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Biomaterials

Biomaterials. Tim Wright, PhD FM Kirby Chair, Orthopaedic Biomechanics, Hospital for Special Surgery Professor, Applied Biomechanics Weill Cornell Medical College. Requirements for Implant Materials. Biocompatibility Corrosion resistance Adequate mechanical properties Wear resistance

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Biomaterials

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  1. Biomaterials Tim Wright, PhD FM Kirby Chair, Orthopaedic Biomechanics, Hospital for Special Surgery Professor, Applied Biomechanics Weill Cornell Medical College

  2. Requirements forImplant Materials Biocompatibility Corrosion resistance Adequate mechanical properties Wear resistance Quality control Reasonable cost

  3. Stress vs Strain Behavior F F/A Area L/L F

  4. Stress vs Strain Behavior F F/A Area Elastic Modulus L/L F

  5. Stress vs Strain Behavior F Brittle F/A Area Elastic Modulus L/L F

  6. Stress vs Strain Behavior F Ultimate Yield F/A Ductile Area Elastic Modulus L/L F

  7. Elastic Modulus, GPa

  8. Fatigue 150 350 550 Cyclic Stress (MPa) 104 105 106 107 108 Cycles

  9. Fatigue 150 350 550 Cyclic Stress (MPa) 104 105 106 107 108 Cycles

  10. Fatigue 150 350 550 Cyclic Stress (MPa) 104 105 106 107 108 Cycles

  11. Fatigue 150 350 550 Cyclic Stress (MPa) Cast Cobalt Alloy 104 105 106 107 108 Cycles

  12. Metallic Alloys Stainless steel Cobalt chromium alloy Titanium alloy Polymers PMMA UHMWPE Ceramics Alumina Zirconia

  13. 316L Stainless Steel

  14. Stainless Steels Introduced in 1920's (316L during WWII) Not magnetic Total hip stems, fracture & spinal fixation Low carbon content insures resistance to intergranular corrosion

  15. 22-13-5 Stainless Steel Wrought Nitrogen Strengthened Stainless Steel 22 Chromium - 13 Nickel – 5 Manganese - 2.5 Molybdenum Higher strength, better corrosion resistance than 316L

  16. Stress Strain Stainless Steels Work harden easily

  17. Stress Strain Stainless Steels Work harden easily

  18. Cobalt Chromium Alloy

  19. Cobalt Chromium Alloy First used in implant devices in 1930's Casting mostly replaced by forging Corrosion resistance by passive oxide film Total joint components

  20. Titanium Alloy

  21. Titanium Alloy First used in implant devices in 1960's Reactivity of titanium with oxygen forms passive layer for corrosion resistance Poor abrasion resistance Notch sensitive Fracture fixation devices, spinal instrumentation, total joint implants

  22. Mechanical Properties Elastic Yield Ultimate Endurance Modulus Strength Strength Limit Material (GPa) (MPa) (MPa) (MPa) Stainless steels 316L Annealed 190 330 590 250 316L 30% CW† 190 790 930 300–450 22-13-5 Annealed 380 690 22-13-5 CW† 860 1040 Cobalt Alloys As cast 210 450–515 655–890 200–310 Hot forged 230 965–1000 1206 500 Titanium Alloys 30% CW† 110 485 760 300 Forged 120 1035 1100 620–690 †CW = cold-worked

  23. Porous Coatings Trabecular Metal (tantalum deposited on a pyrolytic carbon framework)

  24. Porous Coatings Compressive Strength 50-80 MPa Elastic Modulus ~ 3 GPa Trabecular Metal (tantalum deposited on a pyrolytic carbon framework)

  25. Poly(methyl methacrylate) Mechanical grout that polymerizes in situ • Liquid methacrylate monomer hydroquinone (inhibitor) toluidine (accelerant) • Powder prepolymerized PMMA benzoyl peroxide (initiator) BaSO4 or ZrO2 (radiopaque)

  26. Poly(methyl methacrylate) Brittle material Tensile strength = ~ 35 MPa Compressive strength = ~ 90 MPa Fatigue strength = ~ 6 MPa at 105 cycles

  27. UHMW polyethylene H H H H C C C C H H H H crystalline amorphous

  28. UHMWPE • Fabricated by extrusion compression molding direct molding • Sterilized by gamma radiation (inert gas) ethylene oxide gas plasma

  29. UHMWPE • Degradation recombination chain scission cross-linking • Chain Scission free radicals,  MW ,  density • Cross-linking  wear resistance,  toughness

