Orthopaedic metals

Orthopaedic Metallurgy

Molecular Structure of Metals

  • Metals consist of crystalline structures made up of millions of crystalline grains, which contain molecular lattices.
  • Three types of defects can affect the mechanical properties of bulk metal:
    1. Grain Boundaries: Connections between adjacent crystals; they limit crack and dislocation propagation but are sites of weakness.
    2. Dislocations: Defects within individual crystals where the 2-D lattice stops abruptly; under stress, these dislocations propagate, leading to plastic deformation.
    3. Cracks: Volumetric voids within individual crystals that can significantly impact mechanical properties.
  • Manufacturing processes and post-manufacture quality control aim to manage these defects to create metals with desired properties.

Alloys

  • Alloys are combinations of elemental metals that improve mechanical properties, corrosion resistance, or biocompatibility compared to their individual components.
  • Ductility can be reduced by:
    1. Adding large metal ions that prevent lattice layers from slipping.
    2. Adding small ions (e.g., carbon) that fill gaps between lattices and hinder slipping.
  • Adding carbon to steel increases brittleness, hence low-carbon steel is preferred for implants.

Addition of Other Elements

  • Nickel & Chromium: Increase corrosion resistance.
  • Cobalt & Molybdenum: Enhance material strength.

Manufacture of Metals

  1. Machining: Final finishing process using lathes or mills.
  2. Casting: Molten metal is poured into a mold and allowed to cool. Grain size affects properties:
    • Rapid cooling yields smaller grains (harder but more brittle).
    • Slow cooling produces larger grains (more ductile).
    • Not commonly used in orthopaedics due to control difficulties.
  3. Forging: Most common method for implant manufacture.
    • Applies pressure to shape metal, aligning grain boundaries and improving mechanical properties.
    • Can be done through:
      • Cold Working: Room temperature, increases strength but reduces ductility and increases brittleness.
      • Hot Working: Above crystallization temperature, easier to shape, maintains ductility, and can be hardened later.

Heat Treatments

  1. Annealing: Heats metal above crystallization temperature to alter dislocation patterns and grain boundaries, followed by controlled cooling to improve ductility and workability.
  2. Quenching: Sudden immersion in cold water to trap impurities and harden the material.
  3. Tempering: Re-heating to lower temperatures after quenching to reduce brittleness.

Coatings and Surface Finishes

Finish Process Roughness (µm) Materials
Polished Mechanical abrasion 0.1 Any
Matte Mechanical abrasion 0.8 Any
Grit Blasted Mechanical abrasion 3 Any
Sintered Beads Sintering 150 CoCr
Plasma Spray Plasma spraying 50-150 HA, Tantalum
Porous Metals Vapour deposition 150-250 Tantalum
  1. Grit Blasting: Uses hard metallic crystals to give a rough finish for better cement interdigitation.
  2. Sintered Beads: Increase surface roughness by applying heated metal beads, often CoCr, to implants.
  3. Plasma Spray Coating: Uses high-temperature torches to heat coating material into plasma, which is then sprayed onto implants.

Common Orthopaedic Metals

An ideal orthopaedic material should be fatigue resistant, biocompatible, resistant to corrosion, and sufficiently strong, ductile, and stiff.

  1. 316L Stainless Steel
    • Composition: 60% Iron, 20% Chromium, 3% Molybdenum, 16% Nickel, 0.03% Carbon.
    • Mechanical Properties:
      • Good fatigue resistance and relatively biocompatible.
      • High stiffness and hardness, ductility allows contouring of plates.
      • Susceptible to galvanic corrosion and nickel hypersensitivity.
  2. Titanium 64 (Ti-6Al-4V)
    • Composition: 90% Titanium, 6% Aluminium, 4% Vanadium.
    • Mechanical Properties:
      • Less stiff than CoCr and steel, good fatigue resistance.
      • Low surface hardness and highly notch-sensitive.
      • Excellent corrosion resistance and biocompatibility.
      • MRI and CT safe with minimal image distortion.
  3. Cobalt Chrome (CoCr 60255)
    • Composition: 60% Cobalt, 25% Chromium, 5% Molybdenum, 10% other.
    • Mechanical Properties:
      • High modulus of elasticity and stiffness.
      • Good fatigue strength and ultimate tensile strength.
      • Low surface roughness and relatively corrosion-resistant.
      • Used as articulating components but can lead to ion dissemination and metal hypersensitivity.
  4. Tantalum
    • Composition: Pure Tantalum.
    • Mechanical Properties:
      • Low modulus of elasticity similar to cancellous bone.
      • High porosity (up to 80%) allowing for rapid bone ingrowth.
      • Not suitable for stems due to lower strength and fatigue resistance.

Summary of Material Properties

Property Ti-6Al-4V Co-Cr-Mo 316L Stainless Steel Tantalum
Modulus of Elasticity (GPa) 106–115 210 230 2.5–3.9
Yield Strength (MPa) 860 825 170–690 35–51
Ultimate Strength (MPa) 780–1050 430–1028 515–860 50–110
Toughness (J/m³) 75 (MPa m½) - 88–134 -
Hardness (Rockwell C) 36 42 60 -
Fatigue Strength (MPa at 10⁷ cycles) 480–590 310 180–300 18–20
Mass Density (g/cc) 4.4 8.3 8.0 -
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