713318 - Biomechanical Response to Fatigue of Anterior-only Fixation with Integrated Corpectomy Spacer versus Circumferential Fixation for Treatment of Unstable Three-column Cervical Injuries

Introduction:
Anterior-only corpectomies for unstable cervical fractures provide the advantages of reduced surgical time, blood loss, and pain compared to circumferential approaches. Anterior-only approaches are widely used for anterior cervical discectomy and fusion (ACDF). There is concern that posterior fixation is necessary to provide stiffness and stability against the long lever arm created by corpectomy constructs, and reduce risk of construct failures including spacer displacement, screw pullout, and subsidence, especially in long constructs. The present biomechanical study compares the maintenance of stability of an anterior-only construct using an integrated corpectomy spacer for additional anterior stability to that of a circumferential one, following fatigue.

Methods:
Twelve cervical spines (C2–C7) were divided into equal groups (n=6): a (1) circumferential group instrumented with corpectomy spacer (C), anterior plate (P) and lateral mass screws (LMS); and (2) anterior-only group instrumented with integrated corpectomy spacer (iC) with anterior plate. Single-level corpectomy was performed at C4, and all fixation was extended one level proximally (C3) and distally (C5). Range of motion (ROM) was captured using a custom-built six degrees-of-freedom machine in each of three physiological rotation axes: flexion extension (FE), lateral bending (LB), and axial rotation (AR). Relative motion between the construct’s proximal and distal ends was measured in all three axes for both pre- and post-fatigue. Motion was normalized to that of the average intact specimen for each group. Specimens were fatigued by dynamically loading them for 1000 cycles each in FE, LB, and AR.

Results:
While not statistically significant, FE, LB, and AR motion increased more greatly following fatigue in the anterior-only group (Group 2) than in the circumferential group (Group 1). FE motion was increased following fatigue by 1.3±2.2% and 12.3±13.4%, while LB motion increased by 0.4±0.5% and 7.5±6.8%, and AR motion increased by 3.6±7.0 and 10.0±12.2% for Group 1 and Group 2, respectively (Figure 2). Anterior plate screws were pulled out during fatigue for one specimen in Group 2.

Conclusion:
While motion of both the circumferential (Group 1) and anterior-only (Group 2) fixation increased following fatigue, the effect appeared more pronounced for the anterior-only construct. This trend appears to be consistent across all physiological bending modes. The results of this study suggest that even in single-level cervical constructs, posterior-fixation significantly contributes to maintaining construct stability over time as it is loaded. Anterior-only fixation may be at a higher risk for failure and loss of stability as it is loaded over time.