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University of Southern California Dissertations and Theses

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Three-dimensional assessment of tooth root shape and root movement after orthodontic treatment: a retrospective cone-beam computed tomography study

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Three-dimensional assessment of tooth root shape and root movement after orthodontic treatment: a retrospective cone-beam computed tomography study

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Content Three-‐Dimensional
Assessment
of
Tooth
Root
Shape
and

Root
Movement
After
Orthodontic
Treatment:

A
Retrospective
Cone-‐Beam
Computed
Tomography
Study

By

Dovi
Prero

A
Thesis
Presented
to
the

FACULTY
OF
THE
USC
GRADUATE
SCHOOL

UNIVERSITY
OF
SOUTHERN
CALIFORNIA

In
Partial
Fulfillment
of
the

Requirements
for
the
Degree

MASTER
OF
SCIENCE

(CRANIOFACIAL
BIOLOGY)

May
2014

ii

Dedication

To my devoted wife Naomi,
And to our beautiful children Eliana, Alexandra and David

iii

Acknowledgements

Thank you to Dr. Glenn Sameshima whose guidance throughout this project made
it possible.

iv

Table
of
Contents

Dedication ii
Acknowledgements iii
Abstract v
Chapter 1: Introduction 1

Chapter 2: Rationale of Root Resorption - Cementum 3

Chapter 3: Imaging 11

Chapter 4: Research Objective 24

Chapter 5: Material and Methods 25

Chapter 6: Results and discussion 30
Chapter 7: Conclusion 40
References 41

v

Abstract

Introduction: Root resorption following orthodontic treatment is common and often does
not have deleterious effects on the longevity of the teeth. Previous methods of imaging
root resorption include panoramic radiographs and periapical radiographs. Cone Beam
Computed Tomography (CBCT) is now available to assess root resorption in 3
dimensions. Research Objective: To understand the 3 dimensional pattern of root
resorption on maxillary incisors that underwent orthodontic treatment. Additionally, is
there a correlation between direction of tooth movement and pattern of root resorption?
Methods: 25 patients (100 teeth) were involved in this study. Cone Beam Computed
Tomography (CBCT) scans which had been taken pre and post treatment were analyzed
to assess root resorption pattern and direction of movement. Results: Root resorption can
occur not only at the apex in a vertical manner, but also at the mesial, distal, palatal and
facial surfaces of the root. When the teeth moved in the apical direction (31 instances)
root resorption on the isolated apical surface of the root was found 94% of the time (29
instances). When the teeth moved in the palatal direction (65 instances) root resorption on
the isolated palatal surface of the root was found 20% of the time (13 instances).
Conclusion: Root resorption was observed on the surface of root movement, however it
was also observed on surfaces not related to root movement. The clinician may consider
the direction of root movement in diagnosis and treatment planning of orthodontic
treatment, and in the assessment of risk factors for root resorption.

1

Introduction
External surface root resorption is the active removal of mineralized and non-
mineralized cementum and dentin. Although the outcomes of these root resorptive
processes are similar, orthodontic root resorption is distinct from the other types; the term
“external apical root resorption” (EARR) or ‘‘orthodontically induced inflammatory root
resorption’’ (OIIRR) has been suggested (Brezniak 2002). Extensive root resorption
becomes irreversible when it spreads beyond the cementum layer into dentin; it
compromises an otherwise successful orthodontic outcome. It is thus important to know
more about the mechanism, risk factors, and preventive factors of root resorption. Root
resorption can occur in different scenarios: natural maturation of the dentition, idiopathic,
disease, infection, trauma, periapical infectious lesions, periodontal diseases, pathologic
eruption of adjacent or impacted teeth, and orthodontic tooth movement.
In the natural maturation of human dentition root resorption occurs as a process
by which primary deciduous teeth are replaced by the secondary permanent teeth. The
root of the primary tooth and the surrounding bone are resorbed, the crown exfoliates and
space is made for the succedaneous tooth (Marks and Schroeder, 1996). Root resorption
can also occur as a sequel of disease or infection (Kjaer, 2012). Trauma and luxation and
can also cause resorption to occur (Andreasen, 1995). Idiopathic root resorption occurs
without any etiologic factors (Hedge, 2012).
Root resorption following orthodontic treatment has been described in the
literature as an unpredictable adverse effect of orthodontic treatment (Brezniak and
Wasserstein, 1993). In extreme cases of root resorption after orthodontic care, resorption
can compromise or overshadow the beneficial results of orthodontics treatment outcome.

