Topology Optimization Design And Biomechanical Verification Of Femoral Stem Based On Synergy Prosthesis

Total hip arthroplasty is an important method for treating end-stage hip diseases.Successful total hip arthroplasty could greatly improve the life quality of patients.In recent years,the choice of hip joint prosthesis materials and structural optimization design have been the hotspot at home and abroad and have made great progress.The materials of femoral stem prostheses in total hip arthroplasty are mostly metal such as titanium,cobalt,and their alloys.Due to the difference in elastic modulus between the femoral stem and the natural femur,stress shielding effects often happened after the surgery.Complications such as decreasing bone mineral density around the prosthesis,loosening of the prosthesis and fracture around the prosthesis may happen on account of the stress shielding.In recent years,in order to achieve a more reasonable biomechanical distribution after total hip arthroplasty,researchers have begun to explore different materials and structural optimization design methods to optimize prostheses.Topological optimization is one of the important structural optimization methods.Topology optimization technology is a mathematical method for optimizing material distribution according to load conditions,constraints and objective function.The application of topology optimization technology mainly relies on the development of finite element analysis technology,which is mostly used in the aerospace and automotive field.The topology optimization design of the fuselage and parts is to achieve the purpose of reducing weight by using the least materials under the corresponding working conditions.In recent years,topology optimization technology has gradually been applied to implant design in the medical field,especially in orthopedics.Topology optimization technology could reduce the strain energy of the implant by optimizing the material distribution of implants.In the case of reducing the strain energy,the stress shielding effect of the implant is reduced,and the purpose of finally reducing the postoperative complications of the implant is achieved.The topology optimization designed structure generally has a complicated internal structure,which is difficult to achieve with traditional manufacturing processes.Combined with the rapid development of additive manufacturing technology in recent years,the optimized structure can be manufactured,which promotes the development of topology optimization technology from conceptual optimization to practical application.In this study,topology optimization technology was used to optimize the design of the Synergy prosthesis,and biomechanical verification was performed using finite element analysis.In the simulation modeling stage,the heterogeneous assignment was used to simulate the femur and then the surgical process of assembling the prosthesis and femur was operated.In the stage of working condition simulation,the hip joint force and muscle forces were simulated under two daily working conditions: walking and climbing stairs.Joint force and abductor muscle force are simulated.In the topology optimization design stage,the constraint of the volume fraction of the Synergy prosthesis was applied to minimize the compliance,that is,the minimum strain energy was set as the optimization objective.In the post-optimization stage,in addition to the smooth processing of the optimized model,the two methods of filling the low-density part without material(truss-like prosthesis)and porous material(gradient prosthesis)were implemented according to the material density distribution.In the biomechanical verification stage,finite element analysis was used to compare the biomechanical properties of the three prostheses before and after optimization,and the stress changes in each Gruen zone of the femur in each group were observed,measured and statistically analyzed.Meanwhile,the stress peaks of the femoral stem prosthesis in each group were compared and analyzed.Results showed that the femoral stress: in the two daily working conditions of walking and climbing stairs,the more stress was transferred to the femur in the optimized groups,especially the femoral Gruen 7 zone.It showed that the rate of stress shielding of truss prosthesis(walking: 36%,climbing stairs:42%)< gradient prosthesis(walking: 45%,climbing stairs:46%)< original prosthesis(walking: 72%,climbing stairs:74%).It was obvious that truss-like prosthesis and gradient prosthesis could reduce the stress shielding of prosthesis in femoral Gruen 7 zone.The stress of femoral stem: the stress peaks showed that gradient prosthesis(walking: 120.7 MPa,climb stairs: 196.3 MPa)< truss prosthesis(walking: 131.1 MPa,climb stairs: 201.4 MPa)< original prosthesis(walking: 141.6 MPa,climb stairs: 242.6 MPa).The stress decrease in femoral stem reduced the stress difference in the interface of femur and prosthesis.And the lower stress peaks were expected to reduce postoperative prosthesis loosening and the risk of postoperative pain.Based on the results of this study,it can be concluded that the topology optimization technology can be applied to the design of femoral stem prosthesis.In addition to being lighter in weight,the optimized structure also has better biomechanical properties that both the stress peak of the prosthesis itself and the stress shielding on the Gruen 7 zone of the femur were reduced.