Lipoma arborescens of the knee joint

Clinical History

A 63-year-old man presented with a 2-years history of progressively increasing left-sided knee pain. He reported no trauma. Physical examination of the lower extremity revealed no skin abnormalities, but painless swelling. Based on a suspicion of meniscus pathology, radiographs and magnetic resonance imaging (MRI) were ordered.

Imaging Findings

Imaging studies included radiographs and MRI.
Frontal and lateral radiographs show findings of Osgood-Schlatter disease, osteoarthritis, and a soft tissue swelling in the suprapatellar pouch.
MRI imaging shows intraarticular mass-like synovial proliferation with numerous frond-like projections occupying the suprapatellar pouch, with signal corresponding to fat on T1 and fat-sat proton-density fast spin-echo (PD FSE). Large osteophytes, cartilage loss, meniscal tear and synovitis consistent with osteoarthritis were also seen.

Discussion

Lipoma arborescens is a rare, benign intraarticular lesion in which there is a lipomatous tissue proliferation characterized by replacement of the subsynovial tissue by mature adipocytes, giving rise to prominent intra-articular villous proliferation of the synovium [1]. It may result from a chronic inflammatory synovitis with resultant hyperplasia of the fatty subsynovial tissue, but often there is no recognized history of arthropathy. In fact, the exact aetiology of lipoma arborescens remains unclear and most of the cases arise de novo [1].

The knee joint is most commonly affected and especially the suprapatellar pouch [2], however, other joints can be involved too [1]. It commonly affects a single joint but some cases with polyarticular involvement were also reported [1, 2].
Most of the cases affect people between 50 and 70 years [1].
Clinically, presentation is with painless joint swelling, frequently with an associated effusion.
Laboratory test (ESR, rheumatoid factor and uric levels) are not altered, joint aspirate is usually negative for crystals and cells and culture for joint fluid is also negative [3].

Plain radiograph usually demonstrates chronic changes due to ostheoartritis. We can also see swelling and opacification of the suprapatellar pouch, data that are suggestive of effusion, although occasionally it is able to detect fatty lucencies. These findings are more evident on lateral plain film.

Ultrasound imaging findings can be joint effusion with echogenic frond-like excrescences into the effusion.

CT of a lipoma arborescens will show fatty infiltration of the joint space.
MRI is the diagnostic modality of choice to identify frond-like fatty projections [1, 3]. The lesion follows the signal intensity of fat on all sequences (T1 and T2: high signal; will saturate on fat-suppressed sequences). Where effusions coexist, visualisation of the fronds is improved. Even though intravenous contrast MRI is not necessary to identify the characteristic frond-like fatty projections of this tumour, it is useful to show sinovial proliferation. Bony erosions are uncommon [4]. There are some MRI findings that are pathognomonic for lipoma arborescens (Take home message), like the presence of a synovial mass with a frond-like architecture, which exhibits fat signal intensity on all pulse sequences. It also shows suppression of signal with fat-selective presaturation and associated joint effusion. It is also important to note the absence of magnetic susceptibility effects from haemosiderin, a typical characteristic of pigmented villonodular synovitis [3].

Open or arthroscopic synovectomy with excision of the lesion is the treatment of choice. Recurrence of the lesion is uncommon [5].

Differential Diagnosis List

Final Diagnosis

Lipoma arborescens

URL:	https://www.eurorad.org/case/12754
DOI:	10.1594/EURORAD/CASE.12754
ISSN:	1563-4086