Polymerized into a repeating chain by addition polymerization to give high-density polyethylene (HDP).
Free radicals are produced which attach to the carbon atom and then join to each other, creating a continuous chain.
Adjacent chains are held by weak van der Waals forces.
How Can Strength Be Increased?
Increasing Molecular Weight: Higher molecular weight results in more chains.
Creating Cross-links: Cross-linking between chains increases strength.
Reducing Crystallinity: A lower crystallinity enhances flexibility and strength.
Polyethylene Architecture
Differing structural areas within polyethylene:
Crystalline: Organized 3-dimensional patterns of chains.
Amorphous: Random entanglement of chains with a propensity for cross-linking.
Longer chains lead to a greater proportion of the amorphous area.
A greater amorphous area results in greater strength and wear resistance, forming the basis for UHMWPE (Ultra High Molecular Weight Polyethylene).
Characteristics of UHMWPE:
Increased chain length.
Increased number of chains.
Increased proportion of amorphous areas.
Cross-linking, Sterilization, and Storage of Polyethylene
Sterilization traditionally done by gamma irradiation in air.
Radiation encourages cross-linking between polyethylene chains, improving wear rates and tensile strength.
Irradiation generates free radicals that form cross-links, but:
In the presence of oxygen, free radicals oxidize and degrade polyethylene.
Oxidation reduces wear properties and occurs mainly in the subsurface white band (1-2mm).
Over time, this layer increases in size, allowing further oxidation.
Oxidation occurs if:
Polyethylene is sterilized in air or oxygen.
Oxygen seeps into packaging over time (shelf life > 2 years).
Better Sterilization Methods:
Gamma irradiation in inert gas (argon or nitrogen) or in a vacuum.
Sterilization using gas plasma or ethylene oxide (no radiation, no oxygen).
Highly Cross-linked UHMWPE
Made by prolonged irradiation for even more cross-linking.
In vitro shows better wear rates than UHMPE.
However, increased cross-linking can lead to:
Reduced fatigue resistance.
Reduced fracture toughness.
Caution is advised regarding the use of highly cross-linked polyethylene, especially in total knee replacements (TKR), as failure often results from fatigue fractures rather than wear.
Polyethylene Manufacturing
Traditional Method: RAM Bar extrusion, where a cylindrical ‘bar’ of polyethylene is machined into individual implants (especially for TKR).
Issues with RAM Bar Extrusion:
Properties of produced implants differ according to their location within the bar.
Solutions:
Calcium stearate is added to prevent yellowing and corrosion but may lead to fusion defects and worse wear properties.
Other Manufacturing Options:
Compression molding followed by machining.
Direct compression molding (DCM) is considered the gold standard.
In DCM, polyethylene powder is directly compressed into a mold without further machining or additives, allowing for further irradiation in inert gas to promote cross-linking, resulting in optimal wear rates.
Ceramics
Types Used in Orthopaedics
Zirconia: Not used anymore due to high fracture rates and cost.
Alumina: Main ceramic used, improved quality compared to the first generation.
Oxinium: Oxidized zirconia.
Hydroxyapatite: Osteoconductive ceramic that aids in bone growth and bonding but can shear off implants and is difficult to coat.
Manufacturing
Produced by pressing ceramic powder and water into a pre-fabricated cast.
Sintering (heating) is then done to increase density, but over-sintering leads to porosity and weakens the ceramic.
Strength Dependence: Strength is dependent on the original grain size; smaller grain size results in stronger ceramic.
Material Properties
Hard: Third hardest material known.
Stiff: High modulus of elasticity.
Inert: Particles are not biologically active.
Low Surface Roughness: Smooth, reducing abrasive wear of polyethylene.
Good Scratch Profile: Less scratching leads to reduced wear.
Wettable: Better lubrication properties.
Low Wear Characteristics:
Less linear wear.
Less osteolysis due to smaller, less active particles.
Limitations
Brittle: Virtually no plastic deformation before failure, posing a risk of fracture.
Long-term Data: Not yet available.
Cost: More expensive than alternatives.
Squeaking: May be caused by positioning or aging ceramics.
Oxidized Zirconium
A metallic alloy of zirconium with a ceramic surface.
Oxidation in air converts the surface into ceramic, making it part of the material, not just a coating.
Provides properties similar to ceramics: hard, smooth, scratch-resistant, inert, and wettable.
Suitable for metal allergies due to low nickel content.
In vitro studies show greatly reduced polyethylene wear, but no long-term in vivo trials yet.
Ceramic Fracture
Presence of third-body wear leads to plastic and particulate debris.
Revision should include synovectomy to remove all ceramic debris.
Damage to the Morse taper may necessitate revision to prevent catastrophic failure.
Ceramic particulate debris is typically not biologically active due to its small size, but third-body wear can produce more active debris.
Metal on Metal (MoM) Articulations
Historical Issues: Equatorial bearing had high frictional torque, jamming, and high failure rates.
Current Design: Polar bearing with much lower frictional torque; requires a cup head clearance of 90-200 micrometers.
Main Properties of MoM:
Fracture toughness.
Hardness.
Low roughness (smooth and polished).
Small particle generation leads to low biological activity (50-100 nm).
Self-healing capabilities; small scratches are polished out with articulation.
Very resistant to abrasive wear.
Osteolysis is not a major concern.
Challenges
Produce a greater number of particles, particularly during the initial two years (bedding-in period).
Wear Mode: Primarily adhesive, which can be mitigated by using alloys with greater carbide content (e.g., adding molybdenum).
Lubrication in MoM
Fluid film lubrication exists in MoM bearings, but problems include:
Metal ion dissemination into the bloodstream.
Potential carcinogenesis risks.
Occurrence of metallosis, leading to adverse local tissue reactions (ALVAL) and pseudotumors.
MoM total hip replacements (THR) with large heads may experience wear at the taper due to size mismatch.