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Basic Science > Biomechanics of TKR > Flashcards

Biomechanics of TKR Flashcards

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Biology 101 flashcards
1
Q

Which femoral condyle is larger?

A
  • Medial
  • extends more distally cf lateral
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2
Q

What is the degree and slope of the tibia plateau’s?

A
  • 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
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3
Q

In varus deformity where is centre of the knee cf the weight bearing axis?

A
  • Lateral
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4
Q

In a valgus knee where is the centre of the knee in relation to the weight bearing axis?

A
  • Medial
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5
Q

Where is the normal weight bearing surface of the knee?

A
  • Just anterior to centre of knee
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6
Q

If the weight bearing axis is behind the knee what happens to the knee?

A
  • Then there is hyperextension malignment
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7
Q

If the weight bearing axis infront of the knee what happens to the knee?

A
  • Flexion malalignment
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8
Q

Descibe the flexion-extension movements of the knee?

A
  • 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
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9
Q

What are the biomechanical functions of femoral rollback?

A
  1. Increase the lever arm of the quadriceps
  2. Allow clearance of the femur from the tibia in deep flexion
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10
Q

The flexion- extension axis in the knee is approximated to what?

A
  • 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
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11
Q

What does the screw home mechanism allow?

A
  • 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
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12
Q

What are the biomechanics of the knee?

A
  • Static
    • Alignment of articulating bones
    • Geometry of the WB surfaces
    • Laxity of connecting ligaments
  • Dynamic
    • coordinated activity of the muscles
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13
Q

What is the function of the patella?

A
  • Act as a pulley for the quadriceps
  • Increase the power of the quadriceps by increasing the moment arm
  • patellectomy reduces quads strength by 20%
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14
Q

What is the q angle?

A
  • 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
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15
Q

Describe the classification of patella geometry?

A
  • 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

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16
Q

What patella effects makes the pt susceptible to OA?

A
  • 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
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17
Q

How do you measure hte tibial tuberosity- trochlear groove TT_TG?

A
  • 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
18
Q

Describe patellofemoral joint movement ?

A
  • 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
19
Q

What is the patellofemoral contact pressure during walking, ascending,descending the stairs?

A
  • 0.5x body weight- walking
  • 2.5 -3.3 x on ascending/descending the stairs
20
Q

What are the primary restraints to anterior translation?

A
  • 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
21
Q

What are the primary restraints to posterior translation?

A
  • 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
22
Q

What is the primary restraint to IR?

A
  • ACL- primary
  • Politeal oblique ligament (POL) and Posteriomedial complex (PMC)= secondary restraints
23
Q

What are the primary restraints to ER?

A
  • Popliteofibular ligament
  • LCL
  • PLC at 30 degrees of flexion
  • MCL important at degrees of flexion
24
Q

What are the primary restraints to Valgus?

A
  • Superifical MCL- at all angles
  • ACL secondary restraint
25
Q

What are the primary restraints to varus ?

A
  • LCL - in all position of flexion
  • greatest effect is at 30 degrees, least at full extension
26
Q

What is the posteriolateral corner composed of?

A
  • 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
27
Q

What range of motion does a normal knee allow?

A
  • 6 degrees
28
Q

What range of motion does a rigid hinge THR knee allow?

A
  • 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
29
Q

How is sagittal plane stability maintained in the cruciate sacrifing TKR?

A
  • ie AP translation
  • by a curved tibial articulating surface
  • ie conforming as the radii of the tibial and femur are similar
30
Q

How is coronal plane stability maintained in the cruciate sacrifing TKR?

A
  • Median intercondylar eminence
31
Q

What is conformity ?

A
  • A static mechanical concept
32
Q

What is constraint?

A
  • A purely dynamic kinetic concept
33
Q

Why is the PCL retaining TKR meant to be good?

A
  • 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
    *
34
Q

What is the disadv of the PCL retaining in the coronal plane?

A
  • 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 **
35
Q

What is the adv of PCL subsituting TKR?

A
  • 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
36
Q

What makes a good TKR?

A
  • 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
37
Q

How is flexion-ext controlled in TKR?

A
  • 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
38
Q

what is the condylar compromise?

A
  • 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
39
Q

What does femoral and PE radii determine ?

A
  • 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
40
Q

What are the factors that leads to polyethylene wear?

A
  • 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
41
Q

What is backside wear?

A
  • Is wear of the non articulating surface of the tibial insert
  • observed in all designs of TKR independent of capture mechanism of PE inserts
    *
42
Q

What factors in TKR design increase the probability of loosening?

A
  • 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

Basic Science (31 decks)

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