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BMSc - mine > Implant technology Unit 1 > Flashcards

Implant technology Unit 1 Flashcards

1
Q

What 5 points must an implant address?

A

An implant must;
be tolerated by the human body with no risks of adverse toxic effects such as carcinogenesis

relieve pain and enable the patient to achieve sufficient mobility to carry out daily activities

function without failure until its no longer required

be designed for insertion such that a predictable outcome can be reasonably guaranteed by a competent surgeon

be cost effective

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2
Q

What is the main associated complication of implants?

A

Infection - bacteria are attracted to the implant material and the body struggles more to deal with infection in the presence of this foreign material. Often associated with bacteria found on the skin

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3
Q

What are the two main structural requirements for an implant?

A

Strength and stability

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4
Q

The performance of an implant can be looked at under 3 main categories - Structural factors, kinematic factors and biocompatibility. What are the structural factors?

A

Strength - must be able to withstand loads without deforming permanently or breaking
Stiffness - Must be rigid enough to bear loads without excessive deflection while not being so stiff that they adversely affect the loading on adjacent tissues
Lubrication
Wear - rate of wear must not cause failure or damage body tissues
Fatigue - fatigue life should not be greater than the intended life of the implant

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5
Q

What is the kinematic factors relative to the performance of an implant?

A

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

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6
Q

What is the biocompatibility factors relative to the performance of an implant?

A

Biological integration - harmful reactions of implant materials with the body must not exceed accepted safe levels; corrosion of materials by the body should not cause the implant to fail

Functional integration - The implant should not adversely affect the function of other parts of the body

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7
Q

Most orthopaedic implants are attached to bone and form a composite structure with the bone. What is a composite structure?

A

A structure consisting of more than one material

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8
Q

What are the two types of bone?

A

Compact (cortical) and cancellous

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9
Q

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

A

Diaphyseal - Compact bone. It is rigid and resistant to deformation under loading.
Bone in the region of the joint is cancellous and the trabeculae are aligned along directions of greatest stress. It is less rigid and has good shock absorbing properties. Bones are wider at the joints than the diaphysis.

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10
Q

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

A

Strongest under compressive and weakest under shear.

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11
Q

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

A

Stress shielding is the reduction of the load (and therefore stress) that would normally be taken by a bone, due to the presence of an implant. Bone resorbs when under-stressed and this can lead to loosening of the implant.

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12
Q

The stiffness property of a material is defined by its Young’s modulus. What is this?

A

A ratio of stress to strain. It is approximately constant up until its elastic limit.

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13
Q

Most non-biological structural materials are isotopic. What does this mean?

A

Their mechanical properties are the same no matter which direction they are loaded.

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14
Q

Is bone isotropic or anisotropic?

A

Anisotropic - its Young’s modulus depends on the direction in which it is being loaded.

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15
Q

The stiffness of bone changes according to the rate at which it is loaded. What behaviour does this display?

A

Viscoelastic behaviour. The faster it is loaded, the stiffer it becomes

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16
Q

When using an internal bone fixation plate, what are the load transfer regions and the load sharing region?

A

Load transfer - proximal to the fracture site, the applied load in the bone is transferred to the plate. Distal to the fracture site, the plate transfers the load back to the bone.

Load sharing - at the fracture site, part of the load is taken by the bone and part by the plate

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17
Q

What type of load is there at the middle portion of an intramedullary nail?

A

Load sharing between the nail and bone

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18
Q

In all cases of load transfer, the load passes between the materials across the interface. This generates either interface stresses or relative movement. When do each of these occur?

A

Interface stresses are created when the two materials are bonded together whereas relative movement occurs when they are not or if a bond becomes loose. Hence loosening has a serious implication with joint replacements.

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19
Q

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

A

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

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20
Q

When under a load, two materials that are joined will create a shear stress along their interface. What impact does the difference between the two Young’s modulus have on the shear stress?

A

The greater the difference in the Young’s modulus of the two materials, the greater the shear stress will be.

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21
Q

What happens when the interface is not bonded and is lubricated, such as that in a joint?

