Unit 3: Knee joint replacemen Flashcards
What design is successfully used in knee replacement?
total condylar
Is the knee joint inherently stable? What provides this?
no
- soft tissues provide stability eg ligaments, posterior joint capsule, musculature
What is the posterior joint capsule?
a tendinous band that resists hyperextension
What does subluxation of a joint mean?
partial or complete dislocation of the joint
What type of knee subluxation does the anterior cruciate ligament prevent?
posterior
What type of knee subluxation does the posterior cruciate ligament prevent?
anterior
Which ligament resists knee joint adduction?
lateral collateral
Which ligament resists knee joint abduction?
medial collateral
What quality do ligaments have as they move ?
isometric = keep same length
How does the position of the instantaneous centre of rotation change as the knee moves from extension to flexion?
It moves posteriorly by upwards of 10 mm and distally by a few mm.
Why does the femur not roll off the tibia as the knee flexes?
Because the cruciate ligament and joint capsule prevents it from doing so.
What is full knee flexion?
140
What movements does the femur do on tibia?
sliding due to rotation
rolling motion
If a prosthetic has a flat tibial plateau and circular condyles. What else does it need? Why?
retain cruciates during surgery
to control sliding of femur on tibia
What type of loading mainly occurs at knee?
compressive
Explain why the joint reaction force at the knee increases as the sideways horizontal component of the ground reaction force increases.
As this force increases, a greater patella tendon force and a greater hamstring force are required to balance its effects which adds to the joint reaction force.
What adverse effect could a high contact force in the medial compartment of the knee have on a joint replacement?
It will cause high local stresses medially which could cause the underlying cancellous bone to fail.
What components does the joint reaction force at the knee have?
vertical
fore-aft
horizontal
What does the medial horizontal component of the joint reaction force generate at the knee?
a turning moment
List the three important functional kinematic requirements of a replacement knee.
fully extend to 180
flex to 90
slight axial rotation
During knee replacement surgery, what must be done to
(i) the posterior capsule
(ii) the collateral ligaments?
iii) bearing surfaces?
i) dissected out so knee can extend
ii) tension balanced
iii) cut parallel
Is the cost of knee more or less than hip?
5x more than hip
What is the femoral and tibial component usually made of/
femoral = cobalt chrome tibial = HDP
What does constraint mean?
the relationship between tibial and femoral bearing surface geometries.
If there are no intact ligaments, which design of prosthesis should be used?
hinged
Describe movement with a hinged prosthesis
single axis of rotation
no give under lateral and long axis rotational loading
What disadvantage does a hinged prosthesis have?
transmits high shear forces at the interface
Which ligaments are usually intact before a knee replacement?
collaterals fine and can be balanced
ACL destroyed by OA
PCL usually there
What motion
is the PCL important for?
controlling rolling motion of tibia
allows femur to rotate allowing flexion
If the PCL is not preserved, what must be done?
substitute within the prosthesis with a simple stop or cam-like device
What are the advantages of retaining the PCL?
- AP stability
- proprioception
- walking on stairs
- better ROM
What are the disadvantages of retaining the PCL?
- constricts removing the posterior capsule so full extension can be limited
- femoral component sliding over tibial bearing may increase wear
- means a congruent joint surface isnt used that limits HDP wear
What features are needed if the PCL is retained?
flat tibial plateau
correct PCL tension
sloped tibial component 10degrees posterior
What negative effect can prosthesis designs that retain the PCL have?
HDP wear and fatigue problems
Why does a replacement knee need to have a fairly flat tibial plateau when the PCL is retained?
Because the PCL could otherwise become lax or too tight during flexion- extension movement.
Which mechanical factor affects load transmission?
degree of constraint
Which mechanical factor affects the wear of the HDP tibial component?
surface contact
Which mechanical factor affects stresses in the HDP tibial component?
surface contact area
How does constraint affect load transmission?
a rigid joint cannot transmit any load via the soft tissues so it all transfers through the prosthesis-bone interface
What type of constraint exists in most replacements?
axial rotation
lateral motion
Which type of knee replacement is the most constrained?
hinge
How are cement and prosthesis materials affected by constrained load transmission?
they are stiff and not viscoelastic so sudden loads cause large instantaneous stresses at the interface that can cause failure
What helps constrained load transmission?
large contact area using stems and pegs in the components
What determines the useful life of the prosthesis?
the rate of wear of the HDP component
What is the volume of wear of a knee prosthesis proportional to?
sliding distance moved
What formula shows the volume of wear in a knee prosthesis? why?
v = K.s
because c, N and p were constants
What effect does diameter of bearing have on volume of wear material produced?
