PROPERTIESSTRENGTH:The ability of a material to resist anapplied force without rupture.ELASTICITY:Ability of a material to recover itsoriginal shape after deformation.
STIFFNESS:Resistance of a material todeformation.PLASTICITY:Ability of a material to be formed into a new shape without any fracture and retainthat shape after load removal
DUCTILITY:Ability of a material to be stretched withoutfracture. Ability to absorb relatively large amountof plastic deformation before failing.Provides safety factor, opportunity to detectoverloaded implants by X rays
TOUGHNESS:Ability to withstand suddenly applied forceswithout fracture.BRITTLENESS:No evidence of plasticity prior to fracture.
MODULUS OF ELASTICITYThe slope of the stress-strain curve in the elasticregion.Dividing stress applied to a material by the resultingstrain.Stepper the curve, higher MOE-stiffer the material.Young’s modulus.
ALLOYSMaterial composed of two or more elements, one ofwhich is a metalAlloys of same metal with different composition willdiffer in physical, mechanical and chemicalproperties.
MECHANICAL PROPERTYDepends partly on its composition and partly on itsgrain structure.Metals with Finer grain is both stronger and moreductile.G.S is affected by method of fabrication of the metalin to its finished shape.
COMMON METALSStainless steel or Iron based alloyCobalt-chromium alloyTitanium based alloyNickel-titanium alloy
STAINLESS STEELFirst modern alloy used.Iron based alloy. 60 % ironASTM F-55,-56[grades 316 & 316 L]Contains chromium,nickel,molybendumcarbon,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 acidsin body fluids.Increases the passivity by decreasing the rate ofdissolution of Cr oxide.
CARB0N: [0.03%]Increases the strength.Decreases the corrosion resistance.Chromium carbide precipitate –increases thecorrosion,degrade the mechanical properities.Mixing some Ti or niobium,reduces carbideformation.
NICKEL [10-17%]Keeps the austenitic structure of steel stable at roomtemperature.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, todecrease the fatigue failure. cleaner metal.[ASTMF-138].AISI: American iron and steel institute.ASTM: American society for testing and materials.
ADVANATAGESGood mechanical strengthExcellent ductility.Common techniques of production.Available in different strength.Time-testedModerate price.
DISADVANATAGESSlow but Finite corrosion rate.Long term effects of nickel.Inferior to cobalt and Ti alloys in terms of corrosionresistance, biocompatibility and fatigue failure.No method to apply porous surface.
USESShort term implantation in the body as in fracturefixation.THR Implants in elderly Pts in whom physicaldemands are low and cost is a major issue.
DRILL BIT STEELExtremely hardSharpened wellNot ductile breakNot corrosion resistantIf breaks contacts with implant galvaniccorrosion.
COBALT BASED ALLOYASTM F-90: Cobalt-chromium-tungsten-nickelalloy.# fixation implants.ASTM F-75: Femoral prosthesisVitalliumLongest and broadest history of use in arthroplasty.
Casting process overly large grain size,inhomogeneties and porositystress risers fatiguefailure.Modern tech: mold inocultion,forging, hot isostaticpressing.
ADVANTAGESInertIncreased modulus of elasticityHigher strength than steel.Biocompatibility, satisfactory fatigue life andtoughness.Wear resistant.
DISADVANTAGES:Difficult to machineExpensiveLow ductility [screw made of alloy bond well to boneif tried to remove head tends to break].
TITANIUM BASED ALLOYTitanium-aluminum-vanadium Ti6Al4V widely used.Impurities O 2,H 2,N 2 Brittle.ELI(extra low interstitial): limits O2 conc to low levelimproved mechanical properities .Ti6Al4V ELI:used for making implants
PROPERTIESAl stabilizes alpha formVanadium stabilizes beta form.Two phase alloy good strength.EM1/2 that of S.S & CoLower stiffnessreduces stress shelding and corticalosteoporosis.
Corrosion resistance:very dense and stable layer ofTio2.Ductility:considerably lower than S.SIn unstable fixation fretting and produce metaldebrisdiscolouration harmless.
Ti alloys :not good bearing materialsLow wear resistance and high coefficient offriction. Ti-Ti articulating surfaces not used.New tech,nitriding and nitrogen ionimplantationincreases surface hardness andwear resistance.New alloys understudy, to decrease notchsensitivity.
Comparison of S.S and Ti for # fixationHigher elastic modulusHigher ductility butsimilar endurance limitsMachinabilitycheaperCorrosionresistanceLack of toxic ionsNo allergic reactionM.P close to boneNo 2ndoperation.
NITINOLNICKEL-TITANIUM ALLOYSMA [shape memory alloy]Relative amounts of Ni & Ti varied by few % inorder to control the phase change responsible forsmart behaviorNixTi1-x, x % of Ni in alloy.Shape changed at low tempeature,butheated to achieve original shape.
