Back Locked? Blame It on to the Facet Joints.
THE FACET JOINTS
These are the junctions formed where the vertebrae notch together at the back of the spine. Each motion segment has two facet joints forming part of the back compartment. They flank the back corners of each disc, across the Gulf of the vertebral canal. Neighbouring vertebrae contribute two opposing surfaces to make a pair of facet joints. Two notches of the bone project up from the lower vertebral ring interfacing with two projecting down from the upper vertebral ring.
Thus, two inter notching junctions are made.The facet joints do not bear a lot of weight unless the disc is thin or the lumbar lordosis extreme, but they suffer constant wear and tear in controlling the movement of their vertebrae. To protect the joint surfaces a properly functioning joint needs to exist where the two bones meet.The most capable and resilient joints in the body are synovial joints, and all of them-whether in the fingers, knees or facet joints-share common properties. They have extremely strong joint capsules which knit two sides of the joint together. The capsules have a well-weird nervous network to make the joint highly sensitive and they also have a prolific blood supply. The inner lining of the capsules (called the synovial membrane) floods the joint with synovial fluid which both dampens impact and lubricates the working surfaces.
The facet joint capsules are usually strong. With bending forward they provide nearly as much soft-tissue restraint as then discs in glueing the spinal segments together.Researchers have removed the discs from cadavers in a laboratory and shown that almost twice body weight can be suspended by the facet capsules alone. Thus they are more than simple joint capsules: they are more like ligaments and for this reason, they take the name ‘capsular ligaments’.
Each facet joint has a smooth-interfacing congruency of its opposing joint surfaces. These fit snugly together like two cupped palms, the valleys of one side matching the hills on the other. The surfaces of the opposing bones are covered by hyaline cartilage which is a smooth semi-compliant buffer with a rich mother-of-pearl sheen.Cartilage allows the bones to skid over one another and has the yielding consistency of dense plastic which allows it to deform imperceptibly whenever the bones make contact. The direct contact also squeezes fluid out, but when the pressure releases and the cartilage un-dints, it sucks water back in. In this way, the bloodless cartilage keeps itself healthy by creating a ‘circulation’ to pull in nutrients and expel waste products.The slippery cartilage interfaces are lubricated by synovial fluid just as tears flush the eyes and this synovial fluid has astonishing qualities of lightness and slipperiness.
The joint capsule keeps the fluid contained under pressure that springs the joint surfaces apart and softens the impact of bone on bone. It also means the joint operates on a cushion of fluid (in a hydraulic sack) which streamlines movement and takes out the jerkiness.Synovial fluid also cleanses the joint space by clearing away cartilage particles eroded off the main bed during activity.
The synovial membrane liberates large cartilage-eating cells into the tide of floating debris. These cells surround each particle, like an amoeba trapping its food, and dissolve it. It is essential cleaning-up work. Without it the joints would slit up with cartilaginous grit acting like a pot-scourer, grinding away the joint surfaces until nothing was left.
The chain of facet joints down the spine provides a primitive interlinking hook-up which notches the spinal segments together.
The upper facet surfaces are convex and the lower ones concave. This allows the upper vertebrae to lock in place when its convex pillars fit snugly into the concave cups of the ones below.If the facets were not there the vertebrae could roll around on their discs and the neurocentral core could tie itself in knots like a cartoon character of an India-rubber man.
However, the definite front-back alignment of the lumbar facets means our only generous movement of the low back is bending forward. Their configuration means the vertebrae only move forward and back, like the wheels of a train moving down the track, never twisting left or right (although they can side-bend a little). This lets us lower the rest of our body down, like a stooping mechanical crane, putting our hands and face at the right height to be useful.
There is a good reason for the facets otherwise restricting movement: it keeps things from wearing out. The twisting action in the low back is especially hard on the discs. It challenges the inherent weakness of their walls, especially if there is lifting as well. With only every alternate layer of the onion-skin disc wall offering restraint (while the fibres of the other half go on the slack offering no help), repeated twisting can be destructive.