Download Age-dependent external root resorption during tooth movement in rats

Acta Odontologica Scandinavica, 2008; 66: 9398
Downloaded By: [Ren, Yijin] At: 17:10 3 May 2008
ORIGINAL ARTICLE
Age-dependent external root resorption during tooth
movement in rats
YIJIN REN1, JAAP C. MALTHA2, ROBERT S. B. LIEM3, IETSE STOKROOS3 &
ANNE MARIE KUIJPERS-JAGTMAN2
1
Department of Orthodontics, University Medical Centre Groningen, University of Groningen, The Netherlands, 2Department
of Orthodontics and Oral Biology, Radboud University Nijmegen Medical Centre, The Netherlands and 3Department of Cell
Biology, Section Electronmicroscopy, University Medical Centre Groningen, University of Groningen, The Netherlands
Abstract
Objective. To investigate the effect of age on root resorption and distribution along different parts of the root during
prolonged light force application. Material and methods. Orthodontic appliances were placed in two groups of 30 rats
(one group 6 weeks old, the other 912 months old), with contralateral sides as controls. Groups of animals were killed at 1,
2, 4, 8, and 12 weeks. At the study site, incidence was counted as either 1 (with resorption) or 0 (without); severity was
measured as the summed length of all resorption lacunae as a percentage of study total root length. Results. Young and
adult rats had the same incidence of root resorption in the early phase (B4 weeks), and both increased in the late phase (4
12 weeks) to the same level. Severity of resorption increased with prolonged tooth movement only in adult rats. However,
there was no age-related difference in either the early or the late phase. In both groups, the middle part of the root had the
highest incidence of resorption; the most severe resorption occurred exclusively at this part. Conclusion. Orthodontic
intervention even with light forces increased both the incidence and severity of root resorption, the more so in the middle
part of the root. Adult rats had increased incidence and severity with prolonged tooth movement.
Key Words: Age, orthodontics, rats, root resorption, tooth movement
Introduction
External root resorption has been recognized as a
frequent iatrogenic consequence of orthodontic
treatment [1]. Although it also occurs in permanent
teeth under physiological conditions, the incidence is
higher among treated individuals [2,3]. The risk
factors related to higher susceptibility are generally
categorized as patient-related and treatment-related
factors, among which age of the patients, force
regimen, and treatment duration are closely involved
[4,5].
Age has been suggested as a predisposing factor
for root resorption [6,7]. Increased age is concomitant with differences in bone density and capacity for
bone remodelling [8,9]. A common perception has
been that adults may be at greater risk of root
resorption [3,10]. However, some studies have
demonstrated that orthodontic treatment does not
place adults at greater risk for root resorption and
that it is the periodontal condition at the start of
treatment that merits attention [1113]. To date, no
experimental study has investigated quantitively the
impact of age on root resorption during prolonged
tooth movement.
In previous clinical studies, the focus has mainly
been apical root resorption this partly attributable
to the fact that on radiographs only shortening of
root length can be recognized. These studies showed
increased incidence of apical resorption in orthodontic patients [6,14]. Histology studies in patients
have shown more root resorption in the apical region
after tipping tooth movement [15,16]. These findings concur with those of Follin et al. [17], in whose
study localization of root resorption appeared to
be related to type of movement. Localization might
be explained by the differences in stress/strain
distribution in the periodontal ligament [1]. Studies
on beagle dogs have shown more resorption in the
middle and cervical parts of the root after bodily
Correspondence: Yijin Ren, Department of Orthodontics, University Medical Centre Groningen, Triade gebouw 24, Hanzeplein 1, 9700 RB Groningen, The
Netherlands. Tel: 31 50 3610050. Fax: 31 50 3919150. E-mail: [email protected]
(Received 27 October 2007; accepted 12 February 2008)
ISSN 0001-6357 print/ISSN 1502-3850 online # 2008 Taylor & Francis
DOI: 10.1080/00016350801982522
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Y. Ren et al.
tooth movement [18,19]. However, quantitative
experimental data are lacking on the distribution of
root resorption over the root in standardized conditions and on whether this distribution pattern is agerelated.
The present study was designed to address the
above-mentioned unexplored areas and to overcome
a number of the drawbacks of previous experimental
studies, i.e. a mixing-up of the incidence and severity
of resorption [19], investigation only of the pressure
surfaces [18], no discrimination of the pressure and
tension regions [19], and application of heavy forces
[20]. Our aim was to test the following hypotheses
over a 12-week observation period with a light
continuous force in rats: orthodontic tooth movement increases the incidence and severity of root
resorption; both incidence and severity are higher in
adult rats than in young rats and higher in the middle
part of the roots than in the apical and cervical parts.
Material and methods
Experimental tooth movement
Two groups of 30 male Wistar rats, aged 6 weeks and
912 months, respectively, were used. The animals
were acclimatized for 2 weeks before the start of the
experiment. They were housed under normal laboratory conditions and fed powdered laboratory rat
chow (Sniff, Soest, The Netherlands) and water ad
libitum. Ethical permission was obtained from the
Radboud University Nijmegen Medical Centre, The
Netherlands.
