Posterior instrumentation is a common fixation method used in the treatment of spinal diseases. However, the role of different models of fixation system in improving fixation stability in these fractures has not been established. Comparative investigation between posterior rigid fixation (pedicle screw) and four models of posterior dynamic fixation (B Dyne, Elaspine, Bioflex, Coflex rivet) may elucidate the efficacy of each design. The purpose of this study was to investigate the biomechanical differences between rigid fixation and dynamic fixation implantation by using finite element analyses. The goal of the present study was to evaluate the efficacy of five fixation systems mounted on L4-L5 motion segment. In this numerical study, finite element model of an L4-L5 segment was developed from computed tomography image datasets. Five fixation devices were also implanted internally to the motion segment. Another model with an intact intervertebral disc was also analysed for comparison. Loads simulating the physiological flexion, extension and lateral bindings were applied to the superior surface of L4. Results showed that the Elaspine, Bioflex, Coflex rivet and pedicle screw fixation implantation could provide stability in all motions and reduce von Mises stress in the cortical and spongy bone at the surgical segment L4-L5. Moreover, maximal von Mises stress in the annulus disc was observed in dynamic systems but within the safe range. The greater movement of the motion segment was 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 could considerably reduce the load on the discs and prepare conditions for healing of the injured ones. Furthermore, dynamic modes of fixation conferred the possibility of movement to the motion segments in order to facilitate the spinal activities. The numerical results showed that the posterior fixation system (rigid and dynamic) played a very important role in the absorption and minimization of stresses. On the other hand, the tow systems (rigid fixation and dynamic fixation) played such a great role in reducing the stress compared to other synthetic discs. In general, the posterior fixation system gave a lower level of stress in the cortical bones and the spongy bones of the L4-L5 lumbar

Lumbar interbody fusion is a common procedure for treating lower back pain related to degenerative disc diseases, the following two scenarios of posterior lumbar interbody fusion cage (PLIF) were usually used, i.e., Model (I) posterior lumbar intersomatic fusion cage bilateral approach filled with bone graft (1) and (2), (Model II) PLIF with cage made of PEEK or titanium (Ti) materiel filled with bone graft. But the benefits or adverse effects among the two surgical scenarios were still not fully understood. In this regard, we installed these discs between the two vertebrae L5 and S1 of the spine, to ensure spinal stability and avoid slipping, we have used a posterior attachment system (6 screws plus 2 rods) at the pedicular levels of the lumbar vertebra (S1-L5, L5-L4). Finite element analysis (FEA), as an efficient tool for the analysis of lumbar diseases, was used to establish a three-dimensional nonlinear TH1-pelvic FE model (Intact Model) with the ligaments of solid elements. Then it was modified to simulate the two scenarios of PLIF. Two anterior bending moments (P2 and P3) with a P1 compression loading were applied to the 3D model of the spine (TH1-pelvic), respectively. Different mechanical parameters were calculated to evaluate the differences among the three surgical models. The results of numerical values show that these disks played a very important role in the absorption of the stresses and to minimize, On the other hand, the lumbar inter-somatic cage (Model II) filled with cancellous bone is too great a role in reducing the stress compared to another synthetic (Model I) disc. In general, the new model of the inter-somatic cage filled with cancellous bone and reinforced by a posterior fixation system has given a lower level of stress in the cortical bone and the spongy bone of the lumbar vertebra (L5) compared to the healthy disk (D1). The findings provide theoretical basis for the choice of a suitable surgical scenario for different.