Implant technology - unit 2 deck 4 Flashcards
what is the acetabular component made of
HDP which +/- a metal backing between it and its interface with cement or bone
new acetabulum are orientated the same as normal acetabulum with a few degrees margin of error - what is this orientation
about 45 degrees relative to the coronal and sagittal planes of the body and facing slightly backwards
[usually acetabulum is sited first by the surgeon and then the femoral component orientated relative to that acetabular position, making allowances for neck length]
what are features of the acetabular design that can be changed and what do they all affect?
- size of femoral head and acetabular cup
- HDP +/- lined with a metal backing plate on its outer surface
- thickness of HDP layer
- outer dimension of the acetabulum
- acetabular cup can be placed more centrally by deepening the original socket
These all affect joint loading and interface stresses
Joint loading and wear is also influenced by the design of the acetabular cup and the contact surface area between the femoral head and the cup - what 2 factors affect this ?
The clearance between femoral head and the cup should be small:
- contact pressure between head and cup increases as diameter of head reduces thus increased diameter head can reduce contact pressure and wear
- Surface contact angle of 120-180 degrees to minimise contact pressure on the cup
The radial clearance = the difference in the radius between the cup and the head, the cup always being slightly bigger.
- If the thickness of the HDP cup is reduced or a stiffer material used, then the radial clearance must also be reduced to spread the point contact load over a greater area in order to avoid excessive contact stress on the HDP
what does the larger outer diameter of the cup matter for the hip prosthesis
- larger the outer diameter of the cup the better its anchoarge to the pelvis
- and the greater the difference between the frictional torque [twisting motion] at the prosthesis-bone or prosthesis-cement-bone interface
acetabular cups vary in diameter from 40mm to 65mm
What are the different ways in which an acetabular cup can be held in place (4)?
- cemented
- fixed with scews
- push fitted
- incorporate a threaded stem which is screwed into a thread cut in the bone
what is the problem with using of threaded cups?
shown to result in bone resorption due to high stress concentrations in the region of the sharp threads
why is a metal backing used in acetabular replacements?
- The cup helps to hold the plastic in place and reduces its tendency to creep and distort, thereby avoiding high contact stresses and focal wear on the HDP that can occur due to reduced head-socket contact area.
- The plate evens out the loading on the acetabulum ==> reduce high stress concentrations
what is the disadv to stiff metal backing?
stiff metal back to the HDP increases head-cup contact pressure
which depends on thickness of the HDP layer; thinner layers give higher contact pressure
what is loosening of the cup of a hip prosthesis due to?
- mechanical overstressing
- biological reaction to the ingress of HDP wear particles leading to resoprtion of the trabecular bone at the bone-cement interface
how can stresses at the bone-implant (acetabular cup-bone) interface be reduced?
- by retaining the subcondral bone (think cortical sandwhich)
- using a thick layer of HDP and a thick layer of cement
- also by using a metal backed cup
These all create a stiffer structure, thereby reducing areas of high contact stress
Which of the 2 causes of cup loosening is probably the main cause and describe the stages of it occuring
biological reaction to the ingress of HDP wear particles
- HDP wear particles come into contact with bone
- This causes tissue reaction and bone reabsorption
- HDP wear particles migrate from the rim of the cup along the bone interface,
- causing progressive absorption over a number of years of the trabecular bone at the bone-cement interface
- The integrity of the bone cement interface is lost and a fibrous membrane forms between the materials in the gap where the bone is resorbed
This is becoming a major problem in implanted prostheses after 15 years
what happens in the region where the fibrous membrane forms in cup loosening ?
cement fractures as a secondary effect due to high stress conc associated with area of bone loss
[cementless cups are equally prone to loosening via HDP wear particles]
if the acetabular cup is moved nearer to the mid-line in results in lower loads at the joint hip - why is this technique not utilised in practice
- Its effect is probably marginal
- In order to move the cup more centrally the strong cortical bone below the joint surface of the acetabulum [subchondral bone plate] has to be breached which means the prosthesis is mounted on softer and weaker bone. The acetabulum is generally deepened only sufficiently to remove any residual articular cartilage.
- Deepened cup leads to earlier impingement of the femoral neck on the rim of the acetabulum, limiting motion and increasing the risk of the head being prised from the socket and dislocating
Name three important features in the design of modern acetabular cups.
- The thickness of the HDP layer
- The size of the head and cup
- Radial clearance between the head and cup
- The bearing surface materials
- The method of cup fixation to the bone
