KNEE

Spontaneous osteonecrosis of the knee

Radiographically this is very difficult to differentiate from osteochondritis dissecans. Spontaneous osteonecrosis of the knee (SONK) is characterized by an immediate onset of pain in the knee which is almost always confined to the medial aspect of the joint. The typical patient is an older female with pain, tenderness, swelling, and restricted motion. It is not uncommon for the initial radiographs to be normal and a period of weeks or months may pass before the subtle flattening of the weight bearing articular surface of the medial femoral condyle is visualized. After a subtle flattening occurs, a radiolucent defect in the condyle is detected and often times within this lucent area a radiodense line, which consists of cartilage and subchondral bone, can be frequently identified. If no opacity is seen, often times the osseous fragment may be visualized with magnetic resonance imaging. Typically, if this is not treated, progression will occur with the collapse and fragmentation of the weight bearing surface of the medial femoral condyle leading to quite advanced secondary degenerative joint disease in this region. Often times, the secondary degenerative joint disease can become so prominent that it obscures the underlying osteonecrosis making this diagnosis difficult to make over time. Secondary progressive varus deformity can also occur as the rapid joint space narrowing occurs at the medial joint margin.

The precise location of SONK is the weight bearing surface of the medial femoral condyle in contrast to osteochondritis dissecans which is typically present at the lateral aspect of the medial femoral condyle at a non weight bearing portion of the articular margin.

SONK

Classically, osteochondritis dissecans occurs in adolescence with a rotational type of shearing trauma to the knee. Osteochondritis dissecans will lead to intraarticular cartilaginous loose bodies over time which can lead to significant symptoms such as joint locking. Since spontaneous osteonecrosis of the knee is not visualized after the initial trauma, it seems appropriate that with the classic clinical findings, advanced imaging should be considered to make the appropriate diagnosis in the early stages of true osteonecrosis in this region. In this regard, scintigraphic examination using bone-seeking radio pharmaceutical agents has shown promise in discovering spontaneous osteonecrosis about the knee when radiographic findings are not visualized. The problem with bone scanning is this is quite sensitive but not specific for a specific cause of increased bone turnover in this region. This also does not reveal underlying ligamentous instability and \ or associated mensal tears which seem to be extremely common at the medial aspect of the joint in patient's with SONK. MR imaging is probably the most appropriate follow-up for spontaneous osteonecrosis of the knee.

MRI - T1 Weighted

Classically, low signal intensity in the affected region on T1 weighted images and surrounding high signal intensity on T2 weighted images, likely representing bone marrow edema, is characteristic. Other findings on MRI include the presence of cystic lesions, bone collapse, and, quite interestingly, tears or degeneration of the adjacent meniscus. MRI can also give an accurate measurement of the size of the lesion which may have prognostic significance in that larger legions are typically associated with the poor clinical outcome. MRI can also identify the separate cartilaginous and/or osseous fragments lying in situ within the defect, slightly displaced, or even free floating within the articular cavity.

The etiology of spontaneous osteonecrosis of the knee is still not extremely clear. Typically there is at least minimal traumatic insult which probably produced micro factors in the subchondral bone plate which can lead to overlying osseous and cartilaginous collapse. Another interesting correlation is the association with meniscal injury on the same side as the spontaneous osteonecrosis of the knee. Meniscal tears have been questioned in the pathogenesis of SONK and the impact of the articular surface against a fragmented meniscus during activity could result in the initial ischemic event at the medial femoral condyle. It is uncertain whether the sudden onset of pain which is characteristic of SONK could also be related to an acute tear of the meniscus in some cases.