BIOMATERIALS ENT 311/4 Lecture 4 Metallic Material Prepared by: Nur Farahiyah Binti Mohammad Date:...

BIOMATERIALSENT 311/4

Lecture 4 Metallic Material

Prepared by: Nur Farahiyah Binti Mohammad

Date: 28th July 2008

Email : [email protected]

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Teaching Plan

METALLIC BIOMATERIALS

Review types of metallic biomaterials

Define & describe the biomedical application of metallic biomaterial

Evaluate effect of the physiological environment on metallic material

DELIVERYMODE

Lecture

LEVEL OF COMPLEXITY

KnowledgeRepetitionEvaluation

COURSE OUTCOMECOVERED

Ability to describe the concept of biocompatibility & basic concepts of materials used in medical application

Ability to select biomaterials that can be used for different medical applications and explain the criteria that will lead to a successful implants

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1.0 Introduction

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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Metallic material usually used in 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.

1.0 Introduction

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Metals commonly used in Biomedical Application

Metals ApplicationsCobalt-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

Summary

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Stainless steel Cobalt-chromium alloys Titanium alloys Gold and platinum Silver-tin-copper alloys

2.0 List of metals

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3.0 Stainless Steels

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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3.0 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


0.08

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Most widely used for implant fabrication: Austenitic stainless steel

316 stainless steel 316L stainless steel


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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.

http://en.wikipedia.org/wiki/Yield_strength

http://en.wikipedia.org/wiki/Ductility

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3.1 Stainless steel alloy application

Stainless steel alloy applicationDevices Stainless steel type

Orthopedic Fixation Devices: Jewitt Hip nails and platesIntramedullary pins Mandibular staple bone platesSchwartz clips (neurosurgery)Implant prosthesesCardiac pacemaker electrodesHeart valvesStent

316L316L316L420

304316316L (new: nickel free stainless steel)

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3.1 Stainless steel alloy application Jewitt Hip nails and plates

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3.1 Stainless steel alloy application

Intramedullary pin

Mandibular staple bone plates

Schwartz clips (neurosurgery)

Cardiac pacemaker electrodes

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4.0 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.

4.0 Cobalt-Chromium Alloys (cont)

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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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This table adapted from Biomaterials, Sujata V.Bhat

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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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5.0 Titanium alloys

Titanium is a light metal. Density = 4.505 g/cm3 at 26oC

Alloys Density (g/cm3)

Ti and its alloys316 stainless steelCoCrMoCoNiCrMoNiTi

4.57.98.39.26.7

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5.0 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.


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Modulus elasticity of titanium and its alloy is about 110GPa except for the Ti13Nb13Zr.

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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.


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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.


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Biomedical application: Artificial heart valves, dental implants, artificial joint components, orthopedic screws (less desirable), pacemaker cases, vascular stents


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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

6.0 Silver-tin-copper alloys (Amalgam)

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Mechanical properties of dental amalgams

Amalgam Compressive strength (MPa)

Tensile strength (MPa)

Lower CopperAdmixSingle

composition

343431510

604864

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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.


http://en.wikipedia.org/wiki/Dental_composite

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Has bacteriostatic effects Can interfere the bacterial protein

production, DNA replication, or other aspects of bacterial cellular metabolism

http://en.wikipedia.org/wiki/Bacteriostatic_agent

http://en.wikipedia.org/wiki/Protein

http://en.wikipedia.org/wiki/DNA

http://en.wikipedia.org/wiki/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.


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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.

7.0 Corrosion of Metallic Implant

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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.

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Electrolyte contains ion in solution, serve to complete the electric circuit.

Anions = negative ion which migrate toward anode

Cations = positive ion which migrate toward cathode

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At anode: Metal oxidizes by losing valence electron

M → M+n + ne-

At cathode: M+n + ne- → M


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As one moves down the table, the metals tendency for oxidation becomes higher and they are more active (anodic)

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The galvanic series has been developed from data collected on metal corrosion in seawater.

Provides a good indication of the relative activity of various metals in salt solution, similar to that found in the human body.

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7.1 Where does corrosion occur?

In region where two dissimilar metals in contact Implantation of dissimilar metals (mixed metals) is to be

avoided, because: Galvanic corrosion may occur One which is most negative in the galvanic series will

become anode, and the other one will become cathode.

Region where there are variations in material homogeneity. Occur within a single material Grain boundaries are anodic with respect to the

grain interior. Crack in material- matrix will become cathode,

crack will become anode.

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CORROSION DUE TO DEFECTS DURING FABRICATION. Crevice corrosion Pitting Corrosion Intergranular corrosion

CORROSION DUE TO EFFECTS OF MECHANICAL ENVIROMENT Stress and Galvanic corrosion Stress corrosion cracking Fatigue corrosion Fretting corrosion

7.2 Types of corrosion

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CORROSION DUE TO DEFECTS DURING FABRICATION.

Crevice Corrosion Corrosion occurring in spaces to which the

access of the working fluid from the environment is limited.

These spaces are generally called crevice which is narrow, deep crack.

Example: In between the screw and plate of a bone fixation device.

Usually exhibit in stainless steel orthopaedic applications.

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Oxidation of the metal occur in crevice.

The remainder of the piece become cathode.

Depletion of oxygen in the crevice. Diffusion of Cl- ions into the crevice

to balance the charge of the M+n ions created.

The compound formed can react further to produce insoluble hydroxide and librate H+:

MCln + nH2O → M(OH)n + nH+Cl-

Decrease in pH →A shift to acid conditions in the crevice → provide more corrosive environment

Anode

Cathode

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Pitting corrosion Caused by same mechanism as crevice corrosion. Small defects on the surface of the material (e.g

scratch). Passivation layer on the surface is disrupted. Leading to the formation of a relatively small

anode and a large cathode. The anodic region undergo significant dissolution. This is dangerous type of corrosion because it ca

undetected until device failure due to the small overall material loss.


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7.2 Types of corrosion

Intergranular Corrosion Devices fabricated by casting often have

multiple grains Thus susceptible to intergranular

corrosion. Grain boundaries will become anodic

regions of the material. Grain will become cathode.

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Grain (cathode)

Boundaries (anode)

Intergranular corrosion


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CORROSION DUE TO EFFECTS OF MECHANICAL ENVIROMENT

Stress and Galvanic corrosion Occur when two dissimilar metals in

contact. One which is most negative in the galvanic

series will become anode, and the other one will become cathode.

Bending of metal rod or plate will cause: Tensile side → anode Compression side → cathode

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Stress corrosion cracking Occurs to a metal that is both under

tension and subjected to a corrosive environment.

Results in small crack that form perpendicular to the direction of applied stress.

Crack propagate due to anodic dissolution at crack tip.

This lead to brittle fracture.

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Fatigue corrosion Continued bending, loading or motion

around the implant may disrupt the passivating film formed on the metal and exposed the underlying surface, leading to corrosion of this area.

This type of corrosion may significantly shorten the fatigue life of the implant

Results in premature device failure.


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Fretting corrosion Not related to loading. Dependent on motion near the implant

(abrasion, friction). Involves removal of the metal’s

passivating layer by mechanical means. Can be seen around connection between

fixation plates and bone screws.


BIOMATERIALS ENT 311/4 Lecture 4 Metallic Material Prepared by: Nur Farahiyah Binti Mohammad Date:...

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