Osteoblast differentiation during orthodontic tooth movement

Objectives To analyse the expression of osteoblast differentiation markers and osteoclast activity in the periodontal ligament ( PDL ) following 2, 4 and 7 days of orthodontic tooth movement ( OTM ) in an animal model. Setting and Sample Population Eighteen C57 BL /6 wild‐type mice. Material and Methods For the OTM model, orthodontic force was applied to the maxillary right first molar using a closed‐coil NiTi spring activated between the molar and incisors. The left side served as the control. Following OTM , the dissected tissues were scanned for micro‐computed tomography (micro‐ CT ) analysis and processed for histology. Histological stains included tartrate‐resistant acid phosphatase ( TRAP ) staining for osteoclasts and immunohistochemistry for osteoblast markers alpha‐smooth muscle actin (α‐ SMA ), osteopontin ( OP ) and osteocalcin ( OC ). Results Micro‐ CT analysis showed increasing OTM on days 2, 4 and 7 days as well as decrease in bone volume and per cent bone volume at 4 and 7 days. Statistically significant increases in osteoblast marker expression were seen in all groups when compared to the control. TRAP expression was highest at 4 and 7 days, α‐ SMA was highest at 2 days and OP / OC was highest at 4 days. Conclusion During OTM , proliferation of pre‐osteoblasts peaks at 2 days while mineralization of the osteoid peaks at 4 days. The osteoclast response is delayed.