AKT · Musculoskeletal/Metabolic bone & tumours

Septic arthritis

Bacterial infection of a joint — Staph. aureus commonest (gonococcus in young sexually active adults)

Overview

Bacterial infection within a joint space — an orthopaedic EMERGENCY because pus rapidly destroys articular cartilage. Staphylococcus aureus is the commonest organism; Neisseria gonorrhoeae is important in young sexually active adults. Any acutely hot, swollen, painful joint must be aspirated and treated as septic until proven otherwise.

Recognise

Acutely hot, swollen, red, exquisitely painful joint with marked pain on any movement; the patient holds the joint still
Fever and systemic upset; usually monoarticular (knee commonest); prosthetic joints at particular risk
Risk factors: pre-existing joint disease (RA), prosthetic joint, immunosuppression, diabetes, IV drug use, skin breach

Red flags

A hot swollen joint = septic arthritis until proven otherwise → URGENT aspiration before antibiotics → joint washout
Prosthetic joint infection → orthopaedic emergency (revision surgery)

Differentials & how to tell them apart

Gout / pseudogoutcrystals on aspirate (but can coexist with infection — culture too)

Reactive arthritissterile aspirate, post-infectious

Haemarthrosisblood on aspirate, trauma/coagulopathy

Cellulitis / bursitisinfection of soft tissue/bursa without true intra-articular involvement

Investigations

URGENT joint aspiration BEFORE antibiotics — send synovial fluid for urgent Gram stain, culture and crystals (high WCC, organisms); blood cultures; FBC/CRP/ESR; X-ray (baseline); identify the source (e.g. gonococcal — genital swabs).

Management

Urgent aspiration → empirical IV antibiotics → joint washout

1Treat any acutely hot, swollen joint as septic until proven otherwise: URGENTLY aspirate (Gram stain/culture/crystals) BEFORE antibiotics, then start empirical IV antibiotics (flucloxacillin; vancomycin if MRSA risk; ceftriaxone if gonococcal).Gate: Aspirate BEFORE giving antibiotics (to capture the organism) — but do not let that delay treatment; arrange joint drainage/washout for source control.
2Continue a prolonged antibiotic course targeted to culture; prosthetic joint infection requires orthopaedic revision surgery.

Aspirate, THEN empirical IV antibiotics (e.g. flucloxacillin; vancomycin if MRSA risk) — do not delay antibiotics after aspiration; cover gonococcus (ceftriaxone) if suspected; targeted to culture

Joint drainage/washout (arthroscopic or open) — source control — pus must be drained

Prolonged antibiotic course — often several weeks, guided by microbiology

Prosthetic joint infection → revision surgery — orthopaedic management

Key points

Acutely hot, swollen, exquisitely painful joint = septic arthritis until proven otherwise → ASPIRATE before antibiotics, then IV flucloxacillin (vancomycin if MRSA, ceftriaxone if gonococcal) + joint washout. Pus destroys cartilage fast — an orthopaedic emergency. Crystals don't exclude infection.

Monitor & prognosis

Clinical/CRP response, repeat aspiration if needed, microbiology-guided antibiotic course.

Good with prompt drainage and antibiotics; delay causes permanent joint destruction.

Source: BSR septic arthritis; cross-ref sexual_health (gonococcal)