Total hip arthroplasty offset measurement: is C T scan the most accurate option?

Femoral offset is difficult to precisely evaluate with conventional X-ray techniques. Femoral offset characterizes the balance between body weight and the resistance provided by the abductor muscles. Total hip arthroplasties should respect this balance.

Computed tomodensitometry (CT-scan) is more accurate than conventional X-ray to evaluate femoral offset.

Sixty-one patients who received unilateral total hip arthroplasties were prospectively included in the study. Femoral offset was measured by three-dimensional CT-scan reconstruction using the "Hip Plan" (Symbios) software. Offset was also determined with conventional X-ray and results were compared. This software can be used to measure leg length by frontal telemetry. It was developed for preoperative-planning of cementless femoral stem implants with modular necks of various lengths and angles. All pre- and postoperative measurements were made according to the same protocol.

Femoral offset values in this study were very similar to anatomical values found in the literature. They were significantly higher than values obtained by conventional X-ray by an average of 8%. Implantation of hip replacements resulted in a significant increase in offset (1.88+/-4.71 mm) with a slight variation in leg length. Pre- and postoperative leg length increased slightly in the operated leg by an average of 1.66+/-5.63 mm. Seventeen percent of these femurs had high offset associated with small or average sized proximal medullary canals. This preoperative planning software made it possible to identify these difficulties and to adapt implant components using modular long 8 degrees varus necks to restore high offset. In most of these cases, only small femoral stems could be implanted because of the small size of the intramedullary femoral canal. These individual differences were identified with 3D CT-scan reconstruction and included in the preoperative planning. Moreover, leg length could also be evaluated with this method and included in the preplanning.

Compared to conventional X-ray, measurements obtained with this preoperative planning method using 3D CT-scan reconstruction are easy to obtain and not dependent upon test conditions because the frame is placed on the femoral axis. Measurements are not influenced by position inconsistencies or if the hip is fixed in external rotation. The significant number of cases with above average offset confirms the importance of obtaining these measurements and the necessity of adapting the strategy in these cases by using lateralized stems, or, as in our series, modular necks to adjust femoral offset and neck angle.

Level III diagnostic prospective study.