Polyethylene in Arthroplasty: UHMWPE and XLPE

Remelting Method (First Generation XLPE)

Process:

1. Irradiation: Gamma or e-beam (50-100 kGy) in inert atmosphere
2. Crosslinking: C-C bonds form between chains
3. Remelting: Heat above melting point (over 137C, typically 150C)
4. Cooling: Controlled cooling to room temperature
5. Machining: Fabricate into liners

Purpose of remelting:

• Eliminate all free radicals: Heating allows radicals to recombine or be quenched
• Prevent oxidation: No residual radicals means no oxygen reaction
• Shelf stability: Can be stored indefinitely without degradation

Trade-offs:

• Crystallinity reduced: Melting destroys crystalline lamellae, reduces from 45% to 30-35%
• Toughness reduced: Lower crystallinity means lower fracture toughness and impact strength
• Fatigue resistance reduced: More prone to crack propagation under cyclic loading

Clinical implication: Remelted XLPE requires minimum 6-8mm thickness to prevent rim fracture.

This completes the remelting method description.

Annealing Method (Sequential XLPE)

Process:

1. Irradiation: Gamma or e-beam (50-100 kGy) in inert atmosphere
2. Crosslinking: C-C bonds form between chains
3. Annealing: Heat below melting point (below 137C, typically 130C for several hours)
4. Cooling: Controlled cooling to room temperature
5. Machining: Fabricate into liners

Purpose of annealing:

• Reduce free radicals: Heating allows some radicals to recombine (not all)
• Preserve crystallinity: Temperature below melting point maintains lamellae structure
• Better toughness: Maintains 40-45% crystallinity (closer to conventional PE)

Trade-offs:

• Residual free radicals: Not all radicals eliminated (5-10% remain)
• Slow oxidation: Radicals can react with oxygen over years (but much slower than conventional PE)
• Shelf life: Should be packaged in barrier packaging to prevent shelf aging

Clinical implication: Annealed XLPE has better mechanical properties than remelted, but slight oxidation risk. Still vastly superior to conventional PE.

This completes the annealing method description.

Vitamin E Stabilized XLPE (Third Generation)

Concept:

• Antioxidant: Vitamin E (alpha-tocopherol) scavenges free radicals
• Prevents oxidation: Vitamin E reacts with radicals instead of oxygen
• Preserves toughness: No need for high-temperature remelting

Methods:

1. Diffusion method: Soak irradiated PE in vitamin E (diffuses into surface)
2. Blending method: Mix vitamin E into UHMWPE powder before compression molding

Advantages:

• Oxidation resistance: Vitamin E eliminates free radicals without remelting
• Better mechanical properties: Can use lower radiation dose or skip remelting (higher toughness)
• Long-term stability: Vitamin E provides ongoing antioxidant protection

Considerations:

• Lower crosslink density: Some formulations use lower radiation dose (better toughness, slightly higher wear)
• Vitamin E migration: Diffusion method may have non-uniform distribution
• Clinical data: 10+ year data emerging, excellent wear performance

Clinical implication: Vitamin E XLPE may represent optimal balance of wear resistance and mechanical properties.

This completes the vitamin E stabilization description.

Polymer Morphology and Microstructure:

Detailed Ultrastructure:

Lamellae Organization:

• Primary lamellae: Thick (20-50nm), formed during slow cooling
• Secondary lamellae: Thin (10-20nm), formed during recrystallization
• Lamellar thickness: Proportional to crystallization temperature
• Spherulites: Radially arranged lamellae visible under polarized light

Tie Molecule Concept:

• Long polymer chains traverse multiple crystalline and amorphous regions
• Tie molecules: Segments of chain that connect adjacent lamellae through amorphous phase
• Critical for mechanical integrity (transfer stress between crystalline regions)
• More tie molecules = better toughness and fatigue resistance
• High molecular weight (3-6 million Da) ensures abundant tie molecules

Effect of Processing on Morphology:

Chain Mobility and Wear:

The wear resistance of XLPE relates directly to molecular mobility:

• Uncrosslinked PE: Chains can slide past each other (adhesive wear)
• Crosslinked PE: C-C bonds lock chains together (restricted sliding)
• Crystalline regions: Already restricted mobility (inherently wear-resistant)
• Amorphous regions: Crosslinking here provides greatest benefit

Why 50-100 kGy dose is optimal:

• Creates sufficient crosslinks in amorphous phase to restrict chain mobility
• Preserves enough chain mobility for adequate toughness
• Higher doses (greater than 100 kGy) cause excessive chain scission and embrittlement

Classification of Polyethylene Types in Arthroplasty:

Detailed Classification by Brand:

Classification by Radiation Source:

Gamma radiation:

• Cobalt-60 source
• Penetrates throughout material uniformly
• Dose rate: lower (takes hours)
• Temperature: rises during irradiation (may affect crosslinking)

Electron beam (e-beam):

• Accelerated electrons
• Limited penetration depth (may need multiple passes)
• Dose rate: higher (minutes)
• Temperature: can be controlled more precisely

Vitamin E XLPE Sub-classification:

Blending method:

• Vitamin E (0.05-0.3% by weight) mixed into UHMWPE powder
• Uniform distribution throughout material
• Can be irradiated after blending
• Example: E1 (Biomet)

Diffusion method:

• UHMWPE irradiated and crosslinked first
• Soaked in vitamin E solution (diffuses in)
• Gradient concentration (highest at surface)
• Example: Vivacit-E (Zimmer)

Quality Control and Clinical Assessment Methods:

Laboratory Testing Methods:

Mechanical Testing:

1. Tensile testing (ASTM D638):

   • Ultimate tensile strength (UTS)
   • Yield strength and elongation at break
   • XLPE slightly lower UTS than conventional (35-45 vs 40-50 MPa)

2. Impact testing:

   • Izod impact strength (notched specimen)
   • Charpy impact test
   • XLPE reduced 30-40% compared to conventional

3. Fatigue testing:

   • Cyclic loading to simulate walking cycles
   • Crack propagation rate
   • XLPE more susceptible to fatigue crack growth

Oxidation Assessment:

FTIR (Fourier Transform Infrared Spectroscopy):

• Measures carbonyl absorption peak (1718 cm⁻¹)
• Oxidation index = carbonyl peak / reference peak ratio
• Fresh PE: OI less than 0.1; Oxidized: OI greater than 1.0
• Used for shelf aging assessment and retrieval analysis

ESR (Electron Spin Resonance):

• Detects unpaired electrons (free radicals)
• Quantifies residual free radicals after manufacturing
• Remelted XLPE: near zero; Annealed: detectable residuals

Crosslink Density Testing:

Retrieval Analysis (Explanted Liners):

When liners are revised, laboratory analysis provides valuable information:

• Visual inspection: Wear patterns, scratches, rim damage
• Dimensional measurement: Compare to original dimensions (CMM)
• Oxidation profiling: FTIR through thickness (surface to bulk)
• Mechanical testing: Tensile, impact if sufficient material
• Microscopy: Subsurface cracking, delamination, third-body wear

Clinical Imaging for Osteolysis:

Plain radiographs:

• Annual follow-up X-rays recommended
• Look for: periarticular lucencies, component migration, liner wear

CT scan (if osteolysis suspected):

• More sensitive than X-ray for detecting osteolytic lesions
• Quantify lesion volume for surgical planning
• Metal artifact reduction sequences helpful

Implant Selection and Decision-Making:

Advanced Selection Considerations:

XLPE Type Selection:

Dual Mobility Considerations:

In dual mobility constructs:

• Large outer head (40-48mm+ effectively) provides stability
• Smaller inner articulation (22-28mm)
• Some dual mobility cups use conventional PE (design-specific)
• XLPE dual mobility increasingly available and preferred

TKA Polyethylene Selection:

XLPE in TKA - Different considerations:

• Lower cross-shear wear than THA (more sliding, less rotation)
• Conventional PE still acceptable in TKA (less wear concern)
• XLPE increasingly used but evidence less clear than THA
• Cruciate-retaining vs posterior-stabilized affects loading pattern

Managing PE Wear in Follow-up:

X-ray assessment:

• Annual AP pelvis and lateral hip X-rays
• Compare to immediate post-op baseline
• Look for linear penetration, osteolysis, component migration

If lucency detected:

• CT scan to quantify osteolysis volume
• MRI if soft tissue concern (ALTR, pseudotumor)
• Plan intervention based on lesion size and progression

Consider revision if:

• Measurable linear wear greater than 0.1mm/year with XLPE (unusual)
• Progressive osteolysis greater than 1-2cm lesions
• Component loosening or impending fracture
• Symptomatic (pain, instability)

Liner Exchange Considerations:

When liner wear or osteolysis requires intervention:

• Isolated liner exchange possible if shell well-fixed and well-positioned
• Bone grafting of osteolytic lesions through screw holes
• Always upgrade to XLPE if revising from conventional PE
• Consider head exchange (new head on existing stem) to optimize articulation

Liner Selection and Intraoperative Considerations:

Advanced Liner Considerations:

Modular vs Fixed Bearing Liners:

Locking Mechanism Considerations:

Different manufacturers use different liner locking mechanisms:

• Peripheral snap-fit: Ring around liner edge
• Central post: Central locking mechanism
• Screw holes: Can allow osteolysis access (some designs cover them)

Important: Match liner to shell design - they are not interchangeable between manufacturers.

