Guidelines for implant placement to minimize impingement during activities of daily living after total hip arthroplasty.

Impingement, both prosthetic and bony, precedes the vast majority of dislocations after total hip arthroplasty and may adversely impact component wear. Reconstructed computer hip models of 8 subjects were used to evaluate hip range of motion for activities of daily living (ADLs) associated with posterior instability and anterior instability. Variables examined included acetabular position, femoral offset, and head size. The majority of flexion ADLs (associated with posterior instability) encountered prosthetic impingement, whereas extension ADLs demonstrated bony impingement with the 45/20 cup placement position. Cup placement in natural anteversion and adduction allowed normal joint motion in anterior and posterior impinging activities. Insufficient femoral offset and smaller head size negatively impacted range of motion. Any anterior cup and posterior cup protrusions greater than 5 mm should be avoided.