Load transfer mechanisms in the stem Flashcards
How is load transferred in an intramedullary stem?
Stem transfers some of its load to the bone proximally, the rest distally with a central region of load sharing
What is the advantage of a low stiffness stem?
Reduces stress shielding
What is the disadvantage of a low stiffness stem?
High shear stresses at the proximal bone-stem interface or in the cement or at the bone-cement-stem interface
Why is cement not included in load sharing calculations?
Because its rigidity is low can be assumed it takes a very small proportion of the load
How is load transferred from the stem to the bone?
As a shear force
What is an isoelastic stem?
Stem with the same stiffness as bone
Why are isoelastic stems not viable?
Better in terms of stress shielding but shear stresses or the interfaces arising from high proximal load transfer can be high enough to cause failure
What are the two ways of potentially varying the rigidity of a stem?
Using a material which has varying stiffness along its length
Making the stem hollow
What are the magnitude of stresses in the cement dependent on?
Thickness of cement
Stiffness of cement
What are the problems if the cement layer is too thin?
Very high cement stresses
Bone resorption at proximal end of the femur due to cement debris)
What are the problems if the cement layer is too thick?
High cement stresses
Why is a more flexible cement preferred?
Stiffer cement results in higher proximal and distal cement and interface stresses
Why does Harris argue that a proximal collar is good?
Allows compressive load transfer from the stem to the bone - reduces stress sheilding - lowers the stresses in the cement in the proximal medial region
What is the calcar?
Thick cortical part of femoral neck
How should a proximal collar be fitted?
Cut the calcar very accurately to ensure that the collar rests on a substantial part of the bone
Why does Ling argue that proximal collars are bad?
Collar-calcar contact area acts as a pivot about which the stem rotates - distal end prone to high stress conc and may fail particularly if cement mantle is deficient
Also debris due to fretting of the collar against the cement or bone
How is the stem-cement bond strengthened in practice?
Coating the stem with PMMA
What does Ling argue about the cement-metal interface?
Should be free to slide - as it interface is weak so there is a risk of bone reaction to wear particles
What does Harris argue about bonding stems?
A collar should be used to prevent unnecessary sinkage - also argues that the bone-cement interface is usually well interlocked so there is no need to the stem-cement interface to have to slip to protect it
How does a press fit reduce stress shielding?
By promoting hoop stresses in the bone
Why are modern cementless stems coated with hydroxyapatite?
Help bone ingrowth
Potentially eliminates metal debris from bone-metal abrasion
What is the downside of fully HAp coated stems?
Promote stress shielding of the bone
What issue doe sthe lack of distal contact in cementless stems cause?
Thigh pain
How is thigh pain treated in cementless stems?
Custom made plastic sleeves (used to provide good distal contact)
