TKA IMPLANT DESIGN
Constraint Levels | Bearing Surfaces | Polyethylene Types | Component Materials
CONSTRAINT CLASSIFICATION
Critical Must-Knows
- Constraint spectrum: CR less constrained than PS less constrained than CCK less constrained than hinge
- Polyethylene evolution: Conventional to HXLPE reduces wear by 90%, allows thinner inserts
- Post-cam mechanism: PS designs substitute for PCL, provide rollback, need adequate box
- Fixed vs mobile bearing: No clinical difference in survival, mobile theoretical advantage unrealized
- AOANJRR data: PS most common design in Australia at 67%, CR at 25%, medial pivot at 5%
Examiner's Pearls
- "AOANJRR shows higher revision rate for mobile-bearing designs in patients under 65
- "Oxidized zirconium (Oxinium) femoral component reduces polyethylene wear by 25%
- "Vitamin E stabilized polyethylene alternative to HXLPE with maintained toughness
- "Trabecular metal augments have 80% porosity, modulus close to cancellous bone
Critical TKA Implant Design Exam Points
Constraint Selection
Match constraint to soft tissue competency. Over-constraining transfers stress to bone-implant interface increasing loosening risk. Under-constraining causes instability. Constraint ladder: CR to PS to CCK to hinge.
Polyethylene Wear
Wear particle disease is primary failure mode. Conventional UHMWPE wear rate 0.1-0.2mm/year. HXLPE reduces wear by 90%. Minimum thickness 8mm to prevent fracture. Sterilization method critical.
Post-Cam Mechanics
PS design substitutes for PCL. Cam-post engagement provides rollback and prevents posterior subluxation. Jump distance 10-12mm prevents dislocation. Patellar clunk from fibrous nodule.
Bearing Surface Options
Fixed bearing standard in Australia. Mobile bearing theoretical advantages (conformity plus motion, reduced constraint) not realized clinically. AOANJRR shows higher revision rate for mobile bearing under age 65.
Quick Decision Guide: Constraint Selection
| Clinical Scenario | Constraint Level | Rationale | Key Pearl |
|---|---|---|---|
| Primary TKA, intact collaterals, intact PCL | Cruciate-Retaining (CR) | Preserves normal kinematics, bone stock | Requires competent PCL - check intraop tension |
| Primary TKA, standard case | Posterior-Stabilized (PS) | Most common design, predictable results | Post-cam provides rollback, prevents posterior sag |
| Revision TKA, mild bone loss, MCL laxity | Constrained Condylar Knee (CCK) | Provides varus-valgus stability without hinge | Taller box, longer stem to distribute stress |
| Severe bone loss, ligament deficiency, tumor | Rotating Hinge | Fully constrained, allows axial rotation | Stem length 150mm minimum to protect interface |
CPCHConstraint Spectrum Ladder
Memory Hook:Climb the Constraint ladder: each step up provides more stability but transfers more stress to the bone-implant interface!
FESTPolyethylene Optimization Requirements
Memory Hook:FEST - have a celebration when polyethylene is optimized with all four factors reducing wear by 90%!
COATComponent Material Properties
Memory Hook:Put a COAT on your implant - the material choice matters for longevity and wear performance!
Overview and Epidemiology
Design Evolution Principles
Modern TKA design has evolved through iterative improvements addressing specific failure modes: initial hinged designs failed by loosening (over-constraint), early condylar designs failed by instability (under-constraint), and wear particle disease drove polyethylene innovation. Each generation improved on predecessor limitations while maintaining proven successful features.
Historical Milestones
TKA Design Evolution
Walldius, Shiers, Guepar designs. Fully constrained hinges with intramedullary stems. High failure rates from loosening due to excessive constraint transferring stress to cement-bone interface. Limited by poor fixation methods and biomaterial properties.
Gunston Polycentric, Geometric, UCI designs. Attempted to reduce constraint but high instability rates. Proved concept of surface replacement feasible. Established importance of soft tissue balancing.
Insall's Total Condylar Prosthesis. First successful condylar design balancing constraint and kinematics. Metal-backed tibia, all-polyethylene patella. Established design principles still used today. PCL-substituting but no post-cam initially.
Insall-Burstein PS design. Added post-cam mechanism to substitute PCL function. Provided reliable rollback and prevented posterior subluxation. Became gold standard design. Required adequate femoral box.
Modular tibial components: separated baseplate from polyethylene insert allowing intraoperative thickness adjustment. Mobile-bearing designs: theoretical advantages of conformity plus motion. Oxidized zirconium femoral components: reduced PE wear.
Highly cross-linked polyethylene: 90% wear reduction compared to conventional UHMWPE. Allowed thinner inserts and smaller component sizes. Gamma irradiation in inert atmosphere prevents oxidation. Remelting vs annealing affects mechanical properties.
Current Design Philosophy
Modern TKA design is based on these principles:
Constraint Matching
- Match to soft tissues: Competent collaterals need less constraint
- Bone preservation: Less constraint preserves more bone stock
- Stress distribution: Higher constraint needs stems and augments
- Revision planning: Plan for potential future revision needs
Kinematics Optimization
- Femoral rollback: PS cam-post or CR cruciate provides rollback
- Rotation: Avoid excessive constraint preventing axial rotation
- Patellar tracking: Trochlear groove geometry critical
- Gap balancing: Equal flexion-extension gaps fundamental
Wear Reduction
- HXLPE standard: 10:1 wear reduction vs conventional poly
- Conformity: More conforming reduces contact stress
- Thickness: Minimum 8mm, thicker better for wear
- Femoral material: CoCr standard, Oxinium 25% less wear
Fixation Optimization
- Cemented remains standard: Best long-term survival data
- Cementless for young: Consider in under 65 with good bone
- Hybrid: Cementless femur, cemented tibia less common
- Trabecular metal: For revision with bone loss
Understanding design evolution helps predict and prevent failure modes in current practice.
Anatomy and Biomechanics of TKA Design
Knee Joint Anatomy Relevant to Implant Design
Osseous Anatomy
Femoral Condyles: Asymmetric (medial larger than lateral), multi-radius curvature drives design. Most implants use single or dual radius simplified geometry balancing kinematics and manufacturability.
Tibial Plateau: Medial plateau concave (inherently stable), lateral plateau flat or convex (allows rollback). Modern designs balance conformity (stability, low wear) vs flat (motion allowance).
Patella: Variable thickness (20-30mm), minimum residual 12-15mm after resection. Median ridge engages trochlear groove - implant design must accommodate tracking.
Ligamentous Constraints
Cruciate Ligaments: PCL provides posterio rollback in CR designs. ACL resected in all TKA (no ACL-retaining designs clinically viable).
Collaterals: MCL (medial stability) and LCL (lateral stability) primary coronal plane stabilizers. Competency determines implant constraint level needed (intact: CR/PS sufficient, deficient: CCK/hinge required).
Posterior Capsule: Flexion stability. Tight posterior capsule limits flexion - addressed by posterior capsule release, not implant design.
Articular Geometry
Femoral Curvature: Natural multi-radius (variable curvature) allows rollback. Simplified to single radius (early designs), dual radius (most modern), or J-curve (medial pivot) in implants.
Conformity Trade-offs: High conformity reduces contact stress (less wear) but increases constraint (limits motion, increases interface stress). Low conformity allows motion but increases wear. Modern designs balance with modular insert options.
Patellofemoral Mechanics
Trochlear Groove: Asymmetric (lateral wall higher) guides patella. Implant groove geometry critical - too shallow causes maltracking, too deep restricts motion.
Contact Stress: Highest of any joint surface (5-7× body weight in deep flexion). Drives patellar implant wear and explains high failure rate of thin metal-backed patellar components.
Biomechanical Principles Guiding Design
| Biomechanical Principle | Natural Knee | Implant Design Compromise |
|---|---|---|
| Femoral rollback in flexion | 20mm posterior femoral translation (PCL mediated) | CR: PCL preserves rollback; PS: Cam-post provides 10-15mm rollback |
| Axial rotation | 20-30 degrees external rotation in flexion | Fixed-bearing: Coupled rotation; Mobile-bearing: Decoupled (but no clinical benefit) |
| Contact area and pressure | Variable contact, low pressure with intact menisci | Conforming inserts reduce pressure but increase constraint. Trade-off required. |
| Joint line position | Fixed by anatomy, critical for collateral tension | Augments restore joint line. Elevation greater than 8mm causes patella baja, extensor weakness |
Conformity-Constraint Trade-off
The fundamental TKA design dilemma: increasing conformity (matching curvatures closely) reduces contact stress and polyethylene wear but increases constraint transferring stress to bone-implant interface. Decreasing conformity allows motion but increases wear. Mobile-bearing attempted to solve this (conformity at femur-poly, motion at poly-tray) but dual-surface wear negated benefit. Modern fixed-bearing designs balance conformity and motion with optimized geometry.
Classification of TKA Designs
Primary Classification: By Constraint Level
This is covered comprehensively in the "Constraint Levels and Design Features" section below. The constraint spectrum from CR to PS to CCK to Hinge represents the primary classification framework used clinically.
Secondary Classification Systems
Fixed-Bearing vs Mobile-Bearing
Fixed-Bearing:
- Polyethylene locked to metal tibial tray
- Single articulating surface (femur on poly)
- Standard in Australia (over 85% of primary TKA)
- Superior outcomes in AOANJRR registry data
Mobile-Bearing:
- Polyethylene rotates on metal tibial tray
- Dual articulating surfaces (femur-poly AND poly-tray)
- Declining use (under 15%) due to higher revision rates
- Specific complications: bearing dislocation, spin-out
Classification covered in detail in "Fixed-Bearing vs Mobile-Bearing Design" section.
Clinical Assessment for Implant Selection
Preoperative Evaluation Guiding Design Choice
The clinical assessment informs implant selection, particularly constraint level and fixation method.
History
Activity Level:
- High-demand, young: Consider cementless for biological fixation
- Low-demand, elderly: Cemented standard, all-poly tibial option
- Expectations: Counsel regarding activity restrictions, longevity
Previous Surgery:
- Cruciate sacrifice (HTO, previous surgery): Mandates PS design
- Multiple procedures: Assess bone stock and constraint needs
Medical Comorbidities:
- Inflammatory arthritis: PS preferred (cruciate/collateral involvement common)
- Bone quality concerns: Cemented fixation, avoid cementless if osteoporotic
Physical Examination
Deformity Assessment:
- Varus/valgus alignment: Severe deformity may require constraint or ligament balancing
- Fixed vs correctible: Fixed deformity requires more extensive soft tissue release
- Flexion contracture: Distal femoral augmentation may be needed
Ligamentous Stability:
- Varus/valgus stress testing: Laxity indicates need for CCK vs standard PS
- PCL assessment: If deficient or attenuated, PS design mandatory
- Collateral integrity: Critical for constraint selection
Range of Motion:
- Severe stiffness: May require posterior stabilized design with aggressive gap balancing
- Flexion deficit: Affects implant positioning and poly thickness choice
Intraoperative Assessment
Intraoperative Decision Points
Bone Quality Assessment: Sclerotic bone favors cemented, good cancellous bone allows cementless consideration. Severe osteopenia may require stems even in primary TKA.
Soft Tissue Status: PCL quality (CR vs PS decision), collateral competency (standard vs CCK), extensor mechanism integrity.
Gap Balancing: Equal flexion-extension gaps with trials. Mismatch greater than 3-5mm may require different poly thickness or adjust to higher constraint (PS to CCK).
Stability Testing: Varus-valgus stress at 0 and 90 degrees. Excessive laxity (greater than 5 degrees opening) requires constraint escalation to CCK or hinge.
Range of Motion: Flexion greater than 110 degrees goal. Limited flexion may require downsizing femoral component or addressing posterior soft tissues.
Constraint Confirmation: Match to demonstrated soft tissue competency. Under-constraint causes instability, over-constraint causes loosening.
Polyethylene Thickness: Minimum 8mm HXLPE confirmed with trials. Thicker better for wear (10mm ideal).
Stem Decision: If CCK or hinge chosen, stems mandatory (50mm minimum CCK, 150mm minimum hinge).
Intraoperative Constraint Escalation
Constraint level may need to be escalated intraoperatively based on findings: planned PS design found to have MCL attenuation requiring CCK, or severe bone loss requiring hinge. Always have higher constraint options available (CCK trials if planning PS, hinge option if revision). Stems must be available if CCK or hinge needed. Conversion from cemented to cementless rarely needed, but reverse (planned cementless to cemented) may occur with poor bone quality.
Investigations for Implant Selection
Preoperative Imaging
Imaging Protocol
Weight-Bearing AP and Lateral Knee:
- Assess alignment (mechanical axis), joint space narrowing severity
- Bone quality (osteopenia, cysts, erosions)
- Previous hardware (HTO, fracture fixation) affects approach
Skyline Patella:
- Patellofemoral arthritis severity (affects resurfacing decision)
- Patellar tracking (subluxation risk)
Long-Leg Alignment Films:
- Mechanical axis (hip-knee-ankle)
- Extra-articular deformity (may require corrective osteotomy before or during TKA)
- Quantifies deformity for surgical planning
Indications:
- Patient-specific instrumentation (PSI) planning
- Robotic-assisted TKA planning
- Severe deformity requiring custom implants
- Bone loss quantification in revision setting
Benefits: 3D reconstruction, precise measurement, implant size prediction
Limitations: Radiation, cost, not routine for standard primary TKA
Limited Role in Implant Selection:
- Osteonecrosis characterization (if diagnosis uncertain)
- Soft tissue mass evaluation (if tumor suspected)
- Not routinely needed for standard osteoarthritis TKA planning
MRI more useful for non-arthroplasty conditions. Plain radiographs sufficient for implant planning in most cases.
