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Ortho Implant technology > Unit 1: Mechanics and Materials > Flashcards

Unit 1: Mechanics and Materials Flashcards

1
Q

List five criteria for a successful orthopaedic implant

A

be tolerated by the human body with no short term and little long term risk of adverse effects

relieve any pain and enable the patient to achieve sufficient mobility to perform everyday activities

function without failure until it is no longer required

be designed to be implanted successfully by a surgeon of average ability

be of acceptable cost to the purchaser

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2
Q

What are the two main structural requirements for an orthopaedic implant?

A

strength and stability

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3
Q

What are the three main factors affecting implant design?

A

structure
kinematics
biocompatibility

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4
Q

What are the structural factors that must be considered in the design of all orthopaedic implants?

A 
strength
stiffness
lubrication
wear
fatigue
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5
Q

What are the kinematic factors that must be considered in the design of all orthopaedic implants ?

A

range of motion must allow for activities of daily living but direction and pattern must be controlled in order to ensure its stable

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6
Q

What are the important requirements of an orthopaedic implant that are essential for biocompatibility?

A

biological and functional integration

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7
Q

What is biological integration?

A

any harmful reaction between body tissues and a material must be below the accepted level

corrosion shouldnt cause failure

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8
Q

What is functional integration?

A

implant performance should not adversely effect other body parts

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9
Q

What does anisotropic mean?

A

it has different mechanical properties in different directions

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10
Q

What is a composite structure?

A

composed of more than one material

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11
Q

What are the main differences in structure between bone in a diaphysis and bone in the region of a joint?

A

Diaphyseal bone

  • compact bone
  • rigid
  • resists deformation under loading

Joint bone

  • cancellous
  • trabeculae align along directions of stress
  • less rigid
  • shock absorbing
  • wider at joints
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12
Q

Under which type of loading (compressive, tensile or shear) is bone strongest and weakest?

A

Bone is strongest under compressive loading and weakest under shear loading.

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13
Q

What does isotropic mean?

A

mechanical properties are the same no matter what direction

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14
Q

Give an example of an

a) anisotropic material?
b) isotropic material?

A

a) bone

b) most non-biological materials

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15
Q

In what ways is bone viscoelastic?

A

stiffness increases with increasing rate of loading

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16
Q

In what direction of loading is cortical bone stiffest and strongest?

A

longitudinal loading

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17
Q

What is the meaning of stress shielding and how can it adversely affect bone?

A

Stress shielding is the reduction in the load (and therefore stress) that would normally be taken by a bone, due to the presence of an implant. This occurs because some of the load is taken by the implant. Bone resorbs when understressed and this can lead to loosening of the implant.

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18
Q

What is the major role of all orthopaedic implants?

A

to provide structural support

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19
Q

What is load sharing?

A

the regions where the load is partly taken by the bone and partly taken by the implant are called regions of load sharing.

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20
Q

What is load transfer?

A

The regions where load is transferred from an implant to a bone (or from a bone to an implant) are called regions of load transfer

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21
Q

In a bone fixation plate how does load transfer and share between bone and plate?

A

above the fracture: screws fix plate to bone and load transfers from bone to plate

at fracture: broken bones supported by the plate so load is shared between plate and bone

below fracture: screws fix plate to bone and load transfers back to the bone

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22
Q

In a joint replacement stem how does load transfer and share between bone and stem?

A

the load transfers from the stem to the bone
there is a midpoint where stem and bone share this load
then load transfers more on to the bone

below the stem the bone takes all the load

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23
Q

Where does load transfer take place for a bone plate?

A

at bone screw regions

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24
Q

Where does load transfer take place for an intramedullary stem?

A

at end regions of the stem

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25
Q

In a composite material, where does load pass from one material to another?

A

at the interface between them

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26
Q

What can interface loading generate if

a) the materials are bonded ?
b) the materials are not bonded?

A

a) interface stresses

b) relative movement which can cause loosening

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27
Q

What is load transfer across 2 materials under compressive stress dependent on?

A

the stiffness aka young’s modulus

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28
Q

If two materials are subjected to a compressive strength and the youngs modulus of the top material is greater what happens to the materials?

