METALLIC IMPLANTS

Stainless steel

Cobalt chromium alloys

Titanium alloys

Nitinol

Smitha ck

Assistant professor

Srinivas institute of technology

LECTURE 3 BIOMATERIALS 2

CLASSIFICATION OF BIOMATERIALS

Biomaterials can be divided into three major classes of

materials:

Polymers

Metals

Ceramics (including carbons, glass ceramics, and glasses).

Metallic implants

Metals make attractive biomaterials because they possess the following properties:

Excellent electrical

Mechanical properties

Closely packed atomic arrangement resulting in high specific gravity and good strength

High melting points

BIOMATERIALS 3

Applications in the human

body:

Hip and knee joints , for

fracture healing aids as

bone plates and screws,

spinal fixation devices, and

dental implants

In devices such as vascular

stents, catheter guide wires,

orthodontic arch wires , and

cochlear implants

Two primary purposes

As prosthesis to replace a portion of the body such as:

Joints, long bones & skull plates

Fixation Devices to stabilize broken bones while the normal

healing proceeds

Bone plates, intramedullary nails, screws and sutures

Problems:

1.Biocompatibility: The ability of a material to perform with an

appropriate host response in a specific situation

2.Corrosion

3.Design of metallic implants

4.Design limitations the of anatomy

5.Physics properties of the tissue and reactions of the tissue to

the implant and of the implant to the tissues (Host Response

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Different Metallic Biomaterials

Stainless Steel

SS 316

SS 316L

CoCr Alloys

the castable CoCrMo

alloy

The CoNiCrMo alloy

which is usually

wrought by (hot)

forging

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Ti alloys

Pure Ti

Ti6Al4V

TiNiAlloys

Nitinol

Shape Memory effect

Platinum group metals

(PGM)

Pt, Pd, Rh, Ir, Ru, and Os

extremely corrosion

resistant

poor mechanical

properties

pacemaker tips

conductivity.

BIOMATERIALS 6

METALLIC IMPLANT MATERIALS

Metallic implants are used for two primary purposes.

Implants used as prostheses serve to replace a portion of

the body such as joints, long bones and skull plates.

Fixation devices are used to stabilize broken bones and

other tissues while the normal healing proceeds.

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METALLIC IMPLANT MATERIALS

Though many metallic implant materials are available

commercially. The three main categories of metals which are

used for orthopedic implants

Stainless steels

Cobalt-chromium alloys

Titanium alloys

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METALLIC IMPLANT MATERIALS

The Metallic implant materials that are used should have the

following characteristic features:

Must be corrosion resistant

Mechanical properties must be appropriate for desired

application

Areas subjected to cyclic loading must have good fatigue

properties

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STAINLESS STEEL

Stainless steel is the predominant implant alloy.

This is mainly due to its ease of fabrication any desirable

variety of mechanical properties and corrosion behavior.

But, of the three most commonly used metallic implants

namely

Stainless steel

Cobalt chromium alloys

Titanium alloys,

Stainless steel is least corrosion resistant.

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STAINLESS STEEL

The various developments which took place in the development

of steel in metallic implants are discussed below.

Stainless steel (18Cr-8 Ni) was first introduced in surgery in

1926

In 1943, type 302 stainless steel had been recommended to

U.S. Army and Navy for bone fixation.Later 18-8sMo stainless

steel (316), which contains molybdenum to improve corrosion

resistance, 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 resistance.

Development of SS for use in human

body The first metal alloy developed specifically for human use was

the was used to manufacture bone fracture plates and screws.

Vanadium steel is no longer used in implants since its corrosion resistance is inadequate in vivo.

The first stainless steel utilized for implant fabrication was the 18-8 (type 302 in modern classification), which is stronger and more resistant to corrosion than the vanadium steel.

Later 18-8sMo stainless steel was introduced which contains a small percentage of molybdenum to improve the corrosion resistance in chloride solution (salt water). This alloy became known as type 316 stainless steel .

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The chromium content of stainless steels should be least 11.0% to enable them to resist corrosion.

Chromium is a reactive element.

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

The AISI Group III austenitic steel especially type 316 and 316L cannot be hardened by heat treatment but can be hardened by cold working.

This group of stainless steel is non-magnetic and possesses better corrosion resistance than any of the others.

