# In Vitro Model for Lumbar Disc Herniation to Investigate Regenerative Tissue Repair Approaches
## 4 Discussion  
In this study, a new physiological test method was developed that allows the replication of different physiological activities in vitro. With this dynamical test method, it was possible to provoke lumbar disc herniations under experimental conditions. A lumbar disc herniation model was developed that can be used for the biomechanical investigation of new regenerative therapies in order to prevent or treat intervertebral disc herniations. Figure 3: Intradiscal pressure (IDP) in MPa in flexion-extension, left/right lateral bending and left/right axial rotation for the intact disc, after setting the vertical (\(\square\)) or round (o) defects, respectively, assessed with the universal spine tester under pure moments of \(\pm\)7.5 Nm. The simulation of different physiological daily-life activities of patients resulted in intravertebral disc herniations, but only after an annular defect was previously set. The annular defects mimicked a structural failure of the annulus fibrosus which has also been observed in patients with herniated intervertebral discs through clinical MRI. Moreover, annular tears have also been identified in asymptomatic individuals. It could be concluded, that an intervertebral disc herniation with nucleus extrusion through an annular defect might be the consequence of substantial annular fissures or tears [28]. However, a herniation could not be provoked in every specimen, even with a large annular defect. The observation of this specimen with ultra-high field MRI might indicate that structural changes have already occurred inside the disc. Compared to the other intervertebral discs, the nucleus pulposus did not look as homogeneous as in the herniated discs and showed disturbances in the signal response of the MRI (Figure 4). It can be assumed that the ability of the disc to create hydrostatic pressure was already slightly impaired. This might underlie the assumption of [PERSON] et al. that the risk of getting a herniation is higher in younger patients with non-degenerated discs [42], but only if annular defects coexist. This goes in accordance with the findings of [PERSON] et al. that the prevalence of disc herniations increases with age because the annulus fibrosus or endplate junction becomes increasingly injured over time [2]. Additionally, the findings of this study confirm the hypothesis that the risk of an intervertebral disc herniation might be generally higher in the morning after the intervertebral disc has been rehydrated by recovering during night rest [43]. In all specimens, the extrusion of nucleus pulposus occurred either to the central region or to the opposite site to where the combined motion was exerted. This behavior was already observed by [PERSON] and [PERSON] [2]. Three discs with a round annular defect were already herniated during the flexibility test before dynamic loading. During the flexibility test, the IDP values created by pure motion, were comparable to the in vivo IDP in a standing position. A reason for the early occurrence of a nucleus extrusion in the discs with round annular defects could be that the initial damage led to a larger expansion of the defect due to high fiber strains generated by the dynamical loading [44], compared to the thinner initial vertical defect. Hence for further experiments, we would suggest using a model with a round defect in order to guarantee the successful provocation of a nucleus extrusion. The defect and the extrusion of nucleus pulposus material through the aforesaid led to a migration of nucleus material. Interestingly, this migration only caused a small decrease of IDP and increase of ROM. From former in vitro studies [42; 45] it was known,  
Figure 4: Specim in which no herniation could be provoked (**a**) endoscopic view of the posterior annulus fibrosus and imaging from ultra-high field MRI (11.7 T) (**b**) in the transverse and (**c**) sagittal plane cutting through the defect that can be clearly seen, indicated with a green arrow in (**b**).