyles. Also, in 2005, Hutchinson showed that linear and angular
dimensions are more accurate using a CBCT derived panoramic radiograph, compared to
traditional radiographs.
39
Since, the development of the first CT in 1967 by an engineer named, Sir. Godfrey
Hounsfield, CT technology has rapidly evolved. The first generation of CT scanners
used a single detector element to capture a beam of X-rays. Both the detector and source
rotated one degree, a design known as the "translate-rotate" or "pencil-beam" scanner. In
1975, a second generation of CT known as "hybrid" CT was introduced. These machines
used more than one detector and a small fan-beam X-ray, as opposed to a pencil-beam.
In 1976, a third generation CT scanner was introduced. These scanners used a large, arc-
19
shaped detector that acquires an entire projection without the need for translation. This
rotate-only design utilizes the power of the X-ray tube much more efficiently than the
previous generations. A fourth generation scanner shortly followed, replacing the arc-
shaped detector with an entire circle of detectors. In this design the X-ray tube rotates
around the patient, while the detector stays stationary.
The introduction of cone beam computed tomography (CBCT) specifically dedicated to
imaging the maxillofacial region heralds a true paradigm shift from 2D to a 3D approach
to data acquisition and image reconstruction.
1
CBCT was initially developed for
angiography
40
, but other applications have included mammography
41
. CBCT was
originally developed as an alternative to conventional CT to provide a more rapid
acquisition of an image by using a larger field of view (FOV), and a comparatively less
expensive x-ray detector. Advantages of the CBCT are a shorter radiation exposure,
faster x-ray acquisition time, reduced image distortion due to patient movements, and
increased x-ray efficiency. The use of a larger FOV increases the amount of x-ray
scatter. Since the CBCT uses a larger FOV, the image quality related to noise and
contrast resolution is limited.
Figure 8) Traditional CT vs. CBCT; Traditional CT uses a “Fan” of x-rays
and need multiple helical passes around the subject; CBCT uses a “cone of x-
rays and only needs a single pass around the subject.
20
Figure 9) Conventional CT vs CBCT; reconstructions of the base projections
differ, CBCT scans are subject to much more radiation scatter
1
21
How Cone Beam Computed Tomography (CBCT) works
Typically, CBCT machines scan patients in one of three positions: sitting, standing, or
supine. Unlike having the patient upright, a patient supine during a scan can alter the 3D
soft tissue profile, thus it is important to look at additional photographs to accurately
evaluate the soft tissue.
CBCT imaging is accomplished by using a rotating gantry to which an x-ray source and
detector are fixed. A divergent cone-shaped source of ionizing radiation is directed
throughout the middle area of interest onto an x-ray detector on the opposite side. To
acquire an image, a single rotation of the x-ray source and detector rotates around a fixed
fulcrum within the center of interest (figure 8). During the rotation, multiple sequential
planar projection images of the FOV are acquired in a complete or sometimes partial arc.
This procedure varies from a traditional medical CT, which uses a fan-shaped x-ray beam
in a helical progression to acquire individual image slices of the FOV. A computer stacks
the acquired slices to obtain a 3D representation. Each slice requires a separate scan and
separate 2D reconstruction. Because CBCT exposure incorporates the entire FOV, only
one rotational sequence of the gantry is necessary to acquire enough data for image
reconstruction (figure. 9).
1
Figure 10) Isotropic Voxels vs. Anisotropic Voxels
1
The resolution of CBCT imaging is gauged by the volume elements or voxels produced
from the 3D volumetric data. The voxel dimension depends on the pixel size on the
detector. The detector typically detects pixels that range from 0.09 mm to 0.4 mm.
Therefore, the detector determines the final resolution and clarity of the 3D volumetric
image. Because CBCT raw data is obtained in one rotation of the x-ray source, CBCT
voxels are isotropic, which means that all three dimension are of equal length.
Traditional CT scanners rotate multiple times around the object to produce an image. To
create CT voxels, the computer must combine all the obtained slices, and construct a z
dimension depending on slice thickness. These resulting compositional voxels are
anistropic, which is of unequal dimensions. The voxel shape is columnar and has an
equal x and y dimension, but an unequal z dimension (figure 10). Once the patient has
been scanned with a CBCT scanner, data must be processed to create a 3 dimensional
22
23
volume. This process of reconstruction combines a number of individual base
projections, each frame resembling a lateral cephalogram that contains more than one
million pixels (figure 9). Each pixel is assigned 12 or 16 bits of data. A computer
processes the multiple base projections to reconstruct the desired 3D volume. 3D image
processing time varies based on voxel size, FOV, number of projections, and computer
processing speed.
