What could be causing a hot sensation in an elderly man's knee after an injury?

Hot, Swollen Knee After Injury in an Elderly Man

Septic arthritis must be excluded immediately in any elderly patient presenting with a hot, swollen knee after injury, as delayed diagnosis can result in permanent joint destruction, disability, and death. 1, 2, 3

Immediate Diagnostic Approach

Step 1: Obtain Plain Radiographs First

• Plain X-rays are the mandatory initial imaging study to exclude fractures, establish baseline anatomy, detect joint effusion, identify gas in soft tissues, and rule out other pathology before proceeding with further workup 1
• Early radiographs may appear normal in acute infection but remain essential to exclude alternative diagnoses 1

Step 2: Laboratory Testing (Concurrent with Radiographs)

• Order ESR and CRP together immediately - when at least 2 of 3 inflammatory markers are abnormal, this achieves 93% sensitivity, 100% specificity, and 97% accuracy for septic arthritis 4, 1
• Obtain blood cultures (aerobic and anaerobic) if fever or systemic signs are present 1
• Serum interleukin-6 combined with CRP provides higher predictive value than most other markers 4, 1
• Do not rely on peripheral WBC count - it is not elevated in most patients with infected joints and cannot exclude infection 4, 1

Step 3: Joint Aspiration (Critical Next Step)

• Perform image-guided joint aspiration for synovial fluid analysis after radiographs if inflammatory markers are elevated or clinical suspicion remains high 4, 1
• Withhold antibiotics for at least 2 weeks prior to aspiration if clinically feasible, as preaspiration antibiotics cause false-negative cultures 1
• Synovial fluid analysis must include: total and differential cell counts, aerobic and anaerobic bacterial cultures, leukocyte esterase, alpha-defensin, CRP, and nucleic acid amplification testing 1
• Synovial fluid WBC count >50,000/mm³ strongly suggests septic arthritis in the absence of prosthetic joints 5

Critical Clinical Pitfalls to Avoid

Do Not Be Falsely Reassured by Absent Classic Signs

• Chronic infections frequently present with pain alone without fever, erythema, or warmth - these classic signs are often absent in elderly patients 4
• Night pain or pain at rest is more characteristic of infection, while pain on weight-bearing suggests mechanical loosening 4
• Pain is the most common presenting symptom but is nonspecific 4, 2

Most Common Causative Organisms

• Staphylococcus aureus (including MRSA) and coagulase-negative Staphylococcus are the most common pathogens in septic arthritis of native joints 4, 2, 3
• MRSA joint infection is associated with worse outcomes 3

Alternative Diagnoses to Consider

Charcot Neuro-osteoarthropathy (If Diabetic)

• Always suspect active Charcot in a diabetic patient with neuropathy presenting with a unilateral red, hot, swollen foot or knee 4
• Temperature difference ≥2°C between affected and unaffected limb supports diagnosis 4
• Infection does not preclude underlying Charcot - both can coexist 4
• MRI shows bone marrow edema without fractures in stage 0 Charcot 4

Crystal Arthropathy (Gout/Pseudogout)

• Acute gout can present with sudden onset pain, often beginning at night 6
• Joint aspiration for crystal analysis is definitive 6
• Pseudogout is more common in elderly patients and may show chondrocalcinosis on radiographs 6

Post-Traumatic Complications

• Fracture (excluded by initial radiographs) 1
• Hemarthrosis from trauma 1
• Acute exacerbation of underlying osteoarthritis 4

When Advanced Imaging Is Needed

• MRI is not indicated for initial evaluation but should be considered if radiographs and aspiration are inconclusive 1
• MRI has 100% negative predictive value for excluding osteomyelitis 1
• CT and ultrasound are not routinely used for initial septic arthritis evaluation 1

Treatment Considerations Once Infection Confirmed

• Start empiric antibiotics immediately after synovial fluid is obtained if clinical concern for septic arthritis exists 2, 5
• Vancomycin for gram-positive cocci, ceftriaxone for gram-negative cocci, ceftazidime for gram-negative rods 5
• If Gram stain is negative but suspicion high, use vancomycin plus ceftazidime or aminoglycoside 5
• Oral antibiotics are not inferior to IV therapy in most cases 2
• Total antibiotic duration: 3-4 weeks for uncomplicated bacterial arthritis, extended to 6 weeks if osteomyelitis present 3
• Evacuation of purulent material with arthrocentesis or surgical drainage is mandatory 2, 3, 5
• Most patients respond within 72-96 hours; persistent symptoms beyond 4 days warrant reassessment 1