Eight cadaveric cervical spine specimens (C2-T1) were dissected and prepared for biomechanical testing. Each vertebra was instrumented with an optoelectronic marker for motion capture (Optotrak Certus Motion Capture System; Northern Digital Inc. Waterloo, ON, Canada). All specimens were first tested in two interventions: intact or native, followed by implantation of the Mobi-C (Zimmer Biomet Spine, Westminster, CO) TDR at both C4-C5 and C5-C6. The third intervention involved “overstuffing”, replacing an appropriately sized Mobi-C device for another Mobi-C device with the identical footprint but 1 mm taller core (e.g. replacing a 13x15 H5 Mobi-C with a 13x15 H6 Mobi-C). The overstuffing was performed at C4-C5 for four of the eight specimens and at C5-C6 for the remaining four specimens (Figure 1). Mobi-C disc sizes and heights were recorded for all interventions.
The specimens were tested following each intervention using a 6 degree of freedom servo-hydraulic spinal simulator (Figure 2: Bionix Spine Kinematics System, MTS Corporation, Eden Prairie, MN, USA) in 3 cycles of pure-moment flexion/extension (Figures 3 and 4) up to 2 Nm.
The maximum range of motion (ROM) for the entire specimen and individual levels was calculated during the third loading cycle of each test. In addition, the maximum total ROM for each specimen in the native condition was recorded and used as a target to determine deficits in total ROM at the second and third interventions.
A logarithmic curve was fit to the elastic zone of the total rotation vs. moment curve for both flexion and extension of the second and third interventions; the amount of additional moment needed to compensate for the deficiency in ROM between the intervention of interest and the native state was recorded. Finally, additional moment was applied to the levels adjacent to the index levels to determine what increase in motion could be expected using the same logarithmic fitting technique previously described. However, the ROM at which the moment was achieved was recorded, rather than finding the moment at which the target ROM would be reached.
Quality of motion was also assessed for each intervention. A logarithmic curve was fit to the flexion and extension curves for the index levels of each specimen at each intervention. From these curves, an average flexion and extension curve was calculated for each intervention.
To assess for significance differences between groups vs. interventions, a two-way ANOVA with one-way repeated measures was performed. If the effect of group and interaction of group and intervention were found to be insignificant for the value of interest, the groups were combined, a Shapiro-Wilk test was performed to assess normality, and the appropriate (either parametric or non-parametric) one-way repeated measures ANOVA was performed to assess for statistical significance (p<.05).