Review Question - QID 213958

Medialization of the acetabular component will decrease the moment arm of body weight leading to decreased joint reactive force of the hip abductor.

Soft-tissue tensioning during THA is crucial for restoring normal gait mechanics, preventing post-operative instability, and preventing early component loosening. Medializing the acetabular component places the center of rotation of the acetabulum closer to the center of the body thus decreasing the body moment arm (line B in illustration A). Referencing illustration A: AB x A = W x B. Thus if B decreases and A stays the same the force the abductors need to generate is decreased. With medialization of the acetabular component typically an appropriate increase in femoral offset is required to compensate for the medial location of the center of rotation and allow for appropriate tensioning of the abductor muscles. Thus increasing A in illustration A improves the abductor moment arm. Unfortunately, the femoral offset can not be coupled with cup medialization with native femoral anatomy during hip resurfacing arthroplasty.

Queen et al. performed a gait analysis study of 10 THA patients and 10 hip resurfacing arthroplasty (HRA) patients 18 months after surgery. They reported THA patients had decreased peak hip extension and ground reaction forces compared to HRA patients. The authors concluded THA patients do not display similar gait characteristics as HRA patients.

Terrier et al. performed a finite element analysis of preoperative CT scans on 15 patients undergoing THA to assess the effect of cup medialization on gluteus minimus and medius muscle moment arms. They reported that positive increases from 10 to 85% in muscle moment arms in abduction with cup medialization, but large variability in flexion-extension moment arm ranging from -35% to 50%. Additionally, femoral anteversion was inversely correlated with muscle moment arm. The authors concluded that the benefits of cup medialization are dependent on the patient's proximal femur anatomy.

Figure A is an AP pelvis radiograph demonstrating severe right hip osteoarthritis. Illustration A is a diagram depicting the moment arm of the abductor in addition to other forces acting on the hip.

Incorrect Answers:
Answer 1: A femoral stem with an increased neck-shaft angle will result in decreased offset, decreasing the tension of the abductors and a weaker moment arm.
Answer 3: A decrease in femoral offset results in a decreased moment arm which necessitates an increase in abductor muscle force to maintain a balanced pelvis. The increased abductor muscle force results in increased joint reactive forces.
Answer 4: Increased femoral anteversion has a minimal effect on the moment arm of the abductor, but with increasing femoral anteversion there is more posterior rather than lateral displacement of the muscle force vector.
Answer 5: A larger femoral head to neck ratio decreases the risk of dislocation by increasing the arc of motion before impingement of the neck with the acetabular rim; it does not affect the moment arm of the abductors.