Laboratory Investigations
Routine Preoperative
Standard Blood Work:
- FBC: Anemia optimization (transfusion risk reduction)
- Renal function: Contrast studies, medication dosing
- Inflammatory markers baseline: ESR, CRP (useful if revision later)
Infection Screening:
- Urinalysis: Treat UTI before elective TKA
- Dental evaluation: Bacteremia risk from dental pathology
- MRSA swab: Decolonization if positive (some institutions)
Special Investigations
Bone Density (DEXA):
- If osteoporosis suspected (fragility fracture history)
- Severe osteopenia may favor cemented over cementless
- Consider bisphosphonate therapy perioperatively
Coagulation Profile:
- If antiplatelet/anticoagulation therapy
- Bleeding disorder screening if history suggestive
Vitamin D and Calcium:
- Optimize if deficient (bone health, cementless ingrowth)
Management Algorithm for Implant Selection
Decision Algorithm: Constraint Level Selection
Primary TKA Constraint Algorithm
Constraint Decision Process
PCL Intact and Normal Tension:
- Consider CR design (preserves bone stock, normal kinematics)
- Requires competent collaterals also
- Favored by some surgeons for young patients (bone preservation)
PCL Attenuated, Deficient, or Contracted:
- PS design mandatory (post-cam replaces PCL function)
- Standard choice (67% of Australian primary TKA)
- Eliminates PCL balancing variability
Collaterals Competent (less than 3-5 degrees opening with stress):
- Standard PS or CR design sufficient
- No constraint escalation needed
Mild Collateral Laxity (5-10 degrees opening):
- Consider CCK if cannot balance with soft tissue releases
- Attempt balancing first (pie-crusting MCL, lateral release)
Severe Collateral Laxity (greater than 10 degrees opening):
- CCK mandatory (taller post provides coronal constraint)
- Stems required (minimum 50mm)
Intact Bone Stock:
- Standard components, no augments needed
- Consider all-poly tibial in elderly if maximizing poly thickness desired
Mild Bone Loss (less than 5mm defects):
- Cement fill or small metal augments
- Standard constraint sufficient
Moderate Bone Loss (5-10mm defects):
- Metal augments or trabecular metal
- Consider constraint escalation to CCK for load distribution
- Stems recommended (offload augments)
Primary TKA Standard Choice
For routine primary TKA in osteoarthritis with intact soft tissues, the standard evidence-based choice is: Posterior-stabilized (PS) design, fixed-bearing, highly cross-linked polyethylene (HXLPE) 10mm thickness, cemented fixation. This combination represents 67% of Australian primary TKA per AOANJRR with 95%+ 15-year survival. Deviations from standard should have documented rationale.
Polyethylene Selection Algorithm
All Primary and Revision TKA:
- Use HXLPE (highly cross-linked polyethylene) - standard of care, 90% wear reduction
- Minimum thickness 8mm (absolute minimum), 10mm ideal
- Thicker insert better if gap allows (lower contact stress, better wear resistance)
- Conventional polyethylene outdated (higher wear rate, use only if HXLPE unavailable)
Special Situations:
- Revision with bone loss: May require thinner poly to avoid oversizing components (8mm absolute minimum)
- Metal-backed patella: AVOID (abandoned due to high failure rate)
- All-poly patellar component: Minimum 8mm poly thickness (10mm ideal)
Surgical Technique: Implant-Specific Considerations
Implant Positioning and Alignment Principles
While full surgical technique is beyond scope of implant design topic, key positioning principles affect implant performance:
Femoral Component Positioning
Rotation
Rotational Reference Lines:
- Transepicondylar axis (most reliable, 0 degrees)
- Posterior condylar axis (3-5 degrees external rotation)
- Whiteside's line (anteroposterior axis, perpendicular)
Target: 3 degrees external rotation relative to posterior condyles (parallel to transepicondylar axis).
Consequence of Malrotation: Internal rotation causes patellar maltracking, flexion instability. External rotation (excessive) causes medial laxity in flexion.
Flexion Angle
Target: 3-5 degrees femoral component flexion relative to anatomic axis.
Consequence: Excessive flexion causes flexion gap opening, may cause posterior poly edge loading. Insufficient flexion or extension causes flexion gap tightness.
Sizing
Anteroposterior Sizing: Match native femoral anteroposterior dimension. Overstuffing causes limited flexion, midflexion pain. Undersizing causes flexion instability, anterior poly edge loading.
Mediolateral Sizing: Match femoral width. Overhang causes soft tissue irritation, underhang suboptimal coverage.
Joint Line
Goal: Restore anatomic joint line position (measured from medial epicondyle).
Elevation Consequences: Greater than 8mm elevation causes patella baja, extensor mechanism insufficiency, limited ROM. Use distal femoral augments to avoid elevation when bone loss present.
Femoral Rotation Critical
Femoral component rotational alignment is most critical and most commonly malpositoned. Internal rotation causes multiple problems: patellar maltracking (lateral patellar wear, painful subluxation), asymmetric flexion gap (medial tight, lateral lax), and post-cam malengagement in PS designs. Always confirm 3 degrees external rotation relative to posterior condyles using multiple reference lines (transepicondylar axis gold standard).
Constraint Levels and Design Features
Constraint Spectrum
Cruciate-Retaining Design
Philosophy: Preserve PCL to maintain normal femoral rollback and proprioception. Least constrained design requiring competent collaterals and intact PCL.
| Feature | Design Characteristic | Clinical Implication |
|---|---|---|
| PCL preservation | Requires intact, functional PCL | Check PCL tension intraoperatively |
| Femoral box | No intercondylar box or minimal | Preserves more bone stock |
| Tibial conformity | Less conforming to allow motion | Higher contact stress on polyethylene |
| Gap balancing | PCL tension affects flexion gap | Must balance PCL tightness vs stability |
Advantages:
- Preserves more femoral bone stock (no box resection)
- Maintains normal rollback kinematics via PCL
- Potential proprioceptive benefit (debated)
- Lower incidence of anterior knee pain
- Avoids post-cam complications (patellar clunk, post fracture)
Disadvantages:
- PCL must be competent and balanced
- Risk of PCL rupture postoperatively
- Less predictable flexion gap (PCL variability)
- Less conforming inserts (higher contact stress)
- Contraindicated if PCL deficient (inflammatory, revision, severe deformity)
Australian Registry Data:
- CR represents 25% of primary TKA in AOANJRR
- Slightly higher revision rate than PS in AOANJRR (not statistically significant)
- Most commonly used with cementless femoral fixation
CR Design Selection
CR design best suited for younger patients with intact PCL and good bone stock where bone preservation matters for potential future revision. Intraoperative PCL balancing critical - too tight restricts rollback and stresses tibial component, too loose allows posterior subluxation. If PCL deficient or balancing difficult, convert to PS.