2

However, in most cases, root loss resulting from orthodontic treatment does not decrease
the long-term mortality or the function of the involved teeth (Pizzo et al., 2007). Root
resorption most commonly shows itself as apical root shortening or surface resorption
(Al-Qawasmi et al., 2003). It is that same controlled inflammatory process can affect the
surface of the root itself (Brezniak, 2004)
Additionally, from a medico-legal perspective root resorption is an important
topic of study. In the state of California it is one of the most common complaints that
patients have from their orthodontic treatment (CAO, 2005).
For the purpose of this work, the type or root resorption that will be examined in
depth is root resorption secondary to orthodontic movement of the root and specifically
resorption that occurs at the external apex of the tooth. This is commonly referred to as
external apical root resorption. External apical root resorption (EARR) is a common side
effect of orthodontic treatment and it is frequently examined in the literature (Brezniak
1993, Brezniak 2002, Weltman 2010).
There are multiple factors associated with this phenomenon. Among the factors
are genetic background (Al-Qawasami 2003 and Darendeliler 2004), length of treatment,
(McFadden 1989 and Liou 2010), magnitude of orthodontic forces (Costopoulus 1996,
Darendeliler 2006), early vs. late orthodontic treatment (Mavragani 2000 and Brin 2003),
the kind of orthodontic movement (Parker, 1996), trauma, (Brin 1991 and Kindelan
2008), extraction therapy (Brin 2011), and the continuity of force; intermittent vs.
continuous (Ballard, 2009).

3

The Role of Cementum
Current research demonstrates that the mechanism of root resorption is intimately
related to the cementum and its protective layer (Malek, 2001). External root resorption
during orthodontic tooth movement is thought to be caused by ischemic necrosis of the
periodontal ligament during compression, with initial damage to the root cementum layer
(Pizzo, 2007).
Cementum has been described as a non-uniform mineralized connective tissue
(Bosshardt, 1997). Cementum is the less mineralized (65%) than dentin (70%) or enamel
(92%). Two types of cementum are cellular and acellular cementum (Freeman,
1998) Cellular cementum is less mineralized and is deposited around the apical third of
cementum, whereas acellular cementum covers the coronal two thirds of the root.
Acellular cementum consists of only mineralized layers. (Malek, 2001)

Earlier studies have shown that cellular cementum at the apex of shows the
lowest elastic modulus and hardness values, whereas the acellular cementum at the
middle third shows higher hardness and elastic modulus (Malek, 2001). Research has
concluded that hardness was correlated with the amount of mineralization in mineralized
tissues (Hodgskinson, 1989).
Treatment results can sometimes be jeopardized by the surface loss of root
cementum (Brezniak 1993). Even though root resorption has been studied thoroughly it is
still a poorly understood. The only protection that the cementum has from resorption by
osteoclasts is thought to be from the formative cell layer covering it (Rabie 1988,
Wesselnick 1988, Jones 1988). When there is a disruption of the naturally protective

4

formative cell layer, resorption can occur. This may also occur when the cementum is
mechanically damaged (Darendeliler 2004).
Cementum has different qualities depending on the location that it is on the tooth.
It has been shown that the cementum on the apical third of the root is softer than the
middle third. Cementum is the softest at the apical portion of the root (Darendeliler
2001).

Lingual surface Distal surface
(Malek and Darendelier, 2001)
Figure 1
The constancy of the force applied to the apex has shown to directly correlate to
the amount of root resorption. Intermittent force has been shown to have less root
resorption that constant force (Ballard
and
Darendeliler, 2009) as illustrated below.

Figure 2 (Darendeliler 2009)

5

There appears to be a slight increase in the mineral composition of cementum
after the application of light orthodontic forces. There is an overall decrease in the
Calcium concentration of cementum with the application of heavy orthodontic forces that
corresponds to areas of PDL tension (Rex and Darendeliler, 2006).
In tooth movement, after compression in the periodontal ligament, hyalinization is
a common consequence (Iino, 2007). Hyaline has been termed sterile necrosis and forms
in the interstitial space within the periodontal ligament after a compressive load is placed
on a tooth root. Hyalinization of the periodontal ligament usually occurs after a few days,
and lasts up to four to eight weeks. Resorption of the alveolar socket is prevented, and
root resorption near the areas of hyalinization will occur. Hyaline is remove by
macrophages and tooth movement can occur. During hyalinization, resorption craters or
lacunae are found along the length of the root surface (Kokich, 2008)
Cementoblasts secrete cellular cementum and repair the resorption lacunae and
thereby reversing root resorption. When the equilibrium is disrupted, radiographic
evidence of severe root resorption will appear (Kokich, 2008)
The histologic representation of root resorption is illustrated below.

6

Figure 3 (Proffit, 2007, p. 349.)
Factors Related to Root Resorption
The cause for root resorption is multifactorial (Weltman, 2010). Genetics,
independent variability in tissue response, systemic factors, hormonal imbalance,
nutrition, age, habits, and intrusion, can affect root resorption (Brezniak, 1993).
EARR is a complex condition influenced by many factors, with the IL-1B gene
contributing an important predisposition to this common problem. Research by Al
Qawasami has shown that IL-1B accounts for 15% of the total variation of maxillary
incisor EARR. Patients homozygous for the IL-1B allele 1 have a 5.6 fold increased risk
of EARR greater than 2 mm as compared with those who are not homozygous for the IL-
1allele 1. Allele 1 on the IL-1B gene decreases the production of IL-1 cytokines and
correlates with an increased risk of EARR (Al Qawasami, 2003).

