[Finite element analysis of evaluating the change of the hip joint biomechanics following femoral neck shortening after cannulated screw fixation for osteoporotic femoral neck fracture].

Objective

To evaluate the stress status of femoral head and neck, screws and acetabulum caused by femoral neck shortening after internal fixation of femoral neck fracture with finite element method, and to analyze the stress of proximal femoral neck and acetabulum from the mechanical point of view.

Methods

CT scan data of hip of a healthy adult female were collected. Three-dimensional reconstruction MICs and related module function simulation was used to establish the postoperative shortening model of femoral neck fracture with Pauwells angle>50°, which was treated with cannulated screws. The models were divided into four groups: normal femoral neck without shortening, shortening for 2.5 mm, shortening for 7.5 mm and shortening for 12.5 mm. The finite element analysis software MSC. Nastran2012 was used to do the mechanical analysis. The acetabulum surface, femoral head surface, proximal femur and the maximum stress, stress nephogram and other relevant data were collected.

Results

The maximum tensile stress and the maximum stress at the fracture site of the femoral neck increased gradually with the increasing of shortening of femoral neck, however, the maximum compressive stress under the femoral neck and the medial cortex decreased gradually; the maximum stress on the surface of the femoral head was 14.9, 15.0, 16.3 and 16.3 MPa, respectively; the maximum stress on the surface of the acetabulum was 10.1, 10.1 and 10.5,11.7 MPa, respectively.

Conclusion

The mechanical environment of the hip joint changes with femoral neck shortening. With the increasing of femoral neck shortening, the peak stress of the acetabulum increases continuously. When the femoral neck is shortened seriously, the load distribution is uneven and the complex mobility of hip joint is decreased. In addition, the change of shortening might play a role in the necrosis of femoral head.