Implant Technology Unit 1 Flashcards Preview

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Flashcards in Implant Technology Unit 1 Deck (112):
1

what are ortho implant devices and their function

devices made from non-biological materials to improve structure and/or function

- either provide structural support after an injury (i.e. bone fixators)
=- replace or modify injured, diseased and painful joints

2

what qualities must an implant have

- be tolerated by human body with no short or little long term risk (e.g. carcinogenesis)

- relieve pain and enable sufficient mobility for ADL

- adequate strength

- function w/out failure for as long as it is required. Ideally should last expected life space.

- practicability of insertion; predictable outcome reasonably guaranteed by competent surgeon

- cost effective

3

what are most implants made of

metal e.g. stainless steel, titanium alloy

[in joints, bearing surface is made of a plastic material as metal-to-metal causes unsatisfactory result]

4

what is the main problems associated with implants

infection
- bacteria attracted to metal/cement surfaces
- commonly the normal bacteria found on the skin the culprit
- implant has to be removed

5

from a structural standpoint, what is the most important factors in the design of an implant

strength
stability

- its fixation to body tissue should be free from movement
- should function in harmony with the natural structures of the body, especially bone

6

what is important to remember about the cost of the implant

same implants have different prices in different countries

despite low cost of hip replacement it still remains outside the reach of the majority of countries

7

what are the 3 categories of performance of an implant technology

structural factors
kinematic factors
biocompatibility

8

what are the factors under 'structural factors'

strength
- Components must withstand loads acting on them w/out deforming permanently or breaking

stiffness
- components must be rigid enough to bear load without excessive deflection, while not being so stiff that they adversely affect the loading on adjacent tissues

lubrication
- Moving parts must be adequately lubricated or require no lubrication

wear
- The rate of wear of bearing surfaces must not cause failure or generate wear particles which damage body tissues

fatigue
- fatigue life should be greater than the intended life of the implant

9

what are the factors under 'kinematics factors'

Motion
- ROM must be sufficient to enable daily living functions, even if it is less than normal joint ROM
- The directions and patterns of motion must be controlled to ensure stability

10

what are factors under 'biocompatibility'

biological integration
- Harmful reactions of implant materials w/ body tissues shouldn't exceed accepted safe levels
- corrosion of materials by the body should not cause the implant to fail

functional integration
- implant should perform such that it does not adversely affect the function of other parts of the body

11

what are the 2 types of bones

compact bone
- a.k.a cortical bone

cancellous bone

12

how are bones designed to bear load

- 5 things

- end regions of the bones are shaped so as to accommodate the joint i.e. wider at the ends

- end regions of the bones contain cancellous bone which is more porous and less stiff (more flexible) than cortical bone, giving shock absorbing properties

- in cancellous bone, trabecular are aligned along the directions of greatest stress

- region below articular surface is more dense than the cancellous bone below it, provides a rigid underlying surface for the joint to bear on w/out causing excessive deformation

- shafts of bones contain dense compact bone, more rigid than cancellous bone, provides the necessary resistance to deformation under bending and torsional loads

13

how is stiffness measure

Young's modulus (E)
- ratio of stress to strain

14

how does Young's modulus change in most material when loaded

remains approximately constant irrespective of the load applied or the rate of loading

15

What does is mean when a material is said to be isotropic

that their mechanical properties are the same no matter which direction they are loaded

16

what does anisotropic mean and what materials exhibit this behaviour

it's young's modulus depends on the direction in which it is being loaded

bone

17

what direction is cortical bone stiffest and strongest

when loaded longitudinally
- [main direction in which it is loaded naturally]

18

what is meant when a bone is described as viscoelastic

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

19

when is cortical bone strongest and weakest

strongest - under compressive loading

weakest - under shear loading

20

how are implants designed in terms of loading the bone

try to load the bone in compression

avoid shear stress especially but also try avoid tensile

avoid excessive stress shielding

21

what is stress shielding/stress protection and what is an example in orthopaedics

when bone is reabsorbed because of reduced loading

i.e. Wolff's Law

caused in orthopaedics by bone plates; bone around plate gets reabsorbed; can lead to loosening of fixation screws meaning it is no longer effective in supporting the bone

22

what is the main difference in structure between bone in diaphysis and bone in the region of a joint

Diaphyseal bone
- made from compact bone
- rigid and provides resistance to deformation under loading.

Bone in the region of a joint
- cancellous
- the trabeculae are aligned along directions of greatest stress
- much less rigid than cortical bone and has good shock absorbing properties
- Bones are generally wider at the joints than at the diaphyses.