7

Illustration of how Polygenic (complex) traits have a variable number of
influencing genetic factors, some of which may have more influence than
the others, but none are capable of producing the trait by itself. The
potential relationship between various genes and the environmental
factors is complex as denoted by the arrows (Abbas, 2007)
Figure 4
The most commonly resorbed teeth are the maxillary incisors. The average
resorption is around 1.2–1.5 mm per incisor, 10% of the root and the average length
ranges from approximately 12–15 mm (Sameshima, 2004). Root shape also has shown to
be associated with resorption. Roots with abnormal root shapes have a higher propensity
to resorption. Dilacerated maxillary lateral incisors and pointed teeth, showed greater root
resorption. Blunted teeth had less resorption.
Ethnicity has shown that root resorption does not occur equally across the board.
Asian patients statistically exhibit significantly less resorption than white or Hispanic
patients.
Age also plays a role. In mandibular incisors adults have more resorption than
children. The sex of the patient has no impact on the rate of root resorption (Sameshima
2001).
Brin et al has shown that maxillary lateral incisors are unique in that they guide
the path of eruption of the maxillary canines. Brin also showed that lateral incisors

8

undergo radiographically undetectable resorption on the lingual apical surface (Brin
1993).
Another study in adult maxillary incisors revealed that the amount of root
movement, as well as long roots, narrow roots, abnormal root shape, and use of Class II
elastics were significant risk factors. However, the statistical model had a low explained
variance, strongly suggesting a weak prediction power. Root resorption was not a factor
and effected by initial malocclusion, length of treatment time, use of rectangular arch
wires, proximity of the root to the palate or treatment with maxillary osteotomy
(Mirabella, 1995).
The surface of the apex also plays a role. The apex is not a smooth or uniform
surface. (Malek and Darendeliler 2001)
(Fong and Darendeliler, 2006)
Figure 5
Repair of the damaged root restores its original contours unless the attack on the
root surface produces large defects at the apex that eventually become separated from the
root surface. Root resorption lacunae that are not repaired may cause a sequestration of
the apex, which when resorbed results in external apical root resorption as pictured below
(Al-Qawasami, 2004).

9

Once an island of cementum or dentin has been cut totally free from the root
surface, it will be resorbed and will not be replaced. On the other hand, even deep defects
in the form of craters into the root surface will be filled in again with cementum once
orthodontic movement stops. Therefore permanent loss of root structure related to
orthodontic treatment occurs primarily at the apex. Sometimes there is a reduction in the
lateral aspect of the root in the apical region. (Proffit 2007) p. 349

Figure 6
In a study, Sameshima found there were no statistically significant differences
found for extractions, use of Class II and finishing elastics, transverse treatments, overjet,
overbite, vertical, tooth length, and habits. Higher estimated risk was found for abnormal
root shape for both maxillary incisors, and tongue thrust (Sameshima, 2004).
How to prevent, stop or treat root resorption
Identification of orthodontic patients at risk of severe apical root resorption can be
identified based on the amount of resorption at initial stages. This supports the suggestion
of taking a screening panoramic radiograph after the leveling and aligning stage of
treatment (Artun, 2009). Some advocate taking periapical radiographs 6 months into
treatment if resorption is suspected (Weltman, 2010).

10

When there is evidence of root resorption greater than 2mm, treatment should be
paused. There is some evidence that a 2 to 3 month pause in treatment can decrease total
root resorption (Weltman, 2010). Others advocate waiting up to 6 months before active
treatment resumes. During this pause, it is recommended to place passive archwires and
monitor the patient every 6-weeks to check hygiene.
Root canal therapy has been shown to treat root resorption. After trauma, patients
may exhibit root resorption and root canal therapy has been shown to be a treatment to
arrest the resorption (Ghafoor, 2013). There has been no indication that teeth previously
treated with root canal therapy show any more susceptibility for resorption.
Pharmacologic agents have shown an effect on root resorption. There have been
studies on the effects of L-thyroxine on root resorption that are still controversial. Low
doses of this hormone in rats decreased the amount of root resorption by about 50%
relative to that in a control group (Vázquez-Landaverde 2002) Bisphosphonates have had
mixed results in inhibiting or increasing root resorption (Igarashi 1999), as well as
Corticosteroids (Ashcraft 1992 and Ong 2000). Alcohol consumption in adults during
orthodontic treatment tends to increase root resorption through vitamin D hydroxylation
in the liver (Levander 1998 and Brezniak 2002).
Follow up after resorption
Some patients may exhibit severe root resorption yet no mobility post treatment
and necessitate no intervention at all. For patients with mobility a lingual bonded, braided
wire can be used to retain incisors. When there is severe root resorption and severe
mobility the tooth can be extracted and replaced prosthetically. Caution should be taken
when using incisors that have undergone root resorption as abutments. A fixed partial

