Study design: Basic scientific investigation using radiologic, histochemical, and microscopic dissection techniques.
Objective: To document the process of mechanically induced disc herniation from repetitive loading exposure.
Summary of background data: Current knowledge of the mechanism of disc herniation is limited to only a few postmortem studies with even fewer attempts to document the process of damage during the developing stages of herniation.
Methods: Sixteen porcine cervical spine motion segments (C3-C4) were mounted in a custom servo-hydraulic testing machine. The specimens were exposed to 1472 N of compressive load and cyclically flexed-extended in angular positional control to a minimum of 4400 cycles and a maximum of 14400 loading cycles. Measurements from radiologic, histochemical, and microscopic dissection techniques were used to document the progressive trauma.
Results: The experiment produced 8 complete herniations and 4 partial herniations, of which only 4 were diagnosed by contrast discogram. The progressive damage appears to develop with a small cleft (within layer spreading) inside the first inner layer of the anulus. The nuclear material was pressed through this cleft to create a fluid-filled, delaminated pocket between collagen fibers within a lamellar bundle in an anular layer. This was the first stage of damage and disc herniation production at a microscopic level. In full anular herniation, this process is repeated until the nucleus pulposus had tracked completely through the anulus.
Conclusion: The herniation process appears to proceed with nuclear material progressing through small clefts, which accumulates causing delamination within each lamella rather than between anulus layers. No rupture of anulus fibers was found. This knowledge will assist in the development of prophylactic interventions. These data also suggest discordance between discographic indicators and other evidence confirming anular damage.