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Kinematics |
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Flexion/extension |
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Flexion/extension:
The knee joint has two compartments, a medial one on the inner side and a lateral one on the outer side.
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The articulating surfaces of the tibia have different shapes. The medial surface is concave (cup-shaped, in red), while the lateral is convex (bump-shaped, in cyan). As a result, the rotational axes (in yellow) of the two surfaces fail to overlap; they are separated by a significant distance. |
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The condyles of the femur do not share the same axis; rather, the axis of the lateral condyle lies behind the axis of the medial condyle. |
These four axes, which couple to each other via bones and tendons, form what in biomechanics is called a link quadrangle. Because this type of gear system is well understood, biomechanical theory can precisely predict the knee's path of motion
during flexion and extension.
For low angles of flexion the joint surfaces roll on each other. From about 30 deg, a
sliding motion sets in, which completely dominates the movement as flexion angles
exceed 60 deg.
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Path of motion in a natural knee |
Why did Nature develop a link quadrangle? Why does it use a motion that combines rolling and sliding? While it was shown more than 100 years ago by German medical scientists that the knee combines rolliing and sliding motions, the purpose of this design was only recently understood by researchers from the University of Göttingen . Far from an arbitrary feature, the special shape of the joint surfaces turns out to be a critical design principle for the function of the knee:
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Rolling: this type of movement is characterized by very low resistance and thus minimal wear and tear. This is the reason why cars use rolling wheels instead of sliding tracks.
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Rolling is very stable, i.e. rolling means directed motion, which does not involve random or chaotic movements. This is why the game of pool is
(mostly) an art and not pure chance.
Rolling means that one surface rotates and translates relative to another surface. In the human joint, the femoral condyles roll backwards on the tibial joint. The tendons and muscles are tuned to support this “roll-back”. The roll-back helps reduce the loads on the cartilage and thus aids durability.
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Sliding: in this type of movement one of the surfaces has a fixed contact point. As a result, it is subjected to a significant level of wear and tear, especially when large loads are applied.
In this case, friction, which needs to be overcome during sliding, becomes a very serious issue. It causes shearing forces, heat and wear and tear on the surface, all of which negatively impact overall durability. .
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The link quadrangle `automatically´ prevents excessive stretching. As a result, normal stretching is limited in a springing fashion, i.e. there are no hard stops in the knee joint.
It is important to note that during walking and running, i.e. when huge loads are applied to the knee joint, the rolling motion predominates.
The sliding motion only happens during squatting or bending forward, and sitting down, i.e. activities, which usually involve much smaller loads.
One could argue that Nature combined these two types of movements (rolling during loads, sliding without loads) in an optimal way. This special kind of gear system represents what is called a link quadrangle in biomechanics.
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