Introduction: The progression of the collapse is influenced by the extent and location of the necrotic focus of the necrotic femoral head. The authors studied the extent of the necrotic focus on the joint surface by conventional antero-posterior radiographs in neutral position, and antero-posterior radiographs at the 45 degrees flexion position. The differences of the lesions in these two types of radiographs were analyzed.

Materials and Methods: The conventional antero-posterior radiographs of the neutral position, and the antero-posterior radiographs at the 45 degrees flexion positions in 115 hip joints of 86 patients with non-traumatic osteonecrosis of the femoral head were obtained for this study. These 115 hips showed extensive lesions and could be divided into two groups: Type C-1 or C-2 by classification of the Specific Disease Investigation Committee under the auspices of the Japanese Ministry of Health Labor and Welfare. Type C1 lesions occupy more than the medial 2/3 of the femoral head and C2 lesions extend laterally to the acetabular edge.

Results: At the neutral position, Type C-1 lesions were noted in 42 joints (36%). In these 42 hips, 13 showed Type C-1 (30%) at the 45 degrees flexion position. In contrast, 29 joints (70%) were Type C-2. Type C-2 lesions in 73 joints (64%) were shown in the neutral position. In these 73 hips, Type C-2 lesions were disclosed on 69 joints (95%) in the 45 degrees flexion position and Type C-1 was noted on 4 hips (5%).

Conclusions: Based on these results, the authors propose that location of the lesion on the joint surface varies with different hip positions.

Introduction: Osteonecrosis of the femoral head with joint space narrowing in young patients aged less than fifty years old was treated with posterior rotational osteotomy. Changes of the acetabular roof were observed after operation. Early remodeling of the acetabular roof after operation was studied radiographically.

Materials and Methods: Eighteen hips in 15 patients treated by posterior rotational osteotomy were subjects for this study. The age of the patients ranged from 15 to 50 (mean of 36 years). Eleven patients were women, 4 were men. The etiologic factors were steroid administration in 9 patients, alcoholic abuse in 1 patient, trauma in 7 patients, and no apparent factor in 1 patient. Changes of the acetabular roof on antero-posterior radiographs were observed at 6 months, 1 year, and 2 years postoperatively. Atrophic change and uniform shape of the acetabular roof was studied in each period.

Results: At 6 months after operation, atrophic change was noted on all 18 hips. None of uniform shape was found. Atrophic change of 10 hips (56%) was found 1 year postoperatively. Shape of acetabular roof was improved and uniformed in 8 hips. In all 18 hips 2 years after operation, shape of acetabular roof was remodeled and uniformed.

Discussion: From these results, we thought acetabular roof of femoral head involving necrosis with joint narrowing was remodeled at an early period postoperatively.