Disc herniation, wherein a portion of the soft nucleus pulposus passes through the outer layer(s) of the vertebral disc’s sturdy annular fibrosis, occurs over time and is considered a cumulative response to hydraulic pressure. It is estimated that approximately half of the adult population may have herniated, or partially-herniated discs, though only about 2 percent of the population experience pain or numbness when the infiltrating material contacts the spinal nerves. The condition is twice as common in males as in females and is associated with repetitive load-bearing compression on the spine. Due to the gradual nature of the injury, few reliable studies have been made that demonstrate the internal processes of disc trauma as it occurs, though there have been several disc herniation hypotheses proposed.
One study concluded that the process of disc herniation begins when the lamellae of the annulus develops a distortion and fissures in response to applied repetitive compression and bending. The authors of the study observed that the nuclear pulp of the subjects seeped through the fissures and contacted the spinal canal. A similar study used lower levels of repetitive compression to produce nuclear extrusion and annular protrusions. Both studies suggest that disc prolapse occurs over time, from the inside-out.
It appears clear that macroscopic and microscopic processes within the annulus are common features of disc herniation, but exactly what those features are and the exact sequence of events leading to the development of annular lesions remains somewhat of a mystery. One post-mortem clinicopathologic study noted the migration of nuclear fragments in correlation to tears in the annulus. This appeared to be proceeded by the formation of a large cleft in the center of the nucleus as it loses moisture and shrinks in size—a process associated with early mid-life aging. Since the presence of these fragments are more commonly found within the nuclear material than in extruding materials, the supposition is that herniation protrusions with annular material only may be caused by a separate process than that of herniation involving the nucleus pulposus. Another study suggested that evident herniated disc material might actually be newly-produced fibrocartilage, rather than any type of disc or nucleus tissue.
To better understand the progression of disc herniation, a radiologic, histochemical, and microscopic dissection study 1investigated the process of mechanically-induced herniation. Sixteen porcine cervical spine motion segments were mounted in a servo-hydraulic testing device and exposed to progressive loads of compression and differing angular positions to document the progression of trauma. Though only four of the subject samples produced compression trauma that could be diagnosed via contrast discogram, there were a total of 12 herniations in all—4 of which were partial herniations.
The conclusion of this study suggests that the damage incurred during progressive disc herniation develops first in the innermost layer of the annulus. Repeated bending creates clefts and pockets within the lamellae. As the pressure creates more clefts in each annular layer, full disc herniation occurs. Because the clefts form through the pressure created by pockets of nuclear fluids, they do not form a straight path that would be easily identifiable in discography images. This may account for the large discrepancy in the detection and diagnosis of disc herniation using various methods.