  30. Alternative Sterilization No irradiation Gas plasma Ethylene oxide Irradiation w/o O2 Ar, Ni, Vacuum

  31. Poor abrasive/adhesive wear, but less cracking Excellent wear behavior Alternative Sterilization No irradiation Gas plasma Ethylene oxide Irradiation w/o O2 Ar, Ni, Vacuum

  32. Preclinical Test Results Knee simulators 43% to 94% reduction Hip simulators Zero wear McKellop, et al, JOR, 1999 McEwen, et al, J Biomech, 2005;

  33. Clinical Results THA: 30% to 96% reduction at 2 – 5 yrs Digas et al, Acta Orthop. Scand, 2003; Heisel et al, JBJS, 2004; Martell et al, J Arthroplasty, 2003; Dorr et al, JBJS, 2005; D’Antonio et al, CORR, 2005; Manning et al, J Arthroplasty, 2005

  34. Decreasing Toughness Elevated Cross-linked PE Gillis, et al, Trans ORS, 1999

  35. Elevated Cross-linked PE Cup Impingement Holley, et al, J Arthroplasty, 2005

  36. “Second Generation”Elevated Cross-linked PE’s • Mechanical deformation • Doping with vitamin E • Repeated cycles Cross-link & thermally treat Muratoglu, Harris, et al. Wang, Manley, et al. Improve mechanical properties while maintaining gains in wear resistance

  37. Ceramics Solid, inorganic compounds consisting of metallic and nonmetallic elements held together by ionic or covalent bonding Aluminum + Oxygen  Alumina (Al2O3) Zirconium + Oxygen  Zirconia (ZrO2)

  38. Advantages of Ceramics • High elastic modulus (2-3x metals) • High hardness • Polished to a very smooth finish • Excellent wettability (hydrophylic) • Excellent scratch resistance Even with the presence of third bodies • Inert/biocompatible

  39. Disadvantages of Ceramics • Weak in tension • Brittle No ability to deform plastically • Fracture! Fractures in THA femoral heads 1 in 2000in the 1970s 1 in 10000 to 1 in 25000in the 1990s

  40. Ceramics Mechanical properties depend on: Grain size Porosity Impurities • 1970’s Today • 4 to 5 μ 1 to 2 μ • No HIPing  HIPing • 95% purity  99% purity

  41. Alumina & Zirconia • About 20% of femoral heads • Of ceramic heads, 60% alumina, 40% zirconia • Alumina heads introduced in the 1960s • Zirconia introduced in 1980s in response to alumina head fractures ~4x the fracture strength

  42. Zirconia • Stabilized (yttrium oxide) • Unstable crystalline structure tetragonalmonoclinic • Sterilized by ethylene oxide do not resterilize with steam • Excellent wear resistance but not against ceramics, metals

  43. Ceramic-UHMWPE Couples Linear penetration (mm/yr) Hernigou and Bahrami, JBJS Br 2003

  44. Ceramic-UHMWPE Couples Linear penetration (mm/yr) Retrieved heads showed  monoclinic content Hernigou and Bahrami, JBJS Br 2003

  45. Ceramic-UHMWPE Couples Significant reduction in polyethylene wear YH Kim (JBJS, 2005) Prospective, randomized study with 7 yr follow-up Wear rates: Zirconia = 0.08 mm/yr & 351 mm3/yr Co-Cr-Mo = 0.17 mm/yr & 745 mm3/yr

  46. Ceramic-Ceramic Couples Significant reduction in wear & osteolysis Hamadouche, et al (JBJS, 2002) Minimum 18½ year follow-up of 118 alumina-alumina THAs Wear undetectable; 10 cases of osteolytic lesions

  47. Oxidized Zirconium (Oxinium) Metallic alloy (Zr-2.5Nb) with a ceramic surface (ZrO2) intended to provide wear resistance without brittleness • Good V et al, JBJS 85A (Suppl 4), 2003

  48. Oxidized Zirconium (Oxinium) oxygen enriched metal metal substrate ceramic 16 Courtesy: R. Laskin 12 Nano-hardness (GPa) 8 4 0 10 5 15 0 Depth from surface (µ)

  49. Oxidized Zirconium (Oxinium) Wear Rate (mm3/million cycles) • Good V et al, JBJS 85A (Suppl 4), 2003

  50. Oxidized Zirconium (Oxinium) Short term clinical results 42% less wear than Co alloy against PE in knee simulator tests Ezzet, et al., CORR, 2004

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