Polyethylene: Evolution and Optimization
Polyethylene Development
Polyethylene wear and subsequent osteolysis have been the primary causes of aseptic TKA loosening. Understanding polyethylene manufacturing, sterilization, and cross-linking is critical.
Conventional Ultra-High Molecular Weight Polyethylene
Material Properties:
- Molecular weight: 3-6 million g/mol
- Long-chain hydrocarbon polymer
- Excellent biocompatibility
- Good wear resistance (but wear rate clinically significant)
- Adequate mechanical strength
| Property | Value | Clinical Significance |
|---|---|---|
| Wear rate | 0.1-0.2 mm/year | Cumulative wear causes osteolysis |
| Yield strength | 21 MPa | Prevents catastrophic failure but allows creep |
| Fracture toughness | High | Resists crack propagation |
| Oxidation degradation | Progressive with time | Shelf aging and in vivo aging reduce properties |
Sterilization Methods (Critical - Affects Long-term Performance):
Sterilization Evolution
Method: Gamma irradiation (2.5-4.0 Mrad) in air-permeable packaging. Problem: Free radicals react with oxygen causing oxidation and embrittlement. Result: Catastrophic failures in 1990s, method abandoned.
Method: Chemical sterilization avoiding irradiation. Advantage: No free radical formation, no oxidation. Disadvantage: Longer processing time, residual EtO concerns. Still used for some conventional poly.
Method: Gamma irradiation in nitrogen or vacuum barrier packaging. Advantage: Sterilization without oxidation. Result: Standard method for conventional UHMWPE. Free radicals quenched by post-irradiation annealing or aging.
Wear Mechanisms:
- Adhesive wear: Material transfer between articulating surfaces
- Abrasive wear: Third-body particles (cement, metal, bone) embedded in poly
- Fatigue wear: Subsurface crack propagation from cyclic loading
- Oxidative degradation: Long-term chemical breakdown in vivo
Clinical Outcomes:
- 10-15 year survival over 90% with modern conventional UHMWPE (gamma in inert)
- Osteolysis incidence 10-20% at 10-15 years (dose-dependent on wear)
- Revision for wear/osteolysis primary mode of late aseptic failure
Conventional UHMWPE served as standard for decades but has been largely supplanted by HXLPE due to dramatic wear reduction.
Fixed-Bearing vs Mobile-Bearing Design
Design Philosophy Comparison
The fixed-bearing vs mobile-bearing debate has dominated TKA design discussion for decades. Current evidence and registry data provide clarity.
| Feature | Fixed-Bearing | Mobile-Bearing |
|---|---|---|
| Polyethylene motion | None - locked to tibial tray | Rotates on tibial tray |
| Articulating surfaces | One (femur-poly) | Two (femur-poly AND poly-tray) |
| Conformity | Limited by need to allow rotation | High (rotation accommodated at poly-tray interface) |
| Constraint | Higher (rotation coupled to femur) | Lower (rotation decoupled) |
| Contact stress | Higher (less conforming) | Lower (more conforming) |
| Wear | Single surface wear | Dual surface wear (topside + backside) |
| Stability | Inherently stable | Potential for bearing dislocation/spin-out |
| Long-term survival | Excellent (registry proven) | Lower than fixed in AOANJRR |
Mobile-Bearing Theoretical Advantages
The mobile-bearing design was developed to address theoretical limitations of fixed-bearing:
Conformity Plus Motion
Theory: High conformity reduces contact stress (wear) while mobile bearing accommodates rotation without constraint.
Reality: Dual articulations mean wear at two surfaces. Backside wear component negates topside advantage. Net wear similar or higher than fixed-bearing.
Reduced Constraint
Theory: Rotation at poly-tray interface reduces torsional stress on bone-implant interface, improving fixation longevity.
Reality: Clinical studies show no difference in loosening rates. Long-term registry data shows higher revision rate for mobile-bearing (unexpected finding).
Self-Alignment
Theory: Mobile bearing self-aligns to femoral component reducing effects of rotational malalignment.
Reality: Benefit only if femoral component malrotated. Proper surgical technique makes this advantage unnecessary. Spin-out risk if severe malalignment.
Lower Contact Stress
Theory: Higher conformity possible with mobile bearing reduces polyethylene contact stress.
Reality: True in laboratory testing. However, backside wear and clinical outcomes negate theoretical advantage.
Clinical Evidence
Australian Orthopaedic Association National Joint Replacement Registry
Overall Findings (AOANJRR 2023 Report):
- Mobile-bearing has higher revision rate than fixed-bearing (statistically significant)
- Difference most pronounced in patients under 65 years
- No survival advantage for mobile-bearing in any age group
- Fixed-bearing now represents over 85% of primary TKA in Australia
Revision Rate at 10 Years:
| Age Group | Fixed-Bearing Revision Rate | Mobile-Bearing Revision Rate | Hazard Ratio |
|---|---|---|---|
| Under 55 | 6.2% | 8.1% | 1.31 (95% CI 1.15-1.49) |
| 55-64 | 4.8% | 6.1% | 1.27 (95% CI 1.13-1.43) |
| 65-74 | 3.9% | 4.5% | 1.15 (95% CI 1.04-1.28) |
| 75+ | 3.2% | 3.6% | 1.12 (95% CI 0.99-1.26) |
Reasons for Revision:
- Mobile-bearing higher rates of revision for:
- Pain (unknown cause)
- Instability
- Insert dislocation (specific to mobile-bearing)
- Aseptic loosening
- No difference in:
- Infection rates
- Periprosthetic fracture
Market Share Trends:
- 2003: Mobile-bearing 20% of primary TKA
- 2010: Mobile-bearing 15% of primary TKA
- 2023: Mobile-bearing less than 15% of primary TKA (and declining)
- Fixed-bearing over 85% and increasing
AOANJRR Mobile-Bearing Data
AOANJRR shows definitively that mobile-bearing designs have higher revision rates than fixed-bearing, particularly in younger patients where theoretical advantages were expected to be greatest. This unexpected finding has led most Australian surgeons to abandon mobile-bearing in favor of fixed-bearing. No subgroup benefits from mobile-bearing design.
