Teaching Case

A 44-year-old male patient presented with chronic swelling along the anterior aspect of the right knee for at least 5 or 6 years. He had a history of trauma 10 years back. There was no associated pain, but a decreased range of motion was found on physical examination.

The X-ray showed right knee swelling, involving mainly the suprapatellar region with decreased opacity and lateral displacement of the patella (Figures 1a and 1b).

The MRI revealed moderate joint effusion with marked distension of the joint capsule due to the presence of diffuse vegetations with high T1 signal (Figure 2), low signal on T2* (Figure 3) and high PD signal that becomes low signal on fat-suppressed (FS) sequences (Figures 4a, 4b, 4c and 4d). There is also an extension of the effusion into a Baker’s cyst measuring approximately 63 x 23 x 14 mm (Figures 5a and 5b).

Background

Lipoma arborescens is a rare, benign intra-articular lesion characterised by villous proliferation of the synovial membrane and replacement of the subsynovial tissue by mature fat cells. This condition typically affects the knee joint but can occur in other joints, such as the shoulder, elbow, wrist, hip, and ankle [1,2].

Pathophysiology

The condition involves the transformation of normal synovial tissue into hypertrophied villi with significant fat deposition. The exact cause of lipoma arborescens is unclear, but it may be linked to chronic inflammation, trauma, or degenerative joint disease [2,3]. Recent studies identify two forms: primary and secondary. The secondary form is the most common and is typically associated with degenerative joint pathology or meniscopathy, often occurring in older patients. In contrast, primary lipoma arborescens, characterised by exuberant hypertrophied villi, typically occurs in younger adults without other joint changes [4]. In our case, despite a history of knee trauma, there were no signs of old fractures or degenerative arthritis. The extensive hypertrophied villi suggest a primary pathology.

Clinical Perspective

Patients often present with chronic joint swelling, variable arthralgia, and decreased range of motion. The symptoms can mimic those of other joint diseases, such as arthritis or synovitis [5,6].

Imaging Perspective

Plain radiograph – Sometimes can detect fatty lucencies within a soft tissue lesion, but they are usually overshadowed by the associated effusion. Degenerative changes are often present, while osseous erosions are rare [2].

Ultrasound – Typically shows a joint effusion with echogenic “frond-like” projections.

CT – It reveals a low-density intra-articular mass, indicating fat, with minimal or no enhancement [3].

MRI – MRI is the most effective imaging modality for diagnosing lipoma arborescens. It shows characteristic features such as:

• High signal intensity on T1-weighted images due to the presence of fat.
• Decreased signal on fat-suppressed sequences, confirming the fatty nature of the lesion.
• Villous projections extending into the joint space, resembling a “frond-like” or “tree-like” appearance [1,2,7].

Outcome

Conservative Management – Initial treatment may include anti-inflammatory medications and physical therapy to manage symptoms.

Surgical Intervention – Arthroscopic synovectomy or open synovectomy is often required to remove the fatty villous proliferation. This is particularly necessary if the condition causes significant symptoms or functional impairment.

Prognosis – After surgical treatment, it is generally good, with most patients experiencing relief from symptoms. Recurrence is uncommon [5].

Take Home Message

Lipoma arborescens is a rare, benign intra-articular condition characterised by fatty villous proliferation within the synovial membrane, typically affecting the knee. It presents with chronic joint swelling and possible pain, often resembling arthritis. MRI is the diagnostic gold standard, revealing high T1 signal and low signal on fat-suppressed sequences. Surgical removal is often necessary for lasting relief.