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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3. Results and discussion In the present research, 3D finite element models were built with linear material properties to investigate the response of the functional spinal unit under axial compression loads. The model was examined in three cases: without nucleus pulposus, intact model (with nucleus pulposus), and with partial removal of the nucleus pulposus. Validation of the model enables the quality and reliability of the FEM and the modeling methodology. The models are examined by changing the status of the nucleus pulposus, i.e. the central gap of the annulus fibrous was being filled with 95%, and 100% of NP. Figure 4 shows the results of the models. It can be seen that the response of the model accordingly alters with reducing the volume of the filling material, i.e. NP. It can also be concluded that the response of the functional spinal unit is susceptible to the condition of the NP. The disc without nucleus pulposus experience higher axial displacement than the intact disc at the same loading value. This could lead to higher bulging and shear stress in the annulus fibrosus. Subsequently, the disc herniates or a tear in annulus fibrosus presents. In addition, changing the NP condition affects the overall stiffness of the disc. The response of the intact model shows higher stiffness, whereas the disc without nucleus pulpous experiences lower stiffness, as shown in Fig. 4. The model show nonlinear behavior and these findings agree with (Kulak et al. 1976). The pressurization of the nucleus pulposus creates the stiffening effect of the model.

Fig. 4. The response of the model under axial compression loads with different conditions of the nucleus pulposus.

Figure 5 shows comparisons between the mechanical response of the model (intact model) under compression loads with literature results (Brown, Hansen, and Yorra 1957; Joshi et al. 2006; Kulak, Belytschko, and Schultz 1976; Shirazi-Adl et al. 1984). These data are human, and simulated data using FE, in such (Brown et al. 1957) for segments L2/L3 to L4/L5 and (Kulak et al. 1976) for finite element results, while (Joshi et al. 2007) data are for an intact spinal functional unit. The responses of the models confirm that load-displacement behavior falls within the range of reported values for human tissue samples. It is worthy to mention that the results of the literature vary widely because the biological properties of the functional spinal unit are not similar between studies (Tencer and Ahmed 1981). Thus, we have shown that a simple representation of the functional spinal unit through a simple FE model provides a realistic response similar to the native tissue. There are many approaches for improvement, such as model the annulus fibrous as a composite compartment that consists of collagen fibers.