Image Resolution: Pixel and Voxel Size
A pixel represents the smallest sampled 2D element in an image. The image has
dimensions along two axes in millimeters, dictating in-plane spatial resolution. A voxel
is the volume element, defined in 3D space. Its dimensions are given by the pixel,
together with the thickness of the slice (the measurement along the third axis).
Resolution which determines the ability to distinguish structures as separate and distinct
from each other is inherently related to the acquired voxel volume. The FOV, acquisition
matrix, and the slice thickness determine voxel volume. The pixel size (FOV/matrix)
determines the in-plane resolution. Reducing the FOV, increasing the matrix number, or
reducing the slice thickness results in an image with reduced voxel volume. A smaller
voxels produces images with a higher resolution, but a lower signal-to-noise ratio (SNR),
which may cause the image to look grainy.
24
Temporary Orthodontic Mini Screws and CBCT
To enhance the successful implantation of a mini screw, the knowledge of root
positioning is critical. Because CBCT images are free of magnification distortion,
images allow more accurate and dependable views of the interradicular relationships than
panoramic x-rays.
39
With CBCT images an orthodontist can accurately and effectively
treatment plan where to place mini screws, so that proper force vectors can be used
during orthodontic treatment. Also, CBCT images can be used to construct placement
guides for positioning the mini screw between the roots of adjacent teeth in anatomically
difficult sites.
42
To maximize stability of the mini screw during placement, the bone
quality of the proposed placement site can also be evaluated before insertion. CBCT
images have shown to be an accurate way to assess the volume of bone present at the
proposed location.
43
CBCT Radiation Dosage
Previous studies indicate that the effective radiation dose varies, ranging from 29 to 477
mSv, depending on the type and model of CBCT and FOV selected.
44, 45
Comparing
these doses with multiples of a single panoramic dose or background equivalent radiation
dose, CBCT provides an equivalent patient radiation dose of 5 to 74 times that of a single
film based panoramic x-ray, or 3 to 48 days of background radiation. The use of a
thyroid collar and chin position can substantially reduce the dose by up to 40%. In
comparison to a traditional CT, CBCT radiation exposure of the maxillofacial region is
76.25% to 98.5% less.
44-46
25
CBCT Accuracy
Traditional orthodontic analysis typically requires a 2D lateral cephalogram and
panoramic x-ray of the skull, which can be highly inaccurate due to magnification,
anatomic superimposition, beam projection angle, and patient position.
47
These
distortions were unavoidable, until the advent of CBCT imaging. In a CBCT imaging,
this distortion can be avoided, because the x-ray beams emitted are almost parallel to one
another and the object is very close to the sensor. After the CBCT scan is complete, the
computer software processes the data, and the resulting image has a 1-to-1 measurement
ratio to the original object. To ensure that error is kept to a minimum and that other
operational systems are functioning correctly, a water phantom is used to calibrate the 3D
scanner.
48
In 2005, Hilgers studied the accuracy of CBCT in the TMJ. He found that
CBCT accurately and precisely depicts the TMJ complex in a 3D model. The
measurements were reproducible and significantly more accurate than those made with
conventional cephalograms in all 3 dimensional planes.
49
In 2004, Lascala studied the
accuracy of linear measurements obtained by cone beam computed tomography. The
group studied 13 measurements in 8 dried skulls and recorded measurements with an
electronic caliper. The skulls were then scanned and analyzed with a NewTom QR-DVT
9000. The radiographic
distance measurements of the same dry skull were then
compared. They concluded that the CBCT image underestimates the real distances
between skull
sites. Differences were only significant for the skull base, and
therefore, it
is reliable for linear evaluation measurements
of other structures more closely associated
with maxillofacial
imaging.
50
CBCT scans are highly accurate and provide a 3
26
dimensional image of the skull, which is paramount to effective orthodontic treatment
planning.
