Functional Design Factors in Knee Replacements Flashcards
if there are no intact ligaments, which type of prosthesis is used?
hinged
What constraint is offered by a hinged prosthesis?
single axis of rotation with total stability - therefore no need for ligaments
what problem is there with hinge prostheses?
it ha no give under lateral and long axis rotation and transmits high shear forces to implant-cement and cement-bone interfaces
What is performance in hinged prostheses like overall?
not good
Which cruciate is commonly preserved in surgery?
PCL
What advantages are there by retaining the PCL?
proprioceptive feedback
stability on stairs
what are the disadvantages of PCL retention?
constricts dissection of the posterior capsule
encourages the femoral component to slide over the tibia which can increase wear
What happens if the PCL is too tight?
restricted ROM
too much sliding if it is too loose
Why do tibial components in PCL retaining prostheses need to be relatively flat?
to allow the PCL to function properly
What mechanical factors influence the surface shape of a knee prosthesis?
the effect of constraint on load transmission and the generation of high shear stress
the effect of surface contact on wear of the HDP tibial component
the effect of surface contact area on the stresses in the HDP tibial component
Why does the HDP surface become stiffer in time?
gamma radiation sterilisation
oxidation
What problem is encountered if the surface of the HDP stiffness increases?
increases joint contact stress under loading and increases wear
how does fatigue failure occur in HDP?
cyclical loading causes surface delimitation and larger fragments of the material to break off leading to failure.
If there is no loosening or failure of components and no infection, what ultimately causes failure of the prosthesis?
HDP wear
What is the equation for volume of wear?
v = cNs/p
p = hardness of surface
v = A/t v = area of contact/ depth of wear
How is wear decreased in the HDP component?
minimise sliding distance
maximise contact area
What is the solution for HDP wear?
development of a new better material
What kind of loading conditions make a material susceptible to fatigue?
cyclical loading
What 4 factors contribute to the load transfer from the tibial component to the underlying bone?
the thickness of HDP
use of a metal backing plate
use of a tibial component stem
stiffness of the HDP
Why is a thinner HDP component bad?
stresses cannot be distributed evenly in the material
How do tibial components influence ligament tightening?
different thicknesses can alter the tension on the ligaments, and if they are inadequate, a thicker component can be used
Which side of the tibia naturally takes most load?
medial side
how is the metal backing plate held in place?
tibial peg
what is the function of the metal backing plate?
distributes high contact stresses under the condyles to provide even loading to the underlying bone
How does a tibial peg affect the incidence of loosening?
reduces incidence
What happens to total load in the presence of a tibial peg?
the peg relieves the underlying bone about 25% of the total load
How do lateral forces interact with tibial pegs?
lateral reaction force acts to resist the rotating influence of the vertical load by providing lateral resistance
How does stiffness of the HDP component influence contact stress?
higher the stiffness, greater the contact stress.
2x = about 40% increase
How much larger is the medial condyle than the lateral?
1.5mm greater radius
