Effect of mechanical loading on osseointegration combined with degradation behavior of magnesium bone screw in vivo

Mechanical environment is one of the critical factors that affect the degradation and osseointegration performance of magnesium-based orthopedic implants. In this study, the effects of in vivo static and dynamic loadings on osseointegration and biodegradation behavior of magnesium screw were investigated and compared. A total of 54 male New Zealand White rabbits were divided into three groups randomly, with their femoral condyles implanted with high-purity magnesium screws (Implanted Group), only drilled holes (Drilled Group) or without any surgery (Healthy Group) respectively. Half of the rabbits in each group stayed in cages all the time (as static subgroup), while the rest performed treadmill exercise (as dynamic subgroup). Dynamic loadings during rabbits’ exercise were obtained based on kinematic analysis. Femoral condyles implanted with screws in Implanted Group were harvested at 8, 12 and 16 weeks after surgery to evaluate the biodegradation behavior and osseointegration. To compare the mechanical property of bone tissues, three femoral condyles of each subgroup in Implanted, Drilled, and Healthy groups were randomly selected to perform nanoindentation testing at 8, 12 and 16 weeks. Dynamic loading improved osseointegration performance obviously but did not affect the biodegradation behavior of magnesium screws. Elastic moduli of new-born bone surrounding magnesium bone screws were higher in the dynamic groups than that in static groups. Metabolism of released Mg ions from bone screw was normal both under static and dynamic loadings. The effects of mechanical loadings on osseointegration combined with degradation of magnesium screws in vivo were understood deeply in this study, providing guidance for patient's rehabilitative exercise postoperatively.