**5. Meaningfulness and limitation of MBS modeling**

It is clearly pointed out that modeling has to be understood as an approximation to the reality. In near future, it will probably not be possible that all influencing factors can be simulated completely as in reality. Often, input parameters based on literature data representing the average of a specific cohort. Because each person has a very specific anthropometric and morphological characteristic, the standard deviations of such investigations may be relatively large. For example, the standard deviation for geometric parameters that describe the sagittal alignment of the spine is partially enormous. The deviation from average of the pelvic incidence is partly more than 20% [31], of the pelvic tilt 53% [32] respectively 70% [33], and of the total lumbosacral lordosis 19% [34], etc. But based on sensitivity analysis, the exact influence of individual parameters can be determined. However, because of the complexity of the models, it is very important to know the exact configuration of the model and the limita‐ tions of its input parameters. Therefore, the modeling and the validation process should to be understood as an evolving process and will be advanced in future research.

Opportunities for improvement and expansion of the different model components will be summarized in the following:

In the presented models, the biomechanics of the intervertebral disc is defined by a force law, which can be understood as an initial approach. To precise the biomechanical properties of the annulus fibrosis and nucleus pulposus, a 3D hybrid model consisting of MBS and FE units will be built. The aim is to analyze whether the implementation of FE-disc features contributes to a significant gain of knowledge concerning load distribution of the intervertebral discs and the adjacent spinal structures.

Currently, a highly accurate modeled individual cartilaginous contact layer for all facet joints is in development. These individually formed layers of cartilage (**Figure 17**) allow us to do a 3D calculation of the resulting forces.

**Figure 17.** Facet joints with a 3D layer of cartilage.

An extremely important spinal structure that describes the dynamics of the musculoskeletal system is the spinal musculature. Up to now, the muscle groups of the erector spinae and the rectus abdominis implemented in the models are very rudimentary. In focus of current work are corresponding muscle models that describe the dynamics of muscle contraction in an appropriate manner [50, 51].

The aim of our research is the creation of patient-specific 3D hybrid computer models of the human spine for preoperative planning in neurosurgical and orthopedic stabilizing spinal operations. Known and alternative surgical procedures are simulated and a comparison of these possible surgical procedures is presented to determine the biomechanical effects. The simulation is realized highly efficient with short computation times, which will allow a later use in real systems. That means, the 3D coordinates of the optimized implant position can be exported out of the individual models as data sets and can be transferred, to position the implants very accurately, to a navigation system.
