Patients typically present in the 5th-7th decades, but the condition has also been reported in the young 5. Usually these lesions are sporadic, however, they can be seen in the setting of osteoarthritis, collagen vascular disorders, or previous trauma 3.
The clinical presentation is of joint swelling, variable arthralgia, and frequently with an associated effusion 8.
The knee is by far the commonest affected joint (particularly at the suprapatellar bursa), and involvement is usually unilateral 1-3. Occasional reports of hip, shoulder, wrist elbow are also reported. Other joint involvement is uncommon. Involvement of the tendon sheath is even rarer.
The normal synovium is replaced by hypertrophied villi demonstrating marked deposition of mature lipocytes within them 4,5.
Many cases have associated pathologies to varying degrees. Described associated pathology in the knee include 9:
- joint effusion: very common
- degenerative changes: common
- meniscal tears: common
- synovial cysts: uncommon
- bone erosions: uncommon
- chondromatosis: uncommon to rare
- patellar subluxation: rare
- discoid meniscus: rare
Occasionally plain films are able to detect fatty lucencies within a soft tissue lesion, although usually, the largely associated effusion dominates the film. Coexistent degenerative changes are frequently present. Osseous erosion is uncommon 4.
If performed, ultrasound will demonstrate a joint effusion with echogenic 'frond like' projections into the effusion.
CT is able to demonstrate a low density intra-articular mass. As the joint fluid is volume-averaged with the lesion, it is of higher density than fat, but lower than water. Little if any enhancement is seen 6.
MRI is the modality of choice for diagnosis. A typical appearance is of a fat-containing frond-like synovial mass, usually outlined by concurrent joint effusion. The lesion follows the signal intensity of fat on all sequences 4,5.
- T1: high signal; will saturate on fat suppressed sequences
- T2: high signal; will saturate on fat suppressed sequences
- gradient echo (GE): chemical shift artefact is sometimes seen at the fat-fluid interface 6
Treatment and prognosis
The condition is benign and is cured by synovectomy.
Recurrence is uncommon 5.
History and etymology
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”) 7,8.
General imaging differential considerations include
- often calcified (hypointense on all MRI sequences)
synovial osteochondromatosis/synovial chondromatosis
- circumscribed loose bodies
- osseous erosion is common 6
- may calcify
pigmented villonodular synovitis (PVNS)
- low signal on T2 weighted MRI 6
- no fat signal
- enhancement is more conspicuous
- occasional fluid-fluid levels
- thickened synovium, without fat signal
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