import { OnePagerSummary, ExamWarning, InfoCardGrid, InfoCard, MnemonicCard, EvidenceCard, VivaScenarioSection, VivaScenario, ExamCheatSheet, ContentSection, ExamPearl, SafetyAlert, NumberHighlightRow, NumberHighlight, Tabs, Tab, ComparisonTable } from '@/components/mdx' ### Ideal Candidate (FRCS Key Criteria) - **Age**: Over 60 years (lower revision rates in older patients) - **Activity level**: Low to moderate demand - **BMI**: Less than 35 (obesity increases failure risk) - **Arthritis**: Isolated medial or lateral compartment (Ahlbäck grade 1-3) - **ACL**: Intact ACL (ABSOLUTE requirement) - **Deformity**: Correctable to neutral (less than 15° varus/valgus) - **ROM**: Flexion over 90°, extension to neutral - **Patellofemoral**: Minimal or no PFJ arthritis ### Specific Indications - Isolated medial compartment OA (95% of UKR cases) - Isolated lateral compartment OA (5% of cases) - Post-traumatic unicompartmental arthritis - Osteonecrosis confined to one compartment - Medial OA following lateral meniscectomy ### Advantages Over TKR - Minimally invasive approach (6-8cm vs 15-20cm) - Bone stock preservation - Faster recovery (often same-day discharge) - Better proprioception and kinematics - Lower perioperative morbidity - More natural feeling knee ### Absolute Contraindications - **ACL deficiency** (ABSOLUTE - causes bearing dislocation and failure) - Inflammatory arthritis (RA, psoriatic, gout) - Tricompartmental osteoarthritis - Fixed deformity over 15° (uncorrectable) - Significant patellofemoral arthritis - Flexion contracture over 10° - Flexion less than 90° ### Relative Contraindications - Age under 60 (higher revision rates in younger patients) - BMI over 35 - Very high activity demands - Moderate PFJ symptoms - Chondrocalcinosis (CPPD crystal deposition) - Prior high tibial osteotomy (altered bone stock) - Varus/valgus over 10° (higher failure risk) ### Decision Making - If unsure preoperatively, consent patient for possible TKR - Assess ACL intraoperatively before proceeding - Examine opposite compartment and PFJ during arthrotomy - If findings not suitable - ABORT and convert to TKR ### Imaging Protocol 1. **Standing AP radiographs** (full leg or standard) - Assess mechanical axis - Measure varus/valgus angle - Grade medial compartment (Ahlbäck) - Assess lateral compartment integrity 2. **Weight-bearing lateral knee** - Patellofemoral joint assessment - Posterior tibial slope measurement - Anterior femoral osteophytes 3. **Skyline/Merchant view** - Rule out significant PFJ arthritis - Outerbridge grade 3-4 PFJ is relative contraindication 4. **MRI** (selective use) - If ACL integrity questionable on examination - Assess meniscal status - Opposite compartment cartilage evaluation ### Component Selection **Mobile Bearing (Oxford)**: - Most studied design (over 95% 10-year survivorship) - Polyethylene bearing moves freely between components - Self-centering with conforming surfaces - Lower contact stress and wear - Requires precise ligament balance - Risk: bearing dislocation 0.5-2% **Fixed Bearing**: - Polyethylene fixed to tibial component - More forgiving technique - Lower dislocation risk - Higher contact stress and wear rates - Survivorship 85-90% at 10 years ### Templating - Measure AP dimension of affected condyle - Select femoral component matching native condyle - Plan tibial component size and position - Calculate expected gap with minimal bone resection ### Essential Instrumentation - **Oxford UKR instrument set** (or equivalent) - **Tibial resection guide**: Extramedullary or IM - **Femoral sizing guide**: Links femoral position to tibial cut - **Gap spacers**: For bearing thickness selection - **Trial components**: Full range of sizes - **Cement gun and pressurizer** - **Pulse lavage system** ### Component Inventory - Femoral components (sizes A-G typically) - Tibial components (sizes small, medium, large) - Polyethylene bearings (3-8mm thickness in 1mm increments) - Cemented components standard ### Positioning Equipment - Radiolucent table - Lateral post for valgus stress - Leg holder (optional) - Tourniquet (optional - many surgeons prefer bloodless field) ### Fluoroscopy - C-arm available for component position verification - True AP and lateral projections ### Surgical Alternatives 1. **Total Knee Replacement** - More predictable outcomes - Addresses all compartments - Longer recovery - More bone resection 2. **High Tibial Osteotomy** - Younger patients (under 60) - Higher activity demands - Preserves native joint - Longer recovery, rehabilitation 3. **Cartilage Restoration** - Focal defects only - Younger patients - MACI, osteochondral autograft - Limited evidence for OA ### Non-Surgical Management - Weight loss - Activity modification - Physiotherapy and muscle strengthening - Intra-articular injections (corticosteroid, HA, PRP) - Bracing for varus thrust - NSAIDs and analgesia **Location**: Infrapatellar branch crosses 1-2cm anterior to medial parapatellar incision, 2-3cm below joint line. **Protection**: Keep dissection at joint line level, warn patient about numbness risk (common). Avoid anterior extension of incision. **Location**: Directly in surgical field medially, blends with capsule at joint line. **Protection**: Sharp dissection staying on bone, avoid extensive soft tissue release. MCL injury causes valgus instability and failure. **Location**: Posterior to knee joint, 1-2cm behind posterior capsule. **Protection**: Keep dissection anterior, avoid posterior capsular disruption. Retract gently if exposure needed posteriorly. **Location**: Tibial eminence, 10-15mm from anterior tibial cortex. **Protection**: Preserve tibial eminence during tibial resection. Guide wires must be posterior to eminence. ACL injury is catastrophic failure. **Location**: Forms hinge for tibial component, 1cm posterior to anterior cortex at joint level. **Protection**: Horizontal tibial cut perpendicular to axis. Avoid posterior slope (increases resection depth posteriorly). Minimal resection preserves posterior cortex strength. ### Step 1: Patient Positioning and Preparation **Position**: Supine with operative leg free to flex and extend. Leg hanging off side of table or in leg holder. **Lateral post**: Position at mid-thigh level to allow valgus stress during tibial preparation. **Tourniquet**: Optional - many surgeons prefer bloodless field for visualization (250-300 mmHg). **Draping**: Standard knee draping allowing full access from mid-thigh to ankle. **Technical Tip**: EXAM KEY - 'I position supine with leg hanging freely to allow full ROM assessment. Lateral post for valgus stress during tibial cuts. Tourniquet optional but helps visualization for minimally invasive approach.' - Inadequate positioning preventing full ROM assessment - Lateral post too distal - prevents adequate valgus stress - Tourniquet pressure injury if over 90 minutes --- ### Step 2: Skin Incision and Arthrotomy **Incision**: MINIMALLY INVASIVE 6-8cm medial parapatellar incision (contrast with 15-20cm for TKR). Start at superior pole of patella, extend distally over medial border to 1cm below joint line. **Approach options**: - **Medial parapatellar**: Standard, versatile - **Subvastus**: Preserves extensor mechanism, better proprioception, limited in obese patients **Arthrotomy**: Capsular incision at joint line. Minimal or NO patellar eversion needed. **Fat pad**: Preserve to protect patellar blood supply. **Technical Tip**: EXAM KEY - 'Minimally invasive 6-8cm incision. Subvastus approach preferred when feasible - preserves extensor mechanism and improves recovery. Fat pad preservation protects patellar vascularity. Minimal patellar eversion unlike TKR.' - Saphenous nerve injury (infrapatellar branch) - warn patient of numbness - Excessive patellar mobilization - devascularization - Inadequate exposure - compromises component positioning --- ### Step 3: Intraoperative Assessment (CRITICAL DECISION POINT) **ACL INTEGRITY**: Test with Lachman - ABSOLUTE requirement. If deficient, ABORT UKR and convert to TKR. **Opposite compartment**: Assess cartilage - Outerbridge grade 2 acceptable, grade 3-4 is relative contraindication. **Patellofemoral joint**: Assess cartilage and tracking - significant disease suggests TKR better option. **Deformity**: Confirm correctable to neutral with manual stress. **Decision**: If ACL deficient OR tricompartmental disease - STOP procedure and convert to TKR. **Technical Tip**: EXAM KEY - 'ACL must be INTACT - I test with Lachman before proceeding. Opposite compartment should have reasonable cartilage (Outerbridge grade 2 acceptable). If ACL deficient or tricompartmental disease - I ABORT and convert to TKR. Patient consented preoperatively for this possibility.' - Proceeding with ACL deficiency - GUARANTEED early failure and bearing dislocation - Missing opposite compartment or PFJ disease - early progression requiring revision - Overestimating correctability of deformity --- ### Step 4: Osteophyte Removal Remove ALL peripheral osteophytes systematically: - **Anterior femoral osteophytes** - prevent impingement in extension - **Posterior femoral osteophytes** - prevent impingement in flexion - **Tibial rim osteophytes** - medial and lateral - **Tibial spine osteophytes** - clear around ACL attachments **Purpose**: Reveals true joint space, allows assessment of correctable deformity, prevents bearing impingement. **Caution**: Preserve ACL tibial and femoral attachments - stay peripheral to ligament insertions. **Technical Tip**: EXAM KEY - 'I remove ALL osteophytes first - this reveals the true joint space and allows proper assessment of correctable deformity. Systematic removal prevents postoperative impingement. Critical to preserve ACL attachments.' - ACL injury during aggressive osteophyte removal - use rongeur not saw near attachments - Residual anterior osteophytes - cause extension impingement and stiffness - Residual posterior osteophytes - cause flexion impingement --- ### Step 5: Tibial Resection (CRITICAL STEP) **Principles for Mobile Bearing**: - MINIMAL resection - only 2-4mm below deepest erosion - HORIZONTAL cut - no posterior slope (mobile bearing has built-in 7° slope) - Perpendicular to tibial axis - avoid varus/valgus tilt - Preserve tibial eminence and ACL attachment **Technique**: 1. Insert extramedullary or intramedullary guide perpendicular to tibial shaft 2. Position at 2-4mm below deepest point of erosion 3. Apply valgus stress using lateral post 4. Resect with oscillating saw - horizontal cut 5. Check cut surface is perpendicular to mechanical axis **Oxford-specific**: Horizontal cut because polyethylene bearing has conforming surfaces with inherent slope. **Technical Tip**: EXAM KEY - 'Minimal tibial resection (2-4mm below deepest erosion) - preserve bone stock for potential future TKR conversion. HORIZONTAL cut because Oxford mobile bearing has built-in posterior slope in the bearing design. Preserve tibial eminence absolutely - ACL attachment critical.' - Excessive resection - subsidence risk, bone loss complicates future TKR - Tibial eminence violation - ACL injury and catastrophic failure - Varus/valgus cut - component malalignment and bearing dislocation - Posterior slope - over-resects posteriorly, risks ACL and posterior cortex --- ### Step 6: Tibial Component Positioning and Trialing **Position principles**: - Cover entire tibial resection surface - Sit on strong cortical rim (avoid overhang) - Vertical alignment matches tibial shaft axis - Rotation neutral (parallel to anteroposterior axis) **Technique**: 1. Place tibial trial on resected surface 2. Position medially (for medial UKR) to cover cut bone 3. Check no overhang (soft tissue irritation) 4. Verify vertical alignment with rod down tibial shaft 5. Mark position with drill holes or marker **Oxford technique**: Use sizing spoon to determine optimal tibial component size. **Technical Tip**: EXAM KEY - 'Tibial component positioned to cover entire cut surface, sitting on strong cortical bone. Avoid overhang medially or laterally (soft tissue irritation). Vertical alignment matches tibial axis - prevents bearing dislocation. I verify with alignment rod.' - Malposition - primary cause of bearing dislocation (accounts for 70% of dislocations) - Overhang - MCL irritation medially, soft tissue irritation