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