Multiplanar Creep Characterizations of the Functional Spinal Units of the Subaxial Cervical Spine

Abstract Spinal biomechanical testing protocols for specimen preconditioning and characterization of biomechanical creep vary widely and are only reported for pure moment, planar loading protocols, which do not factor in the complex, multiplanar loading pathways present in in situ spinal behavior. This study explored: (1) whether the gain tuning process for stabilizing robotic control for experimental testing can also serve as a biomechanical preconditioning phase and (2) the multidimensional creep properties of the cadaveric human subaxial cervical spine. Specimens were subjected to system gain tuning and two distinct loading trajectories combining flexion–extension and lateral bending loading over a 16-hour period. Key outcomes included changes in range of motion (ROM), neutral zone size, resultant load, stiffness, and mechanical degradation. A nonstatistically significant increase in ROM in the first five boundary capture cycles suggests that gain tuning a robotic system for cadaveric testing may be sufficient preconditioning. Specimens experienced uniform increases in ROM about the craniocaudal axis over the course of each testing day, with the only statistically significant increase occurring around flexion-dominant loading (p = 0.038). The increase in ROM was not retained between days of testing, suggesting that these changes may be due to viscoelastic adaptation rather than structural damage. An increase in neutral zone area across testing days may be linked to intervertebral disc dehydration over long-duration testing. These findings suggest that long-duration testing be conducted on postmortem human subjects with caution. Furthermore, multiplanar protocols can characterize multidimensional spinal creep properties, supporting their use in preclinical spinal testing.