Content uploaded by James Kennedy Hartsfield
Author content
All content in this area was uploaded by James Kennedy Hartsfield
Content may be subject to copyright.
15(2):115-122 (2004) Crit Rev Oral Biol Med 115
(I) Introduction
B
asic descriptors of root resorption are based on the anatom-
ical region of occurrence—i.e., internal root resorption and
external root resorption (cervical root resorption and external
apical root resorption). Additional classification may involve
two types of internal resorption: root canal (internal) replace-
ment resorption and internal inflammatory resorption.
External resorption can be classified into four categories
according to its clinical and histologic manifestations: external
surface resorption, external inflammatory root resorption,
replacement resorption, and ankylosis. External inflammatory
root resorption has been further categorized into cervical
resorption with or without a vital pulp (invasive cervical root
resorption) and external apical root resorption (EARR) (Ne et
al., 1999).
This paper reviews EARR and its association with ortho-
dontic treatment, and examines a new paradigm for its multi-
factorial etiology. EARR is a frequent iatrogenic outcome asso-
ciated with orthodontic treatment, especially in the maxillary
incisors, and may also occur in the absence of orthodontic treat-
ment (Harris and Butler, 1992; Harris et al., 1993). Depending
on the methodology, the incidence of EARR without orthodon-
tic treatment has been reported to range from zero to 90.5%
(Brezniak and Wasserstein, 1993). From 7% to 13% of individu-
als who have not had orthodontic treatment show some EARR
on radiographs (Rudolph, 1936; Harris et al., 1993), presumably
as a function of occlusal forces. There is an association of EARR
in those who have not received orthodontic treatment with
missing teeth, increased periodontal probing depths, and
reduced crestal bone heights (Harris et al., 1993). Individuals
with bruxism, chronic nail biting, and anterior open bites with
concomitant tongue thrust may also show an increased extent
of EARR before orthodontic treatment (Harris and Butler,
1992). Dental trauma, especially with re-implantation of an
avulsed tooth, is also associated with increased EARR
(Donaldson and Kinirons, 2001). For the most part, EARR is
asymptomatic unless substantial tooth structure is affected, so
early detection is unlikely unless radiographs are used
(Brezniak and Wasserstein, 1993, 2002b).
(II) Root Resorption and External Apical
Root Resorption
EARR is the loss of root structure involving the apical region
to the extent that it can be seen on standard radiographs (Fig.
1). EARR is distinct from root resorption (RR). Microscopic
areas of resorption lacunae hallmark root resorption. These
microscopic lesions lack clinical significance and are not
detected by standard radiographs (Brezniak and Wasserstein,
1993). The resorption lacunae develop on the cementum root
surface and can be visualized by histological techniques (Fig.
2). Although EARR and RR during orthodontic tooth move-
ment are believed to be related conditions, a distinction should
be made between these two conditions when incidence and
prevalence are studied. Orthodontic force applied to teeth
GENETIC FACTORS IN EXTERNAL APICAL ROOT
RESORPTION AND ORTHODONTIC TREATMENT
J.K. Hartsfield, Jr.
1
*
,2
E.T. Everett
1,2
R.A. Al-Qawasmi
1
1
Department of Oral Facial Development, Indiana University School of Dentistry, 1121 West Michigan Street, Indianapolis, IN 46202-5186, USA; and
2
Department of Medical and Molecular Genetics,
Indiana University School of Medicine, Indianapolis, IN, USA; *corresponding author, jhartsfi@iupui.edu
ABSTRACT: External apical root resorption (EARR) is a common sequela of orthodontic treatment, although it may also occur
in the absence of orthodontic treatment. The degree and severity of EARR associated with orthodontic treatment are multifac-
torial, involving host and environmental factors. Genetic factors account for at least 50% of the variation in EARR. Variation in
the Interleukin 1 beta gene in orthodontically treated individuals accounts for 15% of the variation in EARR. Historical and con-
temporary evidence implicates injury to the periodontal ligament and supporting structures at the site of root compression fol-
lowing the application of orthodontic force as the earliest event leading to EARR. Decreased IL-1
 production in the case of IL-
1B (+3953) allele 1 may result in relatively less catabolic bone modeling (resorption) at the cortical bone interface with the PDL,
which may result in prolonged stress concentrated in the root of the tooth, triggering a cascade of fatigue-related events lead-
ing to root resorption. One mechanism of action for EARR may be mediated through impairment of alveolar resorption, result-
ing in prolonged stress and strain of the adjacent tooth root due to dynamic functional loads. Future estimation of susceptibil-
ity to EARR will likely require the analysis of a suite of genes, root morphology, skeleto-dental values, and the treatment
method to be used—or essentially the amount of tooth movement planned for treatment.
Key words. Root resorption, EARR, orthodontics, genetics, heritability.
over a short period of time can produce resorption lacunae in
the absence of EARR (Kvam, 1972). An increase in RR can be
accomplished with increased duration of orthodontic force
application and force, with the higher magnitude of moments
producing exposure of root dentin (Casa et al., 2001).
EARR may occur preferentially in the apical region, since
more than three-quarters of resorption lacunae occur in the api-
cal region of the root (Henry and Weinmann, 1951), a fact that
could be explained by the following: (1) Forces are concentrat-
ed at the root apex because orthodontic tooth movement is
never entirely translatory, and the fulcrum is usually occlusal to
the apical half of the root (Harris, 2000); (2) periodontal fibers
assume a different direction in the apical end, which might
explain the increased stress in the region (Henry and
Weinmann, 1951); and (3) the apical third is covered with cel-
lular cementum, whereas the coronal third is covered with acel-
lular cementum. The active cellular cementum depends on a
patent vasculature; accordingly, periapical cementum is more
friable and easily injured in the case of trauma and concomitant
vascular stasis (Henry and Weinmann, 1951; Baumrind et al.,
1996; Harris, 2000).
(III) Incidence of EARR in Association
with Orthodontic Treatment
Depending on the methods, EARR associated with orthodontic
treatment has been noted to range between zero and 100%
(Brezniak and Wasserstein, 1993). Although orthodontic treat-
ment is associated with some maxillary central incisor EARR in
most patients, and more than one-third of those treated experi-
ence greater than 3 mm of loss, severe EARR (more than 5 mm)
occurs in 2% to 5% of the population (Taithongchai et al., 1996;
Killiany, 1999).
(IV) The Effect of Tooth Movement
and the Role of the PDL
The amount of orthodontic movement is positively associated
with the resulting extent of EARR (DeShields, 1969; Sharpe et
al., 1987; Parker and Harris, 1998). Orthodontic tooth move-
ment, or "biomechanics", has been found to account for approx-
imately one-tenth to one-third of the total vari-
ation in EARR (Linge and Linge, 1991;
Baumrind et al., 1996; Horiuchi et al., 1998),
although, in one study (Parker and Harris,
1998), up to 90% of the variation has been
attributed to the extent of tooth movement.
In turn, the required amount of tooth
movement is a function of severity of maloc-
clusion. For example, the greater the incisor
overjet, the greater the amount of retraction
during treatment, and the greater the amount
of incisor resorption (Beck and Harris, 1994;
Harris et al., 1997). Moreover, it was found that
the deeper the overbite, the greater the incisor
intrusion and resorption, and the greater the
root resorption of the distal root of the maxil-
lary first molar (Beck and Harris, 1994).
Extraction patterns can influence the degree of
EARR because of the increased tooth move-
ment, compared with non-extraction cases,
required to close extraction spaces (Blake et al.,
1995; McNab et al., 2000). For example, cases in
which 4 first premolars were extracted have more EARR than
cases with no extractions, or only extractions of the 2 maxillary
first premolars (Sameshima and Sinclair, 2001b).
The amount of time spent in orthodontic treatment can be
a factor in EARR (Taithongchai et al., 1996), but not necessarily
(Beck and Harris, 1994; Taner et al., 1999). Even when duration
of treatment is a factor, it, along with several significant dento-
facial structure measurements, does not account for enough of
the observed variability to be useful as a predictor of EARR by
itself (Taithongchai et al., 1996).
The suitability of the rat as a model for studying tooth
movement was demonstrated almost 50 years ago (Macapanpan
et al., 1954; Waldo and Rothblatt, 1954). Macapanpan and co-
workers (1954) positioned elastic materials between the first and
second maxillary molars, tipping the first molars mesially and
the second and third molars distally. Tipping the teeth created
sites of compression, where cellular death and hyalinization
occurred, and tension, where cellular proliferation was observed
in the periodontal ligament (PDL). Subsequent histological stud-
ies in rodents support an association between root resorption
and the presence and active removal of the hyalinized tissues at
the sites of compression following experimental tooth move-
ment (Rygh, 1977; Williams, 1984; Brudvik and Rygh, 1993a,b,
1994; Hellsing and Hammarström, 1996). Normal healthy PDL
may modulate the cascade of root resorption. Cultured primary
dentition PDL fibroblasts have been shown to inhibit the differ-
entiation of osteoclast-like cells in mouse bone marrow cultures
(Wu et al., 1999). Root resorption therefore may require damage
to the cementoblastic layer in combination with necrosis or
inflammation. Interestingly, when orthodontic force is stopped,
root resorption continues until a functional PDL is established
(Brudvik and Rygh, 1995a,b).
(V) Influence of Dental Anomalies
and Root Morphology
While Kjaer (1995) suggested, as risk factors, the increased
prevalence (as compared with published population frequen-
cies) of dental anomalies-such as tooth agenesis, peg-shaped or
small maxillary lateral incisor crown, crown invaginations,
ectopic eruption, and taurodontism-in a sample of 107 ortho-
116 Crit Rev Oral Biol Med 15(2):115-122 (2004)
Figure 1. Pre- and post-treatment radiographs (A and B, respectively) of maxillary cen-
tral incisors from a female patient. The treatment lasted 4 yrs. The root apices are indi-
cated by asterisks.
dontic cases presenting with "excessive" (more than one-third
of one or more roots resorbed during treatment) EARR, Lee et
al. (1999) did not find increased EARR in 84 orthodontic cases
exhibiting at least one dental anomaly, compared with 84
matched cases in which there were no dental anomalies. The
presence of more than one dental anomaly did not appear to be
associated with a greater risk of EARR. The divergent out-
comes may be due to the difference in the ascertainment of the
study sample as well as other aspects of the two investigations.
Root length and shape have also been variables that have
been studied for their association with EARR. It has been sug-
gested that teeth exhibiting relatively short roots prior to ortho-
dontic treatment tend to develop more resorption during
orthodontic treatment (Ketcham, 1929; Becks, 1936; Massler
and Malone, 1954; Massler and Perreault, 1954; Jakobsson and
Lind, 1973; Goldson and Henrikson, 1975; Newman, 1975;
Kjaer, 1995; Taithongchai et al., 1996; Harris et al., 1997;
Thongudomporn and Freer, 1998). Even though some have
inferred that this is due to increased root resorption activity
prior to the orthodontic treatment, it is not clear that all rela-
tively short roots prior to orthodontic treatment are the result
of active pre-treatment resorption.
Although it has been held that a valid prognosis can be
made by the clinician as to the amount of EARR that could be
expected in the majority of cases on the basis of a careful analy-
sis of relatively short pre-treatment root lengths on the radio-
graphs taken before treatment (Massler and Malone, 1954), it
has also been said that EARR does not increase in teeth with
short roots (Levander and Malmgren, 1988; Goldin, 1989), or
that the tendency for EARR increases with increasing tooth
length (Mirabella and Årtun, 1995; Sameshima and Sinclair,
2001a). The latter finding may be related to longer teeth need-
ing stronger forces to be moved, and the fact that the actual dis-
placement of the root apex is larger during tipping or torquing
movements of longer teeth, although the apical shortening that
occurs in the shorter root is of greater concern (Mirabella and
Årtun, 1995; Sameshima and Sinclair, 2001a).
