The intervertebral discs

The intervertebral discs are resilient, water-filled pillows between the vertebrae. They have scant nerve supply and no blood supply; indeed discs are the largest avascular structures in the body. This also means that other mechanisms must be utitilised to transport raw materials and waste products to keep the discs alive, which will be discussed below, but even at the best of times discs struggle to remain viable.

Discs are designed to give us flexibility at low load and stability at high load and it is significant that our vertical posture helps achieve this by enhancing the tensile strength of the disc walls. It adds to the pressurising of the fluid sacks and converts the spine into a whippy spring-loaded rod of many segments which can flip up straight again after bending.

Figure 1.8 The strap-like anterior and posterior longitudinal ligaments encase the front and back of the 'cotton reels' like a ligamentous strait-jacket.

disc nucleus disc nucleus

Figure 1.9 The disc's nucleus acts like an hydraulic sack distributing forces outward and evenly in all directions. The greater the pressure, the greater the tensile strength generated in the outer disc walls.

Without these tensile properties, the human back would not be the long slender thing it is. We would need a hugely muscular apparatus to haul us up straight again once we had doubled over. However, vertical posture does have its down side. It means the segments at the bottom of the stack are always more loaded by the weight of the rest of the body towering above.

High intradiscal hydrostatic pressure of the discs thrusts the vertebrae apart while at the same time as it helps glue them together. Each has a vigorous incompressibility, like standing on a breadboard balanced on a beachball. They give the spine a quivering up-thrusting romp, while making it secure enough to bend without getting stuck, flopping over like a broken reed.

The squirting liquid nucleus of each disc is contained by a tough, circular containing wall called the anulus fibrosis. The anulus is like an onion skin, made up of approximately 12-15 thin fibrous layers called lamellae. The fibres of each successive lamella run in diagonally opposing directions at an angle of 65 degrees to the horizontal. This provides maximum strength, yet freedom for the wall to pull up, like a lattice stretching, as the spine bends. This fibre angulation makes the disc harder to stretch than if they ran transversely, but easier than if the fibres were aligned vertically to the direction of movement.

The lamellae at the back of the disc are thinner and bunched more closely together, making it possible for the wall to stretch vertically by

50 per cent. This provides freedom for the interspaces to gap open at the back so the spine can bend forward freely. It also means it is weaker, introducing a precarious trade-off between freedom to bend and the possibility that over-bending can break down the wall.

Often the L5 discs are kidney shaped, which exposes a longer flank to increasing the holding power of the back wall. However, kidney-shaped discs have the disadvantage of runkling more in the acute back corners when torsional strains are applied to the disc. You will see later how heavy-duty lifting and twisting can make the disc wall perish at these points.

The roles of the inner and outer disc walls are very different. The middle/inner anulus creates a super-strong capsule which helps hold the watery nucleus contained under pressure. The lamellae here continue in a circular fashion, around both sides of the disc and through the endplates above and below, thus trapping a buoyant hydrostatic pressure within the disc to force the segments apart. This part of the disc bears, indeed rebuffs, compressive axial load. Conversely, the outer annulus works like a tensile ligamentous 'skin' that holds adjacent spinal segments together. Just like any other ligament in the body, this part of the anulus restrains the bones from moving apart and has no role to play in shouldering load. Indeed a low intradiscal pressure can cause the outer disc wall to buckle outwards under load (or when compressed by muscle spasm) and this is often wrongly construed as a prolapsed disc.

Figure 1.11 The middle-inner part of the disc wall acts like a capsule to bear load whereas the outer part holds the segments together and is more like a tensile skin. Like any ligament, this 'ligamentous' part of the disc can scar and adaptively shorten, which makes that segment become a stiff link in the spine.

(Illustration acknowledgement: N. Bogduk, 'Clinical Anatomy of the Lumbar Spine')

Also like any other ligament, this part of the disc has a nerve supply and readily registers pain. The outer anulus complains if wrenched or overstretched by trauma (recall the pain of a twisted ankle) and also becomes painful if it adaptively shortens and cannot 'give' with movement. This is what happens when a disc dries and loses height and this part of the back then becomes painful to bend. As you will read in the next chapter, discomfort is invoked as the spinal segments attempt to pull apart and eventually inflammation sets up at that segmental level, which I believe is the nub of simple back pain.

In the healthy state however, both parts of the disc wall work well to complement each other. A high intradiscal hydrostatic pressure 'inflates' the central capsule and pre-tensions the outer ligamentous nucleus

Figure 1.11 The middle-inner part of the disc wall acts like a capsule to bear load whereas the outer part holds the segments together and is more like a tensile skin. Like any ligament, this 'ligamentous' part of the disc can scar and adaptively shorten, which makes that segment become a stiff link in the spine.

(Illustration acknowledgement: N. Bogduk, 'Clinical Anatomy of the Lumbar Spine')

endplate endplate wall, thus making it hold more securely with the right amount of hold-and-give. Just as inflating an inner tube of a tyre gives the outer wall suitable tensile strength, the spine can bend and sway freely with the internal pressure of the disc matched by the tension of the restraining disc walls, and everything moves under control. If a lack of internal disc pressure fails to invoke sufficient holding wall tension of one of the discs, that segment can become loose and shear forward as the spine bends. This is the main cause of an unstable spinal segment.

