Design of Joint Surfaces Flashcards
What 2 factors is the frictional force dependent on?
The surface properties of the 2 materials in contact (coefficient of friction)
the magnitude of the load pressing them together
Why is HDP used over other materials even tho they have a lower coefficient of friction?
Other materials are either toxic or have poorer wear properties
What is a bad consequence of replacement joints having a higher contact friction than the normal joint?
Much higher shear force transmitted to the acetabulum (can cause loosening at fixation interface)
Why do most hip replacements nowadays use small heads?
Less interface force
How does the frictional force related to the are of contact?
Frictional force is independent of the area of contact
What are the 2 main types of joint wear?
Abrasive wear
Adhesive wear
What is adhesive wear?
When 2 bearing surfaces stick to each other when they are pressed together and one, usually the softer one, is torn off by the harder one
How can adhesive wear be reduced?
Bearing surfaces should be made of materials that have a low level of adhesion
Lubrication
What causes abrasive wear?
When surfaces are not perfectly smooth
Why is a good circulation of lubricant important in terms of abrasive wear?
to remove wear particles
Equation to calculate volume of wear
V = c.N.s / p
v = volume of wear c = coefficient of wear N = applied load across the bearing surfaces s = distance the bearing slides p = hardness of the surface being worn
What problems can wear particles cause in a replacement joint?
Cause intense inflammatory tissue reactions
Cause aseptic loosening
What is the process by which wear particles cause aseptic loosening?
Particles cause adverse tissue reactions
Results in local inflammation
Over a number of years bone resorption and loosening
What are the 3 methods for reducing wear?
Reduce loading o the joint
Keep sliding distance as small as poss
find alternative materials (reduce coefficient of wear)
Why does a femoral head with a small radius wear less than a head with a large radius?
Bearing surface moves less as it slides
Which materials have been found to reduce HDP wear?
Ceramics
Examples of ceramics
Alumina
Zirconia
What are the disadvantages of alumina?
More brittle and prone to fracture than metals
What are the advantages of Zirconia over Alumina?
Tougher
More fracture resistant
Harder
More scratch resistant
What are the disadvantages of a smaller diameter head?
Greater rate of depth of wear
Increased likelihood of dislocation in post op period
Why is the rate of depth of wear greater for a smaller head?
Contact area is less
Why is the increased rate of depth of wear often not an issue?
Prosthesis fails for other reasons first
What problem is caused as the head wears into the cup?
Reduced range of motion (it is necessary to reduce the diameter of the neck as the head diameter is reduced to maintain motion)
What are two advantages of small diameter head?
reduce cup-bone interface shear stresses (lessens risk of loosening)
Produce less volume of HDP wear particles
