Implant technology - unit 2 deck 2

Question 1

Along with a compressive stress what does the joint force acting on the normal hip also produce ?

Answer 1

A bending stress

Question 2

Why does the joint force acting on the normal hip produce a bending stress ?

Answer 2

Because the direction of the joint force vector is not along the neutral axis so the femur is subjected to a bending moment and therefore a bending stress

Question 3

State the equation used for calculating bending stress

Answer 3

• M = the applied bending moment
• y = the distance from the neutral axis to the section of interest
• I = the second moment of area.

Question 4

State how to calculate the bending moment

Answer 4

It is equal to the product of the applied force and the distance from its line of action to the neutral axis

Question 5

What is the assumption made when calculating the joint force at the hip

Answer 5

assumption that the only active muscle was the abductor group joining the greater trochanter to the pelvic

[the force required by this muscle group was found to be about twice body weight]

Question 6

What does the bending moment produce on the femur

Answer 6

tension on the lateral side

compression on the medial side

Question 7

What is the effect of inserting a femoral stem on bending stresses

Answer 7

To reduce the stresses in the proximal end of the femur because the stem takes some of the bending load from the bone.

Question 8

What is required in order to keep the femoral stem in a hip prosthesis in static equilibrium?

Answer 8

The applied load due to the joint force must be balanced by reaction forces due to contact between the stem and the femur.

Fig. A shows that the proximal area on the medial side of the femur provides one main contact point and the lateral distal side provides another

Question 9

What is the purpose of the contact points of the femoral stem with the femur ?

Answer 9

They counteract the tendency for the stem to rotate due to the bending action of the joint force.

Question 10

What is the maximum bending moment on the femoral stem in a hip prosthesis equal to ?

Answer 10

The applied joint force, J, multiplied by its moment arm, d

Question 11

Moving down from proximal to the distal end of a femoral stem what happens to the bending moment ?

Answer 11

From point A It decreases down to 0 at the distal end

Question 12

What does bending stress in the femoral stem vary according to ?

Answer 12

It varies along the length of the stem

Question 13

Why are modern femoral stems much stronger than older stems?

Answer 13

Because they are forged rather than cast

Question 14

What is the main cause of femoral stem failure ?

Answer 14

• Is if it loosens proximally
• In which case the bending moment at the distal end increases drastically and failure can occur.

Question 15

Why is the femoral stem more highly stressed compared to the adjacent bone to it ?

Answer 15

Because its value of I for the stem at any point along the stem is smaller than that of the adjacent bone (because its cross sectional dimensions are smaller)

Question 16

What does the magnitude of the bending moment and hence bending stress on the femoral stem depend on ?

Answer 16

The magnitude and direction of the joint force and the abductor muscle force. This depends on the type of activity being undertaken and the angular position of the thigh relative to the pelvis.

Question 17

Why should a substaintial proportion of the load in a hip prosthesis be transferred from the bone to the stem proximally?

Answer 17

In order to prevent stress shielding at the proximal end of the femur and to ensure the stem takes less load distally and is therefore less stressed

Question 18

What design features of a femoral stem help ensure that the stem does not fail under a bending load?

Answer 18

• By designing it with a large enough second moment of area
• By designing its shape to limit the magnitude of the bending moment due to the joint force

Question 19

What are the design features of a femoral stem which help avoid loosening under a bending load?

Answer 19

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

Question 20

What design feature of a femoral stem helps reduce stress shielding of the bone under bending loads?

Answer 20

• by selecting a suitable rigidity for the stem.

Question 21

What are the three quantities that influence the maximum bending stress in a structure?

Answer 21

• Bending moment
• Second moment area
• distance from the neutral axis

Question 22

How does the presence of a femoral stem affect the magnitude of the bending stresses in the femur?

Answer 22

The stresses are lower because the stem takes some of the load, which means that the bone is less stressed

Question 23

What other stresses are generated under the action of a bending load ?

Answer 23

Radial and circumferential (hoop) stresses

Question 24

Define what a radial stress is and state where it is greatest

Answer 24

Radial stresses are stresses that are directed radially outwards from a central point

They are greatest at the points of bone-stem contact at the proximal and distal ends and are less in between.

Question 25

What do radial stresses cause ?

Answer 25

Hoop stresses in the bone which are primarily tensile stresses that act in a direction that tends to split the bone

Question 26

In Figure 10A, which represents a cementless prosthesis, points A and B have the highest radial stress. These stresses cause tensile hoop stresses around the circumference. In Figure 10B the stem has a loose fit - what happens if a stem has a loose fit in the bone?

