What Is Segmental Instability

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An unstable spinal segment is the antithesis of a stiff spinal segment. It comes about when the two main stabilising structures of a motion segment—the intervertebral disc and facet joints—become stretched and weakened, usually as a result of the degenerative process. Fullblown segmental instability is rare, and can be testing to cure by conservative means because so many systems are in trouble at once— all fuelling one another—and the sources of pain may be multiple.

There is always fleeting 'muscular' instability brought on by temporary under-activity of the deep spinal muscles whenever a segment is inflamed. It is a type of reflex inhibition which switches off the local (intrinsic) muscles to spare the painful segment from additional compression, but it causes mischief when it exposes a structurally weakened segment to excessive movement.

This is the usual route a stiff segment takes when it becomes unstable, and though it may never eventuate, all stiff segments are vulnerable to this course of events unfolding. Once symptoms of frank instability have developed, they can be kept under control as long as the deep-acting segmental muscles do their job in keeping the weak link secure. From this, you can see how imperative it is to keep

Figure 6.1 When the fibrous union of both disc and facet capsules becomes stretched, the segment must rely on the primitive bony notching of the facet joints and muscle power to keep itself in place.

spines working properly once they have developed bad movement patterns from pain. You can also appreciate the scope of intrinsic exercises, through all stages of spinal dysfunction, in keeping instability problems at bay.

Segmental instability develops when primary weakness of the disc eventually translates across to the facets, or when laxity of the facets translates back to the disc. Both structures share almost evenly the job of gluing the spinal segments together and holding them secure when the spine bends. If one fails, greater responsibility is transferred to the other. As soon as the segment is loose in both front and back compartments, it can jostle around in the column with only the primitive bony notching mechanism of the facets and the power of the intrinsic muscles to keep it in place.

The confusing thing about instability is that it is often hard to pick up. The defensive response from the spine can be so total that your back can simply feel rigid. It is usually impossible to sense a loose segment because they all jam together en masse and the spine operates as a splinted rod. The weak link may only come to light when the spasm starts to fade and the spine goes to bend. As it goes over without being properly braced, the loose vertebra goes to slip forward, like a drawer slipping out of a chest of drawers as it tips over.

There are other circumstances where a loose vertebra moves, in a much more low-grade way, every time the spine bends. This may not be especially worrying; you may sense a tiny click or slip in your lower back as you go over, or a small wriggle in the movement, which you cannot prevent happening. In other circumstances, there may be a small arc of pain, just after your spine leaves the vertical, whereafter it moves freely and you can go right to your toes. Returning to upright, there is a similar painful phase in the movement, just as you near the top. This is exactly the point where the segment slips minutely.

Sometimes your spine continues for months with the segment slipping this way every time you bend. There is always a degree of background grumbling, achey stiffness but if you do something to hurt your back, it will suddenly stiffen more. As the muscles become more rigid, your back takes to moving more awkwardly and the clicking gets louder—until the stiffness becomes so limiting there is very little movement at all, and the clicking then stops. At this point, depending on other circumstances (such as how tired you are; whether you are unwell) the defenses of the spine can reveal themselves as not up to the job of protecting the weak internal link. If an action is awkward as well as weighty, or if there is another mishap as you go to bend, you will have an ominous sense of your spine starting to give way. Before you can stop it, you are caught and your whole back collapses, doubled over like a broken reed.

Although the degree of uncontrolled movement of the vertebra is minuscule, it still constitutes 'unstable' activity. The micro-trauma from the repetitive slippage and the giving-way incidents all add up to inflame the structures trying to hold everything in place.

The aberrant movement is usually forward glide (segmental shear), because all lumbar vertebrae have scant restraint for controlling this. Usually, the forward tipping of the vertebra takes up the slack in the posterior ligamentous system, and in this way forward shear is controlled. As the vertebrae tip, their aft section lifts away from the neighbour below which disengages the bony facet lock. This frees it up to glide forward more until the two facet surfaces butt up against one another once again and the lock re-engages. In the clinical setting we simply control shear by building up the muscles that control tip, and by this I mean multifidus.

Figure 6.2 We have few soft-tissue restraints for controlling forward shear of the lumbar segments—only the bony block of the facets engaging. To invoke a soft-tissue brake earlier in range we can control shear by humping the back to control tip.

When the loose vertebra goes to slip, there can be a feeling of the weak link about to give way. This usually happens when the spine moves forward from the vertical position, like a stack of children's building blocks coming undone, as it leaves the relative stability of the vertical. It can also happen when the spine is stretched across in the slung-out position, such as when leaning across to make a bed.

It is usually a lapse in the guarding role of the tummy and spinal muscles, and then their too-late over-reaction, that brings you undone. They all clench instantaneously which takes your breath away, but unlike the out-of-the-blue facet locking episode described in Chapter 4, you have a sense of the familiar; you feel danger coming and you can stop it before it reaches the critical point. The muscles growlingly go on guard, flickering in a menacing sort of way as you veer into your 'weak territory'; a warning sign to stop the movement and backtrack out of it before you go too far.

If time passes without the back folding up under pressure, the incremental slippage can stop happening. One day you realise the stiffness has gone, and the clicking beneath the surface has also faded.

Your back moves better as the harmony of the muscles returns. This usually happens with a fitness or weight-control binge, particularly if it also involves tummy strengthening. It means the weak link has made itself more secure by becoming stiffer, or the segmental muscles across the link have taken to working better. This precarious truce is more or less maintainable (unless you do heavy pushing work), but the former rumblings cannot be wholly ignored. The weak link will always be the first to give way when your back is next put to the test.