A

There will be no shear stress generated.

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22
Q

If there are two materials being compressed from above, what will the stress pattern at the interface look like if;

a) the bottom material is stiffer?
b) the top material is stiffer?

A

a) the load will result in a more concentrated stress in the central region
b) there will be a more even stress pattern and the bottom material will expand laterally more than the top material

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23
Q

How does the shear stress change along the length of an interface of two materials under a shear loading? E.g. the type that occurs between intramedullary stems and the surrounding bone

A

Shear stress is not constant across the whole length of the interface. Load transfer takes place at the end regions of the join between the bars. There is no shear stress at the central portion, which is a region of load sharing. I’d load was transferred across the whole length of the interface, then the shear stress could be found by dividing the applied force by the area being sheared. In reality, this is not the case and shear stress is much greater.

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24
Q

What are stress concentrations?

A

Created when there are sharp corners, notches and holes which induce these high localised stress concentrations.

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25
Q

What can result from bone resorption and stress shielding?

A

Osteopenia

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26
Q

What does the amount of bone transferred between bone and implant depend on?

A

How the loads are shared in the load sharing region. This depends on the relative stiffness of the two components.

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27
Q

The structural stiffness of a component is determined by two factors. What are they?

A

The material stiffness

Its geometrical stiffness - this is to do with the shape of the cross-section

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28
Q

Under shear loading, including torsion, the material stiffness is measured by the shear modulus rather than the Young’s modulus. What is the shear modulus ratio?

A

G = shear stress/shear strain

This can be calculated by applying a twisting load to a material and measuring how much it rotates.

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29
Q

What is stiffness?

A

It is a measure of how much a material deflects under load. Mathematically, it is defined as the force required to produce a unit deflection.
S = F/𝛿 which can be further adapted to S = EA/L

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30
Q

if S=EA/L, what effect does increasing the;

a) area have?
b) Young’s modulus have?
c) length have?

A

a) More stiff
b) More stiff
c) Less stiff - it deflects more under loading

Hence, the geometrical properties that affect the stiffness of a bar under loading are the cross-sectional area and the length of the bar.

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31
Q

When comparing the stiffness of two implants of the same length, we can refer to the term rigidity as length isn’t considered. What is this equation?

A

R = EA

where R = axial rigidity

32
Q

What is the equation for bending rigidity?

A

R = EI

where I is the second moment of inertia

The further the material is from the neutral axis, the greater the I

33
Q

What is the equation for torsional rigidity?

A

R = GJ

Where G is the shear modulus and J is the polar second moment of inertia

34
Q

How does the bone load to stem load ratio compare to the ratio of their rigidities in the load sharing region?

A

In a load sharing region, the ratio of the load taken by the bone, Lbone, to that taken by the stem, Lstem is the ratio of their rigidities, i.e.:
Lbone/ Lstem = Rbone/Rstem

35
Q

How does the proportion of total load taken by the bone equate to its rigidity?

A

Also the proportion of the total load taken by the bone is equal to the ratio of the rigidity of the bone to the total rigidity of the section, i.e.:
Lbone/Ltotal = Rbone/Rtotal = Rbone/Rbone + Rstem

36
Q

If Lbone/Ltotal = 0.4, what would this imply regarding the percentage of total load at different parts of the bone?

A

So, in the load sharing region (Region 2) bone takes about 40% of the total load, which means that 40% of the load must have been transferred from the stem to the bone in Region 1, so the other 60% is transferred from the stem to the bone in Region 3. The proximal bone is therefore heavily stress shielded because it takes just 40% of its normal load.

37
Q

What is one of the main determinants of how an implant is fixed to the bone?

A

Whether it is permanent or temporary

38
Q

What type of fixator tends to be used for fractures?

A

Screws are commonly use to attach an implant to a fracture as they can be easily removed once the fracture site heals

39
Q

What is interference fit?

A

A method of fixation which doesn’t require an implant fixator. Instead, it relies on friction between the surfaces of the bone and implant to prevent movement at the interface. The inner component is slightly larger than the outer component and so the bone and implant are pressed together

40
Q

When is an interference fit method used?