Figure 1.12 Only the outer tensile ligamentous 'skin' of the disc wall is pain sensitive.

(Illustration acknowledgement: N. Bogduk, 'Clinical Anatomy of the Lumbar Spine')

Proteoglycans is the magical x-factor that attracts and holds fluid in the discs. Its unique molecular make-up exerts a powerful osmotic pull on water that counters the effects of gravity bearing down and squeezing the discs dry. Healthy nuclear jelly taken from a disc and set in a saucer of normal saline solution swells by 300 per cent and this force within the discs becomes more potent through the day as they lose fluid and the proteoglycans concentration rises. Even so, all discs lose about 20 per cent of their fluid each day, which they recoup when we lie flat to sleep overnight. This steady seepage of discal fluid by day, with fresh quantities absorbed again overnight, is the main way discs nourish themselves; this stately exchange is only possible because the metabolic rate of discs is so slow. Healthy discs have

grey ramus

Figure 1.12 Only the outer tensile ligamentous 'skin' of the disc wall is pain sensitive.

(Illustration acknowledgement: N. Bogduk, 'Clinical Anatomy of the Lumbar Spine')

a high concentration of proteoglycans whereas degenerated discs do not. This means that degenerated discs are drier, lose their fluid more rapidly during upright hours, and are slower to regain it when compression comes off. Low proteoglycans discs become thinner, poorer spacers, poorer shock absorbers and weaker spinal connectors.

Very importantly from a therapeutic point of view, synthesis of proteoglycans is stimulated by pressure changes through the discs. Normal on-off gravitational forces sustained by the spine as it bends, twists and lifts everyday things enhance both the fluid content of discs and the movement of fluid through them, as well as stimulating disc cell metabolism in general. This simple physiological truism (called 'mechanobiology') gives discs their best chance to regenerate or heal and its significance is essential to the understanding and implementation of effective 'spinal therapy'. It underlines the main focus of self-help treatment and the various means of achieving it are explained in each of the following sections.

The nutrition of the discs

The tenuous viability of discs, even in their healthy state, means that nutrition is critically important for coping with the ongoing incidental trauma of everyday life. Poor disc nutrition has long been held as the chief cause of disc degeneration, so it follows that physical therapy is about improving disc nutrition—specifically by enhancing the disc's ability to hold and circulate water.

There are two main engines of fluid exchange whereby nutrients are absorbed and waste products expelled from discs. The first is the 'diffusion' mechanism described previously, which is transacted as a diurnal exchange over 24 hours; stale fluid slowly excretes through the day and fresh quantities are imbibed overnight. During the day, fluid is pressed out through the disc walls and vertebral endplates, and as they become drier there is a vertical settling of the column as the discs flatten and the spine as a whole loses height (we are all approximately 2 cm shorter by the time we go to bed). This engine is mainly effective in transporting smaller molecules involved in disc metabolism, such as glucose and oxygen.

The second engine is more active through the daylight hours.

The 'convection' method relies on pressure changes induced by grand-scale spinal movement. It creates a rhythmic suction and compression of the discs which shunts small quantities of fluids in and out from the rich capillary beds in the neighbouring vertebral bodies. This method transports larger molecules which have poor diffusivity, the most important of which are cytokines involved in the manufacture of proteoglycans.

This secondary 'pump imbibition' is an invaluable backup mechanism for nourishing discs, especially in circumstances where they suffer undue compression. For people who sit for many hours, particularly sedentary workers or long-distance drivers (who, incidentally, have back problems at four times the national average), squatting or curl down (touching toes and unfurling back up to vertical) exercises are particularly important ways of introducing pressure changes through the low back. The lumbar discs are subject to greater pressure in the early part of the bend but as the spine gets to the bottom and hangs the effect is one of relative suction. Activities of daily life are just as useful. No matter how humble the task, dynamic loading and releasing during spinal movement sucks and pumps the discs through the day, recouping small amounts of fluid and thereby retarding the inexorable settling of the spine.

Whole-scale spinal movement and its shunting effect on discal fluids become even more necessary in three important circumstances:

• when ageing causes occlusion of the small holes in the endplates through which fluids are exchanged from within the vertebral body

• when ageing causes fibrosis of the disc walls making them less permeable to fluid traffic

• when disc degeneration lowers the concentration of proteoglycans which lessens the disc's water-binding capacity.

Flamboyant spinal movement (with as little jarring impact as possible) keeps the spine young and 'gives the discs a drink'. Sadly, one of the oldest directives in orthopaedic medicine and the management of bad backs has been to limit movement—and this has directly hampered disc nutrition and the regeneration of spinal health.

Psoas Crooked Pelvis
Figure 1.13 Pressure changes induced by everyday bend-and-stretch activities help the discs suck and blow to feed themselves. They also stimulate the metabolic activity of discs.
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