The vertebral movements

The movements of the vertebrae in the spine are a combination of gliding, tipping and twisting, although each one individually contributes only a small degree. Superimposed one on one however, the net result is the grandiose wide-ranging mobility of the spine, which is so well known to us. From our towering height we can arch backwards under a limbo bar and bend over to cut our toenails. Well, some of us can.

Of the vertebral movements, gliding is the least generous. The upper vertebra slides transversely forwards, backwards and from side to side on the vertebra below but the actual distances travelled are minute. Glide is more of a background movement which better positions the vertebra for action; it sets the stage and puts the

Figure 1.14 The vertebral bodies roll around on their disc while the facets at the back act like guide rails to keep the movement in check.

vertebra at its optimum starting point for the more adventurous tipping and twisting activity to follow.

As we bend to touch our toes, for example, each vertebra moves forward incrementally on the one below, bringing the upper one to its perfect starting point for tipping. The element of glide contributes the typical flowing quality to the way all living creatures move (a cheetah on the run has it in spades). Without it, all living actions are much more clipped and jerky and not nearly as streamlined and expansive. An elderly lady tottering along the footpath has very little glide in her joints.

The right amount of vertebral glide is important; it is what healthy backs have. Too little or too much glide leads to trouble. If a segment has too little it will be stiff. And significantly, when the degenerative process sets in, it is the first movement to go. Although you cannot necessarily see it—first you only feel it—this lack of background movement makes your spine feel tighter; more restricted and laboured in everything it does. In short it makes you feel rigid and older than you are.

At the other extreme, a vertebral segment is unstable if it has too much glide. It comes to light when the spine bends over and the top vertebra slips forward on the lower one. This is known as segmental instability, and it actually stems from stiffness of a segment. What distinguishes the former from the latter is the total degeneration of the motion segment. It is the stuff of this book and discussed step by step throughout.

With normal segmental mobility, all the vertebrae roll around on their liquid ball-bearing discs whose fibrous walls keep everything in place. As the upper vertebra moves off-centre, tension takes up in the wall mesh and brakes the action. And as soon as bend is incorporated into the action, another brake comes on from the squirting pressure of the nucleus.

With bending forward, which is our most-often repeated action, a healthy nucleus squidges under pressure towards the back of the disc. The migration of this buoyant bubble of fluid thrusts up the back of the vertebra above, thus 'bending' the segment in a tipping action. The opening of the back of the interspace also increases the tension-hold of the mesh of the disc wall and invokes powerful restraint from the capsular ligaments of the facet joints.

Figure 1.15 The bending action invokes its own brake when the backward migration of the nucleus pre-tenses the back wall of the disc. This makes it harder for the two vertebrae to pull apart.

Thus, a well-hydrated disc provides a high intradiscal pressure which adds dynamic stiffening to both the disc wall and the posterior ligamentous system. It also provides good cushioning and clearance between its two neighbouring vertebrae. So, you can see how a wellhydrated disc improves the restraint functioning of both the front

Figure 1.15 The bending action invokes its own brake when the backward migration of the nucleus pre-tenses the back wall of the disc. This makes it harder for the two vertebrae to pull apart.

tensed disc wall lattice and back compartments and in fact creates 'stability' of a working segment. On the other hand, if a nucleus lacks sufficient water, the disc narrows and the segments will settle together, bone on bone. It also fails to provide sufficient oomph to spring-tip the upper segment as the spine bends and the segment will shear forward instead of tipping. As you will read in Chapter 6, this is usually how a degenerating disc becomes over-mobile in the spinal column and eventually becomes unstable. Segmental instability is the end stage of segmental breakdown.

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