A rare case of an intra-articular true lipoma of the ankle.

Background: Most of the foot and ankle soft tissue tumors are benign. Although lipomas are the most common soft tissue tumors, intra-articular lipomas are an extremely rare entity. The majority of described cases of intra-articular lipomas occur in the knee joint. In particular, an intra-articular true lipoma of the ankle has not been reported.

Case Presentation: We report the case of a 45-year-old woman who presented with a progressively growing ankle mass over 5 years, which started to restrict the range of motion. It was not preceded by any trauma or sports activity. On examination, a non-tender firm mass was evident on the lateral aspect of the right ankle joint with no signs of inflammation. It was non-compressible, non-mobile, and did not transilluminate. The clinical diagnosis was probable for a soft tissue ganglion. A radiograph revealed a soft tissue opacity over the anterolateral aspect of the tibiotalar joint. Magnetic resonance imaging (MRI) demonstrated a well-defined, multilobulated, encapsulated lesion located at the lateral aspect of the tibiotalar joint; with intra- and extra-articular components and an analogous signal intensity to fat. The patient underwent surgical excision of the tumor, and the histopathological examination showed mature adipose tissue representing an intra-articular lipoma. At the follow-up visit, the patient had complete resolution of symptoms and no active complaints.

Conclusion: An intra-articular true lipoma of the ankle is an extremely rare tumor. MRI is an invaluable diagnostic tool to make a reliable diagnosis of intra-articular masses.