Disc Herniation Pathology

As these forces act upon the posterior elements of the fibro-cartilaginous laminations, the individual fibers are caused to deform and bow posteriorly outward. This, combined with the stretching occasioned by flexion's separation of the posterior surfaces, rapidly takes these ligamentous structures to their full length. During rapid and excessive forces (such as in falls or heavy lifting) these fibers stretch beyond their tensile limit and tear. The first tears occur in the more central regions of the annulus fibrosus that have no nerve supply (so the damage is often not perceived) and that have no capillary blood supply (so that the cells of the inflammatory process cannot go inside to lay down scar or healing granulation tissue). These events constitute the initiation of the radial tear (Figure 26) and, thereupon, the degenerative process begins.

These tears can occur at an early age and, in and of themselves, go unnoticed due to the annulus's lack of a nerve supply; however, the injury (if it occurs after the blood supply has retreated during the maturation process) never heals and the structural weakness most probably will remain for a lifetime. It seems strange, but a significant fall that lands a young person on his buttocks in flexion and causes no more than a mild discomfort, although easily passing as a few days with a sore back, can, in fact, set up the damaged disc unit for easier future damage of which the individual has no conscious awareness. Nevertheless, the tear is there, waiting for a future traumatic WEIGHT-BEARING FLEXION event to result in advancing it to a frank herniation.

If the forces are sufficient, in injuries after an individual matures, the desiccated and hardened nucleus pulposus can itself fragment or cause the laminations of the inner annulus fibrosus to shatter and fragment. These materials once separated from their contiguous and attached cartilaginous structures become cut off from their nutrient supply and subsequently harden (when cut off from their water supply). Later, they can further break down and degenerate into what has been described as "crab meat" in advanced cases. They can also break off and become loose bodies within the spaces created by the fissuring and the tearing. This degeneration can become so severe that the disc loses its structural integrity causing one vertebral body to slide relative to the adjacent one. This results in a "listhesis" which can be painfully beyond the scope of this book's ability to manage and necessitating a surgical fusion.

More commonly, these loose bodies, especially the hardened nucleus pulposus, upon the successive, forceful flexion events, can be forced further peripherally and extend the tearing. If the forces are directed linearly along the path of a prior injury and towards the periphery, this creates elongations in the existing radial tears (See Fig 27A,B,C) and extensions of circumferential tears (See Fig. 27 D), if the forces are created during a forcefully traumatic circumduction-like event such as twisting while lifting.

If the anterior compressive forces are generated more to the right side of the body, the opposite side of the posterior portion of the disc bears the brunt of most of the expanding force, that being the left side of the posterior aspect of the disc. This explains why a lifting injury caused by a heavy weight on the right side of the body causes the disc to be prolapsed or extruded more to the left. The above series of injuries (Figure 27) would have had to have been due to a flexion injury towards the right. It is not so much the twisting component of a lifting injury that results in damage, it is the fact that the more compressive force generated on one side of the body communicates more lateral force to the opposite posterior side where the ligaments are stretched to their maximum length already and cannot tolerate this force.

There is another reason why discs tend to herniate to one side or the other rather than directly posterior. As that force is felt more and more postero-laterally, the resistance of the ligamentous structures become weaker due to the gradual diminution of the posterior longitudinal ligament's fibers as they anatomically become thinner at the lateral aspects of the discs. This is why a person should never attempt to lift a heavy weight off to the side of the body or twist while lifting (this action is well-documented to be the cause of frequent disabling back injuries). Also, twisting while lifting is damaging because the rotation of one vertebral body relative to another stretches the fibers more and predisposes them to breakage easier because a ligament that is already stretched to its limit is much more likely to rupture when force is applied than one which still has some play in it.

This phenomenon explains why it is a good recommendation that, when lifting, the weight should be placed as close to the midline of the body as possible and lifted directly upwards. In that way, the disc units are not in flexion, and the annulus fibers are not stretched by twisting or posteriorly directed pressure. Instead, the weight is sustained ideally by the shock absorber function of the disc while it is in line with the vertical axis of the spine. Therefore, if a weight has to be moved to a place to one side or the other, it is better to move the entire body by repositioning the legs rather than leaning to one side and twisting to accomplish the same task.

As described above, these forces have been measured by pressure transducers and they are quite phenomenal. They are tolerable in the young, flexible, well-hydrated discs of youth as evidenced by the nearly incomprehensible ability of a child to sustain falls in which they land on the buttocks with the spine flexed forward yet seemingly suffer no untoward consequences. The equivalent fall would most likely cripple an adult because, as a body ages, the liquid center of the nucleus pulposus becomes solidified as the cartilage component loses water with age and actually forms a solid disc-shaped structure. Also, up until the age of eight years, there are small blood vessels supplying the more centralized disc material which help heal it when it is injured. These are gradually obliterated as a person ages. By the time growth has stopped, the nucleus pulposus and the inner regions of the annulus fibrosus no longer have an active blood supply. Thereafter, only the outer most ligamentous structures have a blood supply and the rest of the disc must obtain sustenance from the diffusion of tissue fluid across the cartilaginous end plates. The central disc material becomes solidified and hardened with age probably due, in large part, to its loss of nutrient blood supply.

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

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