Joint Pain OSCE - Septic Arthritis

Case Overview

• Age/Sex: 77-year-old male
• Occupation: Retired postal worker
• Setting: Primary care walk-in/urgent assessment (could be ED)
• Chief complaint: "Painful joints"

Patient Script

Who I Am

I'm a 77-year-old retired postal worker who lives alone and looks after myself at home.

What Brings Me In

My right knee is unbearably painful and swollen and I can't put any weight on it — it's been getting worse over the last few days and I feel feverish.

My Story

It started about 3 days ago with a bit of stiffness in my right knee, which I blamed on the arthritis I've always had. Over the next day it became much worse — severe pain and swelling — and by yesterday I could hardly step on that leg. I've had fever and chills since yesterday and woke up this morning sweating. The pain is constant, worse with any movement, and I can't straighten the knee properly. I had a milder knee flare (gout) years ago, but this feels nothing like that. I also had a small scratch on my lower leg from gardening about 10 days ago which got a little red then seemed to settle. I fell off my bike about 2 weeks ago but only had some bruising and no big problems.

My Medical Background

• Past medical history: Type 2 diabetes diagnosed 12 years ago; long-standing osteoarthritis (knees); prior gout attack 2 years ago
• Medications: metformin 500 mg twice daily; atorvastatin 20 mg at night; paracetamol as needed
• Allergies: none known
• Social: lives alone, daughter nearby who helps sometimes; drinks 1–2 units alcohol most evenings; ex-smoker (stopped 20 years ago)
• Family: Father had heart disease, mother had osteoarthritis

What I Think & Worry About

• I think I might have done something to the knee when I fell, and I'm worried it might be badly damaged.
• I'm scared this is some sort of infection and I might need surgery.
• I'm worried about being alone if I need to be admitted.

If You Ask Me About Other Symptoms...

• Fever/chills: "Yes — started yesterday. I've felt hot and had chills a few times."
• Other joints: "My other knee is a bit stiff sometimes from the arthritis, but only the right one is so painful now."
• Recent infections: "I had a bit of a cough a month ago but it went away. I had a dental cleaning about 3 weeks ago and my dentist said there was some bleeding."
• Urinary symptoms: "No burning, no blood in urine, nothing like that."
• Skin: "Just that small scratch on my shin from gardening about 10 days ago — it was sore for a day and then seemed to calm down."
• Mobility: "I can't really put any weight on the right leg now — I use a walking stick but not today."
• Medication adherence: "I take my pills most days — sometimes I forget at the weekend."
• Pain relief tried: "I took extra paracetamol but it barely touches it; I haven't taken any antibiotics or steroids."

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Clinical Summary

Examination

• General: elderly man, appears unwell, sweaty, visibly uncomfortable
• Vital signs: Temperature 38.9°C; Heart rate 108 bpm; Blood pressure 100/62 mmHg; Respiratory rate 22/min; SpO2 96% on room air
• Right knee: markedly swollen with a tense effusion, overlying erythema and warmth, skin intact apart from a small healed superficial abrasion on the lower leg; severe tenderness on palpation; marked limitation of both active and passive range of motion due to pain
• Other joints: left knee with bony crepitus consistent with osteoarthritis but no acute warmth or swelling
• Cardiovascular/respiratory/abdominal: no murmurs, lungs clear, abdomen soft
• Neurovascular: distal pulses present, neurovascular exam of the limb intact

Investigations

• CBC: WBC 15.8 x10^9/L (neutrophils 13.2 x10^9/L) (leukocytosis with neutrophilia)
• CRP: 220 mg/L (markedly elevated)
• ESR: 85 mm/hr (elevated)
• Blood glucose: 12.5 mmol/L (elevated; known diabetes)
• Blood cultures x2: pending
• Plain X-ray right knee: large effusion; chronic degenerative changes; no acute fracture
• Ultrasound right knee: large joint effusion with synovial thickening
• Joint aspiration (right knee): turbid, yellow-green fluid
  • Synovial fluid WBC: 120,000 cells/mm3 with 95% neutrophils (purulent)
  • Gram stain: Gram-positive cocci in clusters seen
  • Synovial fluid culture: pending (empiric therapy started)

Diagnosis

• Primary: Septic arthritis of the right knee, most likely due to Staphylococcus aureus

  • Evidence: acute monoarticular hot swollen joint with systemic features (fever, tachycardia), markedly elevated inflammatory markers, purulent synovial fluid with very high neutrophil count and Gram-positive cocci on Gram stain

• Differentials:

  • Acute gout flare: less likely because synovial fluid WBC is very high and Gram stain shows organisms; patient has history of gout (red herring) but clinical severity and systemic signs favour infection
  • Flare of osteoarthritis: unlikely to cause fever, systemic inflammatory response, or purulent effusion
  • Cellulitis over the knee: may coexist but exam shows a deep joint effusion and severe limitation of passive movement suggesting intra-articular process
  • Reactive arthritis/post-infectious arthritis: less likely given purulent fluid and positive Gram stain

Management

• Immediate hospital admission for urgent management and monitoring
• Empiric IV antibiotics started promptly after blood cultures obtained: e.g., IV vancomycin (for MRSA coverage) plus IV ceftriaxone or similar broad-spectrum agent (adjust once culture and sensitivities available)
• Orthopedics/acute surgery consult for joint drainage: urgent needle aspiration and likely arthroscopic washout if large effusion or if inadequate clinical response to needle drainage
• Repeat joint aspiration as needed for ongoing drainage and symptomatic relief
• Analgesia: opioid and paracetamol as needed; immobilize limb between procedures
• Monitor: repeat inflammatory markers, blood cultures, renal function and glycaemic control (optimize diabetes control)
• Review antibiotics when synovial and blood culture results available; tailor therapy accordingly
• Social: arrange inpatient support, physiotherapy after infection control, and discuss home support on discharge (daughter involvement)

Key Learning Points

• A single hot, swollen joint with systemic features (fever, raised WBC/CRP) in an older patient should prompt urgent consideration of septic arthritis — joint aspiration is diagnostic and must not be delayed.

• Synovial fluid WBC >50,000/mm3 with neutrophil predominance and purulent appearance strongly suggests septic arthritis; Gram stain/culture guide targeted antibiotic therapy.

• Early empiric IV antibiotics and prompt drainage (needle aspiration or surgical washout) are critical to reduce joint destruction and systemic complications; tailor management based on culture results and clinical response.