Biomechanical Comparison of Facet Versus Laminar C2 Screws

BACKGROUND: Transpedicular or transisthmic screws for C2 instrumentation represent the gold standard; however, the anatomy is not always compatible (hypoplastic pedicles, procidentia of the vertebral artery). Laminar screws (LS) have been proposed as a rescue technique and recently, bicortical facet screws (FS). To date, the biomechanical property of FS remains unknown. OBJECTIVE: To compare the pull-out resistance of bicortical facet (FS) vs laminar (LS) C2 screws. METHODS: Thirty-two human cadaveric C2 vertebrae were screened by CT scan imaging and dual x-ray absorptiometry before receiving both techniques and were randomized according to side and sequence (FS or LS first). Screw positioningwas validated using 2-dimensional x-rays. Sixty-four mechanical tests were performed using pure tensile loading along the axis of the screws until pull-out. Mean pull-out strengths were compared using paired tests, multivariate and survival analysis (Kaplan-Meier curves). RESULTS: The morphometric data were consistent with previous studies. Over 64 tests, the mean pull-out strength of LS (707 +/- 467 N) was significantly higher than that of FS (390 +/- 230 N) (P =.0004). Bone mineral density was weakly correlated with pull-out strength (r = 0.42 for FS and r = 0.3 for LS). Both techniques were mechanically equivalent for vertebrae in which intralaminar cortical grip was not achievable for LS. The mean pull-out strength for LS with laminar cortical grip (1071 +/- 395 N) was significantly higher than that of LS without (423 +/- 291 N) (P <.0001). CONCLUSION: Our results suggest that bicortical FS of C2 offer less mechanical resistance than LS.