Implant technology - unit 2 deck 1

Question 1

What does a hip joint arthroplasty involve and what is the aim of doing one?

Answer 1

The replacement of the damaged bearing surfaces of the femoral head and the acetabulum. In order to give the patient a new joint which permits pain free motion and a stable hip.

Question 2

What are the 2 main methods a hip joint arthroplasty could be done and which is the perferred method ?

Answer 2

1. Replace the bearing surfaces alone - results in unsatisfactory prosthesis fixation
2. Replace the bearing surfaces and some underlying bone - this is the peferred method because it ensures good prothesis fixation (commerical hip joints ustilise this one)

Question 3

Describe commerical hip joint arthroplastys used nowadays

Answer 3

• The femoral head is anchored to the femur by a metal stem inserted into the femoral medullary cavity.
• The new acetabular cup is made to fit into the exisiting one after reaming.

The femoral and acetabular components are then held in place using either bone cement or by direct cementless contact between the prosthesis and the bone.

(using cementless requires cutting a reciprocal shape into the bone into which the prosthesis is hammered or screwed into.

Question 4

Why is there tremendous scope for improvement in the development of joint prostheses ?

Answer 4

Because no prosthesis behaves like a normal joint

Question 5

Attempts to replace damaged joints began as soon as it was possible to do what?

Answer 5

Provide safe and prolonged anaesthesia

Question 6

Why did early joint replacements fail and what due to?

Answer 6

• Surgeons didnt appreciate the large loads placed across joints
• Didnt consider toxicity or immunology

All efforts failed due to wear, rejection and high infection rates

Question 7

What method did early joint replacements focus on and specifically within hip joint replacements what was done?

Answer 7

Early attempts concentrated on replacing the surfaces of joints

Most efforts concentrated on the hip and materials were regularly placed between the bearing surfaces known as inter-position arthroplasty

Question 8

What was the first recorded successful hip replacement and state why?

Answer 8

A hemi-arthroplasty known as the ‘Thompson’ arthroplasty

Because it relied on a press-fit as it involved replacing the femoral head and having a long stem driven down the femoral canal

Question 9

State what a ‘press-fit’ joint replacement is and what is relies on

Answer 9

• A press-fit is when an object of a particular shape is pushed into another object of the same shape but slightly smaller.
• The then relies on close surface contact between the stem and bone - there are no screws or other devices for fixation

Question 10

Define what is meant by a total arthroplasty

Answer 10

This is when both bearing surfaces are replaced

Question 11

What design features make the Charnley hip replacement the ‘gold-standard’ ?

Answer 11

• A smaller femoral head to reduce problems of loosening associated with bearing surfaces
• Introduced bone cement (made from PMMA) to help distribute loads between bone and prosthesis
• Introduced high density polyethylene (HDP) as a bearing material which in combo with a metal femoral head (stainless steel or cobalt chrome) and lubrication by the body fluids, results in a low friction bearing
• Produced a system of instrumentation for inserting and aligning his prosthesis

Question 12

Why can a whole prosthesis not be made of ceramic ?

Answer 12

Because although the have favourable frictional and wear properties they are brittle and subject to sudden failure

Question 13

What are the benefits of using metals compared to ceramics?

However what are the problems with using metals?

Answer 13

The are less brittle than ceramics but are very stiff and give rise of stress sheilding

Question 14

What is one way of providing a less stiff metal prosthesis and therefore reduce stress sheilding?

Give an example of a hip prosthesis which utilises this

Answer 14

• Using composite plastic materials e.g. carbon fibre reinforced polymers
• The Bombelli hip which is made from a metal core which gives it strength and a carbon fibre reinforced outer layer

Question 15

What are the problems with using the Bombelli hip prosthesis and lowering the stiffness of the stem used ? (by using the Bombelli)

Answer 15

Lowering the stiffness creates high shear stresses as the load is transferred from the stem to the femur

Question 16

Why is getting new licensed bone cement into commercial use difficult

Answer 16

Because it is classified as a drug and therefore, huge cost to get onto market

Question 17

List some of the constraints when designing an implant

Answer 17

• Small scope of materials tolerated within the body
• The need to withstand large forces
• The discrepancy between highly structured, heterogeneous bone and homogeneous implant materials

Question 18

Name the most commonly used implant materials

Answer 18

• Cobalt chrome alloy
• Stainless steel
• Titanium and titanium alloys
• High density polyethylene (HDP)
• Polymethylmethacrylate (PMMA) bone cement

Question 19

What are the general criteria for hip joint replacements?

