metals in orthopaedics
TRANSCRIPT
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.
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.