Head-Liner Selection Sequence:

After final reaming, confirm cup size (typically 1-2mm underream for press-fit).

Insert final cup with appropriate press-fit, confirm stable seating.

Liner thickness = (Cup OD - Head size) / 2 + liner wall thickness

Example: 54mm cup with 36mm head = (54-36)/2 = 9mm dome thickness (acceptable)

Insert trial liner and head. Assess:

• Stability (shuck test, ROM testing)
• Leg length and offset
• Impingement (test full ROM)

Insert real liner (confirm full seating). Insert real head (confirm taper engagement).

Handling and Storage:

XLPE handling precautions:

• Keep in sterile packaging until ready to insert
• Avoid prolonged exposure to room air (some formulations)
• Do not drop or damage (may cause rim defects)
• Follow manufacturer storage recommendations

Vitamin E XLPE:

• May have yellow tint (normal for vitamin E)
• Same handling as other XLPE

Detailed Complication Analysis:

Rim Fracture:

Mechanism:

• Edge loading concentrates stress at liner rim
• Reduced toughness of XLPE (crosslinking restricts chain mobility)
• Thinner liners have less material to resist fracture
• Cup malposition increases edge loading risk

Risk factors:

• Liner thickness less than 6mm
• Cup abduction greater than 55 degrees (edge loading)
• High activity level (more loading cycles)
• Impingement (concentrated stress at rim)

Prevention:

• Minimum 6-8mm liner thickness
• Optimal cup position (40-45 degrees abduction)
• Avoid elevated lip liners if cup malpositioned
• Consider larger cup if acetabulum permits

Oxidation and Degradation:

Backside Wear:

Mechanism:

• Micromotion between modular liner and metal shell
• Generates additional particles
• Especially problematic with screw holes (particle access to bone)

Prevention:

• Secure liner locking mechanism
• Confirm complete liner seating
• XLPE reduces but doesn't eliminate backside wear concern
• Some designs use liner covers for screw holes

Dislocation (Related to Liner Choice):

Liner factors affecting stability:

• Elevated lip liners increase jump distance
• Larger head size improves stability (but thinner liner)
• Lateralized liners affect offset
• Constrained liners for high-risk patients

XLPE enables larger heads (low wear penalty) which improves stability.

Surveillance and Monitoring for PE Wear:

Advanced Surveillance Considerations:

When to Increase Surveillance Frequency:

Activity Recommendations:

Patient counselling regarding PE longevity:

• XLPE is designed for active patients
• No specific activity restrictions related to PE wear
• General THA precautions apply (avoid high-impact, contact sports)
• Higher activity = more cycles, but XLPE handles this well

Weight management:

• Higher BMI increases joint loading
• Weight loss beneficial for implant longevity (and general health)
• Not specific to PE type, but reduces overall wear

Managing Symptomatic Patients:

If patient develops symptoms (pain, instability, clicking):

1. Clinical examination:

   • Gait, ROM, stability testing
   • Impingement tests
   • Rule out infection (aspiration if needed)

2. Imaging:

   • X-rays (AP pelvis, lateral hip)
   • CT scan for osteolysis assessment
   • MRI if soft tissue concern

3. Differential diagnosis:

   • Liner wear/osteolysis
   • Component loosening
   • Liner fracture
   • Infection
   • Adverse local tissue reaction
   • Other (bursitis, tendinopathy)

Long-term Survivorship Expectations:

With XLPE:

• 10-year survivorship: greater than 95%
• 15-year survivorship: greater than 90%
• 20-year survivorship: Data emerging, excellent

Main revision reasons with XLPE:

• Instability/dislocation (not PE-specific)
• Infection (not PE-specific)
• Osteolysis (dramatically reduced with XLPE)
• Liner fracture (rare, preventable with proper thickness)