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Implant technology > implant mechanics & materials > Flashcards

implant mechanics & materials Flashcards

1
Q

An implant must…

A

…

  • be tolerated with no short term and little long term risk of adverse toxic effects
  • relieve pain and allow sufficient mobility
  • function w/o failure until no longer required
  • predictable outcome reasonably guaranteed
  • acceptable cost
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2
Q

Why is a plastic material almost always used rather than metal?

A

metal to metal contact has proved to result in an unsatisfactory bearing surface

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3
Q

what is the stiffness of plastic implant materials similar to

A

cancellous bone

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4
Q

what does anisotropic mean

A

different mechanical properties in different directions

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5
Q

structurally what are the 2 most important factors in the design of an implant

A

strength and stability

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6
Q

what are the 2 requirements for biocompatibility of implants

A

biological integration - harmful reactions with body tissues must not exceed accepted safe levels and corrosion of materials by the body must not cause it to fail
functional integration - the implant should not adversely affect function of other parts of the body

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7
Q

what are the 5 structural factors important in implants

A 
strength
stiffness (not too stiff that it affects loading on adjacent tissues)
lubrication
wear
fatigue
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8
Q

why are most bones wider at the ends

A

to accommodate the joint

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9
Q

what is the purpose of the end regions of bone containing cancellous bone

A

shock absorbing properties

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10
Q

what type of load is there on the main body of the femur

A

considerable bending load due to the joint at the head of femur being displaced laterally from the bone shaft

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11
Q

what can be said about the region of bone directly beneath articular surfaces

A

more dense than the cancellous bone below it to provide a rigid enough surface for the joint to bear on

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12
Q

why do bone shafts contain dense contact bone

A

resistance to deformation

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13
Q

what does isotropic mean

A

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

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14
Q

is bone isotropic or anisotropic

A

anisotropic

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15
Q

what defines the stiffness of a material

A

youngs modulus

ratio of stress to strain

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16
Q

how does the strength of cortical bone vary from the metaphysis to the diaphysis

A

only half as strong at the metaphysis

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17
Q

The faster bone is loaded, the ? it becomes

A

stiffer

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18
Q

which type of stress is bone least tolerant to

A

shear stress

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19
Q

The greater the difference in youngs modulus between 2 materials, the greater the ?

A

shear stress

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20
Q

In a join between 2 bars, where would the shear stress be?

A

at the end regions

there is no shear stress in the central portion because this is an area of load sharing

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21
Q

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

A

they have different values of Young’s modulus&raquo_space; they try to deform by different amounts but cant do this if they are joined together so a shear stress is generated

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22
Q

In what case is osteopenia most likely to develop

A

when load sharing is combined with load transfer

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23
Q

what has greater stiffness - material with small or large cross sectional area?

A

larger cross sec area > stiffer

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24
Q

as length increases does a material become more or less stiff?

A

less stiff as length increases

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25
Q

what is geometric stiffness dependent upon

A

cross sectional area and length

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26
Q

diff between material stiffness and geometric stiffness

A

material stiffness is not dependent on shape

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27
Q

equation for shear modulus (G)

A

shear stress / shear strain

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28
Q

Axial rigidity = ?

A

E x A

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29
Q

Bending rigidity = ?

A

E x I

30
Q

Torsional rigidity = ?

A

G x J

G is shear modulus and J is polar 2nd moment of area

31
Q

what is rigidity?

A

stiffness of the cross section of a material

32
Q

What is G (shear modulus)

A

shear stress / shear strain

33
Q

will a stiffer implant take more or less load in the load sharing region

A

stiffer implant will take more load because it is more rigid relative to bone

34
Q

what is the contact area between a fixator and bone called

A

bone-implant interface

35
Q

what causes less trauma to soft tissues - screws or nuts and bolts?

