An in vivo study 1 of the effects of shear force loading applied to the L5-L6 spinal segment of lab rats revealed histological evidence of IVD degeneration in the unit and surrounding discs of sacrificed study rats that had been exposed to shear force via a custom-designed loading device, while no such evidence was evident in the post-mortem rat control group. The results of the study showed that shear force, applied at .33 MPa (a lower level of compressive stress than previously shown to cause IVD degeneration in rat tail discs), creates degeneration of rat IVDs. This information may be instrumental in providing preventative and treatment-oriented care for people who may be at risk of developing IVD degeneration.
The Study of Shear Force
Researchers tested the hypothesis that sustained shear force on a spinal segment would create IVD degeneration in rat lumbar spines. They used 15 young male rats divided into three groups—one sham control group, and two experimental loading groups that would be exposed to loading for one, and two weeks. The shear loading device used was created especially for the experiment and was made of stainless steel. It was applied to the L5 and L6 vertebral rat bones and delivered a static shear load of up to 4 N.
When the shear loading experiment was completed (1 week, and 2 weeks), the rats were sacrificed. The lumbar segments were removed, viewed microscopically, and tested histologically. A degenerative score from 0 to 3 was assigned each sample, with “0” representing no changes, “1” showing minimal changes, “2” representative of samples with moderate changes, and “3” assigned to those samples showing severe changes, including some with NP disappearance. The slides were blinded and randomized to prevent observer bias.
All the rats involved in the study survived the surgery and post-op period, with no signs of distress. Each of the rats that underwent shear loading had IVD degeneration in most of their lumbar discs, across all levels. The sham control rats, however, demonstrated no degeneration after the experiment.
There were differing levels of degeneration in the IVDs of the shear stress-exposed rats. After the shear loading, the posterior annulus of the exposed rats curved into the dorsal area of the NP, creating a reduction in demarcation in these samples and a disappearance of notochordal cells. The anterior NP remnants were disaggregated, collapsing into smaller sections composed of multiple cells, which, along with the NP, later disappeared. There was also a blending of NP, AF, and CE, and it was difficult to see where one began and another ended. The lamellar wall of the inner and middle annulus dissolved, creating disorganization in the AF.
Isolating the effects of different loading modes on IVD degeneration and response is helpful in developing a more complete understanding of IVD biomechanics. Understanding the consequences of shear force applied during compressive spinal loads through in vitro studies can elucidate how shear applied during bending and torsion loading can cause damage to the IVD at the microstructural level and contribute to AF degeneration and failure.
The results of this in vivo study on the disc segments of rats undergoing shear force stress on the L5-L6 IVD segment demonstrated evidence of degenerative changes in all the rats exposed to shear force, while no degeneration occurred in the rat sham control group. The disc damage noted in the experiment groups occurred not only at the L5-L6 levels, but was also evident at adjacent levels (L3-L4, L4-L5, L6-S1). This is further confirmation that the effects of shear force can create damage, proteoglycan depletion, NP content loss and/or collapse, and severe degeneration to disc segments within one week of exposure.