Biomechanics of TKR Flashcards
(42 cards)
1
Q
Which femoral condyle is larger?
A
- Medial
- extends more distally cf lateral
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
3
Q
In varus deformity where is centre of the knee cf the weight bearing axis?
A
- Lateral
4
Q
In a valgus knee where is the centre of the knee in relation to the weight bearing axis?
A
- Medial
5
Q
Where is the normal weight bearing surface of the knee?
A
- Just anterior to centre of knee
6
Q
If the weight bearing axis is behind the knee what happens to the knee?
A
- Then there is hyperextension malignment
7
Q
If the weight bearing axis infront of the knee what happens to the knee?
A
- Flexion malalignment
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
9
Q
What are the biomechanical functions of femoral rollback?
A
- Increase the lever arm of the quadriceps
- Allow clearance of the femur from the tibia in deep flexion
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
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
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
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%
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
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
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
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
What are the primary restraints to varus ?
* **LCL** - in all position of flexion
* greatest effect is at 30 degrees, least at full extension
26
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
27
What range of motion does a normal knee allow?
* 6 degrees
28
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
29
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
30
How is coronal plane stability maintained in the cruciate sacrifing TKR?
* Median intercondylar eminence
31
What is conformity ?
* A static mechanical concept
32
What is constraint?
* A purely dynamic kinetic concept
33
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
*
34
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 **
35
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
36
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
37
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 s**mall posterior radius in contact during flexion**
* **most TKR approximate this**
* but some e,g Scorpion have 1 radius
38
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
39
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
40
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**
41
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
*
42
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
