2D1 Artificial Organs & Implants II

Abstract

IntroductionAn interspinous process decompression device has been developed to treat spinal stenosis by placing the stenotic segment in slight flexion and prevents extension. This restriction of motion by inserting the interspinous device may affect the intradiscal pressure, foramina area, and rotational angle of the instrumented level [1][2][3]. The aim of this study was to understand the biomechanical effects of lumbar spine with the interspinous device in different loading conditions. Materials and MethodsA finite element model of the lumbar vertebral body (L1-L5) was generated from the digitized CT scans using the image software-Amira (Mercury Computer Systems, Inc., Massachusetts, U.S.A.). The surface models of the vertebral bodies and discs were transferred to a finite element preprocessing program -Mentat (MSC Software Corp., Los Angeles, U.S.A.) and the finite element mesh of the intact L1-L5 vertebrae was generated with 44,906 8-node hexahedral elements as shown in Fig. 1. The interspinous process decompression device was inserted between the spinous process of L3 and L4. The material properties of the interspinous process decompression device were a titanium alloy. Accordingly, a modulus of 110 GPa and a Poisson's ratio of 0.3 were defined for the titanium alloy. The nucleus pulposus were modeled as incompressible element. The curved facet joints had a gap of 0.5 mm and could transmit only compressive forces. The ligaments were represented by tension-only cable elements with linear material properties. The inferior side of L4 endplate was regidly fixed. The model was loaded with the upper body weight (300N) to simulate four loading cases: compression, flexion, extension, lateral bending and torsion (10Nm). A commercially available FEA software -MARC (MSC Software) was used as a solver for these analyses. Analyses were performed using the computing facilities at the National Center for High-Performance Computing (NCHC, Taiwan) via internet connection. Analysis results were retrieved back and processed on a local personal computer. All analysis results for the models with interbody spacers were compared with the intact ones. Results and DiscussionThe intradiscal pressure at the L3-L4 disc was decreased in the posterior annulus and nucleus under the extension moment with interspinous process decompression device. (fig.2) In extension, the implant increased the foraminal height by 11%, and the foraminal width by 10%. The range of motion of the instrumented level was significantly reduced in extension. However, the range of motion of the instrumented level were not affected in flexion, axial rotation and lateral bending. (fig.3) The range of motion in flexion-extension, axial rotation, and lateral bending at the adjacent levels were not have significantly changes by inserting the interspinous process decompression device. The interspinous process decompression device could decrease the intradiscal pressure and rotational angle of the upper vertebra in extension but without changing in other loading conditi...