Evaluation of Bony Impingement in Regard to Internal Rotation Limit after Total Hip Arthroplasty Using Rotation Matrix

OBJECTIVES Bony impingement of the proximal femur on the pelvis is an important factor for dislocation after total hip arthroplasty (THA). We evaluated bony impingement after THA using the rotation matrix derived from postoperative computed tomography (CT) images. PATIENTS AND METHODS One hundred and seven hip joints were subjected to primary THA via a posterolateral approach. We used the rotation matrix derived from CT images to calculate internal rotation (IR) limit prior to bony impingement, and compared this limit with the intraoperative limit. RESULTS The average calculated IR limit was 63 degrees (range: 30 to 85 degrees). The average intraoperative IR limit was 49 degrees (range: 20 to 70 degrees). The correlation between the intraoperative IR limit (Y) and the calculated IR limit (X) was expressed as Y=8.9+0.66X (R=0.73; p < 0.0001). CONCLUSIONS We could show a patient's safe range of motion prior to bony impingement, and this will be a good indicator for dislocation not occurring during postoperative rehabilitation.