# 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 **Systematic Approach:** **First Step:** Characterise the pain **Pain Types:** **Type:** Start-up pain **Suggests:** Loosening **Type:** Night pain **Suggests:** Infection, tumour (rare) **Type:** Anterior knee pain **Suggests:** Patellofemoral problem, patellar clunk **Type:** Diffuse pain **Suggests:** Low-grade infection, CRPS, referred pain **Type:** Mechanical symptoms **Suggests:** Instability, component malposition **Differential Diagnosis:** **Intrinsic:** - Infection (most important to exclude) - Aseptic loosening - Instability - Malalignment/malrotation - Polyethylene wear - Patellofemoral complications (clunk, subluxation, loosening) - Stiffness/arthrofibrosis - Component impingement **Extrinsic:** - Referred pain (hip OA, lumbar spine) - Soft tissue problems (pes anserine bursitis, IT band) - Neurological (neuroma, CRPS) - Vascular (claudication, popliteal pathology) **Patient Factors:** - Unrealistic expectations - Psychological factors - Fibromyalgia/chronic pain syndrome **Golden Rule:** INFECTION is the most important diagnosis to EXCLUDE > **Consultant Tip:** Always exclude infection first. Have a systematic approach - intrinsic, extrinsic, patient factors. **Blood Tests:** **Test:** CRP **Threshold:** >10 mg/L suspicious for infection **Test:** ESR **Threshold:** >30 mm/hr suspicious **Test:** IL-6 **Use:** More specific, less affected by systemic conditions **Test:** D-dimer **Use:** Emerging marker, part of ICM criteria **Imaging:** **Xray:** **Views:** AP standing, lateral, skyline, long-leg alignment **Look For:** - Progressive radiolucent lines (loosening) - Component position and rotation - Osteolysis - Patellar tracking - Overall alignment **Ct Scan:** **Indication:** Assess rotation, subtle loosening **Protocol:** Metal artefact reduction (MARS/SEMAR) **Rotational Assessment:** - Femoral component rotation (relative to TEA) - Tibial component rotation (relative to tibial tubercle) **Bone Scan:** **Use:** Nonspecific, may help localise problem **Caveat:** Positive for 1-2 years after surgery normally **Joint Aspiration:** **Importance:** ESSENTIAL to rule out infection **Icm Criteria:** **Synovial Wcc:** >3000/μL (chronic infection) **Polymorphs:** >80% **Leucocyte Esterase:** ++ (dipstick) **Alpha Defensin:** Positive (highly specific) **Culture:** Hold antibiotics 2 weeks before aspiration if possible **Referred Pain:** - Hip X-ray and examination - Lumbar spine assessment > **Consultant Tip:** ALWAYS aspirate before revision. Use ICM 2018 criteria for infection diagnosis. Alpha-defensin is highly specific. **Interpretation:** Likely NOT infected (below ICM thresholds) **Further Assessment:** **Rotational C T:** - Assess femoral and tibial component rotation - Combined malrotation >3-5° can cause PF problems and pain - Internal rotation of femoral component is common cause of failure **Hip Assessment:** - Examine hip, check hip X-ray - Hip OA commonly causes referred knee pain - Hip injection may be diagnostic **Spine Assessment:** - Lumbar spine examination - Consider MRI if radicular symptoms **Repeat Aspiration:** - If suspicion persists, repeat aspiration - Consider alpha-defensin testing **Possible Causes:** - Malrotation causing PF maltracking - Residual soft tissue impingement - Oversized component (overstuffing) - Component malalignment - Patient factors (expectations, chronic pain) > **Consultant Tip:** With negative infection workup, look at mechanical causes. CT rotation protocol is key. **Mechanism:** - Internal rotation of femoral component moves trochlear groove laterally - Patella tracks abnormally (lateral tilt/subluxation) - Increases Q-angle effect - Leads to anterior knee pain, patellar instability **Associated Problems:** - Tight lateral retinaculum - Increased lateral compartment pressure - Patellar maltracking - IT band tightness **Normal Rotation:** **Femoral Component:** 0-3° external rotation relative to posterior condylar axis (matches transepicondylar axis) **Tibial Component:** Slight external rotation (covers tibial tubercle) **Management:** **Non Operative:** - Physiotherapy (VMO strengthening, lateral release stretch) - Lateral patellar taping - Activity modification **Operative:** **Indication:** Significant malrotation causing symptoms refractory to conservative **Procedure:** Revision arthroplasty with corrected component rotation **Complexity:** Significant undertaking - ensure patient understands > **Consultant Tip:** Internal rotation is a common technical error. It's hard to correct and revision results are less predictable. **ICM 2018 Diagnostic Criteria for PJI:** **Major Criteria (1 positive = infected):** - Two positive cultures with the same organism - Sinus tract communicating with joint **Minor Criteria (score-based):** | Test | Score | |------|-------| | Elevated serum CRP (>10 mg/L) OR D-dimer (>860 ng/mL) | 2 | | Elevated ESR (>30 mm/hr) | 1 | | Elevated synovial WCC (>3000/μL) | 3 | | Elevated synovial PMN (>80%) | 2 | | Positive alpha-defensin | 3 | | Positive leucocyte esterase (++) | 3 | | Single positive culture | 2 | | Positive histology | 3 | | Positive intraoperative purulence | 3 | **Interpretation:** - ≥6 points = INFECTED - 2-5 points = INCONCLUSIVE (repeat testing, consider repeat aspiration) - 0-1 points = NOT INFECTED **This patient's score:** - CRP 8 (borderline, score 0) - Synovial WCC 1200 (score 0) - PMN 40% (score 0) - Alpha-defensin negative (score 0) - **Total: 0 points = NOT INFECTED** > **Consultant Tip:** ICM 2018 uses a point-based system. Alpha-defensin has the highest specificity (~98%). Always aspirate before revision surgery. **Surgical Options for Malrotation:** | Option | Indication | Procedure | |--------|------------|-----------| | **Isolated tibial component revision** | Tibial malrotation, well-fixed femur | Revise tibial component to correct rotation | | **Isolated femoral component revision** | Femoral malrotation, well-fixed tibia | Revise femoral component to correct rotation | | **Full revision TKR** | Combined malrotation, other issues | Revise both components | | **Lateral release ± VMO advancement** | Mild malrotation with PF symptoms | Soft tissue procedure only | **This Patient (15° combined internal rotation):** - Significant malrotation requiring component revision - Most likely need femoral component revision (10° IR is the major contributor) - May revise tibial component simultaneously (5° IR) **Surgical Technique Considerations:** | Aspect | Detail | |--------|--------| | **Femoral rotation references** | Transepicondylar axis (TEA), posterior condylar axis, AP axis (Whiteside's line) | | **Tibial rotation references** | Medial 1/3 of tibial tubercle, PCL insertion | | **Intraoperative check** | Navigation, patient-specific instruments, or templating | | **Bone loss management** | May need augments if bone loss from revision | **Patient Counselling:** **What to Explain:** - The femoral component is internally rotated, causing abnormal patellar tracking - This is likely the cause of ongoing pain - Surgery can correct this but is a significant undertaking **Expected Outcomes:** | Outcome | Rate | |---------|------| | Pain improvement | 60-70% | | Complete pain relief | 30-40% | | No improvement | 20-30% | | Revision complications | 5-10% | **Honest Discussion:** - Revision for malrotation has less predictable outcomes than primary TKR - Not all patients get complete relief - Risk of stiffness, infection, instability higher than primary - Alternative is continued conservative management **Risk Discussion:** | Risk | Rate | |------|------| | Infection | 2-4% | | Stiffness requiring MUA | 5-10% | | Instability | 3-5% | | Ongoing pain | 20-30% | | Need for further revision | 5-10% at 5 years | **Decision Framework:** - Ensure patient understands this is for quality of life - Document failed conservative measures - Set realistic expectations - Consider psychological assessment if concerns about pain amplification > **Consultant Tip:** Revision for malrotation has less predictable outcomes than revision for other causes. Set expectations carefully - 30% may have ongoing pain. --- ## 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 1. **Infection workup is mandatory** - CRP, ESR, aspiration with alpha-defensin 2. **Get previous X-rays** - look for progressive changes 3. **CT rotation protocol** - assess component rotation 4. **Examine hip and spine** - referred pain is common 5. **Hold antibiotics 2 weeks before aspiration** if possible 6. **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: 1. **Identify organism** - prolonged cultures, consider PCR/NGS 2. **Plan for two-stage revision** - standard for chronic PJI 3. **First stage:** Remove components, debride, insert antibiotic spacer 4. **Antibiotic course:** 6 weeks IV typically 5. **Repeat aspiration** to confirm eradication 6. **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