To observe the imaging manifestations of osteonecrosis of the femoral head (ONFH) during pericollapse stage and make classification and summary.

The research recruited ONFH patients who had complete image data from the First Affiliated Hospital of Guangzhou University of Chinese Medicine in the pericollapse stage, and the patients with congenital hip deformities and other diseases affecting the hip were exluded. The shape of sclerotic zone and its corresponding relations with "eyebrow arch" of acetabulum and anterolateral column of the femoral head were observed on bilateral hip standard radiograph and lateral radiograph of "frog position" respectively. The classification was performed according to the manifestation patterns.

A total of 266 patients with 435 hips were enrolled. Six patterns of manifestation of the sclerotic zone were observed on bilateral hip standard radiograph, including type A (the edge of sclerotic zone was within the medial 1/3 of acetabular "eyebrow arch" ), type B (the edge was within the middle 1/3), type C1 (the edge was within the lateral 1/3), type C2 (the edge extends laterally to the acetabular edge), type D1 and type D2 (all the sclerotic zones presented ring shape locating at the center of the femoral head and the subchondral bone plate of the weight-bearing area of femoral head). Altogether 19 hips (4.4%) were type A, 89 hips (20.1%) were type B, 155 cases (35.6%) were type C1, 87 cases (20.0%) were type C2, 47 cases (10.8%) were type D1 and 38 cases (8.7%) were type D2. Five manifestation patterns of the sclerotic zone were also observed on bilateral hip frog lateral radiographs, including type 1 (the edge of sclerotic zone located at the posterior medial or central column of the femoral head), type 2(the edge was similar to type 1, and the anterolateral column was involved in the deep), type 3 (the edge located at the anterolateral column), type 4 and type 5 (all the edges presented ring shape locating at the center of the femoral head and the subchondral bone plate of the weight-bearing area of femoral head). Altogether three hips (0.7%) were type 1, 51 hips (11.7%) were type 2, 316 hips (72.6%) were type 3, 32 hips (7.4%) were type 4 and 33 hips (7.6%) were type 5.

Developing the anteroposterior and lateral images of "frog position" classification system based on the radiographic manifestation patterns of sclerotic zone in necrotic femoral head while considering the necrotic involvement of lateral and anterolateral column will be conducive to better understanding the nature of disease and provide useful information for clinical diagnosis, therapy and prognosis.