23

what is the major role of orthopaedic implants

provide structural support

24

in an orthopaedic implant:

BONE FIXATOR

- what are the 3 regions of the implant, assuming the bones touch at the fracture site

Region 1 = LOAD TRANSFER
- where the screws fix the plate to the bone
- here, part of the applied load in the bone is transferred to the plate

Region 2 = LOAD SHARING
- the fracture site, where the broken bones are supported by the plate
- here, part of the load is taken by the plate and part by the bone

Region 3 = LOAD TRANSFER
- where the screws fix the plate to the bone at the other side of the fracture

25

in an orthopaedic implant:

INTRAMEDULLARY STEM OF A CEMENTED JOINT REPLACEMENT

- what are the 3 regions of the implant, assuming the bones touch at the fracture site

Region 1 = LOAD TRANSFER
- where part of applied load is transferred from the stem to the bone

Region 2 = LOAD SHARING
- load sharing between the bone and the stem

Region 3 = LOAD TRANSFER
- remaining part of the load is transferred from the stem to the bone
- the bone below this region, takes all the load

26

where is the load transferred for bone plate and for the intra-medullary stem

bone plate
- load transferred at bone screw region

intra-medullary stem
- load transferred at end regions of the stem only

27

what is meant by load sharing and load transfer

Load sharing region
- For an implant attached to bone, the regions where the load is partly taken by the bone and partly taken by the implant

Load transfer region
- where load is transferred from an implant to a bone (or from a bone to an implant)
- load passes across interfaces between them

28

what does the load generate at the interface

and what serious complication can occur due to this

stresses or relative movement at the interface

stresses = occur when the two materials are bonded together

relative movement = occurs either if they are not bonded or if a bone comes loose

Loosening - serious complication in joint replacement

29

if there was two materials, one on top of another, and the bottom material was more flexible than the top

what would happen under loading

the bottom half would compressed more under loading and expand laterally more than the top

30

if there was two materials, one on top of another, and the bottom material was more flexible than the top

what would happen under loading if the two materials were bonded together

any lateral strain at the interface is the same for both materials, so a shear stress is generated at the interface, because one material is trying to expand more than the other one

31

if there was two materials, one on top of another, and the bottom material was more flexible than the top

what would happen under loading if the two materials were not bonded together

if it is also lubricate

sliding can occur freely so there are no shear stresses

32

what is the general rule about the difference in the young's modulus and shear stress

greater the difference in young's modulus then the greater the shear stress generated

33

if there was two materials, one on top of another, and the top material is less stiff than the bottom

what is stress at the interface like

stresses at the interface under the region of an applied load from above will be much greater

34

if there was two materials, one on top of another, and the top material is less stiff than the bottom

what is the advantage of using a stiff tibial component for a knee prosthesis

distributes loads more evenly over the underlying bone than a material with a lower stiffness, such as polyethylene

35

why does shear stress occur at a bone-implant interface

occurs at a bone-implant interface because the bone and implant each have a different material stiffness i.e. young's modulus

so they try to deform by different amounts under a load

If joined together they cannot deform separately so a shear stress develops between them along line of interface

36

what is important to note about shear stress at the interface

shear stress is not constant across the whole length of the interface

[there is no shear stress in the central portion, which is a region of load sharing]

37

what is the equation for shear stress

shear stress = applied force / area being sheared

38

what causes stress concentration

sharp corners, notches, holes

39

what is a consequence of stress shielding

osteopenia due to bone reabsorption

40

structural stiffness is determined by what 2 factors

material stiffness
- basic property of the material
- i.e. its Youngs Modulus

geometrical stiffness
- shape of the cross section of the structural component

41

how does metal's properties compare to bone

10 times stiffer than cortical bone

many more times stiffer than cancellous bone

are isotropic unlike bone

42

how do you calculate shear modulus

G = shear stress/shear strain

[calculated by applying a twisting load to a material]

43

what do you measure when you are measuring how stiff something is

measure of how much it deflects under load

defined as = force require to produce a unit deflection

[stiffness = S or k ]

44

what is the equation for stiffness

S = force / displacement

S = EA/L

E = youngs modulus
A = area
L = length

45

how does young's modulus of the material affect stiffness

becomes stiffer as young's modulus increases

[becomes stiffer as area increases]

[becomes less stiffer as its length increases]

46

to summarise what geometrical properties affect the stiffness of a bar under axial loading

cross sectional area
length

47

when is rigidity used

If we want to compare the stiffnesses of two implants of the same length

as the length is the same it would have no impact so we refer to the RIGIDITY of the 2 rather than stiffness

48

what is the equation for axial rigidity

R = EA

49

what is the equation for bending rigidity

R = EI

I = second moment of area

50

what is the second moment of area

geometrical property of the cross section which is based on the area of material it contains and also on how far it is away from the neutral axis

51

how does the distribution of the material affect its rigidity whilst bent

the further away a material is placed from the neutral axis the more rigid it is when bent

52

what are the equations for I

Rectangle:
I = bd^3 / 12

Circle:
I = pieD^4 / 64

Hollow rod:
I = pie/64 (D^4 - d^4)

