MAJ VIVEK MATHEW

OVERVIEW History

Properties of metals

Metal working methods

Specific implant materials

Corrosion

Material associated complications

Recent advances

Conclusion

TIMELINE Bone pegs -1500

Brass wire -1775

Ivory rod -1890

Steel plate (Lane) -1905 (Vanadium steel)

Silver rod -1913

Steel alloys -1926 (18-8 type SSMo)

Vitalium (Stellite) -1929 (CC)

Titanium - 1950s

Ceramics -1970

Biodegradable -1980

HISTORY Initially pure metals: corrosion

Developments in metal refining and processing - first half of the 20th century, - wartime needs

Led to - improved materials that were rapidly, although empirically, adapted by surgeons for use in fracture fixation.

PROPERTIES OF IMPLANTS Mechanical properties

Composition

Grain structure: finer grain, stronger and more ductile

Non Mechanical properties

Corrosion resistance

Biochemical inertness

MECHANICAL PROPERTIES The mechanical properties are described in terms of

the deformation or strain produced by an applied stress.

Such behavior can be plotted on a stress-strain diagram

Based on Modulus of elasticity(E)

STRESS & STRAIN

STRESS-STRAIN DIAGRAM Yield stress: maximum stress that can be applied

without producing permanent deformation after removal of the Stress.

end of the elastic (linear) portion of the stress- strain curve.

The ultimate stress, is the stress associated with complete mechanical failure of the test specimen

The area under the stress-strain curve reflects the toughness or the energy absorbed (work required) to produce failure

Stiffness: resistance to deformation

Work done: Force x distance the construct bends.

represented by the area under the force displacement graph

Toughness : work done to fracture a construct or material, including both the elastic and plastic regions of deformation.

Flexible and tough -e.g. rubber, or a child's bone

stiff but brittle (e.g., glass, elderly bone),

YOUNG'S MODULUS Named after Thomas Young, a 19th century British

scientist

Calculates the change in the dimension of a bar made of an isotropic elastic material under tensile or compressive loads

Calculated by dividing the tensile stress by the tensile strain in the elastic (initial, linear)portion of the stress-strain curve:

CALCULATION

Where

E is the Young's modulus (modulus of elasticity),

F is the force exerted on an object under tension,

A0 is the original cross-sectional area through which the force is applied,

ΔL is the amount by which the length of the object changes

L0 is the original length of the object.

YOUNG'S MODULUS Predicts how much a material extends under tension

or shortens under compression.

Determines selection of materials for particular structural application.

The SI unit of Young’s Modulus/Modulus of elasticity (E) is Pascal (Pa or N/m²)

Practical units - Megapascals or Gigapascals.

Material

UltimateStrength

Tensile (MPa)

UltimateStrength

Compressive(MPa)

YieldStrength

0.2% Offset(MPa)

ElasticModulus

(MPaTendon 70 400

Cortical bone 100 175 80 15,000

Cancellous bone 2 3 1000

Polyethylene 40 20 20 1000

PTFE Teflon 25 500

Acrylic bone cement 40 80 2000

Titanium (pure, cold worked) 500 400 100,000

Titanium (Al-4V) (alloy F 136) 900 800 100,000

Stainless steel (316 L) (annealed) >500 >200 200,000

Stainless steel (cold worked) >850 >700 200,000

Cobalt chrome (cast) >450 >50 20,000

Cobalt chrome (wrought, annealed) >300 >300 230,000

Super alloys (CoNiMo) 1800 1600 230,000

HARDNESS Mechanical property of hardness refers to a material's

resistance to indentation either

by a ball (Brinell test) or

by a pyramidal diamond (Vickers test).

NONMECHANICAL PROPERTIES Inertness

Ideally an implant material should not degrade at all.

In reality – it is unachievable

Acceptable if

no significant impairment in the mechanical strength of the device

no release of deleterious local or systemic by-products.

FATIGUE Fatigue refers to a mode of failure that results from

repeated stress at magnitudes lower than that required to cause failure in a single application (ultimate stress).

Implant materials necessarily must have a high degree of fatigue resistance to perform over the long term

CREEP Progressive deformation with time under constant

stress.ie decay of strain under constant stress

Non-crystalline materials such as polymers, are particularly prone to this time-dependent form of deformation

Minimized by using metal alloys having a high melting point.

In plastics by the addition of fillers or other reinforcing materials that raise the viscosity

STRESS RELAXATION Stress relaxation is a time-dependent, decay of applied

stress under conditions of constant strain (in contrast to creep, which is a time-dependent strain under conditions of constant stress).

Time-dependent reduction in applied interfragmentary compression at a fracture site that is fixed and compressed with a DCP.

