The facet joints

Each motion segment has two facet joints forming the back compartment. These are the junctions formed where the vertebrae notch together at the back of the spine. They flank the back corners of each disc, across the gulf of the vertebral canal. The bony notching of the facets provides a primitive interlinking chain down either side of the spine which slots the spinal segments together. If they 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. The lumbar 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.

Figure 1.16 The lower facet surfaces are concave to receive the convex surfaces of the upper facets notching in. The paired facets flank the back of the spinal canal at each segmental level.

Neighbouring vertebrae contribute two opposing surfaces to make a pair of facet joints. Two notches of bone project up from the lower vertebral ring (superior articular pillars) interfacing with two projecting down from the upper vertebral ring (inferior articular pillars). The smooth-interfacing congruency of these four abutting pillars forms the two facet joints. They fit together snugly like two cupped palms, the valleys of one side matching the hills on the other. Each is covered by a buffer of hyaline cartilage with its ultra-smooth semi-compliance and rich mother-of-pearl sheen.

The convexity of the upper facet surfaces and the concavity of the lower ones allow the upper vertebrae to slot into place. The definite front-back alignment of the lumbar facets facilitates forward bending but restricts all other movements of the low back. It 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). It lets us lower the upper body forward, like a stooping mechanical crane, putting our hands and face at the right height to be useful.

Figure 1.17 The front-back alignment of the cupped lumbar facets facilitates forward bending. It also blocks twisting actions which weaken the discs.

There is good reason for the facets restricting lumbar twisting movement: it keeps things from wearing out. The twisting action in the low back is especially harmful to the discs. It challenges the inherent weakness of their walls, especially if there is lifting as well.

the inferior articular pillar of the vertebra above the superior articular pillar of the lower vertebra

Figure 1.17 The front-back alignment of the cupped lumbar facets facilitates forward bending. It also blocks twisting actions which weaken the discs.

the inferior articular pillar of the vertebra above the superior articular pillar of the lower vertebra

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.

The most capable and resilient joints in the body are synovial joints, and all of them—whether of the fingers, knees or facet joints— share common properties. They have a smooth cartilage buffering on opposing interfaces to minimise friction and they are knitted together by a strong fibrous sleeve or capsule, the internal lining of which secretes a lubricating synovial fluid.

Cartilage allows opposing facet surfaces to skid over one another with the yielding consistency of dense plastic, so that it deforms 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.

Synovial fluid—which has astonishing qualities of lightness and slipperiness—lubricates the cartilage interfaces and flushes them clean, similar to the way tears water the eyes. The strong 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 an 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 (macrophages) 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 silt up with cartilaginous grit acting like a pot-scourer, grinding away the joint surfaces until nothing was left.

The facet joint capsules are stronger than most synovial joint capsules and for this reason they usually go by the name of the 'capsular ligaments'. Almost equally with the discs, the capsular ligaments share the role of restraining the lumbar segments as the spine tips forward to bend, the disc holding the front compartments together and the capsular ligaments the back. In addition to their cartilage buffer joint space joint capsule synovial membrane synovial membrane cartilage buffer joint space joint capsule

Figure 1.18 The facet joints are synovial with their opposing cartilage-covered surfaces fitting snugly together like two pressed-together palms. The facet capsule holds the joints in place.

important ligamentous role, the facet capsules are fully innervated (well supplied with nerves) and have a prolific blood supply. Quite unlike the discs, the facets are wired for pain and have the propensity for florid inflammatory reaction in response to physical assault. For this reason, the facet joints are a common source of low-back pain, especially if the disc has started to degenerate and, in losing pressure, transferred a greater restraint role to the facet capsules.

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