الفهرس 
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Abstract
Orthodontic-induced inflammatory root resorption (OIIRR) has been reported to be one of the most common unfavorable side effects of orthodontic treatment. Different chemicals and pharmacological agents, in addition to, some surgical and nonsurgical procedures have been investigated to overcome and treat this problem. However, most of these treatment modalities are not clinically suitable, due to their possible side effects on orthodontic tooth movement and on the general health and comfort of the patient.
Low-level laser therapy (LLLT) has been studied in many fields of dentistry due to its reparative and anti-inflammatory effects. In the field of orthodontics; LLLT has been recently demonstrated to play a pivotal role in accelerating the tooth movement, reducing the orthodontic-induced pain and enhancing bone remodeling. Recently, LLLT has been investigated as a non-invasive and clinically suitable method to prevent and/or repair the OIIRR.
The aim of the present study was to assess histologically and histomorphometrically the effect of LLLT on OIIRR under relatively light- and heavy- orthodontic forces in rats. The null hypothesis was that there is no effect of LLLT on OIIRR under different orthodontic force magnitudes.
A total of thirty-two male Albino rats weighing from (250-300 g) were randomly assigned to four equal groups (8 rats each). Control low-force group (CL): The maxillary left first molar was subjected to orthodontic force of 10-15 g by nickel titanium coil spring stretched between the maxillary left first molar and maxillary incisors. Control high-force group (CH): The maxillary left first molar was subjected to heavier orthodontic force of 50 g. Laser low-force group (LL): The maxillary left first molar was subjected to orthodontic force of 10-15 gin addition to LLLT. Laser high-force group (LH): Teeth were subjected to orthodontic force of 50 g in addition to LLLT.
The laser irradiation in LL and LH groups was performed using a GaAlAs diode laser (λ = 980 nm, 100 mW output power). Oral mucosa at buccal and palatal sides of maxillary left first molar was irradiated every other day at four points (two points at buccal side and two at the palatal side) with a continuous non-contact irradiating mode for 12 seconds for each point (energy of 1.2 J/point, fluence of 16.9 J/cm2).
The rats were euthanized after 7- and 21-days of force application in each group (four rats at each time point) and the maxillae were dissected out. The obtained specimens were placed in 10% neutral buffered formaldehyde and demineralized by trichloroacetic acid, then dehydrated in ascending concentrations of alcohol, cleared in xylene, infiltrated, and embedded in paraffin wax. 4-μm serial transverse sections at the cervical two thirds of maxillary left first molar’s roots were obtained and stained with H&E stain.
Histomorphometricanalysis was performed by mean of a computer program ImageJ 1.50e. For each specimen, two sections, with the most expressive tissue events and best material quality, from the cervical two thirds of the distal-buccal root, were chosen for the histomorphometric analysis, from which root resorption percentage was calculated, and the capillaries count was performed. The results were expressed as means and standard deviations, tabulated and statistically analyzed.
I. Histological results
After 7-days of force application, the control groups (CL and CH) showed initial hyalinization of the periodontal ligament (PDL) at the compression site, in addition to minor areas of bone and root resorption in the direction of the tooth movement. On the other hand, laser groups (LL and LH) showed mature areas of hyalinization of the PDL which were detected in few sections, some of them appeared as remnants of necrotic tissues after being invaded by the odontoclasts. Besides, active areas of bone and root resorptionwere easily detected in the laser groups, mainly in the direction of the tooth movement. Also, odontoclasts were frequently observed in the root resorption craters in the laser 7-days subgroups,indicating active root resorption processes. Almost all the sections in the laser 7-days subgroups showed wide spread blood vessels engorged with RBCs in the PDL space. For the LL 7-dayssubgroup, some of the root resorption craters were covered by thin layer of reparative cementum depositions, which were less frequently found in the LH subgroup, indicating for early start of the reparative events.
After 21-days of force application, the control groups (CL and CH) showed irregular orientation of the PDL fibers, but they were more organized than the control 7-days subgroups. Remnants of hyalinized tissues were frequently seen in the CH 21-days subgroup and were less frequently detected in the CL subgroup. Most of the specimens showed extensive areas of root resorption, most of them extended to the dentine layer. No reparative cementum deposition wasfound in the control subgroups, except for few sections in the CH subgroup showed thin layer of reparative cementum. Odontoclasts were frequently observed in the CL and CH subgroups indicating still active root resorption processes. Few blood capillaries were seen in the compression site of the PDL. Moreover, extensive bone resorption was observed.
On the other hand, laser groups (LL and LH), at day 21, showed proper re-arrangement of the PDL fibers especially for the LH group. Root resorption areas, confined to the cementum only or extended to the dentin, were detected, especially in the LH 21-days subgroup. Most of the root resorption craters were covered by reparative cementum, especially for the LH subgroup which showed the most distinct cementum depositions over all the other control and laser subgroups. No odontoclasts nor hyalinized tissues were detected in all the lased specimens. All the sections in the laser groups showed wide spread of blood vessels engorged with RBCs in the PDL space. The most extensive bone resorption was detected in the laser 21-days subgroups.
II. Histomorphometric results
The percentage of OIIRR
There was no significant difference in the amount of OIIRR between the control groups (CL and CH) and the laser groups (LL and LH) at day 7. However, LLLT decreased the amount of OIIRR significantly at day 21. Moreover, the amount of OIIRR significantly increased from day 7 to day 21 in the control groups (CL and CH), while the laser groups (LL and LH) showed no statistically significant change in the amount of OIIRR from day 7 to day 21. Besides, the high-force subgroups showed an increase in the amount of OIIRR over the low-force subgroups for both the control and laser groups. However, this increase in the amount of OIIRR was statistically not significant.
Capillaries count
LLLT significantly increased the number of blood capillaries in the laser groups (LL and LH), in comparison to the control groups (CL and CH). Moreover, there was no statistically significant change in the number of blood capillariesfrom day 7 to day 21 in all control and laser groups. Besides, there was no statistically significant difference in the capillaries count between the low-force and high-force subgroups in both laser and control groups.
Based on the results of the present study, the null hypothesis was rejected.