Chronic disease involving the imbalance between wear and repair of articular (hyaline) cartilage leading to progressive cartilage loss and accompanying periarticular change
Aetiology
Primary OA
Primary OA is defined as a 'common complex disorder' with multiple risk factors
Biomechanical factors - joint injury, occupational/recreational usage, reduced muscle strength, joint laxity, joint malalignment (e.g. genu varum can lead medial knee OA, genu valgum can lead to lateral knee OA)
Affects weight-bearing or active joints
Presents > 50 years
Secondary OA
Secondary OA occurs when OA affects an unexpected site due to overuse, previous injury or previous arthritis
Examples of conditions which can lead to secondary OA include rheumatoid arthritis and gout
Pathophysiology
Metabolically active dynamic process that involves all joint tissues - cartilage, bone, synovium/capsule, ligaments and muscle
Involves an imbalance between the wear and repair of cartilage between joints
Key pathological changes include localised loss of hyaline cartilage and remodelling of adjacent bone with new bone formation (osteophyte) at joint margins
Leads to increased pressure on the bony surfaces and inflammation leads to pain, swelling (inflammatory effusion), thickening of the capsule and stiffness
Any synovial joint can be affected but knees, hands and hips are the most commonly affected joints
Development of OA
Chondrocyte injury - genetic and biochemical factors
Chondrocytes proliferate - release inflammatory mediators, proteases, collagen and proteoglycans
Remodelling and degradation of cartilage
Stimulates inflammatory changes in synovium and subchondral bone