Functional Anatomy

The nucleus pulposus in combination with the annulus fibrosus affords shock absorbency function to the disc and allows the functional disc unit its dynamic flexibility. With compressive forces, the nucleus pulposus stretches the annular fibers somewhat like a jelly filled tennis ball would flatten when a weight was placed on it. Flexion or extension of the functional unit occurs in part because of the horizontal shift of fluid within the disc, resulting in the expansion of the annular fibers posteriorly or anteriorly, respectively. The elastic nature of the annular fibers tend to oppose and contain the movement of the nucleus pulposus, thereby, tending to restore the functional disc unit to its resting state when the compressive forces are relieved. Sadly, as the disc ages, this elasticity is gradually lost and the capability of the disc to recoil from compressive forces decreases.

Fibro-cartilaginous structures like those comprising the annulus fibrosus have some measure of elasticity; but their fibers are largely fixed in their maximum length and, if stretched beyond that, especially if done rapidly, they can fracture or fragment. When these types of tissues tear or break, they (as a rule after childhood) do not repair themselves. Even the peripheral ligamentous structures do not heal by re-making equivalent tissue (as is often incorrectly assumed) but are replaced by scar. Unfortunately, the scar tissue is never quite as strong as the previously intact ligament. The ramifications of this fact will become painfully apparent as an understanding of disc "disease" unfolds later.

Thus, the combination of the annulus fibrosus and the ligamentous capsular structures surrounding it functions to contain the shock absorbing, weight-bearing, liquid center, yet at the same time, allowing an incredible amount of flexibility. If one wishes to appreciate just how flexible, dynamic, adaptable, and amazing a structure the homo sapiens vertebral column is, just watch an Olympic gymnastics competition and compare the flexibility seen there with the strength displayed in the weight-lifting sessions. For all its supposed faults, the vertebral column still garners an awesome depth of respect from this author.

The intact spine is no anatomical or physiological wimp by any stretch of the imagination. The lower lumbar discs can sustain loads of 1,000 Kilograms (2,200 lbs) when stressed with pure compressive forces. In a rather narrow-visioned perspective, many "experts" criticize the evolutionary design of the back by asserting that the vertebral column has not adequately developed evolutionarily to support man's upright posture. I would argue that it is doing a fantastic job, and a more adequate structure-for-function relationship doesn't exist within the parameters of biological design.

Thinking about it from an evolutionary survival standpoint, humans do not really need their back after they have reproduced and successfully raised their children to the age when they can fend for themselves. That's just about the time the discs start to age and break down. I think Evolution and/or God designed it pretty well considering what it is capable of accomplishing. It is only when humans are no longer useful (from a Darwinian perspective) does the spine fail.

Reportedly, aboriginally-living peoples experience a much lower prevalence of disc disease. The peaceful, archetypal, hunter-gatherers infrequently need put the forces on the spine sufficient to herniate discs. Only when man socially "evolves" to the level wherein he routinely builds structures out of heavy materials, fights one another, eats so much excess food that he becomes twice the weight he was designed to accommodate, or performs feats of athletic largess beyond the limits for which it was intended, does he destroy the integrity his spine. Our problem is not inferior spinal construction, the design is perfect for "Gardening in Eden." Our problem is that the evolution of our tools and their use has not caught up with the design specifications imposed by an otherwise perfectly suited creation. Thus far, unable to find "the right tool for the right job," we have resorted to using our backs as tools--mostly in pursuit of purposes for which they were not constructed to accommodate. One can hardly charge Nature with the sin of imperfection when it is our own chosen misgivings at fault.

This point is exemplified, and the reader can gain an appreciation of the magnitude of the forces applied to the disc, by an enterprising researcher who once convinced a number of average-sized study subjects (70 kg male volunteers) to allow him to stick needles in their intervertebral discs. He attached the needles to a pressure measuring device and put the people in varying positions. He found that the load on the lumbar disc varied from between 25 kilograms (55 lbs) when lying down to over 250 kilograms (550 lbs) in the seated, forward bending position. (Figure 14)

Interestingly, simply sitting generated load magnitudes equal to standing stooped forward in moderate flexion (about 150 kg). Sitting with the back in flexion was worse, generating about 180 kg of load force. The greatest load was experienced when the subjects held a weight while seated and flexed. This position developed disc loads in excess of 275 kg.--that's over 600 pounds of force! This explains why even the simple task of lifting a garbage can may result in putting your back "out" as well as why driving occupations carry such a high risk for disc disease. When driving, one is essentially sitting in a forward flexed position while the bumps and vibration of the road intermittently magnify the stresses at high frequency.

When these numbers are expressed as percentages of the pressureexperienced at the disc while standing (See Figure 15), one can see that the simple act of flexion increases the hydraulic pressure of the disc by fifty percent. Sitting and flexing increases it to one hundred and eighty percent. The significance of these pressures and their increases with flexion will become manifest later when this pressure is felt on the capsule in the form of pain.

Note also that the supine position does not completely remove pressure from the disc, and it remains at 25 percent of the standing pressure. This is an important consideration because if a man weighs 150 lbs., there is still 55 lbs. of pressure acting upon the disc unit in the absence of traction. The significance of this will become apparent later when one considers other back pain programs' exercises or the activity of chiropractors in which the supine position is utilized.

Further Reading:

Spinal Anatomy
Directional Terminology
Structural Anatomy
Functional Anatomy
Pathological Anatomy
Disc Hydraulics / Mechanics
Compression Forces
Correlation of Mechanical Anatomy with Disc Pain
Traction Forces

SUBSCRIBE NOW !

PURCHASE BOOK NOW !