Normal initial magnetic resonance imaging in aseptic bone necrosis of the knee.

Magnetic resonance imaging (MRI) is considered the most sensitive tool for the detection of osteonecrosis at early stages. We report about 3 patients with high suspicion of osteonecrosis at the knees in whom an initial MRI was normal, but the condition was apparent in a subsequent study. The time between the initial insult and the appearance of diagnostic alterations in MR studies remains undefined and, in patients at risk an initial MRI, if done too early, may fail to detect this condition.