A

bottom is more flexible so it expands laterally

even stress pattern at interface

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29
Q

If two materials are subjected to a compressive strength and the young’s modulus of the top material is greater

what is the stress at the interface

a) if they are bonded
b) if they are not + lubricated ?

A

a) even lateral stress pattern at interface but sheer stress generates
b) sliding occurs so no sheer stress

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30
Q

How does youngs modulus affect shear stress?

A

the bigger difference in E for the two materials the greater the shear stress

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31
Q

Are interface pressures higher when

a) top material has a greater young’s modulus than bottom?
b) bottom material has a greater young’s modulus than top?

A

B

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32
Q

How do we calculate stress?

A

force/area

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33
Q

What is shear stress?

A

slippage of surfaces or planes within a material

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34
Q

Why is there a shear stress at a bone-implant interface?

A

the bone and implant each have a different material stiffness (Young’s modulus) so they try to deform by different amounts under the action of a load. If joined together they cannot deform separately so a shear stress develops between them - along the line of the interface.

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35
Q

How can shear stress at bone-implant interface be calculated?

A

normally shear stress= force/area but in reality the load transfer occurs more at the end of the interface so values are higher than theoretical values

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36
Q

Give 3 reasons that stresses between two materials can be higher in reality than in calculation

A
  • nonuniform contact
  • nonuniform mechanical properties eg. stiffness and
    variations in surface contours
  • inc stress concentration due to sharp corners, notches
    and holes
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37
Q

How does length of interface alter shear stress in reality?

A

smaller area being shared, higher stress but closer to the expected value

38
Q

What is the approximate ratio of the material stiffness of stainless steel to that of cortical bone?

A

200/20 = 10

39
Q

What is material stiffness ?

A

Material stiffness is the same as Young’s modulus and is a constant value for a particular material irrespective of its shape.

40
Q

What is geometric stiffness?

A

Geometrical stiffness is associated with the stiffness of a particular structural component and depends on its cross sectional shape and its length.

41
Q

What are the geometrical factors that affect the axial stiffness of a structural component?

A

cross sectional area and length.

42
Q

what are the two components of structural stiffness?

A

material stiffness

geometrical stiffness

43
Q

in which type of loading do metals not fail?

A

compression

44
Q

Metals have high … stiffness but low …. stiffnes

A

structural is high

geometric is low

45
Q

What represents material stiffness under shear loading?

A

shear modulus (G) = shear stress/shear strain

46
Q

What term is used to compare the stiffness of two implants of the same length?

A

rigidity

47
Q

What is meant by rigidity?

A

the stiffness of the cross section of a material. It is similar to structural stiffness but ignores the length of the structure. It is a particularly useful quantity for calculating the loads taken by a bone and an implant in a load sharing region

48
Q

What are the two factors affecting the rigidity of a structure?

A

Material stiffness and a geometrical property of the cross section

49
Q

How do we calculate

a) axial rigidity?
b) bending rigidity?
c) torsional rigidity?

A

a) young’s modulus x cross sectional area
EA

b) young’s modulus x 2nd moment of area
EI

c) shear modulus x polar 2nd moment of area
GJ

50
Q

What is the polar second moment of area?

A

2I

51
Q

In a load sharing region the ratio of loads taken by bone and stem is what?

A

a ratio of their rigidities

52
Q

will a stiff stem give more or less stress shielding of bone in Region 1, than a less stiff stem?

A

The stiffer stem gives a greater degree of stress shielding in Region 1. This can be explained as follows.
In the load sharing region, Region 2, the stiffer implant takes more load than the less stiff one because it is more rigid relative to bone. This means that less load will be transferred from the stem to the bone in Region 1 for the stiff implant, i.e. the bone in Region 1 takes less load and is therefore more stress shielded.

53
Q

What is the proportion of the total load taken by the bone equal to?

A

the ratio of the rigidity of the bone to the total rigidity of the section

54
Q

Why are orthopaedic implants fixated to bone?

A

to prevent loosening and failure

55
Q

What are the main advantages of using screws instead of nuts and bolts in the fixation of implants to bone?

A

access from one side reduces tissue damage

project less

56
Q

What is an interference fit?

A

dimensions of inner component larger than those of outer component so it is pressed into the bone to prevent loosening

57
Q

Why are prostheses stems tapered?

