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
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
cm3 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.