Comparison of 4 and 6 weeks of rest period for repair of root resorption
Mehta et al. Progress in Orthodontics
Comparison of 4 and 6 weeks of rest period for repair of root resorption
Sneh A. Mehta 0
Shailesh V. Deshmukh 0
Ravindranath B. Sable 0
Amol S. Patil 0
0 Department of Orthodontics and Dentofacial Orthopaedics, Bharati Vidyapeeth Dental College and Hospital, Bharati Vidyapeeth Deemed University , Dhankawadi, Pune 411043, Maharashtra , India
Background: The study was designed to evaluate and compare the rest periods of 4 and 6 weeks for healing of orthodontically induced root resorption craters. Methods: The study was conducted with a split-mouth design, with the right and left mandibular first premolars of 14 subjects serving as the two groups of the study. The right premolars constituted group A and the left ones, group B. Intrusive force was applied on these teeth for a period of 6 weeks, followed by retaining the teeth for 4 weeks (group A) and 6 weeks (group B) as rest periods before extraction. The extracted teeth were prepared for histologic examination with haematoxylin and eosin staining and studied under a light microscope. The histological sections were scored based on the level of repair (none, partial, functional, or anatomic) seen in the deepest craters in the apical third region of the roots. The mean values of the scores in the two groups were compared using Mann-Whitney U test. Results: All the teeth showed healing in their deepest craters. The teeth in group A showed partial repair more frequently (84.6%), with the remaining (15.4%) showing functional repair. The teeth in group B showed anatomic repair more frequently (60%), with the remaining (40%) showing functional repair. The mean level of repair was higher in group B (2.6 ± 0.5) as opposed to that in group A (1.15 ± 0.37). The difference between these values was of very high significance (P < 0.05). Conclusions: Longer rest period of 6 weeks showed more advanced healing than a shorter rest period of 4 weeks. Six weeks of rest period is adequate only for the functional repair of resorption craters.
Root resorption; Repair; Intrusion; Histology
Orthodontic tooth movement is always accompanied by
some amount of root resorption [
]. When the
orthodontic force application is stopped, the active root
resorption is known to transition into a process of
repair. This repair is believed to be of three kinds: partial,
functional, and anatomic . Partial repair occurs when
the exposed dentin is only partially covered by new
cementum with some area of the exposed dentin remaining
uncovered; functional repair occurs when the exposed
dentin has been covered completely by a thin layer of
repair cementum without the re-establishment of its original
contour; and anatomic repair is characterized by the
restoration of the root surface to its original contour.
Amount of root resorption has been studied extensively
in relation to the magnitude as well as the duration of
application of orthodontic forces [
]. The general
finding is that continuous forces are more detrimental as they
lead to more amount of root resorption. Discontinuous
forces (interrupted and intermittent) on the other hand
allow for healing of these resorption cavities during the
rest periods when the orthodontic force is not being
1, 2, 4, 5
]. When resorption has reached appreciable
levels, it is advisable to discontinue the orthodontic forces
for some amount of time to allow for repair of the craters.
However, few studies have quantified the amount of time
for which the orthodontic force should be discontinued so
as to permit full anatomic repair of the resorption craters.
It would be beneficial to limit these rest periods to the
shortest possible time so that it does not create a big
impact on the total treatment time. Owman-Moll et al.
suggest that only partial repair of the craters occurs by
waiting for 4 weeks, while functional and anatomic repair
predominate with a rest period of 5 to 8 weeks [
et al. observed that orthodontic resorption undergoes
anatomic repair with a rest period of 8 weeks [
This study was an attempt to evaluate the following:
(i) if a period of 6 weeks is enough for the
orthodontically induced resorption craters to heal anatomically, (ii)
the progress of repair from 4 weeks to 6 weeks after
discontinuation of orthodontic forces, and (iii) the
difference in the thickness of repair cementum after the
two time periods as measured in micrometer.