11

denture can be utilized as a method to restore the dentition and splint the compromised
teeth. Orthodontic restoration includes canine substitution of the lateral incisor (Savage
and Kokich, 2002)
Imaging
Conventional periapical radiography is usually the first choice for imaging of
suspected eruption disturbances, but is an inaccurate method for diagnosing root
resorption. The shortcoming of conventional radiography for the assessment of incisor
roots adjacent to impacted canines, is the overlying structures. In buccally or lingually
positioned canines, resorption can be through to the pulp yet no radiographic evidence
may occur (Ericson, 2000).
Panoramic radiographs have also been used to assess root resorption. Panoramic
radiographs, however, can mask dilacerations and abnormal shapes of root that are easily
discernible in a periapical film. They can also underestimate or overestimate the amount
of root resorption by 20% (Sameshima, 2001).
Other Researchers have also concluded that apical root resorption after
orthodontic tooth movement is underestimated when evaluated on panoramic radiograph.
Cone Beam Computed Tomography (CBCT) is a more accurate method to conventional
radiography, to detect root resorption (Dudic, 2009).
CBCT was first developed for use in angiography and Mozzo et al reported the
first CBCT unit developed specifically for dental use (Mozzo, 1998). Hashimoto et al
reported that the newer versions of CBCT scanners produce images of higher resolution
and lower radiation than previous models (Hashimoto, 2003).
CBCT overcomes the limitations of conventional radiographic methods. It proves

12

to be a useful method for diagnosing the positions and complications of ectopically
erupting teeth, and has been used with increased frequency since 1988. Ericson (Ericson,
2000) showed that CBCT is a reliable method of revealing resorption on maxillary root
incisors caused by ectopic eruption of the maxillary canines (Ericson, 2000).
Both high- and low-resolution CBCT scans can also be used to more accurately
measure external apical root resorption defects than periapical radiographs (Lund, 2010
Li, 2013). CBCT machines now a more affordable and ubiquitous option for general and
specialty dentistry (Ponder, 2013). They have higher resolution, are safer for the patient
with its lower radiation than conventional CT (Schulze, 2004).
The shortcomings of intraoral periapical radiography include magnification, the
position of the x-ray source and patient movement (Katona, 2007).
CBCT allows for visual recreation of all structures in the field of view.
Reconstruction and visualization of each tooth in all 3 planes allows for accurate
measurement of root resorption. CBCT has been shown to be an accurate tool to measure
root length and bone levels in vivo (Lund 2010). The CBCT can be analyzed in all 3
dimensions by using the 3 anatomic planes of the subject (axial, coronal, sagittal). In the
method picture below, researchers made linear measurements of the teeth pre and post
treatment to determine the change in tooth length after orthodontic treatment. (Lund,
2010) Figure 7

13

In addition to tooth length, CBCT was also used to develop a methodology in
measuring inclination and angulation of teeth, employing CBCT and the 3 anatomic
planes of space. Tong et al calculated the mesio-distal angulation and facio-lingual
inclination of all teeth (Tong, 2012). This method was an improvement upon earlier
methods that only used panoramic radiographs (Mayoral 1982 and Ursi 1990) or plaster
cast study models (Andrews 1972).

Tong et al Figure 8

14

Lund et al used CBCT to study root resorption comparing time points of 6 months
into treatment and post-treatment. They confirmed previous studies that the maxillary
incisors are the teeth most often affected by root resorption (Makedonas and Lund 2013).
They used the Malmgren index (Malmgren, 1982) to assess root resorption. The
Malmgrem index is a linear measurement of apical root resorption that is categorized into
4 categories (see image below): 1) Irregular root contour, 2) Minor - less than 2mm or
resorption, 3) Severe - less than 1/3 root resorption and 4) Extreme - more than 1/3 root
resorption (Malmgren, 1982). Lund et al found there was no correlation between the
amounts of root resorption at the 6-month time point compared to the end of treatment.

Figure 9
Leite et al used CBCT to study root resorption of maxillary and mandibular
incisors comparing self-ligating vs. conventional pre-adjusted brackets. A CBCT was
taken pre-treatment and 6 months into treatment. This study again used only linear
measurements (see image below). On average all teeth resorbed 0.4mm or less. There
was no correlation or difference found between the self-ligating vs. conventional pre-
adjusted brackets (Leite, 2012)

15

Figure 10 - Linear measurement or root resorption using CBCT
Lund et al used CBCT in a prospective study of root resorption. In this study, below in
Fig. 11, of the teeth in each patient were measured at 3 time points pre-treatment, during
treatment and post-treatment. In addition to the linear measurement of loss of root length
from the apex, they also isolated the buccal, palatal/lingual and proximal surfaces (see
image below). This was the first study using CBCT to identify resorption on all surfaces
of the root and is referred to it as slanted surface resorption. For the maxillary central
incisor, it was found that the percentage of slanted surface resorption for the buccal was
6.9%, palatal 15.1%, and proximal was 6.6%. For the maxillary lateral, it was found that
the percentage of slanted surface resorption for the buccal was 2.0%, palatal 11%, and
proximal 9.9%. As previous studies indicated, they found maxillary incisors had more
resorption. In addition, they found that there was no significant association between root
resorption and sex, pre-treatment root length, or treatment duration (Lund 2012).

