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