Changes in cup inclination impact impingement-free hip motion after canine total hip replacement.

Objective: To evaluate the effects of changes in prosthetic cup inclination on impingement-free hip abduction, adduction, and internal and external rotation after simulated total hip replacement in dogs.

Methods: For 6 dogs, CT scans of the hip region were used to prepare 3-D surface models of cementless total hip replacement. For each dog, 15 models with cup inclination ranging from -35° to 35° in 5° increments were prepared using computer-aided design software. For each implant position, impingement-free hip motion in abduction, adduction, and internal and external rotation was evaluated using a custom-built computer program for hip flexion/extension angles ranging from 50° to 160° in 5° increments.

Results: A total of 7,920 computer simulations were conducted. Increased cup inclination led to decreased impingement-free abduction in hip extension and increased abduction in hip flexion. Decreased cup inclination led to decreased abduction at all hip angles. Maximal inclination led to increased external rotation in full hip extension, and maximal declination led to increased internal rotation and adduction in full hip flexion.

Conclusions: During total hip replacement, changes in cup inclination influence hip abduction and, to a lesser extent, internal rotation and adduction in flexion and external rotation in extension. The assessment of intraoperative impingement should include abduction, extension combined with external rotation, and flexion combined with adduction and internal rotation.

Clinical relevance: The inclination of a truncated cup influences impingement-free abduction. Inclination should be controlled during cup insertion to keep the prosthetic neck in the central portion of the cup truncation during abduction.