Development Of Biodegradable Zn-Zn Mg-Mg Gradient Alloy Intramedullary Nails And Study On Its Biomechanical Healing Mechanism During Femoral Shaft Fracture Fixation

With rapid development of economic,transportation and construction industry,fractures caused by falling and traffic accidents increased frequently.Meanwhile,with the increasing of aging population,patients with osteoporosis and obese who received bone defect treatment have been increasing year by year,thus there is a huge demand for bone repair materials.Internal fixation is main conventional plan and echnique for treatment.Therefore,we designed a biodegradable Zn-ZnMg-Mg gradient alloy fixation.It can provie rigid fixation at the early stage of fracture healing,and then it can be gradually degraded in vivo to avoid stress shelter.In the end,the material can be absorbed in the body to avoid secondary operation.Our research included the gradient alloy preparation and corrosion evaluation.Mechanical test and finite element analysis were used to evaluate the mechanical properties of the alloy.SD rats femoral fracture fixed by gradient alloy intramedullary nail under physiological load were performed.We designed a biodegradable Zn-ZnMg-Mg gradient alloy according to fracture healing biomechanical mechanism and degradation corrosion features of magnesium and zinc.We used magnesium as matrix material.Zinc powder infiltration method was used on magnesium surface through diffusion infiltration and plating to form zinc-magnesium transition layer,then electroplating a zinc coat layer as the protective layer.The component,thickness and corrosion rate of the Zn-ZnMg-Mg gradient alloy were evaluated by surface morphology observation,phase analysis,immersion test and electrochemical test.The results indicated the corrosion ability can meet the biodegradability requirements for orthopaedic implants.The biomechanical properties of iron,magnesium,zinc,gradient alloy and human cortical bone were measured by mechanical test and finite element analysis.The results denmonstrated the elastic modulus of gradient alloy could reach the standard for intramedullary nailing material and it was lower than iron,but higher than magnesium and zinc.It was concluded that Zn-ZnMg-Mg gradient alloy performed better biomechanical properties,and provided reliable mechanics reference in biodegradable material design.Established SD rats femoral fracture model with gradient alloy and iron intramedullary nail.X-ray,micro-CT,histopathological and biomechanical test were performed to evaluate the fracture healing process.Tested the rats fracture callus and biological characteristics of biodegradable gradient alloy intramedullary nail during different periods in the process of fracture healing.Observed fracture healing process under physiological load and the effect after surgical treatment.The results showed that unstable biomechanics environment of gradient alloy group went against with fracture healing at the early stage,but the volume of callus growed bigger to maitain the fracture stabilitiy.At the end of fracture healing,the alloy was absorbed and biomechanical properties and mineralization level of callus were increased gradually.Finite element analysis was performed to evalute the effect of iron and gradient alloy intramedullary nail on fracture healing.At the early stage,the maximal load and stress distribution of intramedullary nail and femoral were similar in alloy and iron group,then the stress of gradient alloy intramedullary nail gradually decreased,until it was significantly lower than iron nail.These demonstrated that the gradient alloy intramedullary nail could provide appropriciate fixation at the early stage,then gradually degraded without appearing stress shelter to achieve healing.