Induced narrowing and back adaptation of lumbar intervertebral discs in biomechanically stressed rats.

The purpose of this work was to develop an animal model of chronic low-back dysfunction induced by biomechanical stress. The intervertebral disc size was measured by radiography and histology in control rats, in rats intermittently forced to ambulate on a flat surface or in cylinders. Forced ambulation in cylinders caused rats to assume a relative extensive posture in the low back. Lumbar discs in biomechanically stressed rats initially narrowed, and after months of continued intermittent stress, regained near-normal size in vivo, shown by radiography. By using histologic methods, discs from rats that ambulated in any posture were shown to be larger than discs from rats not forced to ambulate. The results were consistent with the hypothesis that in vivo discs in biomechanically stressed rats initially narrow due to compressive loading and then widen back toward normal size because of an altered matrix which has increased imbibing properties. According to this hypothesis altered matrix synthesis in the nucleus pulposus was an adaptation induced by the cyclic compressive loading associated with ambulation.