laterally - Inadequate coverage - subsidence into weaker cancellous bone - Rotation error - bearing tracking abnormality --- ### Step 7: Femoral Preparation and Sizing **Sizing principle**: Match native femoral condyle size (AP dimension). Do NOT oversize. **Oxford technique** (unique feature): 1. Femoral drilling guide sits on tibial resection surface 2. Links femoral component position to tibial cut 3. Maintains balanced flexion-extension gaps automatically 4. Pin placement determines femoral component rotation and AP position **Steps**: 1. Place femoral sizing guide on exposed condyle 2. Select size matching native condyle AP dimension 3. Position drilling guide on tibial cut 4. Insert guide pins for femoral component 5. Prepare femoral condyle with milling/sawing per technique 6. Check no anterior femoral notching (fracture risk) **Technical Tip**: EXAM KEY - 'Femoral component sizing matches native condyle - oversizing causes overstuffing, pain, and stiffness. Oxford technique LINKS femoral position to tibial cut using guide sitting on tibial surface - this ensures balanced gaps throughout flexion-extension arc. Avoid anterior notching absolutely.' - Femoral notching - periprosthetic fracture risk (2-3% incidence if notched) - Malalignment - instability, bearing dislocation, accelerated wear - Oversizing - overstuffing causes persistent pain, stiffness, early failure - Undersizing - instability, excessive laxity --- ### Step 8: Bearing Selection (CRITICAL FOR MOBILE BEARING) **Measurement technique**: 1. Insert trial femoral and tibial components 2. Flex knee to 90° (CRITICAL - measure in flexion not extension) 3. Use spacer blocks/feeler gauges to measure gap 4. Select bearing thickness that fills gap with slight tension **Target balance**: - Gap in 90° flexion: Snug fit with no gross laxity - Extension: 1-2mm opening acceptable (mobile bearing self-centers) - Should resist distraction but not be over-tight **Mobile bearing specifics**: - Polyethylene bearing moves freely between components - Self-centering design accommodates slight laxity - Too thick: Stiffness, pain, cartilage damage opposite compartment - Too thin: Instability, dislocation, abnormal wear **Technical Tip**: EXAM KEY - 'I measure gap in 90° FLEXION with trial components using spacer blocks. Mobile bearing (Oxford) has excellent 10-year survivorship over 95% but needs precise balance. Too thick bearing causes tight medial side, pain, stiffness. Too thin causes instability and dislocation. Slight laxity in extension (1-2mm) is expected and acceptable - bearing self-centers.' - Bearing dislocation (0.5-2%) - from undersizing or malposition - Over-stuffing - persistent medial pain, stiffness, opposite compartment degeneration - Measuring gap in wrong knee position (extension vs flexion) - Selecting bearing without adequate trialing --- ### Step 9: Trial Reduction and Assessment **Systematic trial assessment**: 1. Insert trial components with appropriate bearing thickness 2. Take knee through full ROM (0° extension to 135° flexion) 3. Check bearing stability - should not dislocate or move excessively 4. Assess for impingement - anterior in extension, posterior in flexion 5. Patella tracking - should track smoothly without lateral tilt 6. Gap opening in extension - 1-2mm laxity acceptable for mobile bearing **Red flags requiring adjustment**: - Bearing dislocation on ROM - undersized or malpositioned - Unable to achieve full extension - overstuffed or anterior impingement - Tight in flexion - oversized bearing or posterior impingement - Gross instability - undersized bearing **Technical Tip**: EXAM KEY - 'Meticulous trialing is critical. I take knee through full ROM checking stability, impingement, and patellar tracking. Slight opening in extension (1-2mm) is NORMAL and expected for mobile bearing - it self-centers in function. Bearing should track smoothly without dislocation.' - Over-stuffing not recognized - leads to persistent pain and early failure - Under-stuffing not recognized - leads to instability and dislocation - Impingement not identified - causes pain and