Biomechanical effects of a spinal implant - Investigation through MBS computer modelling

Abstract

Excessive mechanical loads in the lumbar spine often lead to the necessity of surgical intervention and the insertion of implants to provide a load sharing stabilization. An appropriate method for the estimation of the mechanical effects of such implants is the simulation with a bespoke programmed computer model. In order to calculate the load in the different structures of the lumbar spine a computer model was created by the Multi Body System (MBS) tool SIMPACK. The MBS-model consists of rigid bony parts for the vertebrae L1-L5, os sacrum and os ilium as well as intervertebral discs and ligaments. All structures are defined by their physical properties therefore a realistic reproduction of the mechanical behaviour of the lumbar spine can be assured. The validation of the model was carried out in conjunction with various in vitro experiments and experimental data taken from biomechanical literature. The MBS-model has been programmed to calculate the load in the different structures of the spine before and after implementation of the dynamic stabilization system Elaspine (Spinelab AG, Winterthur, Switzerland). Under the influence of external forces the mechanical behaviour of the lumbar spine specimens were tested from L2 to L5, with the segment L3-L4 stabilized by Elaspine. The comparison of the simulation and the biomechanical measurements shows a high degree of similarity. An implantation with Elaspine reduces the range of motion (RoM) in the stabilized segment, whereas the adjacent segments are not affected. For flexion/extension the RoM is reduced to 26 percent, for lateral bending to 46 percent and for the axial rotation to 75 percent. The results show that MBS modelling can serve as a useful device for pre-operative surgical planning. © 2012 by Walter de Gruyter Berlin Boston.