Pathogenesis of osteoarthritis
Osteoarthritis has traditionally been considered a noninflammatory ‘wear and tear’ disease process but it is increasingly recognised that local and systemic inflammation play a key role in its pathogenesis. Unlike autoimmune arthritis (eg rheumatoid arthritis), the systemic inflammation seen in osteoarthritis is not the primary driver of the disease process. It is likely a secondary process induced by cartilage damageMahmoudian, 2021.
Osteoarthritis results from dynamic and complex interactions that affect the whole joint complex. These interactions involve inflammatory, mechanical, molecular and metabolic changes that lead to an imbalance between tissue injury and degeneration, and tissue repair mechanisms, and result in articular cartilage degeneration. The ability of the damaged cartilage to absorb and distribute the mechanical load through the joint is impaired, which leads to increased mechanical stress and further joint damage.
The pain experienced by patients with osteoarthritis is not due to the loss of articular cartilage, because cartilage does not contain neural or vascular structures. Rather, pain is associated with a range of intra-articular and periarticular sources of nociception, including synovitis, subchondral bone changes (reported as bone marrow oedema on magnetic resonance imaging [MRI]), periosteum disruption associated with osteophyte formation, microfractures, ligament degeneration, capsular distension (due to an effusion), muscle spasm, nerve compression, and inflammation of bursa, tendon, muscle, or fascia. As the disease advances, these structures are affected and changes including bone remodelling, osteophyte formation, weakening of periarticular muscles, laxity of ligaments and synovial effusion can become evident.
For patients with chronic osteoarthritic pain, central sensitisation can also contribute to the experience of pain. Pain sensitisation has been shown to be independent of radiographic severity, or symptom or disease duration; rather, it seems to be related to synovitis and effusion.