Septic Arthritis

Septic arthritis is a bacterial infection of a joint. It is an orthopaedic emergency because cartilage destruction occurs rapidly (within 24-48 hours).

Pathophysiology

Bacteria enter the joint via haematogenous spread (most common), direct inoculation (injection, surgery), or spread from adjacent osteomyelitis.

Inflammatory response and bacterial enzymes cause rapid cartilage destruction.

Risk Factors

• Rheumatoid arthritis
• Diabetes mellitus
• Immunosuppression
• Recent joint injection or surgery
• IV drug use
• Prosthetic joint

Routes of Infection

Bacteria reach the synovial space through three main routes:

1. Haematogenous spread (most common, 70-80%)

   • Bacteraemia from distant focus (skin, UTI, pneumonia)
   • Synovium is highly vascular with no basement membrane
   • Bacteria lodge and proliferate rapidly

2. Direct inoculation (15-20%)

   • Joint injection or aspiration
   • Arthroscopy or open surgery
   • Penetrating trauma

3. Contiguous spread (5-10%)

   • Adjacent osteomyelitis
   • Soft tissue infection spreading to joint

Pathological Cascade

Within hours:

• Bacterial proliferation triggers inflammatory response
• Neutrophil infiltration releases proteolytic enzymes (collagenase, elastase)
• Cytokines (IL-1, TNF-alpha) amplify destruction

Within 24-48 hours:

• Cartilage matrix degradation begins
• Proteoglycan loss impairs load-bearing capacity
• Chondrocyte death from hypoxia and enzyme damage

Beyond 48 hours:

• Irreversible cartilage damage
• Pannus formation
• Bone erosion at joint margins

Microbiology

Clinical Features

• Painful, swollen joint - acute onset over hours to days
• Unable to bear weight (if lower limb)
• Limited ROM (held in position of comfort - flexion for knee, abduction/ER for hip)
• Warmth and erythema over joint
• Fever (may be absent in elderly, immunocompromised)
• Usually monoarticular (knee most common, then hip, shoulder)

Joint Distribution

Differential Diagnosis

The acute hot, swollen joint has a wide differential. Crystals and infection can coexist, so identifying crystals NEVER excludes sepsis if the clinical picture fits.

Joint Aspiration (Gold Standard)

Aspiration technique:

• Aseptic technique essential
• Mark anatomical landmarks
• Aspirate BEFORE antibiotics if possible (but do not delay treatment)

Synovial fluid analysis:

Important: WCC greater than 50,000 has 90% sensitivity but crystals do NOT exclude infection - can coexist.

Blood Tests

• WCC - elevated in 50-60%
• CRP - elevated in greater than 90% (most sensitive)
• ESR - elevated but slow to change
• Blood cultures - positive in 40-50%
• Procalcitonin - may help differentiate from crystal arthropathy

Imaging

X-ray:

• Often normal early
• Soft tissue swelling, joint effusion
• Late: joint space narrowing, erosions, destruction

Ultrasound:

• Detects effusion (especially hip, shoulder)
• Guides aspiration
• Cannot differentiate septic from sterile effusion

MRI:

• Most sensitive for early changes
• Shows bone marrow oedema, soft tissue involvement
• Useful for deep joints (hip, SI joint)

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Algorithm

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Guidelines, Registries & Global Practice

Global epidemiology:

• Annual incidence of native-joint septic arthritis is roughly 4-10 per 100,000 in the general population, rising to 30-70 per 100,000 in patients with rheumatoid arthritis or prosthetic joints.
• S. aureus dominates worldwide; the proportion that is MRSA varies widely by region (low in much of Scandinavia and the Netherlands, considerably higher in parts of the USA, South Asia and among community strains in remote and Indigenous populations).
• Gonococcal arthritis remains an important cause in young sexually active adults globally; tuberculous and fungal joint sepsis are disproportionately common in high-TB-burden and resource-limited settings and in the immunocompromised.

Side-by-side guidance:

Regional empirical regimens differ chiefly in how aggressively they cover MRSA — driven by local resistance data rather than true clinical disagreement. The universal principles (aspirate first, urgent drainage, source control, ID/microbiology involvement) are consistent across all bodies.

