The hydrostatic pressure of the nucleus pulposus plays a key role in the biomechanical properties of the thoracic spine, reducing compressive stress gradients in the anulus fibrosus (Stefanakis et al., 2014) and affecting the segmental stability more than any ligamentous or bony structure (Wilke et al., 2020). Several investigators explored the characteristics of intradiscal pressure in various in vitro, in vivo, as well as numerical studies, starting with Nachemson, who analyzed the intradiscal pressure in human lumbar spinal specimens (Nachemson, 1959, 1960). Since then, in vitro intradiscal pressure measurements were predominantly performed on the cervical (Pospiech et al., 1999; Dmitriev et al., 2005; Kretzer et al., 2012; Barrey et al., 2015; Liu et al., 2016; Welke et al., 2016; Bell et al., 2018) and lumbar spine (Wilke et al., 1996; Rohlmann et al., 2001; Molz et al., 2003; Schmoelz et al., 2006; Heuer et al., 2007; Gao et al., 2011). On the thoracic spine, in contrast, solely in vivo measurements in mid- and lower thoracic segmental levels (Polga et al., 2004) and few in vitro measurements on specific levels in polysegmental setups (Cheng et al., 2015; Anderson et al., 2016; Metzger et al., 2016) have been conducted so far. For the validation of numerical models and for ensuring high comparability of experimental setups, however, intradiscal pressure data from tests with clearly defined boundary conditions is essential. The purpose of this in vitro study therefore was to quantify the intradiscal pressure in human thoracic spine specimens under pure moment loading.

In general, pressure-moment curves exhibited symmetrical, V-shaped characteristics (Figure 2), especially in axial rotation. Intradiscal pressure was generally highest in flexion movement at the pure moment of 7.5 Nm, which represented a significant increase compared to extension and right lateral bending for the same applied moment (Figure 3). Furthermore, the intradiscal pressure significantly increased for all applied moments compared to all previous steps in flexion direction, which was also found for bilateral axial rotation with the exception of the intradiscal pressure at 2.5 Nm in right axial rotation. Significant increases of the intradiscal pressure were also found for all loading steps in extension and at an applied moment of 5.0 Nm in left lateral bending, each compared to the intrinsic intradiscal pressure at 0.0 Nm. Median intrinsic pressure was below 0.06 MPa in all motion planes, while being significantly increased during lateral bending movement (0.058 MPa) compared to axial rotation (0.037 MPa) and flexion/extension (0.022 MPa). Variation range was generally lowest in axial rotation and flexion, while being highest in lateral bending, where the average v-shape of the pressure-moment curves was rather flattened. In extension, the median intradiscal pressure even tended to decrease from 5.0 to 7.5 Nm, leading to a non-linear pressure-moment behavior.

Comparing the intradiscal pressure regarding thoracic spinal regions, significantly higher pressures were found in flexion of the upper thoracic spine (T1–T5) compared to the lower thoracic spine (T8–T12) for all loading steps as well as compared to the mid-thoracic spine (T5–T8) at the applied moment of 7.5 Nm (Figure 4). In contrast, the intradiscal pressure in extension was significantly higher in the lower thoracic spine compared to the upper thoracic spine at 7.5 Nm. Significant linear correlation was detected between thoracic spinal level in flexion for all applied moments and in extension at 7.5 Nm, while correlation was generally found being negative in flexion (r = −0.629/−0.561/−0.548 for 7.5/5.0/2.5 Nm, respectively) and positive in extension (r = 0.500) (Figure 5). Additional regression analysis exhibited an approximate intersection point of intradiscal pressure trend at T12 level with about 0.4 MPa in the flexion/extension motion plane. No significant differences between the single thoracic spinal regions or correlations between the single segmental levels and the intradiscal pressure were found in lateral bending and axial rotation. Variation range tended to be lowest in the lower thoracic spine in case of flexion and axial rotation movements, while it tended to be lowest in the mid-thoracic spine in case of lateral bending movements (Figure 4). In extension direction, the pressure-moment characteristics tended to alter in craniocaudal direction, with the median intradiscal pressure peak tending to shift toward higher applied moments.

No statistical significant effects of sex and age on the intradiscal pressure were found in the present study. However, the intradiscal pressure tended to be higher and exhibited larger variation ranges in specimens from female donors compared to those of male donors (Figure 6). Moreover, slight trends were detected toward decreasing intradiscal pressure with increasing donor age for all applied moments in all motion directions (Figure 7). Although age had no statistically significant effect, indications were found that the intradiscal pressure was affected by the tissue quality of the intervertebral disc. While not showing substantial signs of degeneration in the x-ray images, fibrotic changes of the nucleus pulposus and the anulus fibrosus were especially detected in specimens from elderly donors, overall leading to low or even negative intrinsic pressure and more flattened pressure-moment characteristics compared to specimens from younger donors (Figure 8).

A summary of all evaluated parameters including the statistical analysis is given in the Supplementary Excel file attached to this publication.

