hip joint replacement Flashcards
what are the 2 methods for holding the femoral and acetabular components in place
bone cement or cementless contact - the latter involves cutting a reciprocal shape in the bone and hammering the prosthesis (or occasionally screwing it) in
what does ‘press fit’ rely on
close surface contact between the stem and the bone
what does bone cement help to do
it helps to distribute loads between bone and prosthesis
give a pro and a con to using ceramics
good frictional and wear properties
brittle dan subject to sudden failure
what characteristic of metals gives rise to stress shielding
very stiff relative to bone
most commonly used implant materials
cobalt chrome alloy stainless steel titanium and titanium alloy high density polyethylene (HDP) polymethylmethacrylate (PMMA) bone cement
HDP provides a good _____?
bearing surface
disadvantage of HDP
undesirable tissue reaction when fragmented
For daily living, what must the hip be able to do
extend slightly
flex to at least 30 degrees
abduct when weight bearing
rotate when in full extension
what part of the pelvis is the normal weight bearing part when sitting
ischial part of the pelvis
how many groups of muscles and ligaments act across the hip joint?
7
why is the hip joint termed indeterminate?
more than one muscle being active at one time means that forces acting on the femur and the pelvis and across the joint cannot be calculated precisely and must be approximated
why is standing on one leg commonly used for analysis of stresses in hip prosthesis
muscle forces can be estimated to some degree of confidence because some muscles aren’t active at all
also its believed to generate high bending stress in the femur
highest hip moments occur in which plane?
coronal
how can we calculate compressive stress at a section of the bone
compressive force component divided by cross sectional area of the bone
describe 4 ways of preventing a stem from sinking into the medullary canal
tapering the stem
proximal collar
fixing the bone to the stem (bone ingrowth / adhesion)
using cement strong enough to withstand the shear stress
2 ways of reducing stem implant interface shear stresses
proximal collar
tapering
- both allow some of the load to be taken in compression
why does the hip produce a bending stress on the femur
the joint force vector is not along the neutral axis
how to calculate bending stress
My / I
M = bending moment, y = distance from neutral axis, I = second moment of area
how does the presence of a femoral stem affect the magnitude of the bending stresses in the femur?
the stem takes some of the bending load, reducing the stresses in the proximal end of the femur
what is a radial stress and where are they greatest
stresses directed radially outward from a central point
greatest at the points of bone stem contact
what do radial stresses cause in turn
hoop stresses
what type of stress are hoop stresses (primarily)
tensile
under what circumstances does the femur experience high hoop stresses due to the presence of a femoral stem
- when a loose prosthesis experiences a bending load
- when a tapered stem is loaded axially and presses against the femur
how to avoid excessive hoop stresses
ensuring the stem is long enough
providing a good fit of the stem in the medullary cavity
why is it beneficial to use non-circular section for a femoral stem
helps resist rotational shear forces
why does it cause a problem if the subchondral cortical bone is broken during a joint replacement
below the cortical bone is cancellous bone which is normally not highly stressed and if the cortical bone is broken, the load will have to be taken by the underlying cancellous bone
why does there tend to be higher stress concentrations in contact regions between replacement head and cup, compared to natural hip joint
the replacement head and cup usually have a smaller diameter»_space; contact area smaller»_space; higher stress concentrations
what is more toxic, cement monomer or polymer?
monomer
what happens as the cement polymer sets that can damage body tissues?
exothermic reaction and a lot of heat is generated
can PMMA bone cement bond chemically to bone and the prosthesis?
