Biomechanics of the Spine - Essay Example

In their study on the biomechanics of the spine, Eck et al. (2002) set out to determine the effect of cervical spine fusion on adjacent-level intradiscal pressure. The authors based their research on previous clinical studies of spinal fusion that have reported spinal fusion as a risk factor for premature disc degeneration and pathology. In response to this research, Eck et al. hypothesized that limiting motion at one level leads to hypermobility and increased forces at adjacent levels, thus escalating the speed of disc pathology.

To test this hypothesis, they carried out their study on the cervical spines of six human cadavers. The researchers subjected the cadavers’ spines to 20º of flexion and 15º of extension, then measured intradiscal pressures, along with segmental motion, at the intact C4-C5 and C6-C7 vertebrae of their specimens respectively. Subsequently, they measured these variables on the C5-C6 vertebrae following anterior cervical plating. By performing a paired measures t-test on their measurements of changes in intradiscal pressure and segmental motion, the authors arrived at their results.

The research findings demonstrated that intradiscal pressures (the dependent variable) increased considerably during the flexion (the independent variable) imposed upon them. Pressures at the C4-C5 vertebrae increased by roughly 73.2% during the course of this flexion, and at the C4-C5, an increase in pressure of approximately 45.3% was observed. Although intradiscal pressures at adjacent levels increased during pressure, this effect failed to reach significance. Both during extension and flexion, the researchers’ measurements of segmental motion during flexion increased at these adjacent levels.

Based on these results, the authors cite substantial intensification of intradiscal pressure and segmental motion at levels adjacent to fusion as primary causes of early disc degeneration and pathology. In part, these measured effects account for one mechanism of early disc degeneration at levels adjacent to cervical spine fusion, which is in line with the author’s hypothesis. Issues relating to a criticism of Eck et al. (2002) fall generally between two categories: (a) potential confounding variables in the design of the experiment, and (b) methodological limitations that may prevent more accurate results.

One potential confound in the experiment is the possibility of inconsistency between the cadaver specimens. In the Materials and Methods section, no mention is made of how the cervical spines of these specimens were controlled for, or how the specimens’ conditions were similar or different. This relates to an implicit, unresolved question in the author’s introduction, which suggests that a “natural history” may retain greater significance in disc degeneration than fusion effects. If this is the case, it is a question of (1) how the authors controlled for “natural history”, and (2) how large of a role (if significant at all) this “natural history” play.

In the context of the Eck et al. (2002) article, these questions are not answered and are left assumed for the reader. In terms of methodological limitations, there are two mentioned within the Discussion section of the article. The first refers to the fact that the researchers measured pressures for only flexion and extension movements; although these methods have the greatest effect on intradiscal pressure, there may be other forms of movement (such as internal and external rotation) that could be measured with respect to the spine that could affect the results of their study.

The second limitation refers to the methodological inability to exert pressure on the precise location between each of the cadaver specimens. Individual differences between the cadaver specimens may influence the results in unexpected ways. Wisely, the researchers conducted three sequential loading trials to counterbalance measurement effects, but the problem of confounds in the cadaver specimens makes this problem apparent. Still, the results of this study seem to be internally and externally valid; they could prove useful in helping the clinical prevention (and further studies) of early disc degeneration at levels adjacent to spinal fusion.