Unit 2: Hip joint replacement Flashcards
what is a “press fit” anchorage?
A press fit is obtained when an object of a particular shape is pushed into another object of the same shape but slightly smaller.
Name the most commonly used implant materials.
Cobalt chrome alloy, stainless steel, titanium and titanium alloys, high density polyethylene (HDP), polymethylmethacrylate (PMMA) bone cement.
What were the important contributions to joint replacement design made by John Charnley?
the low friction hip,
- small femoral head to reduce interface shear forces between the acetabular cup and the adjacent bone
- a low friction metal-HDP bearing
- bone cement.
- special instrumentation for inserting and aligning the prostheses.
What materials are most hip joint replacements made out of?
cobalt chrome
titanium
high-density polyethylene
Give a positive and a negative of high density polyethylene
good bearing surface
fragments to cause a tissue reaction
What type and range of motion are required from the hip in order to be able to stand, walk and sit down?
Slight extension
flexion to a minimum of 30 degrees
abduction during weight bearing
the ability to rotate when in full extension
What are the forces on a normal hip primarily due to?
external loads and muscle forces
How can stresses on the normal hip be estimated?
measure strain with gauges and use young’s modulus to find out the stress
finite element analysis
Give two reasons why it is so difficult to determine accurately the stresses in the components of a replacement hip.
- bone is an anisotropic material and its exact mechanical properties are difficult to determine.
- it is difficult to know the true forces acting on the prosthesis due to lack of knowledge about which muscles are active during a particular activity.
Which of these activities generates the largest and smallest joint reaction force: walking, ascending stairs, rising from a chair, descending stairs?
highest first: ascending stairs, walking, descending stairs, rising from a chair.
What is meant by an indeterminate structure?
more unknown forces than there are equations to solve them. The forces can sometimes be estimated quite well by making certain assumptions about the likely proportions that each muscle will contribute, such as minimising total muscle energy consumption. This requires some rather complicated mathematics, called optimisation methods
in which plane do the highest moments occur in the femur?
coronal
How does the diagonal hip joint force cause compressive stress?
Fc component causes compressive force
How can compressive stress be calculated in the femur?
compressive force / cross-sectional area
What affects calculating compressive stress in the femur?
pull of muscles
What does the compressive joint force become in a prosthetic hip joint?
transfers from stem to femur as a shear force
What happens if the bond between the bone and stem is not strong?
the prosthesis loosens and sinks down the medullary cavity
Why doesn’t it matter if compressive stresses cant be accurately calculated in a prosthesis stem?
stems dont fail in compression
What design considerations are important for a prosthetic hip joint stem under the action of a compressive axial load?
prevent sinking
reduce shear by converting it to compression
avoid stem fracture
avoid excessive stress shielding
allow bone to form structural composite with the stem
Name 4 ways of preventing a stem from sinking into the medullary canal under the action of a compressive axial load
- tapering the stem
- using a proximal collar
- bonding the stem to the bone
- strong cement
Name two ways of reducing stem implant interface shear stresses under the action of a compressive axial load
By using a proximal collar or by tapering the stem.
These both allow some of the load to be taken in compression, as opposed to a straight stem where all the load is transferred as shear forces.
Name two ways of avoiding stem fracture under the action of a compressive axial load
large cross-section
high-strength material
Why does the joint force on the hip produce a bending moment?
joint force vector not along neutral axis
how can bending stress be calculated?
bending moment x distance from neutral axis / 2nd moment of area
My/I
What is the magnitude of the bending moment equal to?
applied force x its distance from neutral axis
Which side of the femur is in
a) tension?
b) compression?
a) lateral
b) medial
What are the three quantities that influence the maximum bending stress in a structure?
The magnitudes of the bending moment, the second moment of area and the distance form the neutral axis to the outer edge of the cross section.
How does the presence of a femoral stem affect the magnitude of the bending stresses in the femur?
The stresses are lower because the stem takes some of the load, which means that the bone is less stressed.