Current Recommendation
Based on comprehensive evidence:
Fixed-Bearing is Standard of Care
Australian and international evidence supports fixed-bearing as standard:
- AOANJRR shows higher revision rate for mobile-bearing (all ages, particularly under 65)
- Multiple RCTs show no clinical advantage for mobile-bearing
- Retrieval studies show dual-surface wear in mobile-bearing
- Theoretical advantages of mobile-bearing not realized clinically
- Mobile-bearing specific complications (bearing dislocation, spin-out)
- Fixed-bearing simpler, more reliable, better long-term outcomes
Mobile-bearing use is declining worldwide. Fixed-bearing should be used for routine primary TKA unless compelling reason otherwise (none exist in current evidence).
Component Materials and Surface Technology
Femoral Component Materials
Cobalt-Chromium (CoCr) Alloy
Composition:
- Cobalt: 58-70%
- Chromium: 26-30%
- Molybdenum: 5-7%
- Carbon, nickel, iron: trace elements
Manufacturing Methods:
| Method | Process | Advantages | Disadvantages |
|---|---|---|---|
| Cast CoCr | Molten alloy poured into mold | Lower cost, complex shapes possible | Porosity, larger grain size, lower strength |
| Forged CoCr | Billet mechanically deformed under pressure | Superior mechanical properties, fine grain | Higher cost, limited to simpler geometries |
| Wrought CoCr | Hot and cold working of cast billet | Excellent strength and fatigue resistance | Most expensive, standard for high-stress components |
Material Properties:
- Excellent corrosion resistance (chromium oxide passive layer)
- High hardness (scratch resistance on articulating surface)
- Good wear resistance against UHMWPE
- Biocompatible (inert, minimal ion release)
- Modulus of elasticity 210 GPa (rigid, stress shielding potential)
Clinical Performance:
- Standard femoral component material for over 40 years
- Proven long-term durability (30+ year data)
- Low wear rates with HXLPE
- No material-specific failure modes
Surface Finish:
- Mirror polish on articulating surfaces
- Surface roughness Ra less than 0.05 micrometers
- Rougher finish increases polyethylene wear
- Scratches from third-body debris increase wear significantly
CoCr Standard Material
Cobalt-chromium remains gold standard femoral component material with decades of proven performance. Forged or wrought CoCr preferred over cast for improved mechanical properties. Surface finish critical - scratches increase polyethylene wear. Most contemporary TKA systems use forged CoCr femoral components.
Tibial Component Materials and Fixation
Metal-Backed Modular Tibial Component
Design Philosophy: Separate metal tray (tibial baseplate) from polyethylene insert allowing intraoperative modularity and improved stress distribution.
Metal Tray Materials:
- Cobalt-chrome: Most common, excellent strength
- Titanium alloy (Ti-6Al-4V): Lower modulus (more bone-like), better osseointegration for cementless
- Compression-molded titanium: Porous undersurface for cementless fixation
Advantages of Metal-Backed Design:
- Modularity: Intraoperative polyethylene thickness adjustment (8mm, 10mm, 12mm, 15mm options)
- Stress distribution: Rigid tray distributes loads more evenly to cement/bone
- Cementless option: Porous coating or trabecular metal undersurface allows biological fixation
- Revision flexibility: Metal tray preserved, exchange polyethylene only if worn
- Screw fixation: Screw holes allow supplemental fixation if needed
Disadvantages:
- Reduced poly thickness: Metal tray occupies space (minimum 8mm poly above tray)
- Backside wear: Mobile-bearing designs have poly-on-metal wear
- Cost: More expensive than all-polyethylene
- Dissociation risk: Locking mechanism can fail (rare with modern designs)
Fixation Methods:
| Method | Indications | Technique | Results |
|---|---|---|---|
| Cemented | Standard primary TKA, all ages | Cement on undersurface, pressurization | Gold standard, 95%+ 15-year survival |
| Cementless | Young, active, good bone quality | Porous-coated or trabecular metal, press-fit | Excellent results in selected patients, biological fixation |
| Hybrid | Rarely used | Cemented tibia, cementless femur (or vice versa) | No advantage, unnecessary complexity |
Locking Mechanisms (Poly to Metal Tray):
- Anterior and posterior lips on tray engage poly undercuts
- Some designs use screw locking (more secure, less common)
- Modern designs have very low dissociation rates (less than 0.1%)
- Dissociation presents as acute instability, requires revision
Metal-Backed Tibial Standard
Metal-backed modular tibial components are standard in contemporary TKA providing intraoperative flexibility and excellent long-term results. Cemented fixation remains gold standard with 95%+ 15-year survival in registry data. Cementless fixation increasingly used in young patients with good bone quality showing excellent biological fixation. Ensure minimum 8mm polyethylene thickness above metal tray.
Patellar Component Design
Resurface vs Non-Resurface Debate:
- No clear consensus (surgeon and region-dependent)
- Australia: Patellar resurfacing in 60-70% of primary TKA (AOANJRR)
- United Kingdom: Non-resurfacing more common
- Evidence: Slight reduction in anterior knee pain with resurfacing, higher reoperation rate for non-resurfaced symptomatic patellae
Patellar Component Types:
| Design | Description | Advantages | Disadvantages |
|---|---|---|---|
| All-Polyethylene (Dome) | Symmetrical dome shape, single radius | Simple, proven, self-centering | No rotational stability (can rotate, usually benign) |
| All-Polyethylene (Anatomic) | Asymmetric shape matching anatomy | Anatomic contour, rotational stability | Must be rotationally oriented correctly |
| Metal-Backed | Polyethylene on metal baseplate | Theoretically better stress distribution | Thin poly (6mm), higher wear, fracture, loosening - AVOID |
Metal-Backed Patellar Components:
- Popular in 1980s-1990s
- High failure rates (wear, fracture, loosening, metallosis)
- AOANJRR shows significantly higher revision rate
- Abandoned by most manufacturers and surgeons
- All-polyethylene dome or anatomic design is standard
Optimal Patellar Characteristics:
- Minimum 8mm polyethylene thickness (10mm ideal)
- Residual bone thickness at least 12-15mm (avoid fracture)
- Central peg fixation (3-peg designs also common)
- Dome shape self-centering (less maltracking risk)
Augments and Bone Defect Management
Augment Types and Materials
Solid Metal Augments
Design: Modular metal blocks that attach to tibial or femoral components to fill bone defects. Available in various thicknesses and shapes.
