## Most Common Site of Osteoarthritis in Lower Limb **Key Point:** The knee is the most frequently affected weight-bearing joint in the lower limb, followed by the hip. This is due to the high mechanical load and stress transmitted through the knee during standing and walking. ### Epidemiology of Lower Limb OA | Site | Frequency | Reason | |------|-----------|--------| | Knee | Most common (40-50% of symptomatic OA) | Highest load-bearing stress, large surface area | | Hip | Second most common (15-20%) | Weight-bearing, but less frequent than knee | | Ankle | Uncommon (2-5%) | Usually post-traumatic | | 1st MTP joint | Rare in isolation | More common in hallux limitus | **High-Yield:** In the **lower limb**, knee > hip > ankle > other joints. In the **upper limb**, the first carpometacarpal (CMC) joint of the thumb is most common, followed by the proximal interphalangeal (PIP) joints. **Clinical Pearl:** Knee OA is often bilateral and symmetrical. The medial compartment is more frequently affected than the lateral compartment due to greater load transmission through the medial side during weight-bearing. ### Why Knee is Most Common 1. **Mechanical load:** Knee bears 3–6 times body weight during walking 2. **Joint surface area:** Large articular surface with complex biomechanics 3. **Age-related changes:** Cartilage degenerates progressively with age 4. **Prior injury:** Meniscal tears, ligamentous injuries increase risk **Warning:** Do not confuse hand OA prevalence (most common site overall across all joints) with lower limb OA prevalence (knee is most common in lower limb specifically).
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