A study 1 of isolated and adapted bovine spinal segments examined the extent to which flexion, hydration, and loading rates were responsible for the breakdown of the nucleus pulpous structure and found that the rate of loading had little effect on disc damage with full annular division. Instead, the degree of flexion and the hydration level of the disc were the most influential biochemical measures of nucleus polyposis disruption, protrusion, and breakdown. The study concluded that the nucleus is most at-risk of damage when fully hydrated and fully flexed.
To ensure the discs being studied were healthy and in no way degenerated, study samples were obtained from recently slaughtered two-year-old cows and frozen, then thawed and fully-hydrated just prior to use. The intervertebral discs were then isolated and all ligaments and muscles were removed. The vertebral segments were sawn partially through and attached to a stainless-steel plate using an adhesive.
The segments were radiographed shortly after collection and prior to freezing using a rubber band to create the flexion state and inverting the tail so that it straightened under its own weight, creating the non-flexed state. After freezing, the tail segments were thawed and hydrated fully or partially. A computer-controlled hydraulic device was used to compress the disc segments. Time, load, and the amount of displacement were recorded as flexion, hydration levels, and loading rates in separate tests with contrasting (flex or non-flexed) conditions.
Following the sectioning and initial compression testing, the nucleus pulposis of each of the 96 samples was split, examined, and photographed to locate any signs of disruption. The researchers concluded that the height of intervertebral discs increases at a slower rate when the disc is more hydrated. Because each of the discs studied had statistically similar hydration levels, the research suggests that the duration of a static load will influence disc height and hydration levels. This means that hydration levels within a disc can be manipulated by static preloads—a finding in agreement with previous studies. For the purposes of this study, the researchers concluded that disc height was stabilized after approximately 20 hours, and the disc was considered fully hydrated at that time.
Following the disc analysis, the study samples were categorized into five groups based upon the amount and type of damage that had occurred to the disc nucleus. The damage was assigned a weighting (W) value between 0-4 (“O” representing no observable damage and “4” representing complete or partial nuclear displacement), and values of 2 percent and 5 percent were used to distinguish between discs that had been moderately or severely impacted and whether they had sustained any disruption or movement of annular materials. An average damage weighting (ADW) scale was then applied to each biochemically-tested sample group.
The disc samples that were fully flexed and fully hydrated had damage that included nuclear sequestration or the formation of a cleft, or both. Discs that had been fully flexed and partially hydrated experienced some, though less severe, damage than those in the fully hydrated/flexed study group. There was little variance between the levels of damage in the non-flexed, fully hydrated and non-flexed, partially hydrated sample groups. The study authors concluded that the degrees of flexion and hydration were significant factors in intervertebral disc damage, but loading rate had little bearing on the severity of nuclear disruption.
While nuclear prolapse is reproducible in healthy disc samples by fully hydrating and flexing them, the compressive loading rate has little or no bearing in nucleus prolapse of discs with annular wall degradation. This study challenges the presumption that disc prolapse is caused by a degenerative process involving repeated mechanical stress. Instead, the healthy disc nucleus is at risk of annular tears or prolapse when fully hydrated and flexed.