Teaching Case

A 51-year-old male presented with a three-year history of left knee pain and sensation of knee instability. Physical examination revealed joint effusion and pain in the medial and lateral compartments of the left knee. There was no history of trauma.

A plain radiograph of the left knee revealed degenerative changes in the medial femorotibial and patellofemoral compartments, evidenced by joint space narrowing, marginal osteophyte formation, and a subchondral cyst in the patella. Furthermore, in the lateral view, there was an opacification in the suprapatellar pouch of the knee (Figures 1a and 1b).

Magnetic resonance imaging (MRI) showed a large-volume joint effusion, with frond-like diffuse synovial thickening, most prominently in the suprapatellar pouch (Figures 2a, 2b and 2c). The thickened synovium demonstrated a signal pattern consistent with fat, exhibiting high signal intensity on T1-weighted sequences and low signal intensity on fat-saturated sequences (Figures 2a, 2b, 2c and 3). Additionally, medial and lateral meniscus tears were identified (Figures 4a, 4b and 4c).

Background

Lipoma arborescens (LA) is a rare benign disorder characterised by the proliferation of mature adipocytes within the synovial membrane [1]. This condition usually affects a single joint, most commonly the knee joint, specifically the suprapatellar pouch [2]. However, it has also been documented in other joints, such as the shoulder, hip, elbow, and wrist [3].

The aetiology of LA is not completely understood, but current theories suggest a reactive fatty proliferation of the synovium in response to degenerative or inflammatory arthritis [4,5].

Clinical Perspective

LA is most commonly diagnosed in the fifth to seventh decades of life, although it can occur in younger people with arthritis [6]. Patients usually present with insidious and painless swelling of the affected joint, with intermittent joint effusion [1,7]. Due to its nonspecific clinical presentation, imaging is crucial for accurate diagnosis [8].

Imaging Perspective

Plain radiograph is usually the first image modality to evaluate musculoskeletal pathology. However, in the case of LA, the findings are unspecific. Osteoarthritic changes, which include joint space narrowing, osteophytes, subchondral sclerosis, and subchondral bone cyst formation, are the most common associated imaging findings. Regarding the knee joint, a soft-tissue opacity in the suprapatellar pouch is frequently seen [3].

MRI is the imaging modality of choice for diagnosing LA due to its superior contrast resolution and ability to characterise fatty tissues [5,8]. MRI shows frond-like synovial projections with fat signal intensity on all sequences, namely high signal intensity on T1 and T2-weighted images, which suppress on fat-saturation sequences [2,5]. Following gadolinium administration, peripheral enhancement is observed, corresponding to the thickened synovium, while the subsynovial fatty tissue shows no enhancement. Frequently associated imaging findings include joint effusion and degenerative joint changes, as well as meniscal tears in the knee [5].

There are no magnetic susceptibility artefacts from haemosiderin deposition, which is a typical feature of tenosynovial giant cell tumours [7]. The presence of multiple intra-articular cartilaginous loose bodies, exhibiting varying degrees of calcification and ossification, is more suggestive of synovial chondromatosis [2]. Rheumatoid arthritis shows diffuse synovial thickening and enhancement but lacks the characteristic fat signal intensity of LA [9].

Outcome

The primary goals of the treatment are symptomatic relief and prevention of joint damage. Intra-articular injection of corticosteroids can be used to manage a symptomatic LA. However, synovectomy is the gold standard treatment, allowing the removal of the lipomatous tissue and the improvement of joint function [1]. Follow-up is typically advised to monitor for potential recurrence, which is rare [5].

Take Home Message

• Lipoma arborescens (LA) is characterised by the proliferation of mature adipocytes within the synovial membrane, most commonly affecting the knee joint.
• MRI is the preferred imaging modality for accurate diagnosis, showing characteristic frond-like synovial projections with fat signal intensity.
• Synovectomy is the treatment of choice, with patients typically experiencing low recurrence rates.

Written informed patient consent for publication has been obtained.