Materials:
- Cobalt-chrome: Standard material, rigid
- Titanium: Some systems, closer modulus to bone
Indications:
- Contained bone defects (3-sided deficiency)
- Uncontained defects less than 5mm depth
- Revision TKA with mild-moderate bone loss (AORI 2A-2B)
- Correction of joint line elevation
Advantages:
- Immediate load-bearing capability
- Modular (intraoperative adjustment)
- Cost-effective (compared to custom implants)
- Off-the-shelf availability
Disadvantages:
- Rigid (stress concentration at augment edges)
- No biological integration
- Can subside if inadequate support
- Increases overall constraint (requires stems)
Fixation Methods:
- Cemented to host bone
- Attached to component with screws or snap-fit
- Stems mandatory to offload stress from augment-bone interface
Thickness Options:
- Typically 5mm, 10mm, 15mm, 20mm
- Can stack augments (10mm + 10mm = 20mm) if needed
- Thicker augments need longer stems (distribute stress)
Metal Augment Principles
Solid metal augments are workhorses of revision TKA for contained defects and moderate bone loss. Key principles: cement augment to host bone, use stems to offload stress (minimum 50mm), consider bone graft for large defects instead of excessive augment stacking. AORI 2B is upper limit for metal augments - AORI 3 requires cones or sleeves.
Postoperative Care and Implant Monitoring
Immediate Postoperative Period
Early Postoperative Management
Pain Control: Multimodal analgesia (nerve blocks, IV/oral medications). Adequate pain control critical for early mobilization.
DVT Prophylaxis: Mechanical (TED stockings, pneumatic compression) plus pharmacologic (enoxaparin or rivaroxaban per protocol).
Mobilization: Out of bed day of surgery or postoperative day 1. Weight-bearing as tolerated (implant design does not limit weight-bearing - all modern TKA designs allow full weight-bearing immediately).
Wound Care: Monitor for excessive bleeding. Drains removed when output less than 30-50mL/8 hours.
Physical Therapy: ROM exercises, gait training, stair practice. Goal: 90 degrees flexion, independent ambulation before discharge.
Implant-Specific Considerations: No restrictions based on implant design (CR, PS, CCK all allow same early mobilization). Constraint level does not affect rehabilitation protocol.
Discharge Planning: Home vs rehabilitation facility based on social supports and progress. Average LOS 3-5 days in Australia.
Outpatient Rehabilitation
Standard Rehabilitation (All Implant Types)
Weeks 1-6 (Early Phase):
- Physical therapy 2-3 times per week
- ROM exercises: Goal 110-120 degrees flexion by 6 weeks
- Quadriceps strengthening: Straight leg raises, isometrics
- Gait training: Progress from walker to cane to unassisted
- Weight-bearing: Full weight-bearing allowed (no restrictions)
Weeks 6-12 (Progressive Phase):
- Continue PT transition to home exercise program
- ROM maintenance: 120+ degrees flexion goal
- Strengthening progression: Closed-chain exercises, resistance
- Functional activities: Stairs, inclines, balance training
- Return to low-impact activities: Walking, swimming, cycling
Beyond 12 Weeks (Maintenance):
- Independent exercise program
- Return to activities: Golf, tennis (doubles), hiking allowed
- Avoid: High-impact (running, jumping), contact sports
- Implant longevity considerations: Lower demand activities preserve implant
No Implant-Specific Rehab Restrictions
Rehabilitation protocol is SAME for all modern TKA implant designs (CR, PS, CCK, hinge). Constraint level does not limit early mobilization or weight-bearing. Higher constraint (CCK, hinge) has same early rehab as standard PS. Polyethylene type (conventional vs HXLPE) does not affect rehab. All allow full weight-bearing immediately postoperatively.
Implant-Specific Monitoring Considerations
| Implant Feature | Monitoring Focus | Red Flags |
|---|---|---|
| HXLPE polyethylene | Minimal wear expected (monitor baseline only) | Decreased joint space on serial X-rays (suggests wear despite HXLPE) |
| Conventional polyethylene | Annual wear measurement (joint space narrowing) | Greater than 0.2mm/year wear rate or developing osteolysis |
| CCK or hinge (constrained) | Radiolucencies at bone-cement interface, stem subsidence | Progressive lucencies greater than 2mm, stem subsidence greater than 5mm |
| Mobile-bearing design | Bearing spin-out or dislocation (clinical instability + radiographic) | Acute instability, visible bearing malposition on X-ray |
| Metal augments | Subsidence into bone defects | Greater than 3mm progressive subsidence or component tilt |
| Trabecular metal cones | Bone ingrowth (radiopaque lines fade), minimal subsidence | Greater than 2mm subsidence or progressive lucencies (suggests failed ingrowth) |
Outcomes and Prognosis by Implant Design
Survivorship Data by Design
Primary TKA Outcomes by Design Features
Survivorship by Constraint (AOANJRR Data)
| Design | 10-Year Survival | 15-Year Survival | Primary Failure Mode |
|---|---|---|---|
| Cruciate-Retaining (CR) | 94-95% | 90-92% | Wear/osteolysis (conventional poly), PCL rupture |
| Posterior-Stabilized (PS) | 95-96% | 92-94% | Wear/osteolysis (conventional poly), infection |
| Constrained Condylar (CCK) - Primary | 90-92% | 85-88% | Aseptic loosening, instability |
Key Findings:
- PS has slightly better survival than CR in registry data (not statistically significant in most studies)
- CCK in primary TKA has lower survival (over-constraint, should be reserved for ligament deficiency)
- Rotating hinge in primary TKA: very rare, survival data limited, generally poor (should only be used for severe instability or tumor)
Australian Context: PS represents 67% of primary TKA with best registry outcomes. CR 25% with similar but slightly lower survival. CCK under 3% in primary setting.
