164 / 324
between the femoral condyle and the anterior
slope in the 0-30° range to the posterior flat
tibia in the 30-120° range without antero-pos-
terior motion of the femoral condyle. If the
medial femoral condyle had single radius cur-
vature [as in the Oxford UKR] the condyle
would have to move posteriorly with its arti-
cular contact point. In the case of the Oxford
UKR this produces a kinematic compromise
rather than ‘normal’ knee kinematics.
Our model of tibio-femoral kinematics
explains how available flexion, and length of
patello-femoral lever-arm are maximised. Also
the external rotation of the femur aids patellar
tracking.
How is Such a Great Range of
Flexion Possible Whilst
Maintaining Stability?
If, as in the ‘old’ roll-back model, ‘roll-back’
occurred both medially as well as laterally sur-
ely the tibio-femoral joint would be prone to
dislocation.
The limit to the range of flexion is determined
by tension in the extensor mechanism anterior-
ly and posterior tibiofemoral impingement
posteriorly. Laterally posterior impingement is
delayed by near subluxation of the femoral
condyle as it descends the ‘down slope’ of the
posterior tibia. It comes to lie in a precarious
position. Stability is the result of motion in the
medial compartment, and because in the coro-
nal plane the femoral condyle is stabilised in
the concave tibial surfaces.
Medially stability is maintained by the fact
that the medial femoral condyle remains arti-
culating with the tibial plateau until very deep
in flexion when it does go back to ride up on
the well fixed posterior horn of the medial
meniscus. Only then do the posterior
tibia/femur near contact. So how then does the
medial compartment delay this impingement
until maximal flexion is achieved i.e. when the
calf and thigh soft tissues compress? The ans-
wer lies with the shape of the medial articular
shapes: the larger medial femoral condyle pos-
terior offset; and the net posterior tibial slope.
Tension in the extensor mechanism is reduced
by the anterior bevel on the tibia which accepts
posterior displacement of the patellar tendon
as the patella moves posteriorly with lateral
femoral roll-back and the simple fact that the
A-P size of the femoral condyles is greater in
extension than flexion. Furthermore the exten-
sor mechanism is further de-tensioned in the
coronal plane, and kinematics aided, by reduc-
tion of ‘Q-angle’ accompanying external
femoral rotation during knee flexion.
The Concept of ‘An Envelope of
Motion’
The work so far presented was based upon the
knee flexing when the tibia was placed in a
‘comfortable’ neutral position of rotation.
During one of our first studies we made mea-
surements with the tibia not only in the ‘neu-
tral’ position but also in maximal internal and
external rotation [4]. The findings for tibial
internal rotation were not dissimilar from the
findings above but slightly more marked axial
rotation was seen. In the external rotation
group results showed no overall axial rotation
at all- the knee moved almost as a unaxial
hinge (fig. 2) ! Only flexion ranges up to 90°
could be studies as subject felt that their knees
were unable to move much further. This is an
example of how tibial internal rotation is so
vital for deep knee flexion. Due to this variabi-
lity of motion that we demonstrated it is impor-
ted that it is stated that the model of tibio-femo-
ral kinematics in which axial rotation of the
two bones accompanies knee flexion is a ten-
dency rather than obligatory. It is, however,
required for full range of flexion, and to opti-
mise patello-femoral kinematics/kinetics.
NORMAL KNEE KINEMATICS
163