LIVRE JDG 2010

How are the Kinematics Driven?

How is the Apparent Paradox of

Differential Bony and Contact

Point Motion Explained?

The ‘four-bar’ linkage theory is fundamentally

flawed: it is a mathematical model in a single

plane, relying on all ‘bars’ being taut throughout

knee motion. The reality is that the cruciate

ligaments are arranged in 3 planes, and not

taut/straight during the whole of knee motion.

It is the shapes of the articular surfaces which

drive knee motion and explain the paradox

medially of negligible anteroposterior move-

ment of the femoral condyle but posterior

transfer of the articular contact point [3].

Laterally, in sagittal section, there are effecti-

vely two convex surfaces of the tibia and

femur, which favour motion. However, the

coronal section of the tibia laterally is concave

so providing stability. The sagittal section of

the lateral femoral condyle is almost a single

radius arc. As a result, as the condyle moves

back so does the articular contact point.

Medially the arrangement is complex. In the

sagittal plane the medial femoral condyle

actually describes arcs of 2 circles [not single-

radius curvature, not ‘J-shaped closing-helix’

as in many prosthetic designs]. The tibia is flat

in the posterior half with an anterior upslope.

This arrangement allows transfer of contact

JOURNÉES LYONNAISES DE CHIRURGIE DU GENOU

162

Fig. 1 : Schematic showing imaginary lines for each position in the knee flexion arc connec-

ting the reference points on medial and lateral condyles, used for measuring femoral posi-

tion relative to corresponding tibial condyle, superimposed on a tibial plateau. The tibia was

in ‘neutral’ rotation.