Pull-out resistance of Facet versus Laminar C2 screws – an experimental comparative biomechanical investigation

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Background: Trans-pedicular or trans-isthmic screws for C2 instrumentation represent the gold standard; however, the anatomy is not always compatible (hypoplastic pedicles, procidentia of the vertebral artery). Alternatives have been proposed, using laminar screws or, recently, bicortical facet screws.

Research question: To date, no biomechanical studies have compared facet versus laminar screws of C2. We propose a morphometric analysis and a pull-out strengths comparison of both techniques. 

Study design: In vitro randomized comparative biomechanical study.

Methods: Thirty-two human cadaveric C2 vertebrae beneficiated from a CT-scan imaging and a Dual X-Ray Absorptiometry before receiving both techniques, randomized according to side and sequence. Screws positioning was verified using 2D X-rays. Sixty-four mechanical tests were performed using pure tensile loading along the axis of the screws until pull-out. The 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. Regarding the 64 tests, the mean pull-out strength of laminar screws (707 ± 467 N) was significantly higher than that of facet screws (390 ± 230 N) (p = 0.0004). Bone mineral density was moderately positively correlated with pull-out strength (r = 0.42 for facet screws and r = 0.3 for laminar screws). Both techniques were mechanically equivalent when an intra-laminar cortical grip was not achievable for laminar screws. 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 < 0.0001).

Discussion: Our results strongly suggest that bicortical facet screws of C2 offer less mechanical resistance (almost half lower) than laminar screws, except if the intra-laminar grip is not achievable. Alternative loading biomechanical scenarios may be investigated to further confirm these results.