Septic Arthritis

1. Clinical Overview

Summary

Septic arthritis is an orthopaedic emergency. It involves bacterial invasion of the synovial joint space, leading to rapid destruction of articular cartilage (within hours/days) through chondrocyte death and matrix metalloproteinase release.

The classic presentation is a short history (less than 1 week) of a single hot, swollen, painful joint with restricted range of movement and systemic upset. Inability to weight bear is a key sign.

The gold standard investigation is Joint Aspiration before starting antibiotics. Management is Surgical Washout (Lavage) and prolonged antibiotics.

In prosthetic joints (PJI), the presence of a Biofilm makes eradication difficult, often requiring removal of the implant ("Two Stage Revision").

Key Facts

• Mortality: 11% in native joints. Higher in polyarticular disease.
• Knee: The most common site (50%). Hip is second.
• Organism: Staphylococcus aureus is the cause in 60% of cases.
• Cartilage Death: Proteolytic enzymes released by neutrophils destroy cartilage within 48 hours. Time is Cartilage.
• Sex: Gonococcal arthritis is the commonest cause in young, sexually active adults (often polyarticular + rash).

Clinical Pearls

The "Pseudoparalysis" Pearl: A joint with septic arthritis is held in the position of maximum volume (30 degrees flexion for knee, Flexion/Abduction/External Rotation for hip) to minimise pressure. Any movement causes screaming pain ("Pseudoparalysis"). If a child can move the joint through full range, it is unlikely to be septic arthritis.

The "Antibiotic" Pearl: NEVER give antibiotics before aspirating the joint (unless the patient is clinically septic/unstable). Giving antibiotics sterilises the fluid within 4 hours, making culture impossible and blinding definitive treatment.

The "X-Ray" Pearl: X-Rays are usually normal in early septic arthritis (soft tissue swelling only). Joint space narrowing takes days/weeks. Do not be reassured by a normal X-Ray.

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2. Epidemiology

Risk Factors (The "S" Factors)

• Skin Infection (Cellulitis/Ulcers) - Source.
• Surgery (Recent joint injection/operation).
• Systemic Disease (Diabetes, RA, CKD).
• Sexual Activity (Gonorrhoea).
• Steroids (Immunosuppression).
• Social (IV Drug Use).

Organisms

• Staph aureus: Most common (60%).
• Streptococcus: (Group A/B). Second most common.
• Neisseria gonorrhoeae: Young adults. Disseminated gonococcal infection.
• Gram Negatives (E. coli / Pseudomonas): IVDU, Elderly, UTI source.
• Kingella kingae: Children less than 4 years. (Requires PCR).

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3. Pathophysiology

Routes of Infection

1. Haematogenous: Most common. Bacteremia seeds the highly vascular synovium.
2. Direct Inoculation: Bite wound, Nail penetration, Iatrogenic (Injection).
3. Contiguous Spread: From Osteomyelitis (in children where metaphysis is intra-capsular e.g. Hip) or Abscess.

Joint Destruction Mechanism

1. Colonisation: Bacteria enter synovial fluid.
2. Inflammation: Synovium becomes hyperaemic. Neutrophils flood the joint (WCC > 50,000).
3. Pressure: Pus accumulates under pressure, causing tamponade of synovial vessels -> Avascular Necrosis (especially Femoral Head).
4. Enzymatic Lysis: Neutrophils die, releasing lysosomal enzymes and cytokines (IL-1, TNF) which digest cartilage collagen.
5. Pannus: Granulation tissue erodes bone.

Biofilms (Prosthetic Joints)

• Bacteria (especially Staph epidermidis) adhere to metal/plastic.
• They secrete a sugar matrix (Glycocalyx / Slime).
• Bacteria inside the biofilm enter a dormant "planktonic" phase.
• Resistance: They become 1000x more resistant to antibiotics and immune attack.
• Implication: You cannot kill biofilm bacteria with drugs. You must physically remove the metal.

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4. Clinical Presentation

History

• Acute Onset: less than 2 weeks.
• Pain: Severe, throbbing, constant. Worse on movement.
• Swelling: Visible effusion.
• Systemic: Fever, Rigors, Malaise.

Examination

• Look: Red (Erythema), Swollen, Held in "position of ease".
• Feel: Hot (calibrate with other knee). Extremely tender.
• Move:
  • Active: None (Pseudoparalysis).
  • Passive: Extremely painful and restricted.
  • Weight Bearing: Usually impossible.

Variant: Disseminated Gonococcal Infection (DGI)

Triad in young adults:

1. Migratory Polyarthralgia: Pains moving from joint to joint.
2. Tenosynovitis: Pain along tendons (wrists/ankles).
3. Dermatitis: Pustular/Vesicular rash on extremities (sparing face). Note: Joint cultures often negative. Need mucosal swabs (Throat/Rectal/Urethral).

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5. Diagnostic Criteria (Kocher Criteria)

(For The Child with a Limp) Differentiates Septic Arthritis from Transient Synovitis (Irritable Hip).

Probability:

• 1/4: 3%
• 2/4: 40%
• 3/4: 93%
• 4/4: 99%

Differential Diagnosis in Children

• Transient Synovitis: Post-viral ("Irritable Hip"). Kocher less than 2. Self limiting.
• Perthes Disease: Avascular necrosis. Insidious onset.
• SUFE (Slipped Epiphysis): Obese adolescent. Pain in knee.
• Juvenile Idiopathic Arthritis (JIA): Chronic (> 6 weeks).

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6. Investigations

Joint Aspiration (Arthrocentesis)

The Definitive Test.

• Appearance:
  • Normal: Clear / Straw.
  • Septic: Turbid / Opaque / Frank Pus.
• Microscopy:
  • WBC: > 50,000 cells/mm3 (often > 100,000).
  • Polymorphs: > 90%.
  • Gram Stain: Positive in 50% (Guide initial therapy).
  • Crystals: Rule out Gout (can coexist).
• Culture: Gold standard.

