Are all MB knees the Are all MB knees the same?same?

Is MB really better than FB?Is MB really better than FB?

Jean-Louis BriardJean-Louis Briard

classificationclassification Conformity: contact areas & Conformity: contact areas &

designs to reduce fatigue weardesigns to reduce fatigue wear Motion of the bearing relative Motion of the bearing relative

to the tibial tray +++ and its to the tibial tray +++ and its controlcontrol

StabilityStability

#1 Maximum ConformityCan be achieved through:

Multi-radius sagittal femoral geometry with a distal sphere

Constant radius sagittal FT geometry with a posterior sphere

A - SPHERICAL DESIGNS

B -OVOID DESIGNS

Never spherical

Coronal & sagittal radius are different

Multi-radius sagittal femoral geometry: J curve with a distal sphere: S2

Extensive area contact near to extension (R:50mm)

But circular line contact in Flexion

LCS & Generics, Interax ISA, Genesis II/ Profix, Innex, Link TACK, Cinétique

A1 SPHERICAL DESIGNS

S2

Constant radius sagittal FT geometry with a posterior sphere

Contact area more limited in extension (R:25mm)

Larger contact in flexion

Rotaglide, SAL, TRAC, MBK, Oxford TMK

A2 - SPHERICAL DESIGNS

B -OVOID DESIGNSB -OVOID DESIGNS Never sphericalNever spherical Coronal & sagittal radius Coronal & sagittal radius

are differentare different Smaller contact areas Smaller contact areas

(than spherical)(than spherical) often it is an often it is an "" extension extension ""

from the fixed bearing from the fixed bearing line: line: PFCPFC RP, Natural RP, Natural, HLS , HLS …as most of the …as most of the companies want to have a companies want to have a mobile bearing knee in mobile bearing knee in their systemtheir system

Moving from a fixed bearing to Moving from a fixed bearing to a mobile bearing without a mobile bearing without

changing the femoral implantchanging the femoral implant

Allows to increase the conformity Allows to increase the conformity of the tibial insert (in rotation & of the tibial insert (in rotation & ML), ML),

And so, to increase the contact And so, to increase the contact area which should be very area which should be very beneficial for the wear issuebeneficial for the wear issue

It’s like an UPGRADE !It’s like an UPGRADE !

Contact area:(At which flexion angle?

Through full ROM)

Large difference if:

-Multiple sagittal radius

--Single sagittal radius

--- PS (smaller coronal radius to allow V/V tilting)

530 mm2 407 mm2

902 mm2

But in face of V/V tilting and But in face of V/V tilting and subsequent lift-off which subsequent lift-off which divides the area by 2, divides the area by 2, 900 900

mmmm2 2 seems saferseems safer

Remember with LCS and poor PE, wear is exceptional !

#2 Motion of the Bearing relative #2 Motion of the Bearing relative to the tibial tray (H.Mc Ewen)to the tibial tray (H.Mc Ewen)Uni-directional Uni-directional

motionmotionLCS MB/ RPLCS MB/ RPNatural knee (MB)Natural knee (MB)Genesis II/Profix Genesis II/Profix

(rotating only)(rotating only)

TRAC, Interax ISATRAC, Interax ISAPFC PFC RP RP

Multi-directional motion

SAL, Rotaglide,

LCS AP Glide

MBK/LPS Flex, Oxford

Genesis II/Profix (Rot/Translating)

NB: There is a potential for increase wear due the "crossing patterns" motion at the BACK SIDE of the tibial bearing (file!)

Multidirectional Unidirectional

Study conducted at Leeds University by Hannah McEwen, J.Fisher

Has proven the value of the LCS concept designed 30 y ago !

UHMWPE Kinematics

UHMWPE orientates in

principle direction of sliding (Pooley & Tabor 1972, Proc.Roy.Soc.Lon.A, 329, 251)

Strength increased

parallel to sliding but reduced

strength transverse to sliding (Wang et al. 1996, Proc.IMechE, 210H, 141)

+ Multidirectional works as a file !

:

Consequence :

# 1

Abrasive Wear

Mono

directional

Multi

directional

# 2

Limited furrows

Huge amount removed

Fixed vs RP

Fixed bearingMultidirectional motion of femoral

component relative to bearing

RP Mobile bearingBearing rotation decoupled:

Linear rotation at tibial counterface with

reduced rotation at femoral counterface

Not to speak of Backside wear !

