Implant technology - unit 1 deck 1

Question 1

Define what orthopaedic implant devices are

Answer 1

They are devices are made from non-biological materials placed in the body to improve the structure and/or function of an impaired component of the locomotor system

Question 2

In general what do most orthopaedic implant devices do ?

Answer 2

They provide either structural support after an injury (these are mostly bone fixators of one kind or another), or replace or modify injured, diseased and painful joints.

Question 3

What are the key requirements of an implant ?

Answer 3

• Be biocompatible (Be tolerated by the human body with no short term and little long-term risk of adverse toxic effects such as carcinogenesis)
• Relieve pain when it is present and enable the patient to achieve sufficient mobility to be able to manage daily living activities
• Be of adequate strength (Function without failure until it is no longer required. For a bone fixator this may be a few months while a joint replacement ideally should last the expected life span of the individual patient)
• Can be practicability inserted (Be designed for insertion such that a predictable outcome can be reasonably guaranteed by a competent surgeon)
• Be cost effective
• Safety for the surgeon and patient

Question 4

The range of materials used in implants is limited what are the main materials used and what is the difference between the stiffness of these materials compared to bone?

Answer 4

• Homogeneous materials, usually metals (stainless steel, titanium alloy or cobalt chrome alloy)
• They are much stiffer than bone.

Question 5

What is the main exception to homogeneous metal materials being used in implants and why and what is used instead?

Answer 5

Joint implants - this is metal to metal contact has proved to result in an unsatisfactory bearing surface so a plastic material (high molecular weight polyethylene) is almost always used.

Question 6

Describe the likeness of high molecular weight polyethylene stiffness compared to bone?

Answer 6

It has a similar stiffness to that of cancellous bone

Question 7

What are the small range of flexible (not stiff) materials used in implants ?

Answer 7

A small range of soft or flexible devices such as man-made fibre ligaments or darning materials, designed to strengthen diseased or repair damaged structures such as the tendinous rotator cuff

Question 8

Define what is meant when a structure is described as being anisotropic ?

Answer 8

It has different mechanical properties in different directions

Question 9

Describe what composite materials in orthopaedic implant technology are and there similarity to bone

Answer 9

• They are newer and still largely experimental structural materials which are being widely investigated because they can be designed to have mechanical properties more like bone and can be made to be anisotropic (with different mechanical properties in different directions) like bone.
• They are a long way from being accepted clinically as superior to traditional implant materials

Question 10

What is the main associated problem associated with implants

Answer 10

Infection! - the rates are much more common in procedures involving insertion of an implant than those which don’t involve implants

Question 11

Describe how implants increase the risk of infection development and the consequence of infection developing

Answer 11

1. Bacteria are attracted to metal and cement surfaces and rapidly adhere to them and, as the immune defence systems of the body work less efficiently in the presence of implants, this leads to infection.
2. Such infections are often caused by skin commensals and can go on to cause cataclysmic infections which can only be stopped by removing the implant so defeating the initial purpose of the operation.

Question 12

What are the 2 key structural requirements of an implant ?

Answer 12

Strength and stability

An implant should not break, its fixation to body tissue should be free from movement and it should function in harmony with the natural structures of the body, especially bone

Question 13

Why is the cost of an implant important to consider?

Answer 13

The cost of an implant may vary considerably from country to country

Question 14

List 4 criteria for a successful orthopaedic implant

Answer 14

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

Question 15

What are the 5 structural considerations of an implant ?

Answer 15

1. Strength - Components must withstand loads acting on them without deforming permanently or breaking
2. Stiffness - Components must be rigid enough to bear load without excessive deflection (bending), while not being so stiff that they adversely affect the loading on adjacent tissues
3. Lubrication - Moving parts must be adequately lubricated or require no lubrication
4. Wear - The rate of wear of bearing surfaces must not cause failure or generate wear particles which damage body tissues
5. Fatigue - Fatigue life should be greater than the intended life of the implant

Question 16

What is the important kinematic factor to consider ?

Answer 16

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

Question 17

What are the important biocompatibility considerations

Answer 17

1. Biological Integration - Harmful reactions of implant materials with body tissues should not exceed accepted safe levels; corrosion of materials by the body should not cause the implant to fail
2. Functional Integration - The implant should perform such that it does not adversely affect the function of other parts of the body

Question 18

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

Answer 18

1. Strength
2. Stiffness
3. Lubrication
4. Wear
5. Fatigue.

Question 19

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

Answer 19

1. Biological integration
2. Functional integration.

Question 20

what are the 2 main types of bone ?

Answer 20

1. Compact bone (also called cortical found on the outer layer of bone)
2. Cancellous bone

Question 21

Different areas of loading bearing bones have different arrangements of bone material due to variations in the required functions as shown in the pic below

Describe the arrangement of bone material in end regions of load bearing bones

Answer 21

• End regions are shaped to accommodate the joint - most bones are wider at the ends for this reason
• End regions contain more cancellous bone which is more porous and less stiff (more flexible) than cortical bone, giving desirable shock absorbing properties

Question 22

Describe the arrangement of trabecula within cancellous bone

Answer 22

• They create complex patterns because they are aligned along the directions of greatest stress.
• These directions depend on the directions of the natural loads placed on the bone.

e.g. In the femur there is a considerable bending load on its body because the joint at the head of the femur is displaced laterally from the bone shaft rather than being in line with it; so the trabecular lines are different from those in the tibia, where the load is mainly compressive in a vertical direction

Question 23

Describe the arrangement of bone material in the outer layer of bone directly beneath the articular surface

Answer 23

The region directly beneath the articular surfaces is more dense than the cancellous bone below it, in order to provide a fairly rigid underlying surface for the joint to bear on without causing excessive deformation of the bearing surfaces

Question 24

Describe the arrangement of bone material in the shafts of bones

Answer 24

The shafts of the bones contain dense compact bone, which is much more rigid than cancellous bone and provides the necessary resistance to deformation of the bone as a whole under bending and torsional loads

Question 25

Appreciate - The considerable complexity and variation in the structure of bones, particularly in the region of the joints, makes it practically impossible to design prosthetic components with the same load transfer properties. Any replacement part is at best a compromise and it is a matter of determining which materials and design combinations produce a satisfactory useful life for the component without causing mechanical failure or loosening in the regions of attachment to the bone.

