1 Introduction

A joint's normal function depends on the intra-articular mesenchymal tissue called the synovium. Lipoma Arborescens (frond-like) is a very uncommon benign intra-articular tumour of unknown etiology that is characterized by diffuse villous proliferation of the synovium and replacement of the sub-synovial tissue by mature adipocytes.¹ It was originally described by Albert Hoffa in 1904 and later was called with various terminologies such as “diffuse articular lipomatosis”, “villous lipomatous proliferation of synovial membrane” or “diffuse lipoma of joint”.² They are commonly diagnosed among individuals between 4th-6th decades with equal gender prevalence.³ The knee is the most frequently involved, but other joints such as the elbow, wrist, hip, and ankle can also be affected. Although it has been linked to traumatic joint injury, osteoarthritis, psoriatic arthritis, rheumatoid arthritis, collagen vascular disorders, septic arthritis, obesity, and diabetes mellitus; a clear etiology has not been established due to the low incidence. We report a case of lipoma arboroscens of the knee in a patient with a history of HIV infection.

2 Case report

A 46-year male came for evaluation with a 2-year history of progressive left knee swelling which was associated with mild intermittent pain for the last 6 months only during moderate activities prompting him to seek treatment.

3 Clinical findings

A fluctuant swelling of 20 ​× ​12 ​cm was palpated over the anteromedial aspect just above the knee joint (Fig. 1a). Tenderness was present in the medial joint space, the range of motion restricted to 0–90°, special tests being normal and with intact distal-neovascularity. The patient denied any history of previous trauma but was diagnosed with retroviral disease (HIV) for 10 years and was on regular antiretroviral therapy. There were no other significant associations.

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Fig. 1a showing left knee swelling; 1b: showing knee radiograph with medial compartment osteoarthrosis with mild soft tissue density in the suprapatellar pouch.

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4 Diagnostic assessment

The left knee radiograph showed mild medial compartment osteoarthrosis with mild soft tissue density in the suprapatellar pouch (Fig. 1b). Blood investigations were normal. Synovial fluid analysis showed an inflammatory picture with WBCs >20,000 count/c mm with lymphocytic predominance. MR imaging showed frond-like altered signal intensity along the synovial outpouching extending from the suprapatellar pouch encroaching up to the popliteal fossa (Fig. 2a–d). A synovial biopsy was inconclusive.

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Fig. 2a–d T2-weighted sagittal and coronal sections in MR imaging showing multiple frond like altered signal intensity lesions along the synovium.

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DIFFERENTIAL DIAGNOSIS (if any).

5 Therapeutic intervention

The patient underwent an open total synovectomy. Intraoperatively we found multiple yellowish-brownish polypoidal projections ranging from 0.3 to 0.5 ​cm with adjacent irregularities in the suprapatellar pouch (Fig. 3a&3b). The lesions also involved around the cruciate ligaments. Histopathological specimen revealed hyperplastic synovial tissue with synovial lining, with sub-synovial tissue showing lobules of mature adipocytes with dense chronic inflammatory infiltrates containing lymphocytes, eosinophils, and plasma cells suggestive of lipoma arborecens (Fig. 4a–f).

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Fig. 3a,b showing intraoperative images with frond like multiple lobulated projections in knee.

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Fig. 4a Gross photograph showing yellow frond like polypoidal projections from the synovectomy specimen from knee joint. 3b,3c: Microphotograph show polypoidal structures with synovial lining with subsynovial tissue showing lobules of mature adipocytes with intervening blood vessels. 3d–f: Multiple areas show small ill-defined epithelioid granulomas with langhans type giant cells surrounded by lymphocytes. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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6 FOLLOW-UP and outcomes

Post-operatively patient was started on full weight bearing as tolerated. At one-year and two-year follow-up patient was asymptomatic though repeat MR imaging could not be done to evaluate for recurrence due to the patient's financial constraints (Fig. 5a–e).

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Fig. 5a postoperative scar with no knee effusion; 5b–d: 2 year postoperative following with near normal range of movements; 5e: Knee radiograph with no advancement of osteoarthrosis post-surgery.

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7 Discussion

There isn't much current literature on lipoma arboroscens, and what there is mostly consists of case studies. Intriguingly, although this entity was first defined over 100 years ago, most of them have only recently been published. This might be a result of improved contemporary imaging techniques and greater awareness of this disease. Some authors have suggested two distinct categories for lipoma arborescens: an idiopathic “primary” type appearing in younger patients and a much more prevalent “secondary” type most frequently observed in elderly patients associated with arthritis.⁴ Patients with lipoma arborescens often have a gradually worsening swelling of the affected joint, which may also be accompanied by effusion, a restricted range of motion, and pain. Laboratory tests are typically normal. The various differential diagnosis with their characteristic features is described in Table 1. MR imaging narrow downs the possible diagnosis. MRI identifies hyperintense thickened synovium with villous configuration on T1-weighted images and suppression of fatty synovium of fat-saturated proton density imaging.⁵ Synovial reaction brought on by trauma has been hypothesized, but neither our patient nor the majority of lipoma arborescens patients had a history of trauma.³ Additionally, it has been suggested in a study that synovial lipomatosis may be caused by excessive fat deposition or fatty changes brought on by obesity. It was determined that an abnormally high body mass index (BMI) could contribute to abnormal fat deposition and the ensuing lipoma arborescens.² Another hypothesis of lipoma arborescens is a reaction to chronic inflammation of the underlying synovial membrane; which is usually associated with various inflammatory conditions.⁶ Recently there is increasing evidence that systemic immune activation plays an important role in leading to the pathogenesis of acquired immunodeficiency syndrome which in turn causes chronic inflammation.⁷ Though we did not find any association of lipoma arborescens with HIV infection in any of the previous literature; we believe that since our patient has HIV infection, chronic inflammation of synovium would have led to the occurrence of lipoma arborescens. It is also believed that untreated lipoma arborescens could lead to secondary osteoarthritis. Natera et al. in their study concludes by prompt synovectomy, the progressive joint disease can be delayed.⁸ In our case, we found that patient had an early arthritic knee. It has been speculated that persistent synovial thickening and effusions brought on by recurrent mechanical trauma to the proliferating villi eventually result in osteoarthritis. An open or arthroscopic synovectomy is recommended for those with symptomatic lipoma arborecens. Since lipoma arborecens does not undergo complete regression synovectomy is the treatment of choice. The preference for surgical technique mainly depends on the size of the tumour and also the surgeon's preference. Since the tumour was large enough we went ahead with an open synovectomy. As a result, we conclude that lipoma arborecens is indeed a rare condition and necessitates a high index of suspicion for such a precise diagnosis. HIV infection can be one of the associations with lipoma arborecens. Early treatment is necessary to stop the condition's progression toward joint degeneration, and open synovectomy with a very low recurrence rate provides excellent functional outcomes.

FINANCIAL SUPPORT and sponsorship

Nil.

Informed consent

Written and informed consent has been signed by the patient institutional ethical committee approval. No institutional ethical committee approval was required.

Authors contribution

Ramachandra Kamath: Literature search, Manuscript editing, Guarantor. Keerthan Ranga Nayak.U: Concepts, Design, Definition of intellectual content, Manuscript review, Guarantor. Pooja K Suresh: Concepts, Design, Definition of intellectual content, Literature search, Manuscript preparation, Manuscript editing, Manuscript review, Guarantor. Prem Kotian: Literature search, Manuscript preparation, Manuscript editing, Manuscript review. Kshitiz Singh: Literature search, Manuscript preparation.