Unit 3: Knee Joint Replacement Flashcards
What is the knee dependent on for stability?
Ligaments
Integrity of the posterior joint capsule
Good musculature
What is the purpose of the ACL?
Resists posterior subluxation of the femur
What is the purpose of the PCL?
Resists anterior subluxation of the femur
What is the purpose of the LCL?
Resists ADDuction of the joint
What is the purpose of the MCL?
Resists ABDuction of the joint
What is the purpose of the posterior capsule?
Resists hyperextension
How are the ACL and PCL named?
According to their anterior and posterior attachments to the tibia, not the femur
What is meant when it is said that the ligaments in the knee move nearly isometrically?
They keep the same length as they move and do not lengthen or shorten
How does the centre of rotation and point of contact between the femur and tibia change as the knee flexes?
The centre of rotation and point of surface contact both move posteriorly
How do the medial and lateral compartments of the tibia plateau differ?
Medial compartment = slightly concave
Lateral compartment = slightly convex
What is the “screw home” mechanism?
Tibia externally rotates 5 degrees in the last 15 degrees of extension because the medial tibial plateau articular surface is longer than the lateral tibial plateau
What is the purpose of the “screw home” mechanism?
“locks” knee decreasing the work performed by the quadriceps while standing
What is the full range of knee flexion?
About 140 degrees
How does the knee move from flexion to extension?
Combination of rolling and sliding motion
What limits the rolling distance to allow the knee to flex fully without rolling up against the posterior capsule?
Cruciate ligaments
Approximately what magnitude of forces go through the knee joint during normal daily activities?
2-6X body weight
What causes the compressive joint force in the knee?
Action of the quadriceps acting via the patellar ligament
What combines to form the total joint reaction force of 4 BW in the knee joint?
Patellar force of 3 BW + Ground Reaction force of 1 BW
What are the other components other than the vertical component of ground reaction force?
Fore-aft GRF = 20% BW
Medial horizontal component = 5% BW
How does the knee combat low magnitude sideways medial reaction forces?
Quads acting via the patellar tendon pull the joint together hard enough to keep both condylar surfaces in contact with the tibial plateau
How does the knee combat high magnitude sideways medial reaction forces?
Lateral collateral ligament balances the turning moment
What implications do the high loads that act on the medial compartment of the knee have on joint replacement design?
Tibial component needs to be able to transfer igh medial compartment loads on its upper surface to the underlying bone without causing high compressive stresses
What are most commercially available knee replacements made of?
Femoral component = cobalt chrome
Tibial component = HDP
What problems were seen when trying titanium components instead of cobalt chromium?
Metal wear particles were taken up by the synovial membrane causing blackening
What are the minimum functional kinematic requirements of a knee replacement?
Fully extend to 180⁰
Stand without quad effort
Flex to 90⁰
Allow slight axial rotation to maintain natural ligament tension in flex/ext
How is it ensured that the two bearing surfces are cut parallel in a knee replacement?
Use a radiograph of the feur and draw a line from the centre of the femoral head to the knee centre
In knee replacement, why must the posterior capsule be dissected off the back of the femur?
To ensure that the replacement knee can fully extend
How are ligaments balanced in knee replacement surgery?
Lengthened tightened ligaments to match slack ones
What is meant by the term ‘constraint’?
Relationship between tibial and femoral bearing surface geometries - more constrained = less freedom of movement to slide and rotate in different directions
What type of prosthesis is used if there are no ligaments intact?
Hinged prosthesis
What are the downsides of a hinged prosthesis?
Only one axis of rotation - under lateral and log axis rotation transmits high shear forces to implant-cement and comeent-bone interfaces
Which ligament is usually either destroyed or of no mechanical value in OA?
ACL
What are the theoretical advantages of retaining the PCL?
Provides a degree of ant-post stability
May preserve some proprioception
What are the disadvantages of retaining the PCL?
Constricts free surgical dissection of the posterior capsule which may limit full extension
Encourages the femoral component to slide over the tibial bearing which may have detrimental surface wear effects
Why is it normal to slope the tibia posteriorly by about 10⁰?
Ro encourage the femoral component to roll bback on the tibial component
What happens if the PCL is too loose or too tight?
Too loose - allows forard movement on the femur on the tbia so that the normal rolling back motion no longer works
Too tight - restricted flexion, excessive rolling back of the femur on the tibia and possible compression of the 2 prosthetic joint surfaces together posteriorly generating high contact stresses
What is the main problem that have been found with CL retaining prostheses?
HDP wear and fatigue problems
What are the 3 important factors to consider when looking at the surface shape of a knee prosthesis?
Effect of constraint on load transmission
Effect of surface contact on wear of the HDP tibial component
Effect of surface contact area on the stresses in the HDP tibial component
How does the stiffness of HDP change and how does this affect the joint?
Becomes stiffer due to an increase in density after sterilisation with gamma radiation and over time after implantation due to oxidisation
Increased stiffness increases joint contact stress making the HDP more prone to wear
Why is it the HDP not CoCr component that is first affected by wear?
HDP is softer than CoCr
Why is HDP fatigue and wear more of a problem in knee replacements than hip?
