## Pathological Features of Osteoarthritis **Key Point:** Osteoarthritis is a degenerative joint disease characterized by progressive loss of articular cartilage and reactive bone changes. Pannus formation is NOT a feature of OA — it is pathognomonic of inflammatory arthropathies like rheumatoid arthritis. ### Characteristic Pathological Features of OA | Feature | Pathology | Clinical Significance | |---------|-----------|----------------------| | Cartilage fibrillation | Progressive loss and softening of articular cartilage | Early sign; leads to joint space narrowing | | Osteophytes | Bony projections at joint margins | Radiographic hallmark; may limit motion | | Subchondral sclerosis | Increased bone density beneath cartilage | Reactive change; seen on X-ray | | Subchondral cysts | Fluid-filled cavities in subchondral bone | Result of cartilage loss and synovial fluid intrusion | | Synovial inflammation | Mild chronic inflammation | Secondary; not primary feature | ### Why Pannus Is NOT OA **High-Yield:** Pannus is a layer of inflammatory tissue (granulation tissue with fibroblasts, macrophages, and blood vessels) that erodes cartilage and bone. It is the hallmark of **rheumatoid arthritis**, not osteoarthritis. - **RA pannus:** Aggressive, erosive, inflammatory origin - **OA:** Mechanical degeneration, non-inflammatory (or minimally inflammatory) **Clinical Pearl:** The distinction matters clinically — RA requires DMARDs and biologics; OA is managed with analgesics, physical therapy, and intra-articular injections. ### Mnemonic for OA Pathology **LOSS:** - **L**oss of cartilage (fibrillation) - **O**steophytes (bone spurs) - **S**clerosis (subchondral) - **S**ubchondral cysts **Warning:** Do not confuse OA with RA. RA has pannus, systemic symptoms, and elevated inflammatory markers; OA is localized and non-inflammatory.
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