METALLIC

MATERIALS LECTURE 4

Metallic Implant Materials Metallic implants are used for two primary

purposes.

Prostheses: serve to replace a portion of body such as joint, long bones and skull plates.

Fixation devices: used to stabilize broken bones and other tissue while the normal healing proceed. E.g. bone plates, rods, intramedullary nails, screw and sutures.

INTRODUCTION

September 28, 2014 3

Characteristics of a desirable metal

implant include:

bio- compatibility,

strength,

resistance to corrosion, and

imaging transparency.

METALLIC IMPLANT MATERIALS

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Metals and its alloys widely used as biomaterial

because:

Strong material

Ductile : Relatively easily formed into complex

shape

High modulus and yield point : make them suitable

for bearing large load without leading to a large

deformations and permanent dimensional change.

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Metals commonly used in Biomedical Application

Metals Applications

Cobalt-chromium alloys

Artificial heart valves, dental prosthesis, orthopedic fixation plates, artificial joint components, vascular stents

Gold and platinum

Dental fillings, electrodes for cochlear implants

Silver-tin-copper alloys Dental amalgams

Stainless steel Dental prosthesis, orthopedic fixation devices (such as plates and screw), vascular stents

Titanium alloys Artificial heart valves, dental implants, artificial joint components, orthopedic screws, pacemaker cases, vascular stents

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Stainless steel

Cobalt-chromium alloys

Titanium alloys

Gold and platinum

Silver-tin-copper alloys

LIST OF METALS

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STAINLESS STEEL

Predominant implant alloy.

In 1926-The first stainless steel (18Cr-8Ni)

was utilized for implant fabrication, which

is stronger and more resistant to corrosion

than the vanadium steel.

In 1943, type 302 stainless steel had been

recommended to U.S Army and navy for

bone fixation.

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Stainless Steels (cont)

Later 18-8sMo stainless steel or known as 316 stainless steel, which contains a small percentage of molybdenum to improve corrosion in chloride solution (salt water) was introduced.

In the 1950s – 316L stainless steel was developed by reduction of maximum carbon content from 0.08% to 0.03% for better corrosion reduction especially to physiological saline in human body.

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Chromium content of stainless steel should be at least 11% to enable them resist corrosion.

Chromium oxide on the surface of steel provide excellent corrosion resistance.

This table adapted from Biomaterials, Sujata V.Bhat

Stainless Steels (cont)

0.08

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Most widely used for implant fabrication:

Austenitic stainless steel

316 stainless steel

316L stainless steel

Stainless Steels (cont)

Stainless Steels (cont)

Not hardenable by heat treatment, but can be hardened by cold-working.

Nonmagnetic and possess better corrosion.

Inclusion of Mo enhances resistance to pitting corrosion in salt water.

May corrode under certain circumstance.

Suitable to use in temporary devices such as fracture plates, screws and hip nails.

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Stainless Steels (cont)

Strengthens the alloy Weakens the alloys (low mechanical strength)

Cold working ( material is exposed at low temperature relative to the melting point, This leads to an increase in the yield strength of the material and a subsequent decrease in ductility.

Annealing (material is exposed to high temperatures for long period in order to increase ductility and toughness of the material.

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Stainless steel alloy application Stainless steel alloy application

Devices Stainless steel type

Orthopedic Fixation Devices:

Jewitt Hip nails and plates

Intramedullary pins

Mandibular staple bone plates

Schwartz clips (neurosurgery)

Implant prostheses

Cardiac pacemaker electrodes

Heart valves

Stent

316L

316L

316L

420

304

316

316L (new: nickel free stainless steel)

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Stainless steel alloy application Jewitt Hip nails and

plates

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Stainless steel alloy application

Intramedullary pin

Mandibular staple bone

plates

Schwartz clips (neurosurgery)

Cardiac pacemaker electrodes

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COBALT-CHROMIUM ALLOYS The ASTM list four types of CoCr alloys which

are recommended for surgical implant

application:

cast CoCrMo alloy (F75)

Wrought CoCrWNi alloy (F90)

Wrought CoNiCrMo alloy (F562)

Wrought CONiCrMoWFe alloy (F563)

At present only two are used extensively in

implant fabrication, which are cast CoCrMo

alloy and wrought CoNiCrMo alloy

ASTM= The American Society for Testing and Materials

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Molybdenum is added to produce finer

grains = results in higher strengths.

Chromium = enhance corrosion

resistance.

Cobalt-Chromium Alloys (cont)

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Cobalt-Chromium Alloys (cont)

Advantages of CoNiCrMo

Highly corrosion resistance to seawater

(containing chloride ions) under stress.

Has a superior fatigue and ultimate tensile strength than CoCrMo → suitable for

application which require long service life

such as stems of the hip joint prosthesis

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Femoral component of hip joint prosthesis

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Cobalt-Chromium Alloys (cont)

This table adapted from Biomaterials, Sujata V.Bhat

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Cobalt-Chromium Alloys (cont)

Problems with CoCr alloys: Particulate Co is toxic to human osteoblast and

inhibits synthesis of type I collagen.

