Pedicle-screw-based motion preservation systems are often used to support a slightly degenerated disc. Such implants are intended to reduce intradiscal pressure and facet joints forces, while having a minimal effect on the motion patterns.
In a probabilistic finite element study with subsequent sensitivity analysis, the effects of 10 input parameters, such as elastic modulus and diameter of the elastic rod, distraction of the segment, level of bridged segments, etc. on the output parameters intervertebral rotations, intradiscal pressures, and facet joint forces were determined. A validated finite element model of the lumbar spine was employed. Probabilistic studies were performed for seven loading cases: upright standing, flexion, extension, left and right lateral bending and left and right axial rotation.
The simulations show that intervertebral rotation angles, intradiscal pressures and facet joint forces are in most cases reduced by a motion preservation system. The influence on intradiscal pressure is small, except in extension. For many input parameter combinations, the values for intervertebral rotations and facet joint forces are very low, which indicates that the implant is too stiff in these cases. The output parameters are affected most by the following input parameters: loading case, elastic modulus and diameter of the elastic rod, distraction of the segment, and angular rigidity of the connection between screws and rod.
The designated functions of a motion preservation system can best be achieved when the longitudinal rod has a low stiffness, and when the connection between rod and pedicle screws is rigid.
In contrast to rigid spinal stabilization devices, pedicle-screw-based motion preservation systems are intended to maintain most of the natural intervertebral range of motion. Slightly degenerated discs are often treated with motion preservation systems to slow down or even stop further degeneration. It is assumed that a dynamic stabilization system allows for a motion pattern similar to that of a healthy motion segment, a strong reduction in facet joint forces during extension, and a reduced intradiscal pressure during flexion and extension. Thus, pedicle-screw-based implants for dynamic stabilization of the lumbar spine are becoming more and more popular (Ahn et al., 2008, Schmoelz et al., 2006, Stoll et al., 2002 and Wilke et al., 2009). The stabilizing effect of the early and wide-spread Dynesys device, however, differs only slightly from that of a rigid metallic fixation device, (Rohlmann et al., 2007).
Schmoelz et al. (2009) experimentally studied the behaviour of the novel Elaspine non-fusion implant (Spinelab AG, Winterthur, Switzerland), which is comprised of pedicle screws and a clip mechanism connected to a polycarbonate-urethane (PCU) rod with a 360° form-fit. Six fresh lumbar spine specimens were tested and the results of the Elaspine implant were compared to those of the Dynesys system (Zimmer, Winterthur, Switzerland) as well as to the intact situation. Compared to published data for the Dynesys system (Schmoelz et al., 2003), the Elaspine implant allowed greater motion in lateral bending and flexion/extension while still exhibiting a limited range of motion compared to the intact spine. Both implants had a minor effect during axial rotation.
The influence of implant stiffness on the mechanical behaviour of the lumbar spine was evaluated in a deterministic finite element study (Rohlmann et al., 2007). It was found that only implants with a very low stiffness allow significant motion in the treated segment during flexion and extension.
The influences of an implant on intervertebral rotation, facet joint forces, and intradiscal pressure may depend on several factors including implant stiffness, distance of the longitudinal rod from the vertebra, number of treated segments, extent of the bony defect, distraction of the segment, etc. These influences may be strongly increased by combinations of several factors. The combination of all possible values for these factors leads to a very large number of possibilities making it impractical to investigate. In probabilistic finite element studies, calculations are performed for a large number of random parameter combinations and the possible range of results is estimated (Dar et al., 2002 and Haldar and Mahadevan, 2000). A subsequent statistic sensitivity analysis allows the determination of those factors, which mainly explain the variance of the results.
The aims of the present study were twofold: (1) to determine in a probabilistic study the possible ranges of the output parameters intervertebral rotation, facet joint force, and intradiscal pressure for 10 input parameters, such as elastic modulus and diameter of the longitudinal rod, angular rigidity of the stabilization device, number of stabilized segments, etc.; and (2) to calculate in a subsequent sensitivity analysis the coefficients of importance (CoIs) in order to determine the influence of single input parameters on the variance of the output parameters.