The appliance design has been described in detail
in a previous publication [20]. Briefly, a split-mouth
design was used with the experimental side chosen
randomly, and with the contralateral side as the
control. An orthodontic appliance was placed in the
experimental side after general anesthesia (Figure 1).
At 1, 2, 4, and 8 weeks, 5 or 6 rats from each group
were killed, and at 12 weeks also the remaining
animals.
Material preparation
The rats were given an overdose of anesthetic prior
to being killed. They were then perfused with 4%
formaldehyde solution at 378C. The maxillae were
dissected and immersed in the same fixative for 24 h
at 48C. After decalcification in 10% EDTA and
paraffin embedding, serial parasagittal 7 mm sections
were cut. Every 25th section was collected on
SuperFrost/Plus slides (Menzel-Gläser, Braunschwieg, Germany) and stained with H&E.
Registration of root resorption
Three roots (two from the 1st molar, i.e. the middle
and distal roots, and one from the 2nd molar, i.e. the
mesial root) per section and three sections per study
side were selected. Total root length (RL), measured
as the distance along the long axis of the root from
the cemento-enamel junction to the apex, was
divided equally into cervical, middle, and apical
parts (Figure 2). The experimental and control sides
each had mesial and distal regions, which, depending on the direction of tooth movement, were
pressure or tension regions; each had three areas
(cervical, middle, and apical). Only dentin resorption was taken into consideration. In each region/
area, incidence was counted as either 1 (with
resorption) or 0 (without resorption); severity was
measured as the summed length of all resorption
lacunae as a percentage of RL or 1/3 RL (Figure 2).
For average severity, the mean of all 9 measurements
(3 roots 3 sections) per region/area was calculated;
for maximal severity, the largest measurement was
selected as representing the most severe resorption in
the region/area per animal. All resorption lacunae
were directly registered in the microscope.
Statistics
Medians were calculated for each region/area. First,
comparisons across time were performed using the
Figure 1. The orthodontic appliance. A transverse hole was drilled through the alveolar bone and maxillary incisors at mid-root level. A
preformed ligature wire enclosing all three molars was bonded. A coil spring was attached to a ligature wire through the hole to move the
molars mesially with a 10 cN force. At the control side, molars drifted distally, thus the distal and mesial regions were pressure and tension
regions. The opposite was the case at the experimental side, where molars moved mesially.
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Age-related root resorption
95
Figure 2. Registration of root resorption. Total root length (RL) was measured as the distance along the long axis of the root from the
cemento-enamel junction to the apex. It was divided into three equal parts (cervical, middle, and apical), the root length of each part being
1/3 RL. The experimental and control sides each had two mesial and distal regions, which, depending on the direction of tooth movement,
were pressure or tension regions and each region had three areas (cervical, middle, and apical). The severity of root resorption per region
was the summed length of all resorption lacunae present (abc. . .) as a percentage of RL; the severity per area was the summed length
of all resorption lacunae present (abc) as a percentage of 1/3 RL.
Kruskal-Wallis non-parametric (ANOVA) test followed by the TukeyKramer multiple comparison
test. Based on our previous study [20], tooth movement was divided into early phase (weeks 1, 2) and
late phase (weeks 4, 8, 12). As no time-dependent
difference was found within the early and late phase
groups, the data were pooled and Mann-Whitney
tests were used to compare the two phases. The
same tests were used for comparisons between the
experimental and control sides, and between young
and adult rats. Individual animals were the unit for
calculations in all statistics. Differences were considered significant if p B0.05. Data from the early
and late phases were pooled for a general distribution pattern of the incidence and severity of root
resorption per area.
Results
At control, side teeth had drifted distally, and thus
the distal and mesial could be considered as pressure
and tension regions. During orthodontic intervention, teeth had moved mesially; the pressure region
was thus at the mesial and the tension region at the
distal (Figure 1). No difference was found between
weeks 1 and 2 or between weeks 4, 8, and 12 for any
of the study parameters in any region or in either age
group. The data were therefore pooled for the early
and late phases.
Incidence (Figure 3)
The two age groups showed similar changes in
incidence, which increased from the early to the
late phase only in the experimental mesial regions
(p B0.01). Incidence in these regions in the late
phase was higher than in the experimental distal and
control mesial regions in both age groups (pB0.01).
There was no difference between young and adult
animals in any region or in either phase of tooth
movement (Figure 3a,b). In almost all regions, the
middle part of the root had a higher incidence than
the apical and cervical parts (Figure 3c).
Average severity (Figure 4)
Only in adult rats was there any increase in severity
from the early to the late phase in the experimental
mesial region (p B0.01), where severity was significantly higher than in experimental distal regions in
the late phase in both age groups (p B0.01). It was
also higher than the control counterpart (p B0.05)
and both phases (p B0.01) in young rats, and only in
the late phase (p B0.01) in adult rats. There was no
difference between young and adult animals in any
region or in either phase (Figure 4a,b).
Maximal severity (Figure 5)
Figure 4 shows the maximal severity of root resorption per region (Figure 5a,b) and per area
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Y. Ren et al.
