Imaging of early stages of osteonecrosis of the knee

It is important to consider SONK or SON in the differential diagnosis in patients with acute knee pain. Early-stage SONK is difficult to diagnose because of pitfalls in the clinical setting: (1) conventional radiographs are normal at initial presentation; (2) pain over the medial condyle can mimic meniscus symptoms; (3) the incidence of a concomitant medial meniscus tear is high in the elderly; and (4) there is a diagnostic window for the MRI to detect osteonecrosis. The differential diagnosis of evolving but undiagnosed SONK must be considered in elderly patients who present with meniscuslike symptoms with or without a concomitant meniscal tear. In these cases, early imaging of evolving SONK could prevent a presumably unnecessary arthroscopic surgery. Recently, MRI criteria on T2-weighted images were noted that allow differentiation between transient epiphyseal lesions and early irreversible SONK [17]. In cases with suspected SON, MRI and bone scintigraphy seem to be valuable tools to establish a diagnosis. A demarcation rim seems to be indicative of SON rather than SONK. There are no pathognomonic MRI changes indicative of early-stage SON so far. A careful history should be taken to look for risk factors for SON.