import { OnePagerSummary, ExamWarning, InfoCardGrid, InfoCard, MnemonicCard, EvidenceCard, VivaScenarioSection, VivaScenario, ExamCheatSheet, ContentSection, ExamPearl, SafetyAlert, NumberHighlightRow, NumberHighlight, Tabs, Tab, ComparisonTable } from '@/components/mdx' **Evolution of Polyethylene in THA** | Generation | Processing | Advantages | Disadvantages | |------------|------------|------------|---------------| | Conventional UHMWPE | Gamma sterilised in air | Established track record | High wear (0.1-0.2mm/yr), osteolysis | | 1st Gen HXLPE | High-dose irradiation + remelting | 70-90% wear reduction | Reduced fracture toughness | | 2nd Gen HXLPE | High-dose irradiation + annealing | Better mechanical properties | Some residual free radicals | | 3rd Gen HXLPE | Vitamin E stabilisation | Maintains mechanical properties, low oxidation | Limited long-term data | **Cross-Linking Mechanism** - High-energy irradiation (gamma or electron beam) breaks C-H bonds - Creates free radicals on polymer chains - Free radicals recombine to form C-C cross-links between chains - Cross-linked network resists adhesive and abrasive wear **Irradiation Dose Effects** - Higher dose = more cross-links = lower wear - But also = more free radicals = oxidation risk - Optimal range: 50-100 kGy (typically 75-100 kGy for HXLPE) - Conventional PE sterilised at 25-40 kGy **Examiner Question**: "What is the difference in irradiation dose between conventional polyethylene sterilisation and HXLPE cross-linking?" **Model Answer**: "Conventional polyethylene is sterilised at **25-40 kGy** - sufficient for sterilisation but creates minimal cross-linking. HXLPE uses **50-100 kGy** (typically 75-100 kGy) specifically to create extensive C-C cross-links between polymer chains. This higher dose creates a densely cross-linked network that dramatically improves wear resistance - reducing linear wear from 0.1-0.2 mm/year to 0.03-0.05 mm/year (70-90% reduction). However, the higher dose also creates more free radicals which must be managed through post-irradiation processing (remelting, annealing, or vitamin E stabilisation) to prevent oxidative degradation." - **Oxidised PE failure** - free radicals cause embrittlement and delamination if not managed - **Confusing dose ranges** - cross-linking is 50-100 kGy, NOT the same as sterilisation (25-40 kGy) - **Shelf aging** - even packaged PE can oxidise over time; check expiry dates - **1st Gen HXLPE trade-off** - remelting eliminates oxidation but reduces fracture toughness 20-30% **Method 1: Remelting (1st Generation)** - Heat above melt temperature (150°C) - Eliminates free radicals completely - But destroys some crystalline structure - Result: Excellent oxidation resistance, reduced fracture toughness - Examples: Longevity (Zimmer), Marathon (DePuy), XLPE (Smith & Nephew) **Method 2: Annealing (2nd Generation)** - Heat below melt temperature (120-130°C) - Reduces free radicals (not eliminates) - Preserves crystalline structure - Result: Better mechanical properties, some oxidation risk - Examples: X3 (Stryker), E-Poly (Biomet) **Method 3: Vitamin E Stabilisation (3rd Generation)** - Vitamin E (alpha-tocopherol) added as antioxidant - Can be diffused into finished liner OR blended before moulding - Quenches free radicals without heat treatment - Result: Maintains mechanical properties AND oxidation resistance - Examples: E1 (Biomet), Vivacit-E (Zimmer) **Property Comparison** | Property | Remelted | Annealed | Vitamin E | |----------|----------|----------|-----------| | Wear resistance | Excellent | Excellent | Excellent | | Fracture toughness | Reduced | Moderate | Preserved | | Oxidation resistance | Excellent | Good | Excellent | | Fatigue strength | Reduced | Moderate | Preserved | **Examiner Question**: "Why is remelted HXLPE associated with reduced fracture toughness and how might this affect your clinical practice?" **Model Answer**: "Remelting involves heating above the melt temperature (~150°C), which **completely eliminates free radicals** but destroys the crystalline structure of the polymer. This reduces **fracture toughness by 20-30%** and fatigue strength. Clinically, this is most relevant for **thin liners (less than 6mm)** where rim stress is highest and fracture risk increases. For small acetabula where PE thickness is limited, I would consider: (1) using a **smaller head size** (28mm) to maintain adequate PE thickness, (2) preferring **vitamin E HXLPE** which maintains fracture toughness, or (3) if remelted PE must be used, ensuring **minimum 6mm thickness** and avoiding elevated rim liners which further concentrate rim stress." - **Remelted HXLPE in thin liners** - 20-30% reduced fracture toughness increases