Osteoarthritis aetiopathogenesis

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Osteoarthritis: Aetiopathogenesis

Definition

Osteoarthritis (OA) is characterized by failed repair of synovial joint tissues after intra-articular stress, resulting in breakdown of cartilage and bone with symptoms of pain, stiffness, and functional disability. Despite the suffix "-itis," OA is primarily a degenerative disorder of articular cartilage; inflammation acts as a secondary but important contributor. - Goldman-Cecil Medicine, p. 2763; Robbins & Kumar Basic Pathology, p. 789

Classification

Type	Definition	Cause
Primary (idiopathic)	Appears insidiously as an aging phenomenon	No identifiable predisposing condition; oligoarticular
Secondary	Appears in younger individuals (~5% of cases)	Joint deformity, prior trauma, systemic disease (diabetes, obesity)

Prevalence increases exponentially beyond age 50; approximately 40% of people over 70 are affected. - Robbins, p. 789

Aetiology and Risk Factors

1. Biomechanical Factors

The core initiating event is an imbalance between the mechanical load on articular cartilage and its capacity to bear that load.

Overloaded or incorrectly loaded joints (post-traumatic deformities, joint dysplasia, axis deviations, instabilities)
Sports injuries
Obesity - Mendelian randomization studies confirm a causative role of BMI in OA
Weight-bearing joints (hips, knees, lumbar/cervical spine) are preferentially affected

2. Genetic Factors

Genome-wide association studies (GWAS) with >800,000 samples have identified more than 80 OA risk loci, confirming that OA has a significant polygenic component. Key genes include:

GDF5 (chromosome 20): codes for a growth factor important in chondrogenesis and bone growth; risk allele is inversely associated with height
RUNX2, SMAD3, PTHLH: important in skeletal and bone development - highlighting bone morphology's contribution
DOT1L: another height-inversely-associated locus
Genes affecting the immune/inflammatory response, skeletal development, and joint degeneration are all represented across loci

Different genetic variants associate with different joint sites (e.g., hip vs. knee OA). - Firestein & Kelley's Rheumatology, p. 499

3. Other Risk Factors

Age (cartilage is bradytrophic - avascular and barely regenerates)
Female sex
Lack of exercise, poor nutrition
Prior inflammatory joint disease

Pathogenesis: Step-by-Step

The following diagram from Robbins & Kumar summarizes the sequence:

Schematic of OA pathogenesis showing chondrocyte injury, early OA with MMP/cytokine activation, and late OA with chondrocyte dropout and bone changes

FIG: OA is initiated by chondrocyte injury in a genetically predisposed patient (1), leading to matrix changes; degradation exceeds repair in early OA (2); late OA shows loss of matrix and chondrocytes with subchondral bone damage (3). - Robbins & Kumar Basic Pathology, p. 790

Stage 1 - Chondrocyte Injury (Initiation)

Biomechanical stress disrupts the normal flow of interstitial fluid into avascular articular cartilage. Because cartilage has no blood vessels or perichondrium, it has severely limited regenerative capacity. Initial stress injures chondrocytes and disrupts the type II collagen network.

Stage 2 - Early OA: Degradation vs. Failed Repair

Injured chondrocytes proliferate (forming clusters visible histologically) in a failed repair response
They secrete matrix metalloproteinases (MMPs), particularly MMP-13, which cleave the type II collagen fibers
ADAMTS5 (aggrecanase) degrades proteoglycans (aggrecan)

Key regulatory mechanism disrupted in OA: In normal cartilage, MMP-13 and ADAMTS5 are constitutively produced but kept in check by LRP1-mediated endocytic clearance. In OA:

Chondrocytes overproduce MT1-MMP and ADAM17
These enzymes shed the ectodomain of LRP1
Without LRP1, MMP-13 and ADAMTS5 accumulate and drive matrix destruction
Firestein & Kelley's Rheumatology, p. 183

Consequences of matrix loss:

Water content of the matrix increases
Proteoglycan concentration decreases
Horizontal type II collagen fibers are cleaved, yielding surface fissures and clefts (fibrillation)
Cartilage becomes granular and soft

Cytokine/mediator amplification loop: Chondrocytes, synovial cells, and recruited macrophages release:

Mediator	Effect
IL-1, TNF	Induce MMP expression; directly promote cartilage catabolism
IL-6	Amplifies inflammatory signaling
TGF-β	Paradoxically induces MMP production; also promotes osteophyte formation via BMP pathway
PGE2 (prostaglandins)	Pain, inflammation
Nitric oxide (NO)	Promotes chondrocyte apoptosis
BMPs (Bone Morphogenetic Proteins)	Contribute to osteophyte formation at joint margins

Stage 3 - Late OA: Structural Failure

Chondrocyte dropout by apoptosis
Full-thickness sloughing of cartilage segments
Dislodged fragments form loose bodies ("joint mice") in the joint space
Exposed subchondral bone is polished by friction = eburnation (ivory-like appearance)
Microfractures in subchondral bone allow synovial fluid to enter via a ball-valve mechanism = subchondral cysts (geodes)
TGF-β/BMP signaling drives peripheral osteophyte formation (fibrocartilage-capped bony outgrowths that gradually ossify)
Synovium shows mild congestion, fibrosis, and scattered chronic inflammatory cells

Histopathology

OA histology: fibrillation of articular cartilage (A) and eburnated articular surface with subchondral cyst (B)

FIG 19.32: (A) Fibrillation of articular cartilage on H&E. (B) Eburnated articular surface (ivory-like exposed bone), with subchondral cyst (2) and residual cartilage (3). - Robbins & Kumar, p. 791

In early OA, Safranin-O staining (which stains proteoglycans) shows loss of red staining, indicating proteoglycan depletion. Quiescent chondrocytes become activated and form clusters. The tidemark (interface between articular and calcified cartilage) advances and may duplicate, and vascular penetration from subchondral bone occurs. - Firestein & Kelley's Rheumatology, p. 75

The Whole-Joint Disease Concept

Modern thinking views OA as a disease of the whole joint organ, not just cartilage:

Structure	Changes in OA
Articular cartilage	Fibrillation, proteoglycan loss, full-thickness erosion
Subchondral bone	Sclerosis, bone marrow lesions (BMLs), cyst formation, eburnation
Synovium	Low-grade synovitis, inflammatory cell infiltration (secondary)
Osteophytes	Peripheral bony outgrowths (TGF-β/BMP driven)
Ligaments & periarticular muscles	Laxity, atrophy
Joint capsule	Fibrosis

Pain Mechanisms

OA pain is multifactorial - the poor correlation between structural severity and symptom severity reflects this complexity:

Synovitis - inflammatory mediators sensitize synovial nociceptors; NSAIDs and intra-articular corticosteroids are effective analgesics in OA
Bone marrow lesions (BMLs) - associated with inflammatory cell infiltration, cytokine and NGF expression, and sensory nerve terminal ingrowth into subchondral bone
Biomechanical - weight-bearing pain; exercises, braces, osteotomy, and arthroplasty all reduce pain by altering mechanics
NGF/peripheral sensitization - NGF-blocking antibodies (e.g., tanezumab) effectively reduce OA pain, highlighting peripheral nociceptor sensitization as a key driver
Central sensitization - quantitative sensory testing (QST) demonstrates widespread hyperalgesia; fibromyalgia comorbidity is common. After successful arthroplasty, central sensitization may resolve, suggesting the joint itself drives central pain facilitation

Firestein & Kelley's Rheumatology, pp. 4585-4660

Summary: Key Pathogenic Cascade

Biomechanical stress + Genetic predisposition
          ↓
  Chondrocyte injury
          ↓
  ↑ MMPs (MMP-13) + ↑ ADAMTS5 (LRP1 regulation lost)
          ↓
  Collagen II + Proteoglycan degradation
          ↓
  IL-1, TNF, PGE2, NO, TGF-β, BMP release
          ↓
  Amplification loop → more matrix destruction
          ↓
  Chondrocyte apoptosis → cartilage loss
          ↓
  Subchondral bone exposure → eburnation, cysts, BMLs
          ↓
  Osteophyte formation at joint margins
          ↓
  Synovitis (secondary) + peripheral/central sensitization
          ↓
  Pain, stiffness, functional disability

Sources:

Robbins & Kumar Basic Pathology, pp. 789-791
Firestein & Kelley's Textbook of Rheumatology, pp. 75, 183, 499, 4585-4676
Goldman-Cecil Medicine, p. 2763
General Anatomy and Musculoskeletal System (THIEME Atlas), pp. 1895-1905

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