implant mechanics & materials

Question 1

An implant must…

Answer 1

…

• 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

Question 2

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

Answer 2

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

Question 3

what is the stiffness of plastic implant materials similar to

Answer 3

cancellous bone

Question 4

what does anisotropic mean

Answer 4

different mechanical properties in different directions

Question 5

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

Answer 5

strength and stability

Question 6

what are the 2 requirements for biocompatibility of implants

Answer 6

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

Question 7

what are the 5 structural factors important in implants

Answer 7

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

Question 8

why are most bones wider at the ends

Answer 8

to accommodate the joint

Question 9

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

Answer 9

shock absorbing properties

Question 10

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

Answer 10

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

Question 11

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

Answer 11

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

Question 12

why do bone shafts contain dense contact bone

Answer 12

resistance to deformation

Question 13

what does isotropic mean

Answer 13

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

Question 14

is bone isotropic or anisotropic

Answer 14

anisotropic

Question 15

what defines the stiffness of a material

Answer 15

youngs modulus

ratio of stress to strain

Question 16

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

Answer 16

only half as strong at the metaphysis

Question 17

The faster bone is loaded, the ? it becomes

Answer 17

stiffer

Question 18

which type of stress is bone least tolerant to

Answer 18

shear stress

Question 19

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

Answer 19

shear stress

Question 20

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

Answer 20

at the end regions

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

Question 21

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

Answer 21

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

Question 22

In what case is osteopenia most likely to develop

Answer 22

when load sharing is combined with load transfer

Question 23

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

Answer 23

larger cross sec area > stiffer

Question 24

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

Answer 24

less stiff as length increases

Question 25

what is geometric stiffness dependent upon

Answer 25

cross sectional area and length

Question 26

diff between material stiffness and geometric stiffness

Answer 26

material stiffness is not dependent on shape

Question 27

equation for shear modulus (G)

Answer 27

shear stress / shear strain

Question 28

Axial rigidity = ?

Answer 28

E x A

Question 29

Bending rigidity = ?

Answer 29

E x I

Question 30

Torsional rigidity = ?

Answer 30

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

Question 31

what is rigidity?

Answer 31

stiffness of the cross section of a material

Question 32

What is G (shear modulus)

Answer 32

shear stress / shear strain

Question 33

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

Answer 33

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

Question 34

what is the contact area between a fixator and bone called

Answer 34

bone-implant interface

Question 35

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

Answer 35

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

Question 36

what does interference fit fixation rely on

Answer 36

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

Question 37

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

Answer 37

the inner component should have slightly larger dimensions

Question 38

what is the assumption of biological fixation

Answer 38

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

Question 39

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

Answer 39

1. porous beads of the same material of the implant | 2. a ceramic such as hydroxyapatite (which is the main mineral constituent of bone!)

Question 40

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

Answer 40

plasma spray coating

Question 41

what is the downside of using beads of metal

Answer 41

increases the surface area >> increases the level of corrosion

Question 42

what metal is most commonly used for biological fixation and why

Answer 42

titanium because its the least corrosive and most biocompatible

Question 43

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

Answer 43

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

Question 44

what is corrosion

Answer 44

the progressive unwanted removal of a material by an electrochemical process

Question 45

what aspect of an implant acts as an electrode

Answer 45

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

Question 46

what does an area of corrosion of an implant lead to

Answer 46

high stress concentration which can lead to fatigue and failure

Question 47

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

Answer 47

2 different metals

Question 48

when can corrosion happen within a single metal component

Answer 48

if there are non homogeneous regions such as impurities

Question 49

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

Answer 49

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

Question 50

which metal does not need to be formed into an alloy

Answer 50

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

Question 51

name the 3 alloys used in implants

Answer 51

stainless steel cobalt chrome titanium

Question 52

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

Answer 52

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

Question 53

what is fretting corrosion?

Answer 53

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

Question 54

when does crevice corrosion occur

Answer 54

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

Question 55

name 2 areas particularly prone to crevice corrosion

Answer 55

edges of bone plates and between screws and plates

Question 56

name 2 methods of improving corrosion resistance

Answer 56

1. nitric acid immersion treatment | 2. titanium nitride coating

Question 57

how does nitric acid immersion work

Answer 57

it improves the natural passivation layer

Question 58

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

Answer 58

vanadium and aluminium

Question 59

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

Answer 59

titanium!

Question 60

what is the most common stainless steel used for implants

Answer 60

316L grade | - a low carbon steel

Question 61

what is the reason for low carbon content of implants

Answer 61

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

Question 62

what is the main element in stainless steel

Answer 62

iron

Question 63

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

Answer 63

crevice corrosion | >> less ideal for permanent implants

Question 64

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

Answer 64

forged because it is 4 times as strong

Question 65

does chromium have good or bad corrosion resistance

Answer 65

good

Question 66

name a cobalt chrome alloy used in hip prosthesis stems

Answer 66

MP35N

Question 67

what is the most common titanium alloy used

Answer 67

Ti6A14V | - titanium, aluminium and vanadium

Question 68

what is anodising

Answer 68

a process which increases the thickness of the anti corrosive layer

Question 69

why isn't titanium used in joint replacements

Answer 69

its wear resistance is low

Question 70

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

Answer 70

it has a lower material stiffness than metals

Question 71

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

Answer 71

variations in orientation of trabeculae