Biomechanical cyclical loading on cadaveric cervical spines in a corpectomy model

Approved posterior cervical spinal fixation systems have been submitted to the U.S. Food and Drug Administration with results from standardized test protocols from the ASTM. The bench top mechanical studies are designed to minimize biologic and laboratory variability. However, for implant tests such as ASTM F1717 to be more clinically relevant, anatomical and physiologic considerations must be understood. The specific aim of this study was to determine if a human cadaveric cervical spine with a corpectomy and posterior fixation was effective in maintaining stability prior to and following cyclical loading. Six fresh frozen cadaveric human cervical specimens were harvested and prepared. A C5 corpectomy was performed. Posterior cervical instrumentation was implanted from C3 and C7 spanning across the C4-C6 defect. Each specimen followed an established pure moment test protocol to characterize the instrumented spine in flexion extension, lateral bending, and axial torsion at ±2.5 N m and axial compressive loading to 150 N. Subsequently, each specimen was subjected to 10 000 flexion extension cycles. Following the cyclical loading, each specimen was characterized a second time via the same test protocol. Statistical analyses were then performed on the third cycle data between the two pure moment tests. The mean FE bending range of motion (ROM) was 18.0° ±10.7° prior to the 10 000 cyclical bending protocol. Following cyclical loading, the mean ROM measured 22.0° ±19.9°. In axial compression, the mean ROM was 4.1 mm±1.9 mm prior to cycling and 4.2 mm±1.4 mm post-cycling. A statistically significant difference was detected only in the axial torsion mode of loading (p=0.030). Although the ASTM standard provides consistent test methodologies, biomechanical cadaveric testing remains an important step in the validation of all spinal instrumentation. The pure moment biomechanical cadaveric test protocol for the same construct was capable of detecting significant changes pre- and post-flexion extension fatigue cycling only in the axial torsion mode of loading.© 2011 by ASTM International.