When the weak link is stressed, pain may not come on immediately; returning stiffness may be the first clue. This gets more and more noticeable a few days after chopping down a tree, or taking a car trip over a rough road, and then a gnawing pain starts in the leg. The weak link is more susceptible to knocks and bangs passing through the spine, and slowly the level of inflammation rises as the segment is squeezed and the disc is 'milked' by the tightening muscles.


Diagnosing full-blown instability can be difficult because of the complexity of the pain picture. There can be so much pain, it is hard to know where to start. Action X-rays rarely show the segment opening and closing more than it should do because the spinal defenses are much too clever for that. The surrounding muscles splint the loose vertebra and make it appear stiff. A discogram can show internal degradation of the disc, but the most definitive sign of instability is none of these: it is the presence of tooth-like extensions of bone (osteophytes) around both the interbody and facet joints.

With longstanding instability, the body throws out extra bone around the disc perimeter where it meets the vertebral body. The bony spurs provide a broader base of bone for the disc wall to take hold, creating additional surface area for the individual fibres of the disc wall to tie themselves to, thus providing stronger anchorage for the stretching wall.

Similar changes occur in the facet joints when the primary instability is there. The lower facet surface remoulds itself to make a more enveloping lip of bone which curls up around the upper facet surface to hold it in place. In the business, these are called 'wrap around bumpers' and they too are thought to be an ingenious attempt by the body to make the joint more secure. The bony cupping traps the upper vertebra and thus reduces its ability to slide around in-joint.

Figure 6.3 Claw-like bony outgrowths (osteophytes) of the vertebral body indicate instability of the front compartment whereas wrap-around bumpers of the facets indicate bony re-moulding in response to instability of the back compartment.

With instability problems, a great deal of focus must be given to reeducating proper free-flowing spinal movement, in case some freak action creeps in under the spine's guard and wrenches an unprotected weak link. This will create an unstable segment where none existed before.

In fact, that is the central aim of this book: preventing a stiff spinal link becoming unstable, or better still, preventing a stiff link developing in the first place.

Not uncommonly, the segment soldiers on valiantly with its chronically stretched ligaments, coping quite well with the anomalous movement of both front and back compartments. Often, it is when you do something extra to hurt the back that things flare up; the facet swells more or the bulging disc starts impinging on the nearby nerve root. Savage, intolerable leg pain often brings things to a head—and often to the point of a surgeon's knife.

Figure 6.3 Claw-like bony outgrowths (osteophytes) of the vertebral body indicate instability of the front compartment whereas wrap-around bumpers of the facets indicate bony re-moulding in response to instability of the back compartment.

Spinal surgery

Operating on spinal instability involves surgically joining the loose upper vertebra to the lower one by inserting two large titanium screws through both facet joints and then packing out the evacuated disc space with bone chips taken from the pelvic bone. This is called a spinal fusion. It is usually done after first removing the flaccid disc (and sometimes part of the facet joint) in order to relieve the pressure on the spinal nerve root. These procedures are called discectomy and partial facetectomy.

More recently, a whole slew of less invasive operative techniques have come into vogue to provide sorts of quasi-fusions. They involve using plastic or metal struts to join together either the transverse or spinous processes of two segments to limit excessive movement (and compression) rather than obliterate movement altogether, as the older fusions did. Though their rationale may be straightforward in controlling segmental participation, I have doubts about their efficacy, mainly because their alignment in situ has difficulty in controlling forward shear.

Like all operative procedures, I feel much more stringent selection criteria should be in place with proper biomechanical analysis and diagnosis before patients are shepherded this way as a treatment option. Unfortunately, though not the case with all surgeons, these devices seem to be the flavour of the month at the moment, whatever the spinal pathology and we need to see sound evidence for their deployment.

Furthermore, limiting segmental movement in this way brings almost to a halt any possibility of regenerating disc health by conservative means (because disc nutrition requires as much 'good' segmental movement as possible) so patients have to be sure that the slower route of regenerating disc health has been fully tried and tested before being discarded for the quick fix. And indeed, if surgery is to be contemplated, patients need actual figures of a surgeon's experience in using the device or procedure and the outcomes, without feeling difficult or demanding.

As you might imagine, surgical technique is of the essence with any spinal operation (I liken it to using a hammer and chisel on a

Stradivarius violin) because the spine is never quite the same afterwards; it is hard to 'go back' and conservative treatment is never quite as effective.

Apart from disturbing the delicately poised spinal mechanics, prolific scar formation causes many problems. If the scarring becomes invasive, it can be just as space-occupying and obtrusive as the structure deemed worthy of removal. In particular, the nerve root can be slowly strangled by the growth of adhesions, eventually causing the same symptoms of pressure on the nerve, and the old pain starts up again. Post-operative adhesions are similar to the post-inflammatory condition called 'root sleeve fibrosis' described with facet arthropathy and chronic disc prolapse (see Chapters 3 and 5).

The other complication of spinal fusion is the strain translated to the next working level up (L4 if L5 has been fused, or L3 if L4 and L5 have been fused). Both are almost flimsy compared to the robust L5-S1 junction and are ill-equipped to act as the seat of spinal movement. They are not bedded deeply in the pelvis like L5, nor do they have the august ilio-lumbar ligament to lash them down. Thus they are progressively over-taxed by routine movement. The problem usually takes several years to manifest and affects L3 more seriously than L4. Intrinsic spinal strengthening is therefore a critical part of a post-fusion regimen.

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  • Tomburän
    What is segmental instability?
    1 year ago

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