What knee deformities occur in rheumatoid arthritis and how are they managed?

Knee Deformities in Rheumatoid Arthritis

Rheumatoid arthritis causes progressive, symmetric knee joint destruction characterized by valgus deformity, flexion contractures, and posterior subluxation of the tibia, driven by persistent synovitis and pannus formation that erodes cartilage and subchondral bone from both the joint cavity and bone marrow.

Pathophysiological Mechanisms of Deformity

The knee joint destruction in RA follows a predictable sequence initiated by inflammatory infiltrates:

• Synovial proliferation and pannus formation represent the primary destructive mechanism, with invasive synovial tissue extending across cartilage surfaces and into subchondral bone 1, 2
• Bone marrow edema (BME) occurs in 68-75% of early RA patients and serves as a precursor to rapid disease progression and erosion formation 2, 3
• Dual-direction erosion pathway: Bone erosions develop from both the joint cavity (via pannus invasion) and from within the bone marrow (via inflammatory infiltrates), creating a pincer effect of destruction 3
• Cartilage destruction manifests as fibrosis, thinning, and hyperplasia, with severe destruction typically classified as grade 4 in advanced disease 4

Specific Knee Deformities

Valgus Deformity (Knock-Knee)

• Most common angular deformity in RA knees, resulting from asymmetric cartilage and bone loss with greater destruction of the lateral compartment 4
• The medial femoral condyle shows less destruction than other compartments (average destruction grade 3.6), contributing to valgus angulation 4

Flexion Contractures

• Progressive loss of extension develops from chronic synovitis causing capsular fibrosis and posterior soft tissue contracture 1
• Persistent joint effusion and synovial thickening maintain the knee in a flexed position of comfort, leading to fixed deformity over time 5

Posterior Tibial Subluxation

• Results from destruction of cruciate ligaments and posterior capsular structures by invasive pannus 1
• Synovial infiltration destroys ligamentous attachments, allowing posterior displacement of the tibia relative to the femur 4

Meniscal Destruction

• Severe meniscal damage occurs with average destruction grade of 3.85, with medial meniscus more severely affected than lateral (Z = 2.062, P = 0.039) 4
• Pathological changes include fibrosis and engulfing of calcified debris 4

Imaging Characteristics

Plain Radiography

• Remains the diagnostic standard for assessing joint damage progression 1
• Key findings include: fusiform soft tissue swelling, regional osteoporosis, marginal and central erosions, and diffuse loss of joint space 1
• Symmetry is the hallmark of RA joint involvement 1

Ultrasound Evaluation

• High-frequency transducers (≥10 MHz) detect even minor synovitic lesions and small effusions 5, 6
• The suprapatellar recess examined with knee in 30° flexion provides the most sensitive window for detecting joint effusions and synovial proliferation 6
• Power Doppler demonstrates well-vascularized pannus and its destructive effects on joint structures 3
• Can identify loose joint bodies in suprapatellar, infrapatellar, and popliteal regions 5, 6

MRI Findings

• Superior for detecting early disease activity and differentiating synovial fluid from inflammatory pannus 1
• Bone marrow edema appears as inflammatory infiltrates and predicts rapid erosion development 2, 3
• Stage 3 disease (80.8% of patients) shows extensive cartilage destruction with bone marrow involvement 4
• Inflammatory cysts within subchondral bone precede frank erosions 2

Clinical Staging and Progression

Early Stage

• Synovial proliferation and soft tissue swelling 1
• Regional osteoporosis 1
• Bone marrow edema visible on MRI but minimal radiographic changes 2, 3

Intermediate Stage

• Pannus extends across cartilage surface 1
• Small marginal bone erosions at bare areas 1
• Chondral erosions develop 1

Advanced Stage (Majority of Surgical Cases)

• Grade 4 cartilage destruction affecting medial and lateral femoral condyles and tibial plateaus 4
• Marginal and central erosions 1
• Fibrous ankylosis, subluxations, and dislocations 1
• Fractures and fragmentations in end-stage disease 1

Management Approach

Medical Management

• Disease-modifying antirheumatic drugs (DMARDs) should be initiated within 3 months of diagnosis to prevent progressive joint destruction 7
• Methotrexate remains the standard first-line DMARD 5, 7
• Biological therapies targeting proinflammatory cytokines have dramatically changed treatment outcomes and can prevent deformity progression 7
• Treatment goal is achieving remission defined as: tender joint count ≤1, swollen joint count ≤1, CRP ≤1 mg/dL, and patient global assessment ≤1 on 10-cm scale 5

Surgical Intervention

• Total knee arthroplasty (TKA) is indicated for end-stage disease with severe deformity and functional impairment 4
• Surgical planning must account for bone loss, ligamentous insufficiency, and posterior tibial subluxation 4

Critical Clinical Pitfalls

• Do not delay DMARD therapy beyond 3 months of diagnosis, as patients with positive rheumatoid factor have >70% likelihood of developing erosions within 2 years 1, 7
• Bone marrow edema on MRI demands aggressive treatment, as it predicts rapid disease progression even when radiographs appear relatively normal 2, 3
• Symmetric joint involvement is essential for diagnosis—asymmetric disease should prompt consideration of alternative diagnoses 1
• Ultrasound cannot rule out loose bodies if not visualized; negative ultrasound does not exclude their presence 6
• Clinical examination alone misses early synovitis—even minor effusions detectable by ultrasound may indicate active disease requiring treatment escalation 5, 6