Subchondral insufficiency fracture of the knee

Spontaneous osteonecrosis of the knee, also known as Ahlback disease, SONK or even SPONK, has similar appearances to osteochondritis dissecans of the knee but is found in an older age group.

Epidemiology

SONK is seen more frequently in women (M:F 1:3) and affects older patients, typically over the age of 55.

Clinical presentation

Patients often recall an acute onset of severe pain without significant trauma.

Pathology

Osteonecrosis in SONK has no predisposing factors. However, byBy definition, secondary osteonecrosis of the knee occurs secondary to an insult. SONK is not thought to be caused by bone death but may be causedinstead by osteoporosis and insufficiency fractures, with histopathologically proven origins in weakened trabeculae and applied microtraumatic forces ^6,13. Some authors suggestIt is now accepted that the primary event leading to spontaneous osteonecrosis of the knee is a subchondral insufficiency fracture that has further collapsed ¹³.

Radiographic features

It is almost always unilateral, usually affects the medial femoral condyle (but can occasionally involve the tibial plateau ⁹) and is often associated with a meniscal tear. 

Plain radiograph

In the later stages features seen include:

• flattening of the medial femoral condyle
• subchondral radiolucent focus
• complicating subchondral fracture with periosteal reaction

MRI

Features can vary dependantdepending on the stage, and are best characterised on T2-weighted and proton density-weighted sequences. The following criteria apply to lesions without overlying cartilage abnormalities:

• subchondral bone plate fracture ¹³
  • in the weight-bearing area of the involved condyle
  • subtle flattening or a focal depressive deformity
  • an irregular, discontinuous hypointense line in the subarticular marrow, representing callus and granulation tissue
  • there may be a fluid-filled cleft within the subchondral bone plate (poor prognostic factor) ¹³
  • excavated defect of the articular surface (advanced cases)
• focal subchondral area of low signal intensity subjacent to the subchondral bone plate representing local ischaemia (considered most important in early lesions and a specific MRI finding ¹²)
  • this area shows no enhancement on post-contrast; if it is thicker than 4 mm or longer than 14 mm, the lesion may be irreversible and may evolve into irreparable epiphyseal collapse and articular destruction
  • appears as a thickened subchondral bone plate, which represents a fracture with callus and granulation tissue and secondary osteonecrosis in the subarticular region ¹³
• ill-defined bone marrow oedema and and a lack of peripheral low signal intensity rim as seen in osteonecrosis and and bone infarcts
• focal subchondral area of low signal intensity adjacent to the subchondral bone plate representing local ischaemia (considered as a specific MRI finding ¹²); this area shows no enhancement on post-contrast
• deformity of the subchondral bone plate (flattening or focal depression) in the weight-bearing area of the involved condyle 

Treatment and prognosis

Prognosis varies from complete recovery to total joint collapse ². Treatment can either be operative or nonoperativenon-operative, with initial treatment often conservative and consisting of analgesia and protected weight bearing. Subchondral hypointense fracture lines tend to resolve with conservative therapy. In more advanced cases, subchondroplasty (where a bone substitute is injected) may be considered.

Differential diagnosis

Possible considerations include:

• osteochondritis dissecans of the knee

History and etymology

It was first systematically described by Ahlback in 1968 ²