rim fracture risk - **Annealed PE in long-term implants** - residual free radicals may cause late oxidation (monitor patients) - **Vitamin E concentration matters** - too little provides inadequate protection, too much may affect mechanical properties - **Not knowing product names** - examiners may ask about specific products (Marathon, X3, E1) **Head Material Options** | Head | Wear with HXLPE | Advantages | Disadvantages | |------|-----------------|------------|---------------| | Ceramic (Alumina/BIOLOX) | Lowest | Hardest, smoothest, no metal ions | Fracture risk (rare), cost | | CoCr Metal | Low | Established, cheaper | Metal ion generation, scratching | | Oxinium | Very low | Scratch resistant surface | Limited long-term data, cost | **Head Size Selection** | Head Size | Stability | Volumetric Wear | PE Thickness | Typical Use | |-----------|-----------|-----------------|--------------|-------------| | 28mm | Lower | Lowest | Most | Small acetabula, young active | | 32mm | Moderate | Moderate | Moderate | Standard choice | | 36mm | Higher | Higher | Less | Instability risk, posterior approach | | 40mm+ | Highest | Highest | Least | High dislocation risk only | **Jump Distance** - 32mm head: ~8mm jump distance - 36mm head: ~10mm jump distance - 40mm head: ~12mm jump distance - Larger head = more travel before dislocation **Examiner Question**: "You are performing THA on a 45-year-old active patient with a 50mm acetabular shell. What bearing couple and head size would you choose?" **Model Answer**: "For this young, active patient I would choose **ceramic head on HXLPE**. Ceramic provides the lowest wear rates with HXLPE (approximately 18% lower than metal heads) due to its extreme hardness and surface smoothness. With a **50mm shell**, I would use a **32mm head** which provides approximately 9mm of PE thickness - safely above the 6mm minimum. While a 36mm head might improve stability, it would reduce PE thickness to approximately 7mm, and the volumetric wear increase is unnecessary in a well-balanced reconstruction. For this patient who may have this implant for 40+ years, minimising cumulative wear burden takes priority over the marginal stability improvement of a larger head." - **Oversized head in small acetabulum** - sacrifices PE thickness, increases volumetric wear - **Metal head in young active patient** - generates metal ions, higher wear than ceramic - **Not considering renal function** - CKD patients accumulate metal ions; prefer ceramic on HXLPE - **Ignoring metal sensitivity history** - metal allergy patients need ceramic on HXLPE or CoC **AOANJRR 2023 Data** - HXLPE most common bearing couple (greater than 80% of primary THA) - Metal on HXLPE: 15-year revision rate 6.5% - Ceramic on HXLPE: 15-year revision rate 6.1% - Ceramic on ceramic: 15-year revision rate 5.7% **Wear Rate Comparison (Radiostereometric Analysis)** - Conventional PE: 0.1-0.2 mm/year linear wear - HXLPE: 0.03-0.05 mm/year linear wear (70-90% reduction) - Ceramic on HXLPE: 0.01-0.03 mm/year **Osteolysis Rates** - Conventional PE at 10 years: 20-40% prevalence - HXLPE at 10 years: less than 5% prevalence - Critical threshold for osteolysis: greater than 1 billion particles/year **Key Studies** - Engh CA Jr et al (JBJS 2012): Marathon HXLPE, 10-year follow-up, 85% reduction in wear vs conventional - Bragdon CR et al (CORR 2013): Meta-analysis showing consistent 60-80% wear reduction - Devane PA et al (JBJS 2017): 15-year RSA data confirming sustained low wear with HXLPE **Examiner Question**: "What is the key evidence supporting the use of HXLPE over conventional polyethylene?" **Model Answer**: "The evidence supporting HXLPE is robust at multiple levels. **AOANJRR data** from over 500,000 procedures shows HXLPE is now the standard of care, used in greater than 80% of primary THA in Australia. **Wear rates** measured by radiostereometric analysis (RSA) show HXLPE at 0.03-0.05 mm/year compared to 0.1-0.2 mm/year for conventional PE - a **70-90% reduction**. The **clinical impact** is dramatic: osteolysis rates at 10 years are less than 5% with HXLPE compared to 20-40% with conventional PE. Landmark studies include **Engh (JBJS 2012)** showing 85% wear reduction at 10 years, and **Devane (JBJS 2017)** confirming sustained low wear at 15 years. This translates to fewer revisions for wear - AOANJRR shows 15-year revision rates under 7% for HXLPE bearings." - **Mixing up wear rate numbers** - HXLPE is 0.03-0.05 mm/yr (NOT 0.3-0.5 mm/yr which is 10x higher) - **Forgetting the osteolysis threshold** - ~1 billion particles/year triggers macrophage activation - **Not knowing Australian registry data** - AOANJRR shows HXLPE in greater than 80% of THA - **Claiming HXLPE eliminates osteolysis** - it doesn't (still less than 5% at 10 years), but dramatically reduces it **Irradiation creates free radicals that cause oxidation if not managed** - Oxidised PE becomes brittle and delaminates. All HXLPE must undergo post-irradiation processing. EXAM KEY: Know the three methods - remelting (eliminates radicals but reduces toughness), annealing (partial elimination, preserves properties), vitamin E (quenches radicals, preserves properties). **Particulate debris activates macrophages causing osteolysis** - HXLPE produces fewer but smaller particles. Total particle load reduced by 70-90%. EXAM KEY: Osteolysis threshold approximately 1 billion particles/year. HXLPE typically stays below this threshold, dramatically reducing osteolysis rates (less than 5% at 10 years vs 20-40% conventional). **Cross-linking improves wear resistance but reduces fracture toughness** - Remelted HXLPE has 20-30% reduced fracture toughness. Clinical significance: Theoretically increased rim fracture risk with thin liners. EXAM KEY: Maintain minimum 6mm (ideally 8mm) poly thickness. Avoid excessive cross-linking of thin liners. **Long-term oxidation can occur even in implanted HXLPE** - Lipid absorption, cyclic loading, and body fluids can cause in vivo oxidation over decades. EXAM KEY: Vitamin E HXLPE provides ongoing antioxidant protection. Long-term surveillance required for all HXLPE - not immune to failure. **Patient and Implant Factors** | Factor | Consideration | Bearing Recommendation | |--------|---------------|----------------------| | Age less than 55 | High demand, long life expectancy | Ceramic on HXLPE or CoC | | Age 55-75 | Moderate demand | HXLPE with ceramic or metal head | | Age greater than 75 | Lower demand, lower activity | HXLPE with metal head acceptable | | High activity | Increased wear potential | Ceramic head preferred | | Renal impairment | Avoid metal ion accumulation | Ceramic on HXLPE or CoC | | Metal sensitivity | Avoid metal debris | Ceramic on HXLPE or CoC | | Instability risk | Need larger head | 36mm head on HXLPE | | Small acetabulum | Limited poly thickness | 28mm head, adequate poly check | **Component Size Considerations** | Shell Size | Maximum Head | Available PE Thickness | |------------|--------------|----------------------| | 48mm | 28mm | 10mm | | 50mm | 32mm | 9mm | | 52mm | 32mm | 10mm | | 54mm | 36mm | 9mm | | 56mm | 36mm | 10mm | | 58mm | 40mm | 9mm | **Critical Rule**: NEVER select a head size that results in less than 6mm poly thickness. In small acetabula, use 28mm head to preserve adequate PE. **Liner Design Options** | Liner Type | Design Feature | Indication | |------------|----------------|------------| | Neutral | Standard hemisphere | Default for stable reconstruction | | Elevated rim (10°) | 10° lip extension | Mild instability risk, posterior approach | | Elevated rim (20°) | 20° lip extension | Higher instability risk | | Lateralised | Increased offset | Abductor tensioning, leg length | | Constrained | Capture mechanism | Neuromuscular disease, recurrent dislocation | **Liner Positioning** For elevated lip liners: - Posterior approach: Lip positioned posteroinferiorly - Anterior approach: Lip positioned posterosuperiorly - Can use clock-face positioning (e.g., 7 o'clock for posterior approach) **Elevated Lip Trade-off**: Increases stability in one direction but may cause impingement and dislocation in opposite direction. Position lip to cover most vulnerable arc. ### Liner Insertion 1. **Shell Confirmation** - Confirm shell stable (no toggle) - Clean shell of blood and debris with pulse lavage - Inspect locking mechanism 2. **Liner Selection** - Choose appropriate liner type (neutral vs elevated) - Confirm head size compatibility - Verify adequate PE thickness 3. **Insertion Technique** - Align liner features with shell (locking tabs, anti-rotation features) - Insert at correct orientation (elevated lip position) - Apply firm impaction until audible/tactile click - Confirm full seating - no gap between liner and shell 4. **Verification** - Visual inspection of seating - Check locking mechanism engaged - Test stability by attempting to dislodge liner **Examiner Question**: "How do you confirm proper liner seating and what is the consequence of incomplete seating?" **Model Answer**: "I confirm proper liner seating through a **four-step verification**: (1) **Visual inspection** - no visible gap between liner and shell rim circumferentially, (2) **Audible/tactile click** during impaction indicating locking mechanism engaged, (3) **Digital palpation** around the liner rim feeling for any step-off or gap, (4) **Stability testing** - attempting to manually dislodge