Geometric considerations in hip and knee replacments Flashcards

1
Q

What is the jump distance?

A

Distance the head has to come to pop out

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

How does the fem head size affect the jump distance?

A

Bigger fem hea = bigger radius = increased jump distance

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

How should head offset be changed to decrease joint force?

A

Increase offset to decrease joint force

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

How many modes of wear are there?

A

4

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What effects result from increasing the head offset?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What will happen if the offset is increased too much?

A

Apparent lengthening

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is mode 1 wear?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is mode 2 wear?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is mode 3 wear?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is mode 4 wear?

A

Wear occurs between two non-bearing surfaces

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

How does a morse taper stay fixated/

A

Frictional forces between the 2 surfaces (side loading)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Why are morse tapers deliberately deisned with a mismatch?

A

So designer knows where load is taken

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

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

A

<7 degrees each side

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

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

A

Self releasing

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Describe the process of fretting

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Describe the process of galling

A

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

17
Q

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

A

Will only achieve boundary or at best mixed lubrication

18
Q

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

A

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

19
Q

How are wear and friction related?

A

Wear is proportional to friction

20
Q

Equation for friction

A

F = uW

F = Friction 
u = coefficient of friciton 
W = applied load
21
Q

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

A

Applied laod and sliding distance

22
Q

How does head size affect wear?

A

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

23
Q

What are the pros/cons of a small head?

A

Pros
- smaller head gives less wear

Cons
- smaller head less stable

24
Q

What size of fem head is often best compromise?

A

28mm

25
Q

What materials can improve wear characteristics?

A

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

26
Q

What is the ideal Lambda value in fluid film lubrication?

A

3

27
Q

What are the requirements for fluid film lubrication?

A

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

28
Q

WHat can be used to further enhance fludi film lubrication?

A

Wettable material (such as ceramic)

29
Q

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

A

To leave space for synovial fluid to be entrained

30
Q

What are the problems with a metal on metal bearing?

A

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

31
Q

What are potential problems with ceramic on ceramic?

A

Potential for breakage
Squeaking?
Problem of revision?

32
Q

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

A

Larger head size - achieves greater stability and decreased wear

33
Q

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

A

Hoop stresses

34
Q

What are the options for non-cemented fixation?

A

Porous coated
Hydroxyapatite coated
Porous and hydroxyapatite coated