Study of the Mechanical Behavior Between rigid and dynamic fixation systems analyzed by the FEM.

What is it about?

Orthopedic fixation devices are widely used in treatment of spinal diseases. It is expected that application of dynamic stabilization confers valuable movement possibility besides its main role of load bearing. Comparative investigation between pedicle screw model rigid fixation and (B Dyne, Elaspine, Bioflex, Coflexe rivet) models dynamic fixation systems may elucidate the efficacy of each design. The goal of the present study is to evaluate the efficacy of five fixation systems mounted on L4-L5 motion segment. In this numerical study, a 3D precious model of L4, L5 and their intervertebral disc has been employed based on CT images. five fixation devices have been also implanted internally to the motion segment. Finite element method was used to evaluate stress distribution in the disc and determine the overall displacement of the segment as a measure of movement possibility. The results show that The Coflex rivet implantation can provide stability in all motions and reduce disc annulus stress at the surgical segment (L4-L5), on the other hand, Maximum stress in the disc has been observed in dynamic systems but within the safe range. The greater movement of the motion segment has been also appeared in dynamic fixations. Existence of the fixation systems reduced the stress on the intervertebral disc which might be exerted in intact cases. Use of the fixation devices can considerably reduce the load on the discs and prepare conditions for healing of the injured ones. Furthermore, dynamic modes of fixation confer possibility of movement to the motion segments in order to facilitate the spinal activities.

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Why is it important?

The finite element method (FEM) is a very precise technique used to analyze structural stresses. With its application in engineering, this method can solve many equations to calculate the stresses based on the mechanical properties of the structures being analyzed. FEM has many advantages highlighted by the possibility of including the heterogeneity and irregularity of the contour of the spine in the design of the model and the relative ease with which the loads can be applied to different directions and sizes for one analysis more complete. The Coflexe rivet implantation can provide stability in all motions and can reconstruct the posterior spinal structure for load sharing to reduce disc annulus stress at the surgical segment (L4-L5). However, the Coflexe rivet caused a higher displacement and stress at the disc. As a general conclusion, application of the fixation systems can considerably reduce the load on the IVD and prepare conditions for healing of the injured IVD. Furthermore, dynamic modes of fixation, i.e. B Dyne, ELASPINE, BIOFLEX and Coflexe rivet, confer possibility of movement to the motion segments in order to facilitate the spinal activities.

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