Joint injection / aspiration
Formal Definition
Arthrocentesis; the procedure of aspirating synovial fluid from a joint for diagnostic analysis (cell count, Gram stain, culture, crystal analysis) or therapeutic relief; also refers to injection of corticosteroids or hyaluronic acid for inflammatory and degenerative joint conditions; most commonly performed at the knee, shoulder, ankle, and wrist.
How It's Used on the Ward
"Tap the joint" or "joint tap" or "inject the knee" — aspirating a swollen joint for diagnosis or injecting steroids for pain relief.
Example
""Acutely swollen right knee in a febrile patient with prior gout history: must rule out septic arthritis. Knee aspiration: 80,000 WBC/hpf, predominantly PMNs — septic arthritis until proven otherwise. Starting IV antibiotics, orthopedics called for formal joint washout.""
Clinical Context
Synovial fluid analysis: WBC <200 = normal; 200–2,000 = non-inflammatory (OA, trauma); 2,000–50,000 = inflammatory (gout, pseudogout, RA); >50,000 (predominantly PMNs) = septic until proven otherwise. Crystals: negatively birefringent needles = monosodium urate (gout); positively birefringent rhomboids = calcium pyrophosphate (pseudogout/CPPD). NEVER assume gout in a febrile patient with joint effusion without ruling out infection — septic arthritis is the joint-destructive emergency that cannot wait. Corticosteroid injections: effective for 4–6 weeks in inflammatory arthritis; limit to 3–4 injections per year per joint (cartilage damage risk). Ultrasound-guided aspiration improves accuracy for deep joints (hip, shoulder).