Lipoma Arborescens

Description

Lipoma arborescens is a rare, benign intra-articular lesion characterised by villous, frond-like proliferation of synovium that is infiltrated by mature adipocytes. The name derives from its tree-like (arborescent) morphology. It is a non-neoplastic condition, representing a reactive synovial process rather than a true tumour, and is most commonly found in the suprapatellar pouch of the knee joint.

It is strongly associated with chronic joint irritation and is often seen in the setting of osteoarthritis, rheumatoid arthritis, or prior trauma. While most cases are monoarticular, bilateral or polyarticular involvement can occur, particularly in systemic inflammatory conditions.

The typical presentation is painless joint swelling, often with recurrent joint effusions and mechanical symptoms. Early recognition is important to prevent joint degeneration.

Pathogenesis

Lipoma arborescens results from chronic synovial irritation, leading to villous hypertrophy and replacement of the subsynovial connective tissue with mature adipose tissue. The exact mechanism is unclear, but proposed factors include:

• Chronic synovial inflammation
• Recurrent haemarthrosis
• Degenerative joint disease
• Chronic low-grade trauma

Histologically, the frond-like synovial projections are lined by synoviocytes and filled with mature fat, with no atypia or malignancy. Reactive changes such as synovial hyperplasia and chronic inflammatory cells may also be present.

Subtypes

• Primary: Idiopathic, occurs without underlying joint disease
• Secondary: More common; associated with trauma, osteoarthritis, inflammatory arthropathy

Epidemiology, Risk Factors & Associations

• Peak age: 40–70 years
• Slight male predominance
• Most commonly affects the knee (>90%), particularly the suprapatellar recess
• Less common locations:
  • Hip, elbow, shoulder, ankle
  • Bursa (e.g. subdeltoid, iliopsoas)
• Associated with:
  • Osteoarthritis (most common)
  • Rheumatoid arthritis
  • Psoriatic arthritis
  • Pigmented villonodular synovitis (rare coexistence)
  • Prior trauma or meniscectomy

Clinical Features

• Chronic, painless joint swelling
• Recurrent or persistent joint effusion
• Reduced range of motion or mechanical symptoms
• Crepitus or catching sensation
• No constitutional symptoms
• Symptoms may be misattributed to primary osteoarthritis

Complications

• No malignant potential
• Progressive joint degeneration
• Mechanical impingement
• Secondary synovitis
• Rare association with joint instability or meniscal extrusion due to mass effect

Pathological Features

Histopathology

• Macroscopic:
  • Yellow, villous or frond-like hypertrophic synovium
  • Typically in suprapatellar pouch or intercondylar notch
• Microscopic:
  • Villous proliferation of synovium
  • Subsynovial tissue replaced by mature adipose tissue
  • Lined by hyperplastic synoviocytes
  • Chronic inflammatory infiltrate may be present

Serology

• Non-specific
• ESR/CRP may be elevated if underlying inflammatory arthropathy is present

Biochemistry

• Synovial fluid analysis typically non-inflammatory
• May show mild elevation in protein, low cell count

Immunohistochemistry

• Not routinely required
• CD68: may be positive in reactive macrophages
• S100: positive in mature fat

Molecular

• No known molecular alterations
• No clonal proliferation

Genetics

• No genetic or familial association

Radiological Features

General Features

• Intra-articular, fat-containing, frond-like synovial mass
• Most commonly affects suprapatellar pouch of the knee
• May displace synovial fluid
• May be associated with joint effusion, synovial hypertrophy, degenerative changes
• No internal calcification or haemorrhage
• Does not invade adjacent bone or erode cartilage
• Bilateral (20%)

XR

• Non-specific findings
• May show soft tissue fullness or joint effusion
• Degenerative changes may be present
• No calcification or ossification

CT

• Non-contrast:
  • Low attenuation mass (fat density) within joint space
  • May demonstrate frond-like morphology
• Contrast-enhanced:
  • No enhancement of fat-containing portions
  • Synovial lining may enhance subtly

MRI

• T1:
  • Frond-like synovial proliferation with signal intensity identical to fat
  • Suppresses on fat-saturated sequences
• T2:
  • Hyperintense signal identical to fat
  • Surrounding joint effusion hyperintense on fluid-sensitive sequences
• FLAIR:
  • Fat-suppressed frond-like synovial projections
• DWI/ADC:
  • No restricted diffusion
• T1 C+:
  • No enhancement of fatty fronds
  • Mild peripheral or synovial lining enhancement may be seen
• GRE/SWI:
  • No blooming (differentiates from PVNS)
• In/Out-of-Phase:
  • No signal drop-out (no microscopic fat or haemosiderin)

US

• B-mode:
  • Hyperechoic, frond-like mass within joint
  • May float in joint effusion
• Colour Doppler:
  • Typically avascular or minimal peripheral vascularity
  • Helps differentiate from inflammatory pannus or PVNS

NM

• Not routinely used
• May show increased uptake in associated osteoarthritic changes
• FDG PET: No avidity unless secondary inflammation present

Associated Findings

• Joint effusion, often large and persistent
• Coexistent osteoarthritis or meniscal degeneration
• No bone erosion or marrow oedema
• Meniscal extrusion or mechanical displacement in large lesions
• Occasional synovial cysts or bursitis (e.g. Baker’s cyst)

Grading and Staging

• No formal grading or staging system
• Described based on:
  • Extent of synovial involvement (focal vs diffuse)
  • Compartment (suprapatellar pouch, posterior joint, intercondylar notch)
  • Association with underlying joint disease

Diagnosis

• Clinical suspicion in patients with chronic knee swelling and effusion
• MRI is the diagnostic modality of choice:
  • Fat signal on all sequences
  • Fat-suppressed frond-like synovial mass
  • Absence of enhancement or blooming
• Confirmatory diagnosis with histopathology if excised

Differential Diagnosis

Image-based

• Synovial lipoma:
  • Focal, encapsulated fat mass
  • No frond-like morphology
  • No associated joint effusion
• Pigmented villonodular synovitis (PVNS):
  • Low T1 and T2 signal due to haemosiderin
  • Blooming artefact on GRE
  • Enhancing synovial mass (whereas lipoma arborescens does not enhance)
  • May contain entrapped regions of fat
• Synovial chondromatosis:
  • Multiple intra-articular calcified nodules
  • May have joint effusion
  • CT shows characteristic calcifications
• Inflammatory pannus (e.g. RA):
  • Intermediate T2 signal, may contain rice bodies (low signal on all sequences)
  • Enhances post-contrast
  • Effusion, synovitis and cartilage damage may be present
  • No fat signal or suppression
• Synovitis:
  • Uncomplicated synovitis would not have a frond-like appearance
  • Synovitis would be low signal on T1 and high signal, without areas of fat suppression, on T2

Clinically-based

• Chronic knee effusion from OA or meniscal tear
• Inflammatory arthritis
• Baker’s cyst
• Recurrent haemarthrosis
• Internal derangement (e.g. meniscal tear, loose body)

Management

• Referral to orthopaedic surgery or rheumatology depending on context
• Arthroscopic synovectomy is treatment of choice for symptomatic cases
• Open synovectomy in diffuse or extra-articular disease
• Conservative management:
  • Observation in asymptomatic or minimally symptomatic patients
  • NSAIDs and joint aspiration for effusion relief
• Treat underlying cause (e.g. rheumatoid arthritis, OA)
• Recurrence is rare after complete excision
• Long-term follow-up typically not required unless symptoms recur