Proper mechanical stress promotes femoral head recovery from steroid-induced osteonecrosis in rats through the OPG/RANK/RANKL system

Background Long-term use of steroid may lead to osteonecrosis of the femoral head (ONFH). Mechanical stress may help bone formation and remodeling. This study aimed to probe the role of mechanical stress in the femoral head recovery in rats. Methods Rat models with ONFH were induced by steroid. Rats were subjected to different levels of mechanical stress (weight-bearing training), and then the morphology and bone density of femoral head of rats were measured. The mRNA and protein levels of the OPG/RANK/RANKL axis in rat femoral head were assessed. Gain- and loss-of function experiments of OPG were performed to identify its role in femoral head recovery following stress implement. The ex vivo cells were extracted and the effects of stress and OPG on osteogenesis in vitro were explored. Results Steroid-induced ONFH rats showed decreased bone density and increased bone spaces, as well as necrotic cell colonies and many cavities in the cortical bones and trabeculars. Proper mechanical stress or upregulation of OPG led to decreased RANK/RANKL expression and promoted femoral head recovery from steroid-induced osteonecrosis. However, excessive mechanical stress might impose too much load on the femurs thus leading even retard femoral head recovery process. In addition, the in vitro experimental results supported that proper stress and overexpression of OPG increased the osteogenesis of ex vivo cells of femoral head. Conclusion This study provided evidence that proper mechanical stress promoted femoral head recovery from steroid-induced osteonecrosis through the OPG/RANK/RANKL system, while overload might inhibit the recovery process. This study may offer novel insights for ONFH treatment.