knee joint replacement Flashcards
what movements do the ACL, PCL, LCL and MCL resist
ACL - resits posterior subluxation of femur
PCL - resists anterior subluxation of femur
LCL - resits adduction of the joint
MCL - resists abduction of the joint
the ACL and PCL are named according to their anterior and posterior attachments to the?
tibia (NOT the femur)
what is the posterior capsule and what is its function
band of tendinous material
resists hyper-extension
why does the joint reaction force at the knee increase as the sideways horizontal component of the ground reaction force increases
greater patellar tendon and hamstring forces are required to balance the horizontal force, so this adds to the JRF
what adverse effect could a high contact force in the medial compartment of the knee have on a joint replacement?
high local stress medially which could cause the underlying bone to fail
the greater the horizontal force component, the greater the load transferred from the ? compartment to the ? compartment of the joint
lateral to medial
why does the knee lose contact with the lateral condoye as load increases
the muscles dont have enough strength to maintain contact at both sides
what are the minimum functional kinematic requirements of a knee replacement
- should fully extend to 180 degrees allowing the patient to stand without needing muscular effort by the quadriceps
- it should flex to 90 degrees to allow the patient to walk up and down stairs without having to start each step with the good knee
- should permit slight axial rotation as the knee extends to maintain natural alignment throughout flexion and extension
what angle will the femoral cut need to be in relation to the axis of the femur? why?
6 or 7 degrees
to compensate for the natural angulation of the femur in relation to the tibia
why must the posterior capsule be dissected off the back of the femur during a replacement
to ensure the knee can fully extend
advantages of retaining the PCL
provides a degree of A-P knee stability
may preserve some proprioceptive ability
walking on stairs more stable if PCL retained
disadvantages of retaining the PCL
restricts surgical dissection of the posterior capsule which may limit full extension
encourages the femoral component to slide over the tibial bearing which may increase wear
why is it normal to slope the tibial component posteriorly by about 10 degrees
to encourage the femoral component to roll back on the tibial component
why does HDP wear out faster then CoCr
HDP is the softer material
what determines the life of the HDP bearing:
- rate of volume of wear
or
- rate of depth of wear
rate of depth of wear
what must be minimised and what must be increased in order to minimise the depth and volume of wear
minimise the sliding distance (by reducing the radius) to decrease the volume of wear
increase the contact area to reduce the depth of wear
how does the diameter of a bearing affect the volume of wear
increasing the diameter also increases the volume of wear
why is it preferable to increase bearing width than increase bearing diameter
increasing diameter will increase volume of wear but increasing bearing width will not
the central part of the contact area is always in ? and the periphery is always in ?
central part always in compression
periphery always in tension
low contact designs produce stresses in the HDP high enough to cause what?
fatigue failure
aside from HDP surface shape, what 4 other factors influence prosthesis contact stress
thickness of HDP component
whether or not the HDP had a backing plate
whether the tibial component has a stem
stiffness of the HDP material
what is stressed more - a thinner or a thicker HDP component?
thinner because the stresses cannot be evenly distributed within the material
what is the minimum thickness recommended for HDP tibial components that dont have a backing tray?
8mm
which side does the natural tibial plateau take most load on
medial side
what is the purpose of a metal tray under the tibial component?
more even loading distribution
what are the downsides to using a metal plate?
normally the medial side takes the greatest proportion of the load and the stress conc will be greater in the underling bone if a metal plate is present because it is much stiffer than HDP
there will also be a tensile stress laterally between the bone and the plate
what can be done to obtain a more even load distribution in a HDP component without using a metal plate
using a thicker HDP component - 10mm or more - behaves similarly to a metal tray
what is a useful design feature that reduces the incidence of loosening?
a central peg
how does a central peg help to resist the rotating force of the vertical load
by introducing a lateral reaction force which resists the vertical force
advantage of using a modular tibial component that can take different thicknesses of tibial insert
ligament tension can be set correctly by choosing the right thickness of insert
what is the difference in size between the normal medial and lateral femoral condoles and what does this help with
medial condyle is larger - radius about 1.5 mm greater
this aids in the axial rotation achieved during the last phases of full extension
what is the purpose of a posterior stabilised design (i.e. using a ‘stop’ or a ‘cam’)
prevent posterior subluxation of the femur over the tibia
cause the femur to ‘roll back’ a it flexes
as well as a peg, what can be added to a prosthesis to provide additional rotary control?
projections built into the undersurfaces of the tibial components
what is the ultimate mode of failure
HDP wear and wear particles
how does the absence of a patella affect the knee extension force applied in the quads?
it decreases the lever arm which in turn increases the quadriceps force
advantage of using meniscal bearing in a knee prosthesis
lowers contact stresses
disadvantage of using a meniscal bearing in a knee prosthesis
requires removal of more bone
more difficult op to perform
why are 2 pegs commonly used on unicompartmental surface replacement
ro resist tendency to rotate under torsional movement
whats the function of an augmentation block
to fill the gaps between bone and prosthesis
this is often required in revision procedures
what type of patient may benefit from a knee hemi-arthroplasty
younger patients who have a painful and deformed joint that isn’t severe enough to warrant TKR
what material is replacement meniscus made of
HDP
what is the best shape to make a replacement patella
shape that is contoured to match the femur will wear less than a non conforming shape
patella replacement - is wear worse if metal backed or non metal backed?
worse if metal backed due to the metal being more rigid and the HDP being insufficiently thick to distribute the loads