Answer 26

It gives rise to high local stresses resulting in hoop stresses that are high enough to fracture the bone.

Question 27

Why are stems of short length are prone to cause high radial stresses on the bone.?

Answer 27

Because radial stresses are inversely proportional to the square of the length of contact, L, of the stem with the bone.

(L shown in pic)

Question 28

How are excessive radial stresses and therefore excessive hoop stresses avoided ?

Answer 28

• by ensuring that the stem is long enough
• by providing a good fit of the stem in the medullary cavity

Question 29

Under what circumstances does the femur experience high hoop stresses due to the presence of a femoral stem?

Answer 29

• When a loose prosthesis experiences a bending load
• When a tapered stem is loaded axially and presses against the femur.
• When an oversized component is pushed into a cavity, like forcing a large nail into a bamboo cane - there is a tendency to split the cane.

Question 30

How do torsional stresses arise in the femur in hip joint arthroplasty?

Answer 30

• Torsional loads occur when one end of the femur rotates axially with respect to the other.
• Restraining the ankle while the upper body is rotated, creates large torsional stresses that can tear the menisci of the knee. These torsional loads can also be transferred to the thigh and hip.

Question 31

What can the shear force generated by a torsional load result in ?

Answer 31

1. It is transferred across the stem-bone or stem- cement-bone interface which can lead to loosening and rotating of the stem relative to the bone
2. Once loosening has occurred there is also the possibility of the stem sinking under the action of a compressive load

Question 32

What design features help reduce torsional loads on femoral stems ?

Answer 32

• Use of non circular sections to help resistance to rotational shear forces
• the shear strength of cement, if used
• good bonding at the bone-cement and cement-implant interfaces
• surface treatments of the stem to improve interface bonding.

Question 33

Why is it desirable to use non-circular sections for the stem of a femoral component?

Answer 33

To reduce the rotational shear stresses as some can be taken as compressive stress

Question 34

Describe the bone structure which comprises the acetabulum

Answer 34

It can be considered to be a sandwich of cancellous bone between two layers of cortical bone - one covered with articular cartilage forming the joint bearing surface

Question 35

Describe the relationship of the diameter of the acetabulum and the femur and the shape of the femur

Answer 35

• Acetabulum diameter > head of femur diameter
• Head of femur is approx spherical

Question 36

What loading is the acetabulum placed under?

Answer 36

Compressive load ==> compressive stress, due to joint force (femoral head pressing into the acetabulum)

Question 37

What do the cortical shells of the acetabulum play an important role in ?

Answer 37

Load bearing

Question 38

In hip arthroplasty the replacement femoral head and cup usually have a smaller diameter than the natural components what does this result in ?

Answer 38

Higher stress concentrations due to the contact area being smaller (stress = F/A)

Question 39

What are important design factors for hip prosthesis in regards to stress of the acetabulum

Answer 39

• the size and conformity of the replacement joint surfaces - these affect contact areas and hence contact stresses
• ways to maintain the integrity of subchondral cortical bone
• the stiffness and thickness of the cup
• the thickness of the cement layer, if present
• 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 40

What is used if cement is not used to keep the stem in place

Answer 40

1. press fit
2. rely on bone to grow into it, unless it is screwed in place

[use of screws not generally acceptable due to high stress conc at bone-screw interface leading to bone reabsorption and screw loosening]

Question 41

What is bone cement made of and how is it formed ?

Answer 41

PMMA

1. Consists of a powder which is mixed with a volatile (rapidly evaporating) agent containing a catalyst
2. The cement is mixed to a doughy consistency and remains plastic long enough for it to be inserted into the appropriate cavity and for the prosthetic component to be pressed into place.
3. Additives, such as antibiotics and radio-opaque material are added

Question 42

How does bone cement work?

Answer 42

• It acts as a filler, or grout, not an adhesive
• Interlocking bond can form between the cement and the small trabeculae of the cancellous bone in the femur and on microscopic marks on the metal of the prosthesis
• These bonds help to resist shear and tensile forces.

Question 43

How can prosthesis components be adapted to provide greater resistance to shear forces when using bone cement ?

Answer 43

1. The surface of a prosthesis component, such as a femoral stem, can also be roughened, or coated with beads or wires to provide a larger surface area for better keying of the cement
2. prosthesis components are coated with a layer of PMMA cement during manufaturing, cement filler inserted during surgery will adhere to the implant, forming a stronger bond than it would with untreated metal

Question 44

What are advantages of cemented prostheses?