Advantages of CoCr alloys: Low wear

Hard

Tough High corrosion resistance

Application: Artificial heart valves, dental prosthesis, orthopedic fixation plates, artificial joint components, vascular stents

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TITANIUM ALLOYS

Titanium is a light metal.

Density = 4.505 g/cm3 at 26oC

Alloys Density (g/cm3)

Ti and its alloys

316 stainless steel

CoCrMo

CoNiCrMo

NiTi

4.5

7.9

8.3

9.2

6.7

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Titanium alloys (cont)

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Ti6Al4V is widely used to manufacture

implant.

Has approximately the same fatigue

strength (550MPa) with CoCr alloy.

That’s why it has same application as CoCr alloy.

However it more preferable because it

has low density.

Titanium alloys (cont)

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Titanium alloys (cont)

Modulus elasticity of titanium and its alloy is about

110GPa except for the Ti13Nb13Zr.

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Titanium alloys (cont)

When compared by the specific strength (strength per density) the titanium alloys exceed any other implant material.

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Advantages:

Resistance to corrosion by the formation of solid oxide layer to a depth of 10nm.

Under in vivo conditions the oxide (TiO2) is the only stable reaction product.

Limitation:

Has poor sheer strength → less desirable for bone fixation devices e.g. bone screw and plates.

Tends to gall when in sliding contact with itself or another metal.

Titanium alloys (cont)

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However, micro motion at the cement-

prosthesis and cement-bone are

inevitable, consequently titanium oxide

and titanium alloy particles are released

into the extra cellular fluid and can cause

toxicity or triggers giant cell response

around the implant.

Titanium alloys (cont)

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Biomedical application:

Artificial heart valves,

dental implants,

artificial joint components,

orthopedic screws (less desirable),

pacemaker cases,

vascular stents

Titanium alloys (cont)

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Amalgam is an alloy made of liquid

mercury and other solid metal particulate

alloys made of silver, tin, copper, etc.

Dental amalgam typically contain:

45 to 55% mercury

35 to 45% silver

15% tin

Silver-tin-copper alloys

(Amalgam)

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Advantages over other restorative material It is inexpensive

relatively easy to use and manipulate during placement

it remains soft for a short time so it can be packed to fill any irregular volume, and then forms a hard compound.

Amalgam possesses greater longevity than other direct restorative materials, such as composite. On average, serve for 10 to 12 years, whereas resin-

based composites serve for about half that time.

Silver-tin-copper alloys

(Amalgam)

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Silver-tin-copper alloys

(Amalgam)

Has bacteriostatic effects

Can interfere the bacterial protein

production, DNA replication, or other

aspects of bacterial cellular metabolism

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Its main disadvantages are:

poor aesthetics on anterior teeth

the known toxicity of mercury.

Concerns about possible harmful health effects from the

low levels of mercury released from amalgam have

resulted in a decline in the routine use of amalgam in

recent years.

Silver-tin-copper alloys

(Amalgam)

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Other metals

Tantalum

Found to be highly compatible

high density (16.6g/cm3)

poor mechanical properties

Application restricted to a few applications

such as wire sutures for plastic and

neurosurgery and a radioisotope for

bladder tumour.

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Platinum

Extremely corrosion resistant

Poor mechanical properties

Mainly used as alloys for electrodes in

neuromuscular stimulation devices such as

cardiac pacemaker.

Because of their high resistance to corrosion

Low threshold potential for electrical conductivity.

Other metals

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Corrosion is the unwanted chemical reaction of

metals with its environment.

Tissue fluids in the human body contains water,

dissolved oxygen, proteins and various ions such

as chloride and hydroxide.

As a result the human body presents a very

aggressive environment for metals used for

implantation.

CORROSION OF METALLIC

IMPLANT

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Corrosion of Metallic Implant

Fundamental of corrosion Corrosion is an electrochemical process

that involves transfer of electrons from one substance to another.

Coupling of two reaction: Oxidation (generates electrons)

Reduction (consumes electron)

Corrosion occurs when metal atoms become ionized and go into solution to form a compound which flakes off or dissolves.

Case Study 1

Patient’s arm was x-rayed : a bone plate

had been left in place for 30 years.

The screws had lost their clear outline due

to corrosion and irritating effect of the

corrosion products resulted in osseous

proliferation. The plate was found to be

vanadium steel, a metal considered

suitable in the 1920s but since abandoned

for implants.

Case Study 2

Patient experienced pain and disability in

a repaired shoulder fracture

The screws were removed and examined.

One was found to be Co-Cr-Mo and the

others of stainless steel. Bimetallic corrosion

resulted. Such cases can be avoided with

better efforts by both manufacturers and

surgeons to avoid mix metals.