Elsevier

Journal of Biomechanics

Volume 70, 21 March 2018, Pages 262-266
Journal of Biomechanics

The rib cage reduces intervertebral disc pressures in cadaveric thoracic spines by sharing loading under applied dynamic moments

https://doi.org/10.1016/j.jbiomech.2017.10.005Get rights and content

Abstract

The effects of the rib cage on thoracic spine loading are not well studied, but the rib cage may provide stability or share loads with the spine. Intervertebral disc pressure provides insight into spinal loading, but such measurements are lacking in the thoracic spine. Thus, our objective was to examine thoracic intradiscal pressures under applied pure moments, and to determine the effect of the rib cage on these pressures. Human cadaveric thoracic spine specimens were positioned upright in a testing machine, and Dynamic pure moments (0 to ±5 N·m) with a compressive follower load of 400 N were applied in axial rotation, flexion - extension, and lateral bending. Disc pressures were measured at T4-T5 and T8-T9 using needle-mounted pressure transducers, first with the rib cage intact, and again after the rib cage was removed. Changes in pressure vs. moment slopes with rib cage removal were examined. Pressure generally increased with applied moments, and pressure-moment slope increased with rib cage removal at T4-T5 for axial rotation, extension, and lateral bending, and at T8-T9 for axial rotation. The results suggest the intact rib cage carried about 62% and 56% of axial rotation moments about T4-T5 and T8-T9, respectively, as well as 42% of extension moment and 36–43% of lateral bending moment about T4-T5 only. The rib cage likely plays a larger role in supporting moments than compressive loads, and may also play a larger role in the upper thorax than the lower thorax.

Introduction

The rib cage is integral to the thoracic spine anatomically and believed to add stability or share loads with the thoracic spine. Supporting this view, removal of the rib cage increases ranges of motion and reduces stiffness during mechanical testing of cadaveric thoracic spines under applied moments (Mannen et al., 2015b, Watkins et al., 2005). However, the effects of the rib cage on thoracic spine loading are not well studied, and the contribution of the rib cage to spinal support cannot be easily measured in part due to the difficulty in measuring spinal loading within an intact spine-rib cage construct. Because of this, information remains limited on the amount of load sharing the rib cage may provide, and whether this is similar or varies across levels of the spine.

Intervertebral disc pressures have been measured both in vivo and in cadaveric spine tests to provide insight into spinal loading, as disc pressure is strongly correlated to compressive loading in cadaveric spine tests (Anderson et al., 2016, Nachemson, 1960, Pollintine et al., 2004). Applied moments also increase intradiscal pressures in lumbar cadaveric specimens (Rohlmann et al., 2001, Wilke et al., 1996), and finite element modeling suggests lumbar disc pressure increases with intervertebral flexion angle as well as compression (Ghezelbash et al., 2016). However, few studies have examined disc pressure in thoracic discs, whether in vivo (Polga et al., 2004) or in cadaveric specimens (Anderson et al., 2016, Dolan et al., 2013), and none have yet examined thoracic disc pressures under applied moments.

Examination of thoracic disc pressures could provide insight into how the rib cage shares loading with the thoracic spine. We have previously reported that the rib cage appears to have a limited effect on thoracic intervertebral disc pressures under static compressive follower loads (Anderson et al., 2016). However, examination of thoracic stiffness suggests that the rib cage has more effect on the thoracic spine under applied moments than compressive loads (Watkins et al., 2005). Thus, our objective was to measure thoracic spine intradiscal pressures under applied dynamic moments, to determine the effect of the rib cage on these pressures, and to use these measurements to evaluate the amount of moment supported by the spine vs. the rib cage at different locations in the spine. We hypothesize that disc pressure will increase with applied moments, that this increase will be higher with removal of the rib cage, and that the rib cage will support a significant fraction of the overall moment applied to the spine.

Section snippets

Specimens and testing

Eight fresh-frozen human cadaveric thoracic spines (T1-T12) with the rib cage intact were obtained (4 female, 4 male, age range 61–71). Data from experimental testing of these specimens has been previously reported (Anderson et al., 2016, Galvis et al., 2017, Sis et al., 2016), but the disc pressures that are the focus of the current analysis have not been previously examined. Specimens were potted at T1 and T12, and positioned upright in a testing machine (Applied Test Systems, Butler, PA)

Results

Data from one specimen was excluded entirely due to incomplete pressure data or irregularities in applied moments affecting the majority of tests. Thus, analyses were conducted with data from seven specimens (3 female and 4 male, height 1.71 ± 0.11 m, weight 75.8 ± 20.5 kg, age 66 ± 4 years, age range 61–71).

Pressure typically increased with applied moment, whether positive or negative (Fig. 2). Flexion (positive moment) and extension (negative moment) pressure-moment slopes differed (p < .05),

Discussion

Overall, disc pressure increased with applied moments in the thoracic spine, and removing the rib cage increased the pressure-moment slope for most applied moment conditions at T4-T5, but only for AR at T8-T9. Furthermore, removal of the rib cage had the largest impact in AR, increasing the pressure-moment slope about 2.5-fold, while other significant increases were about 1.5-fold. Pressure-moment slopes were smaller in extension than flexion (and not significant at T8-T9) perhaps due to a

Conflict of interest statement

The authors state they have no conflict of interest to disclose.

Acknowledgments

This study was supported by the National Institute on Aging of the National Institutes of Health (K99/R00AG042458) and by a Mentored Career Development Award from the American Society for Bone and Mineral Research. The study sponsors had no role in the study design, data collection, analysis, manuscript preparation, or the decision to submit the manuscript for publication.

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