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Consequently, the above mentioned question
should be answered negative. The native knee
has a typical passive kinematic pattern, as des-
cribed in several papers using different metho-
dology [4, 6, 7, 11, 17, 19, 20], however, the
largest joint in the human body is capable of
adapting to many different kinematic patterns,
depending on extrinsic conditions [18-20].
HOW DO NATIVE KNEE
KINEMATICS RELATE TO
TKA KINEMATICS?
As the native knee is moving as guided by its
articular geometries, gravitational and muscle
loads, within the limits of its ligaments, the
arthroplasty joint should function within this
so-called ‘envelope of function’. The implant
bearing surfaces and intrinsic constraints
should ideally compensate for the functions of
passive structures that are missing due to disea-
se or eliminated/modified by joint replacement.
This is a delicate exercise as insufficient ‘com-
pensation’ will lead to subtle instability. Many
papers have referred to the so-called ‘para-
doxical motion’, indicating an abnormal for-
ward motion of the femoral condyles during
flexion [21-23]. The clinical consequence of
this abnormal motion is unclear. The surroun-
ding tissues are certainly capable of coping
with this motion pattern but it leads to a less
favorable moment arm for the quadriceps sys-
tem, inducing relative weakness of knee exten-
sion. If the forward translation of the femur
occurs in deeper flexion, posterior tibiofemo-
ral impingement leading to decreased flexion
can result. This is probably the explanation for
the observed lesser flexion of cruciate retai-
ning knees as compared to posterior stabilised
devices [23, 25].
On the contrary, overzealous ‘compensation’ in
an attempt to reproduce the passive kinematic
pattern of the native knee also carries risks. We
helped design a knee arthroplasty mimicking
normal passive knee kinematics through gui-
ded motion. [26] Using the experimental kine-
matic model, the kinematics of the device were
validated in the passive setting, but as the knee
was loaded, the prosthesis did not adapt to the
typical changes induced by muscle loading [19,
24]. The increased posterior translation in the
lateral compartment of the prosthetic knee as
compared to the loaded native knee was belie-
ved to explain the iliotibial traction syndrome
we noticed in 7% of the patients treated with
this motion guided knee design [27].
It is clear that knee kinematics will not only
depend upon the implant design. Geometric ali-
gnment will largely affect post-operative knee
kinematics. Alignment mistakes can be gross
and lead to frank stiffness and largely unsatis-
factory results or subtle, leading to suboptimal
clinical results. An example is shown in
figure 5. The concept of a single radius design
is widespread, and promoted by virtue of its
more natural kinematics. One cannot consider
this geometric feature of the prosthesis having
a single radius, without taking the soft tissues
into account. In the example shown, the femo-
ral component is undersized and the joint line
raised as compared to the native knee (e.g. a
femoral component size 5 in stead of 6). The
surgeon compensates with a thicker polyethy-
lene insert (e.g. 14 mm in stead of 10 mm). In
the normal knee, the MCL is isometric and
inserts close to the centre of rotation of the
medial condyle on a 2D model [28]. The centre
of rotation of the knee is represented by the red
dot. In the prosthetic setting with the smaller
femoral component and the raised joint line, a
new centre of rotation will be created, repre-
sented by the blue dot. The MCL, represented
in clear blue, is nicely tensioned in full exten-
sion. As the knee goes to mid flexion, slacke-
ning of the MCL occurs as the insertion of the
MCL does not coincide anymore with the new
centre of rotation of the medial condyle, repre-
sented by the blue dot. In 90° of flexion, ten-
sion is normal again but in deeper flexion the
MCL is pulled tighter by the anisometric new
centre of rotation.
This example clearly illustrates how a ‘kine-
matic feature’ of a prosthesic design is influen-
ced by subtle surgical variability
COMPARATIVE KINEMATICS BETWEEN THE NATIVE KNEE AND TOTAL KNEE ARTHROPLASTY
169