METALS IN ORTHOPAEDICS

APPLICATIONS

Load bearing material for # fixationJoint replacement devicesSplintsBracesTraction apparatus

REASONS

High elastic modullusDuctilityFabricatableForm alloysGood Resistance to internal & external

enviroment

PROPERTIES

STRENGTH: The ability of a material to

resist an applied force without rupture.ELASTICITY: Ability of a material to recover

its original shape after deformation.

STIFFNESS: Resistance of a material to

deformation.PLASTICITY: Ability of a material to be

formed in to a new shape without any fracture and retain that shape after load removal

DUCTILITY: Ability of a material to be stretched

without fracture. Ability to absorb relatively large amount of plastic deformation before failing.

Provides safety factor, opportunity to detect overloaded implants by X rays

TOUGHNESS: Ability to withstand suddenly applied

forces without fracture.BRITTLENESS: No evidence of plasticity prior to

fracture.

MODULUS OF ELASTICITY

The slope of the stress-strain curve in the elastic region.

Dividing stress applied to a material by the resulting strain.

Stepper the curve, higher MOE-stiffer the material.

Young’s modulus.

ALLOYS

Material composed of two or more elements, one of which is a metal

Alloys of same metal with different composition will differ in physical, mechanical and chemical properties.

MECHANICAL PROPERTY

Depends partly on its composition and partly on its grain structure.

Metals with Finer grain is both stronger and more ductile.

G.S is affected by method of fabrication of the metal in to its finished shape.

COMMON METALS

Stainless steel or Iron based alloyCobalt-chromium alloyTitanium based alloyNickel-titanium alloy

STAINLESS STEEL

First modern alloy used.Iron based alloy. 60 % ironASTM F-55,-56[grades 316 & 316 L]Contains chromium,nickel,molybendum carbon,magnesium.Austenitic – classified metallurgically

Because of their microcrystalline structure.Forged stainless steel.[ASTM F-621]Cast stainless steel.[ASTM F-745]Annealed stainless steel.Non-magnetic.

CHROMIUM [17-20%]

Increases the passivity.Protective regenerating chromium oxide

layer.Protection against corrosion.

MOLYBENDUM[2-4%]

Protects against pitting corrosionCounters the action of chloride ions &

organic acids in body fluids.Increases the passivity by decreasing the rate

of dissolution of Cr oxide.

CARB0N: [0.03%]

Increases the strength.Decreases the corrosion resistance.Chromium carbide precipitate –increases the

corrosion,degrade the mechanical properities.

Mixing some Ti or niobium,reduces carbide formation.

NICKEL [10-17%]

Keeps the austenitic structure of steel stable at room temperature.

Corrosion resistanceHelps in production process. Mn & Si [2.8%]To control manufacturing process.

AISI 316L [ASTM F-56]

Implant steel.AISI 316LVM – produced by vacuum

melting, to decrease the fatigue failure. cleaner metal.[ASTM F-138].

AISI: American iron and steel institute.ASTM: American society for testing and

materials.

AISI 440B

Instrument steelMartensiteNo nickelExtremely hardCan break easilyNon-corrosion resistant.

ADVANATAGES

Good mechanical strengthExcellent ductility.Common techniques of production.Available in different strength.Time-testedModerate price.

DISADVANATAGES

Slow but Finite corrosion rate.Long term effects of nickel.Inferior to cobalt and Ti alloys in terms of

corrosion resistance, biocompatibility and fatigue failure.

No method to apply porous surface.

USES

Short term implantation in the body as in fracture fixation.

THR Implants in elderly Pts in whom physical demands are low and cost is a major issue.

DRILL BIT STEEL

Extremely hardSharpened wellNot ductile breakNot corrosion resistant If breaks contacts with implant galvanic

corrosion.

COBALT BASED ALLOY

ASTM F-90: Cobalt-chromium-tungsten-nickel alloy.