Answer 19

• It should be tolerated within the human body with no short term and little long term risk of adverse toxic effects such as carcinogenesis (cancer inducing).
• It should give pain relief and restore the ADL’s to the patient.
• Last a reasonable length of time, ideally exceeding the expected life span of the individual patient without the need for revision.
• Should be able to be inserted by an average surgeon with a predictable outcome guaranteed
• Should be cost effective

Question 20

What are most hip joints made from ?

Answer 20

Cobalt chrome or titanium, which are fairly corrosion resistant, and high density polyethylene (HDP), which provides a good bearing surface although it does give undesirable tissue reaction when fragmented.

No better alternatives

Question 21

What is the single most effective long term way of relieving pain and restoring function to the hip joint suffering from OA?

Answer 21

Hip arthroplasty

Question 22

What is the essential functional range of movement of the hip required for daily living to allow standing, walking and sitting?

Answer 22

• Extend slightly
• Flex to a min of 30 degrees
• Abduct when weight bearing
• Rotate when in full extension.

Question 23

Excluding the use of hip prostheses in young persons what percentage of hip replacements are expected to last 10 years?

Answer 23

90%

Question 24

What is an advantage of cemented total hip arthroplasty makes it easier to ensure predictable outcomes even when inserted by a surgeon of average ability?

Answer 24

Only an approximate reciprocal shape of the medullary cavity has to be achieved as the remaining space is taken up by PMMA which acts as a grout or filler between bone and prosthesis. For the average surgeon this is well within his abilities

Question 25

What biomechanical and biocompatibility factors must be considered when designing orthopaedic implants ?

Answer 25

Load support mechanisms in relation to stress shielding of bone and bone-implant fixation techniques biocompatibility.

Question 26

What are the forces acting on the normal hip due to ?

Answer 26

• External loads
• The muscle forces acting at the hip

Question 27

What is the importance of knowing the forces acting on the normal hip ?

Answer 27

The stresses can then be calculated and used to design a replacement which can withstand the stresses without failing.

Question 28

What are the 2 ways of estimating the stresses acting at the hip joint and describe them?

Answer 28

1. Fixing strain gauges at important locations on the bone, which is then loaded. The stress is calculated from the strain, knowing Youngs modulus for the strain gauge material
2. Finite element analysis - involves creating two dimensional or three dimensional models of the structure made up of small elements applying joint and muscle loads to the model and letting the computer calculate the stresses.

Question 29

Which is the better method for calculating the stresses acting at the hip joint - using strain guages or finite element analysis ?

Answer 29

• Finite element analysis because it allows stresses to be determined with relative ease for different prostheses under different loading conditions. Designs can be compared and regions of high stress identified.
• Whereas using strain guages is very time consuming process and requires many strain gauges in order to understand how stresses are distributed in the bone and implant

Question 30

What is the main drawback of all methods of calculating stresses ?

Answer 30

They are only estimates because the material properties of bone and the bone-implant interface properties are variable and cannot be determined accurately.

Question 31

What 2 other factors are important to remember which make calculating stresses on the hip only estimates ?

Answer 31

1. Joint loading varies according to the physical activity being undertaken.
2. The magnitude of muscle forces for different activities cannot be determined accurately.

Question 32

Why do the joint forces acting at a joint vary in magnitude and direction according to the activity being undertaken?

Answer 32

because the hip joint has such a wide range of movement

Question 33

Why cant the magnitude of muscle forces for different activities cannot be determined accurately at the hip and what is the name given to the hip because you cannot calculate the muscle forces?

Answer 33

• Because there are 7 groups of muscles and ligaments that act across the hip joint and different combinations of these are active at any one time to balance the external forces to maintain equilibrium
• It is not possible to calculate the muscle forces if more than one muscle is active, because there would be more unknown forces than equations to solve them. The structure is therefore called indeterminate.

Question 34

When the hip is described as indeterminate what does this mean ?

Answer 34

That the forces acting on the femur and the pelvis, and across the joint cannot be calculated precisely and must be approximated.

Question 35

What activity is commonly used when analysing the stresses acting at the hip joint and give 2 reasons why?

Answer 35

Standing on one leg - either stationary or during walking gait cycle

Because the muscle forces can be estimated with some degree of confidence as only the abductor muscle forces need to be calculated as the other muscles are not active. Also it is believed to generate high bending stresses in the femur and femoral component of the prosthesis.

Question 36

Give two reasons why it is so difficult to determine accurately the stresses in the components of a replacement hip.

Answer 36

• Because bone is an anisotropic material and its exact mechanical properties are difficult to determine.
• Because it is difficult to know the true forces acting on the prosthesis due to lack of knowledge about which muscles are active during a particular activity.

Question 37

Which of these activities generates the largest and smallest joint reaction force: walking, ascending stairs, rising from a chair, descending stairs?

Answer 37

1. Ascending stairs
2. Walking
3. Descending stairs
4. Rising from a chair

Question 38

Define what is meant by an indeterminate structure

Answer 38

One where are more unknown forces than there are equations to solve them. The forces can sometimes be estimated quite well by making certain assumptions about the likely proportions that each muscle will contribute, such as minimising total muscle energy consumption. This requires some rather complicated mathematics, called optimisation methods.

Question 39

In what plane does the highest moments occur at the hip joint ?

Answer 39

The coronal plane

Question 40

How does compressive stress arise in the femur ?

Answer 40

The hip joint force, J, has a component, F_C, which causes a compressive force in the femur, giving rise to a compressive stress.

Question 41

State the equation to calculate the compressive stress at any section in the bone

Answer 41

The compressive stress, σ_c, can be calculated by dividing the compressive force (F_c), by the cross sectional area of the bone (A)

Question 42

Along with the hip joint force what also affects the compressive stresses on the hip?

Answer 42

The pull of the muscles particularly at the trochanter, and the head and the neck of the femur

Question 43

How is the compressive joint force in the hip transferred to the prosthesis ?

Answer 43

It is transferred from the stem to the femur as a shear force

• Passing directly from the stem to the bone in a cementless prosthesis
• Or via the cement layer in a cemented prosthesis, causing shear stresses in the cement

Question 44

What will happen if the stem bone bond or stem-cement-bone bond in a hip prosthesis is not sufficiently strong?

Answer 44

The prosthesis will loosen and sink down the medullary cavity

Question 45

How can the compressive stress in the stem of a hip prosthesis be found?

Answer 45

By dividing the compressive load taken by the stem at any section along its length by the area of that cross section.

Question 46

What does the compressive load taken by the stem of a hip prosthesis vary due to ?

Answer 46

It varies along the length of the stem

Question 47

What design features help prevent the stem of hip prosthesis from sinking distally in the medullary canal?

Answer 47

• by tapering the stem
• by using a collar at the proximal end of the stem
• by fixing the bone to the stem, by means of bone ingrowth or adhesion
• by using a cement strong enough to withstand the shear stresses

Question 48

what are design features of hip prosthesis help to reduce interface shear stress by converting shear loads to compressive loads?

Answer 48

• by using a support, such as a proximal collar on the stem
• by tapering the stem

Question 49

what are design features of hip prosthesis help avoid fractures of the stem?

Answer 49

• by selecting a stem with a sufficiently large cross section to resist the stresses
• by selecting a high strength material for the stem.

Question 50

How is excessive stress shielding of the bone in hip prostheses avoided and why is this important?

Answer 50

• by careful selection of the rigidity of the stem under axial loading.

The bone forms a structural composite with the stem so there are important load transfer and load sharing mechanisms in play which affect the proportion of the total load carried by each component. If there is excessive stress shielding by the stem, this will result in bone resorption and possible loosening of the implant.