16

A. Example of an upper central incisor at baseline B. Palatal surface resorption at
endpoint. Figure 11: Resorption on the isolated palatal surface
A study by de Freitas highlighted the difference between CBCT and Periapical
Radiography (PR). In their study, maxillary lateral incisors (94.5%) and mandibular
central incisors (87.7%) were the most affected teeth. Below shows the effect of root
resorption on a maxillary central incisor. In PR (right) the resorption is underestimated
when compared with CBCT (left) (de Freitas, 2013).
Figure 12

17

However in posterior teeth, as pictured below, it was found that PR (pictured top –
Malmgren score of 2) is more likely to overstate the root resorption as compared to
CBCT (pictured bottom – Malmgren score of 1) pictured below.

Figure 13 Periapical vs. CBCT
Now we enter the era of volumetric assessment of the involved structures in
orthodontics. In addition to linear measurements, volumetric assessments can now be
made using CBCT.
Grauer’s landmark article describes in precise detail the complexities and
methodology involved in using CBCT technology to render 2D images, 3D images,
registrations and superimpositions, and threshold segmentations of the virtual surface

18

(Grauer 2009). Research involving 3-D imaging and structures recreated from CBCT is
largely based on the foundations delineated in Grauer’s article.
Grauer explains that the 3 dimensional image seen in the CBCT is recreated from
a “stack of 2D images”. The 3D image is made up of voxels, with a gray-level value that
is based on the amount of x-rays absorbed by the object. The user can then select a preset
or user-defined threshold filter applied to the voxels to recreate the image in 3D. (Grauer,
2009)
The simplest method of segmentation, or selection of the part of the desired
image, is the thresholding method. The software with which the threshold segmentation
is accomplished can make a difference. When the user determines a threshold interval, it
means that all voxels with grey levels inside that interval will be selected to construct the
3D model (segmentation) (Weissheimer, 2012). In a study of the oropharynx and airway,
Weissheimer et al compared the threshold segmentations of the airway to a gold standard
using 6 different software programs. All 6 software programs were found to be reliable,
but with differing levels of accuracy, errors, advantages and disadvantages (Weissheimer,
2012). Below is pictured a selection of 3 software programs processing the same gold
standard yet generating different results (Weissheimer, 2012)

Figure 14: Comparison between Imaging Software

19

CBCT has been used to quantify the volumetric amount in cubic millimeters of
external apical root resorption. Baysal et al was the first to use CBCT to demonstrate the
volumetric changes of the root. In their study a Hyrax expander was banded to maxillary
1
st
molars and 1
st
premolars. They showed that during rapid maxillary expansion the root
changes in all 3 dimensions and has a resultant loss in volume (see image below). In
assessing 1
st
and 2
nd
premolars and 1
st
molars, the most affected root is the mesio-buccal
root of the maxillary first molar, an average of 18.6mm
3
(Baysal 2012).

Figure 15: Volumetric Assessment of the root resorption
Li et al employed a similar technique. In their study they examined root resorption
that took place on maxillary molars after intrusion with the aid of mini-screws. They

20

isolated the maxillary molars in the threshold segmentations (see image below). They
determined that the mesio-buccal root is the most affected by root resorption by an
average of 22.48mm
3
(Li, 2013).

21

All CBCT’s are not created equal. In order to visualize subtle anatomic structures
Molen highlights elements such as voxel size, scatter radiation, gray scale bit depth, and
artifacts, caused by metallic objects as being important (Molen, 2010). Lund specifies
that the smaller the voxel size and the smaller field of view (volume size) the more they
are helpful in assessing root resorption. When the voxel size is smaller, the spatial
resolution is higher. When the field of view is smaller there is less scatter radiation and
less noise (Lund 2012).
Effects of Directional Movement on Root Resorption
Martins et al studied maxillary incisors and the effect of directional movement
and root resorption. Using linear measurements and the Malmgren index they found that
there was more resorption when there was intrusion and retraction when compared to
retraction alone (Martins, 2012)
In a study of maxillary incisors with linear measurements from periapical
radiographs, Dermaut et al found that during intrusion 18% of the root length is lost on
average. There was no statistical significance to the duration of the intrusive force and the
amount of root resorption (Dermaut, 1986).
Figure 17: Intrusion and length of root loss

22

In a 1998 study Parker et al studied the effect of the direction of movement on
maxillary incisors. They found that the direction and magnitude of tooth movement
explained up to 90% of the variation in root resorption. Apical and incisal vertical
movements and an increase in incisor proclination, were strong predictors of external
apical root resorption. When Incisor intrusion was combined with an increase in lingual
root torque, it was the strongest predictor of external apical root resorption. In contrast,
distal bodily retraction, extrusion, or lingual crown tipping had no discernible effect
(Parker, 1998).
Han et al compared intrusive and extrusive forces on the same patient. Scanning
electron microscopy was used to examine the root surface area. It was found that
intrusive forces could produce on average 4 times the amount of resorption on the same
patient. However, they also noted, that patients that experience more resorption on
intrusion also experienced some resorption on extrusion as well. There was also more
resorption noted on the mesial and distal surfaces of roots undergoing intrusive forces
(Han, 2005)

