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:
| 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 |
| 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 |
Image 1: AP and Lateral Radiographs of Right Knee
Radiological features:
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?
This pattern suggests Septic Arthritis:
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!
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:
Q: "What are the risk factors for developing septic arthritis after intra-articular injection?"
Risk factors:
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.