الفهرس 
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Abstract
Background: Relapse after orthodontic tooth development (OTM) happens through the rebuilding of the trans-formed periodontal ligament (PDL), this rebuilding process involves the extracellular matrix (ECM). Low level laser therapy (LLLT) biostimulation might enhance bone redesigning, with expanded collagen union, bone development and mineralization, cell expansion and separation, and angiogenesis. it was hypothesized that LLLT may inhibit relapse due to accelerated bone regeneratio. Mesenchymal stem cells (MSCs) might promote tissue formation through their own differentiation or by ctivating endogenous progenitor cells when exposed to an inflamed microenvironment. Aim of the study: The aim of the presented study was to investigate the effect of low level laser therapy and the bone marrow mesenchymal stem cells on the rate of orthodontic relapse in rats. Methods: Sixty males, 8 weeks old, Wistar rats with their maxillary central incisors had undergone an orthodontic distal movement for 2 weeks. The distally moved central incisors were temporarily fixed using a wire-resin splint retainer for one week. After retainer removal the rats were subdivided to 4 equal groups: group (1) (C) without any intervention, to act as the baseline control group. group (2) (L) low level laser therapy (LLLT) was used to irradiate the mesial aspect of the root of maxillary central incisors. group (3) (SC25) MSCs group, in which (50000) MSCs were applied on the mesial aspect of the root of maxillary central incisors. group (4) (SC50) in which (100000) MSCs were applied on the mesial aspect of the root of maxillary central incisors. group (5) (LSC25) Combined group, with both BMMSCs and LLLT application. In which (50000) mesenchymal stem cells were applied on the mesial aspects of the roots of maxillary central incisors and then irradiated by low level laser. group (6) (LSC50) Combined group, with both BMMSCs and LLLT application. In which (100000) mesenchymal stem cells were applied on the mesial aspects of the roots of maxillary central incisors and then irradiated by low level laser. The rats were euthanized at 4 days, 1, 2, 3 and 4 weeks after retention removal. Relapse percentage and bone mineral density were evaluated by using cone beam computed tomography. Histological and immunohistochemical analyses were done using hematoxylin &eosin stain and tartrate resistant alkaline phosphatase anti body respectively. Osteoclasts number, vasculazation and new collagen fibers formation were assessed histologically. The concentrations of matrix metalloproteinase 2 (MMP2) and tissue inhibitor metalloproteinase1 (TIMP1) in rat‘s blood serum were detected using Enzyme-linked immunosorbent assay (ELISA). Results: L(2) and SCs(3&4) groups significantly showed the highest relapse percentage at 4 days and 1 week period (p-value ≤ 0.05), while they showed the least relapse at other periods. L(2) and SCs(3&4) groups showed significantly the highest bone density through most of the experiment periods. The highest osteoclast count was significantly recorded in group L at 4 days and 1 week periods while SCs (3&4) and LSCs (5&6) groups recorded highest values at 2, 3 and 4 weeks periods while the lowest count was recorded in group C throughout the experiment periods (p-value ≤ 0.05). The results also showed that the L(2) and SCs(3&4) groups showed the highest expression values for TIMP1. Whereas, the SCs (3&4) groups revealed elevated expression values for MMP2. Conclusion: 1- The use of LLLT could reduce the relapse tendency as evidenced by increased bone density and enhanced remodeling of PDL through increased osteoclast number, increased collagen fibers concentration and new vascularization on pressure side. 2- LLLT stimulate the expression of TIMP1 which enhance PDL remodeling and could decrease the orthodontic relapse. 3- The transfer of MSCs at the pressure side had positive effects remodeling of PDL and decreasing but not inhibiting the rate of relapse. 4- MSCs showed their repair ability during the 2 weeks period by increased MMP2\TIMP1 ratio. 5- The combined use of LLLT and MSCs showed their synergistic effects in reducing the relapse rate at the late periods of the experiment. 6- The mixed treatment groups (LLLT + MSCs) showed that 50000 cells with LLLT showed better results than 100000 cells.