**4.1. Effects of different spine alignments in the standing position**

Due to different physical constitution and daily physical stress, an individual characteristic alignment of the spine is developed in the course of life. Therefore, the double-S shape of the human spine is subject to a wide range of variations [31–34]. To investigate the effects of such different spinal curvatures on the intradiscal pressure, five models of the lumbar spine with different lordosis angles are created (**Figure 4**). In this work, the lordosis angle is defined by the line through the upper endplate of L1 and the endplate of os sacrum.

Basics of Multibody Systems: Presented by Practical Simulation Examples of Spine Models http://dx.doi.org/10.5772/62864 35

increase of computation time. Therefore, prior to the model building a specific question should

A further important structure to stabilize the human body in upright position is the muscu‐ lature. According to [7], the four muscle groups, left and right musculus erector spinae and left and right rectus abdominis muscle, are modeled. The force representing the musculus erector spinae is varied so, that additional torque on the upper endplate of the vertebra L1 is no longer necessary to maintain the lumbar spine in a state of equilibrium. The model parameters used for muscles are taken from [7, 26]. These muscular structures are implemented at the moment just in the models of Chapter 4.1. A comparison of the different spine models

For the validation process, there is the difficulty of developing a suitable method, which confirms the accuracy of the modeling. An established method compares the simulation results with results from accepted publications. But it has to be mentioned that not all parameters which may have a significant influence on the result are always published. If such a factual circumstance is known, which is at the same time the model limitation, this has to be considered

The model validation was performed by comparing the simulation results with FE results and in-vivo data as found in references [7, 27–29]. A detailed presentation of the validation is to be taken from reference [24]. Another difficulty lies in the selection of suitable input parameters from the literature. The values of these parameters differ significantly in some cases [30]. The validation and modeling cannot be seen as complete, and are successively continued to

To gain an insight into the practical applications of computer modeling in the field of biome‐ chanical modeling of human structures, selected examples of different spinal simulation cases are shown below. In the first examples, the effects of different spinal alignments and obesity in adults and children on the lumbar spine are discussed. Subsequently practical examples of

Due to different physical constitution and daily physical stress, an individual characteristic alignment of the spine is developed in the course of life. Therefore, the double-S shape of the human spine is subject to a wide range of variations [31–34]. To investigate the effects of such different spinal curvatures on the intradiscal pressure, five models of the lumbar spine with different lordosis angles are created (**Figure 4**). In this work, the lordosis angle is defined by

be defined, in order to decide whether such a detailed facet joint modeling is needed.

with the corresponding muscle models is performed (see Section 4).

**3.4. Validation and sensitivity analysis of the model**

34 Numerical Simulation - From Brain Imaging to Turbulent Flows

in the discussion of the model validity.

develop MBS models that get closer to the reality.

**4. Practical application examples of simulation**

possible use of computer models in medicine will be introduced.

**4.1. Effects of different spine alignments in the standing position**

the line through the upper endplate of L1 and the endplate of os sacrum.

**Figure 4.** MBS lumbar spine models with different alignments: The model located in the middle has to be understood as a basic model concerning the anthropometrical and biomechanical properties for the further models A, B, C, and D. The basic model has a lumbar angle of 60°, model A of 64°, model B of 61°, model C of 59°, and model D 56°. It should be noted that the proportions in this figure should be understood as an approximation to the real model dimensions. Therefore, deviations can occur from the model.

Upright standing of a normal-weight person is chosen as simulation case because it represents the natural load case. The models are applied with the weight of the upper body. The force application point is located in the center of gravity. This gravitational force causes a deforma‐ tion of the intervertebral discs, and a corresponding reaction force in the intervertebral discs is built. Therefore, the average intervertebral disc force, calculated from the different values of corresponding functional spinal units (FSUs) of the five models, increases from the lowest FSU to the uppermost FSU. In this context, the term FSU is understood as the smallest physiological motion unit consisting of two adjacent vertebrae, the corresponding interverte‐ bral disc and all adjoining ligament [25].

**Figure 5** shows the intradiscal pressure of all functional spine units of the different aligned models in comparison. Because of the different spine alignments, it could be assumed that also the intradiscal pressures of the various models should greatly differ from each other. But this

**Figure 5.** Interdiscal pressure of the differed aligned lumbar spinal models.

assumption is not immediately evident from **Figure 5**. However, the percentage difference of the maximum and minimum intradiscal pressure value of an FSU is determined, and the percentage difference of the FSUs L5-Sac and L4-L5 is 8% and of the FSU L2-L3 even 13%. For the intervertebral discs, the resulting percentages of L3-L4 and L1-L2 are 3% and 5% respec‐ tively.

To sum up, it can be stated that the individual spinal alignment has an impact on the load distribution of the intervertebral disc.
