Infectious DiseasesTier 1Disease (DEADMAN)

Septic arthritis

Description

Bacterial infection of a joint space - an orthopaedic and medical emergency
Untreated destroys articular cartilage within days -> irreversible joint damage - the reason for urgent aspiration and drainage, not deferred work-up

Most commonly affects large joints - knee (commonest), hip, shoulder
Monoarticular in ~90%
Higher risk in prosthetic joints, pre-existing joint disease (RA), diabetes, immunosuppression, IV drug use, extremes of age

Routes of infection

Organisms

Staphylococcus aureus - commonest overall, including in prosthetic joints
Streptococci - second commonest
Neisseria gonorrhoeae - consider in sexually active young adults, can present as migratory polyarthralgia/tenosynovitis before localising, or as classic monoarthritis; associated dermatitis
Gram-negative bacilli - older adults, immunosuppressed, urinary source
Prosthetic joint: coagulase-negative staphylococci and S. aureus predominate (see Line and prosthetic infections note)

Clinical

Acute hot, swollen, painful joint with markedly reduced range of motion, often fever
Higher suspicion with risk factors above

The essential test - joint aspiration before antibiotics wherever possible

Synovial fluid: WCC often >50,000/microL with neutrophil predominance (>90%) supports septic arthritis, though no cell count reliably excludes it - send regardless of count if clinical suspicion is high
Gram stain and culture of synovial fluid - culture negative in a meaningful minority (especially gonococcal, or with pre-treatment antibiotics)
Blood cultures - frequently positive, especially with S. aureus
Exclude crystal arthropathy (gout/pseudogout) on polarised microscopy - but crystals and infection can coexist, do not let crystals alone exclude septic arthritis in a high-risk patient
Plain X-ray (baseline, may show effusion/joint space changes late); MRI/ultrasound if diagnosis unclear, adjacent osteomyelitis suspected, or joint (e.g. hip) not easily aspirated at bedside

A. Source control - urgent joint drainage, equal priority to antibiotics

Repeated needle aspiration, arthroscopic washout, or open surgical drainage - orthopaedic involvement essential, especially for the hip (poorly accessible to repeated aspiration) or failure to improve
Prosthetic joint septic arthritis - managed per Line and prosthetic infections note (DAIR vs revision)

B. Empirical antibiotics - start immediately after aspiration, do not await culture

Cover S. aureus (including MRSA if risk factors) and streptococci - e.g. flucloxacillin (or vancomycin if MRSA risk/severe penicillin allergy)
Add gonococcal cover (ceftriaxone) if clinical picture suggests (young, sexually active, migratory arthralgia, tenosynovitis, dermatitis)
Add Gram-negative cover in older adults, immunosuppressed, or IV drug users pending culture
Narrow to targeted therapy once organism/sensitivities confirmed

C. Duration

Typically 2-4 weeks total, often starting IV with transition to oral once clinically improving and cultures guide therapy (native joint, uncomplicated)
Longer if osteomyelitis coexists or prosthetic material involved

D. Supportive

Analgesia, initial joint rest followed by early physiotherapy once infection controlled - prolonged immobilisation worsens functional outcome

Cartilage destruction begins within days of untreated infection - urgency of drainage cannot be overstated
Complications: osteomyelitis, joint destruction/ankylosis, sepsis
Prosthetic joint septic arthritis - high relapse risk without appropriate surgical management
Gonococcal septic arthritis - generally excellent prognosis with prompt treatment, less destructive than staphylococcal disease

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