SONK often starts with a sudden onset of knee pain that may worsen at night. SONK must be differentiated from secondary osteonecrosis, which most commonly results from corticosteroid use. Other conditions that cause secondary osteonecrosis include alcoholism, smoking, and coagulopathy. SONK is thought to result from an insufficiency fracture of the femoral condyle that fails to heal. Early radiographic findings are often normal but can progress to demonstrate subchondral flattening, sclerosis of the femoral condyle, narrowing of the joint space, and progressive arthritis. MRI is the best diagnostic imaging tool for early diagnosis of SONK. MRI findings reveal bone marrow edema consistent with local ischemia. Patients treated with 4 to 6 weeks of protected weight-bearing early in the disease usually recover well; more than 90% of these patients will experience resolution of symptoms with conservative treatment. Patients with persistent pain for several months or those that progress to an arthritic knee may be considered for unicompartment or total knee arthroplasty.1,2
Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor-in-chief of the Journal of Orthopedics for Physician Assistants (JOPA).
1. Mont MA, Baumgarten KM, Rifai A, Bluemke DA, Jones L, Hungerford D. Atraumatic osteonecrosis of the knee. J Bone Joint Surg Am. 2000;82(9):1279-1290.
2. Myers T, Cui Q, Kuskowski M, Mihalko W, Saleh K. Outcomes of total and unicompartmental knee arthroplasty for secondary and spontaneous osteonecrosis of the knee.J Bone Joint Surg Am. 2006;88 Suppl 3:76-82.