# 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 **Diagnosis: Acute Septic Arthritis of the Right Knee** **Newman Criteria for Septic Arthritis (Clinical):** - Joint pain ✓ - Joint effusion ✓ - Elevated inflammatory markers ✓ - At least 2 of 3 required for clinical diagnosis **Modified Kocher Criteria (Paediatric, but principles apply):** | Criterion | This Patient | |-----------|--------------| | Non-weight-bearing | ✓ Present | | Fever >38.5°C | ✓ Present | | WCC >12,000 | ✓ Present (18.5) | | ESR >40 mm/hr | ✓ Present (75) | **Risk Factor Assessment:** | Risk Factor | Present | |-------------|---------| | Diabetes mellitus | ✓ (HbA1c 9.2%) | | Recent intra-articular injection | ✓ (10 days ago) | | Pre-existing joint disease | ✓ (OA) | | Immunocompromise | ✓ (diabetes) | **Key Points:** - Synovial WCC >50,000 with >90% PMN = highly suggestive - Gram-positive cocci = likely Staphylococcus aureus - **This is an ORTHOPAEDIC EMERGENCY** - cartilage destruction within 24-48 hours > **Consultant Tip:** Septic arthritis destroys articular cartilage rapidly. Don't delay treatment waiting for culture confirmation. Clinical diagnosis warrants urgent intervention. **Essential Investigations:** | Investigation | Purpose | Key Findings | |---------------|---------|--------------| | **Joint aspiration** | Diagnostic - BEFORE antibiotics if possible | WCC, PMN%, Gram stain, culture, crystals | | **Blood cultures** | Organism identification (30-50% positive) | Draw before antibiotics | | **FBC, CRP, ESR** | Confirm inflammation, baseline for monitoring | Expect elevated | | **UEC** | Renal function for antibiotic dosing | Adjust gentamicin | | **Glucose, HbA1c** | Diabetes control affects healing | Poor control = worse outcomes | **Synovial Fluid Interpretation:** | WCC (cells/μL) | Diagnosis | |----------------|-----------| | <200 | Normal | | 200-2,000 | Non-inflammatory (OA) | | 2,000-50,000 | Inflammatory (RA, gout) | | >50,000 | Septic arthritis likely | | >100,000 | Septic arthritis highly likely | **Crystal Analysis:** - Urate crystals = gout - Calcium pyrophosphate = pseudogout - Crystals can coexist with infection - don't exclude sepsis if crystals present! **Imaging:** | Modality | Purpose | |----------|---------| | X-ray | Baseline, exclude other pathology, prior OA | | MRI | If osteomyelitis suspected, soft tissue extension | | Ultrasound | Guide aspiration if difficult | > **Consultant Tip:** Aspirate BEFORE antibiotics if possible - 48-hour antibiotic washout may be needed if aspiration delayed. But don't delay antibiotics if aspiration not immediately feasible in sick patient. **Immediate Management (Emergency):** **Step 1: Resuscitation** - IV access, IV fluids - Analgesia (IV morphine) - Blood glucose control (insulin sliding scale) - Blood cultures before antibiotics **Step 2: Empirical Antibiotics (after aspiration)** | Regimen | Coverage | Dose | |---------|----------|------| | **Flucloxacillin 2g IV Q6H** | Staph aureus (MSSA) | First-line | | **+ Gentamicin 5mg/kg IV daily** | Gram-negatives | Especially in diabetics | | OR Ceftriaxone 2g IV daily | Broad spectrum | Alternative | **If MRSA suspected or known colonisation:** - Vancomycin 25-30mg/kg loading, then 15-20mg/kg Q12H **Step 3: Urgent Surgical Washout** | Timing | Within 24 hours of presentation | |--------|--------------------------------| | Method | Arthroscopic washout preferred | | Alternative | Open arthrotomy if severe or inaccessible | | Goals | Drain pus, copious lavage (6-9L), synovectomy | | Repeat | May need 2-3 washouts if not improving | **Surgical Technique (Arthroscopic):** - Anterolateral and anteromedial portals - Systematic lavage (6-9L saline) - Debride infected synovium - Leave drain if significant debris - Splint in extension post-operatively **Step 4: Post-operative Care** - Serial CRP monitoring (should halve every 48-72 hours) - Repeat aspiration if not improving - DVT prophylaxis - Physiotherapy once infection controlled - Continue antibiotics minimum 4-6 weeks > **Consultant Tip:** Arthroscopic washout is preferred (less morbidity, good visualisation). Reserve open arthrotomy for failed arthroscopy, severe infection, or limited resources. **Expected Organisms by Patient Population:** | Organism | Frequency | Typical Patient | |----------|-----------|-----------------| | **Staphylococcus aureus** | 50-70% | Most common overall, especially post-injection | | Streptococcal species | 10-20% | Skin/soft tissue source | | Gram-negative bacilli | 10-15% | Elderly, diabetics, IV drug users | | MRSA | 5-15% | Healthcare-associated, prior colonisation | | Neisseria gonorrhoeae | 5-10% | Sexually active young adults | **This Patient's Risk Factors:** - Post-injection: high risk for Staphylococcus aureus - Diabetes: increased risk for Gram-negatives - Gram-positive cocci on Gram stain: confirms Staphylococcus likely **Antibiotic Selection:** | Empirical Therapy | Duration | |-------------------|----------| | IV Flucloxacillin 2g Q6H + Gentamicin | 2-4 weeks IV | | Then oral flucloxacillin or cephalexin | 2-4 weeks oral | | **Total duration:** | 4-6 weeks minimum | **If Culture Confirms MRSA:** - IV Vancomycin (trough 15-20 mg/L) - Consider adding rifampicin if slow response - Duration: 6 weeks minimum **Monitoring:** - CRP trending (should fall rapidly if treatment effective) - Trough levels for vancomycin/gentamicin - Repeat aspiration if not improving clinically > **Consultant Tip:** Don't stop antibiotics based on CRP alone. Complete the full course (minimum 4-6 weeks). Premature cessation leads to relapse. **Prognostic Factors:** | Factor | Good Prognosis | Poor Prognosis | |--------|----------------|----------------| | **Time to treatment** | <24-48 hours | >7 days delay | | **Host factors** | Healthy, young | Diabetes, immunocompromise, RA | | **Organism** | Streptococcus | Staph aureus, MRSA, Gram-negative | | **Joint involvement** | Native joint, single | Prosthetic, polyarticular | | **Pre-existing disease** | Normal joint | RA, OA, prior surgery | | **Response to treatment** | Rapid CRP fall | Persistent elevation | **Outcomes in Septic Arthritis:** | Outcome | Frequency | |---------|-----------| | Full recovery | 50-60% | | Residual joint damage | 25-40% | | Secondary osteoarthritis | 20-30% | | Chronic infection | 5-10% | | Mortality | 5-15% (higher in elderly, comorbid) | **This Patient's Prognostic Factors:** - Poor: Diabetes, delayed presentation (3 days), likely Staph aureus - Good: Native joint, early treatment now, no prosthesis **Long-term Considerations:** - Risk of secondary osteoarthritis - May require TKR in future - Need to monitor for chronic osteomyelitis - Diabetic optimisation essential for healing > **Consultant Tip:** Delay in treatment is the single most important modifiable prognostic factor. Same-day washout should be the goal. **Assessment of Poor Response:** **Clinical Indicators of Failure:** - Persistent fever - CRP not falling (should halve every 48-72 hours) - Ongoing joint pain and swelling - Failure to mobilise **Differential Diagnosis for Non-response:** | Cause | Action | |-------|--------| | Inadequate surgical debridement | Repeat washout | | Resistant organism (MRSA) | Review cultures, change antibiotics | | Wrong diagnosis (crystal arthropathy) | Review synovial fluid | | Concurrent osteomyelitis | MRI of joint | | Abscess formation | MRI/ultrasound | | Foreign body (retained cortisone) | Consider arthroscopy | **Management Steps:** 1. **Repeat aspiration** - culture, cell count 2. **Review microbiology** - sensitivities, need for antibiotic change 3. **MRI** - exclude osteomyelitis, abscess 4. **Repeat surgical washout** - typically needed in 30-40% of cases 5. **Infectious diseases consultation** - complex cases **When to Convert to Open Arthrotomy:** - Failed arthroscopic washouts (>2-3) - Loculated infection not accessible arthroscopically - Concurrent osteomyelitis requiring debridement - Severe soft tissue involvement **Salvage Options:** - Repeated debridement and antibiotics - Long-term suppressive antibiotics (if debridement fails) - Knee arthrodesis (rarely needed, salvage) - Above-knee amputation (life-threatening sepsis) > **Consultant Tip:** Don't give up after one washout. Expect 2-3 washouts in moderate-severe cases. Get ID involved early if not improving. --- ## 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 1. **URGENT SURGICAL DRAINAGE** - within 24 hours 2. **Aspirate BEFORE antibiotics** - if feasible 3. **Blood cultures** - positive in 30-50% 4. **Staph aureus is most common** - empirical flucloxacillin 5. **Multiple washouts often needed** - 30-40% require repeat 6. **Minimum 4-6 weeks antibiotics** - don't stop early 7. **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