Normal Extrusion Intrusion

23

A CBCT study by Darendeliler’s group examined the effect of intrusion and root
resorption on teeth that were extracted after orthodontic movement. They found the
amount of root resorption was related to the magnitude of the intrusive force. The volume
of resorption craters increased with the larger forces. Mesial and distal surface of the
apex experienced more resorption, due to mesio-distal tipping of the teeth (Harris, 2006).
Figure 19: Effect of magnitude of intrusive force on root resorption

Our study used CBCT scans pre- and post-treatment to identify the surface on
which the resorption took place and to correlate that surface with the direction of root
movement.

24

Research
Objective

The
primary
purpose
of
this
retrospective
study
was
to
evaluate
the

following
research
questions.

1. Can
three-‐dimensional
imaging
be
used
to
observe
root
resorption
on
a

specific
surface
of
the
root?

2. Can
a
small-‐field-‐of-‐view
CBCT
be
used
to
superimpose
on
the
maxilla
to

identify
the
root
displacement
and
direction
of
root
movement?

3. Can
there
be
a
correlation
made
between
direction
of
root
movement
and

surface
resorbed?

25

Materials and Methods
The study sample was twenty-five patients from the Ostrow School of Dentistry
of USC Graduate Orthodontic Clinic who underwent comprehensive orthodontic
treatment. Consecutive patients over the age of 16 years old at the beginning of treatment
were selected. There were no restrictions on type of malocclusion, sex, ethnicity,
crowding, or other common dental and orthodontic measurements or history of root
resorption. For each patient pre-treatment and a post-treatment Cone Beam Computed
Tomography - CBCT images were taken. The University of Southern California
Institutional Review Board approved the study protocol USC UPIRB # UP-13-00512.
The pre-treatment CBCT image was evaluated to assess root surface shape before
orthodontic treatment of the maxillary incisors. The post-treatment CBCT was analyzed
to detect if there was resorption at the apex of these teeth and on what surface in all three
dimensions: apical, palatal, facial, mesial and/or distal. The pre- and post- treatment
CBCT’s were visually compared to determine the direction of movement of the incisors.
Data acquisition
The patients were scanned using a Kodak 9000 3D scanner (Carestream Dental,
LLC) in the Redmond Imaging Center at the Ostrow School of Dentistry of USC by the
x-ray technologist. The CBCT’s were taken with an isotropic voxel size of 0.076mm,
field of view of 35mm in height, 53mm in width and 53mm in depth (98,315mm
3
), tube
potential 10kV, tube current 6mA, and duration of 10.8 seconds. The size of the field of
view was approximately that of the maxilla and maxillary teeth.
Procedure
DICOM images were imported into Dolphin 3D Imaging software. A threshold

26

segmentation technique using Dolphin 3D Imaging was employed similar to a method
described by Hecht, 2014. Dolphin 3D Imaging tools were utilized to manipulate the 3D
renderings so the roots of the teeth could be completely visualized. The 3D renderings
were oriented by visual inspection to a natural head position. The first step was to
remove overlaying structures such as tissue, bone, and other teeth so that each individual
tooth root could be completely analyzed using the volume sculpting, transparency, and
segmentation tools. Also, with rotation of the image in all three planes of space, the root
surface could be appreciated pre- and post- treatment.
To view the roots of the teeth the translucent hard tissue density segmentation was
adjusted. The steps to accomplish this as follows:
1) Select the Hard Tissue radio button.
2) Select the Translucent radio button for a translucent view of the patient's hard
tissue.
3) Adjust the Trans slider to adjust the transparency of the image.
4) Adjust the Seg slider to set the segmentation for the hard tissue view.
The translucent and segmentation adjustments enabled the visualization of structures
that might otherwise be difficult to see.
The volume-sculpting tool was used to remove parts of the volume such as overlaying
images of other teeth, bone, and tissue. The images could be viewed from any angle by
rotating the 3D volume.
The limitations of the method were such that it relied on the researchers subjective

27

observation. 10 patients were re-assessed and found to have the same results. In all
radiography it must be considered that the patient may have moved during the duration of
the scan of 10.8 seconds. This may have introduced errors in capture of the images.
Below are examples of threshold segmentations of incisors pre and post treatment
Pictured below Patient 1 Upper Right Central incisor – Buccal View. The apical
resorption can be seen in a non-linear fashion, sloping to the distal. Arrows point to the
area of resorption.

PreTx PostTx

28

Pictured below is Patient 2 Upper Left Lateral Incisor – Buccal View.
PreTx PostTx
Pictured below is Patient 2 Upper Left Central Incisor – Distal View The root resorption
of the apex can be appreciated

Patient 2 UL2 PreTx PostTx

29

The pre and post CBCT’s were compared to assess the change in root surface as
well as to assess the direction the root traveled. The data was compared and an attempt
was made to correlate direction of root movement and surface resorbed.

