Biomechanics of TKR Flashcards
Which femoral condyle is larger?
- Medial
- extends more distally cf lateral
What is the degree and slope of the tibia plateau’s?
- Posterior tilt
- 7 degrees
- tilt is 1 degree greater on lateral side
- the asymmetry of the tibial plateau allows for rotation of the tibia about the anatomical long axis during knee flexion
In varus deformity where is centre of the knee cf the weight bearing axis?
- Lateral
In a valgus knee where is the centre of the knee in relation to the weight bearing axis?
- Medial
Where is the normal weight bearing surface of the knee?
- Just anterior to centre of knee
If the weight bearing axis is behind the knee what happens to the knee?
- Then there is hyperextension malignment
If the weight bearing axis infront of the knee what happens to the knee?
- Flexion malalignment
Descibe the flexion-extension movements of the knee?
- Contraction of popliteus -> IR if tibia -> unlocks knee from extension ready to initate felxion
- Lateral femoral condyle does rollback across the tibia during flexion, but the medial femoral condyle rolls back to a lesser extent
- ( due to the medial compartment being deeped dished concave tibial plateau and relatively fixed meniscus= anterioposterior extcursion of tibiofemoral contact point of only 1cm)
- The lateral condyle with its convex tibial plateau and more mobile meniscus has a greater exercusion of the tibiofemoral contact point.
- asymmetry allows axis rotation of lateral compartment around medial comparmtent of up to 30 degrees
- ie there is Internal rotation of the tibia cf femur with flexion
- During extension
- the femur rolls forwards increasing the levering arm of the hamstrings to act as a brake on hyperextension
- the tibia externally rotates cf femur
- the final ER of tibia to femur from flexion to extension = Screw home mechanism
What are the biomechanical functions of femoral rollback?
- Increase the lever arm of the quadriceps
- Allow clearance of the femur from the tibia in deep flexion
The flexion- extension axis in the knee is approximated to what?
- The transepicondylar axis
- a lateral of the femur reveals that the posterior projections of the condyles are defined by 2 concentric circles centred on the TEL
What does the screw home mechanism allow?
- It is the final ER of the tibia cf femur in movement of extension to flexion
- it results in tightening of both cruciate ligaments and locking of the knee such that the tibia is in a position of maximal stability with respect to the femur
What are the biomechanics of the knee?
-
Static
- Alignment of articulating bones
- Geometry of the WB surfaces
- Laxity of connecting ligaments
-
Dynamic
- coordinated activity of the muscles
What is the function of the patella?
- Act as a pulley for the quadriceps
- Increase the power of the quadriceps by increasing the moment arm
- patellectomy reduces quads strength by 20%
What is the q angle?
- The angle formed by the intersection of lines joining the centre of patella to the ASIS and tibial tuberosity
- Normal Q angle varies between 5-20
- F > M
- angles > 20 = patellofemoral instability and pain
Describe the classification of patella geometry?
- Wiberg
-
type1
- equal medial and lateral facets
-
Type 2
- concave lateral and smaller concave medial facet
-
Type 3
- smaller medial facet , convex
Wiberg proposed that the patella with the deficient medial facet is more likely to develop OA
What patella effects makes the pt susceptible to OA?
- Uneven pressures on the patella as it tracks thru the femoral sulcus
- failure to distribute loads evenly -> areas of high contact stresses
- patella tracking determined by
- Patella geometry ( Wisberg type3- tracks lateral due to dysplastic medial facet)
- Flattening of the femoral sulcus angle
- laterally placed tibial tubercle
How do you measure hte tibial tuberosity- trochlear groove TT_TG?
- Distance from superimposed 2 CT images thru TT and thru the trochlear indicates laterally displaces Tibial tuberosity when the TT-TG is greater than 20mm
- In such cases a medial transfer to tibial tuberosity made be indicated
Describe patellofemoral joint movement ?
- Patella enagages femoral sulcus at 20 degrees of flexion
- tracks along conforming groove
- at 20 degrees of flexion, distal part of patella makes contact first
- As knee flexes contact area of patella shift **proximally **
- Beyond 90 degrees patella ER & only medial facet articulates
- In extreme flexion patella lies in interocondylar groove
What is the patellofemoral contact pressure during walking, ascending,descending the stairs?
- 0.5x body weight- walking
- 2.5 -3.3 x on ascending/descending the stairs
What are the primary restraints to anterior translation?
-
ACL
- Anteromedial = tight flexion
- posteriolateral = tight extension
- when knee flexed to 30 degrees, acl provides 87% of restraint to anterior translation
- others include
- ILIOTIBIAL BAND= 24%
- MID- MEDIAL CAPSULE= 22%
- MID-LATERAL CAPSULE=20%
- MCL 16%
- LCL 12%
- MENISCI
- hamstrings are dynamic stablisers
What are the primary restraints to posterior translation?