In the 1950s the carbon content of 316 stainless steel was reduced from 0.08 to a maximum amount of 0.03% (weight percent), and hence became known as type 316L stainless steel

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STAINLESS STEEL

Advantage of SS 316 & 316L over other

grades of Steel

Biocompatible

These austenitic stainless steels cannot be hardened by HT but can be hardened by cold working

possesses better corrosion resistance than any other steels

The inclusion of molybdenum enhances resistance to pitting corrosion in salt water.

Therefore 316L is recommended rather than 316 for implant fabrication.

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Type % C %Cr % Ni %Mn % other

elements

301 0.15 16-18 6-8 2.0 1.0Si

304 0.07 17-19 8-11 2.0 1-Si

316, 18-

8sMo

0.07 16-18 10-14 2.0 2-3 Mo, 1.0 Si

316L 0.03 16-18 10-14 2.0 2.3 Mo, 0.75Si

430F 0.08 16-18 1.0-1.5 1.5 1.0 Si, 0-6 Mo

CONSTITUENTS OF STEEL

The Stainless steels used in implants are generally of two types:

Wrought

Forged

Wrought alloy possesses a uniform microstructure with fine grains.

In the annealed condition it possesses low mechanical strength.

Cold working can strengthen the alloy.

Stainless steels can be hot forged to shape rather easily because of their high ductility.

They can also be cold forged to shape to obtain required strength.

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STAINLESS STEEL

Mechanical Properties & Corrosion

Resistance of 316L Even the 316L stainless steels may corrode inside the

body under certain circumstances in a highly stressed and oxygen depleted region, such as the contacts under the screws of the bone fracture plate.

Thus, these stainless steels are suitable for use only in Temporary implant devices such as fracture plates, screws, and hip nails.

Surface modification methods are widely used in order to improve corrosion resistance, wear resistance, and fatigue strength of 316L stainless steel

anodization, passivation ( explained next slide)

glow-discharge nitrogen implantation

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Electroplating has been shown to be

generally superior to a mechanical finish for

increasing corrosion resistance which can

also be produced by other surface

treatments such as

Passivation with HNO3.

The reason why stainless steel implants failed

, indicates a variety of deficiency factors like

deficiency of molybdenum

the use of sensitized steel

Deficiency Factors Responsible

for failure of SS implants Deficiency of Mo

Use of sensitized steel

Inadvertent use of mixed metals and incompatible components

Topography and metallurgical finish

Improper implant and implant material selection

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Devices

Alloy Type

Jewitt hip nails and plates 316 L

Intramedullary pins 316 L

Mandibular staple bone plates 316L

Heart valves 316

Stapedial Prosthesis 316

Mayfield clips (neurosurgery) 316

Schwartz clips (neurosurgery) 420

Cardiac pacemaker electrodes 304

APPLICATIONS OF SS STEEL

The two basic elements of Co-based alloys form

a solid solution of upto 65 wt % of CO and 35 wt

% of Cr

To this Molybdenum is added to produce finer

grains which results in higher strength after

casting or forging .

Cobalt is a transition metal of atomic number 27 situated between iron and nickel in the first long

period of the periodic table.

The chemical properties of cobalt are intermediate between those of iron and nickel.

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COBALT CHROMIUM ALLOYS

The various milestones in the development of

cobalt chromium alloys are discussed below.

Haynes developed a series of cobalt-chromium

and cobalt- chromium-tungsten alloys having

good corrosion resistance.

During early 1930s an alloy called vitallium with a

composition 30% chromium, 7% tungsten and 0.5% carbon in cobalt was found.

Many of the alloys used in dentistry and surgery,

based on the Co-Cr system contain additional

elements such as carbon, molybdenum, nickel, tungsten

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COBALT CHROMIUM ALLOYS

Chromium has a body centered cubic (bcc)

crystal structure and cannot therefore have a

stability of the phase of cobalt.

The solubility of the former in the latter

increases rapidly as the temperature is raised.

Metallic cobalt started to find some industrial

use at the beginning of this century but its pure form is not particularly ductile or corrosion

resistant.

The various milestones in the development of

cobalt chromium alloys are discussed below.

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COBALT CHROMIUM ALLOYS

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COBALT CHROMIUM ALLOYS

Cobalt based alloys are used in one of three forms

Cast,

Wrought

Forged

Cast Alloy

The orthopedic implants Co-Cr alloy are made by a lost wax or

investment casting method which involves making a wax pattern of the desired component

1.A wax model of the implant is made and a ceramic shell is built around it

2.the wax is then melted out in a oven (100~150C),

3.the mold is heated to a high temperature burning out any traces of wax or gas-forming materials,

4.molten alloy is poured with gravitational or centrifugal force

5.the mold is broken after cooled

The mold temperature is about 800~1000C and the alloy is at 1350~1400C.