The two-dimensional shape of the root as delineated on
radiographic film appears to be associated with various
amounts of EARR. The tendency for EARR was found to be
greater in teeth with pipette-shaped roots and apical bends
(Newman, 1975; Levander and Malmgren, 1988; Kjaer, 1995),
bottleneck roots (McFadden et al., 1989), abnormal (pointed,
eroded, blunt, bent, and bottle shape) root shape (Mirabella
and Årtun, 1995), thin or pipette-shaped roots
(Thongudomporn and Freer, 1998), and dilacerated, bottle-
shaped, or pointed roots (Sameshima and Sinclair, 2001a),
although so far there is no systematic way to estimate the like-
lihood of EARR based upon root shape other than to say it is
increased. Teeth with blunted roots have been found to be asso-
ciated with both an increased (Thongudomporn and Freer,
1998) and a decreased (Sameshima and Sinclair, 2001a) occur-
rence of EARR. Overall, the shape of the root does appear to be
associated with the likelihood of EARR, and is best examined
on periapical rather than panoramic radiographs (Sameshima
and Asgarifar, 2001).
(VI) Cellular and Molecular Mechanisms
of Odontoclast/Osteoclast Regulation
Multinucleated cells referred to as "odontoclasts" resorb three
dental hard tissues, i.e., cementum, dentin, and enamel. These
cells have morphological and functional characteristics similar
to those of bone-resorbing osteoclasts (Sahara et al., 1994, 1998).
Osteoclast and odontoclast precursors originate from hemopoi-
etic cells in the bone marrow. Osteoclast formation from hemo-
poietic precursors is induced by the cytokines such as
macrophage colony-stimulating factor (M-CSF) and TRANCE
(also called RANKL, ODF, and OPGL), a membrane-bound lig-
and expressed by bone marrow stromal cells (Lean et al., 2000;
Tsurukai et al., 2000). Osteoclast precursors may also be recruit-
ed from the blood via activated endothelium (McGowan et al.,
2001). As circulating mononuclear cells (monoctyes), these pre-
cursors can be induced to proliferate and differentiate into
osteoclasts (Quinn et al., 1998; Massey and Flanagan, 1999;
Fujikawa et al., 2001). Receptor activator of nuclear factor
kappa B (RANK) and its ligand RANKL have been localized in
odontoblasts, pulp fibroblasts, periodontal ligament fibro-
blasts, and in single odontoclasts, the latter finding suggesting
an autocrine/paracrine role (Lössdörfer et al., 2002a,b). RANK
is coded for by the TNFRSF11A gene.
Osteoclastogenesis is modulated by an inhibitor, osteopro-
tegerin (OPG, also called TNFRSF11B). OPG is a soluble
(decoy) receptor for TRANCE and a member of the TNF recep-
tor superfamily. Osteoclast formation and survival require and
15(2):115-122 (2004) Crit Rev Oral Biol Med 117
Figure 2. Histological section through a mouse maxillary first molar
root tipped mesially with 25 g of orthodontic force for 9 days.
Resorption lacunae are indicated by arrows.
are enhanced by transforming growth factor-beta (TGF-beta),
which is abundant in bone matrix. TNF-alpha can also induce
osteoclast formation in vitro from bone-marrow-derived
mononuclear phagocytes, especially in the presence of TGF-
beta. Chambers (2000) suggests that the osteoclast is a mononu-
clear phagocyte directed toward a debriding function by TGF-
beta, activated for this function by TRANCE, and induced to
become specifically osteoclastic by the characteristics of the
substrate or signals from bone cells. Interleukin 1-beta (IL-1
),
a potent bone-resorptive cytokine, is a component of the com-
plex pathways leading to root resorption. A balance between
IL-1
 activity and interleukin receptor antagonist (IL-1RA)
may be crucial in the development of periapical lesions
(Shimauchi et al., 1998). Interleukin 1 alpha (IL-1
␣) is also a
potent bone-resorptive cytokine and has been found along
with TNF-alpha in periapical lesions (Fouad, 1997).
Orthodontic force leads to microtrauma of the PDL and
activation of a cascade of cellular events associated with
inflammation. Orthodontically induced inflammatory root
resorption (OIIRR) has been suggested (Brezniak and
Wasserstein, 2002a,b).
(VII) Systemic Factors
The intimate relationship between tooth movement and root
resorption suggests that factors affecting tooth movement also
affect root resorption. Tooth movement and changes in perio-
dontal tissue in response to orthodontic force in rats vary
depending on the time of day the force is applied (Miyoshi et
al., 2001). Other systemic factors—such as nutritional factors,
metabolic bone diseases, age, and use of drugs—affect ortho-
dontic tooth movement (Tyrovola and Spyropoulos, 2001).
Cyclic changes in the estradiol level may be associated with the
estrous-cycle-dependent variation in tooth movement through
its effects on bone resorption (Haruyama et al., 2002), and estro-
gen deficiency can cause rapid orthodontic tooth movement
(Yamashiro and Takano-Yamamoto, 2001). Calcitonin can
inhibit odontoclast activity (Wiebkin et al., 1996). The action of
calcitonin on osteoclasts occurs at later stages of osteoclast
development, and it inhibits the fusion of committed pre-osteo-
clasts to form mature multinucleated cells. Bisphosphonates
include potent inhibitors of bone resorption used to treat osteo-
porosis and other bone diseases. Bisphosphonates directly or
indirectly induce apoptosis in osteoclasts, which may play a
role in inhibition of bone resorption (Reszka et al., 1999).
Odontoclasts also undergo apoptosis following exposure to
bisphosphonates (Watanabe et al., 2000).
(VIII) Effects of Treatment and Technique
Although treatment and, specifically, the amount of tooth
movement may contribute to EARR, a comparison of maxillary
central incisor EARR among cases treated with the Tweed stan-
dard edgewise technique, the Begg lightwire technique, and
the Roth-prescription straightwire technique found no differ-
ence among the techniques (Parker and Harris, 1998).
However, another study did find more EARR in patients treat-
ed with Begg appliances than edgewise appliances (McNab et
al., 2000). No difference was also found between patients treat-
ed with the "Speed" appliance system and those who received
the edgewise straightwire appliance (Blake et al., 1995).
Comparisons of different techniques may be confounded by
the treatment protocols or decisions of different practitioners,
since there is also variation of EARR seen among different prac-
titioners using fixed edgewise appliances (Sameshima and
Sinclair, 2001b).
