Hyperbaric-therapy enhances bone-regeneration by altering the OPG/RANKL ratio in osteoblasts

Introduction: Hyperbaric oxygen has been shown to successfully promote healing of osteradionecrosis and anaerobic infections. However the mechanisms of action are still uncertain. Cytokines, TNF-α (tumour necrosis factor alpha), receptor activator of NF-κB Ligand (RANKL) and osteoprotegerin (OPG) have been shown to act directly or indirectly on osteogenic cells and their precursors to control differentiation, resorption and bone formation. RANKL and TNF-α promote osteoclast differentiation and thus bone resorption, whereas OPG promotes osteoblast differentiation and also competes with RANKL for the receptor activator of NF-κB (RANK) receptor present on the osteoclast. Objectives: To compare the effect of hyperbaric oxygen on osteoblasts to hypoxia and normoxia Methods: MG63 Osteoblast-like cells were exposed to hyperoxia (hyperbaric oxygen in pressurised chambers), normoxia and hypoxia for 90 minutes. The medium used in each set of experiments were analysed using ELISA assays for RANKL, OPG and TNF-α Results: The OPG levels were significantly increased by about 63% from cells exposed to hyperbaric oxygen when compared to normoxic conditions. Whereas, the OPG levels in hypoxic conditions were significantly reduced by about 54% when compared to controls. RANKL levels were increased by both hypoxia(~56%) and hyperoxia(~25%), but to a greater extent by hypoxia. There was no TNF-α synthesis. Conclusion: To our knowledge this is the first study that shows the effect of hyperbaric treatment on OPG, RANKL and TNF-α expression. The results suggest that hyperbaric oxygen stimulates bone formation and inhibition of resorption mainly through an increase in OPG. Whereas, hypoxia may stimulate resorption through an increase in RANKL and a decrease in OPG. From these findings, we conclude that hyperbaric oxygen may increase bone regeneration by altering the OPG/RANKL ratio in the bone micro-environment.