Mechanics
Although a biological joint can resemble a man-made joint in being a hinge or a ball and socket, the engineering problems that nature must solve are very different because the joint works within an almost completely solid structure, with no wheels or nuts and bolts.
In general, the bearing surfaces of manmade joints interlock, as in a hinge. This is rare for biological joints (although the badger's jaw interlocks).
More often the surfaces are held together by cord-like ligaments. Virtually all the space between muscles, ligaments, bones, and cartilage is filled with pliable solid tissue. The fluid-filled gap is at most only a twentieth of a millimetre thick. This means that synovium has certain rather unexpected jobs to do. These may include:
- Providing a plane of separation, or disconnection, between solid tissues so that movement can occur with minimum bending of solid components. If this separation is lost, as in a 'frozen shoulder', the joint cannot move.
- Providing a packing that can change shape in whatever way is needed to allow the bearing surfaces to move on each other.
- Controlling the volume of fluid in the cavity so that it is just enough to allow the solid components to move over each other freely. This volume is normally so small that the joint is under slight suction.
Read more about this topic: Synovial Membrane
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