STRESS SHIELDING

Wolff’s law: a healthy bone will remodel in response to a load

Osteopenia as a result of removal of normal stress from the bone due to an implant

METHODS OF METAL WORKING Forging

Casting

Rolling & drawing

Milling

Cold working

Annealing

Case hardening

Machining

FORGING Metal is heated and hammered or squeezed into

shape

Produces an orientation of grain flow making the metal stronger

Drop forging- commonly used

CASTING Melting a metal by

heating and pouring into a mould

ROLLING AND DRAWING Rolled between rollers or

drawn through a hole in a hardened plate

Used to form bars and wires

Material gets plastically deformed and grains gets elongated

MILLING A machining process in

which material is fed into a machine with multiple cutting teeth

Removes materials at high rate

Good surface finish

COLD WORKING Finishing process after hot forging performed

below the recrystallization temperature

Gives smoother surface finish

Higher tensile strength

Uniform grain structure

Superior dimension control

ANNEALING Heating to half the melting point and then controlled

cooling.

Reverses effects of work hardening

Restores ductility and toughness to the metal

CASE HARDENING To make outer surface

harder than the inner core

Harder outer surface resists indentation while inner core is able to absorb more energy

MACHINING For geometric features like holes and grooves

It work hardens the surface of the material

Grain structure is unchanged

BROACHING Cut is made through

single passage of the broach

Geometry is inverse of the machine to be surfaced

Used for machining non circular holes, slots and recesses

SURFACE TREATMENTPOLISHING

Grinded & polished to a specified roughness & cleaned with special cleansing agents

Removes scratches

Reduces local stress

PASSIVATION Metal is immersed in strong

Nitric acid solution

Produces protective oxide layer

Enhances corrosion resistance

Stainless steel forms chromium oxide

Titanium alloys form dioxide

PASSIVATION

Damaged by cold working,scratching,mechanicalTrauma and metal fatigue

Re-passivation: Repair of the passivating layer in the presence of oxygen spontaneously

No passivating layer in Cobalt Chrome implants

NITRIDING Allowing the surface to

react with ammonia or potassium cyanate

Hardens the surface of titanium implants

STANDARDS FOR METALSASTM- American Society for Testing of

Materials : committee f-4

ANSI- American National Standards Institute

BSI- British Standards Institute

IOS- International Organization For Standardization

COMMON METALS PURE METALS:

Titanium

Tantalum

ALLOYS: Stainless steel

Cobalt Chrome

Titanium Alloy

SPECIFIC IMPLANT MATERIALS Three broad categories

Austentic stainless steels (iron, chromium, molybdenum)

Cobalt-chrome alloys and

Titanium alloys

SPECIFIC IMPLANT MATERIALS Those currently considered acceptable are materials

based on iron, cobalt, nickel, titanium, tantalum, zirconium, silver, gold, and the noble metals. Of these, tantalum and the noble metals (including gold and silver) do not possess adequate mechanical properties for structural implant purposes, while zirconium is too expensive.

STAINLESS STEEL Iron based alloy containing

chromium, nickel, molybdenum.

Strong

Cheap

Relatively biocompatible

Relatively ductile therefore easy to alter shape. Useful in contouring of plates and wires during operative procedures.

PRESENT STEEL ALLOY• 316L Alloy (ASTM) 62.5% Fe

17.6% Cr - Main Corrosion Resistance

14.5% Ni - Corrosion Resistance

2.8% Mo - Resists Chloride Corrosion

<0.03% C

Minor alloy additions: Mn ,Si

L=Low Carbon Content =No intergranular corrosion

STAINLESS STEEL

Chromium forms an oxide layer when dipped in nitric acid to reduce corrosion

Molybdenum decreases the slow passive dissolution of this chromium oxide layer by 1000 times

Molybdenum reduces pitting corrosion

Nickel: corrosion resistance and facilitates the production process

L-low Carbon content – low carbide formation

316 LVM: Vacuum melted form: resistant to fatigue failure

STAINLESS STEEL The chromium forms an oxide layer when dipped in nitric

acid to reduce corrosion

Molybdenum decreases the slow passive dissolution of this chromium oxide layer by 1000 times

Molybdenum reduces pitting corrosion

Nickel: corrosion resistance and facilitates the production process

L-low Carbon content – low carbide formation-

STAINLESS STEEL High Young’s modulus - 200 GPascals (10 that of

bone) so can lead to stress shielding of surrounding bone which can cause bone resorption.

Usually annealed or cold worked for increased strength. A range of strength and ductilities can be produced.

A wide range of properties exists depending on the heat treatment (annealing to obtain softer materials) or cold working (for greater strength and hardness).

Used in plates, screws, external fixators, I.M. nails.