30

Results
and
Discussion

Root
resorption
of
the
maxillary
incisors
was
visually
observed
at
the

apex
of
the
root.
The
resorption
pattern
was
visually
described
in
all

dimensions
based
on
the
5
possible
surfaces
upon
which
it
was
observed:

apical,
palatal,
facial,
mesial
and
distal.

The
surface
most
often
resorbed
was
the
apical
surface.
81%
of
the

observed
teeth
had
resorption
on
the
apical
surface.
The
surface
least
often

resorbed
was
the
facial
surface
with
only
6%
of
teeth
exhibiting
resorption
on

the
facial
surface.
The
mesial
of
the
root
was
resorbed
more
often
than
the

distal
of
the
root.

31

When
evaluating
the
total
amount
of
surfaces
there
are
a
possible
125

surfaces
for
each
individual
tooth.
It
was
found
that
the
central
incisors

exhibited
more
total
surfaces
resorbed
than
the
lateral
incisors.

The
Upper
Right
Central
exhibited
slightly
more
total
surfaces

resorbed
than
the
Upper
Right
Lateral.
The
Upper
Right
Central
had
more

resorption
of
the
apical
and
distal.
However
the
Upper
Right
Lateral
had
more

resorption
on
the
mesial.

32

The
Upper
Left
had
more
surfaces
resorbed
in
every
category
except
for
the

apical
surface
(blue).

33

The
following
is
an
illustration
of
the
root
resorption
that
takes
place
in

3D
in
a
more
severe
case.

UR2 Surface of Resorption

Apical Mesial Distal Palatal Facial
Direction of Movement
Apical 7 6 0 0 1 0
Mesial 6 1 5 0 0 0
Distal 5 1 1 3 0 0
Palatal 13 10 0 0 3 0
Facial 2 1 0 0 0 1

34

UR1 Surface of Resorption
A M D P F
Direction of Movement
Apical 10 9 0 0 1 0
Mesial 6 2 4 0 0 0
Distal 4 1 0 3 0 0
Palatal 10 1 0 0 9 0
Facial 4 1 0 1 1 1

UL1 Surface of Resorption
A M D P F
Direction of Movement
Apical 9 9 0 0 0 0
Mesial 6 2 4 0 0 0
Distal 4 0 0 3 1 0
Palatal 20 9 2 5 5 0
Facial 4 2 1 0 0 1

UL2 Surface of Resorption
A M D P F
Direction of Movement
Apical 5 5 0 0 0 0
Mesial 0 4 1 0 0 0
Distal 10 4 1 5 0 0
Palatal 10 7 2 1 0 0
Facial 1 0 0 0 0

When
the
teeth
moved
in
the
apical
direction
(31
instances)
root

resorption
on
the
isolated
apical
surface
of
the
root
was
found
94%
of
the

35

time
(29
instances).
It
is
important
to
note
that
even
when
the
teeth
did
not

move
in
the
apical
direction
there
was
an
additional
54
instances
of

resorption
on
the
apical
surface.

When
the
teeth
moved
in
the
palatal
direction
(65
instances)
root

resorption
on
the
isolated
palatal
surface
of
the
root
was
found
20%
of
the

time
(13
instances).
It
is
important
to
note
that
even
when
the
teeth
did
not

move
in
the
palatal
direction
there
was
an
additional
4
instances
of

resorption
on
the
palatal
surface.

When
the
root
moved
palatally
there
was
noticeable
root
resorption
on

the
isolated
palatal
side
25%
of
the
time.
This
may
be
explained
as
follows.

When
the
root
travels
palatally
there
is
a
palatal
portion
of
the
apex
that
is

The
apex
which
resorbed
contains
a
palatal
side
of
it
and
this
is
resorbed

during
treatment.
In
our
study
this
may
not
have
shown
as
a
isolated
palatal

resorption.

When
the
root
moved
in
the
facial
direction
there
was
no
increase
in

root
resorption
on
the
isolated
facial
surface.

When
teeth
moved
mesial
(25
instances),
root
resorption
was
found

48%
of
the
time
(12
instances)
on
the
isolate
medial
surface.
It
is
important
to

note
that
even
when
the
teeth
did
not
move
directly
mesial
there
was
an

additional
24
instances
of
resorption.

When
teeth
moved
distal
(23
instances)
root
resorption
was
found

36

61%
of
the
time
(14
instances)
on
the
isolated
distal
surface
of
the
root.
It
is

important
to
note
that
even
when
the
teeth
did
not
move
directly
medial

there
was
an
additional
20
instances
of
resorption
occurring
on
the
isolated

distal
surface.

These
results
highlight
that
the
root
resorption
may
occur
on
the

surface
of
which
the
root
moves.
But
also
root
resorption
occurs
on
surfaces

even
when
the
root
does
not
move
in
that
direction.

