Hip mechanics after posterior structure repair in total hip arthroplasty.

This study investigated the rotational response of the hip with different repairs of posterior structures after using the posterior approach in total hip replacement. Five groups were tested: (1) the normal hip without a replacement, (2) the normal hip with a vented capsule, (3) no repair of the capsule and external rotators after total hip replacement, (4) repair of only the piriformis tendon, and (5) repair of the capsule and external rotators as a flap of tissue (capsule, piriformis, obturator internus, gemellae, and quadratus) to the posterior aspect of the greater trochanter. Hemipelvis cadaveric specimens were attached to a joint testing device for testing at full extension, 30 degree, 60 degree and 90 degree hip flexion as internal and external rotation was applied to the femur and the load deflection curves were recorded. Specimens after total hip replacement were externally rotated in full extension until dislocation occurred and the maximum torque was recorded. Each specimen then was rotated internally at 90 degree flexion to the point of dislocation and the maximum torque was recorded for comparison as well. The posterior approach had significantly decreased internal rotational support with no repair or only piriformis repair. When the capsule and external rotators were repaired, a more normal load deflection curve resulted when comparing the full repair group with the normal hip. Significantly higher torque was needed to dislocate the hip in flexion when a full posterior repair was done, and most specimens dislocated in flexion without complete failure of the repair.