Transtrochanteric posterior rotational osteotomy (PRO) is one of the joint-preserving surgeries for osteonecrosis of the femoral head. In general, postoperative femoral neck-shaft varus realignment is planned to obtain a sufficient intact articular surface of the femoral head in the weight-bearing portion. Unlike anterior rotational osteotomy, PRO allows for more than 90° rotation of the femoral head, resulting in more complicated morphology. However, little is known about the potential risk of postoperative femoral retroversion after PRO. This simulation study aims: 1) to assess whether postoperative femoral neck-shaft varus realignment can coexist with preserved femoral anteversion after PRO, 2) and whether postoperative proximal femoral morphology could be predicted with approximation equations.

High degree (>90°) PRO is favourable for femoral neck-shaft varus realignment, but unfavourable for maintaining postoperative femoral anteversion.

PRO was simulated by using CT data from 10 hips in 10 healthy volunteers. During simulation, the intertrochanteric osteotomy plane was determined three-dimensionally based on anteroposterior-view line (the osteotomy line on anteroposterior view) and lateral-view line (the osteotomy line on lateral view). By changing either the angle of anteroposterior-view line or lateral-view line, we simulated 90°, 110°, 130° and 150° PRO. To clarify the effects of various posterior rotation angles on postoperative proximal femoral morphology, we made simplified PRO models through changing only the posterior rotation angle.

In the 90°, 110°, 130° and 150° PRO models, the vertically inclined angle of anteroposterior-view line showed a significant positive correlation with femoral neck-shaft varus realignment (90° PRO, r=0.90; 110° PRO, r=0.95; 130° PRO, r=0.97; 150° PRO, r=0.99), while a significant negative correlation with postoperative femoral anteversion angle (90° PRO, r=−0.97; 110° PRO, r=−0.95; 130° PRO, r=−0.92; 150° PRO, r=−0.7). Likewise, the posteriorly tilted angle of lateral-view line showed a significant negative correlation with both femoral neck-shaft varus realignment (90° PRO, r=−0.81; 110° PRO, r=−0.81; 130° PRO, r=−0.79; 150° PRO, r=−0.72) and postoperative femoral anteversion angle (90° PRO, r=−0.90; 110° PRO, r=−0.89; 130° PRO, r=−0.92; 150° PRO, r=−0.88). In the simplified PRO models, the posterior rotation angle showed a significant positive correlation with femoral neck-shaft varus realignment (r=0.33), while a significant negative correlation with postoperative femoral anteversion angle (r=−0.76). The approximation equations for predicting the proximal femoral morphology after PRO were validated.

It was confirmed that high-degree PRO (>90°) is favourable for femoral neck-shaft varus realignment, but works against preserving femoral anteversion. With the approximation equations developed in the current study, surgeons could examine the feasibility of PRO based on postoperative femoral anteversion. In terms of hip joint function and subsequent total hip arthroplasty, excessive deformities including femoral retroversion and severe varus deformity could be avoided.

IV; case series without control group.