3-Metallic Biomaterials-23-07-2024

Metallic Biomaterials

  • General Information

    • Widely used in various medical applications such as:

      • Load-bearing articulating joints

      • Dental prosthetics

      • Maxillofacial surgery

      • Cardiovascular devices

    • Properties:

      • Metallic bonding leads to excellent electrical and thermal conductivity

      • High fracture toughness and elastic stiffness

      • Malleable, machinable, ductile, and weldable properties

Challenges with Metallic Biomaterials

  • Corrosion and Wear

    • Leaching of Metal Ions:

      • Can cause irritation, inflammation, and loosening of implants

      • Corrosion occurs in physiological environments, risking toxicity from metals like iron (Fe), chromium (Cr), and nickel (Ni)

Modulating Properties of Metals

  • Use of Alloys

    • Definition: Impurities are intentionally added to improve material properties

    • Examples:

      • Copper (Cu) enhances mechanical properties when added to gold (Au) and silver (Ag)

      • Brass (Cu-Zn) and bronze (Cu-Sn)

      • Chromium (Cr) is added to iron (Fe) to produce stainless steel

Metal Alloys

  • Composition

    • Combines multiple metals or metal with non-metallic elements

    • Common Alloys:

      • Brass: Copper and Zinc

      • Steel: Iron with up to 2% carbon

Differences Between Pure Metals and Alloys

  • Atomic Structure

    • In pure metals, atoms can slip past each other easily, while different atom sizes in alloys can impede this, affecting strength and ductility.

Major Alloy Systems in Biomaterials

  • Three Major Systems:

    1. Stainless Steels: Primarily 316L

    2. Cobalt–Chromium–Molybdenum (CoCrMo)

    3. Titanium and Its Alloys (ASTM-F76)

The Big Three Alloys

  • Applications and Benefits:

    • 316L Stainless Steel:

      • Used in surgical instruments, screws, rods, plates, spinal devices

      • Advantages: Cost-effective, widely available

      • Disadvantages: Long-term adverse effects due to high elastic modulus

    • CoCrMo Alloys:

      • High strength, fatigue and wear resistance

      • Disadvantages: High elastic modulus, potential wear issues

    • Ti-6Al-4V:

      • Used in dental implants and total joint replacements

      • Advantages: Corrosion resistance, low elastic modulus

Components of Hip Implants

  • Design:

    • Comprised of acetabular cup (polyethylene or ceramic), femoral head (ceramic or metal), and stem

    • Example applications include joint replacements and orthopedic devices

Ways Metallic Implants Can Fail

  • Common Failure Modes:

    • Fracture: Sudden impact failures (e.g. accidental falls)

    • Fatigue: Implants like titanium may require replacement every 15 years

    • Metal to Metal Abrasion:

      • All-metal hip implants can fail due to wear, releasing metal ions into the bloodstream, causing tissue damage.

    • Corrosion: Highly corrosive environment from blood and bodily fluids

    • Stress Shielding:

      • Differences in mechanical properties between implants and native bone lead to resorption and loosening

Strategies to Prevent Implant Failure

  • Improving Fatigue Resistance:

    • Add oxygen to titanium to enhance fatigue life through stronger bonds

  • Mitigating Metal Abrasion:

    • Incorporate polyethylene inserts to reduce metal-on-metal contact

  • Corrosion Resistance:

    • Adding chromium (Cr) to steel forms passive layers that prevent corrosion

  • Stress Shielding Mitigation:

    • Introduce alloying elements or design porous implants that mimic bone porosity

Titanium Implants in Biomedical Applications

  • Common Uses:

    • Joint replacements, dental implants, bone fixation, pacemakers, stents

    • Dental Applications:

      • Crowns, inlays, bridges, and implants

    • Orthopedic Applications:

      • Hip replacements, maxillofacial and cranial implants

Nitinol (NiTi) - Unique Alloy

  • Properties:

    • Shape memory alloy with equiatomic composition of titanium and nickel

    • Exhibits two crystal structures based on temperature, responsible for its shape memory properties

  • Applications:

    • Used in orthopedic devices, screws, stents

  • Limitations:

    • Nickel can cause allergic reactions at high concentrations

Common Metallic Implants

  • Uses include:

    • Cages, plates, screws, and rods for various orthopedic applications.