This study was conducted on the right and left
mandibular first premolars of 15 subjects who reported for
orthodontic treatment and who were indicated for extraction of
mandibular first premolars for the same. Subjects were
explained the procedure and informed consent was obtained
from each participant. The mean age of the selected
sample was 15.4 years ± 2.95 years (12–23 years). The study
was approved by the ethics committee of the university.
In order to be included in the study, the subjects had
to be without (i) any history of previous orthodontic
treatment, (ii) trauma in the orofacial region, and (iii)
predisposing systemic diseases. The mandibular first
premolars had to be fairly upright as evident visually, free of
any periodontal or endodontic pathology, and with
completely closed apices and without pre-existing root
resorption as seen on the orthopantomogram.
The study was a split-mouth study, with the right and
left mandibular premolars of the subjects forming the
two groups of the study, viz. group A, where the teeth
were retained for 4 weeks post-orthodontic intrusion,
and group B, where the teeth were retained for 6 weeks
similarly. The final study design is shown in Fig. 1.
Mandibular first molars were banded with 0.018″
MBT tubes (3M Unitek, Monrovia, California).
Occlusal grinding of the first premolars was done till enough
clearance was obtained for bonding the spring on to
the tooth. Interproximal clearance was obtained using
single-sided abrasive strips to prevent damage to the
A spring design was chosen which involved a posterior
coil and a cantilever arm resting on the occlusal surface of
the mandibular first premolar to provide an apical
direction of the force. Each spring was prepared using a 0.017″
× 0.025″ TMA wire (Ormco, Orange, California) in which
a coil with 3-mm internal diameter was incorporated in
front of the molar. The spring was activated before
bonding to the premolar by opening the coil so that the active
arm goes apically. The amount of activation was adjusted
to a magnitude of 50 g of force as seen on a dial gauge.
The spring was bonded onto the occlusal surface of the
premolar (Fig. 2).
The active intrusion was carried out for a period of
6 weeks on both sides. These force levels and duration
were used as they are physiologically feasible for premolar
intrusion and have been shown to induce root resorption
in human premolars [
]. The springs were then
removed and deactivated so that no active force acts on the
teeth once they are rebonded in the previous manner. At
this point, the springs were only providing retention of the
teeth. This was necessary to prevent further resorption
with extrusive movements during relapse.
After completion of the allotted retention time (4 weeks
for the right premolars and 6 weeks for the left
premolars), the teeth were extracted atraumatically with forceps
under local anaesthesia. The extracted teeth were
immediately washed under running water and placed in
formaldehyde solution diluted to a 1:10 ratio with distilled water
and stored in this manner for a minimum of 5 days to
The teeth were then prepared for histological sectioning.
A stereomicroscope (XTL 3400E, Wuzhou New Found
Instrument Co., Ltd, Guangxi, China) was used at a
magnification of ×10 to study these teeth. After careful
examination of each surface (buccal, lingual, mesial, and
distal), the largest surface irregularity in the apical third of
each tooth was marked using a toothpick dipped in India
ink solution (Fig. 3). They were decalcified using 5% nitric
acid solution for a period of 10 to 12 days. The specimens
were then cut longitudinally into two halves with a scalpel
through the areas marked previously with the India ink
stain under the stereomicroscope, and one half was
embedded in paraffin. Thin paraffin ribbons of a uniform
thickness of 5 μm were obtained for further processing
with a precision rotary microtome (HM 340E, Thermo
scientific, Waltham, MA). These ribbons were then
mounted on slides and stained with haematoxylin and
eosin. The slides thus obtained were examined under a
light microscope (DM 1000LED, Leica Microsystems
Limited, Wetzlar, Germany). Histological photographs were
obtained for examination using a digital microscope
camera assembly (DFC 290HD, Leica Microsystems,
The repair cementum in each tooth was identified,
delineated by reversal lines and differential staining. The
deepest crater in the section, as verified by visual
examination under the microscope, was selected for
further analysis. In cases of conflict in deciding upon
the deepest crater, an image analysis software (Leica
Application Suite, v4.1.0, Leica Microsystems Limited,
Heerbrugg, Switzerland), which could make linear
measurements on histological images, was used to
compare the depth of the craters from the bottommost point
in the crater to the outer root outline imagined as a
continuation of the adjoining cementum. The following
parameters were studied for each of these craters
Level of repair
The grading system described by Owman-Moll et al. [
was used to quantify the level of repair of these craters
(Fig. 4). Scores were assigned to each level of repair to
facilitate the statistical comparison between the two
groups. The scores were assigned as shown in Table 1.