A

For cement-less joint replacements

41
Q

How does bone cement work as a fixation method?

A

It acts as a filling material or grout. It is not adhesive, but fills the gaps between the bone and implant and so a perfect bone-implant geometrical match is not required. Once it is dry, the bone-implant interface should be free of movement

42
Q

When is bone cement used?

A

Most commonly used in the stems of joint replacements

43
Q

What is biological fixation?

A

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

44
Q

What are the two main methods of biological fixation?

A
  1. Beads of the same material as the metallic implant. Small pores between the beads allow the bone to grow into the implant. Exposing a large surface area of metal in this way increases the levels of corrosion and so this techniques is used mainly with titanium prostheses stems as titanium is the least corrosive and most biocompatible of the orthopaedic implant materials
  2. A ceramic such as hydroxyapatite, which is the main mineral constituent of bone. It can be deposited directly on the implant using a method called plasma spray coating. Long term, this method isn’t great as some of the coating noticeably disappears. Short term it is a good method
45
Q

What are biomaterials?

A

Non-biological materials used within the body, normally to repair or replace body parts that have failed

46
Q

What 3 important features are required of an orthopaedic implant material?

A

High degree of biocompatibility - acceptance of the body tissues and systems
Suitable structural properties
Ease of manufacture and fabrication of implant device

47
Q

What impact can body fluids and tissues affect an implant material?

A

This is most likely to be corrosion of the material, which can lead to mechanical failure

48
Q

What impact can an implant material have on the body tissues?

A

Can cause ulceration, allergy, cancer or tissue death

49
Q

What is corrosion?

A

It is the progressive, unwanted removal of material by an electrochemical process. It occurs when two electrodes (solid material that conducts electricity) are immersed in a liquid that conducts electricity (called an electrolyte)

50
Q

When an electrical current can pass between two electro-conductive materials through an electrolyte, allowing a chemical reaction to occur between the electrodes and electrolyte, what is the process called?

A

Galvanic corrosion

51
Q

What effect can corrosion have on implants?

A

Corrosion causes small areas of loss of material, which often show up as small pits or craters. These lead to areas of high stress concentration and can lead to fatigue failure of the material

52
Q

When is the corrosive process most severe in terms of the electrodes?

A

When the electrodes are different metals to each other. However the process still occurs in electrodes of the same metal. If there are non-homogenous regions within the metal, such as non-uniform distribution of alloy constituents or the inclusion of impurities. Hence quality control of implants is important to minimise corrosion

53
Q

Alloys are used for implants and can reduce the amount of corrosion. What 3 alloys are commonly used for implants?

A

Stainless steel
Cobalt chrome
Titanium alloys

54
Q

What can help improve the resistance to corrosion in the metal alloy implants?

A

The presence of a passivation layer of metal oxide that forms on the surface of the material when exposed to a corrosive environment. This layer is a product of corrosion but helps to seal in the underlying material from corrosion

55
Q

What is fretting corrosion?

A

When there is abrasion of materials in contact that removes the protective metal oxide layer, leading to corrosion. It can occur when two materials that are joined together and not meant to move start to loosen and move. It can commonly occur between screws and plates for example. It is particularly common in modular prostheses

56
Q

What is crevice corrosion?

A

It is a corrosion that occurs when there is a crevice between implants which can fill with body fluid which in turn becomes trapped and looses its usual supply of dissolved oxygen. This in turn leads to high concentrations of acids forming which corrodes the metal implants. It is common at the edges of bone plates or between screws and plates. Careful surgical assembly is therefore important to reducing this risk

57
Q

What two methods are used to improve corrosion resistance in metal implants?

A

Nitric acid immersion

Titanium nitride coating

58
Q

How does nitric acid immersion work?

A

It helps to improve the natural passivation layer. The mechanism isn’t fully understood, however in the case of stainless steel and cobalt chrome alloys (both of which contain chromium), it is thought to be related to an increase in the amount of chromium in the passivation layer, which improves corrosion resistance

59
Q

How does titanium nitride coating help with metal implant corrosion?