(IX) Effect of Sex
Although some questionable studies have found that ortho-
dontically treated females had a greater incidence of EARR
than males (Massler and Perreault, 1954; Kjaer, 1995), several
studies have found no difference in EARR between treated
males and treated females (Beck and Harris, 1994; Blake et al.,
1995; Parker and Harris, 1998; Harris et al., 2001; Sameshima
and Sinclair, 2001a).
(X) Interindividual Variability in EARR
Historically, there has been appreciable variability among
orthodontic patients in susceptibility to EARR, which might be
due to a systemic or innate predisposition to resorption in per-
manent as well as primary teeth (Becks, 1936; Massler and
Malone, 1954; Massler and Perreault, 1954; Reitan, 1957;
Newman, 1975). It was proposed that when extreme suscepti-
bility exists, severe EARR would occur even in the absence of
any demonstrable causes (Massler and Perreault, 1954). An eth-
nic dichotomy has been reported, with Asian patients having
significantly less EARR than Caucasian or Hispanic patients
(Sameshima and Sinclair, 2001a). Familial clustering of EARR
has been reported, although the
pattern of inheritance was not clear
(Newman, 1975; Harris et al., 1997).
This implies that there may be a
genetic component for susceptibil-
ity to EARR.
(X1) Application of
Genetic Analyses to EARR
Before undertaking DNA-based
studies to elucidate genetic deter-
minants of development or disease
expression, one would ideally like
to infer as much as possible about
the role of genetic factors in pheno-
typic expression on the basis of
development or disease patterns
within families and populations
118 Crit Rev Oral Biol Med 15(2):115-122 (2004)
TABLE
Intraclass Correlations and Heritability Estimates for EARR
Average Intraclass F-ratio
Variables
a
Sample Size
b
Correlation,
r
i
h
2
c
(model) P-value
Maxillary central incisor 2.08:82 0.420 0.840 2.5 0.0001
Mandibular central incisor 2.09:80 -0.022 0.000 1.0 0.5800
Mandibular first molar-mesial root 2.11:83 0.299 0.598 1.9 0.0010
Mandibular first molar-distal root 2.11:83 0.246 0.492 1.7 0.0070
a
EARR measurements were made to the nearest 0.1 mm with the use of a technique similar to that described
previously by Harris
et al.
(1997).
b
Average sibship size: number of sibships. The sample consisted of 180 orthodontic patients who had
received full-banded comprehensive treatment in the Graduate Orthodontic Clinic at the Indiana University
School of Dentistry. The 180 cases constituted 83 full sibling pairs (sibships).
c
Data from full sib-pairs were used to calculate heritability (
h
2
) estimates for EARR according to formulae
described by Falconer (1960).
(Lander and Schork, 1994). Heritability (h
2
) is defined as the
proportion of the phenotypic variance attributable to genetic,
as opposed to environmental, variance (factors), and its esti-
mation is one of the first objectives in the genetic study of a
quantitative trait such as EARR.
Exploration of the hypothesis of genetic influence on
EARR according to the sib-pair model found moderately high
heritability (h
2
) for EARR. The h
2
estimate averaged about 70%
for the maxillary incisors and mesial and distal roots of the
mandibular first molars, and this accounts for approximately
half of the total phenotypic variation (Harris et al., 1997). This
means that siblings experience similar levels of EARR in
response to orthodontic treatment. Analysis of variable DNA
markers is required to indicate which areas of the genome con-
tain genes that are at least partly responsible for the variation
seen in EARR associated with orthodontic treatment. Before
embarking on DNA analysis related to EARR, we confirmed
the heritability of the trait in a sample from the Graduate
Orthodontic Clinic at the Indiana University School of
Dentistry (unpublished observations) similar to the findings of
Harris et al. (1997) (Table). The heritability estimates were sig-
nificantly above zero in 3 out of the 4 roots examined, at
␣ =
0.05. Excluding the lower incisors, where h
2
= zero, the heri-
tability estimates for EARR ranged from approximately 50%
and 60% for the distal and mesial roots of the mandibular first
molars, respectively, to 84% for the maxillary central incisors.
These estimates are comparable with those determined by
Harris et al. (1997), who suggested that a low heritability esti-
mate for the mandibular central incisor could be due to little
variation in EARR experienced in the anterior mandibular
teeth. Alternatively, since the power to detect heritability
decreases with increasing measurement error, it is possible that
the difference in heritability estimates between the various
roots could be the result of differences in measurement error
among teeth. It could be that the measurement error is too high
in the case of the lower incisor tooth to permit significant heri-
tability to be detected. This is supported by the mandibular
central incisor root apex being the most difficult to identify, due
to the superimposition of many teeth in that region on cephalo-
graphs in our study. In either case, the lack of heritability for
EARR of the mandibular incisors does not necessarily indicate
that these teeth are behaving differently from the roots of the
other teeth where a genetic component has been documented.
Calculation of heritability estimates is a preliminary step
that should be followed by tests for causative agents. With clin-
ical orthodontics, the preliminary goal has been to define the
relative contributions of genetics and the environment.
(XII) Genetic Factors Influencing EARR
EARR is likely influenced by a combination of environmental
and host factors in a multifactor cascade. Efforts to investigate
host factors have focused on a possible genetic component. The
intrinsic value of the laboratory mouse as a model stems from
several reasons, including: linkage studies; the availability of a
dense and detailed genetic map that makes gene mapping in
mice practical and efficient; synteny or the genomic conserva-
tion of gene order with humans (regions of many mouse chro-
mosomes show conservation of both linkage and gene order
with various segments of human chromosomes); and high
degrees of homology with human gene sequences (Meisler,
1996; Ehrlich et al., 1997; Nadeau and Dunn, 1998). In addition
to the large number of available mutants and inbred strains,
mice are excellent hosts for genome manipulation (i.e., trans-
genic and gene inactivation via gene targeting/homologous
recombination). Finally, from a practical standpoint, mice can
be easily and economically raised in relatively small facilities
and have a short gestation and lifespan, which allows large-
scale and longitudinal studies to be performed.
