Study On Biodegradability Of Novel Biodegradable Mg-Sr Alloys And Their Application As Bone Grafts

Repair for bone defect is a very common operation in orthopedics,requiring the bone substitute to have a strong repair role and then promoting bone healing for the defect.The currently used bone repair materials are not satisfied as an ideal bone substitute due to various shortcomings and challenges.Magnesium alloys are becoming a new class of biodegradable bone implant materials with great potential of clinical applications owing to their characteristics of good biocompatibility,matched mechanical properties to bone tissues and degradadtion in human body.The Mg-Sr alloy has attracted much attention to researchers for its excellent biomedical functions of promoting both osteogenesis and angiogenesis.But the rapid degradation rate of Mg-Sr alloy still hinders its clinical application.This study was aiming at increasing the corrosion resistance of Mg-Sr alloy through adjusting chemical composition and using different processing routes,and further exploring the corrosion mechanism.Based on the above study,a Mg-Sr alloy with optimized treatment was used for animal test in order to preliminarily search the possiblity of its application for bone filling.In this study,strontium(Sr)element with roles of promoting skeletal development and osteoid formation was added in magnesium and Mg-Sr binary alloys with different Sr content(Mg-1.0Sr,Mg-1.5Sr and Mg-2.0Sr)were fabricated.The effect of Sr content on microstructure and corrosion resistance of the extruded Mg-Sr alloy was investigated.The extruded Mg-Sr alloy was composed of alloy matrix and second phase of Mg17Sr2.As increase of Sr content,the grain size was decreased and amount of the second phase was increased.Mg-1.5Sr alloy showed the best corrosion resistance in simultaed body fluid,and Mg-2.0Sr alloy was the worst in corrosion resistance,which resulted from the combine effects of grain size and second phase.According to above study,Mg-1.5Sr alloy was selected for further study.The Mg-1.5Sr alloy with different conditions was obtained through extrusion and heat treatment,and effects of extrusion and heat treatment on degradation behavior of the Mg-Sr alloy,as well as the corrosion mechanism of cast and extruded Mg-Sr alloy in simulated body fluid,were studied.Experimental results indicated that the corrosion behavior of Mg-Sr alloy was largely related to the microstructure,and the smaller the grain size in the alloy,the denser the corrosion layer on surface of the alloy,and the better the corrosion resistance.The second phase of Mg-Sr alloy corroded and cracked in Hank's solution,which cannot hinder the corrosion effectively.The net-like second phase covered all the grains in the cast alloy,and the serious galvanic corrosion resulted in poor corrosion resistance with intergranular corrosion.Second phases in the extruded Mg-Sr alloy were fragmented and the galvanic corrosion was weakened with filiform corrosion and pitting corrosion.Corrosion resistance of the extruded Mg-1.5Sr alloy was much less than that of the cast alloy.The heat treatment for optimized corrosion resistance of Mg-Sr alloy was 450 ?/5 h.The Mg-Sr alloy with optimized treatment was implanted into the artificial bone defect in New Zealand rabbit for 8 weeks,and Micro-CT and histological analysis were used to characterize the bone defect repair and surrounding bone response.The results indicated that,compared with the clinically used TCP implant,the Mg-Sr alloy implant showed better properties of promoting osteogenesis and defect repair.A proper amout of Mg ions release and a stable implant surface would be the key factor for promoting adhesion and growth of new bones.Therefore,the optimally treated Mg-1.5Sr alloy has a great application potential as a new generation bone filling material.