# In Vitro Model for Lumbar Disc Herniation to Investigate Regenerative Tissue Repair Approaches
###### Abstract  
Lumbar disc herniation (LDH) is the most common reason for low back pain in the working society. New regenerative approaches and novel implants are directed towards the restoration of the disc or its biomechanical properties. Aiming to investigate these new therapies under physiological conditions, in this study, a model for LDH was established by developing a new physiological in vitro test method. In 14 human lumbar motion segments, different daily-life and worst-case activities were simulated successfully by applying a physiological range of motion and axial loading in order to create physiological intradiscal pressure. An LDH could be provoked in 11 of the 14 specimens through vertical and round annular defects of different sizes. Interestingly, the defect and the LDH hardly influenced the biomechanical properties of the disc. For the investigation of regenerative approaches in further experiments, the recommendation based on the results of this study is to create an LDH in non-degenerated motion segments by the application of the new physiological in vitro test method after setting the round annular defects to a size of 4 mm in diameter.  
test method; dynamic testing; in vitro; (lumbar) disc herniation; physiological activities; regenerative approaches; tissue engineering +
Footnote †: journal: Medical  
Therefore, it is of considerable interest to understand the mechanisms of when, where, and how lumbar disc herniations develop. It is known that most lumbar disc herniations result from endplate junction failures (65%) rather than an annulus fibrosus rupture (35%) [15; 16]. However, which activities lead to the development of such lumbar disc herniations, are yet unknown. Activities that were associated with a higher risk for a gradual development of lumbar disc herniations include flexional movements or lifting weights [17; 18; 19; 20]. Further, it has been assumed that complex and extreme loading conditions might facilitate disc herniations.  
Various in vitro studies investigated the influence of these different loading conditions or motions on structural failures of the lumbar motion segments [2; 15; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30]. The results of those studies confirmed that the combination of bending forward in combination with lateral bending and axial rotation while lifting heavy objects is considered to be harmful [31; 32; 33]. Nevertheless, in most of these studies, an exaggerated range of motion (ROM) or loading values had to be applied in order to simulate the long-term effects expected from physiological movements [15; 30]. So far, no herniation model could be created that allows the investigation of new regenerative therapies under physiological loading conditions and worst-case scenarios. Furthermore, the precise biomechanical mechanism of the development of lumbar disc herniations is still not completely understood. It is still uncertain during which physiologic activity a disc herniation most likely occurs.  
Therefore, the aim of this study was to establish a lumbar disc herniation model by using a new biomechanical in vitro test method which simulates patients' daily life activities. With this new test method, lumbar disc herniations should be produced through standardized annular defects and dynamic physiological loading. This lumbar disc herniation model should allow the investigation of novel implants and regenerative therapies under worst-case loading scenarios that really happen during patients' daily life activities.