Lipoma Arborescens: A classical case

27-year male presenting with a history of long-term left knee pain and swelling. On examination, knee swelling was evident without any redness or tenderness. Restricted knee movements were seen.

T1 weighted coronal images show hyper-intense tissue filling the supra-patellar bursa and extending into the medial tibial-patellar joint space. Classical frond-like projections were seen. A mild amount of hypointense effusion was seen.T2 weighted coronal images show iso to hyper-intense tissue distending the supra-patellar bursa with frond-like arms. A mild amount of hyper-intense fluid is also seen. STIR coronal and PD fat sat axial images show suppression of T1/T2 hyper-intense tissue. No significant contrast enhancement was noted in gadolinium-enhanced T1 weighted images.

Lipoma arborescens is a rare benign articular lesion arising from subsynovial villous proliferation of mature fat cells [1]. Arborescence is a Latin term for “tree-like appearance”, which classically simulates the frond-like morphology of this condition [2]. Though the exact cause is unknown, it is stated that chronic traumatic or inflammatory stimuli can trigger nonspecific reactive synovial fatty proliferation, which may lead to this condition [3]. These stimuli may be degenerative disease, trauma, meniscus injury and synovitis, usually seen in elderly patients. Idiopathic variety occurs in younger patients in the second and third decades of life [4]. The patient usually presents with insidious painless swelling for many years with intermittent episodes of joint pain and effusions [5]. Worsening of the pain and limited joint mobility may be due to the tapping of hypertrophied villi between the joint surfaces [6]. The knee joint is most commonly involved; however, the shoulder, hip, elbow, ankle and wrist, periarticular bursae and tendon sheath may also be involved [7]. Bilateral and polyarticular involvement is also common [8]. Cases have been observed in age groups between 9 and 68 years, and no gender predilection has been reported [6].

Plain radiography is of little value however may demonstrate mass-like soft tissue opacity with lucent areas of fat globules in intra or periarticular tissue. Secondary degenerative changes may also be evident. USG typically show a high echo pattern of fatty tissue with surrounding effusion. CT demonstrates heavy proliferation with frond-like projection in the bursa and periarticular tissue. MR is the investigation of choice, which shows high T1 and T2 signals in soft tissue mass with classical frond-like projections. The Signals are suppressed in STIR and fat. Sat images. Haemorrhagic products are absent on gradient imaging. No contrast enhancement of abnormal soft tissue is seen. Overlying thickened synovium may enhance. Other abnormalities can be seen, such as joint and bursa L effusions. Associated Meniscal tears and degenerative changes may be seen.

Due to its benign indolent course, no surgical treatment is required; however, surgery may be considered in complex cases with complications. An open or arthroscopic synovectomy is the procedure of choice. Rest cases may be managed by conservative treatment concentrating on underlying precipitating conditions and reducing further progression to secondary/complicated arborescence. Recurrence after surgery is uncommon [8].

Take home message

MRI is invaluable in the early diagnosis of lipoma arborescens, delineating its exact anatomical extent, identifying associated abnormalities and differentiating it from other intraarticular masses.