Review Question - QID 3449

Femoral stem subsidence effectively decreases the neck length of the prosthesis resulting in a lax abductor complex which causes an increase in the joint reactive force. This decrease in leg length can also lead to increased hip instability.

Kim et al performed clinical, radiographic, and computed tomography examinations on 1268 patients to determine the prevalence of and factors contributing to dislocation after using a primary cementless total hip arthroplasty system. The significant risk factors for dislocation (3.6% rate overall) were female sex, advanced age, high ASA score, fracture of the femoral neck, nonrepair of the posterior soft-tissue sleeve, low or high cup anteversion, low or high stem anteversion, and low height of hip rotation center.

Nishii et al evaluated component positioning in a series of THA patients who underwent the same surgical procedure to determine if there was a correlation with the occurrence of postoperative dislocation. They found that cup anteversion is one of the important factors for risk of dislocation, and that intentionally placing the cup at low anteversion to compensate for high femoral neck anteversion may predispose the hip to postoperative dislocation.

Illustration A shows a free body diagram of the hip joint. The magnitude of the joint reaction force depends critically on the ratio of (d1:d2). As d2 decreases due to less offset, such as in this question, and body weight remains the same, the joint reaction forces increase.

Incorrect Answers:
Answer 1: The excursion distance (the distance the femoral head must travel to dislocate) is unchanged.
Answer 2: The primary arc range (arc range before impingement) may be decreased due to early trochanteric impingement, but is not increased.
Answer 4: A more lax abductor complex results in an increased joint reactive force.
Answer 5: Femoral offset (distance from center of femoral head to long axis of femur) is unchanged as the radiographs show subsidence primarily in a caudal direction.