In vivo kinematic analysis of squatting after total hip arthroplasty.

BACKGROUND The in vivo kinematics of squatting after total hip arthroplasty is unclear. The purpose of the present study was to determine the range of motion of the hip joint during squatting after total hip arthroplasty. METHODS Using fluoroscopy, we investigated 15 primary cementless total hip arthroplasties performed using a computed tomography-based navigation system. An acetabular component with concavities around the rim and a femoral component with reduced neck geometry were used. The motion of the acetabular and femoral components based on the neutral standing position was analyzed using a two-dimensional to three-dimensional registration technique. FINDINGS No prosthetic impingement occurred in any hips. The mean maximum hip flexion range of motion was 86.2° (range, 55.1°-117.4°) and was not always consistent with maximum squatting. The mean maximum pelvic posterior tilting angle was 25.7° (range, 5.5°-43.5°). The pelvis began to tilt posteriorly at 50°-70° of the hip flexion range of motion. At maximum squatting, the mean ratio of the pelvic posterior tilting angle to the femoral flexion angle was 23.2% (range, 3.8%-45.7%). The mean minimum angle up to the theoretical prosthetic impingement was 26.2° (range, 11.8°-39.8°). INTERPRETATION Although this is a preliminary study, three-dimensional assessment of dynamic squatting motion after total hip arthroplasty using the two-dimensional to three-dimensional registration technique appears to enable elucidation of the range of motion of the hip joint, the contribution of pelvic posterior tilting, and the minimum angle up to theoretical prosthetic impingement during squatting.