Post on 18-Dec-2015
Biomaterials
Tim Wright, PhDFM Kirby Chair, Orthopaedic
Biomechanics, Hospital for Special Surgery
Professor, Applied BiomechanicsWeill Cornell Medical College
Requirements forImplant Materials
BiocompatibilityCorrosion resistanceAdequate mechanical
propertiesWear resistanceQuality controlReasonable cost
Stress vs Strain BehaviorF
F
Area
F/A
L/L
Stress vs Strain BehaviorF
F
Area
F/A
L/L
Elastic Modulus
Stress vs Strain BehaviorF
F
Area
F/A
L/L
Elastic Modulus
Brittle
Stress vs Strain BehaviorF
F
Area
F/A
L/L
Elastic Modulus
Yield
Ultimate
Ductile
Elastic Modulus, GPaCobalt Alloy 200
Stainless Steel 200
Titanium Alloy 110
Cortical Bone 18PMMA 3
UHMWPE 1
Fatigue
Cycles
Cycl
ic S
tress
(M
Pa)
104 105 106 107 108
150
35
0
5
50
Fatigue
Cycles
Cycl
ic S
tress
(M
Pa)
104 105 106 107 108
150
35
0
5
50
Fatigue
Cycles
Cycl
ic S
tress
(M
Pa)
104 105 106 107 108
150
35
0
5
50
Fatigue
Cycles
Cycl
ic S
tress
(M
Pa)
104 105 106 107 108
150
35
0
5
50
CastCobaltAlloy
Metallic Alloys Stainless steel
Cobalt chromium alloy Titanium alloy
Ceramics Alumina Zirconia
Polymers PMMA
UHMWPE
316L Stainless Steel
Fe (65%)C (0.03%)
Cr (18%)
Ni (14%)Mo (3%)
Stainless SteelsIntroduced in 1920's (316L during WWII)
Not magnetic
Total hip stems, fracture & spinal fixation
Low carbon content insures resistance to intergranular corrosion
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
Stainless Steels
Work harden easily
Str
ess
Strain
Stainless Steels
Work harden easily
Str
ess
Strain
Cobalt Chromium Alloy
Co (63%)
Cr (28%)
Ni (3%)Mo (6%) C (0.4%)
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
Ti (90%)
Al (6%)V (4%)
Titanium AlloyFirst 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 steels316L Annealed 190 330 590 250316L 30% CW† 190 790 930 300–450
Cobalt AlloysAs cast 210 450–515 655–890 200–310Hot forged 230 965–1000 1206 500
Titanium Alloys30% CW† 110 485 760 300Forged 120 1035 1100 620–690
†CW = cold-worked
22-13-5 Annealed 380 69022-13-5 CW† 860 1040
Porous Coatings
TrabecularMetal
(tantalum deposited on a pyrolytic carbon framework)
Porous Coatings
TrabecularMetal
(tantalum deposited on a pyrolytic carbon framework)
Compressive Strength
50-80 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)
BaSO4 or ZrO2 (radiopaque)
Poly(methyl methacrylate)Brittle material
Tensile strength = ~ 35 MPa
Compressive strength = ~ 90 MPa
Fatigue strength = ~ 6 MPa at 105 cycles
UHMW polyethylene
C C C C
H H H H
H H H H
crystalline
amorphous
UHMWPE•Fabricated by
extrusioncompression moldingdirect molding
•Sterilized by gamma radiation (inert gas)ethylene oxidegas plasma
UHMWPE•Degradation
recombinationchain scission cross-linking
•Chain Scission
free radicals, MW , density
•Cross-linking
wear resistance, toughness
Alternative Sterilization
No irradiationGas plasma
Ethylene oxide
Irradiation w/o O2
Ar, Ni, Vacuum
Alternative Sterilization
No irradiationGas plasma
Ethylene oxide
Irradiation w/o O2
Ar, Ni, Vacuum
Poor abrasive/adhesive
wear, but lesscracking
Excellentwear
behavior
Preclinical Test ResultsKnee simulators
43% to 94% reduction
McEwen, et al, J Biomech, 2005;
Hip simulatorsZero 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
DecreasingToughness
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 propertieswhile 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 (Al2O3)
Zirconium + Oxygen Zirconia (ZrO2)
• 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 1970s1 in 10000 to 1 in 25000 in the
1990s
Disadvantages of Ceramics
CeramicsMechanical properties depend
on:Grain sizePorosity
Impurities1970’s Today
4 to 5 μ 1 to 2 μNo 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 oxidedo not resterilize with steam
•Excellent wear resistancebut not against ceramics, metals
0
0.1
0.2
0.3
0.4
0.5
5 years 12 years
32 Alumina
28 SS
32 SS
28 Zirconia
Hernigou and Bahrami, JBJS Br 2003
Lin
ear
pen
etr
ati
on
(m
m/y
r)Ceramic-UHMWPE Couples
0
0.1
0.2
0.3
0.4
0.5
5 years 12 years
32 Alumina
28 SS
32 SS
28 Zirconia
Hernigou and Bahrami, JBJS Br 2003
Lin
ear
pen
etr
ati
on
(m
m/y
r)Ceramic-UHMWPE Couples
Retrieved heads showed monoclinic content
Ceramic-UHMWPE CouplesSignificant 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
Ceramic-Ceramic CouplesSignificant reduction in wear &
osteolysisHamadouche, 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 (ZrO2)
intended to provide wear resistancewithout brittleness
Good V et al, JBJS 85A (Suppl 4), 2003
Oxidized Zirconium (Oxinium)
Oxidized Zirconium (Oxinium)
Courtesy: R. Laskin
16
8
12
4
0 5 15
10 0
ceramicoxygen
enrichedmetal
metalsubstrate
Depth from surface (µ)
Nan
o-h
ard
ness
(G
Pa)
0
10
20
30
40
50
60
70
80
Ox Smooth
Ox Rough
Co Smooth
Co Rough
Good V et al, JBJS 85A (Suppl 4), 2003
Wear
Rate
(m
m3/m
illio
n c
ycl
es)
Oxidized Zirconium (Oxinium)
Short term clinical results
42% less wear than Co alloy against PE
in knee simulator testsEzzet, et al., CORR, 2004
Oxidized Zirconium (Oxinium)
Suggested References
Biomaterials Science: An Introduction to Materials in Medicine (ed B Ratner et al), 2nd Edition, San Diego, Academic Press, 2004
Wright TM and Li S. Biomaterials. In Orthopaedic Basic Science (ed J Buckwalter et al), 2nd Edition, Rosemont, AAOS, 2000