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Lipoma arborescens is a benign, diffuse villous proliferation of the synovium characterized by replacement of the subsynovial tissue by mature adipocytes. Its etiology is unknown and fewer than 100 cases have been reported. It resembles other collections of subsynovial fat, the only difference being its large size and villous macroscopic appearance. It typically presents in patients in their fifth through seventh decades of life. It is most commonly monoarticular and most frequently affects the suprapatellar pouch of the knee.1 There have been reports of involvement of the hip, shoulder, wrist, elbow, ankle, and associated bursae.2,3,4,5,6,7 To our knowledge there have been only three previous cases of lipoma arborescens of the subdeltoid bursa in the literature.8,9,10
We report on a case of unilateral lipoma arborescens of the subdeltoid bursa in an elderly patient presenting as a shoulder mass.
An 80-year-old male patient presented for evaluation of a left upper extremity mass at his shoulder. He had noticed the mass for the previous six weeks. He only had modest pain in the shoulder. The pain was mild in nature and was intermittent. He was able to continue all of his activities of daily living without any restrictions. It was in his nondominant upper extremity. He presented for the sole reason of work up of the mass and not any associated symptoms such as pain or weakness. He had previously seen an outside physician who attempted an aspiration of the mass, which had only revealed blood.
His medical history was significant for a pulmonary embolism for which he was taking warfarin.
Physical examination revealed a soft, palpable, “rac-quetball sized” lesion anterior to his left shoulder. It was nontender. He had full active range of motion of his left shoulder. He was able to put his hand behind his back and reach the top of his head. He did not have any pain with range of motion.
The plain x-rays of his shoulder showed no evidence of fracture, dislocation, nor mass lesion. There were degenerative changes of the acromioclavicular joint noted.
MRI of his left shoulder was performed to further evaluate the mass. The MRI showed a large amount of fluid within the subacromial-subdeltoid bursa. The mass measured 1.2 cm × 3.1 cm and demonstrated a frond-like excrescent appearance. It was hyperintense to muscle and isointense to fat on T1, intermediate signal intensity on T2, and suppresses on the T2 fat-sat images. It was read by our musculoskeletal radiologists as being consistent with lipoma arborescens.
He was also noted to have tendinosis of the supraspi-natus tendon with bursal and articular surface partial tearing involving less than 50% of the tendon without evidence of complete tear.
The diagnosis of lipoma arborescens was discussed with the patient. The option of arthroscopic biopsy and synovectomy was discussed. Given that the patient was asymptomatic he preferred to continue on a course of observation of the mass.
He returned to clinic two months later. He continued to have no pain and no change in the size of the mass. A repeat MRI was performed which showed stable appearance of the mass and was again read as being consistent with lipoma arborescens. Given that the mass was stable and the patient had very minor symptoms in his shoulder he again did not wish to undergo any surgical procedures and wished to continue further conservative treatment. He was scheduled for follow-up in approximately one year.
The etiology of lipoma arborescens is unknown. It has previously been described with degenerative joint disease and chronic rheumatoid arthritis.11,12 Clinically it usually presents as joint swelling, pain, limitations in range of motion, and recurrent effusions.
This case is unique in that the patient's chief complaint was an anterior shoulder mass and not pain or restriction of motion.
The MRI characteristics of lipoma arborescens have previously been described by Vilanova et al.13 There is a morphologic pattern of intra-articular fat deposits with a villous proliferation of the synovial membrane. Subsynovial components have high signal intensity similar to subcutaneous fat on Tl and T2-weighted images and low signal on fat-suppressed images. It does not enhance with gadolinium. In their series all cases of lipoma arborescens of the shoulder had an associated rotator cuff tear. Our patient was also found to have a partial thickness tear of the supraspinatous.
Histologic findings of lipoma arborescens show finger-like villi infiltrated by mature fat tissue, chronic inflammatory cells, and vessels, and lined by synovium.11 The differential diagnosis of lipoma arborescens includes other benign synovial disorders. Other disorders included in the differential diagnosis with lipoma arborescens are pigmented villonodular synovitis (PVNS), synovial hemangiomas, synovial lipoma, and synovial chondro-matosis. These other diagnoses can usual be excluded based on MRI findings. A synovial lipoma is a discrete round or oval mass isointense with fat on all sequences, whereas lipoma arborescens has a villous appearance. PVNS is low intensity on Tl and T2-weighted images and enhances with gadolinium. Synovial hemangioma is low signal on Tl-weighted images and high signal on T2-weighted images and characteristic hypointense linear fibrous septa. Synovial chondromatosis is associated with loose body formation.1
Treatment of lipoma arborescens is open or ar-throscopic synovectomy. Recurrence of the lesions following synovectomy is uncommon. In our case given the patient's age and lack of symptoms he elected to continue observation of the lesion. If it does become more painful, our recommendation would be arthroscopic synovectomy with excision of the lesion.