PSI - Issue 28

Hassan Mansour Raheem et al. / Procedia Structural Integrity 28 (2020) 1755–1760 Hassan Mansour Raheem et al. / Structural Integrity Procedia 00 (2020) 000–000

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degeneration starts at the nucleus pulposus, which is considered as the main source of the low back pain (Leone et al. 2008). Therefore, the structure and mechanical behavior of NP have attractive many scholars to conduct their research on NP replacement by manipulated the mechanical response of promising materials, such as a hydrogel (Pereira et al. 2011; Raheem, Bay, and Rochefort 2019) for replacing the degenerative NP. Raheem et. al (2019) provided a method that could enhance the viscoelastic properties of hydrogels by infusing a biocompatible foam into the hydrogel. The compressive load is the primary load that functional spinal unit experience in routine daily life (Gloria et al. 2007). Thus, the model in this research was examined under axial compression loads. The influence of changes in NP present as intact or partially removed on the mechanical response of the functional spinal unit is also studied. Many approaches have also been carried out on the functional unit of the spinal disc in vitro (Chan et al. 2013), in-situ (Joshi et al. 2006), and modeling using Finite Element Analysis (FEA) (Coogan et al. 2016) to understand the response of the disc under different loading modes. Modeling by using FE approaches takes the advantages of a new revolution in simulation. However, few studies have investigated and compiled the results of all cases of the variations in NP (Meakin, Redpath, and Hukins 2001). Thus, this study aims to investigate the variations of the NP on the overall mechanics of the disc. To achieve this goal, FEA models have developed for the functional unit of the spinal column by assuming many scenarios of NP situation, a disc without, with NP, and with partial removal of the NP. 2. Methods 2.1. The geometry of the functional unit of the spinal column Figure 1 shows the model, which is built up using ANSYS SpaceClaim Direct Modeler. The dimensions of the human spinal intervertebral disc were according to Lei et al. study (Lei et al. 2010). The model consists of two distinctive regions: the upper and lower vertebral bodies, and the intervertebral disc. The disc was modeled as two parts: the nucleus pulposus and annulus fibrosus.

Fig. 1. The 3D model of the functional spinal unit.

2.2. Finite element model 3D FEA models have been developed using ANSYS software. The mechanical behavior of the material is assumed as a linear elastic as it is listed in Table 1. To solve the mathematical model in FEA, the geometry must be divided into elements. In the meshing process for the intervertebral disc with NP, a hexahedral element type with 20 node and other types of elements, such as (quadratic element, etc.) were used as shown in Fig. 2. After assigning the types of elements and meshing process, the boundary conditions were applied. The upper surface of the upper vertebra was subjected to various displacements in compression in the z-direction (U z =0 -2.5 mm) for 10 steps while the displacements in x and y direction were fixed (U x =0 and U y =0). The lower surface of the lower vertebra was fixed in three directions (U x =0, U y =0, and U z =0).