Numerical Study of the Effect of Rigid and Dynamic Posterior Attachment Systems on Stress Reduction in Cortical and Spongy Bones of the Lumbar Segments L4-L5

Abstract

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

Stress Shielding Prediction of Unicortical and Bicortical Screws: A Finite Element Analysis

“…On the other hand, the bicortical fixation always engage with both the near and far cortical bones [19]. Rectangular shape of screw thread was used in this study as it shown to give a uniform surface stress distribution [26]- [27]. The 2D model was constructed by 3 mm homogeneous cortical bone layers at upper and bottom of cancellous bone and 20 mm cancellous bone region as listed in Table 2.…”

“…These situations demonstrate that at the bone-implant interaction, it experienced stress shielding response where normal mechanical stress of bones reduced, resulting in bone loss in the region of implants [26]. Even though loosening of the screw has been well reported in the literature, there are still lacking regarding the bone-screw interaction involving the reduction of stress shielding and bone remodelling of fully threaded cortical screw as most of previous study focused on partial thread screw and dental implant.…”