Al-Qawasmi et al. (unpublished observations) subjected
genetically disparate inbred strains of mice to standardized
orthodontic force magnitude and duration where age, gender,
food, and housing were controlled and found variation in the
susceptibility or resistance to root resorption attributed to
orthodontic force (RRAOF), indicating that genotype is an
influencing factor. Mice were grouped into resistant (A/J,
C57BL/6J, and SJL/J), intermediate (C3H/HeJ and AKR/J),
and susceptible (BALB/cJ, DBA/2J, and 129P3/J) strains. That
study identified a seven-fold difference in susceptibility
between the DBA/2J (susceptible) and A/J (resistant) mouse
strains. It is anticipated that in-depth study of susceptible and
resistant mouse strains will facilitate the dissection of precise
molecular pathways and genetic mechanisms involved in the
pathogenesis of EARR in mice and could provide new infor-
mation that may contribute to our understanding of mecha-
nisms underlying the cause of EARR in humans by orthodon-
tic force.
(XIII) Candidate Gene Analysis
Evidence of EARR heritability in humans and genetic back-
ground in an animal model, along with the present under-
standing of the cellular events and molecular networks/path-
ways implicated in EARR, provides a starting point for the
investigation of candidate genes. Linkage disequilibrium meth-
ods are becoming increasingly more important in the genetic
dissection of complex traits. They facilitate evaluation of candi-
date polymorphisms (Spielman et al., 1993) and the fine-map-
ping of linked regions (Risch and Merikangas, 1996). In addi-
tion to linkage studies, one of the most common means for the
evaluation of evidence of an association, or linkage disequilib-
rium, between a candidate gene and a phenotype of interest is
the case-control design. This approach involves the collection
of a sample of affected and control individuals whose allele fre-
quencies at the polymorphism in a candidate gene are then
compared. A common concern in the case-control design is the
spurious detection of association due to population stratifica-
tion. To avoid the pitfalls of population-based association stud-
ies, investigators developed a family-based association test, the
transmission disequilibrium test (TDT) (Spielman et al., 1993).
The primary advantage of the TDT is that it avoids the necessi-
ty of collecting a matched control sample. As originally pro-
posed, the TDT analyzes a nuclear trio consisting of an affected
individual and his/her parents. These three individuals are
genotyped at a marker in or near the candidate gene. The al-
leles transmitted by the genotyped parents to the affected off-
spring are the "affected" sample, and the alleles not transmitted
from these two parents are then used as "control" alleles.
Through the use of a within-family design, the control sample
of alleles is perfectly matched to the affected sample of alleles,
since they are transmitted from the same two parents. Thus,
spurious association results due to population stratification are
avoided. This approach has been extended to allow for the
analysis of linkage disequilibrium with the use of quantitative
rather than qualitative phenotypes (Allison, 1997; Rabinowitz,
1997; Abecasis et al., 2000; Monks and Kaplan, 2000).
15(2):115-122 (2004) Crit Rev Oral Biol Med 119
Highly significant (p = 0.0003) evidence of linkage disequi-
librium of an IL-1B polymorphism with the clinical manifesta-
tion of EARR has been recently reported (Al-Qawasmi et al.,
2003a). Individuals homozygous for the IL-1B (+3953) allele 1
have a 5.6-fold (95% CI, 1.9-21.2) increased risk of EARR > 2
mm as compared with individuals who are not homozygous
for the IL-1B (+3953) allele 1. The IL-1B gene diallelic variation
at +3953 is associated with variable IL-1
 protein production
(Pociot et al., 1992). Cells from individuals homozygous for IL-
1B (+3953) allele 1 have reduced production of secreted IL-1

compared with individuals homozygous for IL-1B (+3953)
allele 2 and even individuals heterozygous for IL-1B (+3953)
alleles 1 and 2 (Pociot et al., 1992). The IL-1B polymorphism
described above accounts for 15% of the total variation of max-
illary incisor EARR (Al-Qawasmi et al., 2003a). Rossi et al.
(1996) examined IL-1 beta and TNF-alpha production in mono-
cytes from a group of orthodontic patients with severe root
shortening and found no significant differences in mean levels
between resorption and non-resorption groups. This supports
the likelihood that EARR is genetically heterogeneous.
In another linkage disequilibrium study (Al-Qawasmi et
al., 2003b), no evidence of linkage was found with EARR and
the TNF
␣
and TNSALP genes. Non-parametric sibling pair
linkage analysis with the microsatellite marker D18S64 (tightly
linked to TNFRSF11A) identified evidence of linkage (LOD =
2.5; p = 0.02) of EARR affecting the maxillary central incisor
(Al-Qawasmi et al., 2003b). This indicates that the TNFRSF11A
locus, or another tightly linked gene, is associated with EARR.
The TNFRSF11A gene codes for the protein RANK.
(XIV) Working Hypothesis
for Predisposition to EARR
IL-1 is a potent stimulus for bone resorption and osteoclastic
cell recruitment during orthodontic tooth movement
(Alhashimi et al., 2001). Expanding on the possible mechanism
of the association of the IL-1
 allele 1 and EARR, a relatively
decreased IL-1
 production in the case of allele 1 may result in
relatively less catabolic bone modeling (resorption) at the corti-
cal bone interface with the PDL. Stress analysis of orthodonti-
cally stimulated rat molars suggests that the initiation of
mechanically induced bone resorption is due to fatigue failure
within the bone itself (Katona et al., 1995; Roberts, 1999). This
would suggest that a deficiency of IL-1
 inhibits the resorptive
response to orthodontic loads. A slower rate of bone resorption
may result in a prolonged stress that is concentrated in the root
of the tooth, which in turn could trigger a cascade of fatigue-
related events leading to root resorption (Ketcham, 1929).
In summary, EARR may be mediated through impairment
of alveolar resorption, resulting in prolonged stress and strain of
the adjacent tooth root due to dynamic functional loads (Katona
et al., 1995). This scenario contradicts the hypothesis that
increased severity of root resorption after orthodontic treatment
is related to an increase in alveolar bone resorption (Engström et
al., 1988). On the contrary, root resorption may be related to
reduced rates of bone resorption at the PDL interface of the root
and the alveolar socket. This would result in a prolonged induc-
tive (lag) phase associated with compressed necrotic areas in the
PDL prior to alveolar bone resorption. In any event, it is likely
that the genetic factors that influence EARR are heterogeneous,
with different mechanisms operating in affected individuals, or
even site-specific responses in the same individual.
