Comparison of the load-sharing characteristics between pedicle-based dynamic and rigid rod devices

Abstract

Recently, numerous types of posterior dynamic stabilization (PDS) devices have been introduced as an alternative to the fusion devices for the surgical treatment of degenerative lumbar spine. It is hypothesized that the use of 'compliant' materials such as Nitinol (Ni-Ti alloy, elastic modulus = 75 GPa) or polyether-etherketone (PEEK, elastic modulus = 3.2 GPa) in PDS can restore stability of the lumbar spine without adverse stress-shielding effects that have often been found with 'rigid' fusion devices made of 'rigid' Ti alloys (elastic modulus = 114 GPa). Previous studies have shown that suitably designed PDS devices made of more compliant material may be able to help retain kinematic behavior of the normal spine with optimal load sharing between the anterior and posterior spinal elements. However, only a few studies on their biomechanical efficacies are available. In this study, we conducted a finite-element (FE) study to investigate changes in load-sharing characteristics of PDS devices. The implanted models were constructed after modifying the previously validated intact model of L3-4 spine. Posterior lumbar fusion with three different types of pedicle screw systems was simulated: a conventional rigid fixation system (Ti6Al4V, Phi = 6.0 mm) and two kinds of PDS devices (one with Nitinol rod with a three-coiled turn manner, Phi = 4.0 mm; the other with PEEK rod with a uniform cylindrical shape, Phi = 6.0 mm). To simulate the load on the lumbar spine in a neutral posture, an axial compressive load (400 N) was applied. Subsequently, the changes in load-sharing characteristics and stresses were investigated. When the compressive load was applied on the implanted models (Nitinol rod, PEEK rod, Ti-alloy rod), the predicted axial compressive loads transmitted through the devices were 141.8 N, 109.8 N and 266.8 N, respectively. Axial forces across the PDS devices (Nitinol rod, PEEK rod) and rigid system (Ti-alloy rod) with facet joints were predicted to take over 41%, 33% and 71% of the applied compression load, respectively. Our results confirmed the hypothesis on the PDS devices by showing the substantial reduction in stress-shielding characteristics. Higher axial load was noted across the anterior structure with the PDS devices, which could slow the degeneration process of bony structures and lower the possibility of implant failure.

Optimal stiffness of a pedicle-screw-based motion preservation implant for the lumbar spine

“…are used. Stiffness of an implant is determined by its shape and elasticity and therefore varies considerably for existing dynamic implants [1][2][3][4]. Its optimal value depends on local conditions and is still unknown.…”

Effect of a pedicle-screw-based motion preservation system on lumbar spine biomechanics: A probabilistic finite element study with subsequent sensitivity analysis

“…It is assumed that a dynamic stabilization system allows for a motion pattern similar to that of a healthy motion segment, a strong reduction in facet joint forces during extension, and a reduced intradiscal pressure during flexion and extension. Thus, pediclescrew-based implants for dynamic stabilization of the lumbar spine are becoming more and more popular (Ahn et al, 2008;Schmoelz et al, 2006;Stoll et al, 2002;Wilke et al, 2009). The stabilizing effect of the early and wide-spread Dynesys device, however, differs only slightly from that of a rigid metallic fixation device, (Rohlmann et al, 2007).…”

Change of Lumbar Motion after Multi-Level Posterior Dynamic Stabilization with Bioflex System : 1 Year Follow Up

“…These techniques destroy the posterior spinal elements which are much responsible for the mobility and stabilization of a spine 1,10) . Current surgical management needs better functional improvements.…”

“…Load transmission by rigid fixation can make osteoarthritic changes in adjacent joints 14) . Other complications such as fatigue fractures of the vertebral body or pedicle, instrument failure, stressshielding, adjacent segment degeneration and loss of lumbar lordosis have always been a disadvantage of rigid fixation 1) . Posterior dynamic stabilization was first introduced in 1992 by Henri Graf 11) following the need for an instrument that can stabilize operated segments, restore the mobility and prevent the adjacent segment degeneration after a decompressive laminectomy which destroys posterior spinal elements 4) .…”