Mammalian spinal biomechanics. II. Intervertebral lesion experiments and mechanisms of bending resistance

Three-point cyclic bending was applied to intervertebral joint complexes (three vertebrae with two intervertebral discs) of monkey (Macaca fascicularis), wallaby (Wallabia rufogrisea frutica), tiger (Panthera tigris), jaguar (Panthera onca) and seal (Halichoerus grypus). Force-displacement loops were recorded for intact specimens in both extension and flexion. Reductions in peak forces at given displacements were measured, following lesions of ventral ligaments, superspinous ligaments, interspinous ligaments and muscles, ligamenta flava and the articular capsules. Subsequently, the vertebral arches were removed from the specimens to test the bending resistance of the intervertebral discs alone. The results of these lesion experiments, coupled with details of intervertebral joint anatomy, suggest that extension resistance is ultimately due to articular joint impaction for all species tested. The prominent ligamenta flava of the monkey and wallaby contrast with the robust discs of jaguar and tiger and illustrate two distinct mechanisms for resisting flexion in mammalian intervertebral joints. The conspicuous absence of soft structural elements in seal intervertebral joints contributes to their low bending resistance. The implications of these findings for mammalian locomotion, behaviour and scaling are discussed.