It is advocated that to avoid complications associated with femoral stem impingement, acetabular positioning should be within a “safe zone.” However, instability remains prevalent despite accurate cup positioning, with studies showing dislocations of cups despite positioning within safe zones. We assessed cup position angles associated with impingement in a group of subjects during (1) squatting; (2) object pick-up; and (3) low chair rise.
Ten subjects (mean age, 69 years; body mass index, 28.4 kg/m2) performed object pick up, squatting, and low-chair rising. Femur-to-pelvis relative motions were recorded for flexion/extension, abduction/adduction, and internal/external rotation. A previously reported custom-validated hip range-of-motion 3-dimensional simulator was used, set for neutral pelvic tilt and 15° of stem version. Acetabular cup abduction and anteversion combinations were chosen. The software computed minimum clearances between components for any hip position. An idealized tapered wedge stem with a 132° neck angle and a 36-mm femoral head was used.
Eight subjects had impingement on squatting between 21° and 51° of inclination. During object pick-up, 9 subjects had impingement with inclination and anteversion angles within the “safe zone.” In low-chair rise, 8 subjects had impingement at cup inclination angles between 14.5° and 49.5°.
The true acetabular target for impingement-avoidance motion is much smaller than previously believed and varies considerably between patients. Certain activities, such as picking up an object, low-chair rise, and squatting reduce the size of the safe zone. This study supports the need for better individualized preoperative patient-specific planning and intraoperative execution for placement of the components.