Effects of anterior boundary of the necrotic lesion on the progressive collapse after varus osteotomy for osteonecrosis of the femoral head

Background: Anterior localization of the necrotic lesion was recently proposed as an important factor for the occurrence of collapse even in medially located osteonecrosis of the femoral head (ONFH). We examined the effects of the anterior boundary of the necrotic lesion on progressive collapse after varus osteotomy for ONFH. Methods: We reviewed the outcomes of 31 hips in 27 patients with ONFH treated by transtrochanteric curved varus osteotomy (CVO) from 2000 to 2012 with a mean follow-up of 10.5 years. The occurrence of progressive collapse of the anterior necrotic lesion was defined as the presence of ≥2 mm collapse using follow-up lateral radiographs. Postoperative osteoarthritic change was defined as ≥1 mm progression of joint space narrowing on follow-up radiographs. The location of the anterior boundary of the necrotic lesion was assessed using the anterior necrotic angle (the angle between the midline of the femoral neck shaft and the line passing from the femoral head center to the anterior boundary of the necrotic lesion on a mid-slice oblique magnetic resonance image). Results: All hips had a postoperative intact ratio of ≥34% (percentage of the transposed intact articular surface of the femoral head to the weight-bearing area of the acetabulum after femoral osteotomy). Progressive collapse of the anterior necrotic lesion was seen in five hips (16%) during a mean of 2.2 years after CVO. Of these, four hips (80%) proceeded to develop osteoarthritic change at an average of 4.3 years after the collapse. Multivariate analysis revealed that the anterior necrotic angle was independently associated with progressive collapse of the anterior necrotic lesion as well as the postoperative intact ratio. Conclusions: This study suggests that hips with anterior localization of the necrotic lesion have a possible risk of progressive collapse of the anterior necrotic lesion after CVO, which can frequently lead to subsequent osteoarthritic change.