Implant technology - unit 1 deck 3

Question 1

Recall that the amount of load transfer from bone to implant or vice versa in a load transfer region depends on the relative loads taken by them in the load sharing region; and this in turn depends on their relative rigidities.

In a load sharing region state the relationship used to calculate the load taken by the bone and that of the implant

Answer 1

Question 2

In the load sharing region when an implant is inplace state the equation used to calculate the total load taken by the bone

Answer 2

Question 3

Let us consider load transfer under a bending load.

Using as an example the cross section of the bone in Figure 6 and assuming a perfectly fitting circular stem (i.e. with a diameter of 18 mm), the relative rigidities can be calculated using the formula above for I for circular shafts:

Answer 3

Re-arrange end bit to equal L_BONE = 0.4 L_TOTAL

Question 4

What is meant by rigidity?

Answer 4

Rigidity is the stiffness of the cross section of a material. It is similar to structural stiffness but ignores the length of the structure. It is a particularly useful quantity for calculating the loads taken by a bone and an implant in a load sharing region

Question 5

What are the two factors affecting the rigidity of a structure?

Answer 5

Material stiffness and a geometrical property of the cross section. Axial rigidity = EA; Bending rigidity = EI and Torsional rigidity = GJ.

Question 6

What is the ratio of the bending rigidity of an 11 mm diameter and a 10 mm diameter intramedullary nail given the following information:

both are made from stainless steel both have a wall thickness of 1 mm.

Answer 6

Note that if you refer to page 16 and the pic of the hollow circles they have a thickness of 1mm walls but that equates to a 2mm difference in outer and inner diameter

Question 7

Referring to Figure 2B, will a stiff stem give more or less stress shielding of bone in Region 1, than a less stiff stem? Explain why.

Answer 7

The stiffer stem gives a greater degree of stress shielding in Region 1. This can be explained as follows.

In the load sharing region, Region 2, the stiffer implant takes more load than the less stiff one because it is more rigid relative to bone. This means that less load will be transferred from the stem to the bone in Region 1 for the stiff implant, i.e. the bone in Region 1 takes less load and is therefore more stress shielded.

Question 8

Most orthopaedic implants are designed to be fixed to bone, what is the contact area between the fixator and the bone is known as ?

Answer 8

The bone-implant interface

Question 9

What are the requirements for the bone-implant interface and what is the potential consequence if this is not ensured?

Answer 9

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

Question 10

. Fixation of an implant can be achieved using different methods, what is the main factor determining the method used?

Answer 10

By whether or not the implant is intended to be removed at a later date

Question 11

How are fracture fixators usually held in place and do they end up getting removed or not?

Answer 11

They are mostly intended for temporary implantation and are held in by screws, which can be undone and the fixator removed after the bone has healed

Question 12

How long are joint replacements intended on being inplace and therefore what should be avoided?

Answer 12

They are intended to be implanted permanently so loosening, which is the main cause of failure, must be avoided

Question 13

Screws have a wide variety of uses in the fixation of orthopaedic devices, especially fracture fixators, what is the advantages of using screws over nuts and bolts?

Answer 13

1. Screw attachment requires access from one side of a bone only, whereas a nut and bolt combination must be accessed from both sides of the hole - one for the bolt, the other for the nut ==> Using a bolt, therefore, involves more trauma to tissues
2. The heads of nuts and bolts tend to project more than the head of a screw. This can create practical problems if there is only a small distance from the bone to the skin‟s surface, such as in the case of the interior part of the tibia. Nuts and bolts are used very rarely for attaching implants to bone.

Question 14

What does the success of using interference fit implant fixation rely on ?

Answer 14

This fixation technique requires no specific fixation device but relies instead on a tight contact between implant and bone, surface friction between the two materials preventing movement at the interface. The surface shapes must match well if this method of fixation is to work well, otherwise there is a considerable risk of loosening or of the stem sinking into the bone

Question 15

How is the tight contact between implant and bone achieved when using interference fit implant fixation ?

Answer 15

The dimensions of the inner component are slightly larger than those of the outer component, and the implant is pressed into the bone to lessen the risk of loosening. Using a tapered stem allows it to push outwards on the bone, preventing excessive subsidence if the interference fit is not adequate

Question 16

What happens if the interference fit is made too tight in implant fixation ?

Answer 16

The bone will split, so it is necessary to find a compromise that gives a good fit without causing bone damage.

Question 17

What type of implants is interference fit used for ?

Answer 17

Cementless joint replacements

Question 18

In practice is it possible to provide total surface contact between the implant and bone using interference fit technique and is this necessary?

Answer 18

• No - its not yet possible to make a prosthesis to fit exactly an intramedullary bone canal or to cut out the bone to exact dimensions to provide a predetermined level of interference
• However, total contact is not necessary, as long as there is sufficient contact to provide the required friction to prevent motion..

Question 19

How does bone cement work?

Answer 19

It acts as a filling material, or grout; it is not an adhesive, as the word cement would suggest. It is intended to fill gaps between a bone and an implant, so a perfect bone-implant geometrical match is not required. Once the cement has cured (dried), the bone-implant interface should remain free from motion if the cement has properly filled the gaps between the materials

Question 20

What is bone cement used for ?

