Acute Septic Arthritis of the Knee
CIM Case: Acute Septic Arthritis of the Knee
Clinical Scenario
Patient: 58-year-old man Presentation: 3-day history of rapidly progressive right knee pain, swelling, and inability to weight-bear, preceded by corticosteroid injection 10 days ago Relevant history: Type 2 diabetes mellitus (HbA1c 9.2%), hypertension, previous intra-articular steroid injection for osteoarthritis 10 days prior, no recent surgery, no prior joint infections Examination findings:
- Temperature 38.5°C, HR 105 bpm
- Large tense effusion right knee
- Marked warmth and erythema
- Exquisite tenderness to palpation
- Severe pain with any passive motion (0-30° only)
- Held in slight flexion (position of comfort)
- Unable to weight-bear
- No draining sinus
- Intact skin (healed injection site)
- Neurovascularly intact distally
Investigations Provided
Laboratory Results
| Test | Result | Normal Range | Interpretation |
|---|---|---|---|
| WCC | 18.5 ×10⁹/L | 4-11 ×10⁹/L | ↑ Leucocytosis |
| Neutrophils | 85% | 40-70% | ↑ Neutrophilia |
| CRP | 185 mg/L | <5 mg/L | ↑↑ Markedly elevated |
| ESR | 75 mm/hr | <20 mm/hr | ↑ Elevated |
| Hb | 132 g/L | 130-170 g/L | Normal |
| Creatinine | 105 μmol/L | 60-110 μmol/L | Normal |
| Glucose | 14.2 mmol/L | 4-8 mmol/L | ↑ Hyperglycaemia |
| HbA1c | 9.2% | <7% | ↑ Poor diabetic control |
| Blood cultures | Pending | - | Await results |
Synovial Fluid Analysis
| Test | Result | Interpretation |
|---|---|---|
| Appearance | Turbid, purulent | Concerning for infection |
| WCC | 85,000 cells/μL | ↑↑ Highly suggestive of septic arthritis |
| PMN % | 95% | ↑↑ Neutrophil predominance |
| Gram stain | Gram-positive cocci in clusters | Likely Staphylococcus |
| Culture | Pending | - |
| Crystals | None | Excludes gout/pseudogout |
Imaging
Image 1: AP and Lateral Radiographs of Right Knee
Radiological features:
- Large joint effusion
- Soft tissue swelling
- Moderate degenerative changes (pre-existing OA)
- No periarticular osteopenia yet (too early)
- No bony erosions visible (early stage)
- No evidence of osteomyelitis
- No loose bodies
Questions & Model Answers
What is the diagnosis and what clinical criteria support it?
What investigations would you perform and why?
Describe your management plan for this patient.
What organism do you expect and what antibiotics would you use?
What factors affect prognosis in septic arthritis?
The patient is not improving after 48 hours. What is your approach?
Key Teaching Points
Pattern Recognition
This pattern suggests Septic Arthritis:
- Acute monoarticular joint swelling and pain
- Fever and systemic symptoms
- Risk factors (diabetes, immunocompromise, recent injection)
- Unable to weight-bear or range the joint
- Synovial WCC >50,000 with >90% PMN
Septic Arthritis vs Gout vs Pseudogout:
| Feature | Septic Arthritis | Gout | Pseudogout |
|---|---|---|---|
| Onset | Hours to days | Hours | Hours to days |
| Fever | Common | Possible | Possible |
| WCC (synovial) | >50,000 | 10,000-50,000 | 10,000-50,000 |
| Crystals | Absent | Urate (negative birefringent) | CPPD (positive birefringent) |
| Gram stain | Positive (60%) | Negative | Negative |
| Response to NSAID | No | Yes (dramatic) | Yes |
CRITICAL: Crystals do NOT exclude infection!
- Gout attack can be triggered by septic arthritis
- If any doubt, treat as septic until cultures negative
Critical Management Points
- URGENT SURGICAL DRAINAGE - within 24 hours
- Aspirate BEFORE antibiotics - if feasible
- Blood cultures - positive in 30-50%
- Staph aureus is most common - empirical flucloxacillin
- Multiple washouts often needed - 30-40% require repeat
- Minimum 4-6 weeks antibiotics - don't stop early
- Monitor CRP - should halve every 48-72 hours
Common Examiner Follow-ups
Q: "How does septic arthritis in a native joint differ from prosthetic joint infection?"
| Feature | Native Joint Septic Arthritis | Prosthetic Joint Infection |
|---|---|---|
| Presentation | Acute, obvious | May be subtle/chronic |
| Organisms | Staph aureus predominant | Coagulase-negative Staph, Staph aureus |
| Biofilm | Absent | Present on implant |
| Treatment | Washout + antibiotics | Often requires implant removal |
| Antibiotic duration | 4-6 weeks | 6 weeks minimum, often longer |
| Cure rate | 90%+ with prompt treatment | 80-90% with two-stage revision |
Q: "What is the role of arthroscopy versus open arthrotomy?"
| Arthroscopic Washout | Open Arthrotomy |
|---|---|
| Preferred first-line | Reserved for failures |
| Less morbidity, faster recovery | More complete debridement possible |
| Good visualisation of joint | Better access to posterior knee |
| Can be repeated easily | Greater soft tissue disruption |
| Limited by loculations | Useful for concurrent osteomyelitis |
Most centres now prefer arthroscopic washout initially, with conversion to open if not improving after 2-3 attempts.
Q: "When would you involve infectious diseases specialists?"
Indications for ID consultation:
- Unusual organisms (MRSA, Gram-negatives, fungi)
- Not responding to empirical therapy
- Antibiotic allergy
- Concurrent osteomyelitis
- Immunocompromised host
- Prosthetic joint infection
- Need for long-term suppressive therapy
Q: "What are the risk factors for developing septic arthritis after intra-articular injection?"
Risk factors:
- Diabetes mellitus - impaired immunity
- Immunosuppression - RA, steroids, biologics
- Skin infection - injection through infected skin
- Poor sterile technique - rare with proper preparation
- Pre-existing joint disease - damaged synovium
Risk of septic arthritis post-injection: approximately 1 in 10,000-20,000 injections
Prevention: strict aseptic technique, avoid injection through infected skin, use single-dose vials.
Related Topics
- Paediatric Septic Arthritis (Kocher Criteria)
- Prosthetic Joint Infection
- Osteomyelitis
- MRSA Infections in Orthopaedics
- Antibiotic Therapy in Orthopaedic Infections