How does the intervertebral discs parameter variation affect the biomechanical behavior of spinal structures? Results of a detailed study of multibody simulation sensitivity

Sabine Bauer, Dietrich Paulus

At the present there is a variety of ways to investigate the biomechanical properties of human structures. In addition to experimenting and imaging, computer modeling is an established method to calculate the kinematics and kinetics. While many different lumbar spine models have already been created, the sensitivity of the model to input parameters has still not been sufficiently studied. To come up with a realistic modeling, the effects and the interaction of various input parameters must be understood in advance. Particularly for the development of patient-specific models knowledge about the influence of the input parameter constitutes the basis of solid modeling. In the respective literature, especially the performance of the intervertebral disc is described as central to any spine model and thus requires a detailed investigation at first. A multibody simulation model of the lumbar spine that takes the biomechanical properties of the spinal structures such as the intervertebral discs, the ligaments, and the facet joints into account has been created. The model has been validated by comparing the results with results from appropriate literature. The aim of this study is to investigate the impact of the modified disc stiffness onto the spinal structures. For this purpose, stiffness values ​​are used as input parameters that have been published in the literature. To investigate the relationship between the different interspinous structures when changing input parameters the most natural load case, the upright state, is simulated. The disc characteristic of the highest and lowest functional spinal unit shows that the load situation under different stiffness is almost identical. Slight changes are recorded in the central functional spinal units, which are also reflected in the intersegmental rotations. The biggest impact is recorded in the facet joints. The load structure of facets changes in all five functional spinal units. Some are nearly twice as heavily loaded. The study proves that the use of different input parameters does not necessarily lead to major change in the biomechanical behavior of the structures in which the input parameter has been changed, but may have a greater impact on other modeled structures.