Review Question - QID 215115

Figure A demonstrates a template that decreases offset only, thus increasing the risk of bony impingement.

Soft tissue tensioning is one of the most important factors relating to a successful THA. Femoral offset is the perpendicular distance from the femoral head center of rotation to the axis of the femur. As offset increases, the risk of bony impingement decreases as the trochanter is taken further away from the bony pelvis. Additionally, increasing offset leads to decreased joint reactive forces and increased soft tissue tensions. Decreasing femoral offset, however, may lead to instability and abductor weakness.

Malik et al review impingement occurring following THA. They report on various causes of impingement including prosthesis design, component position, and biomechanical factors. They conclude that medialization of the cup and restoration of hip offset and length are two operative factors that can be adjusted to decrease the risk of osseous impingement.

Kristiansen et al review dislocations following THA. They report a dislocation rate of 4.9% following 427 cases. They conclude that acetabular component retroversion and joint laxity were the only predisposing factors leading to dislocation.

Figure A is the preoperative template for a THA in which the femoral offset is planned to be decreased.

Incorrect Answers:
Answer 1: Decreasing the femoral offset will decrease the risk of developing trochanteric bursitis.
Answer 3: Decreasing the offset will increase joint reactive forces.
Answer 4: With less offset you also have less tension on abductors and are at risk for gluteus medius lurch.
Answer 5: Just medializing the offset on the template will not change leg lengths.