Goal of the Study?
In this primary research article 1, the authors aim to establish an animal model that can be extrapolated to the complex mechanical load of a human intervertebral disc during bending and compression loading. They hypothesized that a disc herniation could be gradually induced by static complex loading.
Why are they doing this study?
Disc bulges or protrusions often pre-exist in patients with acute back pain, suggesting that repeated compressive loads may gradually result in a herniated intervertebral disc. To the author’s knowledge, this assumption has never been verified using animal models.
What was done?
Twenty research rats were randomly divided into four equal groups. Group #1 was the control; the other 15 rats had an external device implanted in the coccygeal 8-10 vertebrae to bend the spine at a fixed angle. Group #2 had sham surgery but no compression load, Groups #3 and #4 were subject to a compression load of 1.8 N (0.184 kilogram force) and 4.5 N (0.459 kilogram force) respectively. After 14 days, an MRI was performed on all 20 rats and a Pfirrmann classification system was used to classify the disc images into 5 categories. The rats were then euthanized and the C8-10 vertebral body was removed and the Norcross calcification system was used to evaluate the degeneration on a scale of 10 for no degeneration and 2 for severe degeneration. The dissections were also tested for Gene Expression and various statistical tests were performed.
What did they find?
No statistical difference was found between the control group and the sham surgery group, so bending alone seemed to have little or no effect on disc degeneration. There was a slight difference but it was not significant. The MRI and histological scores for the intervertebral disc degeneration were significantly higher in the two loaded groups than the sham or no load surgery group. Group #4, heavily loaded had significantly more disc degeneration than the lightly loaded group. Both loaded groups had significantly more disorganization in the nucleus pulposus and annulus fibrosus from a histological perspective than the sham surgery group.
Why do these findings matter?
Even though the disc structure of rodent tails are biomechanically and compositionally similar to human lumbar there are significant differences. As such, it is difficult to extrapolate this research to humans in a disc degeneration model comparison. But if it can be extrapolated, it indicates that a static complex loaded can induce posterior intervertebral disc protrusion when combining bending and compression but not during bending alone.
At Dynamic Disc Designs, we believe it can be very helpful to show patients the mechanism of a disc injury to help empower a patient about prevention and the solution strategies to their problems. Our dynamic disc models demonstrate that bending alone does not cause disc herniation but will herniate when compression is combined with flexion.