What is synovitis?

What is Synovitis

Synovitis is inflammation of the synovial membrane—the tissue lining the inner surface of joints—characterized by synovial thickening, increased vascularity, and inflammatory cell infiltration that produces joint swelling, pain, and stiffness. 1

Pathophysiology

Synovitis involves several key pathological changes:

• Synovial membrane thickening with infiltration of mononuclear cells (T cells, B cells, macrophages) into the synovial lining layer 1, 2
• Increased vascularity with neovascularization, which distinguishes inflammatory synovitis from mechanical joint disease 1
• Production of inflammatory mediators including cytokines (IL-12/23, TNF-alpha), prostaglandin E2, nitric oxide, and proteolytic enzymes that contribute to cartilage breakdown 1, 3
• Synovial fluid accumulation (effusion) within the joint space, contributing to joint swelling 1

The inflammatory process creates a destructive cycle where synovial inflammation damages adjacent cartilage, and cartilage breakdown products further amplify synovial inflammation 3.

Clinical Manifestations

Synovitis presents with characteristic clinical features:

• Joint swelling that is soft and boggy on palpation, distinct from bony enlargement 1
• Joint pain that is typically worse with activity and may be present at rest in severe cases 1
• Morning stiffness lasting more than 30-60 minutes, which is a cardinal symptom of inflammatory synovitis 1, 4
• Joint tenderness on palpation, particularly over the joint line 1
• Warmth over the affected joint due to increased blood flow 4
• Reduced range of motion due to pain, swelling, and mechanical limitation 4

Common Locations

Synovitis can affect any synovial joint but shows predilection for certain sites:

• Small joints of hands and feet: metacarpophalangeal (MCP), proximal interphalangeal (PIP), and metatarsophalangeal (MTP) joints are commonly involved in rheumatoid arthritis 1, 4
• Wrists and knees are frequently affected across multiple inflammatory conditions 4
• Tendon sheaths and bursae can also develop synovitis (tenosynovitis and bursitis), particularly in psoriatic arthritis where "sausage digit" (dactylitis) results from combined tenosynovitis and joint synovitis 1

Detection Methods

Clinical examination remains the primary method for detecting synovitis, but imaging significantly improves diagnostic accuracy:

• Clinical examination identifies synovitis through palpable joint swelling, tenderness, and the "squeeze test" for MCP/MTP joints 1
• Ultrasound detects synovial hypertrophy (grey-scale) and active inflammation (power Doppler vascularity), and is superior to clinical examination in detecting subclinical synovitis 1
• MRI is the most sensitive modality, detecting synovial thickening, enhancement with gadolinium contrast, and associated bone marrow edema that predicts erosive progression 1
• Conventional radiography is insensitive for synovitis itself but may show secondary signs like joint space narrowing and soft tissue swelling 1

Imaging detects synovitis approximately 2-3 times more frequently than clinical examination alone, particularly in early disease 1.

Disease Associations

Synovitis occurs in multiple rheumatologic and non-rheumatologic conditions:

• Rheumatoid arthritis: characterized by symmetric polyarticular synovitis with potential for erosive joint damage 1, 4
• Psoriatic arthritis: features synovitis combined with enthesitis and dactylitis, often asymmetric 1
• Osteoarthritis: synovitis is increasingly recognized as a key feature contributing to pain and structural progression, even in "non-inflammatory" arthritis 5, 2, 3
• Crystal arthropathies (gout, pseudogout): acute synovitis from crystal deposition 6
• Septic arthritis: infectious synovitis requiring emergency treatment 7
• Juvenile idiopathic arthritis: synovitis in pediatric populations with unique imaging considerations 1

Clinical Significance

The presence and severity of synovitis has important prognostic implications:

• Persistent synovitis (>3 months duration) predicts progression to chronic inflammatory arthritis and joint damage 7
• Subclinical synovitis detected on ultrasound or MRI predicts future clinical arthritis development in at-risk individuals 1
• Synovitis severity correlates with pain intensity, functional impairment, and quality of life reduction 2, 3
• MRI bone marrow edema associated with synovitis is the strongest independent predictor of subsequent radiographic erosive progression 1

Treatment Implications

Synovitis is a therapeutic target across multiple conditions:

• Early treatment of synovitis improves outcomes and may prevent transition from transient to persistent disease 7
• Anti-inflammatory therapy including NSAIDs, corticosteroids (systemic or intra-articular), and disease-modifying drugs targets synovial inflammation 1, 7
• Imaging-detected synovitis can guide treatment decisions, as persistent inflammation on ultrasound or MRI despite clinical remission may warrant treatment intensification 1
• Synovium-targeted therapy in osteoarthritis holds promise for both symptom relief and structure modification 2, 3