Finite Element Analysis of Biomechanical Effects of Proximal Femur Shortening After Intertrochanteric Fracture

Purpose: Proximal femoral shortening is common after intertrochanteric fractures treated with proximal femoral nail anti-rotation (PFNA) .We investigated the effects of biomechanical stability of the proximal femur and helical blade which on proximal femoral shortening in intertrochanteric fractures treated with PFNA using the finite element method. Methods Using the computed tomography data of the proximal femur of a healthy middle-aged volunteer and Mimics software. We established the proximal femoral shortening model after intertrochanteric fracture (Evans-Jensen type III) treated with PFNA. The models were divided into non-shortening, mild shortening (4mm), moderate shortening (8mm), and severe shortening (12mm) group. Abaqus were used for mechanical analysis and to obtain the overall stiffness of the different models, stress and stress nephograms of the proximal femur, fracture surface, whole femur and helical blade. Results In the non-shortening, mild-, moderate-, and severe-shortening groups, the overall stiffness of the proximal femur was 6491.8N/mm, 7264.8N /mm, 8168.0N /mm and 9222.8 N /mm and the maximum stress in the cancellous bone area of the proximal femur was 4.39MPa, 3.96MPa, 3.63MPa and 3.43MPa respectively. The stress on the fracture surface gradually decreased, while the stress at the end of the helical blade increased. However, the stress at the tip decreased gradually with the increase in shortening of the proximal femur. Conclusion With increased proximal femur shortening, fracture stability gradually increases. Intertrochanteric fractures involving the lateral wall may increase the probability of screw blade nail withdrawal and decrease the probability of proximal cutting as shortening increases.