MR imaging of the knee: can changes in the intracapsular fat pads be used as a sign of synovial proliferation in the presence of an effusion?

Abstract

Synovial proliferation is difficult to detect on MR images when joint effusions are present. The objective of this study was to assess the efficacy of MR changes in intracapsular fat pads (prefemoral, quadriceps, Hoffa's) as a sign of synovial proliferation in patients with knee effusions. MR images were obtained in 70 patients with knee effusions. An effusion was considered present if the suprapatellar bursa was distended 1 cm in anterior to posterior diameter on T2-weighted sagittal images. The experimental group consisted of 32 patients with proliferative effusions (effusions associated with synovial proliferation) diagnosed on the basis of histologic or microbiologic data or strict rheumatologic criteria. This group comprised 11 patients with pigmented villonodular synovitis, six with rheumatoid arthritis, one with psoriatic arthritis, three with hemosideric arthritis, and 11 with septic arthritis. The control group comprised 38 subjects with knee effusions who had no arthroscopic evidence of synovial proliferation. After determining the MR criteria of proliferative effusion, two observers who had no knowledge of these cases evaluated abnormalities of the intracapsular fat pads seen on sagittal intermediate-weighted midline MR images. Characteristic changes in the fat pads were noted on MR images in patients with proliferative synovial effusions. Scalloping or truncation of the prefemoral fat pad was 77% sensitive and 95% specific as a predictor of proliferative effusion. Defects and displacement of Hoffa's fat pad had a sensitivity of 57% and a specificity of 99%, and nonvisualization or irregular margins of the quadriceps fat pad had a sensitivity of 61% and a specificity of 100%. Characteristic changes in intracapsular fat pads seen on MR images of patients with proliferative effusions can help distinguish these patients from those with effusions without synovial proliferation.