A

screws - only need access from one side of the bone and the head projects less from the bone to the skin surface

36
Q

what does interference fit fixation rely on

A

there is no specific fixation device, instead it relies on tight contact between implant and bone (surface friction prevents movement)

37
Q

what is necessary practice for the dimensions of the inner component in an interference fit

A

the inner component should have slightly larger dimensions

38
Q

what is the assumption of biological fixation

A

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

39
Q

what are the 2 most common surface coatings used in biological fixation

A
  1. porous beads of the same material of the implant

2. a ceramic such as hydroxyapatite (which is the main mineral constituent of bone!)

40
Q

what technique is used to deposit hydroxyapatite on to the metal surface

A

plasma spray coating

41
Q

what is the downside of using beads of metal

A

increases the surface area&raquo_space; increases the level of corrosion

42
Q

what metal is most commonly used for biological fixation and why

A

titanium because its the least corrosive and most biocompatible

43
Q

what aspect of stems makes them less likely to subside far into the bone canal

A

they are tapered so that as the begin to subside, the stem forms a tighter fit with the bone

44
Q

what is corrosion

A

the progressive unwanted removal of a material by an electrochemical process

45
Q

what aspect of an implant acts as an electrode

A

metal or some other conductive material such as carbon in carbon fibre reinforced plastics

46
Q

what does an area of corrosion of an implant lead to

A

high stress concentration which can lead to fatigue and failure

47
Q

is the corrosive reaction most severe if the electrodes are the same metal or 2 different metals?

A

2 different metals

48
Q

when can corrosion happen within a single metal component

A

if there are non homogeneous regions such as impurities

49
Q

an alloy can increase levels of corrosion - true or false?

A

false

an alloy rather than a pure metal can reduce the levels of corrosion

50
Q

which metal does not need to be formed into an alloy

A

titanium because even in its pure metallic form it is very resistant to corrosion

51
Q

name the 3 alloys used in implants

A

stainless steel
cobalt chrome
titanium

52
Q

what is the property that provides the good corrosion resistance of the metals used?

A

formation of a thin passivation layer of metal oxide (it forms on the surface of the material when its exposed to a corrosive environment)

53
Q

what is fretting corrosion?

A

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

54
Q

when does crevice corrosion occur

A

when body fluid becomes trapped in a crevice between implants. the fluid loses its normal supply of dissolved oxygen&raquo_space; high concentration acids form which corrode the metals

55
Q

name 2 areas particularly prone to crevice corrosion

A

edges of bone plates and between screws and plates

56
Q

name 2 methods of improving corrosion resistance

A
  1. nitric acid immersion treatment

2. titanium nitride coating

57
Q

how does nitric acid immersion work

A

it improves the natural passivation layer

58
Q

what is titanium nitride coating effective in reducing the release of from the alloy

A

vanadium and aluminium

59
Q

what does titanium nitride coating not prevent the release of from titanium alloys

A

titanium!

60
Q

what is the most common stainless steel used for implants

A

316L grade

- a low carbon steel

61
Q

what is the reason for low carbon content of implants

A

to minimise sensitisation of tissues and to make it more resistant to corrosion

62
Q

what is the main element in stainless steel

A

iron

63
Q

what type of corrosion is 316L stainless steel prone to and what does this make it less ideal for

A

crevice corrosion

|&raquo_space; less ideal for permanent implants

64
Q

what is better for implants - forged or casted steel? why?

A

forged because it is 4 times as strong

65
Q

does chromium have good or bad corrosion resistance

A

good

66
Q

name a cobalt chrome alloy used in hip prosthesis stems

A

MP35N

67
Q

what is the most common titanium alloy used

A

Ti6A14V

- titanium, aluminium and vanadium

68
Q

what is anodising

A

a process which increases the thickness of the anti corrosive layer

69
Q

why isn’t titanium used in joint replacements

A

its wear resistance is low

70
Q

in what way is carbon fibre reinforced plastic more like bone than metals used for implants

A

it has a lower material stiffness than metals

71
Q

what may it be possible to simulate with hydroxyapatite fibre reinforced polymers

A

variations in orientation of trabeculae

Implant technology (10 decks)

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