Figure 3. Incidence of root resorption. (a,b) The number of areas with resorption. (c) The percentage of affected areas per area (early
early phase, latelate phase, expexperimental side, concontrol side, mmesial region, d distal region, yyoung rats, a adult rats).
All data are shown as boxplots: *between early and late phases, or between pressure and tension regions; #between experimental and control
sides in the same region, *#pB0.05, **##p B0.01. The same applies in Figures 3 and 4.
(Figure 5c,d). Similar to the average severity, only in
adult rats was there a significant increase from the
early to the late phase in the experimental mesial
region (pB0.01). Maximal severity in this region
was significantly higher than that in experimental
distal regions in the late phase in both age groups
(pB0.01). It was also higher than the control
counterpart in the early (p B0.05) and late
(pB0.01) phases in young rats, but only in the late
phase (p B0.01) in adult rats. There was no difference between young and adult animals in any region
or in either phase (Figure 5a,b).
In experimental mesial regions, the middle part of
the root had a higher severity than the cervical part
(pB0.05), which counts for both age groups (Figure
4c,d); in the middle part, the level was higher in the
mesial than in the distal region (p B0.01). In young
rats, the control distal region also showed a higher
severity in the middle part (pB0.01); the experimental mesial region had a higher level than the
control counterparts in middle (p B0.01) and apical
(p B0.05) areas in young rats, and in all areas in
adult rats (pB0.05 cervical, p B0.01 middle and
apical). There was no difference between young and
adult animals in any area. In both age groups, the
most severe resorption occurred in the middle part.
Discussion
The present study confirms that orthodontic intervention increases both the incidence and severity of
root resorption in pressure regions, which is in line
with previous studies [2224]. The results indicate
that root resorption is a common phenomenon
accompanying tooth movement even with light
forces. We acknowledge that root resorption is a
three-dimensional phenomenon, and that twodimensional measurements may underestimate its
incidence and severity. The maximal severity measurements were certainly less than or, at most, close
Figure 4. Average severity of root resorption in young (a) and adult rats (b).
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97
Figure 5. The maximal severity of root resorption. (a,b) The maximal severity per region. (c,d) The maximal severity per area.
to the most severe situations. Nevertheless, with a
sufficient number of study roots and sections it may
provide a good representation of the three-dimensional situation and reliable measurements [25].
Severity indicators appeared non-different using
length, relative length, depth and surface area of
the resorption lacunae when the same study roots
were used [17].
The most important finding of the present study
was that prolonged tooth movement increased the
incidence of root resorption in both age groups;
however, only in adult rats did it increase severity.
Limited experimental data are available on agedependent root resorption to compare with our
study. Data on humans show that adults are at no
greater risk for root resorption [1113]. The results
of studies on the effects of treatment duration in
humans are controversial [15,2630], which may be
explained by the following aspects. First, apical
resorption is related to the total distance the apex
moves and the time it takes [31], not to the duration
of general treatment, which does not reflect the
amount of apex displacement. Second, the ages of
the study subjects were not considered, which may
have been important. The present study showed that
a short period of tooth movement is not particularly
disadvantageous to adult rats. Prolonged movement,
however, significantly increased the severity of resorption in older animals but not in younger animals.
Previous studies have suggested that tooth movement and root resorption are related to bone turnover rate and functional state of the periodontium
[23,32]; that higher bone turnover increases tooth
movement without affecting root resorption; and
that decreased bone turnover increases the risk of
resorption [19]. This may partly explain the agerelated differences in our study, since bone turnover
rate decreases with age [33].
Although the severity of resorption increased from
early to late phases in adult rats, there was no
difference between young and adult animals in either
phase. These findings may indeed suggest that both
the average and the most severe situation of orthodontically induced root resorption are comparable in
young and adult animals. On the other hand, they
might be related to the large individual variations
which existed in both age groups. Previous studies
with standardized set-ups using animals from the
same genetic strains have also reported large interindividual differences in the velocity and amount of
tooth movement [21,34]. It is possible that susceptibility to root resorption correlates to the velocity or
amount of tooth movement at the individual level.
This aspect requires further investigation.
Another important finding is that in both age
groups the middle part of the root hosted the highest
incidence and severity of root resorption. In a beagle
dog with prolonged bodily movement, root resorption appeared exclusively in the middle part [19],
which was explained by the sand-glass shaped
periodontal space thinnest in the middle part.
Another study on beagles [18] for a similar observation period showed that in bodily movement root
resorption was also predominantly in the middle and
cervical parts. Together with the results of the
present study this indicates that at the microscopic
level the middle part of the root is most vulnerable
for root resorption. Clinical studies, limited in their
measurement methods, cannot detect resorption at
the cervical or middle part, and may therefore come
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Y. Ren et al.
to a biased conclusion. On the other hand, one has
to bear in mind that histological and clinical
observations are at different levels, and comparisons
need to be made with caution.
To our knowledge, this is the first study describing
the age effects on root resorption as well as its
distribution along different parts of the root. The
results confirm that orthodontic experimental intervention even with light forces increases both the
incidence and severity of root resorption, which were
highest along the middle part of the root. Adult rats
had an increased incidence and severity with prolonged tooth movement, but neither was different
from the youngsters.
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