Ostéonécrose spontanée du genou : le résultat d’une fracture sous-chondrale ?

Abstract

The term osteonecrosis of the knee refers to three entities: spontaneous osteonecrosis of the knee (SONK), post-meniscectomy osteonecrosis, which shares most characteristics with SONK and so-called secondary osteonecroses, which are the counterpart at the knee of femoral head osteonecroses. SONK clinical and radiographic signs are well known since their original report in 1968. Bone scintigraphy shows a hot spot at the site of the lesion (i.e. the medial femoral condyle in most cases). At MRI, SONK appears as a subchondral band of low signal on both T1- and T2-sequences surrounded by an area of low signal on T1-sequences and high signal on T2-sequences (so-called bone marrow edema). Importantly these signs at scintigraphy or at MRI as well have no specificity and can also be found in subchondral insufficiency fractures or in subchondral bone marrow lesions associated with osteoarthritis. Thus, caution is mandatory when the diagnosis relies on scintigraphy or MRI only, without radiographic signs, as well in clinical practice than in the analysis of scientific literature on this topic. Rare histopathological studies have shown that osteonecrosis indeed can be found in late cases. In contrast, in less advanced cases, they show signs of subchondral fracture without osteonecrosis. This explains the numerous similarities between SONK and subchondral fractures around the knee. Subschondral fracture as the primary cause of SONK appears now largely acknowledged, but its own cause is still speculative. Excessive mechanical stress, especially in relation with meniscal tears, on a bone weakened by age appears to be the main predisposing factor, while osteoporosis or other bone diseases do not appear to be more prevalent than expected in this somewhat elderly population. Efficacy of measures such as weight-bearing avoiding or bisphosphonates in view to prevent the evolution of subchondral fractures into true osteonecrosis has not been adequately investigated.