# fixation implants.ASTM F-75: Femoral prosthesisVitalliumLongest and broadest history of use in

arthroplasty.

Casting process overly large grain size, inhomogeneties and porositystress risers fatigue failure.

Modern tech: mold inocultion,forging, hot isostatic pressing.

ADVANTAGES

InertIncreased modulus of elasticityHigher strength than steel.Biocompatibility, satisfactory fatigue life and

toughness.Wear resistant.

DISADVANTAGES:

Difficult to machineExpensiveLow ductility [screw made of alloy bond well

to bone if tried to remove head tends to break].

TITANIUM BASED ALLOY

Titanium-aluminum-vanadium Ti6Al4V widely used.

Impurities O 2,H 2,N 2 Brittle.ELI(extra low interstitial): limits O2 conc to

low level improved mechanical properities .Ti6Al4V ELI:used for making implants

PROPERTIES

Al stabilizes alpha formVanadium stabilizes beta form.Two phase alloy good strength.EM1/2 that of S.S & CoLower stiffnessreduces stress shelding and

cortical osteoporosis.

Corrosion resistance:very dense and stable layer of Tio2.

Ductility:considerably lower than S.S In unstable fixation fretting and produce

metal debrisdiscolouration harmless.

Ti alloys :not good bearing materialsLow wear resistance and high coefficient of

friction. Ti-Ti articulating surfaces not used.New tech,nitriding and nitrogen ion

implantationincreases surface hardness and wear resistance.

New alloys understudy, to decrease notch sensitivity.

Comparison of S.S and Ti for # fixation

Higher elastic modulus

Higher ductility but similar endurance limits

Machinabilitycheaper

Corrosion resistanceLack of toxic ionsNo allergic reactionM.P close to boneNo 2nd operation.

NITINOLNICKEL-TITANIUM ALLOY

SMA [shape memory alloy]Relative amounts of Ni & Ti varied by few %

in order to control the phase change responsible for smart behavior

NixTi1-x, x % of Ni in alloy.Shape changed at low tempeature,but heated to achieve original shape.

•Ts- shape transition temperature.plastically deformed below Ts.

USES: difficult # fixation Compressive staples for scaphoid &

fibula, clamp on bone plates, long bone fixator and patella fixator.

TRIP SteelsTransformation induced

plasticityClass of steels which may be cold worked after heat treatment.

Higher strength retaining ductility.D.A: corrosion

Refractory metals

Tungston,tantalium,molybendumHigh melting point.corrosion resistanceExcellent mechanical properitiesVery hardmachining difficulty.

CORROSION – Clinical significance

• Limit fatigue life of implant• Adverse biological reaction to

products of corrosion• Local pain and swelling• Peri prosthetic bone loss• Excretion of excess metal ions• Toxicity of the metal

TYPES OF CORROSION

• UNIFORM ATTACK• GALVANIC CORROSION• FRETTING CORROSION• CREVICE CORROSION• PITTING CORROSION• INTER GRANULAR CORROSION• LEACHING• STRESS-CORROSION

UNIFORM ATTACK

• Corrosion involves the surface uniformly

• Each consecutive atoms forms a cell

• Occur when metal is immersed in electrolytic solution.

GALVANIC CORROSION

• Inappropriate combination of metals may result in accidental creation of battery G.C,when the material is placed in body fluid.

• Metal of higher potential,cathode cannot corrode and metal of lower potential becomes anode,corrode.

• Rubbing of implants and instruments

• Cold welding-transfer of material from screwdriver to head, drillbit to plate.

FRETTING CORROSION

• Corrosion occurring at contact areas between materials under load subjected to vibration and slip

• Repeated oscillatory motion • screw assemblies where the heads

rubbed on the plate and where the nuts and washers were in contact.

• This is due to disruption of the passivation layer.

