Musculoskeletal and rheumatology · Moderate
This case is fictional and exists for study. It describes no real patient.
58 years · male
A 58-year-old male presents to the emergency department with a 2-day history of rapidly progressive right knee pain and swelling. He reports feeling feverish at home and is now completely unable to bear weight on his right leg. He has a medical history of type 2 diabetes mellitus and hypertension. On physical examination, his right knee is diffusely erythematous, hot to touch, and held in slight flexion. There is a prominent joint effusion. Any attempt at active or passive range of motion produces excruciating pain. No cutaneous entry wounds or surrounding cellulitis are identified, and there is no history of direct trauma.
Vital signs
- HR
- 108 bpm
- BP
- 118/76 mmHg
- RR
- 20/min
- Temp
- 38.6 °C
- SpO₂
- 98%
Examination findings
- Right knee hot, erythematous, with large effusion
- Severe restriction of active and passive movement due to severe pain
- Inability to weight-bear on right lower limb
Investigations
| White blood cell count | 17.2 10^9/L | reference: 4.0-11.0 |
| C-reactive protein (CRP) | 195 mg/L | reference: 0-5 |
| Erythrocyte sedimentation rate (ESR) | 78 mm/h | reference: 0-15 |
Question 1 of 3
Which of the following is the single most appropriate diagnostic investigation to perform immediately?
Synovial fluid aspiration (arthrocentesis) for urgent Gram stain, microscopy, leukocyte count, and culture is essential to establish the diagnosis of septic arthritis before initiating antibiotic therapy. Plain radiographs cannot confirm septic arthritis (though they may rule out fracture). Serum uric acid is unreliable during an acute inflammatory flare and does not exclude septic arthritis. MRI is sensitive for soft tissue and bone involvement but should not delay joint aspiration.
Teaching point: Urgent synovial fluid aspiration for microscopy, Gram stain, and culture must be performed before starting empirical antibiotics in suspected septic arthritis.
Question 2 of 3
Immediately after synovial fluid aspiration is successfully performed, which management step should be initiated without delay?
Once joint fluid is obtained for microbiology, empirical intravenous antibiotics (covering common organisms like Staphylococcus aureus) must be started immediately to prevent irreversible cartilage damage and systemic sepsis. Oral NSAIDs or intra-articular steroids are contraindicated in untreated bacterial joint infections. Outpatient oral therapy is inappropriate for acute septic arthritis, which requires inpatient IV treatment and orthopaedic evaluation for potential joint washout.
Teaching point: Start intravenous empirical antibiotics immediately after joint aspiration in patients suspected of having septic arthritis.
Question 3 of 3
Microscopy of the aspirated synovial fluid demonstrates a white blood cell count of 68,000/mm³ with 92% neutrophils. No crystals are visualised under polarised light. How should these fluid findings be interpreted?
Synovial fluid WBC count > 50,000/mm³ with a neutrophil predominance (> 75-90%) is strongly suggestive of septic arthritis. Normal synovial fluid has WBC < 200/mm³, and non-inflammatory/osteoarthritic effusions typically have WBC < 2,000/mm³. Acute crystal arthropathy displays intracellular negatively birefringent needle-shaped crystals (gout) or positively birefringent rhomboid crystals (pseudogout). Reactive arthritis typically produces moderate inflammatory counts, not typically exceeding 50,000/mm³ with negative cultures.
Teaching point: A synovial fluid white blood cell count greater than 50,000/mm³ with > 75-90% polymorphonuclear leukocytes strongly indicates septic arthritis.
Key learning points
- Septic arthritis presents typically as an acute, hot, swollen, painful monarthritis with restricted range of motion and systemic features.
- Synovial fluid aspiration for Gram stain, microbiology culture, cell count, and crystal analysis must be performed urgently prior to antibiotic administration.
- Empiric intravenous antibiotic therapy should be started immediately following joint aspiration, along with orthopaedic referral for potential surgical drainage or arthroscopic lavage.
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