-
PCL= primary
- posteriomedial- tight flexion
- anteriolateral- tight extension
- provides 95% restraint to post translatio at 90o flexion
- secondary restraint to _ER, varus and valgus _
- others
- LCL= secondary restraint
- PLC= LCL, popliteus muscle/tendon complex/posterolateral capsule
- MCL
What is the primary restraint to IR?
- ACL- primary
- Politeal oblique ligament (POL) and Posteriomedial complex (PMC)= secondary restraints
What are the primary restraints to ER?
- Popliteofibular ligament
- LCL
- PLC at 30 degrees of flexion
- MCL important at degrees of flexion
What are the primary restraints to Valgus?
- Superifical MCL- at all angles
- ACL secondary restraint
What are the primary restraints to varus ?
- LCL - in all position of flexion
- greatest effect is at 30 degrees, least at full extension
What is the posteriolateral corner composed of?
- Superificial layer
- iliotibial band
- biceps femoris tendon
- Deep layer
- LCL
- Popliteus tendon
- Fabellofibular ligament
- arcuate lig
- coronary lig
- post horn of lateral meniscus
- middle third of lateral capsular lig
- posterolateral joint capsule
What range of motion does a normal knee allow?
- 6 degrees
What range of motion does a rigid hinge THR knee allow?
- only one degree rotating about the x axis- flexion/extension
- provides a stable construct but alose incrases force across the implant- cement mantle so increase risk of loosening
- Why modern hinges use sloppy hinges by incorporating metal on polyethylene bushings and rotating platforms
How is sagittal plane stability maintained in the cruciate sacrifing TKR?
- ie AP translation
- by a curved tibial articulating surface
- ie conforming as the radii of the tibial and femur are similar
How is coronal plane stability maintained in the cruciate sacrifing TKR?
- Median intercondylar eminence
What is conformity ?
- A static mechanical concept
What is constraint?
- A purely dynamic kinetic concept
Why is the PCL retaining TKR meant to be good?
- It is thought to replicate the knee mechanics more closely because the naive PCL -> femoral rollback and increased stability
- this is contraversial
- These implants have low conformity with a round on flat design to allow femoral rollback
*
What is the disadv of the PCL retaining in the coronal plane?
- in the coronal plane the disadvantage of the round on lfat design is that lift off can occur followed by ** slam down and edge loading of the PE**
- -> **increased contact stress and wear **
What is the adv of PCL subsituting TKR?
- Increased tibial contouring (Conforming) on both coronal and sagittal planes.
- sagittal plane stability = cam and tibial post
- Coronal plane stability = conforming surfaces and colateral ligaments
What makes a good TKR?
- Fixed to supporting bone
- durable
- allows good knee kinematics
- compromise on obljectives
- replication of rollback/sliding motion of femur during deep flexion requires a low conformity , such seen with a flat PE tibial tray in sagittal plane
- Disadv is reduced contact area and increase high contact stress-> increase wear
- Higher conformity increases contact area, but higher contrainst transmit forces to bone-cement- implant surface to increase loosening
- compromise on obljectives
How is flexion-ext controlled in TKR?
- Geometry of femoral condyles and PE
- Natural femoral condyles have 2 curvature of radii
- large anterior radius in contact with tibia during extension and a small posterior radius in contact during flexion
- most TKR approximate this
- but some e,g Scorpion have 1 radius
what is the condylar compromise?
- Contact stress is inversely porptional to contraint
- Normal kinematics inversely porportional to contrainsr
- High contact stress causes wear
- Increased contraint increases loosening
- solution- compromised contrainst to allow low contact stress and reduce loosening
What does femoral and PE radii determine ?
- Rotation and flexion-extension constraint
- Coronal plane geometry
- flat- flat design-> edge loading, slam diwn dueing varus/valgus whereas curve on curved design reduces edge loading , spreading load over a wide area during varus/valgus
What are the factors that leads to polyethylene wear?
-
Thickness of PE
- if <8mm thick then contact stress is greater than the yeild strength of the PE
-
Articular geometry
- flat PE reduced the contact surface area and maximises the contact load. conforming PE has opposite effect
- method of PE sterilisation
- Increasing conformity-> backside wear
- All PE components
What is backside wear?
- Is wear of the non articulating surface of the tibial insert
- observed in all designs of TKR independent of capture mechanism of PE inserts
*
What factors in TKR design increase the probability of loosening?
- Flexible implant - low thickness to length ratio
- Small contact area
- Load transfer to edges of contact ( cause dby kareg varus- valgus movement, or AP pr medial -lateral instability
- Features that -> concentrated stress e.g.peripheral tibial try pegs
