Chronic back pain has historically been treated through spinal decompression and fusion often accompanied by fixation devices. Concerns regarding the effect of rigid fixation on the surrounding tissue and vertebral levels adjacent to fusion have given rise to a new paradigm based on restoring healthy or natural motion of the operated level. This paradigm revolves around the design and implementation of so-called motion preservation devices. In vitro testing has been and will continue to be an integral step in the design and evaluation process for both rigid fixation and motion preservation devices. However, the metrics commonly used to asses the efficacy of a rigid fixation device are insufficient for the assessment of motion preservation devices. In addition, motion preservation device metrics have not been rigorously defined or characterized in the healthy human spine.

The kinematic response of the human cadaveric spine to loading in a six-degree-offreedom spine testing apparatus can be expressed in terms of Euler angles and the helical axis of motion while the viscoelastic response can be expressed in terms of the energy dissipated by each specimen during a single cycle of testing. Beyond conventional metrics, a new, noninvasive method based on applying test kinematics to a three-dimensional rigid-body model of the spine is developed and used to investigate articulation of the facet joints. Articulation is investigated based on a distance map between adjacent articular surfaces and quantified through the calculation of a parameter describing the proportion of the facet contact area.

Statistically significant differences were found between the facet contact area parameter at full extension and full flexion at every level of the lumbar spine during in vitro testing (p<0.037). Additionally, significant differences were found between the mean helical axis locations of some of the levels. A significant difference was found between the anterior/posterior location of the helical axis during flexion and Extension at the L1-L2 level (p=0.003). The sensitivity of these parameters in describing differences in lumbar kinematics between levels and between different portions of the range-of-motion lends credence to their efficacy in evaluating the quality of motion achieved after implantation of a motion preservation device.