Spontaneous Osteonecrosis of the Knee
Traditionally described as Ahlbäck’s disease, spontaneous osteonecrosis of the knee (SONK) is characterized by sudden onset of severe, often-disabling pain without a remembered causative event in elderly female patients. In most of the cases, the weight-bearing portions of the medial femoral condyle are affected. Radiography reveals well-defined subchondral radiolucency, subsequent flatoftening of the femoral condyle, increasing sclerosis, and associated degenerative change. Often there is an accompanying meniscal lesion.
MRI contributes to early diagnosis and to defining the extent of the lesion (Bjorkengren et al. 1990). In T1-weighted sequences, there is a band or oval-shaped area of subchondral low signal intensity. STIR/T2-weighted sequences show surrounding, often extensive, edema, sometimes even involving the soft tissues. Recently, the spontaneous nature of the lesion has been questioned and there is evidence that the initial event in Ahlbäck’s disease is instead a stress fracture (Zanetti 2003; Yao 2004; Sokoloff 2001; Yamamoto and Bullough 2000).