Limitations of CBCT
CBCT image artifacts arise because of the inherent polychromatic nature of the x-ray
beam. The x-ray source produces a heterogeneous mix of low and high energy photons,
which passes through an object. The low energy photons are absorbed more than the
higher energy photons, resulting in a richer high energy photon penetrating beam or beam
hardening. Beam hardening manifests as a cupping artifact, or streaks and dark bands.
Cupping artifacts occur when x-rays passing through the center of an object become
harder than x-rays passing through the edges of an object. Since the x-ray beam becomes
harder in the center, the image is processed incorrectly and produces the artifact.
Patient motion can cause blurriness of the reconstructed 3D image. This type of error can
be minimized by placing a head restraint on the patient before the scan is taken.
Undersampling can also occur if too few basis projections are used in the reconstruction.
A reduced number of projections can lead to blurriness and a noisier 3D image.
Resolution of fine detail will be affected.
Scanner related artifacts can manifest as a circular or ring shaped artifacts. These
artifacts are caused by poor scanner detection or improper scanner calibration. These
problems will consistently result in circular artifacts.
27
CBCT image noise is mostly caused by scattered radiation. Since CBCT uses a cone
shaped x-ray beam, scattered radiation is unavoidable. This scattered radiation becomes
omnidirectional and is recorded by the CBCT detector. This nonlinear attenuation is
contributes to the overall image noise.
A cone beam effect can be a source of artifacts. Since the x-ray beam diverges and the
beam rotates around the patient, the amount of information for peripheral structures is
reduced. The outer rows of the detector record less than the inner rows, resulting in
image distortion, streaking, and greater peripheral noise.
DICOM
DICOM is a universal file type that facilitates data exchange and viewing between
hardware, regardless of the manufacture. This format was created in response to the need
of a universal file format in medical imaging, such as CT scans, MRI’s, and ultrasound.
Before DICOM was introduced, it was almost impossible to view images by any software
other than manufacture’s software that generated the images. In 1983, the American
College of Radiology (ACR) and national Electrical Manufactures Association (NEMA)
joined forces and released their first standard in 1985, ACR/NEMA 300. Soon after its
release, many vendors started to adopt and accept the standard. After its third standard
revision, the format changed its name to DICOM. Currently, the DICOM standard has
achieved a near universal level of acceptance among medical imaging equipment vendors
and healthcare IT organizations.
28
DICOM files stores massive amounts of data that need to be viewed on dedicated
workstations where the images can be appropriately displayed by a DICOM viewer.
A DICOM file made up of a header and the image itself. The header contains: patient
demographic information, acquisition parameters, referrer, practitioner and operator
identifiers and image dimensions. The remaining portion of the DICOM file contains the
image pixel data. Because DICOM files can contain multiple high-resolution images,
DICOM files sizes tend be large.
DICOM images can be compressed by converting the data into smaller image file types.
Compression is a means to efficiently archive and transfer image data. There are two
main types of data compression: lossless and lossy. Lossless compression allows the file
size to be reduced without any loss of information. This allows all the original data to be
recovered if necessary. This type of compression uses a substantial amount of processing
power and makes files slower to open and save. By contrast, lossy image compression
permanently eliminates some of the file data, which can result in a remarkable reduction
in file size. (figure 11) The goal is to eliminate redundant information from the dataset
without adversely affecting image quality, but excessive compression inevitably results in
image degradation.
Figure 11) Image quality; No image compression vs. image compression
29
30
Chapter 3: Assumptions
1. Mini screw implants are placed at the mucogingival junction.
2. Mini screw implants are placed at the midpoint between the second premolar and
first molar.
3. Exact insertion site of the mini screw from the study model can be accurately
triangulated on the CBCT 3D volume.
4. Mini screws are implanted perpendicular to the cortical bone surface
31
Chapter 4: Limitations
1. Crown and root morphology is not the same for every patient.
2. Location of the mucogingival junction varies in each patient.
3. Patient scans are not of the highest resolution. This can cause errors in measuring
the small increments of distance.
