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

Unit 1: Orthopaedic implant materials Flashcards

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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
How is material stiffness defined under axial and bending loads?
Young's Modulus
26
How is the material stiffness of a material defined under shear loading (including torsion)?
Shear Modulus (G = shear stress/shear strain)
27
How is stiffness defined mathematically?
Force required to produce a unit deflection
28
How do Young's modulus and cross-sec area affect the stiffness of a structure?
As E and A increase the material becomes stiffer
29
How does length affect the stiffness of a structure?
As length increases stiffness decreases
30
How is the stiffness of two implants with the same length compared?
By their rigidity
31
How is axial rigidity calculated?
R = EA
32
How is bending rigidity calculated?
R = EI
33
How is the second moment of area calculated for a rectangular structure?
I = bd3 / 12
34
How is the second moment of area fro a circular structure calculated?
I = πd4 / 64
35
How is torsional rigidity calculated?
R - GJ | G = shear modulus, J = polar second moment of area
36
For circular sections how is J related to I?
J = 2I
37
In a load sharing region, how is the ratio of load taken by the bone to that taken by the stem calculated?
It is the ratio of their rigidities | Lb/Ls = Rb/Rs
38
In a load sharing region, how is the proportion of the total load taken by the bone calculated?
Ratio of the rigidity of the bone to the total rigidity of the section Lb/Lt = Lb/Rt = Rb/Rb + Rs
39
If the stem is less stiff how does this affect load transfer?
More load transferred proximally and less distally
40
What is the benefit of a less stiff stem?
Reduces stress shielding and bone resorption
41
What dictates how a bone is fixated?
Whether or not the implant is intended to be removed at a later date
42
What are the advantages of screws over nuts and bolts?
Requires access from one side of a bone only | Less trauma to tissues
43
What is interference fit?
Relies on tight contact between implant and bone - surface friction between the two materials prevents movement at the interface
44
What shape of stem is particularly useful in interference fit?
Tapered stem
45
What is the purpose of bone cement?
Acts as a filling material to fill gaps between a bone and implant so a perfect geometrical match is not required
46
Why would it be extremely difficult to apply an adhesive from a practical point of view?
Bones are wet and difficult to access for cleaning and preparation
47
What assumption does biological fixation work under?
That bone will grow into a porous coating, mesh or roughened area on the surface of an implant
48
What are the two most common surface coatings used in biological fixation?
Beads of the same material as the metallic implant or a ceramic
49
Beading of the surface for biological fixation is ost commonly used for which metal and why?
Titanium - exposing large surface area of metal increases corrosion (particularly crevice) and titanium is least corrosive and most biocompatible
50
What is the main mineral constituent of bone?
Hydroxyapatite
51
What is plasma spray coating?
A technique to deposit HAp directly on to the metal surface
52
What are the current uses of HAp coating?
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
What are the 3 important features required of a orthopaedic implant material?
High biocompatibility Suitable mechanical properties Ease of manufacture
54
What 2 factors are important to consider when evaluating biocompatibility?
The xtent to which body fluids and tissues affect a material and the extent to which a material adversely affects body tissues
55
Define corrosion
The progressive unwanted removal of a material by an electrochemical process
56
Describe galvanic corrosion
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
What comprises the electrodes & electrolyte in implants?
Electrode - metal or conductive material | Electrolyte - body fluids (contain salts)
58
How does corrosion affect implants?
Causes small areas of loss of material (often show up as small pits and craters) - stress concentrations that lead to fatigue failure
59
How can corrosion be minimised?
Is more severe between different metals Can occur in single metal component so important to reduce impurities Using an alloy
60
What are the only three alloys used in implants?
Stainless steel Cobalt chrome Titanium alloys
61
Why do metal alloys and titanium have good corrosion resistance?
Passivation of metal oxide forms on the surface of the material when it is exposed to a corrosive environment
62
What is fretting corrosion?
When abrasion of materials in contact removes the protective metal oxide layer allowing corrosion to occur
63
When does fretting corrosion tend to occur?
Between screws and plates | Interference fits
64
What is crevice corrosion?
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
Where is prone to crevice corrosion?
Edges of bone plates | Between screws and plates
66
What two methods can be used to improve corrosion resistance?
Nitric acid immersion | Titanium nitride coating
67
How does nitric acid immersion work?
Improves the natural passivation later (in stainless steel and CoCr is though to be related to increased amount of chromium)
68
Why is titanium nitride coating good?
Reduces release of harmful metallic substances in the alloys into the body fluids (in particular vanadium and aluminium from titanium alloys)
69
Where do the products of corrosion of implants appear?
In small quantities in the blood, urine, some tissues, storage organs (liver) and in the nails and hair
70
What are the 7 main biological reactions to implant materials?
``` 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
Why does a thin fibrous layer form between the body and implant and why is this bad?
If there is micromotion at the interface | Stops fixation so bone and implant can't be true composite structure
72
Why does infection occur in implants?
Ingress of bateria before or during surgery - implants tend to suppress body's defence mechanisms to infection
73
Why does inflammation in regions of metal corrosion occur?
Protective oxide layer is lost and small wear particles of the material react with body tissues
74
What percentage of patients may develop sensitivity to cobalt, chromium or nickel?
50%
75
What is the name of the most common stainless steel used for implants?
316L grade
76
Why does 316L stainless steel have a low carbon content?
Minimise sensitisation of tissues | More corrosion resistant
77
Which type of corrosion is stainless steel prone to?
Crevice corrosion
78
What implants is stainless steel most appropriate for?
Temporary implants such as fracture fixation
79
How is stainless steel for implants made? why?
Forged - energy involved increases yield stress it is less ductile than cast steel but 4 times as strong
80
How does the fatigue strength of stainless steel compare to chrome and titanium?
Lower
81
Which part of cobalt chrome provides corrosion resistance?
Chromium
82
What are the 3 main components of a cobalt chrome alloy?
Cobalt Chromium Molybedum
83
Comment on the strength of cobalt chrome
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
What is the advantage of cast CoCrMo?
Complex shapes can be cast more easily than forged
85
What are cast CoCrMo alloys useful for and why?
Bearing surfaces due to low coefficient of friction with polyethylene
86
What is anodising?
A process which increases the thickness of an anti-corrosive protective layer on a metal's surface (increases corrosion resistance)
87
How do the corrosion products of titanium compare to those from stainless steel and cobalt chrome alloys?
Less harmful to the body
88
How do the mechanical properties of titanium compare to stainless steel and cobalt chrome?
Titanium is less dense (lighter) and about half as stiff | Has higher fatigue strength than stainless steel
89
Why is titanium not suitable for bearings in joint replacements?
Low wear resistance
90
What are the properties of fibre reinforced polymers?
Very stiff, high strength but brittle fibres embedded in a much more flexible resin material
91
Why are fibre reinforced polymers good?
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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