Overloaded orthopedic force induces condylar subchondral bone absorption by stimulating Mesenchymal Stem Cells differentiating into osteoclasts via mTOR-regulated RANKL/OPG secretion in osteoblasts.

Appropriate orthopedic force led to bone remodeling of mandibular condyle, while overloaded orthopedic force (OOF) induced condylar bone absorption. Bone absorption is ascribed to the imbalanced activities between osteoclasts and osteoblasts, mechanism of which remains unclear. This study aimed to observe the condylar changes induced by overloaded orthopedic force by mandible advancement appliance and to further investigate the role of mTOR and RANKL/OPG in osteoclastic differentiation of stem cells in vivo and in vitro. In vivo, the results of micro-CT analysis indicated that condylar bone resorption was induced by OOF through mandibular advancement appliance since 2 weeks and worsened time-dependently. Morphologically, cartilage thickness was reduced, subchondral cortical bone line appeared uncontinuous, subchondral bone exhibited irregular-shaped and owned uneven surface. The bone mineral density (BMD), bone volume fraction (BV/TV), trabecular number (Tb.N), trabecular thickness (Tb.Th) decreased accomplished with the increased Trabecular separation (Tb.Sp) determined by micro-CT. Additionally, based on immunofluorescent labeling, OOF activated both osteoclasts and osteoblasts, but osteoclastogenesis prevailed over osteogenesis. The mTOR activation and ratio of RANKL/OPG in osteoblasts (OBs) were elevated by OOF. In vitro, the results of Western blot and PCR consistently suggested that the mTOR and RANKL/OPG ratio were upregulated by overloaded mechanical stretch. Pretreatment with mTOR inhibitor, rapamycin, could attenuate the activation of mTOR and the secretion of RANKL in OBs. Interestingly, based on the Trap staining, the supernatant of OBs exposed to OOF could promote osteoclastic differentiation of mesenchymal stem cells (MSCs), while its role was weakened by inhibition of mTOR in OBs. Collectively, OOF induced condylar bone absorption, in the process, osteoclastogenesis was prominent than osteogenesis. The activation of mTOR and secretion of RANKL/OPG was enhanced by OOF and involved in promoting MSCs differentiating into osteoclasts.