What stops the bending moment from rotating the stem?
main points of contact with the femur on proximal medial and distal lateral stem
Does the bending moment increase or decrease as you move from proximal to distal stem?
decreases
What is the distribution of bending stress along the stem?
pear-shaped
How can proximal stem loosening cause failure?
distal bending moment increases
Is the 2nd moment of area greater for the stem or the bone?
bone because bigger cross-section
Is the stem or the bone stressed more?
the stem because its got a smaller cross-section
Why is it beneficial to have substantial load transfer proximally?
to prevent stress shielding in the proximal femur
distal stem less stressed
Give 3 important design considerations for a stem under a bending load
ensure stem doesnt fail
avoid loosening
mimimise stress shielding
give 2 ways to ensure a stem doesnt fail under a bending load
large second moment of area
shape to limit magnitude of M
How can stem loosening be prevented under a bending load?
strong bond
good press fit
How can stress shielding of bone under a bending load be minimised?
suitable stem rigidity
What are the three quantities that influence the maximum bending stress in a structure?
The magnitudes of the bending moment, the second moment of area and the distance form the neutral axis to the outer edge of the cross section.
How does the presence of a femoral stem affect the magnitude of the bending stresses in the femur?
The stresses are lower because the stem takes some of the load, which means that the bone is less stressed.
What is a radial stress?
directs radially out from centre
Where are radial stresses highest?
points of contact with bone and stem
What is a hoop stress?
aka circumferential
primary tensile stress that acts in a direction that splits bone
Do long or short stems have high radial stress?
short
Under what circumstances does the femur experience high hoop stresses due to the presence of a femoral stem?
short stem
loose stem
How can torsional stresses arise in the femur?
when one end of the femur rotates axially with respect to the other
Why is it desirable to use non-circular sections for the stem of a femoral component?
to help resistance to rotational shear forces
State three important factors to consider in the design of a replacement acetabulum.
- size + conformity of joint surface in replacement affects the contact area and stresses
- subchondral cortical bone should be maintained
- The stiffness and thickness of the cup
- whether or not to use a cup with a metal backing plate
- the method used for fixing the cup into the acetabular socket.
does the acetabulum or head of femur have a larger diameter?
acetabulum
What acetabular structure is very important for load bearing? if this is damaged what structure is loaded?
cortical shell
underlying cancellous bone
Why are screws not an acceptable way to fix a femoral sttem?
high stress concentrations at the bone-screw interface cause bone resorption which loosens the screw
What is bone cement made from?
polymethyl methylacrylate (PMMA)
What is the difference between a monomer and a polymer?
A monomer is a short molecular chain version of a polymer. Monomers react chemically when activated to join together to produce polymers. The types of long molecular chains produced give rise to different mechanical properties
What is the benefit of coating a prosthesis component with PMMA during manufacturing?
to resist shear and tensile forces
In which mode of loading is cement strongest?
compression
State two advantages of using a cemented, as opposed to a non- cemented, prosthesis.
no need for surfaces to be an exact fit
fills gaps so that stress distributions are equal
State 4 problems associated with using PMMA bone cement
exothermic reaction destroys bone tissue
fragments cause inflammatory reactions causing bone resorption, and increase surface wear
low tensile and shear strength because not an adhesive can cause failure and loosening
toxic monomer left over
Why is a non-bonded cement interface of concern?
Because cement particles will be released into the tissues due to abrasive wear; because the prosthesis may loosen.
How does increased stem rigidity affect
a) proximal load taken by stem?
b) proximal load taken by bone?
c) proximal stress shielding?
d) distal load transfer?
e) distal shear stress?
f) proximal sheer stress?
a) increase
b) decrease
c) increase
d) increase
e) increased
f) decreased
What disadvantage does using a less rigid stem have?
increases shear stress
how do we calculate load taken by the bone in the load sharing region?
bone rigidity/ total rigidity
why do isoelastic stems fail?
The proximal shear stresses are too high for cement and hydroxyapatite coatings to withstand, so the interface bonds fail.
how does tapering the stem affect load transfer?
