Changes of center of rotation and femoral offset in total hip arthroplasty

During single-leg standing the body weight is counterbalanced by the abductor muscles strength (1). Any variation in the hip joint center of rotation (COR) after total hip arthroplasty (THA) influences the lever arm and tension of the abductor muscles as well as the lever arm of the body weight, and thus the force required by the muscles to balance the pelvis. Medialization of COR with reduced lever arm of the body weight positively affects hip function (2) and has been associated with increased survival of THA (3-6). Also, the height of COR determines the tension of the abductor muscles (7), and superior and lateral relocation after THA has been connected with an increased rate of aseptic loosening (8,9). Therefore medialization of the cup with a respective increase in femoral offset has been advocated (10,11). However, medialization of the COR in THA may have negative effects on joint reaction forces, and may cause bone loss and changes in proprioception