Question 26

State the equation for youngs modulus and what it indicates

Answer 26

E = stress / strain

The youngs modulus indicates the stiffness of a material

Note - A material with a small youngs modulus requires only a small amount of stress to produce a large strain (flexible) and vice versa

Question 27

What happens to the youngs modulus of most structural materials provided they are not deformed near to or beyond their elastic limit ?

Answer 27

Irrespective of the load applied or the rate of loading, Youngs modulus (E) remains constant

Question 28

What are most non-biological structural materials mechanical properties described as being ?

Answer 28

Isotropic - which means that their mechanical properties are the same no matter which direction they are loaded.

Question 29

What type of loading is cortical bone in the diaphysis region of a bone strongest and stiffest ?

Answer 29

Cortical bone is stiffest and strongest when loaded longitudinally, which is of course the main direction in which it is loaded naturally

Note - The shear strength is based on torsional tests, where the bone is twisted about its long axis, so only one figure for shear strength is given.

Question 30

Describe the difference in strength of cortical bone in the diaphysis region compared to metaphysis

Answer 30

Cortical bone in the metaphysis (near the ends, where the bone grows) is only about half as strong as diaphyseal bone

Question 31

What happens to the stiffness of bone as the rate at which it is loaded changes ?

Answer 31

The faster it is loaded the stiffer the material (materials with this property are called viscoelastic) e.g. when trying to shut a door fitted with a „door closure unit‟, which prevents the door slamming as it shuts. The faster you try to push the door shut the more difficult it is to close, i.e. it becomes stiffer.

Recall - Viscoelastic behaviour is time dependent, which means that the response of the material varies according to the length of time that a load is applied and the rate at which a load is applied. When a load is removed from a viscoelastic material it will return to its original size and shape, like an elastic material, however, the response is not immediate.

Question 32

How much stiffer is cortical bone when loaded rapidly compared to slowly ?

Answer 32

20% stiffer

Question 33

How does the ultimate stress of cortical bone differ under different types of loading ?

Answer 33

• Cortical bone is weakest under shear loading, it is then 2x’s as strong under tensile loading and then 3x’s as strong under compressive loading
• Therefore under different types of loading it will have different ultimate strengths (stresses) which is the max stress it will tolerate before failure

Question 34

How does the youngs modulus (stiffness) of cancellous bone compare to that of compact bone?

Answer 34

Varies from a maximum of about 50% of that of cortical bone to less than 0.5%.

Question 35

The wide range of stiffness of cancellous bone as previously mentioned poses a problem for implants ?

Answer 35

The only way to create a replacement material with such a wide range of stiffness would be to give it a mesh-like structure of variable density, like cancellous bone. This is difficult to achieve and poses an increased risk of infection due to the large surface area of material, but also because this type of structure might not provide a firm enough material for attaching or bonding an artificial joint.

It is important to bear in mind that the structural features of bones, have evolved to make the best use of bone as a load bearing material and not for working in conjunction with implants. Once an implant has been joined to bone, the new composite structure behaves differently under load than the bone alone, so the original bone structure may no longer be ideal in its new structural role.

Question 36

Define wolffs law

Answer 36

• Bone is a living material which actively alters its mechanical properties according to the magnitude of the loads placed on it.
• Bone material is laid down in regions where it is required for structural support and removed (resorbed) in regions where it is not needed

Question 37

Describe what stress shielding is

Answer 37

After the insertion of an implant, parts of the bone to which it is attached tend to experience a reduction of loading, the load now being shared between the bone and the implant. This reduction in loading and stressing of the bone is called stress shielding. Bone that is shielded by an implant from taking its full normal load tends to be resorbed

Question 38

What can stress shielding lead to and give an example of implants it may occur in

Answer 38

It may ultimately lead to breakdown of the interface between implant and bone, causing possible loosening and subsequent fracture of the bone.

e.g. when using bone plates in fracture fixation, the plate is fixed to hold broken bones together at a fracture site and therefore provides structural support which relieves the bone around the fracture of some of its load. This causes the bone to resorb, resulting in less bone material, so more load is taken by the bone plate, which in turn causes more resorption. Fortunately, the process slows down so that a substantial proportion of the bone mass remains, but it can reach the stage where there is such a loss of bone strength that the fixation screws loosen, which means that the fixator is no longer effective in supporting the bone

Question 39

With fracture fixation devices, the bone usually heals before the plate loosens or fails. However, in permanent implants, such as joint replacements why is bone reabsoption a serious issue making it essential that excessive stress shielding is avoided?

Answer 39

Because loosening of the prosthesis can lead to the need to remove. Replacement can then prove difficult since some of the original bone is lost because of resorption, so fixation becomes more of a problem. It is essential, therefore, to design implants to avoid excessive stress shielding.

Question 40

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

Answer 40

• Diaphyseal bone is made from compact bone. It is rigid and provides resistance to deformation under loading.
• Bone in the region of a joint is cancellous and the trabeculae are aligned along directions of greatest stress. It is much less rigid than cortical bone and has good shock absorbing properties. Bones are generally wider at the joints than at the diaphyses.

Question 41

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

Answer 41

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

Question 42

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

Answer 42

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