A true intra-articular lipoma of the knee in a girl.

A 16-year-old girl presented with a soft-tissue mass in the anterolateral aspect of her right knee. Magnetic resonance imaging revealed an intra-articular tumor arising from the anterior fat pad of the knee. The tumor showed low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. It was resected under arthroscopic guidance. Grossly, the tumor was composed of soft, yellowish adipose tissue. Pathologic examination revealed a collection of mature adipocytes with marked myxoid changes in the matrix. A diagnosis of lipoma was made. True intra-articular lipomas should be distinguished from lipoma arborescens, which is considered to be a reactive process. In the English language literature, we found only 7 cases of true intra-articular lipoma reported previously. The present case is the youngest patient with such a tumor, and all the previous reports are of the tumor in adult patients. Intra-articular lipoma should be considered in the differential diagnosis of intra-articular masses in adolescents.