•Ts- shape transition temperature.plastically deformed below Ts.USES:difficult # fixationCompressive staples for scaphoid & fibula,clamp on bone plates, long bone fixator and patellafixator.
TRIP SteelsTransformation inducedplasticityClass of steels which may be cold worked after heattreatment.Higher strength retaining ductility.D.A: corrosion
CORROSION – Clinical significance• Limit fatigue life of implant• Adverse biological reaction toproducts 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 surfaceuniformly• Each consecutive atoms forms a cell• Occur when metal is immersed inelectrolytic solution.
GALVANIC CORROSION• Inappropriate combination of metalsmay result in accidental creation ofbattery G.C,when the material isplaced in body fluid.• Metal of higher potential,cathodecannot corrode and metal of lowerpotential becomes anode,corrode.
• Rubbing of implants and instruments• Cold welding-transfer of materialfrom screwdriver to head, drillbit toplate.
FRETTING CORROSION• Corrosion occurring at contact areasbetween materials under load subjected tovibration and slip• Repeated oscillatory motion• screw assemblies where the heads rubbedon the plate and where the nuts andwashers were in contact.• This is due to disruption of the passivationlayer.
CREVICE CORROSION• This is a form of local corrosion dueto differences in oxygen tension orconcentration of electrolytes orchanges in pH in a confined space,such as in the crevices between ascrew and a plate
• 16 to 35% of modular total hip implantsdemonstrated moderate – to severe corrosion inthe conical head - neck taper connections• corrosion at the junction between screw head andthe plate in 50-75% of all devices• Other typical crevices are scratches on thesurface of an implant, the interface between boneand an implant, the cement - metal interface, andany other sharp interface likely to be depleted ofoxygen relative to another oxygenated area
PITTING CORROSION• form of localized, symmetriccorrosion in which pits form on themetal surface.• Start as defect in the passive layer.• Proceeds into the metal,setting upself-accelerating concentrationgradient.
• on the underside of screw heads• occurs infrequently on the neck orthe underside of the flange ofproximal femoral endo -prostheses
INTERGRANULAR CORROSION• A form of galvanic corrosion due toimpurities and inclusions in an alloy• Stainless steels, if improperly heattreated after fabrication, maycorrode by this mechanism owing to arelative depletion of chromium fromthe regions near the grain boundaries.This phenomenon is calledsensitisation
LEACHING• This form of corrosion results fromchemical differences not within grainboundaries but within the grainsthemselves• The presence of more than one phasein the alloy (multiphasic), e.g., 35% Nicontaining cobalt-base alloy.
STRESS - CORROSIONCRACKING• Involves both mechanical andchemical effects• It is a phenomenon in which a bendmetals in a certain environment,especially those rich in chlorides, issubjected to stress and fails at amuch lower level of stress than usualas a result of corrosion
MEASURES TO PREVENTCORROSION• Manufacturing ProcessSurface treatment– Nitriding can reduce the magnitude offretting corrosion of Ti-6AI-4Vdevices.– Implantation of ions to harden thesurface. This can improve theresistance to wear - acceleratedcorrosion phenomenon
• Passivation to thicken the protectiveoxide layer.• Stainless steel forms a chromium oxide.Tiforms Tio2 layer.• Involves immersion in strong nitric acidsolution for specific time.Polishing to remove scratches,that couldact as stress raiser.
METAL FAILUREBRITTLE FAILURE:A Screw head made of material with poorductility may demonstrate failure when overloadedin torque.
PLASTIC FAILUREImplant bends permanently because of loadingbeyond the yield strength of the material causingloss of surgical alignment.
FATIGUE FAILURE All metallic objects are subjected to F.F under cyclic loading ,hastenedby body fluid.[wt bearing lower limb] Originates in small flaws within material[grain boundaries,voids] ormechanical defects on the surface of the material. Extrinsic defects[scratches,bends] decrease the fatigue life by acting asstress raisers. Inserting a metallic implant in to a situation where load is greater thanendurance limit triggers a competition between the completion ofimplants designed functional task and its fatigue failure.
•# fixation devices are designed to share theload with # boneHealed # bones unloads the # fixation device andprolongs F.lifeF.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 REMOVALBoth advantage and disadvantage.Major drawback:High costRisk of 2ndsurgery[wound complications,N.V injury,anaesthesia.]limiting physical activity.Implant removal shouldnot be done for avoiding airtravelling concerns.
FACTORS FAVOURING METAL REMOVALRisk 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 retainedimplants & no Pt should be guaranteed completepain relief.Explaining the Pt about possible risks of implantremoval.
MIXING OF IMPLANTSUnsound practice.High risk of corrosion.Slight variation exists even in materials of samespecification.Different working methods used by differentmanufactures difference in the mechanicalproperties of metal
PRACTICAL CONSIDERATIONUse of implants and instrumentation of differentdesigns lead to jamming, broken drills & taps,loosefits,gaps.No manufacturer will take responsibility for implantfailure.