Answer 44

• • surfaces of bone and prostheses do not have to be an exact fit because any gaps can be filled with cement e.g. in the femoral stem you dont need to ream or rasp the medullary cavity then (this is the main advantage over cementless)
• • cement fills all gaps between the bone and prostheses, allowing an even stress distribution thus preventing high stress concentrations

Question 45

How should cement be inserted into the femoral canal and why?

Answer 45

Under high pressure to ensure it fills all the gaps

Question 46

What are the disadvantages of using bone cement ?

Answer 46

reaction that takes place to form cement polymer is exothermic, the high temps produced are enough to destory surrounding body tissue

Small cement fragments produce intense inflammatory reactions leading to bone reasorption. There is always some monomer left after the chemical reaction and this is much more toxic than the polymer.

fragments of cement that fall into replacement joints are known to be responsible for increasing surface wear

Bone cement has a low shear and tensile strength (strongest under compression), it is not possible to kept it under compression and tensile stresses due to femur being stresses tensile on its lateral surface. Because cement does not bond chemically to the metal or bone, any tensile loading will tend to part the cement from its contact with the bone and stem. Which can lead to cement fatigue failure and eventual loosening, the main cause of prosthesis failure.

Question 47

What is the only way to avoid micro-motion under tension in cement ?

Answer 47

To provide a true chemical bond between the bone and the prosthesis, which PMMA bone cement cannot do.

[Even pre-coating a femoral stem with cement at best allows a bond between the cement and the stem, but cannot fix the cement to the bone]

Question 48

Has any cementless hip prosthesis shown itself to be better than the charnley cemented low friction arthroplasty?

Answer 48

No

Question 49

Cement does not bond chemically to bone, or to implants unless they are coated w/ a cement layer - true or false

Answer 49

true

Question 50

Describe the non-bonded cement interface problem (so talking about cemented prosthesis)

Answer 50

1. Even if two components are well interlocked there is always a small amount of relative motion between them when they are loaded cyclically and this leads to rubbing, resulting in wear particles being released into the tissues.
2. which can lead to bone resorption ==> loosening

Question 51

. Even in cementless prostheses there will be a significant amount of abrasive wear taking place over a period of years as the bone rubs against the metal - true or false?

Answer 51

True

Question 52

What happens is there is not good bonding between bone and metal or cement?

Answer 52

Layer of fibrous tissue forms which prevent proper ingrowth of bone into hydroxyapatite coated prostheses and has no shear or tensile strength

Question 53

What is done in order to minimise the growth of the fibrous layer

Answer 53

Micromotion should be kept to a minimum after surgery, which means an accurate fit of the prosthesis at surgery followed by controlled weightbearing.

Question 54

What are the ways to improve the longevity of cemented prosthesis ?

Answer 54

1. increase the keying of the cement to the prosthesis - the prosthesis surface can be roughened, or coated with beads or wires
2. coat metal components with PMMA, which can then bond with the inserted cement, ensuring an intimate fit and improved resistance to motion.
3. combine a PMMA surface coating on the prosthesis with a cement that bone can bond to i.e. using a cement containing hydroxyapatite
4. give up altogether on the aim of achieving a metal-cement bond and instead make the stem smooth and allow it to sink down the canal until it forms an interface fit in the cement mantle e.g. exeter hip

Question 55

What is the difference between a monomer and a polymer?

Answer 55

• A monomer is a short molecular chain version of a polymer.
• Monomers react chemically when activated to join together to produce polymers. The types of long molecular chains produced give rise to different mechanical properties.

Question 56

what is the benefit of coating a prosthesis component w/ PMMA during manufacturing

Answer 56

To provide the best possible opportunity for the PMMA inserted during surgery to bond to the stem.

Question 57

State 2 advatnages of using a cemented as opposed to a non-cemented prosthesis

Answer 57

In cemented prostheses, the shape of the stem is not critical as the cement will fill any gaps between the stem and the bone; cement inserted properly will fill all the gaps, providing a even distribution of contact stresses at the interfaces.

Question 58

State 2 problems associated with using PMMA bone cement

Answer 58

PMMA fragments cause an adverse tissue reaction, which can lead to bone resorption; PMMA; the cement is not an adhesive and therefore cannot resist tensile forces.

Question 59

Why is a non-bonded cement interface of concern ?

Answer 59

Because cement particles will be released into the tissues due to abrasive wear; because the prosthesis may loosen.