Bloods

• WBC: Elevated (Neutrophilia).
• CRP/ESR: Almost always elevated. (CRP rises faster, ESR falls slower).
• Blood Cultures: Positive in 50%. Mandatory.

Imaging

• X-Ray: Baseline. Exclude fracture/tumour.
• Ultrasound: Highly sensitive for effusion (Hip). Used to guide aspiration.
• MRI: Most sensitive. Shows bone oedema (Osteomyelitis) and soft tissue abscesses.

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7. Management: Medical

Antibiotics

Start IV immediately AFTER aspiration.

• Native Joint Protocol:
  • Adult: Flucloxacillin 2g IV QDS (High dose for bone penetration).
  • Penicillin Allergy: Clindamycin or Vancomycin.
  • Gram Negative Risk (Elderly/UTI): Add Gentamicin or Ceftriaxone.
  • Gonococcal: Ceftriaxone 1g IV OD.
• Duration: IV for 2 weeks, then Oral for 4 weeks (Total 6 weeks).

Analgesia & Splinting

• Splint the joint in functional position to relieve pain.
• Deep Thrombosis Prophylaxis (LMWH).

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8. Management: Surgical

(The Source Control - "Don't let the sun set on a septic joint")

Native Joint

• Arthroscopic Washout: Keyhole. Large volume saline irrigation (6-9 Litres). Debridement of synovium.
• Open Washout (Arthrotomy): Used for hips (difficult to scope) or if arthroscopy fails.
• Serial Washouts: Often require multiple trips to theatre (every 48 hours) until joint is clean.

Prosthetic Joint Infection (PJI) Strategy

1. DAIR (Debridement, Antibiotics and Implant Retention)

• Indication: Early infection (less than 3 weeks from surgery) OR Acute haematogenous (less than 3 weeks symptoms). Implant is stable.
• Procedure: Open joint, wash, change modular parts (plastic liner - "exchangeable parts"), keep metal.

2. One Stage Revision

• Remove all metal -> Massive wash -> Put new metal in immediately.
• Risk: Reinfection if bug not cleared.

3. Two Stage Revision (Gold Standard for Late Infection)

• Stage 1: Remove all metal. Insert Antibiotic Spacers (Cement loaded with Vancomycin/Gentamicin).
• Interval: 6-12 weeks of IV antibiotics.
• Stage 2: Re-implant new prosthesis once CRP normal and joint clean.

Long Term Suppression (Palliative)

For elderly/frail patients unfit for revision.

• Lifelong Oral Antibiotics: Doxycycline / Cephalexin.
• Goal is to keep infection "quiet", not cure.
• Review every 6 months.

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9. Complications

Local

• Cartilage Destruction: Prominent in Staph aureus. Leads to secondary Osteoarthritis.
• Osteomyelitis: Infection spreads to bone.
• Dislocation: Capsular destruction destabilises joint (esp Hip in babies - Tom Smith's Arthritis: destruction of cartilaginous femoral head).
• Growth Plate Arrest: In children, leading to limb length discrepancy.

Systemic

• Sepsis / Shock.
• Endocarditis: Always listen to the heart.

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10. Evidence & Guidelines

Guidelines

1. BOA (British Orthopaedic Association) BOAST: Diagnosis and Management of the Infected Native Joint.
   • Mandates aspiration before antibiotics.
   • Mandates multidisciplinary care (Ortho + Micro).
2. IDSA Guidelines: Treatment of MRSA in joints (Vancomycin targets).

Key Concepts

• Girdlestone Procedure: (Excision Arthroplasty). A "salvage" procedure for hips where infection cannot be cleared. The head is removed and NO implant is put back. The leg is short and wobbly, but pain-free and infection-free.

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12. Patient Explanation

What is Septic Arthritis?

It involves bacteria getting into the fluid of your knee joint. The joint is a closed space, so the bacteria grow rapidly and turn the fluid into pus.

Why do I need surgery?

Antibiotics travel in the blood, but they struggle to get into the "soup" of pus in the joint. We need to physically wash the pus out (like washing a dirty plate) so the antibiotics can finish the job.

Will my joint be normal?

If we caught it early (today), yes. If the bacteria have been there for a few days, they may have eaten some of the smooth movement cartilage. This might leave you with a "creaky" knee (arthritis) in the future.

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13. References

1. British Orthopaedic Association (BOA). BOAST - The Management of the Infected Native Joint. 2022.

2. Kocher MS et al. Differentiating between septic arthritis and transient synovitis of the hip in children: an evidence-based clinical prediction algorithm. J Bone Joint Surg Am. 1999 Dec;81(12):1662-70. PMID: 10608376

3. Mathews CJ et al. Bacterial septic arthritis in adults. Lancet. 2010 Mar 6;375(9717):846-55. PMID: 20206778

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14. Examination Focus

Common Exam Questions

1. "Patient with possible septic knee but on Warfarin (INR 3.0). Do you aspirate?"
   • Answer: YES. Septic arthritis destroys the joint in hours. Bleeding risk is minor compared to joint destruction. Use small needle/pressure.
2. "Antibiotic for Staph Aureus Septic Arthritis?"
   • Answer: Flucloxacillin 2g IV QDS.
3. "Organism in Sickle Cell patient?"
   • Answer: Salmonella.

Viva Points

• Kocher Criteria: Be able to list all 4: Non-weightbearing, Fever, ESR > 40, WBC > 12.
• Tom Smith's Arthritis: Septic arthritis of the hip in infancy destroying the unossified femoral head.

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Last Reviewed: 2026-01-04 | MedVellum Editorial Team