Accurate femur repositioning is critical during intraoperative total hip arthroplasty length and offset assessment.

Techniques for intraoperative leg length equalization are based on measurements between fixed points on the pelvis and femur. These techniques have not been reliable because they are based on accurate femur repositioning. We examined the error that results from inaccurate femur repositioning during total hip arthroplasty. Total hip arthroplasty was simulated on a calibrated test bench and changes in leg length and femoral offset were measured. Before dislocation, the femur was held in neutral alignment. Total hip arthroplasty was simulated without changing length or offset and the femur was returned to neutral. Length and offset changes were measured with the femur held in 5 degrees and 10 degrees of abduction/adduction and flexion/extension. Five degrees of abduction/adduction malpositioning caused 8 mm of apparent change in leg length. Errors in femoral offset followed a similar trend. When using common techniques for intraoperative leg length equalization and offset restoration, inaccurate abduction/adduction repositioning of the femur with respect to the pelvis can cause substantial errors in the measurement of length and offset change.