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 Quality control Reasonable cost

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

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

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

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

Elastic Modulus, GPa Cobalt Alloy200 Stainless Steel200 Titanium Alloy110 Cortical Bone18 PMMA3 UHMWPE1

Fatigue Cycles Cyclic Stress (MPa)

Fatigue Cycles Cyclic Stress (MPa)

Fatigue Cycles Cyclic Stress (MPa)

Fatigue Cycles Cyclic Stress (MPa) Cast Cobalt Alloy

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

316L Stainless Steel

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

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

Stainless Steels Work harden easily Stress Strain

Stainless Steels Work harden easily Stress Strain

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

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

Elastic Yield Ultimate Endurance Modulus Strength Strength Limit Material (GPa) (MPa) (MPa) (MPa) Mechanical Properties Stainless steels 316L Annealed L 30% CW † –450 Cobalt Alloys As cast – – –310 Hot forged – Titanium Alloys 30% CW † Forged –690 † CW = cold-worked Annealed CW †

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

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

Poly(methyl methacrylate) Mechanical grout that polymerizes in situ Liquid methacrylate monomer hydroquinone (inhibitor) toluidine (accelerant) Powder prepolymerized PMMA benzoyl peroxide (initiator) BaSO 4 or ZrO 2 (radiopaque)

Poly(methyl methacrylate) Brittle material Tensile strength = ~ 35 MPa Compressive strength = ~ 90 MPa Fatigue strength = ~ 6 MPa at 10 5 cycles

UHMW polyethylene C C HHHH HHHH crystalline amorphous

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

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

Alternative Sterilization No irradiation Gas plasma Ethylene oxide Irradiation w/o O 2 Ar, Ni, Vacuum

Alternative Sterilization No irradiation Gas plasma Ethylene oxide Irradiation w/o O 2 Ar, Ni, Vacuum Poor abrasive/adhesive wear, but less cracking Excellent wear behavior

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

THA: 30% to 96% reduction at 2 – 5 yrs Clinical Results 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

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

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

“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

Ceramics Solid, inorganic compounds consisting of metallic and nonmetallic elements held together by ionic or covalent bonding Aluminum + Oxygen  Alumina (Al 2 O 3 ) Zirconium + Oxygen  Zirconia (ZrO 2 )

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 Advantages of Ceramics

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

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

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

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

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

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

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 mm 3 /yr Co-Cr-Mo = 0.17 mm/yr & 745 mm 3 /yr

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

Metallic alloy (Zr-2.5Nb) with a ceramic surface (ZrO 2 ) intended to provide wear resistance without brittleness Good V et al, JBJS 85A (Suppl 4), 2003 Oxidized Zirconium (Oxinium  )

Courtesy: R. Laskin ceramic oxygen enriched metal substrate Depth from surface (µ) Nano-hardness (GPa)

Good V et al, JBJS 85A (Suppl 4), 2003 Wear Rate (mm 3 /million cycles) Oxidized Zirconium (Oxinium  )

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

Suggested References Biomaterials Science: An Introduction to Materials in Medicine (ed B Ratner et al), 2 nd Edition, San Diego, Academic Press, 2004 Wright TM and Li S. Biomaterials. In Orthopaedic Basic Science (ed J Buckwalter et al), 2 nd Edition, Rosemont, AAOS, 2000