The bearing surface contact area is smaller so the stresses in the material are higher
Most knee replacements are designed to provide some constraint in which two directions?
Axial rotation
Lateral motion
Why do sudden loads give rise to large instantaneous stresses at the interfaces?
Cement and prosthesis materials are much stiffer than the soft tissues and not viscoelastic - so energy due to sudden loads is not absorbed gradually
How does the constraint of the prosthesis affect the length of stem required?
The more constrained the motion of the prosthesis the greater the length of stem required
What formula can be used to calculate the volume of wear?
v = c.N.s / p
c = coefficient of wear N = applied load s = distance that the bearing slides p = hardness of the surface being worn
What is the relationship between the rate of volume of wear and the rate of depth of wear?
Volume of wear (v) = area of contact (A) x depth of wear (t)
In summary, how do you minimise the rate of production of wear particles (i.e. the volume rate of wear)?
Minimise the sliding distance
How do you minimise the rate of depth of wear?
Increase contact area (can be achieved by having a wide bearing)
What is lamination cracking at the joint associated with?
A combination of both high contact stress and a cyclical sliding motion between the surfaces
What developments in recent years have helped to reduce HDP fatigue to some extent?
Improved quality of HDP manufacturing process and the use of ultra high molecular weight polyethylene
How does the stress differ from the central contact area to the periphery?
Central part of contact area = always in compression
periphery = always in tension
How does the thickness of the HDP component affect stress and why?
The thinner the HDP component the greater it is stressed - because the stresses cannot be distributed evenly in the material
What are the current recommendations for HDP thickness?
8mm thickness minimum for HDP tibial components without a metal backing tray
On which side does the natural tibial plateau take most load?
On the medial side (60%)
What is the purpose of a metal backing plate?
Distribute the high contact stresses under the condyles in order to provide an even loading on the bone beneath it
What is the downside of a metal backing tray?
stress concentration on underlying medial bone is greater because the metal is so much stiffer
Also tensile stress between the plate and the bone laterally
Why is there now a trend towards all HDP components rather than a metal backing?
Recent research has shown that a thicker HDP (>10mm) behaves similar to a metal tray
What effect does a metal peg when used with a metal backing plate?
Reduces stresses by 20-40% - helps to reduce high contact stresses due to uneven loading that could cause the bone to fail and subside
How does the stiffness of the HDP affect contact stress?
The higher the Young’s modulus of the HDP the greater the contact stress (as the HDP deforms less)
How does the femoral component of a knee replacement differ from a normal knee?
Replacement is symmetrical
Normal knee - medial condyle is larger than the lateral condyle
Why do replacements not copy the natural asymmetry of the knee joint?
V expensive
In practice no shown benefit to asymmetry
Why are partially constrained shapes generally favoured?
They provide the required degree of functional movement
Do not suffer greatly from loosening due to overstressing
They limit the range of sliding motion to help reduce wear
How does PCL affect the shape of the tibial component?
If PCL retained - need flattish surface profile
What are the 2 aims of posterior stabilised designed prostheses?
Prevent posterior femoral subluxation of the femur over the tibia
Cause the femur to ‘roll back’ as it flexes
How are components anchored in knee joint replacement?
PMMA cement
Sometimes projections
Fem component - press fit
Tibial component - screw fixation or a stem
What is the purpose of the patella?
Provides a better leveage for the patellar tendon - lower force required for flexion - lower joint reaction force
How does the shape of the surface of the patella affect HDP wear?
Conforming shapes contoured to match the femur wear less than convex (non-conforming) shapes
Why is wear worse in metal backed patellae?
Because the HDP is insufficiently thick to distribute the loads and is therefore prone to higher contact stresses than the less rigid all-HDP component
In which condition is it preferable to replace all joint surfaces?
Rheumatoid arthritis
What comprises a meniscal bearing prosthesis?
Metal femoral component
Metal tibial component
HDP meniscus
What are the disadvantages of meniscal prostheses?
Increased technical difficulty in achieving ligamentous balance and overall alignment without risking dislocation of the moving bearing
What is a knee hemi-arthroplasty?
Replacing only one side of the tibio-femoral joint
In what situation is hemi-arthroplasty used?
Alternative to osteotomy in younger patients who have a painful and deformed joint that is not severe enough to warrant joint replacement but with too advanced a disease process to permit osteotomy
In hemi-arthroplasty why are pegs or lugs used to limit the loss of bone stock?
In case of the need to perform a revision operation
What are the main problems with revision of TKR?
Loss of bone sotck for anchoring the revision prosthesis
Hard to allign/stabilise due to ligament damage
What design is used for revision of TKR?
Linked hinge design
Describe the linked hinge design used in revision of TKR
Bearing surface with a high degree of constraint built into the tibio-femoral bearing
High peg projecting above the tibial plateau which is partially captured by a central groove in the femoral component
What are augmentation blocks?
Flat or wedge shaped blocks that can be added to prosthesis in revision TKR to achieve contact between the prosthesis and bone
If there is gross loss of bone through the loosening process how can a revision TKR be anchored?
Intramedullary stems
What is reaming?
A process of enlarging a hole using a special tool which removes only small amounts of material