Thickness of repair cementum
The thickness of the repair cementum in these same craters
was measured (in μm) as the perpendicular distance from
the deepest point of the crater till the outer surface of the
layer (Fig. 5). The image analysis software mentioned before
was used for this purpose. Two measurements were taken
by two different operators.
Statistical tests were performed using R statistical
software, v3.2.2. To check the reliability of the system
utilized to measure the thickness of the repair
cementum, the interoperator reliability for the cementum
thickness readings was assessed using Spearman’s rank
correlation coefficient for both the groups. Mean
values and standard deviations were calculated for the
scores of level of repair for both the groups and
compared using Mann-Whitney U test. Similarly, mean
values and standard deviations for the base readings
(first operator) for cementum thickness obtained from
both the groups were calculated and compared using
Mann-Whitney U test. A “P value” of <0.05 was
considered statistically significant for both the comparisons.
Fifteen subjects originally participated in the study.
However, 1 subject dropped out midway before extraction of
the mandibular first premolars. The final sample consisted
of 14 subjects of which, 4 were males and 10 were females.
Another subject could not report for extraction at 4 weeks
Fig. 5 Schematic fig. to show the method of measuring repair
cementum thickness. Measured as perpendicular distance from
the deepest point of the crater till the outer surface of the layer
after force withdrawal. Subsequently, both mandibular
first premolars of this subject were included in group B,
the 6 weeks rest period group. Subsequently, group A
included 13 teeth and group B included 15 teeth. The largest
craters that were analysed for the study were all found in
the apical third of the lingual surfaces of the specimens.
Histologically, all the teeth showed healing in their
deepest craters. Healing was with either type (cellular/
acellular) of cementum, with early repair areas
showing the presence of acellular cementum predominantly
(8 out of 13 in group A) and later areas showing the
presence of cellular cementum more often (13 out of
15 in group B) (Fig. 6). The results of the study are
Level of repair
All the teeth showed healing in their deepest craters.
Thus, none of the teeth in the sample were assigned a
score of 0. A majority of the teeth in group A showed
partial repair (score 1), with the remaining showing
functional repair (score 2). A majority of the teeth in
group B showed anatomic repair (score 3), with the
remaining showing functional repair (see Fig. 7). The
distribution of different levels of repair amongst the two
groups is shown in Table 2. The same table shows that
the mean level of repair in group A was 1.15 ± 0.37 and
that in group B was 2.6 ± 0.5. Mann-Whitney U test
suggests that the difference between the mean values is
significant (P < 0.05).
Thickness of repair cementum
Table 3 shows the correlation coefficients between the
readings obtained by the two operators in both the
groups. As seen here, there was a very high, statistically
significant correlation (rs close to +1) between the
readings in both the groups. The agreement plots (Fig. 8)
show the distribution of the readings. The mean
thickness of repair cementum was 44.59 ± 25.81 μm in group A
and 127.88 ± 77.19 μm as seen in Table 4. Mann-Whitney
U test suggests that this difference between the thickness of
cementum is significant (P < 0.05).
Orthodontically induced inflammatory root resorption is
an unavoidable side effect of orthodontic therapy.