a smaller diameter reduces volume of wear
Why is it better to have a smaller volume of wear?
fewer HDP particles
How does volume of wear relate to depth of wear?
volume of wear = area of contact x depth of wear
v = A x t
How can depth of wear be reduced?
having a large surface area of contact
Why is it advantageous to increase the width instead of the diameter of bearing?
increasing diameter increases volume of wear
Why does HDP fatigue occur?
high contact stresses
cyclical sliding motion between surfaces
What does HDP fatigue cause?
premature wear and subsurface lamination cracking
What is the contact stress dependent on?
increased contact area, due to shape of bearing, will decreases contact stresses
Why is a curved femoral condyle better for contact stresses?
moment due to lateral ground reaction force lifts condyle up and all load goes through the other
a curved condyle has more contact area for the stress to pass through
How does the contact area for the tibial component vary?
from flexion to extension
Which part of the contact area is in
a) compression?
b) tension?
a) centre
b) periphery
Why is the HDP tibial component prone to fatigue failure?
contact points change between flexion and extension
it experiences repetitive loading / unloading and tensile / compressive loading cycles.
List four design features of a tibial component that affect the magnitude of the stresses on the underlying bone.
Thickness of HDP component
If there is a metal backing plate
If there is a tibial stem(peg)
Stiffness of HDP component
How does thickness of HDP component affect contact stresses? why?
thin HDP has greater stresses because they can’t be evenly distributed
How is load distributed on the tibial plateau?
medial side is 60% of load
State one advantage of a modular tibial component that can take different thicknesses of tibial insert.
allows for ligaments to be balanced
What is the advantage of using a metal backing plate?
even distribution of high contact stresses on tibial plateau
What is a disadvantage of using a metal backing plate?
it may cause excessively high stresses on the medial bone if the knee is unevenly loaded because the metal plate is stiff and therefore does not spread out a high contact load as the less stiff HDP.
What advantage does a tibial stem have? How?
prevents sinkage and loosening by reducing high contact stresses as it resists medial vertical load with a lateral reaction force
How has design of knee prosthesis improved since total condylar?
posterior stabilised models have a design that takes over the job of PCL so it doesn’t need to be retained
How are the femoral condyles different in a prosthesis than in a normal knee?
normal = asymmetrical with large medial aiding axial rotation in full extension
prosthesis = symmetrical condyles for ease and cost
Which movement has a smaller radius of curvature: flexion or extension?
flexion
What is the role of the anterior part of the femoral component curvature?
accommodates movement of the patella during flexion and extension
Which component is most important for the degree of constraint?
tibial
Describe a posterior stabilised prosthesis
dished surface
prevents posterior subluxation of femur on tibia
allows femur to roll back as it flexes
uses a stop or cam device
Which method of posterior stabilisation is better? why?
cam device because the curved shape allows for a more smooth, roll-back in flexion
What features of a knee prosthesis provide rotational stability?
Projections on the undersurface of the tibial or femoral components, the stem (if present), screws (if present).
What additional fixation methods are required for cementless knee prostheses?
Screws and/or stem, to provide the required fixation.
How does the absence of a patella affect the knee extension force applied in the quadriceps?
The patella tendon lever arm is reduced, requiring a greater quadriceps force. This adds to the patello-femoral joint reaction force.
Why is a metal backed patella replacement generally undesirable?
The HDP is too thin to prevent high stresses in the material, which can result in extremely fast wear.
What can be done to encourage patellar tracking?
groove anterior femoral component
What 2 things can reduce wear of patellar bearing?
contour to femur
dont use metal backing
In which type of arthritis, is the patellar bearing surface always replaced?
rheumatoid
What is the advantage of using meniscal bearing knee prostheses.
lowers contact stresses.
What is the disadvantage of using meniscal bearing knee prostheses.
requires removal of more bone,
more difficult operation to perform.
Where does the meniscus move during
a) flexion?
b) extension?
a) back
b) forward
How can the risk of dislocation with a meniscal bearing prosthesis be reduced?
limit sliding
What is a hemi-arthroplasty?
when one side of the tibio-femoral joint is replaced
Who benefits from hemi-arthroplasty?
younger patient
severe pain and deformity
in-between osteotomy and TKR
Why are two pegs commonly used on unicompartmental surface replacement?
To resist the tendency to rotate under torsional loading.
Give two problems with revision knees
loss of bone stock
need for alignment and stability
What is the function of an augmentation block?
To fill in gaps between the prosthesis and the bone. This is frequently required during revision procedures, where some bone will be lost as a result of the primary arthroplasty or because it is no longer viable.
If an augmentation block cant account for all the bone loss what can be used instead?
intramedullary stem in femur and tibia
