Geometric considerations in hip and knee replacments Flashcards
What is the jump distance?
Distance the head has to come to pop out
How does the fem head size affect the jump distance?
Bigger fem hea = bigger radius = increased jump distance
How should head offset be changed to decrease joint force?
Increase offset to decrease joint force
How many modes of wear are there?
4
What effects result from increasing the head offset?
Decreases joint reaction force
BUT
Increases the torque forces and cantilver forces on the femoral fixation
What will happen if the offset is increased too much?
Apparent lengthening
What is mode 1 wear?
Between the 2 surfaces that are articulating together in the manner intended by the designer
What is mode 2 wear?
Wear occurs between a bearing surface and a non-bearing surface
What is mode 3 wear?
Wear occurs as a result of third body particles coming between bearing surfaces
What is mode 4 wear?
Wear occurs between two non-bearing surfaces
How does a morse taper stay fixated/
Frictional forces between the 2 surfaces (side loading)
Why are morse tapers deliberately deisned with a mismatch?
So designer knows where load is taken
For a morse taper to be self locking what angle should it be?
<7 degrees each side
If a morse taper angle is >7 degrees each side what does that ean
Self releasing
Describe the process of fretting
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
Describe the process of galling
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
With a metal or ceramic on UHMW Polyethylene what/ lubrication will be achieved?
Will only achieve boundary or at best mixed lubrication
With a metal and metal and ceramic on ceramic what lubrication can be achieved?
It is possible to achieve fluid film lubrication (but only with large diameter heads/sockets)
How are wear and friction related?
Wear is proportional to friction
Equation for friction
F = uW
F = Friction u = coefficient of friciton W = applied load
After bedding in wear rate for any bearing couple what is frictional proportional to?
Applied laod and sliding distance
How does head size affect wear?
Larger femoral head will generate more wear debris than a smaller heaad
What are the pros/cons of a small head?
Pros
- smaller head gives less wear
Cons
- smaller head less stable
What size of fem head is often best compromise?
28mm