4. The placement of the mini screw implant can vary between clinicians
5. Discrepancies may exist in transferring measurements obtained from casts to the
computer generated model
6. Slight malocclusions may cause inaccurate measurements
32
Chapter 5: Delimitations
1. All patients included in this study must have
a. Full adult dentition
b. No major restorations of the maxillary right second bicuspid and maxillary
right first molar
c. No major malocclusions of the maxillary left side
d. Model casts that are free of distortion
e. 3D DICOM images with minimal distortion
33
Chapter 6: Materials and Methods
Sample Description
92 orthodontic patient (varied age and gender) records from the University of Southern
California Graduate Orthodontic program were obtained. The selection for the sample
was as follows:
1. Presence of full adult dentition
2. No major restorations on patient’s right second premolar and first molar
3. Patient study records were free of distortion
4. Minimal distortion of the corresponding 3D volumetric image
Measurement Methods
With an electronic boley gauge, each stone cast model was measured to the nearest
hundredth. 3D image volume distances were measured an InVivo measurement tool.
The measurement tool is a simple ruler that measures the distance between two points in
a 2D plane. Prior to measuring the 2D distances on the 3D image volume, the skull was
oriented, so that the right maxillary buccal plate was perpendicular to the screen.
CT Conditions
Each CBCT scan was taken using a NewTom 3G under the following conditions: tube
voltage: 110kVP, tube current: 15mA, scan time: 36 seconds, grayscale: 12bit, field of
view: 200mm (12 inch). Before the scan was acquired, each patient was supine and
oriented perpendicular to the rotation axis of the CBCT x-ray source.
34
Methodology
Patient study models and photographs (n=92) were used to estimate the desired insertion
point of the mini screw implant on the patient’s right side. The insertion point of the mini
screw implant was selected at the height of the mucogingival junction and at the midpoint
between the maxillary second bicuspid and the first molar (figure 12). After the desired
insertion point was marked on the patient cast, three specific measurements were
measured: maxillary second bicuspid tip to the desired site, maxillary first molar
mesiobuccal cusp tip to the desired site and, maxillary first molar distalbuccal cusp tip to
the desired site (figure 13). These measurements were used to triangulate the exact
location of the point of entry of the mini screw implant. The linear measurements were
measured with an electronic caliper gauge. Corresponding patient 3D volumetric images
(n=92) were previously scanned with an AFP Imaging Newtom 3G CBCT scanner and
were stored in a DICOM format. All patient DICOM files were stored at the University
of Southern California Graduate Orthodontic Program file servers. The 12 inch FOV
sensor setting was used in all patient radiograph scans. Each scan is equivalent to about 7
single film panoramic x-rays.
51
Figure 12) Use of patient photographs to estimate height of the mucogingival
junction.
Figure 13) Measurements of the selected points to the mini screw for triangulation
of the implantation site.
35
36
Patient 3D DICOM data was imported into the Anatomage InVivo software, and then
analyzed. Since the stone models and computed 3D model were in a 1:1 ratio,
measurements can reliably and accurately be transferred directly from the stone model to
the computed 3D image. The insertion site of the mini screw implant was approximated
on the 3D image by triangulation of the 3 measured lines (figure 14). By using the
InVivo software, the patient’s right side was selected from the 3D volume, and then
oriented perpendicular to the screen. Using the InVivo implant module, a 6.0 mm by 1.6
mm virtual mini implant was inserted at the triangulated mini implant insertion point
(figure 14 &15). The virtual mini implant was adjusted until the orientation was
perpendicular to the buccal cortical plate. The virtual mini screw was inserted into the
maxilla until the entire mini screw was in bone. An axial view mini screw is used to
view the anterior posterior position of the virtual mini screw relative to the second
premolar and first premolar (figure 16). By manipulating the 3D image, and by using the
measurement tool, the distances can be measured: anterior/posterior (roots of the adjacent
teeth and virtual mini screw); vertical (first molar root apex); horizontal (lingual cortical
plate to mini screw tip). All measurements were duplicated on the casts, and all
duplicated measurements were transferred to the CBCT volume. All duplicated CBCT
3D distances were averaged.
Reliability of the measurements
The Method Error (ME) was calculated for the distances measured on the 3D image
volume. The 3D image volumes of 92 maxillary arches were selected and measured
twice by the same observer after a one week interval. The ME calculations were
performed using the Dahlberg formula, ME = Σd/2n, where d is the difference between
two measurements of a pair, and n is the number of paired double measurements.
52
The ME of the mini screw to the second premolar is 0.22 (n=92), and the mini screw to
the first molar is 0.23 (n = 92). The ME of the mini screw to the first molar root apex is
0.53 (n = 92).
