Mechanical properties of the cadaveric and hybrid III lumbar spines

This study identified the mechanical properties of ten cadaveric lumbar spines and two Hybrid III lumbar spines. Eight tests were performed on each specimen: tension, compression, anterior shear, posterior shear, left lateral shear, flexion, extension and left lateral bending. Each test was run at a displacement rate of 100 mm/sec. The maximum displacements were selected to approximate the loading range of a 50 km/h Hybrid III dummy sled test and to be non-destructive to the specimens. Load, linear displacement and angular displacement data were collected. Bending moment was calculated from force data.
Each mode of loading demonstrated consistent characteristics. The load-displacement curves of the Hybrid III lumbar spine demonstrated an initial region of high stiffness followed by a region of constant stiffness. The exception to this rule was the tension tests, as the steel cables that run the length of the spine appear to dominate the mechanical response of the Hybrid III spine in tensile loading. The loading curves of the cadaveric spines demonstrated an initial region of low stiffness followed by a region of increasing stiffness, as is typical of soft tissue response. Notable findings included the observation that the whole cadaveric lumbar spine specimens are stiffer in posterior shear than in anterior shear. This finding is in contrast to motion segment studies, in which the opposite trend is observed. This difference is believed to be due to the freedom of vertebrae L1 through L5 to rotate in the whole lumbar spine specimens. In functional spinal unit shear tests, both vertebrae are rigidly potted.