Polyethylene

Chemistry & Manufacture

  • Basic Monomer: C2H4
  • Manufactured by the Ziegler process:
    • Ethylene gas is polymerized into a repeating chain through addition polymerization.
    • Low temperature and low pressure conditions produce Polyethylene powder.
  • Adjacent chains are held together by Van der Waals forces, resulting in High Density Polyethylene (HDP).

Polyethylene Architecture

Polyethylene consists of different structural areas: - Crystalline: Organized 3D patterns of chains. - Amorphous: Random entanglement of chains, offering: - More wear resistance. - A tendency to cross-link.

How to Increase Polyethylene Strength

  1. Increasing the number of chains (molecular weight).
  2. Creating cross-links between chains.
  3. Reducing crystallinity.
  4. Manufacturing method.
  5. Sterilization method.
  6. Storage method.
  7. Removing un-cross-linked free radicals.
  • Longer chains lead to a greater proportion of amorphous areas, enhancing strength and wear resistance.
  • This forms the basis for UHMWPE (Ultra High Molecular Weight Polyethylene):
    • Increased chain length.
    • Increased number of chains.
    • Increased proportion of amorphous areas.

Cross-linking, Sterilization, and Storage

  • Sterilization Methods:
    • Traditionally via gamma irradiation in air.
    • Radiation encourages cross-linking between polyethylene chains.

Effects of Oxidation

  • Oxidation Issues:
    • Degrades polyethylene, reducing wear properties due to less cross-linking and a higher proportion of crystalline areas.
    • Occurs mainly within the subsurface white band (1-2 mm).

Reasons for Oxidation

  1. Polyethylene is sterilized in air or oxygen.
  2. Oxygen seeps into packaging over time (shelf life > 2 years).
  3. In vivo: Some degree of oxidation occurs naturally after implantation (minimal as synovial fluid has low oxygen content).

Better Sterilization Methods

  1. Gas plasma or ethylene oxide sterilization (no radiation) minimizes oxidation.
  2. Gamma irradiation in inert gas (argon or nitrogen) or in a vacuum is preferred.

Highly Cross-linked UHMWPE

  • Made through prolonged irradiation, resulting in:
    • Increased stiffness and hardness.
    • Increased wear resistance.

Concerns with Highly Cross-linked Polyethylene

  • Reduced fatigue resistance.
  • Reduced fracture toughness (UTS).
  • Risk in total knee replacements (TKR) due to fatigue fractures rather than wear.

Methods of Removing Residual Free Radicals

  • Annealing:
    • Heating to above or just below the melting point removes residual free radicals, reducing in vitro or shelf life oxidation.
    • Prolonged annealing alters mechanical properties.
  • Adding Vitamin E:
    • Scavenges free radicals and may prevent long-term oxidation while in situ.
  • X3 Poly:
    • A modern type of highly cross-linked polyethylene produced by 3 short cycles of irradiation and annealing.
    • Exhibits high wear resistance with minimal in vivo alterations in mechanical properties.

Polyethylene Manufacturing Methods

  • RAM Bar Extrusion:
    • Traditional method where a cylindrical ‘bar’ of polyethylene is machined into individual implants.
    • Variability in properties based on the location in the bar.
  • Addition of Calcium Stearate:
    • Prevents yellowing and corrosion, but can lead to fusion defects and worse wear properties.
  • Compression Moulding:
    • Polyethylene produced in individual blocks, then machined to create the implant.
    • Better than RAM bar extrusion but can produce sharp edges and stress risers.
  • Direct Compression Moulding:
    • The current gold standard, directly compressing polyethylene powder into pre-made molds of implants without further machining or additives.
    • The final implant is irradiated to promote cross-linking.
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