Patient: 65-year-old man Presentation: Persistent right knee pain 18 months after primary TKR, worse with activity, preventing return to golf Relevant history: Primary cemented TKR for OA, initial good result for 6 months then gradual onset of diffuse pain, no trauma, no recent dental procedures, diabetic (well-controlled), BMI 32 Examination findings:
| Test | Result | Normal Range | Interpretation |
|---|---|---|---|
| CRP | 8 mg/L | <10 mg/L | Borderline (grey zone for PJI) |
| ESR | 25 mm/hr | <30 mm/hr | Upper normal |
| WCC | 7.5 ×10⁹/L | 4-11 ×10⁹/L | Normal |
| HbA1c | 7.2% | <7% | ↑ Diabetes suboptimal control |
| D-dimer | 1200 ng/mL | <860 ng/mL | ↑ Elevated (part of ICM criteria) |
| Test | Result | ICM Threshold | Interpretation |
|---|---|---|---|
| Synovial WCC | 1200 cells/μL | >3000/μL | Below threshold |
| Polymorphs | 40% | >80% | Below threshold |
| Leucocyte esterase | Negative | ++ positive | Negative |
| Alpha-defensin | Negative | Positive | Negative (highly specific) |
| Culture | No growth (14 days) | - | Negative |
Image 1: Weight-bearing AP, Lateral, and Skyline Radiographs
Radiological features:
Image 2: CT with Metal Artifact Reduction (MARS) Protocol
Rotational assessment:
What is your approach to the painful TKR?
What investigations would you perform?
The CRP is 8, ESR 25, and aspiration shows WCC 1200 with 40% polymorphs. Cultures are negative. What now?
CT shows 10° of internal rotation of the femoral component. How does this cause pain?
What are the ICM 2018 criteria for periprosthetic joint infection?
Given the confirmed malrotation, what are your surgical options and how do you counsel the patient?
This pattern suggests Mechanical Cause (Not Infection):
Systematic Approach to Painful TKR:
| Category | Causes |
|---|---|
| Infection | PJI (most important to exclude) |
| Loosening | Aseptic loosening |
| Instability | Flexion gap imbalance, PCL dysfunction |
| Malalignment | Rotational malalignment (femoral internal rotation) |
| Patellofemoral | Patellar maltracking, clunk syndrome, component failure |
| Stiffness | Arthrofibrosis |
| Referred pain | Hip OA, lumbar spine |
| Soft tissue | Pes anserine bursitis, IT band |
| Neurological | Neuroma, CRPS |
Golden Rule: ALWAYS exclude infection first before considering revision for other causes.
Q: "What is the sensitivity and specificity of alpha-defensin?"
Alpha-defensin is the most specific synovial fluid marker:
Q: "What is the 'normal' femoral component rotation?"
Femoral component should be placed in 0-3° of external rotation relative to the posterior condylar axis, which aligns it parallel to the transepicondylar axis (TEA). Internal rotation is a common error that causes:
Q: "What would you do if the alpha-defensin was positive?"
If alpha-defensin positive (high specificity), this confirms PJI:
Q: "What is combined rotational malalignment and how much is significant?"
Combined rotational malalignment = femoral IR + tibial IR. Normal combined rotation should be neutral to slight external rotation. Significant thresholds: