Implant technology Unit 2

Question 1

Hip joint arthroplasty aims to give the patient pain free movement and stability. What 2 possible solutions are there to achieve this?

Answer 1

Replacing the bearing surfaces alone - this has so far proven unsatisfactory

Replacing the surfaces and some surrounding bone in order to gain a good prosthesis fixation

Question 2

Describe how the surfaces and surrounding bone is replaced.

Answer 2

The femoral head is anchored to the femur by a metal stem inserted into the medullary cavity of the femur. The new acetabular cup is made to fit into the existing socket, after reaming. The femoral and acetabular components are held together by either bone cement or by direct “cement-less” contact between the prostheses and bone

Question 3

What was the first recorded successful hip replacement?

Answer 3

A hemi-arthroplasty replacing the femoral head with steel, termed “Thomson arthroplasty.” It was held in place by a “press fit” - long stem of prostheses was driven down into the femoral canal

Question 4

What developments were made to hip arthroplasty during the 1950s?

Answer 4

McKee and Farrar developed a total hip arthroplasty prostheses involving two metal bearings. It ultimately would fail due to high levels of friction.

In France, the Judet brothers were the first to use PMMA in an arthroplasty.

Success was still rare - it wasn’t until the 60s that a breakthrough was made

Question 5

John Charnley developed a “low friction” total joint arthroplasty in the 1960s. What are the key principles of this prostheses which are still used today?

Answer 5

He designed a smaller femoral head to reduce problems of loosening associated with bearing friction.

He introduced a concept of a third filling material (bone cement) between the bone and prostheses.

He introduced high density polyethylene (HDP) as a bearing material which in combination with a metal femoral head and body lubrication, results in a low friction bearing.

He was the first to produce instruments to match the prostheses to help with the surgery

Question 6

What is the general criteria for hip joint replacements?

Answer 6

It should be tolerated within the human body with no short term and little long term risk of adverse effects such as carcinogenesis.

It should give pain relief and restore the activities of daily living to the patient.

It should last a reasonable length of time which ideally should exceed the life span of the individual patient.

It should be inserted by a competent surgeon such that a predictable outcome can be reasonably guaranteed.

It should be of acceptable cost bearing in mind the relative cost of hospital stay and the economy of the individual and their country

Question 7

What are most hip joint replacements made of?

Answer 7

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

Question 8

The essential functional range of movement of the hip is not as much than the full anatomical range. What is required for normal standing, walking and sitting down?

Answer 8

Extended slightly
Flex to a minimum of 30 degrees
Abduct when weight bearing
Rotate when in full extension

Question 9

What methods can be used to analyse the stresses acting on the hip structure - both normal hips and prostheses?

Answer 9

Traditional method of measuring them

Finite element method - 2D and 3D models of the structure made up of small elements can be made

Question 10

The load transfer mechanisms in normal and replacement hips are quite different. Two key points to remember are; Joint loading varies according to the physical activity being undertaken and The magnitude of muscle forces for different activities cannot be determined accurately. How would the following rank in terms of joint reaction force; Walking, rising from a chair, descending stairs and ascending stairs?

Answer 10

Ascending stairs - 7.2
Walking - 5.1
Descending stairs - 5.0
Rising from a chair - 3.0

Question 11

How many groups of muscles and ligaments cross the hip joint?

Answer 11

7 - combinations of these are active at any one time to balance external forces and moments acting at the joint, so that equilibrium can be maintained

Question 12

Why is the combination of muscles and ligaments known as indeterminate?

Answer 12

The forces acting on the femur and pelvis and across the joint cannot be precisely calculated and so must be approximated

Question 13

How can the compressive stress be calculated at a section of the bone?

Answer 13

By dividing the compressive force component of the hip joint force by the cross sectional area of the bone

Question 14

How does the stresses generated in a hip prostheses differ to normal bone?

Answer 14

The compressive joint force is transferred from the stem to the femur as a shear force. If the stem-bone bond or stem-cement-bone bond is not sufficiently strong, the prostheses will loosen and sink down the medullary cavity

Question 15

How can the stem be prevented from sinking distally in the medullary cavity when under an applied load?

Answer 15

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 16

How can the interface shear stresses be reduced by converting shear loads to compressive loads?

Answer 16

By using a support, such as a proximal collar on the stem

By tapering the stem

Question 17

How can fracture of the stem be avoided?

Answer 17

By selecting a stem with a sufficiently large cross sectional area to resist stresses
By selecting a high strength material for the stem

Question 18

Another important consideration is that the bone forms a structural composite with the stem so that there are important load transfer and load sharing mechanisms in play. If there is excessive stress shielding by the stem, bone will be resorbed with possible loosening of the implant. what considerations for the device help reduce this risk?

Answer 18

To avoid excessive stress shielding of the bone, the careful selection of a stem with the correct level of rigidity under axial loading is important

Question 19

What is the formula to calculate the bending stress in the femur?

Answer 19

The applied bending moment multiplied by the distance from the neutral axis to the section of interest, divided by the second moment of area

Question 20

What typical pattern arises in the bending stress distribution along the stem of a hip prostheses?

Answer 20

Pear-shape pattern

Question 21

How is the joint force balanced to prevent rotation in the prosthetic hip joint due to a moment arm?

Answer 21

Reaction forces at the proximal medial side of the stem and the distal lateral side of the stem

Question 22

How does the value of I for the stem compare to the adjacent bone?

Answer 22

It is smaller (because its cross sectional dimensions are smaller) and so it is more highly stressed

Question 23

How does the bending stress vary with different activities?

Answer 23

The bending moment (and hence bending stress) depends on the magnitude and direction of the joint force and abductor muscle force. As the direction moves away from the midline of the stem, the stress will increase

Question 24

How is it ensured that the stem doesn’t fail under a bending load?

Answer 24

By designing it with a large second moment of area

By designing its shape to limit the magnitude of the bending moment due to the joint force

Question 25

How can loosening of the stem be avoided?

Answer 25

By providing a sufficiently strong bond between the bone and the stem or cement
By providing a good press fit of the stem in the medullary canal

Question 26

How can stress shielding of the bone under bending loads be minimised?

Answer 26

By selecting a suitable rigidity of the stem

Question 27

Radial and circumferential (hoop) stresses are also generated under the action of a bending load. Where are radial stresses at their greatest?

Answer 27

Radial stresses are greatest at the points of bone-stem contact at the proximal and distal ends and are less in between these points

Question 28

Radial stresses in turn cause hoop stresses in the bone. In what way do these act?

Answer 28

They act primarily in a tensile manner in a direction that tends to split the bone

Question 29

What is the relationship between radial stresses and the length of contact between the stem and the bone?

Answer 29

The radial stresses are inversely proportional to the square of the length of contact of the stem with the bone. This means that stems of short length are prone to cause high radial stresses

Question 30

How can excessive hoop stresses be avoided?

Answer 30

By ensuring that the stem is long enough

By providing a good fit of the stem in the medullary cavity

Question 31

How do torsional loads of the femur arise?

Answer 31

When one end of the femur rotates axially with respect to the other

Question 32

How can rotational shear stress be reduced?

Answer 32

The use of non-circular sections to help resistance to rotational shear forces
The shear strength of the cement
Good bonding at the bone-cement and cement-implant interfaces
Surface treatments of the stem to improve interface bonding

Question 33

How should the replacement acetabulum be designed to minimise unwanted stresses?

Answer 33

The size and conformity of the replacement joint surface is important
The integrity of cortical bone should be maintained
Stiffness and thickness of cup
Thickness of the cement layer
Whether or not to use a cup with a metal backing plate
The technique used to fix the cup to the remaining acetabular bone

Question 34

What is bone cement made of?

Answer 34

Polymethyl methacrylate (PMMA)

Question 35

PMMA is a polymer. What does it consist of?

Answer 35

The monomer consists of a powder which is mixed with a volatile (rapidly evaporating) agent containing a catalyst

Question 36

PMMA cement is not an adhesive. How does it bond with the bone and prostheses?

Answer 36

Physical interlocking bond can form between the cement and the small trabeculae of the cancellous bone in the femur and on microscopic scratches and scores of machining and polishing marks on the metal of the implant. These bonds help resist shear and tensile forces. The surface of the prosthesis component can also be roughened or coated with beads or wires to provide a large surface area for better keying of the cement

Question 37

What are the advantages of cement prostheses?

Answer 37

The mating surfaces with bone do not have to be an exact fit

Cement fills all the gaps between bone and implant, allowing even stress distribution and preventing high stress concentrations associated with small bone-implant contact areas

Question 38

What are the problems of using cement?

Answer 38

Chemical reaction taking place to form the cement polymer is exothermic. As it sets, the temperatures are high enough to destroy body tissue next to the cement

Small fragments of cement can induce inflammatory reactions

Tensile loading of the cement can part it from the bone and repetitive loading of this kind can result in cement fatigue failure and eventual loosening

Question 39

What should be done in order to minimise the formation of a fibrous layer at the interface after surgery?

Answer 39

Micro motion should be kept to a minimum , which means an accurate fit of the prosthesis at surgery followed by controlled weight bearing. This approach is particularly important if the bone is to be encouraged to grow into hydroxyapatite coatings

Question 40

What is done to try and improve the longevity of cemented prostheses?

Answer 40

Using techniques to increase the keying of the cement to the prosthesis

Coating metal components with PMMA

Question 41

How is the load transferred from the stem to the femur?

Answer 41

Some is transferred proximal and the rest distally, with a central region of load sharing. A more rigid stem will take more load proximally. This results in proximal stress shielding of the femur.

Question 42

Load is transferred from the stem to the bone proximally and distally with a central load sharing region. In a more rigid stem, is more transferred proximally or distally? What impact does this have on the femur?

Answer 42

A more rigid stem will take more load proximally and transfer distally. This means there is more stress shielding of the femur proximally. A solution to this problem would be to use a less rigid material for the stem

Question 43

What problems can arise when a low stiffness stem is used?

Answer 43

Although it reduces stress shielding of the femur proximally, it does cause other problems. High shear stresses are created at the proximal stem-bone interface and can be large enough to shear them apart. The result will be loosening of the implant. It is therefore important to strike a balance between stress shielding and shear stress when designing the implant and choosing the material

Question 44

What equations are used to give the load of the bone and the load of the stem proximally?

Answer 44

Lb = Rb/Rt
Ls = Rs/Rt

Where Lb is load of bone, Ls is load of stem and R is the rigidities of bone, stem and total.

Knowing the proportion of the load taken by both the bone and the stem in the load sharing region, we can easily determine the load transferred at the proximal and distal ends of the stem

Question 45

What type of force transfers the load from the stem to the bone?

Answer 45

A shear force - the shear is highest at either end of the stem where the load is being transferred with little in the load sharing region

Question 46

What is the problem in using isoelastic stems?

Answer 46

Although they are great in terms of stress-shielding being low, they are susceptible to failure in the interlocking at the interface due to a high shear force proximally

Question 47

What is the recommended stem shape in a cemented prosthesis?

Answer 47

It has two tapers - one at each end. This acts to reduce the proximal and distal cement stresses considerably

Question 48

What effect does cement have on load transfer?

Answer 48

Cement allows good contact between bone and stem which avoids high stress concentrations. Cement is introduced under pressure and the use of centralising devices for locating the stem within the femur allow a good contact to be achieved along the full length of the prosthesis

Question 49

What problems can arise if the cement layer is too thin?

Answer 49

Very high cement stresses and bone resorption at the proximal end of the femur - especially when less than 2mm thick. The resorption is believed to be due to cement debris from the fractured cement layer causing an adverse tissue reaction

Question 50

What problems can arise if the cement layer is too thick?

Answer 50

High cement stresses

Question 51

What is the optimum cement layer thickness both proximally and distally?

Answer 51

3-7mm proximally and minimum of 2mm distally

Question 52

What stiffness of cement is preferred?

Answer 52

A stiffer cement results in higher proximal and distal cement and interface stresses and so a more flexible cement is preferred

Question 53

What effect can a collar have on load transfer?

Answer 53

There are varying views. Some say that a collar allows compressive load transfer from the stem to the bone which reduces stress shielding and lowers the stresses in the cement in the proximal medial region. The calcar must be cut accurately to ensure that the collar rests on a substantial part of the bone

Other argue that the collar-calcar contact acts as a pivot about which the stem rotates, leaving the distal end of the stem prone to high stress concentrations and may fail.

Both methods are used

Question 54

How does the cement-less press fit reduce stress shielding?

Answer 54

It promotes hoop stresses in the bone which reduce the stress shielding

Question 55

What are cement-less stems coated with?

Answer 55

Hydroxyapatite - some are coated all over which helps bone ingrowth and potentially eliminates metal debris due to bone-metal abrasion. However, fully coated stems promote stress shielding so a balance must be found

Question 56

The bending moment can be reduced by reducing the offset distance between the head and the neutral axis. What 2 ways can this be done in?

Answer 56

By reducing the length of the neck of the stem

By increasing the angle between the long axis and the axis of the neck

However modern materials are stronger and can withstand higher bending moments and so these are less so required

Question 57

What are two essential requirements for the bearing surfaces of a replacement hip joint?

Answer 57

Low contact friction between them and the materials must wear as slowly as possible

Question 58

What is the equation for friction force?

Answer 58

F = in where u is the coefficient of friction and N is the normal force which is perpendicular to the frictional force

Question 59

The friction coefficient is specific to the materials involved.

Answer 59

Synovial joints have around 3-100 times less friction than the metal bearing and HDP (high molecular weight polyethylene) combination used in replacement joints

Question 60

What consequences are there of the replacement having a higher contact friction than the normal joint?

Answer 60

A much higher shear force is transmitted to the acetabulum which can be high enough to loosen it at the fixation interface

Question 61

Why is a smaller head used than a larger one?

Answer 61

The friction force is the same in both cases as it is independent of the area of contact. A smaller head therefore results in a smaller interface force which is why they are preferred to larger heads

Question 62

What are the two main types of wear between bearing surfaces?

Answer 62

Adhesive wear and abrasive wear

Question 63

What is adhesive wear?

Answer 63

It occurs when two bearing surfaces stick together when they are pressed together and one, usually the softer one, is torn off by the harder one. Bearing surfaces should therefore be made up of materials that have a low level of adhesion. Lubricants provide a layer between the two materials which reduces the wear

Question 64

What is abrasive wear?

Answer 64

It occurs when the surfaces are not perfectly smooth. Surfaces should be highly polished so to avoid abrasive wear. The entry of particles from outside the bearing can also cause wear. Good circulation of lubricant is important so that wear particles can be removed and not rub against the bearing surfaces, causing more wear

Question 65

What formula represents the volume of wear?

Answer 65

v = (c.N.s)/p

Where v = volume of wear, c = a constant (coefficient of wear), N = the applied load across the bearing surfaces, s = the distance the bearing slides and p = the hardness of the surface being worn

Question 66

HDP wear particles can migrate far from the joint and have been found in the distal end of a cement-less implant. What effects can these wear particles have?

Answer 66

They can cause intense inflammatory tissue reactions. It is one of the major problems associated with aseptic loosening of prosthetic components. The local inflammation can eventually lead to resorption and prosthesis loosening

Question 67

How can wear be reduced?

Answer 67

By reducing the normal force (loading of the joint). This can be done by altering the stem neck angle to reduce the joint reaction force.

Keeping the sliding distance as small as possible. This is achieved by using a smaller prosthetic femoral head.

Finding alternative materials which are more wear resistant and don’t have any adverse, toxic effects

Question 68

What is the disadvantage of a smaller prosthetic femoral head?

Answer 68

The rate of depth of wear is greater than for a larger head because its contact area is less. Although relatively few HDP cups wear through because there is a failure elsewhere in the prosthesis before this, the joint does lose its range of motion as the material wears. This is because the neck of the stem contacts the cup. To reduce this, the diameter of the neck is reduced in order to maintain the required range of motion for longer in smaller diameters of femoral heads.

Another issue is that there is an increased risk of dislocation in the post-op period because of the increased likelihood of neck impingement on the edge of the cup

Question 69

What is the acetabular component usually made of?

Answer 69

HDP which may or may not have a metal backing between it and its interface with cement or bone

Question 70

What is the normal acetabular orientation?

Answer 70

About 45 degrees relative to coronal and sagittal planes and faces slightly backwards

Question 71

What is the recommended contact angle to minimise contact pressure on the cup?

Answer 71

120 degrees

Question 72

What is the radial clearance between the head and the cup?

Answer 72

The difference in the radius between the cup and the head, the cup always being slightly bigger. If the thickness of the HDP cup is reduced or a stiffer material used, then the radial clearance must also be reduced to spread the point contact load over a greater area in order to avoid excessive contact stress on the HDP

Question 73

What are the issues for threading cups to keep the cup in place?

Answer 73

They often result in resorption at the high stress concentrations in the region of the sharp threads

Question 74

Pros and cons of using a metal backing for the cup?

Answer 74

It holds the plastic in place and reduces the creep and distort = less high contact stresses and focal wear

However, a stiff metal back increases the head-cup contact pressure

Question 75

What can loosening of the cup be due to?

Answer 75

Mechanical overstressing

Biological reaction to the ingress of HDP wear particles which ultimately lead to resorption of the trabecular bone at the bone-cement interface. Fibrous membrane forms in the gap where the bone is resorbed. Cement fracture in this region may be secondary to high stress concentrations associated with areas of bone loss

Question 76

If the cup is moved more centrally towards the midline of the boy, the distance x increases between the joint and femur which results in a lower load at the hip joint. Why is this not utilised in practice?

Answer 76

In order to move the cup centrally, very strong cortical bone needs to be breached which means the prosthesis in mounted on softer and weaker bone.

The deepened cup leads to earlier impingement, limiting the range of motion and increasing the risk of dislocation

Question 77

What is the average failure rate of hip replacements?

Answer 77

10% before 10 years and 1% of the remainder per year

Question 78

Most prostheses loosen before they wear out due to wear particles and the role of bacteria and infection. What is the strategy for dealing with failed prostheses?

Answer 78

Failed prostheses is removed as well as all surrounding inflammatory tissue, infection is then excluded and a new prostheses is fitted either immediately or once inflammatory and infective issues are dealt with. It is termed revision arthroplasty and if it is done in one step it is known as exchange arthroplasty