Occult intraosseous fracture: magnetic resonance appearance versus age of injury.

Twenty-two cases of occult intraosseous fracture in the region of the knee are presented. Occult intraosseous fractures have been incidentally detected in the magnetic resonance (MR) evaluation of the knee in the setting of a recent physical injury and normal radiographic studies. There is no unique mechanism of injury, but occult intraosseous fracture presumably results from direct impaction or axial overloading. MR shows speckled or band-like areas of low signal in the intramedullary space of the epiphyses, and occasionally, the adjacent metaphyses. In most cases, T2-weighted images show high signal in corresponding regions of variable size. The relative extent of high signal findings is shown to vary significantly with the age of injury. Individual variability within groups, however, precludes the actual prediction of lesion age on the basis of the MR appearance. Our observations provide indirect evidence that the findings on T2-weighted images resolve earlier than the corresponding findings on T1-weighted proton density images. The primary differential diagnosis in cases of occult intraosseous fracture is stress fracture. The diagnosis of occult intraosseous fracture may be important in explaining persistent pain after injury in otherwise normal knees.