Throughout
the
development
of
the
research
design
there
were
several

attempts
made
to
develop
a
method
of
superimposition
with
small
field
of

view
CBCT.
Our
process
to
obtain
our
results
in
this
study
afforded
great

insight
into
the
processing
and
analysis
of
CBCT’s.

The
first
attempt
was
to
superimpose
using
best-‐fit
of
the
maxillary

molars.
However,
it
was
determined
that
since
most
of
the
landmarks
on
the

maxillary
dentition
would
change
during
treatment,
it
would
be
impossible
or

grossly
inaccurate
to
superimpose
two
landmarks
upon
each
other
that
were

moving
targets.

Another
attempt
was
made
to
superimpose
on
the
incisive
canal.
On

one
test
case
it
showed
a
potential
to
relate
two
small
field
of
view
CBCT's

upon
one
another
with
a
stable
landmark
as
pictured
below.

37

To
test
this
we
used
a
large
field
of
view
CBCT
on
a
non-‐growing

patient
and
superimposed
on
cranial
base,
a
landmark
that
is
known
in
the

literature
to
be
stable.
We
found
that
the
incisive
canal
does
move
and

remodel
even
in
a
non-‐growing
patient
as
pictured
below.
Therefore
we
were

unable
to
use
the
incisive
canal
to
superimpose
a
small
field
of
view
CBCT.

38

Root
resorption
occurred
approximately
50%
of
the
time
on
the

surface
in
which
the
root
travels
mesial
or
distal.
When
the
root
moves

apically
root
resorption
occurred
94%
of
the
time
on
the
apical
surface.
When

the
root
moves
palatally
root
resorption
occurred
20%
of
the
time.

However
it
may
also
occur
on
surfaces
not
associated
directly
with
root

movement.
This
may
be
explained
by
the
concept
that
once
the
resorption

cascade
is
initiated
it
may
continue
without
regard
for
the
surface
of

movement.
Also,
in
our
study,
the
timepoints
included
were
pre-‐treatment

and
post-‐treatment
and
we
observed
the
movement
of
the
root
between
the

two
time
points.
But,
the
root
may
travel
in
one
direction
during
initial
stage

of
treatment
and
end
up
in
another
direction
at
the
end
of
treatment.
For

example,
a
root
that
is
moved
mesially
may
first
move
distally,(due
to
bracket

placement
or
normal
mechanics)
then
mesially.
In
such
a
case,
the
post

39

treatment
would
show
a
mesial
movement,
but
there
may
be
resorption
on

the
distal
because
the
root
first
traveled
distal.

Roots
of
maxillary
incisors
undergo
root
resorption.
The
apex
has

irregular
contours
(Lund
2013)
and
the
force
from
the
orthodontic

movement
is
concentrated
at
the
apex
(Lund
2010).

Our
study
indicates

there
may
be
a
correlation
of
direction
of
movement
and
surface
of

resorption.

Further
studies
along
with
larger
sample
sizes
may
validate
our

findings.
Furthermore,
volumetric
assessment
of
root
resorption
may
lead
to

a
better
understanding
of
the
changes
that
the
root
undergoes
during

orthodontic
movement.

Other
areas
of
study
may
include
using
large
field
of

view
CBCT
,
superimposing
on
cranial
base
to
assess
in
a
more
precise

method
root
resorption
and
root
movement.

40

Conclusion

Roots
of
maxillary
incisors
undergo
root
resorption.
The
apex
has
irregular

contours
(Lund
2013)
and
the
force
from
the
orthodontic
movement
is
concentrated

at
the
apex
(Lund
2010,
Darendeliler
2006).

Our
study
indicates
there
may
be
a

correlation
to
direction
of
movement
and
surface
of
resorption.

When
the
root
travels
mesial
or
distal,
root
resorption
occurred

approximately
50%
of
the
time
on
that
mesial
or
distal
surface.
When
the
root

moves
apically
root
resorption
occurred
94%
of
the
time
on
the
apical
surface.

When
the
root
moves
palatally,
root
resorption
occurred
20%
of
the
time.
However

resorption
may
also
occur
on
surfaces
not
associated
directly
with
root
movement.

Further
studies
along
with
larger
sample
sizes
may
validate
our
findings.

Furthermore,
with
the
advent
of
CBCT,
volumetric
assessment
of
root
resorption
is

now
able
to
be
employed
which
may
lead
to
a
better
understanding
of
the
changes

that
the
root
undergoes
during
orthodontic
movement.

Other
methods
of

superimposition
on
the
incisive
canal
or
best
fit
on
the
maxillary
teeth
were
deemed

inaccurate.
Other
areas
of
study
may
include
using
large
field
of
view
CBCT,

superimposing
on
cranial
base
to
assess
in
a
more
precise
method
root
resorption

and
root
movement.

The
clinician
may
consider
the
direction
of
root
movement
in
diagnosis
and

treatment
planning
of
orthodontic
treatment,
and
in
the
assessment
of
risk
factors

for
root
resorption.

41

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