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Study Design: A basic science study that used a porcine cervical spine model to produce disc prolapse subsequently exposed to an extension protocol., Objective: This study investigated whether extension or combined extension and side flexion could move the displaced portion of nucleus from the anulus towards the nucleus., Summary of Background Data: Previous research has established that repeated flexion can create disc prolapse, the question here is whether repeated extension can reverse the process., Methods: The C3/4 segments of 18 porcine cervical spines were dissected and potted in cups. Specimens were preloaded, then axially compressed (1472 N), and repeatedly rotated in either pure flexion or combined flexion and side flexion at a rate of 0.5 degrees /s. Specimens that prolapsed were axially compressed and repeatedly and rotated into extension., Results: Based on a blinded radiologist's review of the radiograph images, all 18 specimens contained healthy discs before testing, but after testing 2 of the 18 specimens had endplate fractures, whereas 11 of the 18 specimens had prolapsed. Prolapsed nucleus was reduced in 5 of the 11 prolapsed specimens after the reversal testing, whereas the remaining 6 did not change. Subclassification analysis revealed that the prolapsed discs that centralized had significantly less disc height loss (P < 0.01). Neither the classification of the herniation (circumferential or radial) nor the angle of lordosis of the specimens was linked to the behavior of the specimens., Conclusion: This study showed that with repeated flexion, in porcine cervical spines, disc prolapse was initiated and that the displaced portion of nucleus can be directed back towards the center of the disc in response to particular active and passive movements/positions.

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.

Study Design: A repeated measures biomechanical analysis of the effects of abdominal bracing in preparation for a quick release of the loaded trunk., Objectives: To quantify the ability of individuals to abdominally brace the externally loaded trunk, and assess their success in achieving and enhancing appropriate spine stability., Summary of Background Data: Spine stability requires trunk muscle coactivation, which demands motor control skill that differs across people and situations. The quick release protocol may offer insight into the motor control scheme and subsequent effect on spine stability., Methods: There were 10 individuals who sat, torso upright, in an apparatus designed to foster a neutral spine position. They were instructed to support a posteriorly directed load to the trunk in either their naturally chosen manner, or by activating the abdominal muscles to 10%, 20%, or 30% of maximum ability. The externally applied load was then quickly released, thereby unloading the participant. Muscle pre-activation patterns, spine stability, and kinematic measures of trunk stiffness were quantified., Results: Participants were able to stabilize their spine effectively by supporting the load in a naturally selected manner. Conscious, voluntary overdriving of this natural pattern often resulted in unbalanced muscular activation schemes and corresponding decreases in stability levels., Conclusions: Individuals in an externally loaded state appear to select a natural muscular activation pattern appropriate to maintain spine stability sufficiently. Conscious adjustments in individual muscles around this natural level may actually decrease the stability margin of safety.