STAINLESS STEEL

STAINLESS STEEL Completely non magnetic = MRI SAFE

May affect image quality

Majority: Cold worked for strength

Surface treatment: Electropolishing

STAINLESS STEEL

Advantages of SS

Higher Strength

Higher Stiffness

Higher Ductility

Cheaper

Relatively Biocompatible

COBALT CHROME Two forms F-75 and F-90

F-90 – freacture fixation implants( CoNiCrMo)

Co : 55-65 %

Cr : 19-21 %

W : 14-16%

Ni : 09-11%

C : .05 - .15%

F-75: femoral prosthesis(CoCrMo)

COBALT CHROME Basically two types :

CoCrMo alloy –casted

CoNiCrMo alloy- wrought by (hot) forging.

CoCrMo alloy used in dentistry and recently, in making artificial joints.

CoNiCrMo alloy used in the stems of prosthesis for heavily loaded joints

such as the knee and hip.

COBALT CHROME Young’s modulus higher than stainless steel

Stress shielding a theoretical risk.

Usually fixed with cement

Advantages Stronger (less ductile)

More Corrosion Resistant

Resistance to abrasion(resurfaceing implants)

Highly resistant to corrosion,especially crevice corrosion

Lesser degree of galvanic corrosion than in the iron-based alloys.

Quite resistant to fatigue and to cracking caused by corrosion

are not brittle, since they have a minimum of 8% elongation.

The superior fatigue and ultimate tensile strength of the wrought CoNiCrMo alloy: femoral prosthesis stem

Both the cast and wrought alloys have excellent corrosion resistance.

TITANIUM AND ITS ALLOYS Very reactive metal

Rapidly forms an oxide layer-physiologically inert

Two forms cpTi

Alloys

Young’s modulus approximately half that of stainless steel, therefore less risk of stress sheilding

Less prone to fatigue failure

cpTi (Commercially Pure Titanium)

Does not contain major alloying elements

Special anodized surface finish

Decreased tendency towards fibrous capsule formation

Increased resistance to infection

Increased thickness of protective oxide film

Specific colour depends on oxide film thickness

“NO PIGMENTS”

TITANIUM ALLOYSImportant alloy in use in orthopaedics is Titanium-Aluminum-Vanadium

TAV : Titanium Aluminum Vanadium

TAN : Titanium Aluminum Niobium

TiMo : Titanium Molybdenum

Ti6Al4V ELI(ASTM F-136): Al - stabilises alpha form

V - Beta form

ELI - Low interstitial grade

TITANIUM & ITS ALLOYS

Advantages Excellent resistance to corrosion

Excellent Biocompatibility

More MR scan compatible than other metals.

Disadvantages

Poorer wear characteristics than others

Brittle

TITANIUM AND ITS ALLOYS

TITANIUM VS STAINLESS STEEL SS can be produced with higher elastic modulus and

ductility

SS cheaper

Ti is corrosion resistant and free of potential toxic ions

Ti: no allergic reactions

Ni-Ti Alloys Nitinol: Shape memory alloy

Can undergo predictable shape changes with temp

Can return to original shape when temperature rises

Can be bent, compressed or deformed but regain shape on heating

NONMETALLIC MATERIALS Many nonmetallic materials have been advanced for

structural implantation

PMMA

amorphous glasses

crystalline ceramics

carbon composites and

polymeric materials.

PMMA Nonmetallic materials - Polymethyl methacrylate

Charnley in 1960 used it as a grouting agent

Acrylic cement

Often the term "bone cement" is applied to PMMA although it lacks the property

Filler material in total-hip insertion

More used in veterinary orthopedics

PMMA In situ mixing of powdered, prepolymerized

methylmethacrylate with liquid monomer is exothermic-temperatures as high as 1220C

release of the monomer, MMA, into the circulation Hypotension

hypoxemia

occasional fatalities

May induce hypersensitivity reactions (contact-type) some patients,

as well as in orthopaedic surgeons

(extremely lipophilic may diffuse through the gloves)

CORROSION

Gradual degradation of metals by electrochemical reaction

CORROSION STRESS CORROSION

GALVANIC CORROSION

CREVICE CORROSION

PITTING CORROSION

FRETTING CORROSION

INTERGRANULAR CORROSION

STRESS CORROSION Occurs in areas of high stress gradients

High stress gradients cause local areas of relatively high activity in metal

May be a presence of a crack due to stress

The corrosion encourages crack progression.

STRESS CORROSION Saline environment of

the body:stress corrosion can occur.

Stress corrosion combines the effects of the local growth of the crack resulting from cyclic loading with galvanic corrosion

STRESS CORROSION

GALVANIC CORROSION

Electrons flow from the more negative to the more positive material when immersed in a liquid conductor.

Material removed from the plate during galvanic corrosion.

Due to two different metals being used

e.g. stainless steel screws and titanium plate or an impurity in the metal.

GALVANIC CORROSION

CREVICE CORROSION

Results from small galvanic cells formed by impurities in the surface of the implant, causing crevices as the material corrodes.

