Effects of intertrochanteric osteotomy plane and preoperative femoral anteversion on the postoperative morphology of the proximal femur in transtrochanteric anterior rotational osteotomy: 3D CT-based simulation study.

BACKGROUND: Transtrochanteric anterior rotational osteotomy (ARO) is joint-preserving surgery for patients with osteonecrosis of the femoral head (ONFH). During ARO, femoral neck-shaft varus angulation by changing intertrochanteric osteotomy plane is often designed to obtain a sufficient postoperative intact ratio. However, the effect of intertrochanteric osteotomy plane on postoperative femoral anteversion has not been well examined. Therefore, we performed a simulation study of ARO to determine how intertrochanteric osteotomy plane and preoperative femoral anteversion affect both femoral neck-shaft varus angle and postoperative femoral anteversion.

HYPOTHESIS: Both femoral neck-shaft varus angle and postoperative femoral anteversion are predicted by intertrochanteric osteotomy plane and preoperative femoral anteversion in ARO.

MATERIALS AND METHODS: Using CT-data obtained from 10 hips in 10 patients with ONFH, ARO was simulated. On anteroposterior view, basic intertrochanteric osteotomy line (AP-view line) was defined as the perpendicular line to the femoral neck axis. On lateral view, basic intertrochanteric osteotomy line (lateral-view line) made through the cut surface of greater trochanter was defined as the perpendicular line to the lateral axis of the femur. By changing either AP-view or lateral-view line, 49 ARO models/hip were produced, in which femoral neck-shaft varus angle and postoperative femoral anteversion were assessed.

RESULTS: With increase in the vertically-inclined degree of AP-view line, both neck-shaft varus angle and postoperative femoral anteversion increased. With increase in the posteriorly-tilted degree of lateral-view line, neck-shaft varus angle increased, whereas postoperative femoral anteversion decreased. The approximation equations based on the multiple regression analyses were as follows: neck-shaft varus angle≈vertically-inclined degree of AP-view line×0.9+posteriorly-tilted degree of lateral-view line×0.8+preoperative femoral anteversion×0.7; postoperative femoral anteversion≈vertically-inclined degree of AP-view line×1.1-posteriorly-tilted degree of lateral-view line×0.8.

DISCUSSION: The postoperative morphology of proximal femur was nearly defined by intertrochanteric osteotomy plane with preoperative femoral anteversion, which is useful for preoperative planning in terms of both achieving a sufficient postoperative intact ratio and maintaining femoral anteversion.

LEVEL OF EVIDENCE: Level IV case series without control group.