Risk to the vertebral artery during C-2 translaminar screw placement: a thin-cut computerized tomography angiogram–based morphometric analysis

Abstract

The goal of this study was to characterize the anatomy relevant to placement of crossing C-2 translaminar screws, including morphometric data, and to evaluate the risk of violating the vertebral artery (VA) during the screw placement. Placement of bilateral crossing C-2 translaminar screws has become an increasingly popular method for dorsal C-2 instrumentation as it is felt to avoid the known risk of VA injury associated with C1-2 transarticular screw fixation and C-1 lateral mass-C-2 pars screw fixation. The source images from 50 CT angiograms of the neck obtained from October to November 2007 were studied. Digital imaging software was used to measure lamina thickness and maximum screw length, perform angulation of screw trajectories in the axial plane, and evaluate the potential for VA injury. In cases where the VA could be injured, the distance between the maximal screw length and artery was measured. Logistic regression was performed to evaluate lamina width, axial angle, and screw length for predicting the potential for VA injury. Mean lamina thickness, axial angle, and maximal screw length were determined for 100 laminae, and a potential for VA injury was noted in 55 laminae. The anatomically defined ideal screw length was longer in laminae with potential for VA injury than in laminae with no apparent risk (35.2 vs 33.6 mm, p = 0.0131). Only increasing optimal screw length was noted to be a statistically significant predictor of potential VA injury (p = 0.0159). The "buffer zone" (the distance between an optimally placed screw and the VA) was 5.6 ± 1.9 mm (mean ± SD, range 1.8-11.4 mm). A screw limited to 28 mm in length appeared to be safe in all laminae studied. Crossing C-2 translaminar screws have been reported to be safe and effective. In addition to morphometric characteristics, the authors have found that screws placed in this trajectory could jeopardize the vertebral arteries in the foramen transversarium or the C1-2 interval. A C-2 translaminar screw limited to 28 mm in length appeared to be safe in all 100 screw trajectories studied in this series.