Currently, there are no reliable markers to predict either
which patients will develop EARR or the severity of EARR fol-
lowing orthodontic tooth movement (Vlaskalic et al., 1998). The
association of the IL-1B (+3953) allele 1 and EARR, which
accounts for approximately 15% of the total EARR variation
seen in orthodontic patients, has emerged as a potential genet-
ic marker (Al-Qawasmi et al., 2003a). Although the presence of
particular root shapes, genetic markers, marked overjet, or the
need for extractions may all be associated with an increase in
the likelihood of EARR, none is sufficient, alone, to predict
EARR reliably. Future analysis of all of these factors (which will
also require further discovery of genetic factors such as one or
more genes flanking the D18S64 polymorphism, perhaps
TNFRSF11A) together should increase the validity of risk esti-
mation, and may even, within reasonable confidence intervals,
result in a prediction.
Acknowledgment
An American Association of Orthodontists Foundation Biomedical Research
Award to JKH supported this work.
R
EFERENCES
Abecasis GR, Cardon LR, Cookson WO (2000). A general test of
association for quantitative traits in nuclear families. Am J Hum
Genet 66:279-292.
Al-Qawasmi RA, Hartsfield JK Jr, Everett ET, Flury L, Liu L,
Foroud TM, et al. (2003a). Genetic predisposition to external
root apical resorption. Am J Orthod Dentofacial Orthop 123:242-
251.
Al-Qawasmi RA, Hartsfield JK Jr, Everett ET, Flury L, Liu L,
Foroud TM, et al. (2003b). Genetic predisposition to external
apical root resorption in orthodontic patients: linkage of chro-
mosome-18 marker. J Dent Res 82:356-360.
Alhashimi N, Frithiof L, Brudvik P, Bakhiet M (2001). Orthodontic
tooth movement and de novo synthesis of proinflammatory
cytokines. Am J Orthod Dentofacial Orthop 119:307-312.
Allison DB (1997). Transmission-disequilibrium tests for quantita-
tive traits. Am J Hum Genet 60:676-690.
Baumrind S, Korn EL, Boyd RL (1996). Apical root resorption in
orthodontically treated adults. Am J Orthod Dentofacial Orthop
110:311-320.
Beck BW, Harris EF (1994). Apical root resorption in orthodontical-
ly treated subjects: analysis of edgewise and light wire mechan-
ics. Am J Orthod Dentofacial Orthop 105:350-361.
Becks H (1936). Root resorptions and their relation to pathological
bone formation. Part 1: Statistical data and Roentgenographic
aspects. Int J Orthodont Oral Surg 22:445-482.
Blake M, Woodside DG, Pharoah MJ (1995). A radiographic com-
parison of apical root resorption after orthodontic treatment
with the edgewise and speed appliances. Am J Orthod
Dentofacial Orthop 108:76-84.
Brezniak N, Wasserstein A (1993). Root resorption after orthodon-
tic treatment. Part 2. Literature review. Am J Orthod Dentofacial
Orthop 103:138-146.
Brezniak N, Wasserstein A (2002a). Orthodontically induced
inflammatory root resorption. Part I: The basic science aspects.
Angle Orthod 72:175-179.
Brezniak N, Wasserstein A (2002b). Orthodontically induced
inflammatory root resorption. Part II: The clinical aspects. Angle
Orthod 72:180-184.
Brudvik P, Rygh P (1993a). The initial phase of orthodontic root
resorption incident to local compression of the periodontal lig-
120 Crit Rev Oral Biol Med 15(2):115-122 (2004)
ament. Eur J Orthod 15:249-263.
Brudvik P, Rygh P (1993b). Non-clast cells start orthodontic root
resorption in the periphery of hyalinized zones. Eur J Orthod
15:467-480.
Brudvik P, Rygh P (1994). Root resorption beneath the main hyalin-
ized zone. Eur J Orthod 16:249-263.
Brudvik P, Rygh P (1995a). Transition and determinants of ortho-
dontic root resorption-repair sequence. Eur J Orthod 17:177-188.
Brudvik P, Rygh P (1995b). The repair of orthodontic root resorp-
tion: an ultrastructural study. Eur J Orthod 17:189-198.
Casa MA, Faltin RM, Faltin K, Sander FG, Arana-Chavez VE (2001).
Root resorptions in upper first premolars after application of
continuous torque moment. Intra-individual study. J Orofac
Orthop 62:285-295.
Chambers TJ (2000). Regulation of the differentiation and function
of osteoclasts. J Pathol 192:4-13.
DeShields RW (1969). A study of root resorption in treated class ii,
division i malocclusions. Angle Orthod 39:231-245.
Donaldson M, Kinirons MJ (2001). Factors affecting the time of
onset of resorption in avulsed and replanted incisor teeth in
children. Dent Traumatol 17:205-209.
Ehrlich J, Sankoff D, Nadeau JH (1997). Synteny conservation and
chromosome rearrangements during mammalian evolution.
Genetics 147:289-296.
Engström C, Granström G, Thilander B (1988). Effect of orthodon-
tic force on periodontal tissue metabolism. A histologic and bio-
chemical study in normal and hypocalcemic young rats. Am J
Orthod Dentofacial Orthop 93:486-495.
Falconer DS (1960). Introduction to quantitative genetics. New
York: Ronald Press Co.
Fouad AF (1997). Il-1 alpha and TNF-alpha expression in early peri-
apical lesions of normal and immunodeficient mice. J Dent Res
76:1548-1554.
Fujikawa Y, Sabokbar A, Neale SD, Itonaga I, Torisu T, Athanasou
NA (2001). The effect of macrophage-colony stimulating factor
and other humoral factors (interleukin-1, -3, -6, and -11, tumor
necrosis factor-alpha, and granulocyte macrophage-colony
stimulating factor) on human osteoclast formation from circu-
lating cells. Bone 28:261-267.
Goldin B (1989). Labial root torque: effect on the maxilla and incisor
root apex. Am J Orthod Dentofacial Orthop 95:208-219.
Goldson L, Henrikson CO (1975). Root resorption during Begg
treatment; a longitudinal Roentgenologic study. Am J Orthod
68:55-66.
