These images show a low signal intensity defect in the medial talar dome. This represents an osteochondral fracture in the talus. The etiology is unknown in this case, but the most common causes are either trauma or osteonecrosis.
The term commonly applied to this entity is osteochondritis dissecans. In my opinion, this is a term to avoid, since it implies absolutely no information about the actual pathology or etiology of this lesion. In many cases, one can find an antecendent history of trauma or of some condition predisposing the patient to osteonecrosis. However, in many other cases, such antecedent causal information is absent.
MR is quite sensitive for demonstrating osteochondral fractures such as this. It may also be useful in prognosis of the lesion. One of the clinical concerns with such a lesion is whether it is viable or likely to become a devascularized intra-articular fragment. MR may be able to help with this question. On the proton-weighted image (SE 2000/17 msec), one can see a small collection of high signal fluid beneath the fragment. The remainder of the boundary around the fragment appears to be fluid-free, suggesting that this fracture may not be complete, or the fragment may be solidly anchored in its donor site by fibrous tissue. On the other hand, if there were a significant amount of fluid around the margin of the fragment, it would suggest that the fragment might be devascularized, and might become a loose intra-articular fragment.