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For several decades accepted theories of nor-
mal knee kinematics have been dominated by
the flawed concept of femoral ‘roll-back’ [in
which the whole femur, medially as well as
laterally, moves posteriorly on the tibia during
flexion]. This has been said to be driven by
tension in the cruciate ligaments [‘the four-bar
linkage concept’]. This model was said to
confer the advantage of maximising flexion
and elongating the patello-femoral lever-arm
in the flexed knee when it is most needed. This
has so dominated thinking that virtually all
current TKR designs incorporate mechanisms,
with or without the PCL, to achieve ‘roll-
back’. Recent kinematic studies of TKRs show
that, especially PCL sparing designs, do not
replicate normal knee motion and therefore
have function compromised.
Using vertical open-access MRI we have the
capability of studying knee kinematics in 3
dimensions whilst weight-bearing in the living
subject.
The Motion of The Bones
Our studies show that in the 0-120° range there
is negligible anteroposterior motion of the
medial femoral condyle, whilst the lateral femo-
ral condyle moves back, by a combination of
rolling and sliding, by an average 22mm [1].
The motion in the medial compartment is
almost pure sliding. This differential medial
and lateral motion equates to axial external
rotation of the femur on the tibia during
flexion. Thus ‘roll-back’ of the whole femur
does not exist (fig. 1).
120° is the limit of active flexion. Further
flexion is a passive phenomenon, and results in
a different pattern of motion: both medial and
lateral femoral condyles move back about
10mm to 140° [1].
The Motion of the Bones and
Articular Contact Points is Not
The Same
Our studies have shown that in contrast to our
observations of bony motion [above], the
motion of points of contact between the femo-
ral condyles and their respective tibial sur-
faces has a different pattern. Both medially
and laterally the contact points move back to
a similar degree [approx 20mm] in the range
0-120° [2]. Laterally the movement is steady
due to the ‘roll-back’ process but medially it
occurs as a rapid transfer of contact about 30°
by a ‘rocking’ process.
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NORMAL KNEE KINEMATICS
Achieving Deep Flexion Whilst Maintaining Stability
and Optimising Patellofemoral Function
A. WILLIAMS