Intra-articular synovial lipoma of the knee joint (located in the lateral recess): a case report and review of the literature.

This report describes a rare intra-articular synovial lipoma of the knee joint which developed in a 66-year-old female. The patient suffered from sudden knee pain and a catching or slight interruption of normal motion of the left knee and then noticed the tumor. The tumor was located in the lateral recess of the knee joint and showed a signal intensity similar to subcutaneous fat on T1 and T2()-weighted magnetic resonance images. The arthroscopy revealed a smooth, globular, yellowish, encapsulated tumor extending into the lateral recess from the surface of the lateral condyle and the tumor was totally excised under arthroscopic guidance. Histologic examination of the specimen revealed a tumor composed of mature adipose cells covered by a thin fibrous layer of varying thickness and normal synovial lining cells. The diagnosis was intra-articular synovial lipoma. Intra-articular synovial lipomas should be distinguished from other similar lipomatoid conditions such as Hoffa disease and villous lipomatous proliferation of the synovial membrane (lipoma arborescens). Intra-articular synovial lipoma should be considered in the differential diagnoses when examining a patient with sudden knee pain, and a catching or locking knee.