A

It significantly decreases the amount of harmful metallic substances being released from the metal alloys to the body fluid by decreasing corrosion.

60
Q

What two substances is titanium nitride coating particularly good at reducing the release of from titanium alloys?

A

Vanadium and aluminium - both of which are particularly harmful to the body

61
Q

Titanium nitride coating is not helpful in reducing the amount of titanium released from titanium alloys. Why is this not a huge issue?

A

It has no effect on the release of titanium. However, titanium is regarded as the least harmful orthopaedic implant metal.

62
Q

Tissues can react to implanted materials in many ways. The products of corrosion of metallic implants can appear in various ways in the body - in the blood, urine, tissues, storage organs such as the liver, in nails and in hair. What impact can these substances have on the body?

A

Growth of a thin fibrous tissue layer between the implant and body tissue, particularly if there are micro-movements between the two materials. The fibrous tissue isolates the implant from the body and the bone and implant aren’t fixed together.

Local infection - can be at time of implant fitting or later on. Implant materials tend to suppress the body’s ability to defend from infections.

Body sensitisation to metals - many develop sensitivity to metals such as chromium or nickel.

Inflammation of regions where there is metal corrosion.

Tissue necrosis in the region of bone cement use in joint replacements. This generates considerable heat as it cures.

Immunological reactions including “cell-mediated bone resorption.”

Tumour formation - rare

63
Q

What is the most common form of stainless steel used for implants?

A 
316L grade - it has a low carbon content to minimise sensitisation of the body tissues.
Iron 63%
Chromium 18%
Nickel 13%
Molybdenum 3%
Manganese 2%
Also contains traces of other metals
64
Q

Why are stainless steel 316L implants not ideally used for permanent implants such as hip replacements?

A

Under high stress, it may corrode and crack and is prone to crevice corrosion. It is used more for temporary implants such as fracture fixation devices

65
Q

How are stainless steel implants manufactured?

A

They are forged - i.e. hammered into shape. This helps increase the yield stress of the material but makes it less ductile. It is relatively cheap to manufacture

66
Q

What other mechanical properties of stainless steel are there?

A

It suffers more from local pitting corrosion due to failure of the passivation layer than cobalt chrome and titanium, and its fatigue strength is lower

67
Q

What cobalt chrome alloy is used in orthopaedic implants and what is its composition?

A

Stellite 21

Cobalt 65%
Chromium 25-30%
Mylobdenum 6%

68
Q

How does the biocompatibility of Stellite 21 cobalt chrome compare to that of stainless steel 316L?

A

More resistant to corrosion in vivo as chromium provides good corrosion resistance. However it is not as strong. It is preferred for permanent implants

69
Q

How is cobalt chrome manufactured?

A

Casting method

70
Q

Why are alloys of cobalt chrome useful for joint replacements?

A

They are good bearing surfaces due to their low coefficients of friction with polyethylene, which is the plastic used almost exclusively in joint prosthesis bearings

71
Q

Titanium is used in either pure or alloy form. What alloy form is the most common for orthopaedic implants?

A

Ti6A14V - titanium with around 6% aluminium and 4% vanadium

72
Q

Pure titanium is anodised. What does this mean?

A

Anodising is a process which increases the thickness of the anti-corrosive protection layer on the metal’s surface, making it very resistant to corrosion within the body

73
Q

How do the mechanical properties of titanium compare to the other two?

A

It is less dense (lighter) and less stiff. It also has a higher fatigue strength than stainless steel. It has a low wear resistance and so is unsuitable for bearings in joint replacements. It is more commonly used for bone fixation plates

74
Q

Fibre reinforced plastics are being trialled as an alternative to metal implants. What are they?

A

They are very stiff, high strength, but brittle fibres embedded in a much more flexible resin material

75
Q

What metal implant material has the best corrosion resistance?

A

Titanium and its alloys

BMSc - mine (13 decks)

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