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AY BMSc Implant Technology > Unit 1: Orthopaedic implant materials > Flashcards

Unit 1: Orthopaedic implant materials Flashcards

1
Q

What are 6 requirements of an implant?

A 
Biocompatibility 
Relieve pain and allow adequate movement
Adequate strength and lifespan
Cost effective manufacture 
Practicability of insertion 
Safety for the surgeon and the patient
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2
Q

How does the stiffness of HDP compare to bone?

A

Similar stiffness to that of cancellous bone

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3
Q

How does the stiffness of metal compare to bone?

A

Metals are stiffer

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4
Q

What is the main associated problem with the insertion of orthopaedic impants?

A

Infection

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5
Q

What structural factors are important in implant design? (5)

A 
Strength 
Stiffness 
Lubrication 
Wear 
Fatigue
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6
Q

What is the structure of the end regions of bone?

A

Shaped to accomodate the joint (wider at the end)

Contains cancellous bone

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7
Q

Why is is desirable for the end regions of bone to contain cancellous bone?

A

Shock absorbing properties (more porous and less stiff)

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8
Q

Describe the structure of cancellous bone in the knee joint

A

Trabeculae are alligned along direction of greatest stress
In femur - horizontal to stop lateral displacement
In tibia - vertical to resist compressive loads

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9
Q

Why is the region directly beneath the articular surfaces more dense than the cancellous bone below it?

A

To provide a fairly rigid underlying surface for the joint to bear on without causing excessive deformation of the bearing surfaces

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10
Q

What is the structure in the shafts of bones?

A

Contain dense compact bone (more rigid than cancellous so provides resistance under bending and torsional loads)

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11
Q

How is the stiffness of a material described?

A

By its Young’s Modulus (stress/strain)

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12
Q

Most biological materials are isotropic. What does this mean?

A

Their mechanical properties are the same no matter which direction they are loaded

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13
Q

Bone is anisotropic. What does this mean?

A

It’s Young’s modulus depends on the direction in which it is being loaded

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14
Q

In which direction is cortical bone stiffest and strongest?

A

When loaded longituidinally

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15
Q

How does the strength of metaphyseal bone compare to that of diaphyseal bone?

A

Metaphyseal bone (near the ends) is only about half as strong as diaphyseal bone

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16
Q

Describe how bone is viscoelastic

A

The stiffness of bone changes according to the rate at which it is loaded.
The faster it is loaded the stiffer it becomes.

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17
Q

Quantify the difference in the ultimate stress of cortical bone under different types of loading

A

Cortical bone is twice as strong under tensile and three times as strong under compressive than shear loading

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18
Q

How does the varying ultimate stress of cortical bone affect implant design?

A

Need to find ways to load bone under compression and avoid shear stresses especially but also tensile stresses as much as possible

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19
Q

Why might designing a mesh like material to act like cancellous bone not be desirable in an implant?

A

Increased risk of infection due to large surface area

Also this type of structure might not be firm enough for attaching or bonding an artificial joint

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20
Q

What is stress shielding?

A

When reduction in loading and stressing of a bone, due to an implant, leads to bone resorption

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21
Q

What is load transfer and when does it occur?

A

Load transfer where part of the applied load is transferred between bone and implant. It happens at specific regions depending on the implant

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22
Q

What causes interface stresses?

A

Movement at the interface when two materials are not bonded or if a bond comes loose

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23
Q

What can induce stress concentrations?

A

Sharp corners
Notches
Holes

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24
Q

What affects how loads are shared in the load sharing region?

A

Relative stiffness of the two components

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25
Q

How is material stiffness defined under axial and bending loads?

A

Young’s Modulus

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26
Q

How is the material stiffness of a material defined under shear loading (including torsion)?

A

Shear Modulus (G = shear stress/shear strain)

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27
Q

How is stiffness defined mathematically?

A

Force required to produce a unit deflection

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28
Q

How do Young’s modulus and cross-sec area affect the stiffness of a structure?

A

As E and A increase the material becomes stiffer

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29
Q

How does length affect the stiffness of a structure?

