What are the differences in joint deformation between patients with Rheumatoid Arthritis (RA) and Osteoarthritis (OA)?

Joint Deformities in Rheumatoid Arthritis vs Osteoarthritis

Rheumatoid arthritis causes symmetric inflammatory deformities affecting MCPs and PIPs with ulnar deviation and swan neck deformities, while osteoarthritis produces bony enlargement (Heberden's and Bouchard's nodes) at DIPs and PIPs with lateral deviation, sparing the MCPs.

Joint Distribution Patterns

Rheumatoid Arthritis

• Targets MCP and PIP joints while characteristically sparing DIP joints 1
• MCP involvement occurs in approximately 27-29% of cases, with PIP involvement in 29-33% of cases 1
• The 2010 ACR/EULAR classification criteria specifically include MCPs and PIPs in assessment while explicitly excluding DIPs 1
• Wrist involvement is common alongside MCP and PIP disease 2, 1

Osteoarthritis

• Predominantly affects DIP joints, PIP joints, and thumb base (first CMC joint), NOT the MCPs 2, 3
• The prevalence of symptomatic hand OA is highest at DIPs, followed by thumb base, then PIPs, with MCPs being least commonly affected 2, 3
• When MCP involvement occurs in hand arthritis, this should prompt consideration of RA rather than OA 3

Characteristic Deformity Patterns

Rheumatoid Arthritis Deformities

• Ulnar deviation at MCP joints due to inflammatory synovitis and ligamentous laxity 1
• Swan neck deformities (PIP hyperextension with DIP flexion) 1
• Soft tissue swelling rather than bony enlargement 3, 1
• In lower extremities, RA knees demonstrate valgus (knock-knee) deformity with mean alignment more valgus than OA 4
• Greater variability in joint orientation angles with the whole lower limb, femur, tibia, and joint being more valgus in RA than OA 4

Osteoarthritis Deformities

• Heberden's nodes (bony enlargement at DIP joints) 2, 3, 1
• Bouchard's nodes (bony enlargement at PIP joints) 2, 3, 1
• Lateral deviation at interphalangeal joints 2
• Subluxation and adduction deformity at thumb base 2
• In lower extremities, OA knees demonstrate varus (bow-legged) deformity with mean HKA angle of -10.6° compared to -3.4° in RA 4
• Bony ankylosis may occur in erosive OA subset, particularly at interphalangeal joints 2

Key Distinguishing Clinical Features

Type of Swelling

• RA produces soft tissue swelling from inflammatory synovitis 3, 1
• OA produces bony enlargement from osteophyte formation 3, 1

Pattern of Involvement

• RA: symmetric, MCP + PIP involvement with DIP sparing 1
• OA: DIP and PIP involvement, thumb base, sparing MCPs 2, 3

Morning Stiffness

• RA: prolonged stiffness lasting >30 minutes 1
• OA: limited morning stiffness lasting <30 minutes 2, 3

Pain Pattern

• RA: pain at rest, improves with movement initially 1
• OA: pain on usage that worsens with movement and improves with rest 3

Special Considerations

Erosive Osteoarthritis Subset

• Targets interphalangeal joints specifically with subchondral erosion 2
• May progress to marked bone and cartilage attrition, instability, and bony ankylosis 2
• Shows inflammatory symptoms (stiffness, soft tissue swelling, erythema) and mildly elevated CRP, mimicking RA 2
• However, erosive OA had significantly higher scores at DIP, PIP, and thumb IP joints, but not at MCP joints, distinguishing it from RA 2

Lower Extremity Alignment

• RA knees: valgus deformity (mean HKA -3.4°, mLDFA 86.6°, mMPTA 85.9°) 4
• OA knees: varus deformity (mean HKA -10.6°, mLDFA 88.2°, mMPTA 84.3°) 4
• Joint line convergence angle is significantly less in RA (2.7°) compared to OA (6.8°) 4

Common Pitfalls

• DIP joints can occasionally be involved in RA, though rare, so their involvement alone does not exclude RA 1
• OA and inflammatory arthritis can coexist in the same patient, complicating the clinical picture 1
• Psoriatic arthritis can target DIPs and may show asymmetric involvement with dactylitis, requiring differentiation from both RA and OA 2, 1