Components have a relative movement against one another so that the passivating layer is removed

Occurs in fatigue cracks & other crevices where oxygen tension becomes low

Lack of oxygen also inhibits the repair of passivating layer. e.g. a screw head in a plate,

site where a modular head on a hip prosthesis fits on the neck.

CREVICE CORROSION

Corrosion initiated as fine cracks formed between granules of metals

INTERGRANULAR CORROSION

FRETTING CORROSION

Results from small movements between components of a device causing abrasive damage to the passivating layer

CORROSION CAN BE MINIMISED BY Choosing a corrosion resistant material

Treating the surface with a passivating layer prior to use

Not using combinations of metals in close proximity

Careful operating technique to reduce surface scratching

Using non modular implants.

MATERIAL PERFORMANCE & FAILURE

Three categories:

purely mechanical - result of direct overload including impact or fatigue

purely environmental - corrosion & tissue hypersensitivity -"sterile abscess.“

conjoint mechanical-environmental

MATERIAL PERFORMANCE & FAILURE

• Others

Tumor formation (approximately 30 reported cases in the canine of osteosarcoma formation in the vicinity of fracture-fixation devices)

Osteoporosis

RECENT ADVANCES TRIP steel (transformation induced plasticity), which

has a composition of 9% chromium, 8% nickel, 4% molybdenum, 0.3% carbon, and the balance iron

two-phase alloy system

After cold working demonstrates excellent ductility and strength

New nickel-free stainless steels have been recently developed primarily to address the issue of nickel sensitivity. These stainless steels also have superior mechanical properties and better corrosion resistance. The Ni-free compositions appear to possess an extraordinary combination of attributes for potential implant applications in the future

MP35N A multiphase alloy containing a nominal 35% nickel in

addition to cobalt, chromium, and molybdenum

Superior ductility & improved corrosion resistance

By appropriate work hardening and heat treatments

high yield strengths (300,000 psi)

while retaining 10% elongation

CERAMIC MATERIALS High compressive strength and biocompatibility

Poor ductility leading to brittility

Commercially, an ALUMINA total hip (alumina ball and socket with metal stem) has been introduced for human application

UHMWPE Ultra high molecular weight polyethylene (UHMWPE)

low coefficient of friction with metal - used as a bearing surface in several multicomponent total joint devices.

More recently this material has been reinforced with graphite to retard the inevitable in vivo creep and wear process

UHMWPE

OXINIUM It consists of a zirconium metal substrate that transitions

into a ceramic zirconium oxide outer surface.

The ceramic surface is extremely abrasion resistant.

Lower coefficient of friction against ultra-high molecular weight polyethylene.

Combines the abrasion resistance and low friction of a ceramic with the workability and toughness of a metal.

OXINIUM

TANTALUM Chemically stable and

biologically inert

Structure supports bone integration, bone remodeling, and vascularization

High coefficient of friction for enhanced stability

Low modulus of elasticity similar to cancellous bone for more normal physiological loading

BIOABSORBABLE POLYMERS Polylactic Acid

Polyglycolic Acid

Polydioxanone

Advantages Lower incidence of infection

Pediatric fractures

Limit stress shielding of bone

Gradual load transfer to healing tissue

Eliminate hardware removal

Radio-lucent

BIOABSORBABLE POLYMERS Disadvantages

Lower initial fixation strength

More creep and stress relaxation

Uses Lower end radius,hand

Ankle

Pins for children

Protective perforated membrane for bone grafts

Carriers of osteogenic substances without affecting bone healing and imaging

BIOABSORBABLE POLYMERS Complications

Sterile sinus tract infection

Osteolysis

Synovitis

Hypertrophic fibrous encapsulation

Under trial Poly vinylidine fluoride

Used in veterinary orthopedics

Clinically found to be sufficiently biocompatible not to elicit adverse local or systemic sequelae

METALLOSIS Involves deposition and build-up of metal debris in

the soft tissues of the body.

Metals abrade against one another.

Incidence of 5% of metal joint implant patients

The abrasion of metal components may cause metal ions to be solubilized.

Immune system identifies the metal ions as foreign bodies and inflames the area around the implant.

Symptoms

pain around the site of the implant

pseudotumors

a noticeable rash that indicates necrosis.

Can contribute to loosening the implant.

Dislocation of non-cemented implants.

Causes osteolysis.

More in short statured obese ladies

CONCLUSION The ultimate aim of orthopedic biomaterials -

structural integrity of the damaged bone

material properties

device design and Complex interplay

physiologic requirement

It is the surgeon's responsibility to understand the complex interplay and to minimize performance failure.

REFERENCES1. Elements of fracture fixation,Vol 2: A J Thakur

2. Rockwood and Green: Fracture Fixation in Adults

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