Effects of Anterior Shear Displacement Rate on the Structural Properties of the Porcine Cervical Spine

While the individual tissues of the vertebral joint demonstrate viscoelastic properties, the global viscoelastic properties of the lumbar vertebral joint are not well established. This study investigated how changes in displacement rate influenced the mechanical response of the porcine cervical spine (a surrogate or model for the human lumbar spine) exposed to acute anterior shear failure loading. Thirty porcine cervical spine specimens (15 C3-C4 and 15 C5-C6) were placed under a 1600 N compressive load and subsequently loaded in anterior shear to failure at one of three randomly assigned displacement rates (1 mm/s, 4 mm/s, or 16 mm/s). Ultimate anterior shear force, ultimate displacement, average stiffness, and energy stored until failure were calculated. Load rate in the elastic region was also calculated to compare the load rates used in this study to those used in previous studies. Changes in displacement rate affected the C3-C4 and C5-C6 specimens differently. C5-C6 specimens tested at 16 mm/s had an ultimate force that was 28% and 23% higher than at 1 $(p=0.0215)$ and 4 mm/s $(p=0.0461)$⁠, respectively. The average stiffness to failure of the C5-C6 specimens tested at 16 mm/s was 52% higher than at 4 mm/s $(p=0.0289)$⁠. No such differences were found for the C3-C4 specimens. An increase in the anterior shear displacement rate did not necessarily demonstrate viscoelasticity of the vertebral joint. Specimen intervertebral levels were affected differently by changes in anterior shear displacement rate, which may have been a result of anatomical and postural differences between the two levels. Future studies should further investigate the effect of displacement rate on the spine and the inconsistencies between different specimen levels.