Pathophysiology
Clinical meaning
Synovial joints are the most clinically relevant joints, consisting of articular cartilage (hyaline cartilage covering bone ends, 2-4 mm thick, avascular, nourished by synovial fluid diffusion), the synovial membrane (highly vascularized connective tissue lining the joint capsule, producing synovial fluid), the joint capsule (fibrous outer layer providing structural support), and supporting ligaments, tendons, and bursae. Synovial fluid is an ultrafiltrate of plasma enriched with hyaluronic acid (provides viscosity), lubricin (boundary lubrication), and glucose. Normal synovial fluid is clear, viscous, with <200 WBC/mm3. Arthrocentesis (joint aspiration) is the gold standard for diagnosing joint pathology: non-inflammatory (<2000 WBC, >75% mononuclear) = osteoarthritis, traumatic; inflammatory (2000-50,000 WBC, >50% PMN) = rheumatoid arthritis, crystal arthropathy, reactive arthritis; septic (>50,000 WBC, >90% PMN) = bacterial infection requiring urgent treatment. Crystal analysis under polarized light microscopy: monosodium urate crystals (gout) are needle-shaped with strong negative birefringence (yellow when parallel to compensator axis); calcium pyrophosphate dihydrate crystals (pseudogout/CPPD) are rhomboid-shaped with weak positive birefringence (blue when parallel). Osteoarthritis involves progressive degradation of articular cartilage through mechanical wear and enzymatic destruction (matrix metalloproteinases), subchondral bone remodeling, osteophyte formation,...