restricted ROM - Converting to thicker bearing without reassessing balance --- ### Step 10: Bone Preparation and Cementation **Preparation**: 1. Remove trial components 2. Pulse lavage bone surfaces thoroughly (removes debris, blood, fat) 3. Dry with suction and gauze - cement bonds best to dry bone 4. Pack wound to keep field dry during cement mixing **Cementation sequence**: 1. Mix cement to appropriate consistency 2. Cement tibial component FIRST - Apply cement to bone surface - PRESSURIZE with finger/pressurizer into cancellous bone - Seat tibial component in marked position - Hold firmly until cement sets 3. Cement femoral component SECOND - Apply cement to bone cuts - Pressurize into cancellous bone - Seat component, ensure no cement interposition - Hold position until cement cures 4. Remove ALL excess cement meticulously - Anterior recess - Posterior capsule - Intercondylar notch - Tibial cut margins **Technical Tip**: EXAM KEY - 'Cement technique is CRITICAL. I pressurize cement into cancellous bone for optimal fixation. Remove ALL excess cement - loose cement bodies are major cause of bearing wear, impingement, and failure. Pulse lavage and dry bone essential for cement bonding.' - Cement debris - causes bearing wear, impingement, third-body wear, synovitis - Component malalignment during cementation - cannot be corrected once set - Cement interposition - prevents proper seating, creates gap - Inadequate pressurization - early loosening --- ### Step 11: Final Bearing Insertion and Assessment **Bearing insertion**: 1. Ensure all cement fully cured 2. Remove any remaining debris 3. Copious irrigation 4. Insert final polyethylene bearing (size determined during trialing) 5. Bearing should click into position (mobile bearing designs) **Final assessment**: - Full ROM: 0° extension to 135° flexion - Stable bearing through full ROM - no dislocation - No impingement in extension or flexion - Patella tracks smoothly - Bearing self-centers during movement - No clicking or catching **Technical Tip**: EXAM KEY - 'Final checks absolutely critical: ROM 0-135°, stable bearing throughout arc, no impingement. Mobile bearing should self-center during movement. I verify patellar tracking. UKR patients have MUCH faster recovery than TKR - often same-day or overnight discharge, immediate weight bearing.' - Bearing dislocation in early postoperative period - from undersizing or malposition - Residual impingement - causes pain and stiffness - Wrong bearing thickness inserted - verify matches trial --- ### Step 12: Closure and Post-procedure Protocol **Closure**: 1. Copious irrigation (3L minimum) 2. Achieve hemostasis (minimize hematoma) 3. Repair capsule carefully with absorbable sutures 4. Repair retinaculum and subcutaneous layer 5. Skin closure (subcuticular or staples) 6. Infiltrate local anesthetic (improves pain control) 7. Sterile dressing and wool-and-crepe **Drain**: Usually not required (minimally invasive approach with less dead space). **Post-procedure**: - Weight bearing as tolerated IMMEDIATELY - ROM exercises same day - Often outpatient or 1-night stay - Early mobilization to prevent stiffness - DVT prophylaxis per protocol **Technical Tip**: EXAM KEY - 'I repair capsule meticulously. Local anesthetic infiltration improves early pain control. UKR has significantly FASTER recovery than TKR - often same-day discharge, immediate weight bearing, faster return to function. Patients achieve better ROM and proprioception than TKR.' - Wound complications (uncommon with small incision) - Bearing dislocation during early mobilization - DVT/PE risk (lower than TKR but still present) - Inadequate pain control delaying mobilization ### Immediate Post-operative (Day 0-1) - **Mobilization**: WBAT immediately with crutches/walker - **ROM exercises**: Begin same day - active assisted flexion/extension - **Ice and elevation**: Reduce swelling - **Pain control**: Multimodal analgesia (paracetamol, NSAIDs, opioids prn) - **DVT prophylaxis**: LMWH or DOAC per local protocol - **Discharge**: Often same day or overnight stay (contrast with 3-5 days for