The in vitro effects of instrumentation on multilevel cervical strut-graft mechanics

Study Design. Biomechanical study using a programmable testing apparatus that replicated physiologic flexion-extension cervical spine motion, and loading mechanics. Objectives. To determine the influence of anterior, posterior, or combined plating on multilevel cervical strut-graft mechanics in vitro. Summary of Background Data. The addition of instrumentation does not prevent construct failure in multilevel ( more than two levels) cervical corpectomy. Methods. Six fresh human cadaveric cervical spines (C2-T1) were tested in six sequential conditions that included harvested (H), C4-6 corpectomy, strut grafted, strut grafted with an anterior cervical plate (SGAP), strut grafted with posterior plates (SGPP), and strut grafted with combined anterior and posterior plates (SGAPP). A customized force-sensing strut graft (FSSG) was used to measure axial compression-tension, flexion-extension and lateral bending moments, and axial torsion. Parameters of stiffness, segmental vertebral motion, and strut-graft loads were compared, to determine differences among the spine conditions. Results. Flexion of the strut-grafted spine loaded the FSSG, and extension motion unloaded the FSSG. With the anterior plate, flexion of the SGAP spine significantly unloaded the FSSG; extension loaded the FSSG more than flexion of the unplated spine (P = 0.03). The opposite occurred with the posterior plates (SGPP), where flexion of the spine significantly loaded the FSSG (more than the strut grafted spine) and extension unloaded the FSSG (P < 0.03). The combined construct (SGAPP) counteracted the tension band effect of the individual plates and demonstrated significantly less overall FSSG load change than either plate alone (P = 0.03). Conclusions. Multilevel cervical instrumentation effectively increases stiffness after corpectomy. However, anterior or posterior plating alone excessively loads the graft with small degrees of motion, which may promote pistoning and failure of multilevel constructs.