no
why is their a lot of interest in hydroxyapatite coated cementless prostheses
they can bond chemically to bone (hydroxyapatite is a natural constituent of bone)
what is there a tendency for developing at the metal or cement interface with the bone and what can be done to minimise it
fibrous tissue forming creating a layer with no shear or tensile strength
to minimise: micro motion should be kept to a minimum after surgery (» accurate fit at surgery followed by controlled weight bearing)
when is it particularly important to prevent the production of a fibrous layer
when hydroxyapatite coating is being used as the layer prevents proper ingrowth of bone into hydroxyapatite coated prostheses
advantages of a cemented prosthesis
stem doesn’t need to be an exact fit
cement will fill all the gaps so there will be an even stress distribution
what are the problems with PMMA cement
PMMA fragments cause adverse tissue reactions which can lead to bone resorption
the cement does not form an adhesive bone so it cant resist tensile forces
what is the main disadvantage of isoelastic stems
proximal shear stresses too high»_space; interface bonds fail
what is the downside of more load being transferred proximally from the stem to the bone
higher shear forces and interface shear stresses generated
why is thickness of the cement layer important and what is the ideal thickness
- too thin a cement layer (< 2mm) can lead to bone resorption due to cement debris from fractured cement layer causing adverse tissue reaction
- too thick and too thin can both cause high cement stresses
- ideally between 3 and 7 mm proximally and minimum of 2 mm distally
effect of a proximal collar on load transfer from the stem to the femur
the collar transfers load from the stem to the bone proximally, helping to reduce stress shielding and proximal interface shear stresses
whats the main argument against using a collar
it acts as a pivot which causes fretting wear at the pivot and high stem stresses distally
advantage of coating the whole stem with hydroxyapatite
helps bone ingrowth
what is the disadvantage of fully coating a stem with hydroxyapatite
promotes stress shielding of the bone
how does a tapered wedge help proximal load transfer
by transferring much of the load to the bone in compression rather than in shear
- name 2 ways that the neck of a femoral stem can be altered to reduce the bending moment of the stem
- how do these modifications affect hip joint force
- shortening the neck
- increasing the angle of the neck (making it more vertical)
- both of these modifications increase the joint reaction force»_space; increases wear
how does femoral canal geometry change with age
cortical bone thins»_space; the canal becomes wider
why is cobalt chrome preferred to stainless steel or titanium
cobalt chrome and titanium have better corrosion resistance than stainless steel
titanium has poor wear properties
» cobalt chrome is the best option
have ceramics got good corrosion resistance?
yes, excellent
what are the 2 main types of wear that occur at bearing surfaces
abrasive and adhesive
why does adhesive wear occur
the 2 bearing surfaces stick together when they’re pressed together and one (usually the softer one) is torn off by the harder one
why is HDP used as a bearing surface in replacement joints
it has low coefficient of friction with metals
disadvantage of using HDP as a bearing surface
fragments of HDP produce adverse tissue reaction that leads to bone resporption
main disadvantages of using a smaller diameter head
- rate of depth of wear is greater
- increased chance of dislocation in post-op period due to higher chance of neck impingement on the edge of the cup
important features in the design of the acetabular cup
thickness of HDP layer
lining with metal backing plate
outer dimension of the acetabulum
what is radial clearance
difference in radius between the cup and the head
when must radial clearance reduce
stiffer material used or thickness of the HDP cup is reduced
allowing contact load to be spread over a greater area
why is there a minimum recommended thickness of HDP cup
to avoid excessive bearing contact stresses
advantage of using a metal backing plate on the cup
helps hold the plastic in place and prevents creep and distortion
what are the 3 steps that lead to acetabular component loosening due to HDP fragments
- ingress of HDP at bone interface
- bone resorption
- migration of HDP along the interface until loosening occurs
methods of reducing adhesive wear
- reduce joint loading
- keep sliding distance as small as possible i.e. femoral head with small radius
- find alternative materials for the head that reduce wear in the HDP i.e. ceramics such as alumni or zirconia
what are alumni and zirconia
ceramic metals
why is centralisation the cup not common practice despite it lowering the load at the hip joint?
the effect is probably marginal and there are two major disadvantages
- to move the cup more centrally the strong cortical bone would have to be breached and the prosthesis would therefore be mounted on much weaker bone
- would result in earlier impingement of the neck on the rim of the acetabulum which limits motion and increases the risk of dislocation
