This document provides an overview of implant materials, including a history of materials used, properties and testing of metals, and specifics on common implant materials. It discusses the properties of stainless steel, cobalt chrome, and titanium alloys - the three major categories of implant materials. Stainless steel is described as being strong, cheap, and relatively biocompatible but having a high modulus leading to stress shielding. Its chromium oxide layer provides corrosion resistance.

1. MAJ VIVEK MATHEW

2. OVERVIEW
 History
 Properties of metals
 Metal working methods
 Specific implant materials
 Corrosion
 Material associated complications
 Recent advances
 Conclusion

3. 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

4. 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.

5. PROPERTIES OF IMPLANTS
 Mechanical properties
 Composition
 Grain structure: finer
grain, stronger and
more ductile

6.  Non Mechanical properties
 Corrosion resistance
 Biochemical inertness

7. 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)

8. STRESS & STRAIN

10. 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

11.  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),

12. 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:

13. 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.

14. 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.

15. Material
Ultimate
Strength
Tensile (MPa)
Ultimate
Strength
Compressive
(MPa)
Yield
Strength
0.2% Offset
(MPa)
El
Mo
(M
Tendon 70 4
Cortical bone 100 175 80 15
Cancellous bone 2 3 1
Polyethylene 40 20 20 1
PTFE Teflon 25
Acrylic bone cement 40 80 2
Titanium (pure, cold worked) 500 400 10
Titanium (Al-4V) (alloy F 136) 900 800 10
Stainless steel (316 L) (annealed) >500 >200 20
Stainless steel (cold worked) >850 >700 20
Cobalt chrome (cast) >450 >50 20
Cobalt chrome (wrought, annealed) >300 >300 23
Super alloys (CoNiMo) 1800 1600 23

16. 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).

17. 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.

18. 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

19. 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

20. 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.

21. 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

22. METHODS OF METAL WORKING
 Forging
 Casting
 Rolling & drawing
 Milling
 Cold working
 Annealing
 Case hardening
 Machining

23. FORGING
 Metal is heated and hammered or squeezed into
shape
 Produces an orientation of grain flow making the
metal stronger
 Drop forging- commonly used

25. CASTING
 Melting a metal by
heating and pouring
into a mould

26. 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

27. MILLING
 A machining process in
which material is fed
into a machine with
multiple cutting teeth
 Removes materials at
high rate
 Good surface finish

28. 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

29. ANNEALING
 Heating to half the melting point and then controlled
cooling.
 Reverses effects of work hardening
 Restores ductility and toughness to the metal

30. 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

31. MACHINING
 For geometric features like holes and grooves
 It work hardens the surface of the material
 Grain structure is unchanged

32. 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

33. SURFACE TREATMENT
POLISHING
 Grinded & polished to a specified roughness &
cleaned with special cleansing agents
 Removes scratches
 Reduces local stress

34. 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

35. PASSIVATION
 Damaged by cold working,scratching,mechanical
Trauma and metal fatigue
 Re-passivation: Repair of the passivating layer in the
presence of oxygen spontaneously
 No passivating layer in Cobalt Chrome implants

36. NITRIDING
 Allowing the surface to
react with ammonia or
potassium cyanate
 Hardens the surface of
titanium implants

37. STANDARDS FOR METALS
 ASTM- American Society for Testing of
Materials : committee f-4
 ANSI- American National Standards Institute
 BSI- British Standards Institute
 IOS- International Organization For
Standardization

38. COMMON METALS
PURE METALS:
 Titanium
 Tantalum
ALLOYS:
 Stainless steel
 Cobalt Chrome
 Titanium Alloy

39. SPECIFIC IMPLANT MATERIALS
 Three broad categories
 Austentic stainless steels (iron, chromium,
molybdenum)
 Cobalt-chrome alloys and
 Titanium alloys

40. 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.

41. 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.

42. 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

43. 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

44. 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-

45. 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.

46. STAINLESS STEEL

47. STAINLESS STEEL
 Completely non magnetic = MRI SAFE
 May affect image quality
 Majority: Cold worked for strength
 Surface treatment: Electropolishing

48. STAINLESS STEEL
Advantages of SS
 Higher Strength
 Higher Stiffness
 Higher Ductility
 Cheaper
 Relatively Biocompatible

49. 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)

50. 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.

51. 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)

52.  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.

53.  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.

56. 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

57. 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”

58. TITANIUM ALLOYS
Important 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

59. TITANIUM & ITS ALLOYS
 Advantages
 Excellent resistance to corrosion
 Excellent Biocompatibility
 More MR scan compatible than other metals.
 Disadvantages
 Poorer wear characteristics than others
 Brittle

60. TITANIUM AND ITS ALLOYS

62. 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

63. 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

64. NONMETALLIC MATERIALS
 Many nonmetallic materials have been advanced for
structural implantation
 PMMA
 amorphous glasses
 crystalline ceramics
 carbon composites and
 polymeric materials.

65. 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

66. 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)

67. CORROSION
Gradual degradation of metals by
electrochemical reaction

68. CORROSION
 STRESS CORROSION
 GALVANIC CORROSION
 CREVICE CORROSION
 PITTING CORROSION
 FRETTING CORROSION
 INTERGRANULAR CORROSION

69. 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.

70. 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

71. STRESS CORROSION

72. 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.

73. GALVANIC CORROSION

74. 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.

75. CREVICE CORROSION

76. Corrosion initiated as fine cracks
formed between granules of
metals
INTERGRANULAR CORROSION

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

78. 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.

79. 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

80. MATERIAL PERFORMANCE &
FAILURE
• Others
 Tumor formation (approximately 30 reported cases in the
canine of osteosarcoma formation in the vicinity of fracture-
fixation devices)
 Osteoporosis

81. 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

82.  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

83. 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

84. 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

86. 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

87. UHMWPE

88. 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.

89. OXINIUM

90. 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

91. 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

93. 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

94. BIOABSORBABLE POLYMERS
 Complications
 Sterile sinus tract infection
 Osteolysis
 Synovitis
 Hypertrophic fibrous encapsulation

95. Under trial
 Poly vinylidine fluoride
 Used in veterinary orthopedics
 Clinically found to be sufficiently biocompatible not to
elicit adverse local or systemic sequelae

96. 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.

97.  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

99. 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.

100. REFERENCES
1. Elements of fracture fixation,Vol 2: A J Thakur
2. Rockwood and Green: Fracture Fixation in Adults

101. THANK YOU