knee joint replacement

Question 1

what movements do the ACL, PCL, LCL and MCL resist

Answer 1

ACL - resits posterior subluxation of femur
PCL - resists anterior subluxation of femur
LCL - resits adduction of the joint
MCL - resists abduction of the joint

Question 2

the ACL and PCL are named according to their anterior and posterior attachments to the?

Answer 2

tibia (NOT the femur)

Question 3

what is the posterior capsule and what is its function

Answer 3

band of tendinous material

resists hyper-extension

Question 4

why does the joint reaction force at the knee increase as the sideways horizontal component of the ground reaction force increases

Answer 4

greater patellar tendon and hamstring forces are required to balance the horizontal force, so this adds to the JRF

Question 5

what adverse effect could a high contact force in the medial compartment of the knee have on a joint replacement?

Answer 5

high local stress medially which could cause the underlying bone to fail

Question 6

the greater the horizontal force component, the greater the load transferred from the ? compartment to the ? compartment of the joint

Answer 6

lateral to medial

Question 7

why does the knee lose contact with the lateral condoye as load increases

Answer 7

the muscles dont have enough strength to maintain contact at both sides

Question 8

what are the minimum functional kinematic requirements of a knee replacement

Answer 8

• 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

Question 9

what angle will the femoral cut need to be in relation to the axis of the femur? why?

Answer 9

6 or 7 degrees

to compensate for the natural angulation of the femur in relation to the tibia

Question 10

why must the posterior capsule be dissected off the back of the femur during a replacement

Answer 10

to ensure the knee can fully extend

Question 11

advantages of retaining the PCL

Answer 11

provides a degree of A-P knee stability
may preserve some proprioceptive ability
walking on stairs more stable if PCL retained

Question 12

disadvantages of retaining the PCL

Answer 12

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

Question 13

why is it normal to slope the tibial component posteriorly by about 10 degrees

Answer 13

to encourage the femoral component to roll back on the tibial component

Question 14

why does HDP wear out faster then CoCr

Answer 14

HDP is the softer material

Question 15

what determines the life of the HDP bearing:
- rate of volume of wear
or
- rate of depth of wear

Answer 15

rate of depth of wear

Question 16

what must be minimised and what must be increased in order to minimise the depth and volume of wear

Answer 16

minimise the sliding distance (by reducing the radius) to decrease the volume of wear
increase the contact area to reduce the depth of wear

Question 17

how does the diameter of a bearing affect the volume of wear

Answer 17

increasing the diameter also increases the volume of wear

Question 18

why is it preferable to increase bearing width than increase bearing diameter

Answer 18

increasing diameter will increase volume of wear but increasing bearing width will not

Question 19

the central part of the contact area is always in ? and the periphery is always in ?

Answer 19

central part always in compression

periphery always in tension

Question 20

low contact designs produce stresses in the HDP high enough to cause what?

Answer 20

fatigue failure

Question 21

aside from HDP surface shape, what 4 other factors influence prosthesis contact stress

Answer 21

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

Question 22

what is stressed more - a thinner or a thicker HDP component?

Answer 22

thinner because the stresses cannot be evenly distributed within the material

Question 23

what is the minimum thickness recommended for HDP tibial components that dont have a backing tray?

Answer 23

8mm

Question 24

which side does the natural tibial plateau take most load on

Answer 24

medial side

Question 25

what is the purpose of a metal tray under the tibial component?

Answer 25

more even loading distribution

Question 26

what are the downsides to using a metal plate?

Answer 26

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

Question 27

what can be done to obtain a more even load distribution in a HDP component without using a metal plate

Answer 27

using a thicker HDP component - 10mm or more - behaves similarly to a metal tray

Question 28

what is a useful design feature that reduces the incidence of loosening?

Answer 28

a central peg

Question 29

how does a central peg help to resist the rotating force of the vertical load

Answer 29

by introducing a lateral reaction force which resists the vertical force

Question 30

advantage of using a modular tibial component that can take different thicknesses of tibial insert

Answer 30

ligament tension can be set correctly by choosing the right thickness of insert

Question 31

what is the difference in size between the normal medial and lateral femoral condoles and what does this help with

Answer 31

medial condyle is larger - radius about 1.5 mm greater

this aids in the axial rotation achieved during the last phases of full extension

Question 32

what is the purpose of a posterior stabilised design (i.e. using a ‘stop’ or a ‘cam’)

Answer 32

prevent posterior subluxation of the femur over the tibia

cause the femur to ‘roll back’ a it flexes

Question 33

as well as a peg, what can be added to a prosthesis to provide additional rotary control?

Answer 33

projections built into the undersurfaces of the tibial components

Question 34

what is the ultimate mode of failure

Answer 34

HDP wear and wear particles

Question 35

how does the absence of a patella affect the knee extension force applied in the quads?

Answer 35

it decreases the lever arm which in turn increases the quadriceps force

Question 36

advantage of using meniscal bearing in a knee prosthesis

Answer 36

lowers contact stresses

Question 37

disadvantage of using a meniscal bearing in a knee prosthesis

Answer 37

requires removal of more bone

more difficult op to perform

Question 38

why are 2 pegs commonly used on unicompartmental surface replacement

Answer 38

ro resist tendency to rotate under torsional movement

Question 39

whats the function of an augmentation block

Answer 39

to fill the gaps between bone and prosthesis

this is often required in revision procedures

Question 40

what type of patient may benefit from a knee hemi-arthroplasty

Answer 40

younger patients who have a painful and deformed joint that isn’t severe enough to warrant TKR

Question 41

what material is replacement meniscus made of

Answer 41

HDP

Question 42

what is the best shape to make a replacement patella

Answer 42

shape that is contoured to match the femur will wear less than a non conforming shape

Question 43

patella replacement - is wear worse if metal backed or non metal backed?

Answer 43

worse if metal backed due to the metal being more rigid and the HDP being insufficiently thick to distribute the loads