Implant technology - unit 2 deck 3 Flashcards
State what rigidity and stiffness is and their relationship
- Rigidity of a bar is an indication of its ability to resist axial deformation. Rigidity = EA
- Stiffness of a bar is the force required to produce a unit deflection. k = F/ Δl
Relationship is k = EA/ l (EA = rigidity) so stiffness is equal to rigidity per unit length
Describe the load transfer between the stem component of a hip arthroplasty and the femur, with reference to Huiskes explanation of load transfer in a cemented femoral stem
The stem transfers some of the load proximally and the rest distally to the femur/cement with a central load sharing region
Huiskes:
At the proximal end of the femur, the stem takes all the load, F, but some ofthis is transferred to the bone in the proximal load transfer region. Distally, the rest of the load is transferred to the bone
How does the rigidity (or stiffness as referring to their relationship the more rigid the stem the more stiff) of a femoral stem affect load sharing with the bone/cement?
The more rigid the stem, the more load it takes proximally. This means that a more rigid stem results in more proximal stress shielding of the femur.
In order to reduce stress shielding what can be done ?
Use a less stiff stem
what are the problems with using a less stiff stem component and therefore why a compromise between keeping stress-sheilding low and keeping the proximal interface shear stresses low must be made in prosthesis design?
- Using a lower stiffness stem decreases stress sheilding but increases shear stresses generated at the proximal bone-stem or bone-cement-stem interfaces and in the cement itself.
- These shear stresses can be large enough to shear apart the interlocking grip at the bone-cement and stem-cement interfaces resulting in loosening ==> a compromise must be made.
State the equation used to calculate the load (Lb) taken by the bone in the central load sharing region when a femoral stem is used
Rb = rigidity of bone Rt = rigidity of combined bone-stem composite structure respectively
State the equation used to calculate the load (Ls) taken by the stem in the central load sharing region when a femoral stem is used
Rs = rigidity of stem Rt = rigidity of combined bone-stem composite structure respectively
Consdering the equations used to calculate the load taken by the bone and the stem in the load sharing region, why can the rigidity of the bone cement be excluded ?
. The rigidity of the cement is so low so takes a very small proportion of the load in the load sharing region
Knowing the proportion of load taken by both the bone and the stem in the load sharing region (using the equations), how can we determine how much load is transferred from the stem to the bone at the proximal and distal ends of the stem?
e.g. its calculated that 40% of the total load was taken by the bone in the load sharing region
this means that only 40% of the total load was transferred from the stem to the femur at the proximal end, so the other 60% was transferred distally.
How is the load transferred from the stem to the bone (via the cement if present) and where is this highest and what does its magnitude depend on?
Shear stress - highest at the ends of the stem
Its magnitude depends on the magnitude of the shear force, which in turn depends on how much load is transferred to the bone proximally and distally, e.g. the greater the proximal load transfer from the stem to the bone, the greater the proximal shear stress at the stem-bone interface.
Using a stiffer material to increase the rigidity of the stem results in a lower proximal shear stress (because less load is transferred to the bone proximally) but increases stress shielding of the proximal femur. It also increases the shear stress in the distal load transfer region because more load is transferred distally.
what are isoelastic stems and why are they not viable
stems with the same stiffness as bone
reduce stress shielding but cause high shear stress at bone-stem or bone-cement-stem interface causing implant failure
We cannot easily design a femoral stem which gives both low stress shielding and low interface stresses unless it has what?
a collar to rest on the top of the femur
What is the effect of tapers on shear stresses in the cement layer of cemented stems?
They act to reduce the proximal and distal cement stresses considerably as in a non-tapered stem load is transfered soley as shear stresses proximally and distally, whereas in tapered stems it allows some of the load to be transfered by compression, the greater the taper the greater the compression
[The stresses in the stem are higher than those in most commercial designs because there is less material where it is tapered, but it is still strong enough to take the loads placed on it without fracturing. because stress = F/A]
Explain why the relative rigidities of the stem and femur influence the proximal load transfer stresses.
The relative rigidities of the stem and the femur determine how much load is taken by each in the load sharing region.
A stiff stem takes a large proportion of the total applied load in the load sharing region, so less is transferred to the bone proximally. This means that the proximal shear forces, and hence the shear stresses, associated with load transfer are low.
A low modulus stem takes less load in the load sharing region, so more load is transferred proximally, which means higher shear forces and interface shear stresses.
What are the main disadvantages of using an isoelastic stem?
The proximal shear stresses are too high for cement and hydroxyapatite to withstand so interface bonds fail
what does cement allow for in prostheses
allows good contact between bone and stem, avoiding high stress concentrations. Should allow good contact along the full length of the prosthesis
What does the magnitude of stresses within cement depend on ?
