A new study 1 to determine which mechanical conditions were unsafe for the intervertebral disc (IVD) culminated with the presentation of a numerical model that can be used to predict disc failure under all loading conditions. Using a series of complex loading conditions on ovine lumbar IVD segments, researchers were able to examine the state of stress of the IVD and AF, combining numerical results with that of a parallel in vitro study.
Researchers used an FE model of the ovine lumbar intervertebral disc. The geometry of this model was centered around reconstruction of the endplates (Eps), caudal and cranial vertebrae from a 1.3-4 spinal segment. They used a custom Python script to generate the disc. This construct was created to represent the lumbar spine. This disc model contained an annulus fibrosus (AF), nucleus pulposus (NP), and a bony and cartilaginous endplate (EP). Researchers divided the AF in the posterior, lateral, and anterior sections. They composed a mesh of 56,496 hexahedral elements and 60,145 nodes. They then compared its flexibility with previous collected data to validate that the model was in agreement with the results of that study.
The study’s authors simulated five loading scenarios in near-static states to replicate the previous study’s conditions. They chose loading angles double that of those used in the previous study and applied briefly 3.75 Nm without posterior elements. They used a kinetic energy that was less than 10 percent of the total energy in the simulations. They then analyzed the stresses created in the circumferential, axial, and radial direction, as well as the interface stress between the Eps and the three defined, divided, and subdivided sections of the AF. They averaged all stress spatially over the cranial, middle, caudal, inner, and outer AF and calculated the interface stress between the AF and the EP as the ratio between the cross-sectional area and nodal forces. They then studied the role of each type of stress in disc failure to determine a numerical formula for predicting IVD failure.
The results of the study demonstrated that tensile axial stress greater than 10 MPa and a positive circumferential stress greater than 8MPa may cause the annulus fibrosis (AF) to fail and that flexion is the loading condition most often associated with disc failure. The results were in agreement with those of the previous study and were useful in developing a model to predict disc failure. In short, the highest stress states were created by the application of rotations in the main-plains. This observation was most prevalent in the posteo-lateral and anterior regions of the circumferential and axial directions.
By comparing the results of this investigation of how complex loading conditions generate stress in the IVD or failure of the AF to a parallel in vitro study, researchers determined that a tensile axial stress of 10MPs or above and a positive circumferential stress of over 8 MPs may cause AF failure. The most unsafe type of loading for the disc is flexion. This numerical model can be used to predict the risk of disc failure in any type of loading condition. The formula can also be used to determine risks in implantable devices and models of entire motion segments.