Synovial Fat Deposition of the Knee Is Associated with Degenerative Joint Disorder

Synovial fat deposition, also known as lipoma arborescens, is a rare articular disorder with villous synovial proliferation, commonly seen in the knee. We explored the relationship between the degree of synovial fat deposition on the magnetic resonance imaging (MRI) and the severity of degenerative joint disorder, also called osteoarthritis, on plain radiography. The enrolled patients underwent MRI with a 0.4T permanent magnetic unit in a single institution over a 9-month period. The indications of MRI were chronic knee disorder of non-specific cause. Patients with minor trauma were also included. Consecutive 1,091 knees of 1,075 patients were assessed for the degree of synovial fat deposition on MRI and the severity of degenerative joint disorder on plain radiography. The degenerative joint disorder was graded by radiographic features obtained within one month from MRI using Kellgrene-Lawrence (K-L) scores. MRI features of synovial fat deposition were classified as none, mild and severe. Synovial fat deposition was identified in 30 knees of 29 patients (2.7%) (11 men and 18 women; aged from 25 to 86 years, one patient with bilateral lesions): one female patient with osteoarthritis secondary to rheumatoid arthritis and 28 patients with degenerative joint disorder. The K-L grade was 4 in the case of rheumatoid arthritis. There was a moderate positive correlation between the K-L grade and fat deposition grade (correlation coefficient: 0.59, p < 0.001). Thus, synovial fat deposition was noted in the advanced degenerative joint disorder. We propose that fat deposition represents a nonspecific secondary phenomenon of degenerative joint disorder.