Figure 14) InVivo Virtual Implant tool; triangulated measurements were
transferred.
37
Figure 15) Measurements from the casts are transferred to the 3D volumetric data
to triangulate the point of insertion. A 6 mm length by 1.6 mm diameter virtual
temporary orthodontic mini implant is placed at the triangulated point of insertion;
Vertical distance from the mini screw to first molar apex is measured.
38
Lack of
resolution of the
lingual cortical
plate.
Figure 16) Axial view; Verification that the virtual temporary orthodontic
mini implant is placed in between the second premolar and the first molar
roots; Anterior Posterior distance is measured.
39
Lack of
resolution of the
lingual cortical
plate.
Figure 17) Axial view of another 3D volume showing the lack of resolution
of the lingual cortical plate.
40
41
Chapter 7: Results / Data Analysis
Table 1: Sample
Gender Frequency Percent
Female 49 53.26
Male 43 46.74
Total 92 100
Table 2: Sample Age
Gender
Average Age in
Months
Average Age in
Years
Female 199
16.6
Male 232
19.3
Total 215 17.9
Table 3: Female Measurements (Both Sets)
(Direction)
Area
Mean (± SD)
Distance
1st
Measurement
Mean (± SD)
Distance
2nd
Measurement
Average (± SD)
Length of 1
st
&
2
nd
Measurements
Female
(A-P)
mini to
0.82 (.27) 0.83 (.28) .83 (.27)
second
premolar
(A-P)
mini to
0.81 (.23) 0.84 (.23) .83 (.22) 1st molar
(Vertical)
mini to 1st
molar apex 7.9 (.13) 7.98 (1.72) 7.94 (1.39)
42
Table 4: Male Measurements (Both Sets)
(Direction)
Area
Mean (± SD)
Distance
1st
Measurement
Mean (± SD)
Distance
2nd Measurement
Average (± SD)
Length of 1
st
&
2
nd
Measurements
Male
(A-P)
mini to
second
premolar 0.86 (.21) 0.83 (.20) .84 (.20)
(A-P)
mini to
1st molar 0.87 (.25) 0.86 (.20) .87 (.21)
(Vertical)
mini to 1st
molar apex 9.20 (2.5) 9.36 (2.65) 9.28 (2.53)
Table 5: Average of Male and Female Measurements
(Direction) Area Measured Total Average (± SD) length
Male and Female
(A-P)
mini to second premolar 0.83 (.24)
(A-P)
mini to 1st molar 0.85 (.22)
(Vertical)
mini to 1st molar apex 8.6 (2.1)
43
Table 6: Differences in the Average Lengths Between Male and Female
Difference in Length
(Male-Female) P-value
(A-P) mini to second
premolar 0.014 0.78
(A-P) Mini to 1st Molar 0.039 0.39
(Vertical) Mini to 1st Molar
apex 1.34 0.003*
* Significant based on a .05 significance level
Table 7: Pearson Correlation Coefficients ( p-value)
A-P mini to
second pre
A-P Mini to
1st molar
Vertical
mini to
1st molar
root Age Gender
(A-P)
mini to
second
premolar 1 .91 (<.0001) .02 (.82) -.19 (.07) .03 (.78)
(A-P)
mini to
1st molar .91 (<.0001) 1 -.01 (.91) - .19 (.08) .09 (.39)
(Vertical)
mini to
1st molar
apex .02 (.82) -.01 (.91) 1 .14 (.19) .32 (.002)
44
Table 8: Average Distances by Age Group
Age Group
N
Average
A-P mini to 2
nd
premolar
Average A-P
mini to 1
st
molar
Average
Vertical mini to
1
st
molar apex
Younger than 10 1 .72 .78 5.68
10-12 6 .98 .95 8.03
12-14 0
14-16 27 .87 .89 8.85
16-18 21 .67 .88 8.2
18-20 11 .77 .85 8.17
20-22 7 .86 .88 8.90
22-24 1 .50 .58 7.97
24-26 4 .78 .77 8.38
26-28 0
28-30 4 .89 .90 8.91
30-32 2 .60 .60 9.24
32-34 2 .63 .57 9.84
34-36 1 .35 .38 9.02
36-38 1 .06 .725 8.33
38-40 2 .82 .94 8.09
40-42 0
42-44 0
44-46 1 .74 .82 12.45
P-Value .32 .23 .88
* The distances are based on the average distance of the first and second sets.
