Spontaneous osteonecrosis of the knee (SONK), originally described by Ahlbach et al in 1968, is a condition of unknown cause, in its classic form appearing in the medial femoral condyle in middle-aged or elderly subjects (1,2).
On plain radiographs, SONK initially appears as a flattening of the femoral condyle and progresses to a radiolucent osteochondral defect adjacent to the weightbearing area, typically involving the medial femoral condyle
- initial radiographs may be negative, especially if the symptoms are of short duration, and the diagnosis is therefore often missed at this stage (3)
- with time, radiography shows a characteristic half-moon-shaped sclerosis with a radiolucent center adjacent to the joint surface, followed by a joint surface impression.
On magnetic resonance imaging (MRI), SONK lesions appear as a focal, low-signal findingwith linear features in the subarticular bone of the epiphysis.
Aetiology:
- cause of SONK is unknown
- theories include a disrupted vascular supply, repetitive trauma, and postarthroscopic origin
Clinical findings:
- onset of SONK is classically an acute onset of pain without a significant predisposing trauma
- some patients, however, describe a more gradual increase of symptoms
- most common finding on physical examination is localized tenderness over the medial femoral condyle
- usually associated with articular effusion
- most patients presenting in their 50s or older
Koshino Radiographic Classification for Spontaneous Osteonecrosis of the Knee (SONK)
| |
| |
| Weightbearing area with radiolucent oval shadow in the medial femoral condyle, flattening of the condyle |
| Collapse of the subchondral bone plate with formation of a calcified plate and a clear sclerotic halo |
| Osteoarthritic changes, such as spur formation and osteosclerosis, with a shallow concave articular surface at the osteonecrotic region |
SONK can be differentiated from secondary osteonecrosis by the absence of risk factors such as alcoholism, chronic use of corticosteroids, and systemic disorders
In more advanced stages of the disease, patients with larger lesions and osteochondral defects comprising more than 40% of the condylar width have an unfavorable prognosis and tend to rapidly progress to osteoarthritis (1)
Management:
- conservative treatment
- with protected weightbearing, analgesia, and physical therapy is generally successful in early stages, with resolution of MRI alterations and clinical symptoms in up to 93% of patients (4,5)
- surgical treatment
- usually performed after failure of conservative treatment or in poor prognostic lesions and includes core decompression, cartilage repair, high tibial osteotomy, or joint arthroplasty
Reference:
- Aglietti P, Insall JN, Buzzi R, Deschamps G. Idiopathic osteonecrosis of the knee: aetiology, prognosis and treatment. J Bone Joint Surg Br. 1983;65(5):588-597.
- Ahlback S, Bauer GC, Bohne WH. Spontaneous osteonecrosis of the knee. Arthritis Rheum. 1968;11(6):705-733.
- Lotke PA, Abend JA, Ecker ML. The treatment of osteonecrosis of the medial femoral condyle. Clin Orthop Relat Res 1982;109-116.
- Jordan RW, Aparajit P, Docker C, Udeshi U, El-Shazly M. The importance of early diagnosis in spontaneous osteonecrosis of the knee - a case series with six year follow-up. Knee. 2016;23(4):702-707.
- Yates PJ, Calder JD, Stranks GJ, Conn KS, Peppercorn D, Thomas NP. Early MRI diagnosis and non-surgical management of spontaneous osteonecrosis of the knee. Knee. 2007;14(2):112-116.
- Koshino T, Okamoto R, Takamura K, Tsuchiya K. Arthroscopy in spontaneous osteonecrosis of the knee. Orthop Clin North Am. 1979;10(3):609-618.