Clinico-radiological factors affecting the joint space narrowing after transtrochanteric anterior rotational osteotomy for osteonecrosis of the femoral head

The purpose of this study was to investigate the factors that affect the joint space narrowing after transtrochanteric anterior rotational osteotomy (ARO) for osteonecrosis of the femoral head (ONFH). We reviewed 39 cases (43 hips) of ONFH in patients who underwent ARO between 2000 and 2004. Their mean age was 42 years (23-61) at the time of surgery. The mean follow-up period was 7.3 years (3-10). The following clinical and radiological factors were investigated: the preoperative stage, localization and extent of the necrotic lesion, and the postoperative intact ratio (transposed intact articular surface of the femoral head to the weight-bearing surface of the acetabulum). The 43 hips were divided into two groups: a joint space narrowing (JSN) group and a non-joint space narrowing (non-JSN) group. Thirty-seven hips (86 %) were categorized as non-JSN and six (14 %) as having JSN. The preoperative Japanese Orthopaedic Association score was significantly higher in the non-JSN group than in the JSN group (P = 0.01). In the non-JSN group, the rate of early stage disease was significantly higher than in the JSN group (P = 0.03). The postoperative intact ratio was significantly higher in the non-JSN group than in the JSN group (P = 0.002). A multivariate analysis revealed that the postoperative intact ratio was an independent predictor of the progression of joint space narrowing after ARO, and the cutoff point was 39.2 %. The results of this study suggest that the postoperative intact ratio is one of factors determining the progression of joint space narrowing after ARO and that an approximately 40 % or higher postoperative intact ratio is recommended to prevent joint space narrowing.