## Most Common Site of Septic Arthritis **Key Point:** The knee is the most frequently affected joint in septic arthritis, accounting for 40–50% of all cases across age groups. ### Distribution of Septic Arthritis by Joint | Joint | Frequency (%) | Age Group | Pathophysiology | |-------|---------------|-----------|------------------| | **Knee** | 40–50% | All ages | Largest weight-bearing joint; high vascularity; frequent trauma | | Hip | 15–25% | Infants & young children; adults | Intra-articular blood supply in children; deep location limits early detection | | Ankle | 10–15% | All ages | Smaller joint; less common hematogenous seeding | | Shoulder | 5–10% | All ages | Least common; superficial location | | Wrist/Elbow | 5–10% | IV drug users | Associated with injection sites | ### Why the Knee is Most Common 1. **Anatomical factors:** - Largest synovial joint in the body - High vascularity with rich blood supply to synovium - Frequent site of trauma and microtrauma 2. **Biomechanical factors:** - Bears significant body weight during standing and walking - High synovial fluid turnover and metabolic activity - Increased risk of hematogenous seeding during bacteremia 3. **Clinical accessibility:** - Superficial location allows early clinical detection - Easier diagnosis with physical examination and aspiration **High-Yield:** When a patient presents with acute monoarthritis, always examine the knee first — it is the most likely joint to be infected. **Clinical Pearl:** In infants and young children (<3 years), the hip is the second most common site and carries higher morbidity due to risk of avascular necrosis from increased intra-articular pressure. **Mnemonic:** **BIG KNEE** — **B**iggest joint, **I**nfected most, **G**reat vascularity → **K**nee **N**eeds **E**arly **E**xamination.
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