Theoretical Analysis of Total Hip Dislocation and Comparison of Hemispherical Cup and Newly Developed Cup

Dislocation remains a serious complication of total hip replacement. The aim of this study was to analyze the process by which the femoral head is levered out of the acetabular cup when the femoral component sustains a torque after prosthetic impingement. The jump distance of the femoral head and its trajectory were calculated and compared for the common hemispherical cup and a newly developed elongated cup with a preferred impact mode (ChenLin cup). The results show that the latter avoids the short jump distance seen with the former and makes it more difficult for the femoral head to follow a path resulting in dislocation. Thus, it is expected that the ChenLin cup will reduce the risk of dislocation. In addition, this study found that, with both types of cup, the femoral head diameter and the outer diameter of the cup affect the jump distance, whereas the head/neck ratio has almost no effect on the jump distance for the hemispherical cup, but may have a positive effect for the ChenLin cup. This study suggests that the use of a combination of a small cup, large femoral head, and large head/neck ratio will reduce the occurrence of total hip dislocation induced by torque.