The intradiscal pressure of the human thoracic spine has been poorly investigated. While several previous in vivo and vitro studies determined the intradiscal pressure of the cervical and lumbar spine, the hydrostatic pressure in thoracic intervertebral discs was solely explored in a single in vivo study (Polga et al., 2004). For a complete understanding of thoracic spinal biomechanics as well as for the accurate validation of in vitro and numerical models of the thoracic spine, however, data on the thoracic spinal intradiscal pressure is essential. The present in vitro study therefore aimed to quantify the intradiscal pressure of all human thoracic spine segmental levels under multi-planar pure moment loading.

Pressure-moment characteristics revealed a high dependence of the intradiscal pressure on the respective motion direction. While showing almost linear relationship in flexion and bilateral axial rotation, a distinct tendency was found in the upper and mid-thoracic spine toward a non-linear relationship in extension as well as in bilateral lateral bending, respectively. Since all segmental levels were tested under same loading conditions, the non-linear pressure-moment behavior in extension might indicate that in the upper and mid-thoracic spine, physiological loading is generally lower in extension direction compared to the other five motion directions when hypothesizing that the pressure-moment characteristics show linear behavior during elastic deformation of the intervertebral disc. However, due to the fact that the intradiscal pressure even decreased for higher moments in the upper segmental levels, it can be expected that the loads are rather transferred to the facet joints than to the intervertebral disc. Moreover, it was found that the flexion intradiscal pressure significantly decreased from the upper to the lower thoracic segmental levels, whereas the intradiscal pressure significantly increased, while being approximately equal at T12 level, suggesting that the intradiscal pressure divergence between flexion and extension generally decreases toward the thoracolumbar transition region. The pressure-moment characteristics in lateral bending, in contrast, exhibited a high variation range and low median intradiscal pressure values in the upper and mid-thoracic segmental levels, indicating a high impact of individual intervertebral disc properties on the lateral bending intradiscal pressure, such as degenerative, morphological, or biochemical features. In axial rotation, on the contrary, the pressure-moment characteristics were overall comparable across all segmental levels and revealed a low variation range, indicating a low effect of intervertebral disc properties on the intradiscal pressure during axial torsion. In general, thoracic spinal motion segments showed V-shaped pressure-moment behavior during entire loading cycles in all motion planes in the present study, while previous studies on the lumbar spine exhibited more butterfly-shaped pressure-moment characteristics (Wilke et al., 1996; Rohlmann et al., 2001; Schmoelz et al., 2006). This might indicate that in the thoracic spine, bending and torsion of the intervertebral disc leads to a more homogeneous volumetric compression of the nucleus pulposus compared to the lumbar spine, where the nucleus pulposus has a larger play due to higher deformability of the anulus fibrosus (Koeller et al., 1984). Therefore, the intervertebral disc height, which is greater in the lumbar spine compared to the thoracic spine (Pooni et al., 1986), might essentially contribute to the pressure-moment behavior.

In the present study, intrinsic intradiscal pressure values varied between −0.052 and 0.175 MPa with a median value of 0.037 MPa, while not exhibiting any significant effect of segmental level, sex, or age. However, significant effects of the motion plane were found, indicating potential effects of the motion plane on specimen preconditioning, although differences were overall minimal. Data of comparable in vitro studies on the lumbar spine also exhibited higher intrinsic intradiscal pressure in lateral bending compared to flexion/extension and axial rotation (Wilke et al., 1996; Rohlmann et al., 2001; Schmoelz et al., 2006), leading to the assumption that preconditioning in lateral bending might differ from the other two motion planes. Compared to previous in vitro studies on the cervical and lumbar spine, the intrinsic intradiscal pressure values of the present study showed a high variation range, while overall being in the same order of magnitude (Table 2). This large variation range might be explained by effects of intervertebral disc degeneration, which is known to increase with advancing age in the thoracic spine (Goh et al., 2000). Moreover, previous in vitro studies on the lumbar spine found that the intrinsic intradiscal pressure is lower in more degenerated intervertebral discs compared to healthy, young intervertebral discs, which generally show positive pressure values in vitro (Nachemson et al., 1979; Panjabi et al., 1988; Adams et al., 1996). As a consequence, it can be expected that disc degeneration induced the negative pressure values in the present study, although no distinct signs of degeneration were examined on the x-rays prior to the experiments. However, even mildly degenerated intervertebral discs exhibit hydrostatic properties, which was shown in a previous in vitro study on the lumbar spine (Adams et al., 1994). Therefore, specimens with negative pressure values were not excluded from the present study, since the primary aim was to evaluate the intradiscal pressure of average thoracic spinal motion segments. Nevertheless, the effect of intervertebral disc degeneration on the intradiscal pressure of the thoracic spine represents an important research question, which should be further investigated in future studies.

