Anatomie et physiologie : recherches en cours
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Compressive Biomechanics of the Reptilian Intervertebral Joint

Fauble K, Adams J, Van Sickle C, Young BA, Gerdes M

Some reptiles have an intervertebral disc, while others have lost all traces of the disc and have diarthrotic joints between the successive vertebrae. Counter-intuitively, the group of reptiles without intervertebral discs include the largest living taxa capable of bipedal locomotion. This study explored this phenomenon by comparing the pre-sacral intervertebral joints of the American alligator (Alligator mississippiensis) with those from specimens of Varanus. These two taxa were chosen because they have similar number of pre-sacral vertebrae and similar body weights; however, Varanus can move bipedally and has diarthrotic intervertebral joints, whereas Alligator has intervertebral discs and cannot move bipedally. This study consisted of three objectives; 1) to document the anatomy of the intervertebral joint, 2) to quantify the compressive biomechanics of the intervertebral joints and explore which features contributed to compression resistance, and 3) to quantify the impact of compression on the intervertebral foramen and spinal nerves in these two taxa. The experimental results revealed that the diarthrotic intervertebral joints of Varanus were significantly (4x) stiffer than the intervertebral disc of Alligator, and that a significant component of this increased stiffness arose from the facet joints. Compressing the intervertebral joints of the two taxa caused a reduction in foraminal area, but the magnitude of this reduction was not significantly different. We hypothesize that the main factor preventing spinal nerve impingement in Varanus during gravitational compression is the relatively small size of the spinal ganglion/nerve relative to the foraminal area.