Answer 20

It is the most common implant fixation technique used in the stems of joint replacements

Question 21

There are no suitable adhesives for attaching materials to bone - T or F?

Answer 21

True

Question 22

What are biological fixation techniques used in implant fixation?

Answer 22

This technique has been tried for hip joint replacements

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

Question 23

Describe the 2 main porous coatings used in biological implant fixation and the main drawbacks with both techniques

Answer 23

1. Porous bead coating using beads of the same metal as the implant. Pores between the spherical beads allow the bone to grow into the coating. This technique exposes a large surface area of metal making it very vulenrable to corrosion esp service corrosion ==> titanium is used in this technique as it is the least corrosive and most biocompatible of the orthopaedic implant metals.
2. A ceramic, such as hydroxyapatite (HAp) HAp which is deposited directly on to the metal surface using a technique called plasma spray coating. It has not been possible to achieve good long term bonding between the bone and the coating using this technique. After a year or two, sometimes longer, it is noticeable that some of the coating disappears, however short-term the bonding is good. HAp can alternatively be applied to a porous metal coating, such as the titanium beads mentioned above and may produce better long term results.

Question 24

What are the main advantages of using screws instead of nuts and bolts in the fixation of implants to bone?

Answer 24

Screws require access on one side of the bone only and tend to project less than nuts and bolts

Question 25

Why are prostheses stems tapered?

Answer 25

They are tapered so that they cannot subside very far into the bone canal. Subsidence stops because the tapered stem forms a tighter fit in the bone canal as it sinks. A parallel sided stem would continue to subside.

Subsidence = the gradual caving in or sinking of an area of land.

Question 26

What are the two main methods of biological fixation?

Answer 26

Porous beads and ceramic coatings.

Question 27

Define what a biomaterial are

Answer 27

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

Question 28

What are the three important features required of an orthopaedic implant material?

Answer 28

1. A high degree of biocompatibility - i.e. acceptance by the body tissues and body systems
2. Suitable structural (mechanical) properties for the application
3. Ease of manufacture and fabrication of implant devices.

Question 29

Biocompatibility covers all aspects of the interaction between the human body and the implant material, mechanical biocompatibility will not be discussed further as we have discussed stress shielding,

Instead focus on what 2 key factors of biocompatability ?

Answer 29

• The extent to which body fluids and tissues affect a material - this is most likely to be corrosion of the material, which can lead to mechanical failure (Corrosion)
• The extent to which a material adversely affects body tissues - for example its tendency to cause abnormal changes to tissue (such as ulceration, allergy or cancer) or tissue death. (Tissue reaction)

Question 30

Describe what corrosion is and how it occurs

Answer 30

Corrosion is the progressive unwanted removal of material by an electrochemical process. It occurs when two electrodes (solid materials that conduct electricity, usually metals) are immersed in a liquid that conducts electricity (called an electrolyte). The chemical reaction is similar to that which occurs in a battery. An electrical current can flow from one metal to the other through the electrolyte, allowing a chemical reaction to take place between the electrodes and the electrolyte.

This process is called galvanic corrosion.

Question 31

Referring back to what corrosion is and how it occurs, how does corrosion occur in implants i.e. what is the electrode and electrolyte

Answer 31

• In implants the electrodes may be metal or some other conductive material such as the carbon in carbon fibre reinforced plastics.
• The electrolyte is made up of the body fluids, which contain salts of various kinds

Question 32

In implants what is the potential consequence of corrosion occurring

Answer 32

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

Question 33

Why is it generally advisable not to use different implant metals in contact, particularly if one of them is stainless steel?

Answer 33

• Because the corrosive reaction is generally most severe where the electrodes are different metals
• Particularly if one of them is stainless steel, which also corrodes when in contact with carbon fibre reinforced plastics

Question 34

Can corrosion also occur if the two electrodes are of the same metal?

Answer 34

Yes

• If there are differences in material composition due, perhaps, to slight variations in the manufacturing processes of two components. Corrosion can also happen within a single metal component if there are non-homogeneous regions within the material, such as non-uniform distribution of alloy constituents or the inclusion of impurities.
• These regions act as electrodes of slightly different materials, allowing corrosion to occur == manufacturing quality is very important

Question 35

Define what a metal alloy is

Answer 35

The mixing together of certain metals together forming an alloy

Question 36

What is one of the main benefits of metal alloys used in implants ?

Answer 36

They are very resistant to corrosion

Question 37

What are the 3 metal alloys and 1 pure metal which are used in orthopaedic implants due to having acceptable levels of corrosion resistance ?

Answer 37

Alloys:

1. Stainless steel
2. Cobalt chrome
3. Titanium alloys

Pure Titanium

Question 38

What provides the metal alloys and titanium with their corrosion resistant property ?

Answer 38

• A passivation layer of metal oxide that forms the surface of the material when it is exposed to a corrosive environment.
• This layer is resistant to corrosion. The layer itself is a product of corrosion but acts to seal the underlying material from further corrosion