45
Table 9: Pearson Correlation Coefficients for Distances and Age Groups
Distance Measured Pearson Correlation
Coefficient
P-Value
Average
(A-P) mini to 2
nd
premolar
-0.26 .01
Average (A-P) mini 1
st
molar -.22 .03
Average (Vertical) mini to 1
st
molar
.14 .17
After inserting the virtual mini implants between the second bicuspid and first molar,
distances between the virtual implant and specific points were measured to obtain 3D
positions. The measurements (mm) were taken in the anterior/posterior direction and
vertical direction. The transverse distance was not measured due to the lack of resolution
of the lingual plate on the 3D image (figure 16 & 17).
Of the 92 patient records studied, 49 were female (53%) and 43 were male (47%). The
average female age and male age was 6.6 years and 19.3 years, respectively. The overall
average age of all 92 patients is 17.9 years.
In the anterior posterior direction, the male average distance from the mini screw to the
second bicuspid is 0.84 mm (± 0.20 mm), and to the first molar is 0.87 mm (± 0.21 mm).
The female average distance from the mini screw to the second bicuspid is 0.83 mm (±
0.27 mm), and to the first molar is 0.83 mm (± 0.22 mm). The average distance of all
46
patients from the mini screw to the second bicuspid is 0.83 mm (± 0.24 mm), and to the
first molar is 0.85 mm (± 0.22 mm).
In the vertical direction, the male average distance from mini screw to the root apex is
9.36 (± 2.65 mm). The female average distance from mini screw to the root apex of the
first molar is 7.98 mm (± 1.72 mm). The average distance from the mini screw to the
root apex of the first molar is 8.6 mm (± 2.1 mm).
The difference between males and females in the anterior posterior distance is negligible
(Table 6). The data indicates that there is no statistically significant difference between
the average distance of males and females in the anterior posterior direction from the
mini screw to second premolar (p=.78), and there is no statistically significant difference
in the average distance of males and females in the anterior posterior direction from the
mini screw to the first molar (p=.39). However, there is a statistically significant
difference in the distance between the males and females in the vertical direction from the
mini screw to the first molar root (p=.003).
The distance from the mini screw to the second premolar and the distance from the mini
screw to the first molar are significantly correlated (r=.91, p<.0001) (Table 7). The
distance from the mini screw to the second premolar and the distance from the mini
screw to the first molar are marginally significantly negatively correlated (r=-.19, p=.07).
47
The distance in the vertical direction from the mini screw to the first molar apex and
gender are significantly correlated (r=.32, p=.002).
The data shows no statistically significant difference in the average distances in the
anterior posterior direction from the mini screw to the second premolar among the age
groups (p=.32) (Table 8). Also the data shows, that there is no statistically significant
difference in the average distance from the mini screw to the first molar among the
different age groups (p=.23). The data shows no statistically significant difference in the
average distance in the vertical direction from the mini screw to the first molar among the
different age groups (p=.88).
48
Chapter 8: Discussion
The purpose of this study is to obtain an average of all the 3 dimensional coordinates of a
virtual mini screw. With the 3D coordinates, the clinician can estimate the amount of
space is available for implantation, and estimate the amount of tooth movement before
the tooth root contacts the mini screw implant. Due to the lack of clarity in the NewTom
volumetric 3D images, only the position in the anterior posterior direction and in the
vertical direction could be accurately and reproducibly measured. The lack of resolution
can be from a variety of patient specific factors and machine specific factors.
Because the NewTom 3G has one of the longest scan times of 36 seconds, any micro
movement of the head during the scan acquisition stage can cause an orientation shift in
the base projections. During reconstruction, the software will combine the series of base
projections, including the distorted base projections, leading to an inaccurate and blurry
3D volumetric image. To obtain a quality 3D volumetric image, micro-motions of the
head must be kept to a minimum. Motion distortion can be decreased by using head
restraints or oral instructions to the patient to remain still during the scan acquisition.
53
Patient dental restorations, surgical plates or radiographic markers are high-attenuation
metallic objects that can cause streak artifacts in the 3D volumetric image. Since the
metal can attenuate the x-ray beam, the attenuation values behind the object are
incorrectly high. The metal causes the effect of bright and dark streaks in CT images
49
which significantly degrade the image quality. To minimize streak artifacts, all metallic
objects must be removed from the patient prior to the scan.
