A 28-year-old woman presents with acute pain and effusion in the left knee joint. She denies recent trauma or surgery. Laboratory investigation of synovial fluid confirms septic arthritis. Which is the most common site of septic arthritis in adults?

Explanation

## Most Common Site of Septic Arthritis in Adults **Key Point:** The knee joint is the most common site of septic arthritis in adults, accounting for approximately 40–50% of all cases of bacterial arthritis. ### Frequency of Septic Arthritis by Joint | Joint | Frequency in Adults | Frequency in Children | Clinical Notes | |---|---|---|---| | Knee | 40–50% | 40–50% | Most common; large weight-bearing joint; high synovial blood flow | | Hip | 15–20% | 30–40% | More common in children; higher morbidity if delayed | | Shoulder | 10–15% | 5–10% | Often associated with IV drug use or endocarditis | | Ankle | 5–10% | 5–10% | Less common; smaller joint | | Wrist/Elbow | 5–10% | 5–10% | Uncommon; usually follows direct inoculation | | Other (sacroiliac, sternoclavicular) | <5% | <5% | Rare; often in IV drug users (sternoclavicular) | **High-Yield:** The knee is predisposed to infection because of: 1. **Large synovial surface area** — greater exposure to bacteremia 2. **Rich synovial blood supply** — facilitates hematogenous seeding 3. **Frequent weight-bearing stress** — may impair local immune defenses 4. **Large joint volume** — allows rapid accumulation of purulent fluid ### Clinical Presentation at the Knee **Clinical Pearl:** Septic arthritis of the knee typically presents with: - Acute onset of severe pain (often monoarticular) - Marked swelling and warmth - Severe limitation of motion (especially flexion and extension) - Systemic signs: fever, chills, malaise - Positive synovial fluid culture (gold standard for diagnosis) ### Differential by Age and Risk Factor **Mnemonic:** **KHIPS** — **K**nee (most common overall); **H**ip (more common in children); **I**V drug users → shoulder/sternoclavicular; **P**ost-surgical → any joint; **S**eptic → always consider knee first. ### Why Hip is Second Most Common While the hip is the second most common site overall, it is **more frequent in children** (30–40% of pediatric cases) because: - Proximity to the femoral artery (hematogenous seeding) - High vascularity of the femoral neck - Risk of rapid cartilage destruction if diagnosis is delayed [cite:Harrison 21e Ch 330; Robbins 10e Ch 26]