In a non-tapered stem, load is transferred solely by shear forces, at the proximal and distal ends of the stem. In practice, the tapered stem does allow load transfer by compression of the stem on the bone; the more the taper, the greater the compressive load transfer.
What are the risks associated with using a cement layer that is too thin?
Cement failure and prosthesis loosening.
What are the risks associated with using a cement layer that is too stiff?
increases proximal and distal cement and interface stresses
What are the three main disadvantages of using a small diameter head?
They result in higher contract stresses and HDP wear; the rate of depth of wear is greater
increased likelihood of post-op dislocation
What is the main argument against using a collar?
That the collar acts as a pivot, which causes fretting wear at the pivot and high stem stresses distally.
What is an exeter hip? what are its advantages?
no bonding: smooth tapered stem
means no wear particles, free to slide, low friction, puts more compressive stress on the cement
What is the main advantage of completely coating the surface of a stem with hydroxyapatite as opposed to just partial coating?
Complete coating gives the best chance for adequate bone ingrowth to hold the prosthesis in place.
What advantage do cementless stems have?
promotes hoop stresses that decrease stress shielding
What can lack of distal contact in a cementless stem cause?
thigh pain
Why are stems tapered?
to prevent subsidence
How does a tapered wedge help proximal load transfer?
By transferring a significant proportion of the load in compression, rather than shear.
What problems with shape can occur in a cementless implant?
contact important as if gaps form there will be increased stresses
- canal dimensions arent proportional to femur size
- femoral canal changes with age
Name the two ways that the neck of a femoral stem can be modified in order to reduce the bending moment at the stem.
By shortening the neck and by increasing the neck angle, i.e. making it more vertical.
What effect do the modifications which reduce the bending stresses have on the hip joint force?
increase it
What affects the friction force on a bearing?
coefficient of friction (material property of both surfaces) normal load (contact load)
What happens to the acetabular component when there is increased friction in the bearing of a replacement joint?
increased shear load in acetabulum may cause loosening
How can interface forces be reduced?
using head of smaller diameter (charnley)
Why is HDP used as a bearing surface in replacement joints?
What is its main disadvantage?
Because it has a low coefficient of friction with metals.
Because fragments of HDP produce adverse tissue reaction that leads to bone resorption.
What are the two types of joint wear?
adhesive: bearing surfaces stick when pressed together and softer is torn
abrasive: surface is not smooth so particles worn off
State two reasons why small diameter heads are used in hip replacements?
Because they reduce cup-bone interface shear stresses, lessening the risk of loosening.
Because they produce less volume wear of HDP than large diameter head
What are the two main disadvantages of using a small diameter head?
They result in higher contract stresses and HDP wear; the rate of depth of wear is greater.
How can the constant of friction be reduced?
use materials like ceramic and zirconia
What is acetabular component made from?
HDP +/- metal backing
How does the normal acetabulum sit?
45 degrees to coronal and sagittal plane
faces slightly backward
Name three important features in the design of modern acetabular cups.
The metal backing plate the thickness of the HDP layer the size of the head and cup the radial clearance between the head and the cup the bearing surface materials the method of cup fixation to the bone.
List three acetabular design features that affect the contact pressure at the bearing surface of the hip joint.
The diameter of the cup
the radial clearance
the thickness of the HDP layer.
How can cups be fitted? which of these has a bad disadvantage?
cemented
push fitted
incorporate a threaded stem –> bone resorption
Why is there a minimum recommended thickness to the HDP cup?
To avoid excessive bearing contact stresses
What is the advantage of using an acetabular component with a metal backing plate?
The plate evens out the loading on the acetabulum so no high stress conc and holds the plastic in place
What are the three steps that lead to acetabular component loosening due to HDP fragments?
Ingress of HDP at the bone interface; bone resorption; migration of HDP along the interface until loosening occurs.
Why do cups loosen?
primarily biological due to HDP wear particles
also mechanical overstressing
How can stresses at the bone implant interface be reduced?
retain subchondral bone
thick layer of HDP
thick layer of cement
metal backed cup
Why is centralising the cup impractical?
damage cortical bone
earlier impingement of femoral neck causing limited rom and dislocation