TKR) ### Early Phase (Week 1-6) - **Weight bearing**: Progress to full weight bearing, wean aids - **ROM**: Target 0-120° by 6 weeks - **Physiotherapy**: Quadriceps strengthening, proprioception exercises - **Activities**: Stairs, return to driving at 2-3 weeks (right knee sooner) - **Follow-up**: Wound check at 2 weeks, remove sutures/staples ### Intermediate Phase (Week 6-12) - **ROM**: Achieve 0-135° (usually exceeds TKR outcomes) - **Strength**: Progressive resistance exercises - **Function**: Return to light activities, walking unlimited - **X-rays**: Standing AP/lateral at 6 weeks to assess components ### Late Phase (3-12 months) - **Activities**: Return to low-impact sports (golf, swimming, cycling) at 3 months - **High-impact**: Avoid running, jumping sports (increases failure risk) - **Function**: Most patients report "normal" or "near-normal" knee (better than TKR) - **Follow-up**: Review at 3, 6, 12 months, then annually ### Long-term Surveillance - **Annual X-rays**: Assess for progression of OA, component loosening - **Survival**: 95% at 10 years, 85% at 15 years (with good patient selection) - **Conversion**: 5-8% convert to TKR by 10 years (bone stock preserved makes this straightforward) ## References 1. **National Joint Registry for England, Wales, Northern Ireland and the Isle of Man (NJR) 2024 Annual Report**. UK National Joint Registry. Available at: https://www.njrcentre.org.uk - Comprehensive registry data showing Oxford mobile bearing UKR with 95.3% implant survival at 10 years. 2. **Beard DJ, Davies LJ, Cook JA, et al.** The clinical and cost-effectiveness of total versus partial knee replacement in patients with medial compartment osteoarthritis (TOPKAT): 5-year outcomes of a randomised controlled trial. *Lancet*. 2019;394(10200):746-756. Level 1 evidence comparing UKR to TKR in appropriate patients. 3. **Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) 2024 Annual Report**. Adelaide: AOA. Available at: https://aoanjrr.sahmri.com - Australian-specific UKR data showing 14.2% cumulative revision rate at 10 years, Oxford mobile bearing most common design. 4. **Mohammad HR, Strickland L, Hamilton TW, Murray DW.** Long-term outcomes of over 8,000 medial Oxford Phase 3 Unicompartmental Knees - a systematic review. *Acta Orthop*. 2018;89(1):101-107. Systematic review demonstrating excellent long-term survivorship with appropriate patient selection. 5. **Kozinn SC, Scott R.** Unicondylar knee arthroplasty. *J Bone Joint Surg Am*. 1989;71(1):145-150. Classic paper establishing patient selection criteria for UKR - age over 60, weight under 82kg, low demand. 6. **Goodfellow JW, O'Connor JJ, Murray DW.** The Oxford Meniscal Unicompartmental Knee. *J Knee Surg*. 2010;23(1):3-9. Foundational paper on Oxford mobile bearing design principles and biomechanics. 7. **Pandit H, Jenkins C, Gill HS, et al.** Unnecessary contraindications for mobile-bearing unicompartmental knee replacement. *J Bone Joint Surg Br*. 2011;93-B(5):622-628. Challenges traditional contraindications and expands indications based on modern evidence. 8. **Lygre SH, Espehaug B, Havelin LI, Furnes O, Vollset SE.** Does patellofemoral joint status influence survival of unicompartmental knee arthroplasty? *Acta Orthop*. 2010;81(3):337-343. Registry study examining impact of PFJ arthritis on UKR outcomes. 9. **Edmondson M, Atrey A, East D, et al.** Survival analysis and functional outcome of the Oxford unicompartmental knee replacement up to 11 years follow up at a District General Hospital. *J Orthop*. 2015;12(Suppl 1):S105-S110. Real-world outcomes from district hospital showing reproducibility of registry results. 10. **Hamilton TW, Rizkalla JM, Kontochristos L, et al.** The Interaction of Caseload and Usage in Determining Outcomes of Unicompartmental Knee Arthroplasty: A Meta-Analysis. *J Arthroplasty*. 2017;32(10):3228-3237. Meta-analysis demonstrating importance of surgeon experience and patient selection for UKR outcomes - higher volume surgeons have better results.

Unicompartmental Knee Replacement (UKR/Oxford)

Unicompartmental Knee Replacement (UKR/Oxford)