Geometric considerations in hip and knee replacements Flashcards
Why in re-surfacing arthroplasty is femoral head size kept small?
minimise reaming of the acetabulum
What happens when the offset distance is increased?
increasing the offset decreases the joint reaction force
BUT
increases the torque and cantilever forces on the femoral fixation
may lead to apparently lengthening
What are the two main types of wear
mechanical
chemical
What are the 3 subtypes of mechanical wear
abrasive
adhesive
fatigue
What is chemical wear?
corrosion - crevice, pitting
What are the 4 modes of wear?
Mode 1 - between two articulating surfaces as the designer intended
mode 2 - between a bearing surface and a non-bearing surface
mode 3 - result of a 3rd body particle coming between the bearing surfaces
mode 4 - two non-bearing surfaces
How do morse tapers fix?
frictional forces between the two surfaces
What is negative/positive mismatch?
negative - majority of the load is near the top of the trunnion
positive - the majority of the load is near the top of the trunnion
What is fretting corrosion and how does it occur?
wear due to micro movement
parts of the passivation layer abraded off leaving exposed metal. localised oxidation process causes galvanic currents releasing and forming acid molecules.
What is galling?
adhesive wear where softer material gets gouged out and forces lumps - some lumps stick to harder surfaces and may cause jamming
Wear is proportional to what?
friction
Wear rate is proportional to what?
applied load and sliding distance
Will a larger or smaller femoral head generate more wear debris?
larger head generates more wear debris
Is a larger head or smaller head more stable?
smaller is less stable - 28mm often best compromise
Describe fluid film lubrication
needs a hard on hard bearing and sufficient clearance to allow lubrication fluid to be entrained between the bearing surfaces
sufficient speed of motion needed
