STEPTRACKERMUSCULOSKELETAL | PATHOLOGY + IMMUNOLOGY + PHARMACOLOGY | HIGH-YIELD REVIEW

Osteoarthritis vs Rheumatoid Arthritis

Mechanical wear of cartilage versus autoimmune inflammation of synovium — compared feature by feature.

FOCUS ON
OA: weight-bearing joints and DIPs, pain worse with use, brief stiffness, osteophytes, no systemic signs. RA: symmetric MCP/PIP/wrist, morning stiffness > 1 hour that improves with use, pannus erodes bone, anti-CCP, extra-articular disease; methotrexate first-line. Watch for C1–C2 instability, Felty syndrome and juvenile idiopathic arthritis.
FEATURE
OSTEOARTHRITIS
RHEUMATOID ARTHRITIS
CLUE
Mechanism
Mechanical stress → chondrocyte injury → cartilage loss
Autoimmune synovitis (HLA-DR4, smoking → citrullination); T cells, B cells, cytokines (TNF, IL-1, IL-6)
Cartilage first vs synovium first
Who
Older, obese, prior joint injury
Women 30–50
Young woman with symmetric hand arthritis → RA
Joints
Knees, hips, spine, DIP (Heberden), PIP (Bouchard), 1st CMC; asymmetric
MCP, PIP, wrists, feet, C-spine; symmetric; DIPs spared
DIP = OA (or psoriatic)
Pain & stiffness
Worse with use, better with rest; stiffness < 30 min
Morning stiffness > 1 hour, improves with activity; systemic fatigue, fever
Inflammatory pattern = RA
Pathology
Fibrillated cartilage, eburnation, subchondral cysts and sclerosis, osteophytes, joint mice
Synovial hyperplasia → pannus (inflamed granulation tissue) erodes cartilage and bone; fibrous ankylosis
Pannus = RA
Deformities
Bony enlargement, crepitus, genu varum
Ulnar deviation, swan-neck, boutonnière, Baker cyst, atlantoaxial subluxation
Check C-spine before intubation in RA
X-ray
Asymmetric joint-space narrowing, osteophytes, subchondral sclerosis
Periarticular osteopenia, marginal erosions, symmetric narrowing
Erosions = inflammatory
Labs
Normal; noninflammatory synovial fluid
Anti-CCP (specific), RF (IgM against IgG Fc), ↑ ESR/CRP, anemia of chronic disease
RF also in Sjögren, HCV, endocarditis
Extra-articular RA
None in OA
Rheumatoid nodules (central fibrinoid necrosis, palisading histiocytes), ILD, pleuritis, pericarditis, scleritis, Sjögren, vasculitis, AA amyloidosis
Felty: RA + splenomegaly + neutropenia
Treatment
Weight loss, exercise, acetaminophen/NSAIDs (topical), intra-articular steroids, joint replacement
NSAIDs, short-course steroids; DMARDs — methotrexate first; biologics after
Start DMARDs early to prevent erosions
Juvenile idiopathic arthritis
Arthritis before 16, lasting > 6 weeks
Oligoarticular (ANA ⁺ → silent uveitis), polyarticular, systemic (daily spiking fevers, salmon rash)
Regular slit-lamp exams

Mechanism — rheumatoid synovitis

Smoking + HLA-DR4Citrullinated peptides
→
CD4 T cells + B cellsRF, anti-CCP
→
TNF, IL-1, IL-6
→
PannusSynovial hyperplasia
→
RANKL → erosionsJoint destruction

Each cytokine and cell in this chain is a drug target: TNF, IL-6, CD20 B cells, T-cell costimulation, JAK signaling.

RA drugs

DRUG
TARGET
WATCH FOR
Methotrexate
DHFR (anti-inflammatory at low dose)
Hepatotoxicity, pneumonitis, cytopenias, teratogen · give folate
Hydroxychloroquine
Lysosomal, TLR signaling
Retinopathy
Sulfasalazine, leflunomide
5-ASA + sulfa · pyrimidine synthesis
Sulfa reactions · hepatotoxicity
Infliximab, adalimumab, etanercept
TNF-α
Reactivated TB — screen first
Abatacept · rituximab · tocilizumab
CD80/86 · CD20 · IL-6R
Infections
Tofacitinib
JAK
Zoster, thrombosis

How it's tested

COMMON CONFUSIONHeberden (DIP) and Bouchard (PIP) nodes are bony OA findings. Soft, warm swelling of MCPs and PIPs with morning stiffness is RA.
THE TRAPAn RA patient with neck pain, hyperreflexia or weakness may have atlantoaxial subluxation — image the cervical spine before any neck manipulation or intubation.
YOUR APPROACHWhich joints + symmetry → stiffness pattern → systemic features → X-ray (osteophytes vs erosions) → serology → treatment and its toxicity.

If you see… think…

70-year-old obese woman, knee pain worse after walking, crepitus → osteoarthritis
Bony nodules on DIP joints → Heberden nodes (OA)
35-year-old woman, symmetric MCP swelling, 2 h morning stiffness → rheumatoid arthritis
Most specific antibody for RA → anti-CCP
Subcutaneous elbow nodule with central necrosis and palisading cells → rheumatoid nodule
RA + splenomegaly + neutropenia → Felty syndrome
RA patient with neck pain and new hyperreflexia → atlantoaxial subluxation
Ulcers and pneumonitis on a once-weekly RA drug → methotrexate toxicity
Test before starting infliximab → latent TB screen
Child with oligoarthritis and ANA⁺ → JIA — screen for uveitis