Painful Total Knee Replacement
CIM Case: Painful Total Knee Replacement
Clinical Scenario
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:
- Well-healed midline anterior incision
- Mild warmth compared to contralateral knee
- Minimal effusion
- Range of motion 5-100° (lacking full extension)
- Stable to varus/valgus stress in extension and flexion
- Diffuse tenderness, no point tenderness over patella
- Patellar tracking normal
- No distal neurovascular deficit
- Hip examination: painless full ROM
- Lumbar spine: no radicular symptoms
Investigations Provided
Laboratory Results
| 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) |
Synovial Fluid Analysis
| 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 |
Imaging
Image 1: Weight-bearing AP, Lateral, and Skyline Radiographs
Radiological features:
- Cemented TKR in situ
- No progressive radiolucent lines around tibial or femoral component
- Alignment appears satisfactory on AP view
- No significant osteolysis
- Patellar component in situ, tracking centrally
- No evidence of loosening
Image 2: CT with Metal Artifact Reduction (MARS) Protocol
Rotational assessment:
- Femoral component: 10° internal rotation relative to transepicondylar axis
- Tibial component: 5° internal rotation relative to tibial tubercle
- Combined rotational malalignment: 15° internal rotation
Questions & Model Answers
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?
Key Teaching Points
Pattern Recognition
This pattern suggests Mechanical Cause (Not Infection):
- Delayed onset pain after initial good result
- Normal/borderline inflammatory markers
- Negative synovial fluid analysis
- Negative alpha-defensin (highly specific)
- CT showing component malrotation
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.
Critical Management Points
- Infection workup is mandatory - CRP, ESR, aspiration with alpha-defensin
- Get previous X-rays - look for progressive changes
- CT rotation protocol - assess component rotation
- Examine hip and spine - referred pain is common
- Hold antibiotics 2 weeks before aspiration if possible
- Repeat aspiration if inconclusive - don't revise on uncertainty
Common Examiner Follow-ups
Q: "What is the sensitivity and specificity of alpha-defensin?"
Alpha-defensin is the most specific synovial fluid marker:
- Sensitivity: 97-100%
- Specificity: 96-98%
- Not affected by systemic inflammatory conditions (unlike CRP/ESR)
- Can be used with bloody specimens
- Available as lateral flow immunoassay (Synovasure) - point of care
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:
- Lateral patellar maltracking
- Anterior knee pain
- Flexion gap asymmetry (tight lateral)
- Increased polyethylene wear
Q: "What would you do if the alpha-defensin was positive?"
If alpha-defensin positive (high specificity), this confirms PJI:
- Identify organism - prolonged cultures, consider PCR/NGS
- Plan for two-stage revision - standard for chronic PJI
- First stage: Remove components, debride, insert antibiotic spacer
- Antibiotic course: 6 weeks IV typically
- Repeat aspiration to confirm eradication
- Second stage: Reimplantation when markers normalised
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:
- >3-5° combined internal rotation - associated with anterior knee pain
- >10° combined internal rotation - frequently symptomatic
- This patient has 15° - significant malalignment requiring revision
Related Topics
- Periprosthetic Joint Infection
- Two-Stage Revision TKR
- Component Alignment in TKA
- Alpha-Defensin Testing
- Chronic Pain After Arthroplasty