A

As length increases stiffness decreases

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30
Q

How is the stiffness of two implants with the same length compared?

A

By their rigidity

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31
Q

How is axial rigidity calculated?

A

R = EA

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32
Q

How is bending rigidity calculated?

A

R = EI

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33
Q

How is the second moment of area calculated for a rectangular structure?

A

I = bd3 / 12

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34
Q

How is the second moment of area fro a circular structure calculated?

A

I = πd4 / 64

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35
Q

How is torsional rigidity calculated?

A

R - GJ

G = shear modulus, J = polar second moment of area

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36
Q

For circular sections how is J related to I?

A

J = 2I

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37
Q

In a load sharing region, how is the ratio of load taken by the bone to that taken by the stem calculated?

A

It is the ratio of their rigidities

Lb/Ls = Rb/Rs

38
Q

In a load sharing region, how is the proportion of the total load taken by the bone calculated?

A

Ratio of the rigidity of the bone to the total rigidity of the section
Lb/Lt = Lb/Rt = Rb/Rb + Rs

39
Q

If the stem is less stiff how does this affect load transfer?

A

More load transferred proximally and less distally

40
Q

What is the benefit of a less stiff stem?

A

Reduces stress shielding and bone resorption

41
Q

What dictates how a bone is fixated?

A

Whether or not the implant is intended to be removed at a later date

42
Q

What are the advantages of screws over nuts and bolts?

A

Requires access from one side of a bone only

Less trauma to tissues

43
Q

What is interference fit?

A

Relies on tight contact between implant and bone - surface friction between the two materials prevents movement at the interface

44
Q

What shape of stem is particularly useful in interference fit?

A

Tapered stem

45
Q

What is the purpose of bone cement?

A

Acts as a filling material to fill gaps between a bone and implant so a perfect geometrical match is not required

46
Q

Why would it be extremely difficult to apply an adhesive from a practical point of view?

A

Bones are wet and difficult to access for cleaning and preparation

47
Q

What assumption does biological fixation work under?

A

That bone will grow into a porous coating, mesh or roughened area on the surface of an implant

48
Q

What are the two most common surface coatings used in biological fixation?

A

Beads of the same material as the metallic implant or a ceramic

49
Q

Beading of the surface for biological fixation is ost commonly used for which metal and why?

A

Titanium - exposing large surface area of metal increases corrosion (particularly crevice) and titanium is least corrosive and most biocompatible

50
Q

What is the main mineral constituent of bone?

A

Hydroxyapatite

51
Q

What is plasma spray coating?

A

A technique to deposit HAp directly on to the metal surface

52
Q

What are the current uses of HAp coating?

A

Good short term bonding but loosens after a year or two

Can be applied to orous metal coating which may produce better long term results

53
Q

What are the 3 important features required of a orthopaedic implant material?

A

High biocompatibility
Suitable mechanical properties
Ease of manufacture

54
Q

What 2 factors are important to consider when evaluating biocompatibility?

A

The xtent to which body fluids and tissues affect a material and the extent to which a material adversely affects body tissues

55
Q

Define corrosion

A

The progressive unwanted removal of a material by an electrochemical process

56
Q

Describe galvanic corrosion

A

2 electrodes immersed in an electrolyte - current can flow from one electrode to the other allowing a chemical reaction between the electrodes and electrolyte

57
Q

What comprises the electrodes & electrolyte in implants?

A

Electrode - metal or conductive material

Electrolyte - body fluids (contain salts)

58
Q

How does corrosion affect implants?

A

Causes small areas of loss of material (often show up as small pits and craters) - stress concentrations that lead to fatigue failure

59
Q

How can corrosion be minimised?

A

Is more severe between different metals
Can occur in single metal component so important to reduce impurities

Using an alloy

60
Q

What are the only three alloys used in implants?

A

Stainless steel
Cobalt chrome
Titanium alloys

61
Q

Why do metal alloys and titanium have good corrosion resistance?

A

Passivation of metal oxide forms on the surface of the material when it is exposed to a corrosive environment

62
Q

What is fretting corrosion?