Its thickness and stiffness
What happens if cement is too thin or too thick?
- too thin = very high cement stress and bone reabsorption at proximal end of femur
- too thick = high cement stress
==> cement failure and loosening
what is the optimum cement thickness
layer of 3mm to 7mm proximally
minimum 2mm distally
what is the effect of collar on load transfer
2 sides to the debate; one side thinks they work, the other don’t
However, both collared and collarless stems are used succesfully
For using collar:
- allows compressive transfers load from stem to the bone proximally, helping to reduce stress shielding and proximal interface shear stress
Against using collar:
- collar acts as a pivot, causing fretting wear at the pivot and high stem stresses distally
what is the potential mechanism for cement to fail
- cement has low shear strength and its interface bonds are weak, particularly with the stem
- stem-cement bonds loosens, stresses in cement increases, leads to cracks and eventually cement failure
How can the stem-cement bond be strengthened ?
- by coating stem with PMMA
- by roughening the stem surface
- by applying a porous coating to stem surface
How do cementless stems work and how do they reduce stress shielding ?
They rely on a press-fit which promotes hoop stresses in the bone which reduces stress shielding
What is a general rule of thumb for all stem shapes?
- all are tapered to prevent subsidence
- many have a proximal wedge so that the stem can rest of the bone, allowing transmission of compressive forces as well as shear forces
why is the shape of the stem important in cement-less femoral implants
stem needs to be in contact with a large proportion of the femur
If the outer dimensions of the stem at any point along its length are smaller than the corresponding inner dimensions of the medullary canal, there will be a gap that cannot be filled. (seen in fig.)
Careful stem selection is required to overcome potential shape problems
why are different stem shapes needed for the elderly compared to the young?
cortical bone thins and the canal gradually becomes wider as you age
[particularly in women after the age of 50 to 60]
how can the bending moment be reduced in hip protheses to reduce bending stress in the stem
reduce the offset distance from the head to the neutral axis of the stem
can be done by 2 ways:
- reducing the length of the neck of the stem (pic A)
- increasing the angle between the long axis and the axis of the neck i.e. make it more vertical (pic B)
What is the con of utilising either of the 2 ways of reducing bending moments in hip prostheses and explain how they do this?
They increase the joint reaction force, giving rise to greater wear and acetabular bone-implant stresses
Taking moments about point A on the pelvis;
ΣMA = 0 ==> W.d - J.x = 0
so J = W.d/ x
Moving the line of action of the joint force laterally in order to reduce its lever arm, as shown in Fig. A and B, decreases x so J increases.
What is the equation used to calculate friction force ?
F = μN
F = the friction force (parallel to the surface) required to start the block moving
μ = coefficient of friction N = normal contact force
why do most hip replacements use small heads
interface force for the small head is much less than it would be in a bigger head (refer to pic)
- The friction force (F) is the same in both cases as it is independent of area
- you will see that r1/R1 is much less than r2/R2, so the interface force F1 for the small head is also much less than the force F2.
[It is also important to bear in mind from the formula F = μN above that the frictional force at the joint surface depends on the magnitude of the contact force pressing on the joint, and that this is dependent on the magnitude of the joint reaction force.
Why do many authorities favour a return to a anatomical offset on the stem of hip prosthesis ?
- the formula F = mN above that the frictional force at the joint surface depends on the magnitude of the contact force pressing on the joint, and that this is dependent on the magnitude of the joint reaction force.
- many hip replacements, including the Charnley, are designed with a small offset (lever arm) to reduce the bending moment on the hip.
- The consequence of this is a higher joint reaction force. This means that the joint friction force, F, increases and so too does the interface shear force F1. A higher joint reaction force also increases wear in the joint,
3 factors affect the volume of adhesive wear - what is the formula for calculating abrasive wear which incorportates them?
This implies that the softer material will wear away first (refer to P) and that it will wear away more, the softer it is, the more it is loaded and the more it moves.
how is the sliding distance kept as small as possible?
use a femoral head with a small radius
- wears less than one with a large radius as the bearing surface moves less as slides
Pic shows by rotating an angle (θ), the distance, D, moved by the bearing surface of the larger head is much greater than the distance, d, moved by the smaller head.
What are the 2 disadvantages on wear when using a smaller femoral head ?
- The rate of depth of wear is greater than it would be for a larger head because its contact area is less ==> wear through the bearing surface of the cup quicker. Resulting in lose of the joints ROM as the material wears, because the neck of the stem of the femoral component contacts the cup
- An increased likelihood of dislocation in the post-op period. This is because of the increased likelihood of neck impingement on the edge of the cup.