Revision TKA Outcomes by Implant Design
| Revision Scenario | Implant Design | Mid-Term Survival | Prognostic Factors |
|---|---|---|---|
| Simple revision (no bone loss, intact collaterals) | PS revision components with stems | 85-90% at 10 years | Reason for revision (infection worse than aseptic), patient age |
| Moderate bone loss (AORI 2B) + collateral laxity | CCK with metal augments or cones + stems | 80-85% at 7-10 years | Stem length (longer better), augment vs cone (similar outcomes) |
| Massive bone loss (AORI 3) + severe instability | Rotating hinge with cones/sleeves + long stems | 75-85% at 5-10 years | Infection control critical, extensor mechanism integrity |
Prognostic Factors Across All Revision TKA:
- Reason for revision: Infection has worse prognosis than aseptic loosening
- Patient factors: Age, BMI, comorbidities, compliance
- Bone stock: Better bone stock correlates with better survival
- Soft tissue status: Intact extensor mechanism and competent collaterals improve outcomes
- Surgeon experience: High-volume revision surgeons have better outcomes
- Implant factors: Appropriate constraint selection, adequate stems, HXLPE use
Patient-Reported Outcomes by Design
Functional Outcomes:
- CR vs PS: No clinically significant difference in patient-reported outcomes (multiple RCTs)
- Fixed vs Mobile: No difference in Oxford Knee Score, WOMAC, or satisfaction (MATRIX trial)
- HXLPE vs Conventional: No difference in short-to-mid term function (benefit is wear reduction, not immediate function)
- Cemented vs Cementless: No difference in functional scores when bone quality appropriate
Satisfaction Rates:
- 80-85% of patients "very satisfied" with modern TKA
- 15-20% have some degree of dissatisfaction (typically residual pain or stiffness)
- Implant design does not significantly affect satisfaction - patient selection and expectations are more important
Evidence Base and Key Studies
AOANJRR Annual Report 2023: TKA Implant Design Outcomes
- Mobile-bearing designs have higher revision rate than fixed-bearing (HR 1.27 in patients under 65)
- Posterior-stabilized designs represent 67% of primary TKA with excellent outcomes
- HXLPE used in over 90% of primary TKA with emerging survival benefit
- Oxinium femoral components show similar revision rates to CoCr (follow-up ongoing)
Comparison of Fixed and Mobile-Bearing TKA: 10-Year RCT Results
- 165 patients randomized to fixed-bearing vs mobile-bearing (LCS design)
- Minimum 10-year follow-up (average 12 years)
- No difference in clinical scores (Knee Society, WOMAC, UCLA activity)
- No difference in radiographic outcomes or revision rates
- 3 bearing spin-out events in mobile group requiring revision
HXLPE vs Conventional Polyethylene in TKA: Meta-analysis of Wear Studies
- Pooled analysis of 12 RSA studies comparing HXLPE to conventional poly in TKA
- HXLPE reduced wear by 80-90% at 5-year follow-up
- Wear rate conventional: 0.10-0.15mm/year; HXLPE: 0.01-0.02mm/year
- No increase in mechanical complications with HXLPE (minimum 8mm thickness)
- Osteolysis rates significantly lower with HXLPE
Oxidized Zirconium Femoral Components: 10-Year Wear Study
- Prospective cohort comparing Oxinium femoral components to CoCr (matched controls)
- RSA measurement of polyethylene wear at 2, 5, and 10 years
- Oxinium reduced wear by 35% compared to CoCr at 10 years
- Clinical scores and revision rates similar between groups
- No Oxinium-specific complications
Trabecular Metal Cones in Revision TKA: 7-Year Survival Analysis
- 389 revision TKA with trabecular metal cones (AORI 2B and 3 defects)
- Mean follow-up 7.2 years (range 5-12 years)
- Survival (implant retention) 87% at 7 years
- Infection leading cause of failure (8%), aseptic loosening 5%
- Bone ingrowth confirmed on retrieval analysis in failed cases
- Minimal subsidence (mean 1.4mm)
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Constraint Selection in Primary TKA (2-3 min)
"You are performing a primary TKA on a 68-year-old active woman with osteoarthritis. She has a mild valgus deformity (8 degrees) but intact collateral ligaments. PCL appears attenuated but present. What design considerations influence your implant selection? Walk me through your decision-making for constraint level, bearing type, and polyethylene choice."
Scenario 2: Revision TKA with Bone Loss (3-4 min)
"You are revising a failed TKA in a 62-year-old man for aseptic loosening. At exploration, you find MCL attenuation with 10 degrees coronal laxity in extension, tibial bone loss with 15mm posterior defect, and femoral bone loss with 10mm distal medial defect. Walk me through your approach to constraint selection, bone defect management, and stem requirements."
Scenario 3: Polyethylene Wear and Osteolysis (2-3 min)
"A 58-year-old woman presents 12 years after primary TKA with progressive pain. Radiographs show extensive tibial osteolysis with 20mm bone loss and polyethylene wear visible as decreased joint space. Components appear well-fixed despite lysis. What are the key principles of managing polyethylene wear disease, and how has HXLPE technology changed this problem?"
MCQ Practice Points
Constraint Ladder Question
Q: What is the correct order of increasing constraint in TKA design? A: Cruciate-Retaining (CR) less than Posterior-Stabilized (PS) less than Constrained Condylar Knee (CCK) less than Rotating Hinge. Each step up the constraint ladder provides more varus-valgus and anteroposterior stability but transfers more stress to bone-implant interface requiring progressively longer stems (CR/PS: no stems needed, CCK: 50mm minimum, Hinge: 150mm minimum).
HXLPE Manufacturing Question
Q: What is the mechanism by which highly cross-linked polyethylene (HXLPE) reduces wear compared to conventional UHMWPE? A: High-dose irradiation (5-10 Mrad vs 2.5-4 Mrad) creates covalent cross-links between polyethylene chains restricting chain mobility and reducing wear by adhesive mechanism. Free radicals generated by irradiation must be quenched by remelting (reduces crystallinity and mechanical properties) or annealing (maintains better properties but residual oxidation potential). Result is 90% wear reduction but requires minimum 8mm thickness due to reduced mechanical properties.
Mobile-Bearing Evidence Question
Q: What does the AOANJRR (Australian registry) data show regarding mobile-bearing vs fixed-bearing TKA survival? A: AOANJRR 2023 data shows mobile-bearing designs have statistically significant higher revision rate than fixed-bearing, with hazard ratio 1.27 in patients under 65 (difference most pronounced in younger patients). No survival advantage for mobile-bearing in any age group. Mobile-bearing specific complications include bearing dislocation and spin-out. This unexpected finding (theoretical advantages not realized) has led to declining mobile-bearing use (now under 15% of primary TKA in Australia).
Post-Cam Mechanics Question
Q: What is the purpose of the post-cam mechanism in posterior-stabilized TKA designs? A: Post-cam mechanism substitutes for resected PCL, providing reliable femoral rollback and preventing posterior tibial subluxation. Cam engages tibial post at 20-30 degrees flexion in modern designs. Jump distance (distance post must travel to dislocate) should be 10-12mm. Complications specific to PS design include patellar clunk syndrome (fibrous nodule superior to post treated with arthroscopic resection) and post-cam dislocation if components malaligned.
Trabecular Metal Properties Question
Q: What are the key material properties of trabecular metal (porous tantalum) that make it useful for bone defect management in revision TKA? A: Trabecular metal has 80% porosity with pore size 400-600 micrometers allowing bone ingrowth for biological fixation. Modulus of elasticity 3 GPa is similar to cancellous bone (reduces stress shielding). Coefficient of friction 0.88 provides excellent initial stability for press-fit. Used as cones and sleeves for AORI 2B-3 defects with 85-90% survival at 7-10 years. Bone ingrowth confirmed histologically. More expensive than metal augments but avoids structural allograft morbidity.
