Heat-stimuli-enhanced osteogenesis using clinically available biomaterials.

A recent study reported that heat stress stimulates osteogenesis in an in vivo rat model using alginate gel and magnetite cationic liposomes. However, for clinical use, the efficacy for promoting osteogenesis needs to be investigated using clinically approved materials, and preferably with animals larger than rats. The aim of this study was to evaluate multiple heat stimuli-triggered osteogenesis in rat tibial defect models using already clinically applicable materials (Resovist® and REGENOS®) and determine the efficacy also in the rabbit. Fifty-eight rats and 10 rabbits were divided into two groups, respectively, with or without hyperthermia treatment at 45°C for 15 min. (hyperthermia; 20 rats once a week, 8 rats three times a week, 5 rabbits once a week, control; 30 rats and 5 rabbits). Micro-CT assessment at 4 weeks revealed that a significantly stimulated osteogenesis was observed in the once a week group of both rats and rabbits as compared to the control group (p = 0.018 and 0.036, respectively). In contrast, the three times a week group did not show enhanced osteogenesis. Histological examination and image analysis showed consistent results in which the area of mineralized bone formation in the once a week hyperthermia group was significantly increased compared with that in the control group at four weeks (rat; p = 0.026, rabbit; p = 0.031). Newly formed bone was observed in the grafted materials from the periphery toward the center, and more osteoclasts were found in the once a week group. Heat stress also induced enhanced alkaline phosphatase expression in cultured osteoblastic cells, MC3T3, in vitro (p = 0.03). On the other hand, heat stress had no obvious effects on chondrogenic differentiation using ATDC5 cells. Our study demonstrates that heat-stimuli with clinically applicable novel heating materials can promote significant osteogenesis, and may thus be a promising treatment option for diseases associated with bone defects.