Background: The prognosis for a patient with osteonecrosis of the hip is generally considered to be worse if a large volume of the femoral head is involved, the patient is symptomatic, and the stage of the lesion is advanced. In 1990, we began a prospective study to detect collapse in asymptomatic hips with a very small stage-I osteonecrotic lesion in the femoral head. We hypothesized that such patients would have a favorable prognosis. These hips were followed for a minimum of ten years after the diagnosis.
Methods: A small asymptomatic stage-I osteonecrotic lesion (not seen on plain radiographs) was diagnosed with magnetic resonance imaging in forty patients (forty hips) contralateral to a hip with symptomatic osteonecrosis. The criterion for inclusion in the study was a lesion with a volume of <5 cm(3) involving <10% of the volume of the femoral head. Plain radiographs were made annually in six different projections for all patients. At the most recent follow-up evaluation (average, eleven years), patients with a symptomatic hip but without evidence of collapse on plain radiographs underwent a computerized tomography scan.
Results: Thirty-five (88%) of the forty hips became symptomatic, and twenty-nine (73%) demonstrated collapse. The mean interval between the diagnosis and the first symptoms was eighty months. Symptoms always preceded collapse by at least six months. The mean interval between the diagnosis and the collapse was ninety-two months (range, seventy-two to 140 months). The diagnosis of collapse could be made on only one or two of the six radiographic views obtained for each patient at each evaluation. The diagnosis of collapse for two patients was made only on a computerized tomography scan at the most recent follow-up evaluation. At the time of final follow-up, the twenty-nine hips with collapse had symptoms of intractable pain and required surgery.
Conclusions: This study confirms that the diagnosis of collapse is difficult in hips with a very small stage-I osteonecrotic lesion. Multiple radiographic views and computerized tomography scans may be required to demonstrate small areas of collapse. Clinical and radiographic signs of progression of the disease in asymptomatic hips with a very small asymptomatic lesion progress more slowly than do those signs in hips with a large symptomatic stage-II lesion. Because hips with a small area of osteonecrosis do collapse in a large percentage of patients, such patients should be followed carefully over a long period of time.
Level of evidence: Prognostic study, Level I-1 (prospective study). See Instructions to Authors for a complete description of levels of evidence.