A

cannot subside very far into the bone canal. Subsidence stops because the tapered stem forms a tighter fit in the bone canal as it sinks

58
Q

When is interference fit with tapered stem used?

A

cementless joint replacement

59
Q

What is the function of bone cement?

A

it fills the gaps between the bone and implant to reduce movement and loosening

60
Q

What are the two main methods of biological fixation?

A

Porous beads

ceramic coatings

61
Q

What are biomaterials?

A

non-biological materials used within the body to repair or replace body parts that have failed

62
Q

Name three important features required of an orthopaedic material

A

biocompatible
suitable structural properties
easy to manufacture

63
Q

What is biocompatibility?

A

the interaction between body tissue and implant

  • how body fluids affect the material
  • how the material adversely affects the body
64
Q

What is corrosion?

A

when 2 electrodes are immersed in an electrolyte fluid and a current flows between them, allowing a chemical reaction where there is progressive removal of material

65
Q

In an implant what is the

a) electrode?
b) electrolyte?

A

a) metal or carbon

b) body fluid inc. salt

66
Q

How does corrosion result in fatigue failure?

A

loss of material shows up as pits and craters that have high stress concentrations making them more likely to fatigue fail

67
Q

When are corrosive reactions most severe?

A

different metals

68
Q

Under what circumstances can an implant made of one alloy behave as two electrodes?

A

manufacturing problems

  • contains impurities
  • not mixed so non-homogenous regions
69
Q

What materials can reduce corrosion?

A

alloys eg stainless steel, cobalt chrome, titanium alloys

pure titanium

70
Q

What is fretting corrosion?

A

when abrasion of materials in contact removes protective metal oxide leading to surface damage and reduced fatigue life

71
Q

Give examples of when fretting corrosion may occur

A

between screws and plates
when interference fits used
in modular prostheses

72
Q

What is crevice corrosion?

A

when body fluid gets trapped in the crevices between implants it loses its dissolved oxygen and becomes a high concentration acid that corrodes the implant

73
Q

Give examples of where crevice corrosion occurs

A

edges of bone plates

between screws and plates

74
Q

Give two ways to improve corrosion resistance

A

nitric acid immersion

titanium nitride coating

75
Q

What are the adverse effects of implanted materials on the human body.

A

growth of a fibrous layer between the bone and the implant
local infection
body sensitisation to metals
tissue inflammation in regions of implant corrosion
tissue necrosis in the region of bone cement immunological reaction to wear particles
tumours

76
Q

Which metal implant materials are thought to have the best corrosion resistance?

A

titanium and its alloys

77
Q

What is the main element in stainless steel?

A

iron

78
Q

What is the most commonly used stainless steel? What is it used for?

A

316L grade

temporary implants eg. screws and plates

79
Q

What type of manufacturing process makes stainless steel stronger? What does this sacrifice?

A

forged

less ductle

80
Q

What negative quality does stainless steel have?

A

corrodes and cracks
crevice corrosion
local pitting corrosion
low fatigue strength

81
Q

Is cobalt chrome or stainless steel more corrosion resistant?

A

cobalt chrome

82
Q

Is cobalt chrome or stainless steel stronger?

A

stainless steel

83
Q

What is the original cobaly chrome alloy?

A 
stellite 21 
CoCrMo
cobalt
chromium
molybedum
84
Q

What are the differences between forged and casted cobalt chrome alloys?

A

forged are stronger but less corrosion resistant

85
Q

Where are cobalt chrome alloys used? Why?

A

hip joint because strength from the size

bearing surface as low coefficient of friction with polyethylene

86
Q

What property makes pure titanium very corrosion resistant?

A

anodised

ie. more of a protective layer

87
Q

How does titanium compare to stainless steel in its

a) density?
b) stiffness?
c) fatigue strength?

A

a) lower
b) 1/2
c) higher

88
Q

What is titanium used for?

A

bone fixation plates

89
Q

Why isnt titanium used in bearing joint surfaces?

A

low wear resistance

90
Q

In what way is carbon fibre reinforced plastic more like bone than the metals used for orthopaedic implants?

A

It has lower material stiffness than metals - about three times that of cortical bone rather than ten times (steel and cobalt chrome) or five times (titanium and its alloys).

Ortho Implant technology (7 decks)

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