How do you differentiate osteoarthritis (OA) knees from rheumatoid arthritis (RA) knees in terms of history and physical examination (PE)?

Differentiating Osteoarthritis from Rheumatoid Arthritis of the Knee

Osteoarthritis of the knee can be confidently distinguished from rheumatoid arthritis through a composite of clinical features: OA typically presents in patients over 40 with usage-related pain, minimal morning stiffness (<30 minutes), asymmetric involvement of one or few joints, bony enlargement with crepitus, and absence of inflammatory laboratory markers, whereas RA presents with prolonged morning stiffness (>1 hour), symmetric polyarticular involvement including small joints of hands/feet, soft tissue swelling, and positive inflammatory markers. 1, 2

History Taking: Key Distinguishing Features

Age and Onset

• OA: Typically presents in patients over 40 years old, with gradual onset and progressive symptoms 1, 3
• RA: Can occur at any age but commonly presents between 30-50 years, often with more abrupt onset 2

Pain Characteristics

• OA: Pain worsens with activity and improves with rest; usage-related pain is the hallmark feature 1, 4
• RA: Pain may be present at rest and often worse in the morning, improving somewhat with activity 2

Morning Stiffness Duration

• OA: Brief morning stiffness lasting less than 30 minutes, or only mild inactivity stiffness 1, 4
• RA: Prolonged morning stiffness typically lasting more than 1 hour, often several hours 2

Joint Distribution Pattern

• OA: Affects one or few joints at any one time; symptoms are often intermittent and asymmetric 1, 2
• RA: Symmetric polyarticular involvement is characteristic, typically affecting small joints of hands (MCPs, PIPs) and feet, along with wrists 2
• Critical distinction: While RA primarily targets MCPs, PIPs, and wrists in the hands, OA predominantly affects DIPs, PIPs, and thumb base 2

Systemic and Extra-articular Features

• OA: Absence of systemic symptoms; purely mechanical joint disease 3, 5
• RA: May present with constitutional symptoms (fatigue, low-grade fever, weight loss) and extra-articular manifestations 2

Physical Examination: Distinguishing Findings

Joint Characteristics

• OA knee findings:

  • Bony enlargement and hard swelling (osteophytes) 1, 5
  • Joint line tenderness, particularly medial compartment 1, 5
  • Coarse crepitus on passive motion 1, 5
  • Reduced range of motion with passive arc limitation 1, 5
  • Possible varus or valgus deformity in advanced disease 5
  • Cool to touch, no warmth 3

• RA knee findings:

  • Soft tissue swelling (synovial thickening and effusion) rather than bony enlargement 2
  • Warmth and tenderness suggesting active inflammation 2
  • Symmetric involvement when multiple joints affected 2
  • Boggy, spongy feel to joint swelling 6

Hand Examination (Critical for Differentiation)

• OA pattern: Heberden nodes (DIP joints), Bouchard nodes (PIP joints), thumb base involvement with possible lateral deviation or subluxation 2
• RA pattern: MCP and PIP involvement with sparing of DIPs; ulnar deviation, swan neck or boutonniere deformities may develop 2
• Key point: Examining the hands is essential even when evaluating knee complaints, as the joint distribution pattern strongly differentiates these conditions 2

Inflammatory Signs

• OA: Minimal to absent inflammatory signs; may have mild swelling from effusion but without warmth or erythema 1, 3
• RA: Active inflammatory signs including warmth, erythema, soft tissue swelling, and tenderness 2

Laboratory and Imaging Considerations

When to Order Tests

• OA diagnosis: Can be made clinically without imaging or laboratory tests in patients over 40 with typical presentation 1
• RA evaluation: Laboratory tests should be performed when inflammatory features are present, including RF, anti-CCP antibodies, ESR, and CRP 2

Laboratory Findings

• OA: Normal inflammatory markers (ESR, CRP); no autoantibodies 4, 3
• RA: Elevated acute phase reactants (ESR, CRP); positive RF and/or anti-CCP antibodies in majority of cases 2
• Important caveat: Erosive OA (a specific subset) may show mildly elevated CRP, but this targets interphalangeal joints of hands, not knees 2

Radiographic Features

• OA: Joint space narrowing, osteophytes, subchondral sclerosis, subchondral cysts 1, 4, 5
• RA: Periarticular osteopenia, marginal erosions, symmetric joint space narrowing without osteophytes 2

Common Diagnostic Pitfalls

Avoid Single-Feature Diagnosis

• No single criterion has adequate sensitivity and specificity for diagnosis 2
• A composite of multiple features is essential: age, gender, joint distribution, type of swelling (bony vs. soft tissue), stiffness duration, and laboratory findings 2

Recognize Disease Coexistence

• OA and RA can coexist in the same patient, particularly in older individuals 2
• The presence of radiographic OA changes does not exclude concurrent inflammatory arthritis 2

Functional Impairment Is Not Discriminatory

• Both conditions can cause severe functional disability and pain levels may be equivalent 2, 7
• Severity of symptoms alone cannot differentiate OA from RA 7

Age-Related Considerations

• Radiographic OA changes are common in asymptomatic older adults 1, 3
• Presence of radiographic changes must correlate with clinical symptoms for diagnosis 1