Computed Tomography Staging of Osteonecrosis of the Femoral Head

Osteonecrosis of the femoral head (ONFH) usually affects patients younger than 50 years and frequently leads to collapse of the femoral head and subsequent osteoarthritis of the hip. Joint-preserving procedures are attempted in the early stages without collapse, while total hip arthroplasty (THA) is done during more advanced stages with femoral head collapse or a fracture in the necrotic portion. Thus, accurate staging of the disease is mandatory to decide therapeutic strategy. The purpose of this study is to determine the prevalence of occult fracture in the necrotic portion among osteonecrotic femoral heads, which were classified as Association Research Circulation Osseous (ARCO) stage 1 or 2 on radiographs.

A preoperative computer tomography (CT) scan was routinely performed to obtain an adequate cup position in patients undergoing THA at our institution. Radiographs and CT scans of 308 patients, who underwent unilateral THA due to ONFH from January 2011 to December 2014, were assessed by two orthopedic surgeons to compare the stage based on simple radiography and that based on CT scans.

Among the 308 hips, which were classified as stage I or II lesions on simple radiography, fracture inside the necrotic portion was seen in 63 hips (20.5%) on CT scans. Fifty-nine of the 63 patients were followed for two to five years. Further collapse of the femoral head occurred in 41 patients, and 36 of them underwent THA during the follow up.

In this study, 20.5% of hips classified as ARCO stage I or II on simple radiography were found to be ARCO stage III on CT imaging. When a joint-preserving treatment is considered for early stage ONFH, CT examination is necessary for more precise staging of the disease.