Presentation
Six month history of intermittent pain and swelling in proximal right knee.
Patient Data
Plain radiographs of the right knee show increased soft tissue density in the suprapatellar region suggestive of a joint effusion. There is no chondrocalcinosis, erosion or loose intra-articular body seen.
A longitudinal image from an ultrasound of the right knee shows a large, hypoechoic fluid collection containing frond like, hyperechoic soft tissue masses.
Large right knee effusion with an intra-articular frond-like mass of soft tissue that appears to emanate from the posteromedial aspect of the synovium. The frond-like portions of this mass are relatively low on T2 signal and high on T1 signal. The fronds of this mass enhance in a striking pattern with intense peripheral surface enhancement and little, if any, central enhancement of the individual papules covering the surface of the soft tissue mass. There is no blooming artifact on the gradient echo sequences.
The soft tissue mass surrounds the posterior cruciate ligament and distends the posterior aspect of the knee joint. The lateral synovium of the distended right knee joint enhances brightly and has a smooth, non-nodular contour. There are no articular erosions. The menisci, cruciate ligaments, and collateral ligaments are grossly normal in appearance.
Case Discussion
This is an example of lipoma arborescens, a non-infectious synovial proliferation, occurring in the pediatric population. Originally described by Hoffa, the macrospic frondlike appearance was felt to resemble a tree in leaf; hence, the Latin term arborescens (meaning “tree-forming” or “treelike”)1.
The knee is the most commonly involved joint. MRI is the modality of choice for diagnosis; the frond-like lesion follows the signal intensity of fat on all sequences. Synovectomy is the treatment of choice.
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