Harris EF (2000). Root resorption during orthodontic therapy.
Semin Orthod 6:183-194.
Harris EF, Butler ML (1992). Patterns of incisor root resorption
before and after orthodontic correction in cases with anterior
open bites. Am J Orthod Dentofacial Orthop 101:112-119.
Harris EF, Robinson QC, Woods MA (1993). An analysis of causes
of apical root resorption in patients not treated orthodontically.
Quintessence Int 24:417-428.
Harris EF, Kineret SE, Tolley EA (1997). A heritable component for
external apical root resorption in patients treated orthodonti-
cally. Am J Orthod Dentofacial Orthop 111:301-309.
Harris EF, Boggan BW, Wheeler DA (2001). Apical root resorption
in patients treated with comprehensive orthodontics. J TN Dent
Assoc 81:30-33.
Haruyama N, Igarashi K, Saeki S, Otsuka-Isoya M, Shinoda H,
Mitani H (2002). Estrous-cycle-dependent variation in ortho-
dontic tooth movement. J Dent Res 81:406-410.
Hellsing E, Hammarström L (1996). The hyaline zone and associat-
ed root surface changes in experimental orthodontics in rats: a
light and scanning electron microscope study. Eur J Orthod
18:11-18.
Henry JL, Weinmann JP (1951). The pattern of resorption and repair
of human cementum. J Am Dent Assoc 42:270-290.
Horiuchi A, Hotokezaka H, Kobayashi K (1998). Correlation
between cortical plate proximity and apical root resorption. Am
J Orthod Dentofacial Orthop 114:311-318.
Jakobsson R, Lind V (1973). Variation in root length of the perma-
nent maxillary central incisor. Scand J Dent Res 81:335-338.
Katona TR, Paydar NH, Akay HU, Roberts WE (1995). Stress analy-
sis of bone modeling response to rat molar orthodontics. J
Biomech 28:27-38.
Ketcham AH (1929). A progress report of an investigation of apical
root resorption of vital permanent teeth. Int J Orthod Oral Surg
Radiol 15:310-328.
Killiany DM (1999). Root resorption caused by orthodontic treat-
ment: an evidence-based review of literature. Semin Orthod
5:128-133.
Kjaer I (1995). Morphological characteristics of dentitions develop-
ing excessive root resorption during orthodontic treatment. Eur
J Orthod 17:25-34.
Kvam E (1972). Scanning electron microscopy of tissue changes on
the pressure surface of human premolars following tooth
movement. Scand J Dent Res 80:357-368.
Lander ES, Schork NJ (1994). Genetic dissection of complex traits.
Science 265:2037-2048.
Lean JM, Matsuo K, Fox SW, Fuller K, Gibson FM, Draycott G, et al.
(2000). Osteoclast lineage commitment of bone marrow precur-
sors through expression of membrane-bound trance. Bone
27:29-40.
Lee RY, Årtun J, Alonzo TA (1999). Are dental anomalies risk fac-
tors for apical root resorption in orthodontic patients? Am J
Orthod Dentofacial Orthop 116:187-195.
Levander E, Malmgren O (1988). Evaluation of the risk of root
resorption during orthodontic treatment: a study of upper
incisors. Eur J Orthod 10:30-38.
Linge L, Linge BO (1991). Patient characteristics and treatment
variables associated with apical root resorption during ortho-
dontic treatment. Am J Orthod Dentofacial Orthop 99:35-43.
Lössdörfer S, Gotz W, Jager A (2002a). Immunohistochemical local-
ization of receptor activator of nuclear factor kappaB (RANK)
and its ligand (RANKL) in human deciduous teeth. Calcif Tissue
Int 71:45-52.
Lössdörfer S, Gotz W, Jager A(2002b). Localization of IL-1alpha, IL-
1 RI, TNF, TNF-RI and TNF-RII during physiological drift of rat
molar teeth—an immunohistochemical and in situ hybridiza-
tion study. Cytokine 20:7-16.
Macapanpan L, Weinmann J, Brodie A (1954). Early tissue changes
following tooth movement in rats. Angle Orthod 24:79-94.
Massey HM, Flanagan AM (1999). Human osteoclasts derive from
CD14-positive monocytes. Br J Haematol 106:167-170.
Massler M, Malone AJ (1954). Root resorption in human permanent
teeth: a Roentgenographic study. Am J Orthod 40:619-633.
Massler M, Perreault JG (1954). Root resorption in the permament
teeth of young adults. J Dent Child 21:158-164.
McFadden WM, Engström C, Engström H, Anholm JM (1989). A
study of the relationship between incisor intrusion and root
shortening. Am J Orthod Dentofacial Orthop 96:390-396.
McGowan NW, Walker EJ, Macpherson H, Ralston SH, Helfrich
MH (2001). Cytokine-activated endothelium recruits osteoclast
precursors. Endocrinology 142:1678-1681.
McNab S, Battistutta D, Taverne A, Symons AL (2000). External api-
cal root resorption following orthodontic treatment. Angle
Orthod 70:227-232.
Meisler MH (1996). The role of the laboratory mouse in the human
genome project. Am J Hum Genet 59:764-771.
Mirabella AD, Årtun J (1995). Risk factors for apical root resorption
15(2):115-122 (2004) Crit Rev Oral Biol Med 121
of maxillary anterior teeth in adult orthodontic patients. Am J
Orthod Dentofacial Orthop 108:48-55.
Miyoshi K, Igarashi K, Saeki S, Shinoda H, Mitani H (2001). Tooth
movement and changes in periodontal tissue in response to
orthodontic force in rats vary depending on the time of day the
force is applied. Eur J Orthod 23:329-338.
Monks SA, Kaplan NL (2000). Removing the sampling restrictions
from family-based tests of association for a quantitative-trait
locus. Am J Hum Genet 66:576-592.
Nadeau JH, Dunn PJ (1998). Genomic strategies for defining and
dissecting developmental and physiological pathways. Curr
Opin Genet Dev 8:311-315.
Ne RF, Witherspoon DE, Gutmann JL (1999). Tooth resorption.
Quintessence Int 30:9-25.
Newman WG (1975). Possible etiologic factors in external root
resorption. Am J Orthod 67:522-539.