53

what does a higher I value mean

the object will be stiffer when bent

i.e. more difficult to bend

54

what is the equation for rigidity under torsional loading

R = GJ

G = shear modulus
J = polar second moment of area

55

what does the amount of load transfer from bone to implant [or vice versa] depend on

relative loads taken by them in the load sharing region

56

why does the cement in joint prosthesis inserted into bone take very little of the load

as its rigidity is low due to both a low E and a low cross section of material

57

in the load sharing region, what is the ratio of the load taken by the bone to that taken by the stem equal to

ratio of their rigidities

Lbone / Lstem = Rbone / Rstem

58

what is the ratio of rigidity of the bone equal too

total load taken by the bone

Lbone/ Ltotal = Rbone / R total = R bone / R bone + Rstem

[example Q p15]

59

what is the result is the stem of a prosthesis is less stiff

i.e. more like bone

more load would be transferred proximally and less distally, reducing stress shielding and bone reabsorption

60

what is the result is the stem of a prosthesis is more stiff

would transfer less load to the bone

more stress shielding and bone reabsorption

61

what is rigidity

stiffness of the cross section of the material

62

what is the bone-implant interface and what is essential about this interface

contact area between the fixator of an implant and the bone

must remain fixed and free from movement, otherwise the implant will loosen and probably fail

63

what holds in fracture fixators

screws which can be undone

allows the fracture fixator to be removed after healing

64

what is the advantage of screws compared to nuts and bolts

screw attachments only require access from 1 side of a bone only

nuts/bolts need access from both sides

65

what is the main disadvantages of nuts + bolts

more trauma to the tissue

project more than screws - cause issue when small distance from bone to skin's surface [i.e. interior part of tibia]

66

what is the 'Interference Fit' dependant on

required no specific fixation device

relies on tight contact between implant and bone, the surface friction between the two prevents movement at the interface

67

when is the Interference Fit used as a method of implant fixation

when the dimensions of the inner component are slightly larger than those of the outer component

the implant is pressed into the bone to lessen the risk of loosening

used in cementless joint replacements

68

what is a possible side effect of the Interference Fit being made too tight

the bone can split

69

what is the function of bone cement and when is it commonly used

fill gaps between a bone and implant

once cement has dried, the bone implant interface should remain free from motion

commonly used in stems of joint replacement

70

what is the assumption of biological fixation

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

71

what are the 2 main methods of biological fixation

porous beads
- made from same material as implant
- used mostly w/ titanium protheses stems as titanium is least corrosive and most biocompatible

ceramic coatings
- normally with HAp, the main mineral constituent of bone

72

why are prostheses stems tapered

so they cannot subside very far into the bone canal

tapered stem forms a tighter git in the bone canal as it sinks

73

what are 3 important features of an orthopaedic implant

high degree of biocompatibility

suitable structural mechanical properties for the application

ease of manufacture and fabrication of implant devices

74

what is biocompatibility

interaction between the human body and the implant material

75

what are 2 factors of biocompatibility

1) the extent to which body fluids and tissues affect a material
- most likely to be corrosion of the material, which can lead to mechanical failure

2) the extent to which a material adversely affects body tissues
- e.g. its tendency to cause abnormal changes to tissue (such as ulceration, allergy or cancer) or tissue death.

76

what is corrosion and when does it occur

the progressive unwanted removal of material by an electrochemical process

occurs when two electrodes are immersed in a liquid that conducts electricity

77

what is galvanic corrosion

electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte

78

what are the components of corrosion in implants and what is the consequence of it

the electrodes are metal or conductive material like carbon

electrolyte is body fluid

causes small areas of loss of material, makes pits and craters

become high stress concentration areas

can lead to failure fatigue

79

when is the corrosive reaction generally more severe

when the electrodes are different metals

[but can occur still if metals are same material]

80

what is advised in ortho to prevent corrosion

not to use different implant metals in contact, particularly if one is stainless steel which corrodes when in contact with carbon fibre

e.g. stainless steel screws to fix a carbon fibre reinforced plastic bone plate

81

what group metals are resistant to corrosion and why are they resistant

alloys - mixture of metals together

passivation layer
- forms on the surface
- layer itself a product of corrosion
- seals underlying layer from further corrosion

82

what are the 3 alloys used in ortho

stainless steel, cobalt chrome and titanium alloys

83

what is fretting corrosion

corrosion as a response to the removal of the passivation layer by the repetitive rubbing together under a load of 2 materials

occurs between screws and plates and also morse tapers [rely on the friction between two tapered components to prevent motion]

