Factors influencing progressive collapse of the transposed necrotic lesion after transtrochanteric anterior rotational osteotomy for osteonecrosis of the femoral head

Background Transtrochanteric anterior rotational osteotomy (ARO) for osteonecrosis of the femoral head (ONFH) can preserve for a long-time collapsed femoral head. Progressive collapse of anteriorly-transposed necrotic lesion leads to secondary arthritic changes and clinical failure. Critical factors influencing collapse of the transposed necrotic lesion after ARO remain largely unknown. Therefore, we performed a retrospective study of ARO to determine: (1) if preoperative collapse influences collapse of the transposed necrotic area, (2) if any other factor may influence collapse of the transposed necrotic area. Hypothesis We hypothesized the degree of preoperative femoral head collapse influences progressive collapse of the transposed necrotic lesion after ARO. Materials and methods We reviewed 47 hips in 42 patients with ONFH treated with ARO between 2000 and 2005 with a mean follow-up of 11.4 years (10–14 years). The occurrence of progressive collapse of the transposed necrotic lesion after ARO was examined using lateral radiographs taken at least once every year after ARO. The following factors were statistically analyzed: age, sex, body mass index, Harris Hip Score (HHS), preoperative level of collapse, extent of the necrotic lesion and postoperative intact ratio (ratio of the transposed intact articular surface of the femoral head). Results Progressive collapse of the transposed necrotic lesion (progressive collapse group) was seen in 17 hips (36%) during a mean period of 1.8 years (0.5–3.7 years) after ARO, which has developed within 4 years in all cases. Preoperative level of collapse in the progressive collapse group (4.4 ± 1.4 mm) was significantly larger than that in the non-progressive collapse group (2.1 ± 1.0 mm), which was independently associated with progressive collapse of the transposed necrotic lesion in multivariate analysis (P < 0.0001) with cut off point of 2.98 mm. In univariate analysis, lower preoperative HHS, severe extent of the necrotic lesion and the lower postoperative intact ratio were also associated with progressive collapse of the transposed necrotic lesion, but were not associated as independent factors in multivariate analysis. Discussion The current study suggests that progressive collapse of the transposed necrotic lesion after ARO depends mainly on the preoperative level of collapse (cut-off point = 2.98 mm). Level of evidence IV; retrospective case series.