Minimizing this resorption by taking appropriate steps should be
a moral duty for any orthodontist. That intermittent force
is biologically less detrimental than continuous force is
common knowledge [
1, 2, 4, 5
]. However, how
intermittent this force should be is a question that still remains
largely unanswered. This study was a small step towards
understanding the same.
Intrusion is known to be the most harmful type of
tooth movement where root resorption is concerned,
with intrusive movement causing four times the
resorption caused by extrusive movement [
]. Although the
maxillary incisors have been shown to be the teeth most
susceptible to resorption following intrusion [
], an in
vivo study is possible only with premolars as they are
the teeth most frequently extracted for orthodontic
To standardize the morphology of the resorption
craters for comparison in both the groups, the initial
force levels, the duration, and the direction of the force
were kept the same for both the groups. Following active
intrusion, the teeth were retained with a passive spring
instead of leaving them unbonded, avoiding resorption
during extrusive relapse, as the objective was to study
the repair of the craters induced by application of
intrusive forces only.
There was a significant difference between the mean
values for the level of repair in the two groups. Anatomic
repair was seen more frequently in group B while partial
repair was more frequent in group A. This indicates that
healing of craters progresses rapidly with time. Previous
studies have shown similar results. Be it tipping forces [
or intrusive forces [
], level of repair was always shown to
increase with time.
While a majority of the teeth in group B showed
anatomic repair after a rest period of 6 weeks, there were
still six teeth in this group that showed only functional
repair. Based on these results, the hypothesis that full
anatomic repair occurs with 6 weeks of force withdrawal
was rejected. It can be concluded that 6 weeks is close
to, but not enough for complete repair of orthodontic
resorption. Six weeks, however, is enough for the
exposed dentin to get fully covered with new cementum.
At first glance at the previous studies, it might seem that
full repair of orthodontic resorption occurs by 8 weeks.
In the study by Cheng et al. [
], anatomic repair of
craters was only seen in the teeth which were allowed to
rest for a period of 8 weeks, irrespective of the levels of
intrusive force applied to induce the resorption (25 or
225 g). However, the sample size in this study was too
small to be conclusive. As stated earlier, Owman-Moll et
] reported more frequent appearance of anatomic
repair (12%) in the teeth retained for 8 weeks. The
remaining 88% of the teeth in this group however did
not show anatomic repair in spite of waiting for
8 weeks. Ahmed et al. [
] reported greater healing by
8 weeks even though the resorption was secondary to
an acute trauma induced with mini-screw implants.
They also reported only 73.1% repair even after a rest
period of 12 weeks. The findings of the rat study by
Gonzales et al. [
] suggest that minor resorption
craters can be seen on the roots even after 16 weeks of
retention. Ethical limitations, however, do not allow
including such long rest periods in humans as it would
mean delaying the orthodontic treatment in the
subjects for longer periods. Thus, a definite time period at
which full repair can be expected with confidence
cannot be confirmed. A healing period of 6 to 8 weeks
however does seem to fully repair most of the
resorption craters or at least cover the entire surface of the
exposed dentinal tubules at the bottom of all the
resorption craters. Resorption and repair are thought to
occur simultaneously in the presence of orthodontic
load also [
]. The phenomenon that dominates this
cyclic process decides if there will be irreversible
damage to the roots. Intermittent rest periods of 6 to
8 weeks are recommended in between the orthodontic
therapy to give repair a better chance over resorption.
Another interesting observation was that initial repair
(group A) was predominantly acellular in nature while
later repair proceeded with deposition of cellular
cementum predominantly (group B). This finding is in
agreement with previous studies [
3, 6, 16
]. This can be
explained by the phenomena of non-functional retarded
acellular repair and functional rapid cellular repair as
discussed by Vardimon et al. [
]. Acellular cementum
has been known to form more slowly [
]. To fill large
voids as created by orthodontic forces, faster healing is
required, which results in the cementocytes getting
trapped within the lacunae before complete mineralization
of the increments can take place.