84

what can fretting also cause

surface damage to implants

reduces fatigue life

85

what is crevice corrosion

occurs in crevices between implants, where body fluid can become trapped and lose its normal supply of dissolved oxygen

leads to high conc acid forming which corrodes the metal

86

what areas are prone to crevice corrosion and how can it be avoided

- edges of bone plates
- between screws and plates

careful surgical assembly of components to ensure good screw-plate contact

87

what are the 2 methods for improving corrosion resistance

nitric acid immersion

titanium nitride coating

88

what is nitric acid immersion

improves the natural passivation layer

not entirely sure how it works but thought to be related to the increased amount of chromium in the passivation layer, which improves corrosion resistance

89

what is titanium nitride coating

significantly decreases corrosion therefore reduces the releases of harmful metallic substances

effective in reducing the release of vanadium and aluminium from titanium alloys

does not decrease the release of titanium but titanium is regarded at the least harmful implant metal

90

what are tissue reaction to implanted metals

7 things

- growth of thin fibrous tissue layer between implant and body tissue. Fibrous layer is body isolating itself from the foreign body

- local infection

- body sensitisation to metals

- inflammation in regions of metal corrosion, where protective oxide layer is load and small particulars react with body tissues

- tissue necrosis in regions were bone cement is used in joint replacements

- immunological reaction due to wear in the particulars from surface of joint replacement > can lead to cell mediated bone reabsorption

- tumours [rare]

91

why have ceramic materials not been used in ortho implants

fail in a brittle manner
give no advanced warning of failure

92

what are the materials used for implants

stainless steel
cobalt chrome alloys
titanium alloys
fibre reinforced plastics

93

what is the most common stainless steel type used in ortho and what are features of it

316L grade
- low carbon steel content to minimise sensitisation of tissue and make it more resistant to corrosion by the body

- main element is iron

- has high corrosion resistance but can corrode and crack under high stress
- prone to crevice corrosion

94

what is 316L grade stainless steel used for

temporary implants
- fracture fixation (e.g. screws and plates)
- load on the implant decreases as the bone heals and implant can be removed

95

what is 316L not ideal for

permanent implants due to it being prone to crevice corrosion
- e.g. hip replacements

96

what is the strength of stainless steel dependant on

how it is manufactured
- ortho implants normally forged [i.e. heated metal is forced into shape by hammering]
- the work/energy involved in forging process causes metal to harden increasing its yield stress but makes material less ductile

97

what is forged stainless steel 4 times stronger than

steel produced by casting

98

what are advantages and disadvantages of using stainless steel

Adv:
- manufacturing costs a relatively low

Disadv:
- suffers from more pitting corrosion [due to the passivation layer failing] than cobalt and titanium
- fatigue strength is lower

99

what is the main advantage to cobalt chrome alloys and what component gives it this characteristic

more resistant to corrosion in viva than stainless steel

chromium

100

what ortho procedure is cobalt chrome alloys preferred in and what is the preferred composition

permanent implants [even though it is not as strong]
- hip implants

Stellite 21 - 65% cobalt, 25-30% chromium and 6% molybedum

101

what are the other commonly used cobalt alloy used in ortho

MP35N
- 35% nickel, 20% cobalt
- used in hip joint stems

CoCrMo
- used as bearing surfaces because of their low coefficient of friction

102

titanium is used in ortho as either pure metal form or as an alloy - what is the most common alloy form

Ti6Al4V

103

what happens to pure titanium before it is used in ortho

it is anodised
- process which increases the thickness of an anti-corrosive protective layer on metal's surface
- makes it very resistance to corrosion within the body
[better corrosion resistance than stainless steel]

104

what are the mechanical properties of titanium

lighter and half as stiff as steel and cobalt chrome

higher fatigue strength than stainless steel and cobalt chrome alloys

105

what is titanium commonly used in

fracture fixation plates

[low wear resistance makes it unsuitable for bearing in joint replacements]

106

what makes fibre reinforced polymers and what properties does it have

very stiff, high strength but brittle fibres embedded in a flexible resin material

- high strength properties
- stiffness can be selected according to the number and type of fibres used
- no longer brittle

107

what features do carbon fibre reinforced polymers have

most biocompatible

stiffness about one third that of stainless steel, so more mechanically compatible with bone

superior fatigue properties compared to stainless steel

108

what has carbon fibre reinforced polymers been used for

internal bone fixation plates
fracture plates

[superior fatigue properties means that it can overcome the problem of fatigue in metal plates due to movement at the fracture site]

109

what metal implants have the best corrosion resistance

titanium and its alloys

110

in what away is carbon fibre reinforced plastic more like bone than the metals used in ortho implants

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)]

111

what qualities must an implant have

strong enough not to break under use

in regions where loads are shared between a bone and an implant, the rigidity of the implant must be such that it minimises stress shielding of the bone [which can lead to bone reabsorption and loosening of a prosthesis]

112

what are the 2 main problems of implants

corrosion

detrimental effects of the products of corrosion on the body cells, tissues and systems