## Most Common Predisposing Factor for Primary Osteoarthritis **Key Point:** Age is the single most important and most common risk factor for primary (idiopathic) osteoarthritis. The prevalence of OA increases exponentially with age. ### Age as the Dominant Risk Factor **High-Yield:** - Radiographic evidence of OA is present in >80% of individuals >65 years of age. - Symptomatic OA prevalence doubles with each decade after age 50. - Age-related changes in cartilage matrix, reduced chondrocyte function, and accumulated microtrauma all contribute. ### Major Risk Factors for Primary OA | Risk Factor | Mechanism | Prevalence | |-------------|-----------|------------| | **Age (>50 years)** | Cumulative joint wear, reduced cartilage repair | Most common; exponential increase | | **Female sex** | Hormonal factors, post-menopausal changes | Women > men after age 50 | | **Obesity** | Increased mechanical load, inflammatory mediators | Strong modifiable risk factor | | **Genetic predisposition** | Collagen mutations, inherited joint structure | 50% heritability | | **Joint injury history** | Post-traumatic OA (secondary) | Modifiable; preventable | | **Muscle weakness** | Loss of dynamic joint stabilization | Age-related sarcopenia | **Clinical Pearl:** While trauma, inflammatory arthritis, and chronic corticosteroid use can *cause* secondary OA, they are not the most common *predisposing* factors for primary OA in the general population. Age is universal and unavoidable. **Mnemonic:** **AGE-OA** = Age, Gender (female), Environment/Estrogen, Obesity, Anatomy (joint structure).
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