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The combination of numerical analysis and experimental testing can help to better understand the mechanical behavior of the disc, but the anatomical and mechanical differences between the ovine and the human disc prevent the use of the human models for investigating in vitro animal studies. Many numerical studies on the IVD were performed, but the majority of them were focused on modeling human segments. Furthermore, they provide additional advantages in comparison with the in vitro studies by giving information about local stresses, pressures and deformations, which cannot be directly measured in vitro. īy this point of view, in silico models are crucial because they can effectively support experimental studies. The European directive 2010/63/EU on the protection of animals used for scientific purposes is based on the principle replacing, reducing and refining the animal experiments. Animal Sacrificing is strictly regulated and the debate on animal use is a daily issue. Experiments are expensive, time consuming and laborious. Nonetheless, despite animals can be exploited as a surrogate of humans, there are some economic and ethical reasons for which the number of animals sacrificed for research studies must be reduced.
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Little and co-authors performed uniaxial and biaxial compression and shear tests on sheep specimens to investigate the hyperelastic response of the annulus ground substance. In this regard Kasra and colleagues performed tensile tests on lumbar annular specimens to investigate Young modulus, stress and strain to failure. These peculiarities made sheep a good animal model of the human IVD and promoted the development of experimental tests aimed at exploring the IVD behavior under different loading scenarios.ĭespite some investigations about the structure and composition of the ovine IVD are available in the literature, to our knowledge only few groups investigated in vitro the mechanical properties of the ovine annulus fibrosus. Moreover, with respect to cows, the housing and maintenance of sheep are cheaper. Furthermore, the similar histomorphology of the lumbar intervertebral disc (IVD) and good results in fusion techniques made the ovine species also a good model for in vivo studies. Sheep spines were used to study the initial stabilizing effect of spinal implants in cervical and lumbar segments. However, within the animal substituting options sheep are considered as a favored choice because their disc anatomy shows many similarities with the human one, especially in the lumbar and in the thoracic sections. Concerning to that, many studies investigated the spinal mobility and failure properties assessing human and animal models, but the use of different species (humans, sheep, calf, pig) caused a lack of comparability between the results. Similarly, the use of animal models is characterized by intrinsic disadvantages as genetic, biological, anatomical and postural differences in comparison with humans. Therefore, the number of specimens usually collected and assessed in the experiments results reduced, negatively affecting the statistical robustness of the outcomes. In addition, the costs of using human specimens are remarkable and often not nimbly sustainable when planning research studies. Primarily, human specimens are characterized by a strong variability in anatomy and physical properties, and since usually harvested from aged subjects they often present degenerated or pathological features. Human specimens for in vitro experiments on spine biomechanics have been used for decades, despite some intrinsic limitations.