A

When abrasion of materials in contact removes the protective metal oxide layer allowing corrosion to occur

63
Q

When does fretting corrosion tend to occur?

A

Between screws and plates

Interference fits

64
Q

What is crevice corrosion?

A

Occurs in crevices between implants where body fluid can become trapped and lose its normal supply of dissolved oxygen - high conc acids form which corrodes metals

65
Q

Where is prone to crevice corrosion?

A

Edges of bone plates

Between screws and plates

66
Q

What two methods can be used to improve corrosion resistance?

A

Nitric acid immersion

Titanium nitride coating

67
Q

How does nitric acid immersion work?

A

Improves the natural passivation later (in stainless steel and CoCr is though to be related to increased amount of chromium)

68
Q

Why is titanium nitride coating good?

A

Reduces release of harmful metallic substances in the alloys into the body fluids (in particular vanadium and aluminium from titanium alloys)

69
Q

Where do the products of corrosion of implants appear?

A

In small quantities in the blood, urine, some tissues, storage organs (liver) and in the nails and hair

70
Q

What are the 7 main biological reactions to implant materials?

A 
Growth of fibrous layer 
Local infection 
Body sensitisation to metals 
Inflammation in regions of corrosion 
Tissue necrosis from bone cement 
Immune reaction to wear particles 
Tumours
71
Q

Why does a thin fibrous layer form between the body and implant and why is this bad?

A

If there is micromotion at the interface

Stops fixation so bone and implant can’t be true composite structure

72
Q

Why does infection occur in implants?

A

Ingress of bateria before or during surgery - implants tend to suppress body’s defence mechanisms to infection

73
Q

Why does inflammation in regions of metal corrosion occur?

A

Protective oxide layer is lost and small wear particles of the material react with body tissues

74
Q

What percentage of patients may develop sensitivity to cobalt, chromium or nickel?

A

50%

75
Q

What is the name of the most common stainless steel used for implants?

A

316L grade

76
Q

Why does 316L stainless steel have a low carbon content?

A

Minimise sensitisation of tissues

More corrosion resistant

77
Q

Which type of corrosion is stainless steel prone to?

A

Crevice corrosion

78
Q

What implants is stainless steel most appropriate for?

A

Temporary implants such as fracture fixation

79
Q

How is stainless steel for implants made? why?

A

Forged - energy involved increases yield stress it is less ductile than cast steel but 4 times as strong

80
Q

How does the fatigue strength of stainless steel compare to chrome and titanium?

A

Lower

81
Q

Which part of cobalt chrome provides corrosion resistance?

A

Chromium

82
Q

What are the 3 main components of a cobalt chrome alloy?

A

Cobalt
Chromium
Molybedum

83
Q

Comment on the strength of cobalt chrome

A

Cast CoCr is not as strong as stainless steel but stiffness is similar
Used in joints where replacement part is big enough to have sufficient strength

84
Q

What is the advantage of cast CoCrMo?

A

Complex shapes can be cast more easily than forged

85
Q

What are cast CoCrMo alloys useful for and why?

A

Bearing surfaces due to low coefficient of friction with polyethylene

86
Q

What is anodising?

A

A process which increases the thickness of an anti-corrosive protective layer on a metal’s surface (increases corrosion resistance)

87
Q

How do the corrosion products of titanium compare to those from stainless steel and cobalt chrome alloys?

A

Less harmful to the body

88
Q

How do the mechanical properties of titanium compare to stainless steel and cobalt chrome?

A

Titanium is less dense (lighter) and about half as stiff

Has higher fatigue strength than stainless steel

89
Q

Why is titanium not suitable for bearings in joint replacements?

A

Low wear resistance

90
Q

What are the properties of fibre reinforced polymers?

A

Very stiff, high strength but brittle fibres embedded in a much more flexible resin material

91
Q

Why are fibre reinforced polymers good?

A

High strength properties Stiffness can be selected
Mechanically more compatible with bone
Superior fatigue properties to many metals

AY BMSc Implant Technology (13 decks)

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