For this study, it was hypothesized that with an
increase in the time period allotted for the repair, the
greater amount of cementum would be deposited within
the resorption craters. If this hypothesis was true, then
the thickness of repair cementum should have been
greater in the teeth in group B as opposed to that in
group A. There was a significant amount of difference
between the mean values of the repair cementum
thickness seen in both the groups, with the mean value
clearly being more in group B (127.88 μm) as compared
to that in group A (44.59 μm). This indicates that the
amount of cementum deposited is indeed greater at
6 weeks. However, the standard deviation accompanying
these values was very high (25.81 μm for group A,
77.19 μm for group B) with individual values ranging from
11.57 to 77.34 μm in group A and 24.46 and 244.34 μm in
While the high and significant values of Spearman’s
rank correlation coefficient in both the groups validate
the reliability of the method used to measure the repair
cementum thickness in this study, other factors may
have played a role in causing this variability including
(i) variations in root anatomy and surface morphology
which can be seen between contralateral teeth also
], leading to different stress distribution in
PDL. This can cause different levels and patterns of
resorption on different areas of the root ; (ii) errors in
the initial cut after decalcification, which divides the
specimen into two for embedding into paraffin. Utmost care
was taken to make this cut precisely through the marking
made under the stereomicroscope. However, errors can
occur during this procedure as it is a manual step and
depends on the skill, accuracy, and the experience of the
technician; and (iii) non-correlation of the area of the
crater with its depth. The assumption was that the
crater with the biggest surface area under a
stereomicroscope would also be the deepest one, thus
standardizing the depth of the craters. However, the surface
area of the crater is a function of the area upon which
the stresses are concentrated, which again depend on
the morphology of the root surface as discussed earlier
]. Multiple Howship’s lacunae may sometimes
coalesce to form larger craters, as has been evident in
other studies [
], thus increasing the surface area of
the craters. The depth of the crater, on the other hand,
is a function of the constancy of the force. When an
orthodontic force is applied, the stresses generated at a
particular point tend to reduce with time as the bone
and cementum remodel around this point. The more
constant the force, the deeper the resorption craters
]. If these two factors are not correlated, then the
depth of the craters was not truly standardized.
There were certain other limitations to this study. A
cantilever spring such as the one used in this study
would tend to exert extrusive and distal tipping forces
on the molars. Future studies using a similar spring
design could employ the use of anchorage preservation
protocols, like a lingual holding arch for instance, to
maximize the intrusion force on the premolars.
While 6 weeks of rest period was not found to be
adequate for full repair of the craters, it was surely enough
for the exposed dentinal tubules to be fully covered with
repair cementum. Repair of root resorption is a
spontaneous, time-based phenomenon, with a longer rest period
of 6 weeks showing better healing than a shorter rest
period of 4 weeks. This repair begins at the centre of the
resorption crater with deposition of acellular cementum
and progresses further with deposition of cellular
cementum extending to involve the entire surface of the crater.
The thickness of the repair cementum also increases with
time but is possibly only limited to the actual depth of the
The authors would like to thank Dr. Sangeeta Palaskar and Dr. Rasika Pawar,
Department of Oral Pathology and Microbiology, Sinhgad Dental College
and Hospital, Pune, for helping out with the study of the histological slides.
SAM is the primary investigator, carried out the conception of the study, and
performed the study (spring placement, deactivation, histological study) and
manuscript preparation. SVD is the secondary investigator; contributed to the
conception of the study, study design, and spring design; and overlooked
the study and manuscript overview. RBS participated in the spring design
and manuscript overview. ASP overlooked the study and manuscript
preparation. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Ethics approval and consent to participate
The study was approved by the ethics committee of Bharati Vidyapeeth
Deemed University, Pune, Maharashtra, India, with the reference number
As per the recommendations of the committee, all subjects were explained
the need and the procedure of the study and written consent was obtained
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