Previous investigations on the intradiscal pressure of the thoracic spine are limited to few in vitro studies using polysegmental test setups (Cheng et al., 2015; Anderson et al., 2016, 2018; Metzger et al., 2016) and one in vivo study (Polga et al., 2004). While Cheng et al. did not report absolute pressure values and Anderson et al. solely investigated the effect of follower loading on the intradiscal pressure at T4–T5 and T8–T9 levels, Metzger et al. found slightly higher average pressure values at T7–T8 level compared to the present study. Mean pressure values of Metzger et al., however, were within the variation range of the present study and exhibited similar pressure behavior with respect to the different motion directions, including pressure values being highest in flexion (Table 2). Anderson et al. (2018) furthermore identified increased pressure-moment slope after rib cage removal, indicating a stabilizing effect of the rib cage structures, which has to be considered when interpreting the results of the present study, where the anterior rib cage structures were neglected. Moreover, pressure-moment slopes were generally higher at T4–T5 level compared to T8–T9 level in all motion directions, which can be attributed to the use of an additional compressive follower load of 400 N in this study. In an in vivo study, Polga et al. measured the intradiscal pressure in mid- (T6–T8) and lower (T9–T11) thoracic spinal levels. They detected pressure values of about 0.3 MPa in the mid- and of about 0.2 MPa in the lower thoracic spine in prone position, presumably resulting from low muscular activity in axial compression direction, while in both upright sitting and standing, the intradiscal pressure increased to about 1.0 MPa in the mid- and to about 0.9 MPa in the lower thoracic spine. During bending and twisting movements of the participants, however, Polga et al. found increasing intradiscal pressure values from the mid- to the lower thoracic spine. Furthermore, the pressure values of Polga et al. were significantly higher compared to the values of a previous study on the lumbar spine in upright standing position, but significantly lower when compared to forward flexed position while holding a weight of 20 kg (Nachemson and Elfström, 1970). The same trend was also found compared to a further in vivo study on the lumbar spine with one participant (Wilke et al., 1999). Therefore, it might be concluded that in vivo, the intradiscal pressure decreases along the thoracolumbar spine in sole upright position, whereas the pressure increases along the thoracolumbar spine in bending or twisting positions. Comparing these findings with the combined data of the present in vitro study as well as previous in vitro studies, investigating intradiscal pressure in the cervical spine (Pospiech et al., 1999; Dmitriev et al., 2005; Kretzer et al., 2012; Barrey et al., 2015; Liu et al., 2016; Welke et al., 2016; Bell et al., 2018) and lumbar spine (Wilke et al., 1996; Rohlmann et al., 2001; Molz et al., 2003; Schmoelz et al., 2006; Heuer et al., 2007; Gao et al., 2011) without axial preload, a different intradiscal pressure distribution can be observed. While showing almost consistent intrinsic intradiscal pressure along the spine, the intradiscal pressure tends to decrease along the spine under pure moment application in all motion directions, especially when seen in relation to the applied moment (Table 2). This might indicate that additional axial preload alters the intradiscal pressure distribution along the spine, which should be considered in future experimental testing as well as in numerical model development. Moreover, additional compressive preload affects the pressure-moment characteristics, as previously shown for the lumbar spine (Rohlmann et al., 2001), especially leading to increased intrinsic pressure, which has to be taken into account when comparing the in vitro data of the present study to in vivo data and when validating numerical models. Testing without additional compressive load therefore represents a major limitation of the present study. Nevertheless, the data of the present study is essential for the validation of numerical models of the thoracic spine due to its clearly defined boundary conditions and therefore highly reproducible data. Previous investigations suggest a 400 N follower preload in order to create physiological intradiscal pressure values in the thoracic spine (Anderson et al., 2016), while producing potential, hardly controllable artifacts due to coupling and clamping effects at the same time. Moreover, follower load was shown to significantly decrease the flexibility and to alter the three-dimensional motion behavior of the thoracic spine (Liebsch et al., 2018), leading to additional boundary conditions for numerical models. Furthermore, preloading of the specimen might affect the intradiscal pressure measurement due to the distinct decrease of intervertebral disc height, which is lowest in the thoracic spine (Pooni et al., 1986). When using the herein presented data for the validation of a numerical model of the thoracic spine, the costovertebral joints have to be considered due to the presence of the posterior sections of the ribs in the present study. These were left intact because of their stabilizing effect on thoracic spinal motion segments (Liebsch et al., 2017) and thus their potential influence on the intradiscal pressure. Another limitation of the present study is that possible effects of the sensor entry point in the anterior part of the intervertebral disc cannot be fully excluded and may have caused small measuring artifacts especially in flexion direction. However, all tested specimens in the present study showed adequate anterior disc height, suggesting only minimal effects of the sensor entry point.

In conclusion, a large dataset regarding intradiscal pressure values of the human thoracic spine was generated. This dataset can be used as comparative data for future in vitro studies as well as for the validation process of numerical models of the thoracic spine.

All datasets presented in this study are included in the article/Supplementary Material.

The use of human specimens was approved by the ethical committee board of the University of Ulm, Germany, in November 2014 (No. 302/14). The specimens were acquired from the body donation organizations Anatomy Gifts Registry program (AGR, Hanover, Maryland, USA) and Science Care (Science Care Inc., Phoenix, Arizona, USA), which declared that written informed consent of the donors was obtained prior to decease.

H-JW: funding acquisition, discussion, and manuscript review. AH: study design, specimen preparation, and experimental testing. KW: specimen preparation and experimental testing. CL: data analysis, discussion, and manuscript preparation. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.