Biomechanics of normal & replacement hip joint

Question 1

What is “press-fit”?

Answer 1

Long stem driven down the femoral canal (relies on close surface contact - no screws or other devices used)

Question 2

What were the 4 major contributions of Charnley to hip joint replacement?

Answer 2

Smaller head (reduce problems of loosening asoc eith bearing friction)
Introduced bone cement (to help distribute loads)
Introduced HDP as bearing material (low friction)
Produced a system of instrumentation

Question 3

What are the advantages and disadvantages of using a ceramic as a bearing material?

Answer 3

Adv - frictional and wear properties

Disadv - brittle, subject to sudden failure

Question 4

What is the single most effective way of relieving pain and restoring function in sufferers of arthritis of the hip?

Answer 4

Hip arthroplasty

Question 5

What are the essential functional movements of the hip?

Answer 5

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

Question 6

What are the two methods of estimating the stresses on the hip joint?

Answer 6

Measuring - strain gauges (use E to calculate stress)

Finite Element Analysis

Question 7

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

Answer 7

7

Question 8

Why is the hip joint indeterminate as a structure?

Answer 8

Not possible to calculate muscle forces if more than one muscle is active - must be approximated

Question 9

In which plane of the hip joint do the highest moments occur?

Answer 9

Coronal plane

Question 10

how can the compressive stress be calculated at any point in the femur?

Answer 10

Divide compressive force component by cross-sec area

Question 11

How is force transmitted from the stem ot the femur?

Answer 11

As a shear force

Question 12

What 4 methods exist for preventing the stem from sinking distally in the medullary canal?

Answer 12

Tapering the stem
Using a proximal collar
Fixing the bone ot the stem (bone ingrowth or adhesion)
Using a cement strong enough to withstand the shear stress

Question 13

What are the 2 ways of reducing interface shear stresses by converting shear loads to compressive loads?

Answer 13

Using a support (e.g. proximal collar)

Tapering the stem

Question 14

What are the 2 ways of avoiding fracture of the stem?

Answer 14

Use a stem with sufficiently large cross-sec to resist the stresses
Use high strength material for the stem

Question 15

What equation is used to calculate bending stress?

Answer 15

Ϭ = My / I

M = applied bending moment (
y = distance from neutral axis to section of interest
I -= second moment of area

Question 16

When calculating the hip joint force which group of muscles are presumed only to be acting?

Answer 16

Abductor

Question 17

What is the force required from the abductor muscle group to maintain equilibrium?

Answer 17

2 x body weight

Question 18

In terms of bending stress where is failure most likely to occur/

Answer 18

If stem loosens proximally - beinding moment at distal end increases - failure can occur

Question 19

Why is the value for I for the stem at any point along the stem smaller than that of adjacent bone?

Answer 19

Because its cross-sectional dimensions are smaller

Question 20

What are the 2 design factors to ensure that the stem does not fail under a bending load?

Answer 20

Designing it with a large enough seond moment of area

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

Question 21

What are radial stresses?

Answer 21

Stresses that are directed radially outwards from a central point

Question 22

Where are radial stresses in the femur greatest?

Answer 22

At the points of bone-stem contact and the proximal and distl ends

Question 23

What are hoop stresses?

Answer 23

Primarily tensile stresses that act in a direction that tends to split the bone

Question 24

How does radial stress relate to the length of contact of the stem with the bone?

Answer 24

Inversely proportional to the square of the length of contact (stems of short length are prone to cause higher radial stresses in the bone)

Question 25

What are the 2 design factors to avoid excessive hoop stresses?

Answer 25

Ensuring stem is long enough

Provide good fit of the stem in the medullary cavity

Question 26

What are the four important design factors in reducing torsional stress?

Answer 26

Use non-circular sections to help resistance to rotational +shear forces
Shear strength of cement (if used)
Good bonding at bone-cement and cement-implant interfaces
Surface treatments of the stem to improve interface bonding

Question 27

Which type of stress is the acetabulum subject to?

Answer 27

Compressive

Question 28

Describe the structure of the acetabulum

Answer 28

Sandwich of cancellous bone between 2 layers of cortical bone
Lightweight structure with good rigidity under bending load
Cortical shells highly stressed under compressive load

Question 29

Why do replacement femoral head and cup tend to have higher stress concs in the regions of contact?

Answer 29

They have a smaller diameter than the natural components which means stress concs are higher as contact area is smaller