This document discusses metals used in orthopaedics, including their properties, applications, advantages, and disadvantages. It describes common metals like stainless steel, cobalt alloys, and titanium alloys. Stainless steel is inexpensive but has corrosion over time. Cobalt alloys are biocompatible with high strength but expensive. Titanium alloys have excellent biocompatibility properties but lower strength. The document also covers corrosion, metal failure modes, and considerations for metal removal and mixing implants.

1. METALS IN ORTHOPAEDICS

2. APPLICATIONS
Load bearing material for # fixation
Joint replacement devices
Splints
Braces
Traction apparatus

3. REASONS
High elastic modullus
Ductility
Fabricatable
Form alloys
Good Resistance to internal & external enviroment

4. 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.

5. 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

6. 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

7. TOUGHNESS:
Ability to withstand suddenly applied forces
without fracture.
BRITTLENESS:
No evidence of plasticity prior to fracture.

8. 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.

9. 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.

10. 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.

11. COMMON METALS
Stainless steel or Iron based alloy
Cobalt-chromium alloy
Titanium based alloy
Nickel-titanium alloy

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

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

14. CHROMIUM [17-20%]
Increases the passivity.
Protective regenerating chromium oxide layer.
Protection against corrosion.

15. MOLYBENDUM[2-4%]
Protects against pitting corrosion
Counters the action of chloride ions & organic acids
in body fluids.
Increases the passivity by decreasing the rate of
dissolution of Cr oxide.

16. 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.

17. NICKEL [10-17%]
Keeps the austenitic structure of steel stable at room
temperature.
Corrosion resistance
Helps in production process.
Mn & Si [2.8%]
To control manufacturing process.

18. 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.

19. AISI 440B
Instrument steel
Martensite
No nickel
Extremely hard
Can break easily
Non-corrosion resistant.

20. ADVANATAGES
Good mechanical strength
Excellent ductility.
Common techniques of production.
Available in different strength.
Time-tested
Moderate price.

21. 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.

22. 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.

23. DRILL BIT STEEL
Extremely hard
Sharpened well
Not ductile break
Not corrosion resistant
If breaks contacts with implant  galvanic
corrosion.

24. COBALT BASED ALLOY
ASTM F-90: Cobalt-chromium-tungsten-nickel
alloy.
# fixation implants.
ASTM F-75: Femoral prosthesis
Vitallium
Longest and broadest history of use in arthroplasty.

25. Casting process overly large grain size,
inhomogeneties and porositystress risers fatigue
failure.
Modern tech: mold inocultion,forging, hot isostatic
pressing.

26. ADVANTAGES
Inert
Increased modulus of elasticity
Higher strength than steel.
Biocompatibility, satisfactory fatigue life and
toughness.
Wear resistant.

27. DISADVANTAGES:
Difficult to machine
Expensive
Low ductility [screw made of alloy bond well to bone
if tried to remove head tends to break].

28. 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

29. PROPERTIES
Al stabilizes alpha form
Vanadium stabilizes beta form.
Two phase alloy good strength.
EM1/2 that of S.S & Co
Lower stiffnessreduces stress shelding and cortical
osteoporosis.

30. Corrosion resistance:very dense and stable layer of
Tio2.
Ductility:considerably lower than S.S
In unstable fixation fretting and produce metal
debrisdiscolouration  harmless.

31. Ti alloys :not good bearing materials
Low wear resistance and high coefficient of
friction. Ti-Ti articulating surfaces not used.
New tech,nitriding and nitrogen ion
implantationincreases surface hardness and
wear resistance.
New alloys understudy, to decrease notch
sensitivity.

32. Comparison of S.S and Ti for # fixation
Higher elastic modulus
Higher ductility but
similar endurance limits
Machinability
cheaper
Corrosion
resistance
Lack of toxic ions
No allergic reaction
M.P close to bone
No 2nd
operation.

33. NITINOL
NICKEL-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.

34. •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.

35. TRIP Steels
Transformation induced
plasticityClass of steels which may be cold worked after heat
treatment.
Higher strength retaining ductility.
D.A: corrosion

36. Refractory metals
Tungston,tantalium,molybendum
High melting point.
corrosion resistance
Excellent mechanical properities
Very hardmachining difficulty.

37. 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

38. TYPES OF CORROSION
• UNIFORM ATTACK
• GALVANIC CORROSION
• FRETTING CORROSION
• CREVICE CORROSION
• PITTING CORROSION
• INTER GRANULAR CORROSION
• LEACHING
• STRESS-CORROSION

39. UNIFORM ATTACK
• Corrosion involves the surface
uniformly
• Each consecutive atoms forms a cell
• Occur when metal is immersed in
electrolytic solution.

40. 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.

41. • Rubbing of implants and instruments
• Cold welding-transfer of material
from screwdriver to head, drillbit to
plate.

42. 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.

43. 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

44. • 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

45. 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.

46. • on the underside of screw heads
• occurs infrequently on the neck or
the underside of the flange of
proximal femoral endo -prostheses

47. 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

48. 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.

49. 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

50. MEASURES TO PREVENT
CORROSION
• 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

51. • 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.

52. METAL FAILURE
BRITTLE FAILURE:
A Screw head made of material with poor
ductility may demonstrate failure when overloaded
in torque.

53. PLASTIC FAILURE
Implant bends permanently because of loading
beyond the yield strength of the material  causing
loss of surgical alignment.

54. 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.

55. •# 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.

56. 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.

57. 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.

58. 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.

59. 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

60. 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.

61. THANK YOU