Effect of Femoral Neck Anteversion on Postoperative Intact Ratio of Curved Varus Osteotomy for Osteonecrosis of the Femoral Head: A Computed Tomography-Based Simulation Study.

Background: The postoperative intact ratio-referring to the percentage of viable bone in the weight-bearing area of the femoral head after curved varus osteotomy (CVO)-is critical for a successful postoperative outcome. This study aimed to evaluate the effect of femoral neck anteversion on the postoperative intact ratio in CVO.

Methods: We performed a computed tomography-based simulation of 21 hips with type C1 osteonecrosis of the femoral head according to the Japanese Investigation Committee classification. The study simulated 30 and 20° of varus motion around two different axes perpendicular to the femoral neck axis (FNA) and posterior condylar axis (PCA). The difference in postoperative intact ratios between the two CVO types was evaluated.

Results: The median and mean femoral neck anteversion were 21.1 and 15.3°, respectively (range, -12.0 to 38.5). There were 12 hips (57.1%) that had a femoral neck anteversion angle ≥ 20°. A moderate positive correlation was observed between the difference in intact ratios between the FNA and PCA groups and the femoral neck anteversion angle in both the 30° varus simulation (ρ = 0.50, P = 0.020) and the 20° varus simulation (ρ = 0.58, P = 0.006). Notably, in the 30° varus simulation, four cases with femoral neck anteversion ≥ 20° showed a > 10% difference in the intact ratio between the FNA and PCA groups, whereas one case in the 20° varus simulation exhibited a similar difference. The FNA-referenced CVO tended to provide a larger intact ratio than that provided by the PCA-referenced CVO.

Conclusions: Femoral neck anteversion ≥ 20° affected the postoperative intact ratio after CVO performed in reference to the PCA. For patients who had femoral neck anteversion ≥ 20°, setting the osteotomy axis perpendicular to the FNA is recommended to optimize the postoperative intact ratio.