Geometric considerations in hip and knee replacments

Question 1

What is the jump distance?

Answer 1

Distance the head has to come to pop out

Question 2

How does the fem head size affect the jump distance?

Answer 2

Bigger fem hea = bigger radius = increased jump distance

Question 3

How should head offset be changed to decrease joint force?

Answer 3

Increase offset to decrease joint force

Question 4

How many modes of wear are there?

Answer 4

4

Question 5

What effects result from increasing the head offset?

Answer 5

Decreases joint reaction force
BUT
Increases the torque forces and cantilver forces on the femoral fixation

Question 6

What will happen if the offset is increased too much?

Answer 6

Apparent lengthening

Question 7

What is mode 1 wear?

Answer 7

Between the 2 surfaces that are articulating together in the manner intended by the designer

Question 8

What is mode 2 wear?

Answer 8

Wear occurs between a bearing surface and a non-bearing surface

Question 9

What is mode 3 wear?

Answer 9

Wear occurs as a result of third body particles coming between bearing surfaces

Question 10

What is mode 4 wear?

Answer 10

Wear occurs between two non-bearing surfaces

Question 11

How does a morse taper stay fixated/

Answer 11

Frictional forces between the 2 surfaces (side loading)

Question 12

Why are morse tapers deliberately deisned with a mismatch?

Answer 12

So designer knows where load is taken

Question 13

For a morse taper to be self locking what angle should it be?

Answer 13

<7 degrees each side

Question 14

If a morse taper angle is >7 degrees each side what does that ean

Answer 14

Self releasing

Question 15

Describe the process of fretting

Answer 15

Wear due to micromovement (mode 4)
Parts of passivating layer abraded off leaving exposed non-oxidised metal
Both exposed metal and particles abraded off become oxidised (oxide is usually harder)
Localised oxidation process causes galvanic currents releasing H ions and HCl formation
Third body wear from oxidised particles - between bearing surface and at fretting surface - false brindelling

Question 16

Describe the process of galling

Answer 16

Adhesive wear
Typically occurs with relatively converging sliding contact (e.g. Morse taper assembly)
Softer material gets gouged out and forms lumps
Some of lumps stick to harder surface
May cause jamming of surfaces

Question 17

With a metal or ceramic on UHMW Polyethylene what/ lubrication will be achieved?

Answer 17

Will only achieve boundary or at best mixed lubrication

Question 18

With a metal and metal and ceramic on ceramic what lubrication can be achieved?

Answer 18

It is possible to achieve fluid film lubrication (but only with large diameter heads/sockets)

Question 19

How are wear and friction related?

Answer 19

Wear is proportional to friction

Question 20

Equation for friction

Answer 20

F = uW

F = Friction 
u = coefficient of friciton 
W = applied load

Question 21

After bedding in wear rate for any bearing couple what is frictional proportional to?

Answer 21

Applied laod and sliding distance

Question 22

How does head size affect wear?

Answer 22

Larger femoral head will generate more wear debris than a smaller heaad

Question 23

What are the pros/cons of a small head?

Answer 23

Pros
- smaller head gives less wear

Cons
- smaller head less stable

Question 24

What size of fem head is often best compromise?

Answer 24

28mm

Question 25

What materials can improve wear characteristics?

Answer 25

Oxidised Zirconium (ceramic properties at the surface)
Highly cross linked polyethylene
Vitamin E-Polyethylene

Question 26

What is the ideal Lambda value in fluid film lubrication?

Answer 26

3

Question 27

What are the requirements for fluid film lubrication?

Answer 27

Hard on hard bearing
Sufficient clearance
Sufficient speed of motion between bearing surfaces

Question 28

WHat can be used to further enhance fludi film lubrication?

Answer 28

Wettable material (such as ceramic)

Question 29

Why must the radius of the femoral head be slightly smaller than the acetabular component?

Answer 29

To leave space for synovial fluid to be entrained

Question 30

What are the problems with a metal on metal bearing?

Answer 30

Metal ion absorption
Impingement
Head/neck ratio
ALVAL (Aseptic Lymphocyte Vasculitis Associated Lesion)
?Reliant on accurate acetabular implantation

Question 31

What are potential problems with ceramic on ceramic?

Answer 31

Potential for breakage
Squeaking?
Problem of revision?

Question 32

What size of femoral head is better for hard on hard bearings? Why?

Answer 32

Larger head size - achieves greater stability and decreased wear

Question 33

What stresses are generated at the interface in a double taper polished stem?

Answer 33

Hoop stresses

Question 34

What are the options for non-cemented fixation?

Answer 34

Porous coated
Hydroxyapatite coated
Porous and hydroxyapatite coated