Bacterial infection of a joint — orthopedic emergency requiring urgent arthrocentesis and drainage.
Also known as: septic arthritis, pyogenic arthritis, bacterial arthritis
Overview
Pyogenic infection of a joint space, most commonly bacterial. Untreated, it leads to rapid cartilage destruction within days. Native joint and prosthetic joint infections share principles but differ in microbiology and surgical management.
Epidemiology
Annual incidence ~4-10 per 100,000; higher in patients with RA, prosthetic joints, diabetes, or immunosuppression. Knee is the most commonly involved joint in adults; hip is more common in young children.
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Intravenous drug use (sternoclavicular, sacroiliac, vertebral involvement)
Recent joint surgery or intra-articular injection
Skin or soft tissue infection, bacteremia, endocarditis
Age <5 or >65
Sexually active young adult (gonococcal arthritis)
Pathophysiology
Bacteria enter the joint by hematogenous seeding (most common), direct inoculation (trauma, injection, surgery), or contiguous spread from adjacent osteomyelitis or soft tissue infection. Bacterial proliferation drives a brisk neutrophilic synovitis; bacterial enzymes and host inflammatory mediators rapidly destroy articular cartilage.
Ultrasound — guides aspiration, especially for deep joints (hip)
MRI — if osteomyelitis suspected or to evaluate axial joints (SI, sternoclavicular)
Diagnostic algorithm
Synovial Fluid
Normal
Non-inflammatory (OA)
Inflammatory (RA, crystal)
Septic
WBC/mm3
<200
<2,000
2,000-50,000
>50,000 (often >100,000)
% PMNs
<25%
<25%
>50%
>75%
Color/clarity
Clear, pale yellow
Clear, yellow
Yellow, cloudy
Purulent
Crystals
None
None
Possible
May coexist
Culture
Negative
Negative
Negative
Often positive
Synovial fluid analysis — the cornerstone of joint diagnosis.
Complications
Rapid cartilage destruction with permanent joint damage if treatment delayed
Osteomyelitis of adjacent bone
Sepsis and septic shock
Chronic post-infectious arthritis
Avascular necrosis of femoral head (pediatric hip)
Recurrence, especially with prosthetic joints
PANCE pearls
Synovial fluid WBC >50,000 with PMN predominance strongly suggests infection, but no threshold rules it out — culture is definitive.
Gonococcal arthritis often has lower fluid WBC and culture sensitivity; suspect in young, sexually active patients with migratory polyarthralgia, tenosynovitis, and pustular rash.
Crystals and infection can coexist — never exclude septic arthritis just because crystals are found.
A prosthetic joint with persistent pain after the early post-op period is infected until proven otherwise — aspirate before starting antibiotics if at all possible.
Children with refusal to bear weight, fever, ESR/CRP elevation, and high WBC (Kocher criteria) should be evaluated for pediatric septic hip.
References
IDSA 2013 — IDSA Clinical Practice Guideline for the Diagnosis and Management of Prosthetic Joint Infection (Osmon et al., Clin Infect Dis 2013)
AAOS 2022 — AAOS Clinical Practice Guideline on Diagnosis and Prevention of Periprosthetic Joint Infections (2022)
BSR 2006 — BSR/BHPR Guideline for Management of the Hot Swollen Joint (Coakley et al., Rheumatology 2006)
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