Compressive Biomechanics of the Reptilian Intervertebral Joint

This study compared the pre-sacral intervertebral joints of the American alligator (Alligator mississippiensis) with those from specimensof Varanus. These two taxa were chosen because they have similarnumber 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 anatomyof the intervertebral joint, (2) to quantify the compressive biomechanicsof the intervertebral joints and explore which features contributed tocompression resistance, and (3) to quantify the impact of compressionon the intervertebral foramen and spinal nerves in these two taxa. Theexperimental results revealed that the diarthrotic intervertebral jointsof Varanus were significantly (4x) stiffer than the intervertebral disc ofAlligator, and that a significant component of this increased stiffnessarose from the facet joints. Compressing the intervertebral joints of thetwo taxa caused a reduction in foraminal area, but the magnitude of thisreduction was not significantly different. We hypothesize that the mainfactor preventing spinal nerve impingement in Varanus during gravitational compression is the relatively small size of the spinal ganglion/nerve relative to the foraminal area.