BASIC SCIENCE & PATHOLOGY

SPONTANEOUS OSTEONECROSIS OF THE KNEE IN NOT A TRUE OSTEONECROTIC CONDITION

Mears, S.; Mont, M.A.
Johns Hopkins University School of Medicine; Department of Orthopaedic Surgery; Baltimore, MD

Introduction: Spontaneous osteonecrosis of the knee (SPONK) is a condition affecting patients over the age of 55 years. Typically, the patient has sudden medial knee pain. A corresponding lesion is visible on radiographs within the medial femoral condyle. The histopathology of this disease has not been characterized.

Methods: Twenty-two knees in 21 patients consecutively treated for SPONK with either core decompression or total knee arthroplasty were examined histologically in a blinded manner. Diagnosis was confirmed by clinical history, plain radiographs, magnetic resonance images and bone scintimetry.

Results: Patients had a mean age of 67 years, no exposure to steroids, no other diseased joints and there was one patient with bilateral disease. Symptom duration prior to intervention was 3 to 36 months (average 11 months). Only one of 22 specimens demonstrated evidence of bone necrosis. No specimens showed fat necrosis, marrow necrosis, fibrous change or appositional repair of bone. Fourteen of 22 (64%) showed significant osteopenia and 15 of 22 (68%) showed evidence of osteoarthritis. The mean histologic stage, according to the system of Arlet and Durroux, was 0.1, with 21 of 22 specimens (95%) stage 0.

Discussion: These results indicate that SPONK is not an osteonecrotic condition and has been misnamed. The high incidence of osteopenia and osteoarthritis suggest that these conditions may play a role in the pathogenesis of this disease. Perhaps microfracture within an area of weaker bone is the initial event leading to subchondral fracture and collapse of the medial femoral condyle.