CREVICE CORROSION

• This is a form of local corrosion due to differences in oxygen tension or concentration of electrolytes or changes in pH in a confined space, such as in the crevices between a screw and a plate

• 16 to 35% of modular total hip implants demonstrated moderate – to severe corrosion in the conical head - neck taper connections

• corrosion at the junction between screw head and the plate in 50-75% of all devices

• Other typical crevices are scratches on the surface of an implant, the interface between bone and an implant, the cement - metal interface, and any other sharp interface likely to be depleted of oxygen relative to another oxygenated area

PITTING CORROSION

• form of localized, symmetric corrosion in which pits form on the metal surface.

• Start as defect in the passive layer.• Proceeds into the metal,setting up

self-accelerating concentration gradient.

• on the underside of screw heads • occurs infrequently on the neck or

the underside of the flange of proximal femoral endo -prostheses

INTERGRANULAR CORROSION

• A form of galvanic corrosion due to impurities and inclusions in an alloy

• Stainless steels, if improperly heat treated after fabrication, may corrode by this mechanism owing to a relative depletion of chromium from the regions near the grain boundaries. This phenomenon is called sensitisation

LEACHING

• This form of corrosion results from chemical differences not within grain boundaries but within the grains themselves

• The presence of more than one phase in the alloy (multiphasic), e.g., 35% Ni containing cobalt-base alloy.

STRESS - CORROSION CRACKING

• Involves both mechanical and chemical effects

• It is a phenomenon in which a bend metals in a certain environment, especially those rich in chlorides, is subjected to stress and fails at a much lower level of stress than usual as a result of corrosion

MEASURES TO PREVENTCORROSION

• Manufacturing Process Surface treatment– Nitriding can reduce the

magnitude of fretting corrosion of Ti-6AI-4V devices.

– Implantation of ions to harden the surface. This can improve the resistance to wear - accelerated corrosion phenomenon

• Passivation to thicken the protective oxide layer.

• Stainless steel forms a chromium oxide.Ti forms Tio2 layer.

• Involves immersion in strong nitric acid solution for specific time.

Polishing to remove scratches,that could act as stress raiser.

METAL FAILURE

BRITTLE FAILURE: A Screw head made of material with

poor ductility may demonstrate failure when overloaded in torque.

PLASTIC FAILURE

Implant bends permanently because of loading beyond the yield strength of the material causing loss of surgical alignment.

FATIGUE FAILURE

All metallic objects are subjected to F.F under cyclic loading ,hastened by body fluid.[wt bearing lower limb]

Originates in small flaws within material[grain boundaries,voids] or mechanical defects on the surface of the material.

Extrinsic defects[scratches,bends] decrease the fatigue life by acting as stress raisers.

Inserting a metallic implant in to a situation where load is greater than endurance limit triggers a competition between the completion of implants designed functional task and its fatigue failure.

•# fixation devices are designed to share the load with # bone

Healed # bones unloads the # fixation device and prolongs F.life

F.F occurs when loads are excessive, [comminuted # ] and period of load bearing is longer.

F.life is important in delayed union and non-union.

METAL REMOVAL

Both advantage and disadvantage.Major drawback: High cost Risk of 2nd surgery[wound complications,N.V

injury, anaesthesia.] limiting physical activity. Implant removal shouldnot be done for avoiding air

travelling concerns.

FACTORS FAVOURING METAL REMOVAL

Risk of peri-implant #Risk of sensitivity / allergy for Ni & Cr ions

[M.sensitivity in gen public & # surgery is 10-15%].

Carcinogenic risk [sarcoma].Pain relief.

Practical consideration:

Caution in attributing persisting pain to retained implants & no Pt should be guaranteed complete pain relief.

Explaining the Pt about possible risks of implant removal.

MIXING OF IMPLANTS

Unsound practice.High risk of corrosion.Slight variation exists even in materials of

same specification.Different working methods used by different

manufactures difference in the mechanical properties of metal

PRACTICAL CONSIDERATION

Use of implants and instrumentation of different designs lead to jamming, broken drills & taps,loose fits,gaps.

No manufacturer will take responsibility for implant failure.

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