Effects of lower limb and pelvic pin positions on leg length and offset measurement errors in experimental total hip arthroplasty.

Background: Leg length (LL) and offset (OS) are important factors in total hip arthroplasty (THA). Because most LL and OS callipers used in THA depend on fixed points on the pelvis and the femur, limb position could affect measurement error. This study was conducted on a THA simulator to clarify the effects of lower limb position and iliac pin position on LL and OS errors and to determine the permissible range of limb position for accurate LL and OS measurement.

Methods: An LL and OS measurement instrument was used. Two pin positions were tested: the iliac tubercle and the top of the iliac crest intersecting with the extension of the femoral axis. First, the limb was moved in one direction (flexion-extension, abduction-adduction, or internal-external rotation), and LL and OS were measured for each pin position. Next, the limb was moved in combinations of the three directions. Then, the permissible range of combined limb position, which resulted in LL and OS measurement error within ±2 mm, was determined for each pin position.

Results: Only 4° of abduction/adduction caused 5-7 mm error in LL and 2-4 mm error in OS, irrespective of pin position. The effects of flexion-extension and internal-external rotation on LL error were smaller for the top of the iliac crest than for the iliac tubercle, though OS error was similar for both pin positions. For LL, the permissible range of the combined limb position was wider for the top of the iliac crest than for the iliac tubercle.

Conclusion: To minimize LL and OS measurement errors in THA, adduction-abduction must be maintained. The iliac pin position in the top of the iliac crest is preferred because it provides less LL measurement error and a wider permissible range of combined limb position for accurate LL measurement.