What is the medical workup for a patient with knee pain status post Total Knee Arthroplasty (TKA) presenting to the emergency room?

Emergency Department Workup for Post-TKA Knee Pain

Initial Imaging

Begin with weight-bearing knee radiographs (AP, lateral, and Merchant views) as the first-line diagnostic study 1, 2. These radiographs must specifically evaluate for:

• Progressive radiolucent lines >2mm at the bone-cement or cement-prosthesis interface (suggests loosening) 2
• Focal osteolysis or component subsidence/migration 2
• Periprosthetic fractures 1, 2
• Soft tissue gas or abnormal effusions suggesting infection 2
• Component positioning and hardware failure 2

Critical Clinical Assessment

Perform focused examination to differentiate infection from mechanical causes:

• Night pain or pain at rest indicates infection, whereas pain on weight-bearing suggests mechanical loosening 3
• Assess for warmth, erythema, and effusion which suggest infection 4, 5
• Evaluate hip with internal/external rotation to exclude referred pain from occult hip pathology 3

Infection Workup (Most Critical)

If radiographs show concerning features OR clinical suspicion exists for infection, proceed immediately to joint aspiration before any additional imaging 1, 2. This is the critical diagnostic step recommended by the AAOS 1.

Joint Aspiration Protocol:

• Send synovial fluid for cell count with differential (>3000 WBC/μL or >80% PMNs suggests infection) 2
• Gram stain and aerobic/anaerobic cultures 2
• If aspiration is negative but infection remains suspected, the AAOS recommends repeat aspiration 1

Laboratory Studies:

• ESR and CRP (CRP >13.5 mg/L has 73-91% sensitivity for prosthetic infection) 3
• CBC with differential 4, 5

Advanced Imaging (When Initial Workup is Negative/Inconclusive)

For Suspected Infection with Negative Aspiration:

Combined In-111 labeled leukocyte/Tc-99m sulfur colloid bone marrow scan achieves 96% sensitivity and 87% specificity for periprosthetic knee infection 2. This is the most accurate advanced imaging for infection 1.

For Suspected Aseptic Loosening:

• Fluoroscopy with stress views can demonstrate abnormal component displacement and lucent lines not visible on standard radiographs 1
• CT without IV contrast is usually appropriate for evaluating extent of osteolysis, intraosseous or soft-tissue gas, and reactive bone formation not evident on radiographs 1

For Soft Tissue Pathology:

• Ultrasound can assess periprosthetic fluid collections, guide aspiration, and evaluate for extensor mechanism disruption (patella alta/baja, posterior tibial subluxation) 1, 2
• MRI with metal artifact reduction sequences has 86-92% sensitivity and 85-87% specificity for infection when lamellated hyperintense synovitis is present 1, 2

Critical Pitfalls to Avoid

• Do not rely on bone scintigraphy in the first 2 years post-TKR - positive uptake is common in asymptomatic patients and has poor specificity 2
• Never proceed to advanced imaging before obtaining joint aspiration when infection is suspected - aspiration is more specific and cost-effective 2
• Do not assume normal radiographs exclude significant pathology - early loosening and infection may not show radiographic changes 2
• Never proceed to revision surgery without a clear diagnosis - reoperation for unexplained pain is frequently associated with suboptimal results 4, 5, 6

Disposition Algorithm

If Aspiration Cultures are Positive:

• No additional imaging required per AAOS guidelines 1
• Immediate orthopedic consultation for surgical management 1

If Aspiration is Negative but Infection Still Suspected:

• Repeat aspiration as recommended by AAOS 1
• Consider nuclear medicine imaging (In-111 WBC/Tc-99m sulfur colloid) 2

If Infection Ruled Out:

• Evaluate for mechanical causes (loosening, instability, component malposition) with fluoroscopy or CT 1, 2
• Consider extra-articular causes: hip pathology, spine pathology, vascular complications 3, 7
• Orthopedic follow-up for serial evaluations if diagnosis remains unclear 4, 5