Effects of mechanical force on RANKL expression and osteoclasts formation in PDL cells

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We found in the present study that not tensile force but compressive force applied to the periodontal ligament (PDL) cells induces RANKL mRNA. It may be related to the osteoclasts genesis in the compression side during orthodontic tooth movement. As expression of RANKL is usually little, it is so difficult to detect RANKL expression clearly in any experiments. The expression of cox-2 in the PDL cells, the most important enzyme for PGE_2 synthesis and responsible for osteoclasts genesis, is significantly stimulated in response to both compressive and tensile forces, and the intensity of expression is much dependent on the donor age. These data may be involved in the fact that gingival recession and alveolar crest resorption during orthodontic treatment is more frequent in the adult patients than in the young patients. Even through the cox-2 expression of PDL cells in response to mechanical force is high, RANKL expression is unclear or very low. It is known that PGE_2 has been shown to stimulate RANKL expression, but RANKL is mediated not only expression of cox-2 but also other factors. Another experiments show that the certain magnitude of compressive force to the osteoblats (Saos-2) stimulates BSP expression via PGE_2 synthesis, and cox-2 may have important roles of bone formation. The experiments to clarify anti-inflammatory effects of low-energy laser irradiation show that low-energy laser irradiation dramatically inhibits cox-2 expression from PDL cells in response to cyclic tension force. There is high possibility that low-energy laser irradiation to the moved teeth may reduce alveolar crest resorption during orthodontic treatment. Further studies such as effects of different laser dose, frequency and wavelength to PDL cells on bone resorption factors may be necessary for clinical use.