The effect of femoral component head size on posterior dislocation of the artificial hip joint

Background: Posterior dislocation continues to be a relatively common compl ication following total hip arthroplasty. In addition to technical and pati ent-associated factors, prosthetic features have also been shown to influen ce stability of the artificial hip joint. In this study, a dynamic model of the artificial hip joint was used to examine the influence of the size of the head of the femoral component on the range of motion prior to impingeme nt and posterior dislocation following total hip replacement. Methods: Six fresh cadaveric specimens were dissected, and an uncemented to tal hip prosthesis was implanted in each. Each specimen was mounted in a me chanical testing machine and loaded with use of a system of seven cables at tached to the femur and pelvis that simulated the action of the major muscl e groups crossing the hip joint, The hip was taken through a range of motio n similar to that experienced when rising from a seated position. The three -dimensional position of the femur at the points of impingement and disloca tion tvas recorded electronically, The range of joint motion was tested wit h prosthetic femoral heads of four different diameters (twenty-two, twenty- six, twenty-eight, and thirty-two millimeters). Results: Significant associations were noted between the femoral head size and the degree of flexion at dislocation in ten (p = 0.001), twenty (p < 0. 001), and thirty (p = 0.003) degrees of adduction. Increasing the femoral h ead size from twenty-two to twenty-eight millimeters increased the range of flexion by an average of 5.6 degrees prior to impingement and by an averag e of 7.6 degrees prior to posterior dislocation; however, increasing the he ad size from twenty-eight to thirty-two millimeters did not lead to more si gnificant improvement in the range of joint motion. The site of impingement prior to dislocation varied dth the size of the femoral head. With a twent y-two-millimeter head, impingement occurred between the neck of the femoral prosthesis and the acetabular liner, whereas with a thirty-two-millimeter head, impingement most frequently occurred between the osseous femur and th e pelvis. Conclusions: With the particular prosthesis that was tested, increasing the diameter of the femoral head component increased the range of motion prior to impingement and dislocation, decreased the prevalence of prosthetic imp ingement, and increased the prevalence of osseous impingement. Clinical Relevance: These results suggest that femoral heads with a twenty- eight-millimeter diameter increase the range of motion after total hip repl acement. This may be beneficial when additional factors compromising joint stability are encountered.