Study Of The Degradation Behavior Of Mg Alloy Orthopaedic Impant Under Stress In Vivo And In Vitro

Magnesium alloys have become the research hotspot for orthopaedic implant materials because of their biodegradability,good biocompatibility,mechanical compatibility and bone induction.However,the degradation rate of magnesium alloy is too fast,which does not match the rate of bone healing.When the bone injury is not fully healed,the rapidly attenuated mechanical properties of magnesium alloys due to the fast degradation make magnesium alloys can not play a supporting role any more,which leads to the failure of implantation.Therefore,to solve the problem that the degradation rate of magnesium alloy does not match the rate of bone healing will be the key to the clinical application of magnesium alloys.In this paper,the extruded Mg-Zn-Y-Nd-Zr alloys designed by our research group which shows good mechanical properties and corrosion resistance was used to study.Firstly,the degradation behavior of magnesium alloys in SBF and SD rats tibia bone marrow cavity and the changes of mechanical properties caused by degradation were studied in order to understand the evolution law of mechanical properties of magnesium alloy.Then the stress corrosion cracking behavior in SBF was studied in order to investigate the stress corrosion cracking sensitivity of magnesium alloys used in this paper.Finally the degradation behavior of magnesium alloy under axial cyclic compressive stress in flowing SBF was studied and the effect of axial cyclic compressive stress on bone healing was verified.It provided a new solution to solve the degradation rate dismatch between magnesium alloy and bone healing.(1)The degradation products of magnesium alloys which immersed in SBF were cracking blocks and contained C,O,Ca,P,Mg,Zn elements.In the first 14 days of immersion,the corrosion pit at magnesium alloy surface was relatively small and uniform distributed which maked the tensile strength and elongation have not decreased much compared with the reference sample,after 14 days immersion,the tensile strength and elongation decreased by 17.4%and 17.5%respectively.After 21 days immersion,the corrosion pit on the surface of magnesium alloy was larger and deeper,and further expanded after 28 days:immersion,which led to the rapid attenuation of its mechanical properties,after 28d immersion,the tensile strength and elongation decreased by 24.8%and 60%respectively.The fracture type changed from the toughness fracture based mixed fracture of the first 14 days to the quasi-cleavage fracture based mixed fracture of the 21 days and 28 days immersion.(2)The degradation of magnesium alloy in the bone marrow cavity was more uniform than in SBF.After implantation for 28 days,the surface still not large corrosion pit occurred,its degradation rate was 3 to 5 times times smaller than in vitro SBF.The maximum bending load decreased gradually with the prolongation of implantation time and decreases by 3.98%after implantation for 28 day's.(3)The stress corrosion cracking susceptibility of extrusion state Mg-Zn-Y-Nd-Zr alloy used in this paper was not substantial and this aspect should not be a concern for its application in biodegradable orthopaedic implants.(4)The axial cyclic compressive stress did not change the degradation product morphology of magnesium alloy in SBF,the effect of stress on the degradation rate was greater in the first 14d,and decreased after 21d.After 28d,the tensile strength of magnesium alloys decreased by 25.5%,38.3%and 29.4%respectively,the elongation decreased by 58.6%,69.3%and 65.6%,respectively under three stresses(1500??,3000?? and 4500??).(5)The magnesium alloy pin was implanted into the tibial fracture site of SD rats,the fracture line was still clearly visible after the 28 days implantation in the non-stress group which indicated that the healing rate was slow,while in the stress group,after 7days implantation,the callus formation was obvious and the fracture line was blur,with the accumulation of stress time,callus further growed,indicating that stress promotes the healing of fractures.This study used the method of applying stress to solve the problem that the degradation rate of magnesium alloy did not match the rate of bone healing,which provided anew solution for the clinical application of magnesium alloy and theoretical guidance for the degradation of magnesium alloy under simulated physiological stress.