Which axial and bending stiffnesses of posterior implants are required to design a flexible lumbar stabilization system?
Introduction
Nowadays, various posterior implant concepts are available, restoring the mechanical function of the lumbar spine. They can be used to support the spine for a broad range of spinal pathologies and abnormalities. These implant concepts can be classified into rigid, semi-flexible and flexible posterior stabilization devices. (The terminology of rigid, semi-flexible and flexible is defined in the “Material and methods” and was introduced by authors to later classify the calculated implant stiffnesses).
The general goals of rigid stabilization devices are to immobilize or to fuse one or more segments and bridging a dysfunction or instability between vertebral segments. There are numerous clinical studies that reported that the fusion can lead to an accelerated disc degeneration at adjacent segments (Shah et al., 2003; Kumar et al., 2001; Schlegel et al., 1996; Lee, 1988; Lehmann et al., 1987), screw misplacements, pedicle fractures or screw breakages (Esses et al., 1993; McAfee et al., 1991). Dynamic stabilization devices have been introduced to clinics as an alternative to the fusion with the aim of preserving the spinal motion (Mayer and Korge, 2002). This should also alleviate back pain and might prevent an initiation of the adjacent level disease (Sengupta, 2004, Sengupta, 2005; Cunningham et al., 1997). Because of these advantages, a large number of different dynamic stabilization devices have been developed and are currently in clinical use (Stoll et al., 2002).
One of the first dynamic stabilization devices was the Dynesys™ implant (Dynamic Neutralization System for the spine–Zimmer, Minneapolis, MN). This device has been developed to keep the mobility of the instrumented spinal segment to a certain degree and to achieve nerve root decompression. However, biomechanical tests showed that this implant provided a higher stiffness than originally desired (Schmoelz et al., 2006, Schmoelz et al., 2003). This case highlights the actual problem in the process of designing a posterior implant. Currently, there are no literature values, which would provide material stiffness data for implants to yield a predefined amount of stabilization for the spine. Based on this knowledge, we might be able to predict a certain implant stiffness to reduce the range of motion (RoM) by e.g. 30% in comparison to a non-treated segment.
Therefore, the aim of this finite element (FE) study was to provide stiffness parameter ranges for posterior stabilization devices, which would achieve a well-defined flexible, semi-flexible or rigid connection between two vertebrae. In order to examine the validity of the computed results, an in-vitro study was carried out testing the RoM of a dynamic stabilization device: DSS™ (Paradigm Spine; Wurmlingen, Germany) and a rigid stabilization device: internal fixator.
Section snippets
FE-model
A three-dimensional (3D), non-linear FE-model of an intact L4-5 ligamentous human lumbar motion segment was used in this study (Fig. 1). This FE-model has been used previously to investigate a number of clinical relevant issues (Heuer et al., 2008; Schmidt et al., 2008, Schmidt et al., 2007b, Schmidt et al., 2007c). The model validation has been extensively documented in these studies. In the following is a brief description of the FE-model.
The commercial software ANSYS 11.0 (Swanson Analysis,
Results
Generally, the FE-model predicted that the simultaneous variation of ca and cb caused two effects. First, both parameters showed a non-linear response in the stabilization of the spinal segment. That means an increase of ca and cb within a range of 0 and 200 N/mm caused a strong RoM reduction. In contrast, if both stiffness parameters increased starting from 200 N/mm, a lower RoM decrease resulted. The second effect was found, in that the ca of the posterior stabilization device delineates the
Discussion
The development of posterior spinal devices has provided various benefits, but it can be accompanied with severe complications, e.g. screw breakages, screw loosening or initiation of degeneration processes (Chen et al., 2005; Mayer and Korge, 2002; Pihlajamaki et al., 1997; Niu et al., 1996). Some of these problems can be related to high device stiffnesses. Therefore, dynamic implants have had an increase of acceptance in clinical practice. The stiffness of these devices varies widely, whereas
Conclusion
The results of our FE-model demonstrated that there is a certain dependency between axial and bending stiffness of a posterior device on the amount of the segmental stabilization. In contrast to the current state of knowledge, only small fixator stiffness values (nearly ca=45 N/mm and cb=30 N/mm) are necessary to reduce the spinal flexibility by 30%. Using these data, it is possible to develop implants for certain clinical indications. For the delineation of implant stiffness values, we recommend
Conflict of interest
The authors declare that neither the authors nor members of their immediate families have a current financial arrangement or affiliation with the commercial companies whose products may be mentioned in this manuscript. This study was founded by the German Research Foundation and should not be conflicted by commercial interests.
Acknowledgment
This study was financially supported by the German Research Foundation (Wi-1352/12-1). The implant DSS™ was provided by the company Paradigm Spine (Wurmlingen, Germany).
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