the liner which should be completely stable. **Incomplete seating** is a catastrophic error that leads to: micromotion and backside wear generating excessive debris, accelerated PE failure, potential liner dissociation requiring urgent revision, and osteolysis from the wear debris cascade. If there is ANY doubt about seating, I would remove and re-insert the liner after confirming the shell is clean and the liner is the correct size for the shell." - **Incomplete seating** - leaves gap causing micromotion, backside wear, accelerated debris, and dissociation risk - **Malpositioned elevated lip** - creates impingement in wrong arc, paradoxically increases dislocation risk - **Wrong liner size** - locking mechanism fails, leading to liner dissociation - **Blood/debris in shell** - prevents full seating; always lavage shell before liner insertion ### Head Selection and Insertion 1. **Material Selection** - Ceramic head preferred for young/active patients - Metal head acceptable for older/lower demand - Oxinium for metal sensitivity but renal concerns about ceramic 2. **Size Selection** - 32-36mm standard for most patients - 28mm for small acetabula (maintain PE thickness) - Larger heads for instability risk 3. **Neck Length** - Assess leg length and offset with trial - Standard, +3.5mm, +7mm, -3.5mm options typical - Final selection after stability testing 4. **Insertion** - Clean and dry taper (moisture causes corrosion) - Align head on taper - Single firm impaction (do NOT hammer repeatedly) - Confirm full seating **Examiner Question**: "What is the correct technique for head impaction onto the femoral taper?" **Model Answer**: "Proper head impaction is critical for preventing trunnionosis (head-taper corrosion). The technique is: (1) **Clean the taper** - any blood, bone, or debris causes fretting and corrosion, (2) **Dry the taper completely** - moisture trapped at the interface causes crevice corrosion and accelerated metal ion release, (3) **Align the head axially** on the taper, (4) **Single firm impaction** using a head impactor - DO NOT hammer repeatedly as this creates micromotion and damages the taper surface. The 'cold welding' of head to taper occurs with the first impaction; subsequent blows cause fretting damage. After impaction, confirm full seating by attempting gentle axial traction - the head should be completely stable." - **Wet or contaminated taper** - trapped moisture causes crevice corrosion and trunnionosis with metal ion release - **Multiple impaction attempts** - damages taper surface causing fretting, corrosion, and potential head dissociation - **Wrong neck length** - not checking leg length/offset with trial first; leads to LLD or instability - **Ceramic head mishandling** - dropping or impacting against metal instruments can cause microfractures 1. Australian Orthopaedic Association National Joint Replacement Registry. Annual Report 2023. Adelaide: AOA; 2023. 2. Kurtz SM, Gawel HA, Patel JD. History and systematic review of wear and osteolysis outcomes for first-generation highly crosslinked polyethylene. Clin Orthop Relat Res. 2011;469(8):2262-2277. 3. Bragdon CR, Doerner M, Martell J, et al. The 2012 John Charnley Award: Clinical multicenter studies of the wear performance of highly crosslinked remelted polyethylene in THA. Clin Orthop Relat Res. 2013;471(2):393-402. 4. Oral E, Muratoglu OK. Vitamin E diffused, highly crosslinked UHMWPE: a review. Int Orthop. 2011;35(2):215-223. 5. Engh CA Jr, Hopper RH Jr, Huynh C, et al. A prospective, randomized study of cross-linked and non-cross-linked polyethylene for total hip arthroplasty at 10-year follow-up. J Arthroplasty. 2012;27(8 Suppl):2-7. 6. Devane PA, Horne JG, Ashmore A, et al. Highly cross-linked polyethylene reduces wear and revision rates in total hip arthroplasty: a 10-year double-blinded randomized controlled trial. J Bone Joint Surg Am. 2017;99(20):1703-1714. 7. Glyn-Jones S, Thomas GE, Garfjeld-Roberts P, et al. The John Charnley Award: Highly crosslinked polyethylene in total hip arthroplasty decreases long-term wear: a double-blind randomized trial. Clin Orthop Relat Res. 2015;473(2):432-438. 8. Muratoglu OK, Bragdon CR, O'Connor DO, et al. A novel method of cross-linking ultra-high-molecular-weight polyethylene to improve wear, reduce oxidation, and retain mechanical properties. J Arthroplasty. 2001;16(2):149-160. 9. Harris WH. The problem is osteolysis. Clin Orthop Relat Res. 1995;311:46-53. 10. Callary SA, Solomon LB, Holubowycz OT, et al. Wear of highly crosslinked polyethylene acetabular components: a review of RSA studies. Acta Orthop. 2015;86(2):159-168.

Highly Cross-Linked Polyethylene in THA

Highly Cross-Linked Polyethylene in THA