Spontaneous osteonecrosis of knee

Clinical History

A 46-year old male patient with three-week history of right knee pain

Imaging Findings

A 46-year-old fit active male presented with three-week history of right knee pain. No history of direct trauma to the knee. There was no previous history of knee problems to note.
On clinical examination knee is slightly swollen with tenderness over the medial femoral region. Both flexion and extension are slightly restricted.

Image findings:
Plain radiographs were within normal limits. MRI showed joint effusion, medial femoral condyle bone marrow oedema with subchondral linear low signal area consistent with fracture and fibrillation of the articular cartilage of the medial compartment of the joint. Appearances are in keeping with spontaneous osteonecrosis of knee (SONK).

Discussion

Spontaneous osteonecrosis of the knee (SONK) usually presents with acute onset of knee pain despite the patient having no significant history of trauma. It is most commonly seen in the 5th - 7th decade with a male to female ratio of 1:3. The weight-bearing portion of the medial femoral condyle is the most frequent location. However, it has also been reported in the lateral femoral condyle and both tibial plateau. SONK is often found in association with meniscal tears, following prior arthroscopy or meniscal resection, and following a diagnosis of chondromalacia associated with osteoarthritis.

Plain radiographs in the initial stages appear normal. Flattening of the weight bearing surface, subchondral sclerosis and collapse eventually lead to secondary osteoarthritis in the later stages. Triple phase isotope bone scans show increased uptake at the site of lesion. MRI is more sensitive than bone scanning and plain radiography. It provides earlier and more extensive information on the distribution of marrow abnormalities and the presence of cartilage damage, which is relevant to prognosis. Bone marrow oedema (BMO) and subsequent necrosis change the normal fat signal of the marrow with the classical appearance of hypointense regions on T1 and hyperintense regions on T2-weighted sequences. SONK can be further associated with insufficiency fracture. An insufficiency fracture is seen on MRI as subchondral, crescentic or linear dark signal regions on both T1 and T2 weighted images.

A pattern of BMO isolated to one side of a weight bearing articulation in the lower extremity has previously been attributed to many different conditions such as, transient osteoporosis, transient bone marrow oedema syndrome, true osteonecrosis, spontaneous osteonecrosis and shifting bone marrow oedema. The differentiation of these conditions is often difficult and confusing due to overlapping radiological findings and identical clinical presentations. Currently the most accepted proposed etiology is that the weight bearing articular surface is subject to altered stresses which predisposes to the development of insufficiency fractures. Some of the conditions such as transient osteoporosis and transient bone marrow oedema syndrome are merely early manifestations of subchondral insufficiency fractures. Those cases where these fractures fail to heal in the early stages are thought to progresses further to osteonecrosis.

Treatment of SONK is mainly conservative including non-steroidal anti-inflammatory drugs, protected weight bearing with crutches and physiotherapy. Surgical intervention has been shown to have mixed results

Differential Diagnosis List

Final Diagnosis

Spontaneous osteonecrosis of knee(SONK)

URL:	https://www.eurorad.org/case/6861
DOI:	10.1594/EURORAD/CASE.6861
ISSN:	1563-4086