PSI - Issue 28

Hassan Mansour Raheem et al. / Procedia Structural Integrity 28 (2020) 1727–1732 Hassan Mansour Raheem et al. / Structural Integrity Procedia 00 (2020) 000–000

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due to trauma or a disease process. A disc herniation occurs when the annulus fibers are weakened or torn and the central component, i.e. NP, extrudes out of the confined area of the annulus fibrosus (Bogduk 1991). Thus, several studies have been investigated the bulging phenomena in the disc (Heuer, Schmidt, and Wilke 2008; Raheem, Rochefort, and Bay 2020; Reuber et al. 1982a). It is considered that the increasing bulging in the intervertebral disc leads to generate high pressure on adjacent nerve roots and eventually causes low back pain or even paralysis. Raheem et al. (2020) provided a novel device that could test promising materials for nucleus pulposus replacement instead of using traditional methods, i.e. injecting material into a cadaveric spine (Joshi et al. 2006), or using animal models (Omlor et al. 2012). Besides, the bulging in the disc was examined using digital image correlation method. The magnitude of the bulging in the disc depends on the region. In other words, it is not uniform in all areas, i.e. the posterior, posterolateral, and lateral regions, such as the bulging magnitude in the lateral region is nearly similar to the posterolateral region, but the posterior region has smaller bulging magnitude compared with other regions under the same compressive loads (Reuber et al. 1982). Therefore, the bulging (deformation) plays a vital role in patients suffering from disc degeneration. Increasing bulging causes to increase shear stresses at the annulus fibrosus and eventually leads to surgical intervention if classical treatments do not remove the pain. Therefore; knowing the behavior of bulging in the intervertebral disc helps to prevent severe damage to the disc and contributes to finding a suitable treatment for damaged discs. Thus, this study aims to build a three-dimensional finite element (FE) model to investigate the bulging in the lumbar disc due to changing the nucleus pulposus status under axial compression loads. 2. Methods 2.1. The functional unit of the spinal column Figure 1 shows the geometry of the 3D model of the functional unit of the human spinal column of the lumbar region. It is composed of three parts: the upper and lower vertebral bodies, and the intervertebral disc. The disc consists of the central part, the nucleus pulposus, and the annulus fibrosus, which is surrounded by the NP. The dimensions of the model are based on (Lei et al. 2010).

Fig. 1. The functional unit of the spinal column

2.2. Finite element model 3D FEA models have been developed using the ANSYS ® software. The mechanical behavior of the material is assigned as a linear elastic as it is listed in Table 1. To solve the mathematical model in FEA, the geometry must be