Control of the bearing mvtControl of the bearing mvt with cones, with cones, cylinders, slots, stops which may prove to be deleterious cylinders, slots, stops which may prove to be deleterious for wearfor wear

The poly cone (LCS RP) has proven its good The poly cone (LCS RP) has proven its good function for 3 decadesfunction for 3 decades

(uniplanar motion) (uniplanar motion) NB: NB: without the PS shear !without the PS shear !

StabilityStability is an issue is an issue Possibility of dislocations such Possibility of dislocations such

as posterior dislocations which as posterior dislocations which can occur with fixed bearings as can occur with fixed bearings as wellwell

But due to the possibility of But due to the possibility of rotation with MB, this dislocation rotation with MB, this dislocation is usually asymetricalis usually asymetrical

And results in a spin-outAnd results in a spin-out

LCS spinout but LCS spinout but very exceptional very exceptional

(4/2000)(4/2000)

StabilityStability is an issue is an issue

Deep Dish

Jump height: 6 mm

LCS RP prosthetic stabilityLCS RP prosthetic stabilityRelies only on the dish shape (uphill

principle) of the RP insert

jump height reflects :the "forgiven " laxity

But PS is attracting But PS is attracting many surgeons for the many surgeons for the following arguments:following arguments:

-more secure technique, -more secure technique,

-more difficult knees, -more difficult knees,

-better mechanics, with -better mechanics, with roll back after 70° ?roll back after 70° ?

-and hopefully better -and hopefully better flexion as with RPF ?flexion as with RPF ?

So, the PFC So, the PFC ΣΣ RP RP

Seems as an excellent Seems as an excellent compromise with compromise with increased conformity increased conformity and still with the PS and still with the PS ""security security “ but …. has “ but …. has half of the contact half of the contact area of the LCS !area of the LCS !

PS RP (0-70°)PS RP (0-70°)

The PS Post usually comes into contact The PS Post usually comes into contact with the bar only at 70°with the bar only at 70°

Between 0 & 70°, it functions as a dish Between 0 & 70°, it functions as a dish (like LCS) for AP stability :(like LCS) for AP stability :

- - butbut the the side contact between the post side contact between the post and the boxand the box controls the rotation controls the rotation of of the insert and the insert and ""assistsassists"" to maintain good to maintain good relationship between the femur and the relationship between the femur and the insert and prevent spin-outinsert and prevent spin-out

Jump HeightJump Height (engagement depth) of the FT (engagement depth) of the FT articulation.articulation.When it is exceeded, danger of dislocation When it is exceeded, danger of dislocation or spin-outor spin-out

TRACHLS

IB2

Importance of surgical technique to keep good stability and so allow soft tissues to function & dissipate the stresses (load sharing) that can lead to wear & loosening

LCS

PS and MB ++++PS and MB ++++

During (high) flexion, some During (high) flexion, some rotation of the tibial insert rotation of the tibial insert usually takes placeusually takes place

The ability of the insert to The ability of the insert to rotate allows to maintain rotate allows to maintain excellent excellent large contact area large contact area between the postbetween the post and the and the cam avoiding edge contact cam avoiding edge contact and subsequent wear of the and subsequent wear of the post post

Probably, some Probably, some bearing bearing control control

mechanisms mechanisms will fail!will fail!

All MB knees are All MB knees are not equal and not equal and won’t survive won’t survive

equally !equally !

LCS systemLCS system

Unchanged for 30 years and widely copied today !

Rotating Platform

Results Complications LCS RPResults Complications LCS RP

Stability Stability FixationFixation Wear and osteolysisWear and osteolysis Patellar complicationsPatellar complications Survivorship Survivorship

LCS resultsLCS resultsa) complicationsa) complications

Meniscal bearingsMeniscal bearings reports of early reports of early

failures with failures with meniscal bearings: meniscal bearings: dislocations, dislocations, fracturesfractures

Reports of late Reports of late failures with wear failures with wear (MB exchange after (MB exchange after 10-15y)10-15y)

abandoned !

LCS spinout but LCS spinout but very exceptional very exceptional

(4/2000)(4/2000)

StabilityStability is an issue is an issue