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THE DISK: SHOCK ABSORBER AND PRESSURE DIFFUSER

The disk absorbs shock and diffuses pressure on the spine. The farther down the spinal column toward the lumbar region one goes, the more the pressure loading of the column increases.

The superior cervical spine is nevertheless capable of supporting the head and performing precise movements without the aid of intervertebral disks. Thus, in spite of its apparent fragility and thin articular surface, it is sub-jected to considerable loads.

In the normal vertebral spine, the healthy disk is per-fectly homogeneous and very hydrophilic. When it is loaded axially, its height decreases (Fig. 12.1) and the annulus bulges at the periphery like a tire under an exces-sive load. The decrease in the height of the disk space can be 1.5 mm for the lumbar disk of an individual carrying 100 kg on the shoulders. In fact, one must first add to this 100-kg weight, the weight of the trunk, and of the head, which is about 40 kg for a man weighing 80 kg, and then the intrinsic compressive force produced by muscular con-traction that increases compression on the disk. This means that the disk will have to support a load of more than 150 kg when the individual is upright and the disk is in normal position. If the individual bends forward, the pressure on the nucleus pulposus will be transmitted most forcefully to the posterior annular fibers — the weakest of the fibers that make up the annulus fibrosus.

PRESSURE

Some authors have calculated the approximate com-pressive and distractive forces on the mobile segment in different anatomic positions, both with and without extrin-sic loads. Here are a few examples taken from the studies of Herbert:

• Subject standing, without overload: the lumbar disk is submitted to a pressure of 15 kg/cm² in the nucleus, taken in tension by the annulus, and to a strain of shearing of 13 kg in the corresponding facet joints.

• Subject in forward flexion: the disk is submitted to a pressure of 58 kg/cm² in the nucleus, taken in tension by the annulus, and to a shearing of 47 kg in a plane perpendicular to the axis of the column, absorbed in compression of the facet joints.

• Subject in full flexion, lifting up a bar of 100 kg: the disk is submitted to a compression of 144 kg/cm² in the nucleus, taken in tension by the annulus, and to a shearing of 126 kg supported by the facet joints, absorbed in compression.

In this case, theoretically, there is a total pressure of 1000 kg on the nucleus. This demonstrates the enormous pressures exerted on the intervertebral segments.

By measuring intradiscal pressure, Nachemson intro-duced some new notions: (a) load on the nucleus is greater when sitting than when standing (measured on the L3 disk); (b) pressure is maximal during sitting, with the subject bent forward and lifting a load, arms hanging; (c) the lowest pressure is measured with the subject supine;

and (d) intradiscal pressure increases 45% with coughing and 45% with trunk rotation. For a 70-kg man in the upright position (Fig. 12.2), for example, the pressure on the L3 disk is 70 kg. It is about 120 kg if he bends forward 20°, and it is 340 kg if he lifts a load of 20 kg with his legs extended.

In a preliminary study, Drevet, using a material capable of taking both static and dynamic measures (a miniatur-ized piezo-resistive capacitor), noted:

• Important variations in the basal pressures in sub-jects of the same age and of identical morphotype

• Extremely rapid variations according to exogenous factors (suspension, carrying loads, etc.)

• Piezo-resistive measurements ranging from 1 to 3, depending on contraction of the muscles

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The lumbar spine is not alone in being subjected to such stresses. There is, for example, the cervico-occipital junction, with its narrow bony structures and slender artic-ular surfaces that support and allow for the mobility of the head — a large 5-kg ball that rests in equilibrium on two small facets the size of nails and can be mobilized in all directions. Some persons carry on their heads loads of over 50 kg that are transmitted in their entirety to the articulations of the first two vertebrae, since at that level there is no disk to dissipate compressive loads (Fig. 12.3).

ROLE OF ABDOMINAL WALL

Intra-abdominal pressure generated by the contraction of the abdominal musculature has to be taken into account (Bartelink) when evaluating the effective load that the lumbar spine must support. Any lumbar effort in effect

“leans” on contracted abdominal muscles. The intra-abdominal pressure absorbs part of the load: 30%, accord-ing to Morris et al. The contraction of the abdominal muscles can be replaced or supplemented by the resistance

that a corset or belt provides (as with a weight lifter’s belt). A corset or belt helps by providing an inelastic resistance against which an elastic, contracted, or lax abdominal wall cannot expand, thus increasing the intra-abdominal pressure. Thus, good intra-abdominal muscle tone and a strong diaphragm and pelvic floor aid in reducing the load on the lumbar spine by reducing the pressures that it must support. Furthermore, because of their attach-ment to the thoracolumbar fascia, the abdominal muscles create an extensor moment upon the spine when they contract, imparting further stability.

ROLE OF VERTEBRAL BODY AS A SHOCK ABSORBER

The disk may not be the only shock-absorbing element of the spine. In light of the studies of Lamy and Farfan, it appears that the vertebral body itself functions as a shock absorber with variable resistance. First, the authors expressed the liquid contained in a vertebral body, approx-imately 0.8 to 1 mL for each vertebral body. Then they

Figure 12.1 a, b. The axial pressure dissipated by the disk is transmitted to the annulus fibrosus and to the vertebral endplates.

c. During anterior flexion, the nucleus pulposus moves posteriorly.

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studied 60 cadavers. From 30 of them, they squeezed out half of the vertebral liquid, using pipettes placed in the vascular holes (0.5 mL/vertebra). The other 30 cadavers remained untouched and formed the second group. To each of the columns, the researchers applied increased axial loads. Under weak loading, the two groups behaved similarly. When the load was increased gradually, the nor-mal vertebrae of the second group demonstrated increased resistance in contrast to the vertebrae of the first group.

The authors concluded that the vertebral body has an automatic hydraulic system that modifies its resistance according to the load received. They also believed that when the lumbar spine is in a flexed position, the posterior ligaments act like a valve, blocking venous circulation on its return course, while intra-abdominal pressure is suffi-cient to block the external venous system. These two mechanisms that block intervertebral venous circulation act synergistically to increase the elastic resistance of the vertebral body during effort.

ROLE OF SACROILIAC JOINTS

It seems clear that the essential role of the sacroiliac joints is serving as very powerful shock absorbers (see Chapter 11).

Figure 12.2 For a 70-kg standing man, the pressure on the L3 disk is on the order of 70 kg. It changes during forward flexion, when the angle reaches 20° from the vertical, to a pressure on the order of 120 kg. When a weight of 20 kg is lifted with the legs extended, the pressure reaches 340 kg (numbers according to Nachemson). Intradiscal pressure, however, depends not only on the weight lifted, but also on the intensity of the contraction of the muscles involved.

Figure 12.3 The subject is holding on his head a weight of 30 kg. This mass and weight are transmitted to the atlanto-occipital and the atlanto-axial articulations. The first disk to encounter this pressure is between C2 and C3.

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