## Most Common Cause of Hip Osteoarthritis ### Classification of Hip OA **Key Point:** Primary (idiopathic) osteoarthritis accounts for approximately 85–90% of all hip OA cases in the general population, making it the most common cause by far. ### Primary vs. Secondary OA | Cause | Frequency | Mechanism | Age of Onset | |-------|-----------|-----------|---------------| | **Primary (Idiopathic)** | 85–90% | Age-related cartilage degeneration | >50 years | | Developmental dysplasia | 5–10% | Abnormal hip anatomy from birth | 40–50 years | | Post-traumatic | 2–5% | Femoral neck fracture, acetabular fracture | Variable | | Avascular necrosis | 1–3% | Femoral head necrosis | 30–50 years | | Other (SUFE, Perthes, etc.) | <1% | Childhood hip pathology | Variable | ### Why Idiopathic OA is Most Common 1. **Age-related degeneration**: Progressive loss of cartilage elasticity and proteoglycan content with aging. 2. **Cumulative microtrauma**: Decades of normal weight-bearing and gait lead to cartilage fatigue. 3. **No obvious predisposing factor**: Occurs in individuals with normal hip anatomy and no prior hip disease. 4. **Genetic predisposition**: Familial clustering suggests inherited susceptibility to cartilage degeneration. **High-Yield:** In clinical practice, when a patient >50 years presents with hip OA WITHOUT a history of hip trauma, DDH, or childhood hip disease, assume primary idiopathic OA until proven otherwise. **Clinical Pearl:** Primary hip OA typically presents with insidious onset of groin pain, morning stiffness, and progressive loss of internal rotation—classic features of age-related cartilage degeneration. ### Distinguishing Features **Mnemonic: DDFA** — Developmental, Dysplasia, Fracture, Avascular — these are the main secondary causes to rule out, but they account for only 10–15% of hip OA.
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