A 68-year-old woman from rural India presents with progressive hip pain, stiffness, and a limp over 5 years. Radiographs show joint space narrowing, osteophytes, and subchondral sclerosis. What is the most common cause of primary osteoarthritis at the hip joint?

Explanation

## Primary Hip Osteoarthritis: Etiology and Frequency ### Definition and Classification Hip osteoarthritis is classified into: - **Primary (idiopathic):** No identifiable structural predisposing cause - **Secondary:** Results from a known structural or metabolic abnormality **Key Point:** Primary (idiopathic) hip osteoarthritis is the most common form, accounting for 50–60% of all hip OA cases in the general population. ### Frequency of Hip OA Causes | Cause | Frequency | Notes | | --- | --- | --- | | **Primary/Idiopathic** | 50–60% | No structural predisposition; age and genetics are key | | **Developmental dysplasia of hip (DDH)** | 15–25% | Second most common; significant risk factor | | **Femoroacetabular impingement (FAI)** | 10–15% | Increasingly recognized in younger patients | | **Avascular necrosis** | 5–10% | Post-traumatic or idiopathic | | **Rheumatoid arthritis** | 5% | Inflammatory cause | | **Other (SCFE, Perthes, trauma)** | 5–10% | Various secondary causes | ### Why Primary OA is Most Common 1. **Age-related cartilage degeneration:** Cumulative wear and tear over decades 2. **Genetic predisposition:** Family history of OA increases risk 3. **No structural abnormality required:** Occurs in structurally normal hips with advancing age 4. **Female predominance:** Especially in postmenopausal women (as in this case) ### Clinical Pearl **High-Yield:** In an elderly patient with insidious-onset hip OA and normal hip anatomy on imaging (no dysplasia, no FAI, no AVN), the diagnosis is primary idiopathic OA. This is the most likely diagnosis in the general population. ### Mnemonic **PRIMARY OA = Most Common (>50%)** - **P**rimary (idiopathic) - **R**are structural abnormality - **I**nsidious onset - **M**iddle-aged to elderly - **A**ge and genetics - **R**adial joint space loss - **Y**ears of progression **Tip:** If the stem does NOT mention a specific structural abnormality (dysplasia, FAI, AVN, rheumatoid arthritis), default to primary idiopathic OA as the most common cause.