By using the 12 inch FOV sensor, the acquired 3D volumetric image is subject to much
more radiation scatter, which can increase the noise of the 3D volumetric image. A 6
inch FOV sensor could drastically decrease the noise of the 3D volumetric image and
improve image resolution, but such a small field only allows for a small portion of the
subject to be imaged. CBCT manufactures have attempted to improve the clarity, by
creating specific algorithms and filters to remove noise and streak artifacts.
54
The voxel size ultimately determines the resolution of the 3D volumetric image, the
smaller the voxel, the higher the resolution. As the FOV increases, the voxel size
increases, not the number of voxels within the image. With a FOV of 6 inches, 9 inches
and 12 inches, the voxel sizes are 0.2 mm, 0.3 mm, and 0.4 mm respectively. The 3D
volumetric images in this studied were scanned with 12 inch FOV, which gives the
highest voxel size possible with the NewTom 3G. This large voxel size could contribute
to the poor resolution of the images. In this study, the average anterior posterior distance
from the mini screw implant to adjacent teeth is about 0.8 mm. Since all scans were
acquired with the 12 inch FOV, the distance between the mini screw and tooth root is
about 2 voxels. Due to the inherent resolution of the original scan, measurements in the
millimeter range could be inaccurate.
50
The tooth root has a similar bone density to the jaw, and its boundaries are at a low
contrast in the volume data. To improve visualization of the tooth roots, segmentation of
teeth in a 3D volume can be preformed. Manual segmentation is possible, and requires a
clinician to remove the images surrounding the tooth roots in each slice. Noise and
unwanted bone would be removed. The remaining images could be used to construct the
3D image. These images will have a smoother and more defined contour, which can
increase measurement accuracy.
3D volumetric images could have been degraded from exporting them from a proprietary
NewTom format, into a universal DICOM format. To move from a proprietary format to
a universal format, data that could be utilized in the proprietary format is removed to
make DICOM images universal. Also, image compression can also affect the overall
resolution. In DICOM, the two main types of image compression used are lossy and
lossless. If a lossy type of compression is used, the file size is dramatically reduced, but
the quality and resolution can be severely degraded.
Numerous DICOM readers can be purchased or found for free on the internet. The
DICOM reader itself can affect image quality.
55
These readers have different coded
logarithms that process and projects DICOM images. The overall resolution and image
depends on how the reader processes the DICOM data. One DICOM reader may project
the surfaces of the bone accurately, but incorrectly projects the internal structures.
Another DICOM reader may accurately project the internal structures, but in correctly
51
projects the bone surface. Prior to image reconstruction, each DICOM reader should be
used to determine its advantages and disadvantages.
Errors from this study could have come from the measurements of the casts, the
measurements on the 3D volumetric image, or the incorrect use of the 3D imaging
software. Errors in the cast measurements were kept to a minimum by measuring all
distances twice. Errors in the 3D volumetric images were also kept to a minimum by
measuring all distances twice. Measurement errors from the Anatomage InVivo software
could have come from a variety of factors. An incorrect skull orientation could cause
distances to be inaccurately measured. The use of the brightness and contrast controls in
the InVivo software allows the user to adjust the 3D volumetric image density, so that the
amount of hard tissues or soft tissues can be visualized. If the image was not adjusted to
remove all of the soft tissues, the measurement tool would measure the distances based
on the soft tissue, and incorrectly calculate the desired distances.
In this study, the average amount of space between the second bicuspid and first molar is
3.2 mm, which is similar to a study by Carano, in 2004. Carano studied 50 subjects and
concluded that, depending on the level of implantation, about 2.9mm to 3.0mm of
interdental space exists between the second premolar and first molar.
56
From this study the average distance from the virtual mini screw to the second premolar
is 0.83 mm in females, and 0.84 mm in males. The average distance from the virtual
52
mini screw to the first molar is 0.83 mm in females, and 0.87 in males. The average
distance from the virtual mini screw to the root apex of the first molar is 7.94 mm in
females, and 9.28 mm in males. These measurements indicate that the buccal segments
can be distalized approximately 0.8 mm before the mini screw contacts the second
premolar root. Also, the buccal segment can be protracted approximately 0.8 mm before
the mini contacts the first molar root. This information is useful in the treatment of class
II and class III malocclusions.
