The effect of muscle loading on the biomechanics of the fixation interface in femoral shaft fractures: a comparative study of finite element and musculoskeletal system coupled modelling of SS 316 L and Ti-6Al-4 V fixation systems

The current femoral loading model does not explicitly consider muscle loading, which introduces a degree of bias in the prediction of interface biomechanical parameters of the fracture model. Furthermore, the sensitivity of the implant material to muscle loading is not known. This paper investigates the effects of implants of two materials (SS 316 L and Ti-6Al-4 V) on the biomechanical parameters of fixed femoral stem fractures using a joint simulation method of finite element and musculoskeletal modeling. A personalized musculoskeletal loading model and a simplified loading model were constructed and applied to a finite element analysis of fracture fixation. The effects of two fixation systems, SS 316 L and Ti-6Al-4 V, on the biomechanical parameters of the interface of the fracture model were compared, and the sensitivity of the implant material to muscle loading was explored. The results demonstrated that the neglecting muscle loading led to an overestimation of the stresses on the femur, plate, and screws in the fracture model. Furthermore, the sensitivity of screw stress to muscle loading was found to be higher in the Ti-6Al-4 V fixation system than in the SS 316 L. Conversely, the femoral stress was observed to be more sensitive to muscle loading in the SS 316 L model than in the Ti-6Al-4 V model. The omission of muscle loading led to an overestimation of IFS values for the two fixation systems, with an error of up to 24.4%. However, the difference in the sensitivity of IFS to muscle loading between the two fixation systems was minimal. Furthermore, the SS 316 L fixation system exhibits superior shear resistance. The differential sensitivity of the two materials, SS 316 L and Ti-6Al-4 V, to muscle loading in the interfacial biomechanical parameters of the fracture model necessitates the selection of appropriate materials for the optimization of the design for different key parameters. Consequently, the influence of muscle loading on the interfacial biomechanical parameters of the fracture model must be considered during the design of the materials for the implant in order to obtain more accurate clinical predictions.