| Summary: | Background: Various lateral mass screw fixation methods have been described in the literature with various levels of safety in relation to the anterior neurovascular structures. This study was designed to radiologically determine the minimum lateral angulations of the screw to avoid penetration of the vertebral artery canalusing three of the most common techniques: Roy-Camille, An, and Magerl. s and Methods: Sixty normal cervical CT scans were reviewed. A minimum lateral angulation of a 3.5 mm lateral mass screw which was required to avoid penetration of the vertebral artery canal at each level of vertebra were measured. Results: The mean lateral angulations of the lateral mass screws (with 95 confidence interval) to avoid vertebral artery canal penetration, in relation to the starting point at the midpoint (Roy-Camille), 1 mm medial (An), and 2 mm medial (Magerl) to the midpoint of lateral mass were 6.8 degrees (range, 6.3-7.4 degrees), 10.3 degrees (range, 9.8-10.8 degrees), and 14.1 degrees (range, 13.6-14.6 degrees) at C3 vertebrae; 6.8 degrees (range, 6.2-7.5 degrees), 10.7 degrees (range, 10.0-11.5 degrees), and 14.1 degrees (range, 13.4-14.8 degrees) at C4 vertebrae; 6.6 degrees (range, 6.0-7.2 degrees), 10.1 degrees (range, 9.3-10.8 degrees), and 13.5 degrees (range, 12.8-14.3 degrees) at C5 vertebrae and 7.6 degrees (range, 6.9-8.3 degrees), 10.9 degrees (range, 10.3-11.6 degrees), and 14.3 degrees (range, 13.7-15.0 degrees) at C6 vertebrae. The recommended lateral angulations for Roy-Camille, Magerl, and An are 10 degrees, 25 degrees, and 30 degrees, respectively. Statistically, there is a higher risk of vertebral foramen violation with the Roy-Camille technique at C3, C4 and C6 levels, P < 0.05. Conclusions: Magerl and An techniques have a wide margin of safety. Caution should be practised with Roy-Camille's technique at C3, C4, and C6 levels to avoid vertebral vessels injury in Asian population.
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