Registry and resource notes:

• Arthroplasty registries (NJR, AJRR, AOANJRR, the Swedish and Norwegian registers) track PJI as a leading cause of revision and inform implant- and fixation-specific infection risk; deep infection is among the top three reasons for revision knee arthroplasty in most registries.
• High-resource settings favour arthroscopic washout, antibiotic-loaded cement spacers and multidisciplinary bone-infection units. In limited-resource settings, open arthrotomy with serial saline lavage, longer empirical courses, and a lower threshold for considering tuberculous or fungal aetiology are appropriate adaptations.

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Early Complications

Persistent infection:

• Inadequate debridement - failure to remove all infected tissue
• Resistant organism - MRSA, multi-drug resistant GNB
• Biofilm formation (especially PJI) - bacteria protected from antibiotics
• May require repeated washouts (every 48-72 hours until resolved)
• Consider changing antibiotic regimen based on sensitivities

Systemic sepsis:

• Can progress to septicaemia with bacteraemia
• Multi-organ failure in severe cases (ARDS, AKI, DIC)
• Mortality 10-15% overall
• Higher mortality in elderly (up to 30%) and immunocompromised patients
• Requires ICU admission and aggressive resuscitation

Wound complications:

• Wound dehiscence after open washout
• Sinus tract formation with chronic drainage
• Skin necrosis requiring plastic surgery input

Late Complications

Joint destruction:

• Cartilage loss is irreversible once proteoglycans depleted
• Occurs within 24-48 hours without treatment
• Results in secondary osteoarthritis requiring arthroplasty
• Worse outcomes in weight-bearing joints (hip, knee)

Osteonecrosis:

• Particularly hip joint in children
• Septic arthritis can damage blood supply to femoral head
• May develop Perthes-like changes
• Long-term surveillance required

Ankylosis:

• Fibrous or bony fusion of joint surfaces
• More common with delayed treatment beyond 7 days
• May require arthrodesis for pain relief
• Consider arthroplasty if bone stock adequate

Growth disturbance (children):

• Physeal damage if infection crosses growth plate
• Limb length discrepancy (can be several centimetres)
• Angular deformity requiring corrective osteotomy
• Growth arrest lines visible on X-ray

Chronic pain and stiffness:

• Post-infectious arthritis even after eradication
• Reduced range of motion from fibrosis
• May require prolonged rehabilitation

Outcomes and Prognosis

Functional outcomes:

• Good to excellent outcome in 70-80% if treated promptly
• Poor outcome associated with delayed diagnosis
• Hip and shoulder have worse functional prognosis

Prognostic Factors

Poor prognosis associated with:

• Delay in treatment more than 7 days (single most important factor)
• Age more than 65 years
• Pre-existing joint disease (RA, OA)
• Polyarticular involvement (often haematogenous)
• Virulent organisms (S. aureus worse than streptococci)
• Prosthetic joint involvement
• Immunocompromised state (diabetes, HIV, malignancy, steroids)
• Axial joint involvement (hip, shoulder)

Good prognosis associated with:

• Treatment within 24-48 hours
• Single joint involvement
• Streptococcal or gonococcal infection
• Young healthy patient
• Peripheral joint (knee, ankle)

• Antibiotic duration. The Gjika RCT (PMID 30992295) showed 2 weeks non-inferior to 4 weeks after surgical drainage, but the cohort was dominated by hand/wrist joints. How far this extends to large weight-bearing joints, S. aureus bacteraemia, or immunocompromised hosts remains unsettled, and most units still use 3-6 weeks for large-joint or staphylococcal disease.
• Needle aspiration vs surgical washout for native joints. Serial closed aspiration achieves source control in selected accessible joints (notably the knee) in some series, while many surgeons regard urgent arthroscopic or open washout as standard. No RCT settles this; decision rests on joint, organism, loculation and response.
• Arthroscopic vs open drainage. Meta-analytic data (PMID 33939020) favour arthroscopy for knee and shoulder, but the evidence is low-quality and retrospective; the hip is still generally opened.
• DAIR vs early revision in acute PJI. Success of DAIR depends heavily on a short symptom duration, well-fixed implant, susceptible organism and exchange of modular parts. The exact time threshold (often quoted as ~3 weeks) and the role of rifampicin combinations for staphylococci are debated.
• Diagnostic biomarkers. Synovial alpha-defensin, leucocyte esterase and serum/synovial procalcitonin add specificity but are imperfect; none replaces aspiration, culture and clinical judgement. The "WCC over 50,000" threshold is a guide, not an absolute cut-off (sensitivity is well under 100%).
• Crystals plus infection. Demonstrating gout or CPPD does not exclude concurrent sepsis; missed co-existent infection is a recognised pitfall.