Parker RJ, Harris EF (1998). Directions of orthodontic tooth move-
ments associated with external apical root resorption of the
maxillary central incisor. Am J Orthod Dentofacial Orthop
114:677-683.
Pociot F, Molvig J, Wogensen L, Worsaae H, Nerup J (1992). A TaqI
polymorphism in the human interleukin-1 beta (IL-1 beta) gene
correlates with IL-1 beta secretion in vitro. Eur J Clin Invest
22:396-402.
Quinn JM, Neale S, Fujikawa Y, McGee JO, Athanasou NA (1998).
Human osteoclast formation from blood monocytes, peritoneal
macrophages, and bone marrow cells. Calcif Tissue Int 62:527-531.
Rabinowitz D (1997). A transmission disequilibrium test for quan-
titative trait loci. Hum Hered 47:342-350.
Reitan K (1957). Some factors determining the evaluation of forces
in orthodontics. Am J Orthod 43:32-45.
Reszka AA, Halasy-Nagy JM, Masarachia PJ, Rodan GA (1999).
Bisphosphonates act directly on the osteoclast to induce cas-
pase cleavage of mst1 kinase during apoptosis. A link between
inhibition of the mevalonate pathway and regulation of an
apoptosis-promoting kinase. J Biol Chem 274:34967-34973.
Risch N, Merikangas K (1996). The future of genetic studies of com-
plex human diseases. Science 273:1516-1517.
Roberts WE (1999). Bone dynamics of osseointegration, ankylosis,
and tooth movement. J IN Dent Assoc 78:24-32.
Rossi M, Whitcomb S, Lindemann R (1996). Interleukin-1 beta and
tumor necrosis factor-alpha production by human monocytes
cultured with l-thyroxine and thyrocalcitonin: Relation to
severe root shortening. Am J Orthod Dentofacial Orthop 110:399-
404.
Rudolph CE (1936). A comparative study in root resorption in per-
manent teeth. J Am Dent Assoc 23:822-826.
Rygh P (1977). Orthodontic root resorption studied by electron
microscopy. Angle Orthod 47:1-16.
Sahara N, Okafuji N, Toyoki A, Ashizawa Y, Deguchi T, Suzuki K
(1994). Odontoclastic resorption of the superficial nonmineral-
ized layer of predentine in the shedding of human deciduous
teeth. Cell Tissue Res 277:19-26.
Sahara N, Ashizawa Y, Nakamura K, Deguchi T, Suzuki K (1998).
Ultrastructural features of odontoclasts that resorb enamel in
human deciduous teeth prior to shedding. Anat Rec 252:215-228.
Sameshima GT, Asgarifar KO (2001). Assessment of root resorption
and root shape: periapical vs panoramic films. Angle Orthod
71:185-189.
Sameshima GT, Sinclair PM (2001a). Predicting and preventing
root resorption: Part I. Diagnostic factors. Am J Orthod
Dentofacial Orthop 119:505-510.
Sameshima GT, Sinclair PM (2001b). Predicting and preventing
root resorption: Part II. Treatment factors. Am J Orthod
Dentofacial Orthop 119:511-515.
Sharpe W, Reed B, Subtelny JD, Polson A (1987). Orthodontic
relapse, apical root resorption, and crestal alveolar bone levels.
Am J Orthod Dentofacial Orthop 91:252-258.
Shimauchi H, Takayama S, Imai-Tanaka T, Okada H (1998). Balance
of interleukin-1 beta and interleukin-1 receptor antagonist in
human periapical lesions. J Endod 24:116-119.
Spielman RS, McGinnis RE, Ewens WJ (1993). Transmission test for
linkage disequilibrium: the insulin gene region and insulin-
dependent diabetes mellitus (IDDM). Am J Hum Genet 52:506-
516.
Taithongchai R, Sookkorn K, Killiany DM (1996). Facial and den-
toalveolar structure and the prediction of apical root shorten-
ing. Am J Orthod Dentofacial Orthop 110:296-302.
Taner T, Ciger S, Sencift Y (1999). Evaluation of apical root resorp-
tion following extraction therapy in subjects with class I and
class II malocclusions. Eur J Orthod 21:491-496.
Thongudomporn U, Freer TJ (1998). Anomalous dental morpholo-
gy and root resorption during orthodontic treatment: a pilot
study. Aust Orthod J 15:162-167.
Tsurukai T, Udagawa N, Matsuzaki K, Takahashi N, Suda T (2000).
Roles of macrophage-colony stimulating factor and osteoclast
differentiation factor in osteoclastogenesis. J Bone Miner Metab
18:177-184.
Tyrovola JB, Spyropoulos MN (2001). Effects of drugs and systemic
factors on orthodontic treatment. Quintessence Int 32:365-371.
Vlaskalic V, Boyd RL, Baumrind S (1998). Etiology and sequelae of
root resorption. Semin Orthod 4:124-131.
Waldo CM, Rothblatt JM (1954). Histologic response to tooth move-
ment in the laboratory rat: procedure and preliminary observa-
tions. J Dent Res 33:481-486.
Watanabe J, Amizuka N, Noda T, Ozawa H (2000). Cytochemical
and ultrastructural examination of apoptotic odontoclasts
induced by bisphosphonate administration. Cell Tissue Res
301:375-387.
Wiebkin OW, Cardaci SC, Heithersay GS, Pierce AM (1996).
Therapeutic delivery of calcitonin to inhibit external inflamma-
tory root resorption. I. Diffusion kinetics of calcitonin through
the dental root. Endod Dent Traumatol 12:265-271.
Williams S (1984). A histomorphometric study of orthodontically
induced root resorption. Eur J Orthod 6:35-47.
Wu YM, Richards DW, Rowe DJ (1999). Production of matrix-
degrading enzymes and inhibition of osteoclast-like cell differ-
entiation by fibroblast-like cells from the periodontal ligament
of human primary teeth. J Dent Res 78:681-689.
Yamashiro T, Takano-Yamamoto T (2001). Influences of ovariecto-
my on experimental tooth movement in the rat. J Dent Res
80:1858-1861.
122 Crit Rev Oral Biol Med 15(2):115-122 (2004)
