Rheumatoid Arthritis: Etiopathogenesis in Orthopaedics

Definition

• Campbell's Operative Orthopaedics 15th Ed 2026
• Miller's Review of Orthopaedics 9th Ed

Etiology (Risk Factors)

Etiopathogenesis Flow Chart

Step-by-Step Pathogenetic Mechanism

PHASE 1: Triggering of Autoimmunity (Pre-clinical RA)

Environmental trigger (smoking, infection, periodontitis)
       +
Genetic susceptibility (HLA-DRB1, PTPN22)
       ↓
Peptidylarginine deiminase (PAD) enzymes activated
       ↓
Citrullination of self-proteins:
  (fibrin, vimentin, fibronectin, collagen type I/II, α-enolase)
       ↓
Citrullinated proteins recognized as "non-self"
       ↓
Production of Anti-Citrullinated Protein Antibodies (ACPA/anti-CCP)
  + Rheumatoid Factor (RF = IgM/IgG anti-Fc of IgG)

PHASE 2: Antigen Presentation and Lymphocyte Activation

Macrophages + Dendritic Cells (DCs) present citrullinated antigens
       ↓
via MHC Class II (HLA-DR) → CD4+ T-helper cells activated
       ↓
T-cells differentiate:
  • Th1 cells → produce IFN-γ, TNF-α
  • Th17 cells → produce IL-17 (critical for neutrophil recruitment)
       ↓
T-cells stimulate B-cells → Plasma cells
       ↓
Plasma cells produce RF and ACPA/anti-CCP
  (detectable in serum before synovitis begins)

PHASE 3: Synovial Inflammation (Synovitis)

Immune complexes (RF + IgG; ACPA + citrullinated proteins)
       ↓
Deposit in synovial membrane and joint space
       ↓
Activate complement cascade (C3a, C5a)
  + Fc receptor activation on macrophages
       ↓
Release of proinflammatory cytokines:
  • TNF-α → master cytokine (drives all downstream inflammation)
  • IL-1β → cartilage destruction, fever
  • IL-6 → systemic manifestations, acute-phase reactants (CRP, ESR ↑)
  • IL-15 → recruits and activates more T-cells
  • IL-17 → neutrophil recruitment
  • GM-CSF → myeloid cell activation
       ↓
SYNOVIAL LINING CELL HYPERPLASIA
  (lining normally 1-2 cell layers → expands to 6-10 layers)
       ↓
Fibroblast-like synoviocytes (FLS) become activated
  + Macrophage-like synoviocytes proliferate

PHASE 4: Pannus Formation (The Hallmark of RA)

Activated synovial cells (FLS + macrophages)
       ↓
Undergo neo-vascularization (angiogenesis - VEGF driven)
       ↓
Form "PANNUS" = hyperplastic, invasive synovial tissue
  (granulation-like tissue overlying cartilage)
       ↓
Pannus invades articular cartilage and subchondral bone
       ↓
Releases destructive enzymes:
  • Matrix metalloproteinases (MMP-1, 3, 13) - degrade collagen
  • Aggrecanases (ADAMTS) - degrade proteoglycans
  • Cathepsins - lysosomal enzymes
  • Plasminogen activator → activates latent collagenase
       ↓
Cartilage erosion → joint space narrowing

PHASE 5: Bone Erosion (Periarticular Osteoporosis and Marginal Erosions)

TNF-α + IL-1β + IL-17 → Upregulate RANKL expression
  (on FLS, osteoblasts, activated T-cells)
       ↓
RANKL binds RANK on osteoclast precursors
       ↓
Osteoclast differentiation and activation
  (OPG = osteoprotegerin is the decoy receptor that normally inhibits this)
       ↓
In RA: RANKL/OPG ratio ↑ → osteoclast activity predominates
       ↓
Periarticular bone erosions (marginal erosions first)
  + Subchondral bone destruction
  + Periarticular osteoporosis
  + Systemic osteoporosis (IL-6 mediated)

Comprehensive Pathogenesis Diagram (Cellular Level)

Histopathology of the RA Joint

• Synovial lining hyperplasia (villous proliferation)
• Dense lymphocytic infiltrate - CD4+ T-cells with macrophages and dendritic cells
• Plasma cells producing RF (often forming secondary follicles with germinal centers)
• Pannus tissue - fibrovascular granulation tissue at cartilage-pannus junction
• Neovascularization throughout inflamed synovium
• Rheumatoid nodules - central fibrinoid necrosis surrounded by palisading macrophages

Cytokine Pathways (Inflammation Cascade)

Orthopaedic Consequences of Pathogenesis

IgG Rheumatoid Factor and Immune Complex Formation

1. Deposit in synovial tissues and joint fluid
2. Are stabilized by IgM-RF and C1q (polyvalent Fcγ-binding)
3. Attract neutrophils → release reactive oxygen intermediates (ROIs) + lysosomal enzymes (neutral proteases, collagenase)
4. When adherent to cartilage, cause "frustrated phagocytosis" → extracellular hydrolase release

Summary: Etiopathogenesis in One Schematic

GENETIC SUSCEPTIBILITY (HLA-DRB1, PTPN22)
           +
ENVIRONMENTAL TRIGGER (smoking, infection, PAD activation)
           ↓
PROTEIN CITRULLINATION → ACPA + RF production
           ↓
ANTIGEN PRESENTATION (DCs, macrophages via MHC II)
           ↓
CD4+ T-CELL ACTIVATION → Th1 / Th17 differentiation
           ↓
B-CELL ACTIVATION → PLASMA CELLS → RF + ACPA
           ↓
IMMUNE COMPLEX FORMATION in synovium
           ↓
COMPLEMENT ACTIVATION + Fc receptor stimulation
           ↓
CYTOKINE STORM: TNF-α, IL-1β, IL-6, IL-15, IL-17
           ↓
SYNOVIAL HYPERPLASIA + ANGIOGENESIS
           ↓
PANNUS FORMATION (invasive hyperplastic synovium)
           ↓
MMP + Cathepsin release ──→ CARTILAGE EROSION
           ↓
RANKL upregulation ──→ OSTEOCLAST ACTIVATION → BONE EROSION
           ↓
JOINT DEFORMITY + FUNCTIONAL DISABILITY

Key Biomarkers in Pathogenesis

Therapeutic Targets Based on Pathogenesis

• TNF-α inhibitors (etanercept, infliximab, adalimumab) - must be discontinued perioperatively due to infection risk
• IL-6 receptor antagonists (tocilizumab)
• T-cell costimulation blockers (abatacept - blocks CD28/B7 interaction)
• B-cell depletion (rituximab - anti-CD20)
• JAK inhibitors (tofacitinib, baricitinib) - block cytokine signalling
• Methotrexate, hydroxychloroquine, prednisone - can generally be continued perioperatively
• Anti-RANKL (denosumab) - targets the RANKL/RANK/OPG pathway directly