Prospective Design Delineation and Subsequent In Vitro Evaluation of a New Posterior Dynamic Stabilization System

Study Design. Finite element and in vitro study. Objective. Finite element calculations to delineate a dynamic fixator and confirmation with an in vitro experiment. Summary and Background Data. In the last few years, there was a paradigm shift from rigid to dynamic fixation of spinal segments. However, some so-called dynamic implants like the Dynesys performed still stiffer than anticipated. The aim of this study was to optimize a dynamic stabilization system. Methods. The development steps of this implant design can be summarized in a development loop. First, a finite element model of an intact human L4-L5 segment was used to delineate implant stiffness parameters for the implant, in consideration of clinical concerns and safety aspects. These data were used in a second step, leading to the final implant design. This development process was completed with an appropriate in vitro experiment. The optimal axial and bending stiffness were computed to reduce the spinal motion by 30%. For the validation process, in vitro tests were performed on 6 human lumbar spinal segments L2-L3 with a median age of 52. The model and the specimens were loaded with pure unconstrained moments of 7.5 Nm in flexion, extension, lateral bending, and axial rotation. Results. This study demonstrated the advantages of employing a finite element model for the implant parameter delineation. It was possible to prospectively outline the needed stiffness parameters for a desired spinal range of motion achievement. Conclusion. In summary, FEM may accelerate the development and the realization of a new implant design.