Patients typically present in the 5th-7th decades but the condition has also been reported in young 5. Usually these lesions are sporadic, however they can be seen in the setting of osteoarthritis, collagen vascular disorders or previous trauma 3.
Clinically presentation is with painless joint swelling, frequently with an associated effusion.
The most frequent site of involvement is suprapatellar bursa of knee joint, and the disorder is usually unilateral 1-3. Occasional reports of hip, shoulder, wrist elbow are also reported. Other joint involvement is uncommon. Involvement of 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 can be associated with 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 large associated effusion dominates the film. Coexistent degenerative changes are frequently present.
Bony erosions are 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 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. 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
- gradiant echo (GE): chemical shift artefact is sometimes seen at the fat-fluid interface 6
Typically, there is frond-like proliferation of fat-containing cells. Where effusions coexist, visualisation of the fronds is improved.
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
- MRI hyopintense
synovial osteochondromatosis/synovial chondromatosis
- circumscribed loose bodies
- erosions common 6
- may calcify
pigmented villonodular synovitis (PVNS)
- MRI demonstrates low signal on T2 weighted images 6
- no fat signal
- enhancement is more conspicuous
- occasionally fluid-fluid levels are seen
- thickened synovium but no fat signal
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- 2. Giant synovial lipoma arborescence of the right knee in a 76-year-old diabetic woman with purulent joint effusion. Çukur S, Belenli OK, Yücel I, Yazici B. Aegean Pathology Society, APJ, 3, 10–13, 2006.
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- 8. Coll JP, Ragsdale BD, Chow B et-al. Best cases from the AFIP: lipoma arborescens of the knees in a patient with rheumatoid arthritis. Radiographics. 2011;31 (2): 333-7. doi:10.1148/rg.312095209 - Pubmed citation
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- 10. Sanamandra SK, Ong KO. Lipoma arborescens. Singapore Med J. 2015;55 (1): 5-10. Free text at pubmed - Pubmed citation