Coarse ones formed at higher temperatures will decrease the strength.

However, a high processing temperature will result in larger carbide precipitates with greater distances between them, resulting in a less brittle material.

Again there is a complementary (trade-off) relationship between strength and toughness.

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Wrought alloy:

possess a uniform microstructure with fine grains.

Wrought Co-Cr Mo alloy can be further strengthened by cold work .

Forged alloy

It is available in standard shapes and sizes and is annealed at 7300C for 1-4 hours, furnace cooled to 6000C and air-cooled to room temperature.

More uniform microstructure

Usually hot forged

The superior fatigue and ultimate tensile strength of the wrought CoNiCrMo alloy make it suitable for the applications which require long service life without fracture or stress fatigue.

Such is the case for the stems of the hip joint prostheses.

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COBALT CHROMIUM ALLOYS

Forged alloy

Expensivesophisticated press and

tooling

Porous coated Co-Cr implants

Bone in growth applications

1.Sintered beads gravity sintering

2.Plasma flame sprayed metal

powders

3.Diffusion bonded

LECTURE 3 BIOMATERIALS 27

COBALT CHROMIUM ALLOYS

Advantages

The advantage of using titanium based alloys as implant materials are

low density or Lightness (4.5 g/cm3)and good mechanical properties

-Cr Implies greater flexibility

Good mechano-chemical properties

The major disadvantage being the relatively high cost and reactivity.

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TITANIUM BASED ALLOYS

October 3, 2014 29

TITANIUM BASED ALLOYS

Titanium is a light metal having a density of 4.505g/cm3 at 250C. One of the most widely used titanium alloys for biomedical applications:Ti6Al4V The Ti6Al4V alloy has approximately the same fatigue

strength (550 MPa) as that of CoCr alloy. Titanium alloys can be strengthened and mechanical properties varied by controlled composition and thermomechanical processing techniques. Since aluminum is a lighter element and vanadium barely heavier than titanium, the density of Ti-6% Al-4% V alloy is very similar to pure titanium.

The melting point of titanium is about 16650C although variable data are reported in the literature due to the effect of impurities.

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TITANIUM BASED ALLOYS

Titanium exists in two allotropic forms,

The low temperature -form has a close-packed hexagonal

crystal structure with a c/a ratio of 1.587 at room temperature

Above 882.50C -titanium having a body centered cubic

structure which is stable

The presence of vanadium in a titanium-aluminium alloy tends to form - two phase system at room temperature.

Ti-6 Al-4V alloy is generally used in one of three conditions wrought, forged or cast.

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TITANIUM BASED ALLOYS

Forged alloy

The typical hot-forging temperature is between 900C and 930C.

Hot forging produces a fine grained -structure with a depression of

varying phase.

A final annealing treatment is often given to the alloy to obtain a stable microstructure without significantly altering the properties of the alloy.

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TITANIUM BASED ALLOYS

Cast alloy

To provide a metallurgical stable homogenous structure castings are annealed at approximately 8400C .

Cast Ti-6 Al-4V alloy has slightly lower values for mechanical properties than the wrought alloy.

Titanium and its alloys are widely used because they show

exceptional strength to weight ratio

good mechanical properties.

The lower modulus is of significance in orthopedic devices since it implies greater flexibility.

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TITANIUM BASED ALLOYS

To improve tribiological properties of Titanium

there are four general types of treatments made.

Firstly, the oxide layer may be enhanced by a

suitable oxidizing treatment such as anodizing

Secondly, the surface can be hardened by the

diffusion of interstitial atoms into surface layers

Thirdly, the flame spraying of metals or metal

oxides on to the surface may be employed

Finally, other metals may be electroplated onto

the surface LECTURE 3 BIOMATERIALS 34

TITANIUM BASED ALLOYS

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TITANIUM BASED ALLOYS

BONE SCREWS USED FOR IMPLANTATION

Nitinol Alloy of Ni-Ti

Can be designed to change its shape or dimensions in response to an increase in temperature small enough to be tolerated by the adjacent tissues in which it is embedded.

FCC ----Martensite

It has good strain recoverability, notch sensitivity and has excellent fatigue, biocompatibility and corrosion resistance

Applications

Shape memory Stents A stent is a man-made 'tube' inserted into a natural passage/conduit in the body to prevent, or counteract, a disease-induced, localized flow constriction.