Between females and males, there were no significant differences in the anterior posterior
distances of the mini screw in relation to the adjacent teeth, the second premolar, nor the
first molar. But between females and males, there was a significant difference in the
vertical distance of the mini screw in relation to the apex of the first molar. This
difference is most likely due to the sexual dimorphism of teeth, which male teeth tend to
be slightly larger than female teeth.
57
Since the virtual mini screw was placed directly in between the roots of the second
premolar and first molar, the distances measured were very similar, and therefore
significantly correlated. The distance from the mini screw to the root apex of the first
molar and gender was also significantly correlated. In respect to age, there were no
significant differences, nor correlations between age groups and the three distances
measured.
53
Although the 3D position of the virtual mini screw could not be calculated, useful
information can be derived from the obtained results. With this information, the
orthodontist can effectively visualize the position of the mini screw implants to
effectively treatment plan complex tooth movements, thus providing quality and efficient
patient care. The proximity of the mini implant to the roots of neighboring teeth can be
estimated. This is especially useful in treatment planning the amount of the protraction
or retraction of the maxillary teeth with mini implants in correcting for class II or class III
malocclusions.
As CBCT technology continues to evolve, the quality, clarity, and resolution will
improve. Quality 3D imaging will revolutionize orthodontic treatment planning. More
research, in 3D scanning and 3D software, is needed to produce the best patient
radiographic image without increasing radiation dosage.
54
Conclusion
1. Due to the low 3D image resolution, only 2 of the 3 dimensions could be
measured from adjacent structures.
2. In the anterior posterior dimension, the average mini screw distance to the
adjacent roots of the maxillary second bicuspid and first molar is approximately
0.8 mm in both males and females. This is enough space to correct a ¼ step class
II or a ¼ class III malocclusion. Any malocclusion greater will require implant
removal and reinsertion after the buccal segments are retracted 0.8 mm.
3. In the vertical dimension, the average mini screw distance to the first molar root
was 7.94 mm in females and 9.28 mm in males.
4. In the anterior posterior dimension, there was no significant difference in the
distances between males and females.
5. In the vertical dimension, there was a significant difference in the distance
between males and females. This could be due to sexual dimorphism.
6. There was no significant difference in the distance between age groups.
7. CBCT volumetric image resolution could be increased if,
a. The FOV is small, thus allowing for a smaller voxel size
b. Patient micro-movements are eliminated
c. No DICOM image compression, lossless compression
d. The selection of the DICOM reader is based on the structures being
studied
e. Distances measured from proprietary software
55
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Abstract (if available)
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Asset Metadata
Creator
Cheung, Craig
(author)
Core Title
Cirtual 3D placement of temporary orthodontic anchorage implants
School
School of Dentistry
Degree
Master of Science
Degree Program
Craniofacial Biology
Publication Date
05/07/2009
Defense Date
03/23/2009
Publisher
University of Southern California
(original),
University of Southern California. Libraries
(digital)
Tag
cbct,cone beam computed tomography,NewTom,OAI-PMH Harvest,TAD,temporary anchorage device,virtual
Language
English
Contributor
Electronically uploaded by the author
(provenance)
Advisor
Sameshima, Glenn T. (
committee chair
), Moon, Holly (
committee member
), Paine, Michael L. (
committee member
)
Creator Email
craigcheung@gmail.com,crcheung@yahoo.com
Permanent Link (DOI)
https://doi.org/10.25549/usctheses-m2209
Unique identifier
UC1471603
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etd-Cheung-2801 (filename),usctheses-m40 (legacy collection record id),usctheses-c127-235570 (legacy record id),usctheses-m2209 (legacy record id)
Legacy Identifier
etd-Cheung-2801.pdf
Dmrecord
235570
Document Type
Thesis
Rights
Cheung, Craig
Type
texts
Source
University of Southern California
(contributing entity),
University of Southern California Dissertations and Theses
(collection)
Repository Name
Libraries, University of Southern California
Repository Location
Los Angeles, California
Repository Email
cisadmin@lib.usc.edu
Tags
cbct
cone beam computed tomography
NewTom
TAD
temporary anchorage device
virtual