The Effect Of Lower Power Laser Irradiation On Root Resorption Induced By Orthodontically Treatment In Rats

Root resorption associated with tooth movement is an unsolved problem in orthodontics. If such root resorption could be prevented, it would be an important contribution toward reducing risk factors in orthodontic treatment.External apical root resorption is a sterile inflammatory process that is extremely complex and involves various disparate components, including mechanical forces, tooth roots, bone, cells, surrounding matrix, and certain known biologic messengers .It reported external apical root resorption of > 3 mm to occur at a frequency of 30%, with only 5% of treated individuals found to have > 5 mm of root resorption. The etiology of external apical root resorption following orthodontic treatment is not fully understood. We need more study to kown it clearly.ObjectThe purpose of this study was to investigate the root resorption on Wister rats during experimental tooth movement and the effect of lower power laser irradiation on relief the degree of root resorption, which would provide more information on root resorption induced by experimental tooth movement and the mechanism of lower power laser irradiation on relief the root resorption.MethodsFifty-five Wistar rats were used in this study. The animals were divided into 11 groups on average,each group comprised 5 animals: 1 group untreated,5 group animals as irradiated group were exposured in lower power laser irradiation for 15 minutes during the experimental tooth movement,the other 5 group animals as control group were not received lower power laser irradiation during the orthodontics treatment. An orthodontic appliance, consisting of a coil spring connected the right side first molar and the two incisors by ligature wire. 90g force as the heavy force was used in experiment. The animals were sacrificed during 1,4,7,14,21days. The upper jaw, including the molars, was dissected, fixed with 4% paraformaldehyde, decalcified in 10% EDTA for 3 weeks and then embedded in paraffin. The mesio-buccal root of the upper first molar was examined in serial cross-sections. Sections (5μm thick) were stained with hematoxylin and eosin(H&E). Tartrate resistant acid phosphatase (TRAP) staining were used for observed osteoclast cells,its precursor cells and osteoclast-like cells . TNF-α,TRAF6 were detected with immunochemical staining .ResultsAfter 1 day treated by 90g mesial force, the periodontium on pressure side was narrowed obviously. A lot of hyaline zone were observed more or less in pressure side, and the area of this necrosis degeneration zone in irradiated group was smaller than in control group. The earliest root resorption lacunae was detected on day 4 after experimental movement. The area of root resorption mainly happened in 2/3 crown root and bifurcation region. The number of osteoclast-like cells beside cementum got the peak on day 14, and, it is less in irradiated group than control group. On day 14, in the control group, we can found a lot of serious root loss in pressure side arrived dentine. The behavior of resorption stoped after 21days. The total area of dentine resorption lacunae in irradiated group was statistically less than that in the control group. The cells involved in root resorption are mononucleat or multinucleat cells with TRAP positive. In the alveolar bone, the maximum number of osteoclast cell appeared on day 4,7 in the irradiated group, they were significant more than control group. However, the number of odontoclast closed to root in irradiated group was statistically less than control group. The TNF-αimmunochemistry staining showed that expression of TNF-αgot the maximum on the 4 in the irradiated group, but, in control group, the maximum of exression of TNF-αappeared on the 7, however, many of them distributed beside the root of pressure. In the results of TRAF6 immunochemistry staining, TRAF6 positive expression were detected on both osteoclast cells in alveolar bone and odontoclast cells in resorption lacunae on the root surface. That probably meant that they are the same activation pathway。conclusion1. Root resorption related to a hyalinized zone showed a consistent pattern: Root resorption started in the circumference of the necrotic hyalinized tissue. It was detected on day 4.2. Mono- and multi-nucleated TRAP-positive cells were participating in active removal of the hyalinized tissue toward the root surface, and in resorption of cementum and dentine.3. The area of root resorption lacunae on the root surface increased sharply on day 14. And, it was less in irradiated group than in control group.4. Lower power laser irradiation can not only increase the turnover of the bone and periodontal ligament during the tooth movement but also protect root from resorption.5. Lower power laser irradiation can promote root repair. The repair was observed on the 21 in the irradiation group.6. The cells involved in root resorption Mono- and multi-nucleated TRAP-positive cells, both osteoclast cell and odontoclast cell expressed TRAF6 positive production, they were similar cell system, and may be have the same activation pathway.