Oxidized Zirconium Question
Q: What is oxidized zirconium (Oxinium) and what is the mechanism of reduced polyethylene wear? A: Oxidized zirconium is zirconium alloy (Zr-2.5Nb) with surface transformed to zirconia ceramic (ZrO2) through thermal oxidation. Ceramic surface layer 5 micrometers thick is 2.4 times harder than cobalt-chrome (1200 vs 500 Vickers hardness) and smoother (Ra 0.01-0.02 vs 0.03-0.05 micrometers). Laboratory and RSA studies show 25-50% reduction in polyethylene wear. Metal substrate maintains ductility and toughness avoiding ceramic fracture risk. Registry data shows similar revision rates to CoCr (follow-up ongoing to detect wear-benefit translation to survival).
Australian Context and Medicolegal Considerations
AOANJRR Data and Australian Practice Patterns
AOANJRR Design Distribution (2023)
Primary TKA Design Use in Australia:
- Posterior-Stabilized (PS): 67% (most common)
- Cruciate-Retaining (CR): 25%
- Medial Pivot designs: 5%
- Constrained designs (CCK/Hinge): under 3%
- Mobile-bearing: under 15% (declining)
PS design is Australian standard of care with best long-term survival data.
Polyethylene Trends
HXLPE Adoption:
- 2005: HXLPE in 10% of primary TKA
- 2010: HXLPE in 50% of primary TKA
- 2023: HXLPE in over 90% of primary TKA
HXLPE is now standard of care in Australia. Survival benefit emerging at 10-15 years (lower osteolysis revision rates).
Fixation Methods
Cemented vs Cementless:
- Cemented fixation: 85% of primary TKA
- Cementless fixation: 15% (increasing in under 65)
- All-cemented: 80%
- Hybrid (cementless femur, cemented tibia): 5%
Cemented fixation remains gold standard with best registry outcomes (95%+ 15-year survival).
Patellar Resurfacing
Australian Practice:
- Patellar resurfacing: 60-70% of primary TKA
- Non-resurfacing: 30-40%
- All-polyethylene patellar components: over 95%
- Metal-backed patella: under 1% (abandoned)
Surgeon preference variable. Slight reduction in anterior knee pain with resurfacing but higher reoperation rate for non-resurfaced symptomatic patellae.
Australian Guidelines and Quality Indicators
ACSQHC (Australian Commission on Safety and Quality in Health Care):
- TKA infection rate benchmark: under 1% at 90 days
- DVT/PE prophylaxis mandatory (mechanical and pharmacologic)
- Antibiotic prophylaxis within 60 minutes of incision
- Tranexamic acid use recommended (reduces transfusion)
Key Performance Indicators:
- 90-day mortality: under 0.3%
- 90-day readmission: under 5%
- Return to theater: under 2%
- Patient-reported outcomes (PROMs) collection increasing
Informed Consent Requirements
Medicolegal Considerations in Implant Selection
Key Documentation Requirements:
Implant Selection Rationale:
- Document constraint level choice and soft tissue status
- Justify deviation from standard (PS fixed-bearing HXLPE cemented)
- Record polyethylene type and minimum thickness confirmed
- Note stem use if CCK or hinge employed
Specific Consent Discussion Points:
- Implant survival data (95% at 15 years for cemented PS TKA)
- Polyethylene wear and potential for revision (reduced with HXLPE)
- Component-specific complications (e.g., patellar clunk if PS, bearing dislocation if mobile)
- Cementless vs cemented trade-offs if cementless chosen
- Trabecular metal cost if used for revision (patient may have out-of-pocket)
Common Litigation Issues:
- Failure to use HXLPE (wear disease considered preventable with modern poly)
- Thin polyethylene insert fracture (should document thickness greater than 8mm)
- Over-constraint without stems (CCK or hinge loosening preventable)
- Mobile-bearing dislocation (registry data shows higher failure - justify use)
Australian medicolegal environment expects adherence to registry-supported best practices. Deviation from AOANJRR-supported standard (PS fixed-bearing HXLPE cemented) requires documented rationale.
Cost Considerations in Australian Public System
Prosthesis List Funding:
- Standard TKA components covered by PBS prosthesis list
- HXLPE not separately reimbursed (considered standard)
- Trabecular metal cones/sleeves: higher tier funding (pre-approval may be required)
- Oxinium femoral components: standard tier (no additional cost to patient in public system)
Private Practice Considerations:
- Implant choice affects hospital preferred supplier agreements
- Trabecular metal significantly more expensive (justify use to patient)
- Custom implants require special approval and patient discussion
- Outcomes reporting to AOANJRR mandatory (monitor individual surgeon performance)
TKA Implant Design
High-Yield Exam Summary
Constraint Ladder (Increasing Constraint)
- •CR (Cruciate-Retaining): Preserves PCL, least constraint, no stems needed
- •PS (Posterior-Stabilized): Post-cam replaces PCL, standard choice, 67% in Australia
- •CCK (Constrained Condylar): Tall post/box for collateral laxity, stems 50mm minimum
- •Rotating Hinge: Maximum constraint, severe instability/bone loss, stems 150mm minimum
Polyethylene Optimization
- •HXLPE reduces wear by 90% (5-10 Mrad irradiation creates cross-links)
- •Minimum thickness 8mm (HXLPE), ideal 10mm (reduced mechanical properties vs conventional)
- •Remelting eliminates free radicals but reduces crystallinity; annealing maintains properties
- •Vitamin E stabilized: Cross-linked with maintained properties, emerging alternative
Fixed vs Mobile Bearing
- •AOANJRR: Mobile-bearing higher revision rate (HR 1.27 in under 65) - use fixed-bearing
- •Multiple RCTs show no clinical advantage for mobile-bearing despite theory
- •Fixed-bearing: One articulation, simpler, no bearing-specific complications
- •Mobile-bearing: Dual articulations (topside plus backside wear), dislocation risk
Component Materials
- •Femoral: CoCr standard (forged superior to cast), Oxinium reduces wear 25%
- •Tibial: Metal-backed modular standard (cemented or cementless)
- •All-poly tibial: Elderly/low-demand, maximizes poly thickness, cemented only
- •Patellar: All-poly dome or anatomic, AVOID metal-backed (high failure rate)
Bone Defect Management
- •Metal augments: AORI 2A-2B, cemented, requires stems (50mm minimum)
- •Trabecular metal cones: AORI 2B-3, press-fit, 80% porosity, biological fixation
- •Sleeves: Circumferential metaphyseal support, AORI 3, 90% survival at 5 years
- •Stem length: CCK 50mm minimum, hinge 150mm minimum, bypass defects by 2 cortical diameters
Key Evidence and Australian Data
- •AOANJRR 2023: PS 67%, CR 25%, fixed-bearing over 85%, HXLPE over 90%
- •Cemented fixation gold standard: 95%+ 15-year survival
- •Mobile-bearing declining (under 15%) due to registry-proven inferior outcomes
- •HXLPE survival benefit emerging at 10-15 years (reduced osteolysis revision)