Lipoma arborescens of the knee

Clinical History

A 14-year-old woman was referred to rheumatology for polyarthralgias (lumbar, right ankle and bilateral knee pain). After the initial clinical and laboratory studies, no definite diagnosis was achieved.
The most affected joint was the left knee, with progressive swelling and pain during the past 3 months.

Imaging Findings

The initial plain film demonstrated opacification of the suprapatellar fat pad suggesting joint effusion. There were no other relevant radiographic abnormalities.
MRI showed moderate joint effusion and frond-like synovial proliferation that follows fat signal intensity in all pulse sequences. No other abnormalities were found.

Discussion

Lipoma arborescens is an idiopathic joint lesion, characterised by villous lipomatous proliferation replacing the subsynovial tissue [1]. Generally it is monoarticular and most commonly occurs in the knee, but other joints and bursas can be affected.
This condition is thought to represent a non-specific synovial reaction to trauma and inflammation. The strong association with degenerative joint disease, chronic rheumatoid arthritis and seronegative spondyloarthropaties supports this hypothesis [2].
However, in some patients (generally young) it can be lacking other joint findings. So, some authors propose that in these cases these lesions should be categorised rather as primary lipoma arborescens, as opposed to the secondary / reactive form [3].
It has a nonspecific clinical presentation. Although it can be asymptomatic, it is often associated with intermittent pain, swelling and movement range restriction.
The main differential diagnoses include pigmented villonodular synovitis, synovial chondromatosis and synovial haemangiomatosis, but the differentiation by MRI is generally easy [4].
Radiographic findings are nonspecific. It may be noted a soft-tissue density in the suprapatellar pouch. Ultrasound findings include proliferation of the synovial membrane with frond-like masses and effusion.
MRI appearance is diagnostic. Generally there is a frond-like synovial mass that has the same signal intensity of fat in all sequences. Usually, there is also joint effusion and lack of magnetic susceptibility artefacts in gradient-echo sequences.
There have been reports of lipoma arborescens appearing as a more focal pseudo-mass [2].
Generally, treatment consists of curative open or arthroscopic synovectomy. Some authors consider medical management options like intra-articular injection of radio-active compounds such as Yttrium 90 or steroids [5].

Differential Diagnosis List

Final Diagnosis

Lipoma arborescens of the knee

URL:	https://www.eurorad.org/case/10247
DOI:	10.1594/EURORAD/CASE.10247
ISSN:	1563-4086