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Optimal Entry Point and Trajectory for Anterior C1 Lateral Mass Screw.

Research paper by Yong Y Hu, Wei-Xin WX Dong, William Ryan WR Spiker, Zhen-Shan ZS Yuan, Xiao-Yang XY Sun, Jiao J Zhang, Hui H Xie, Todd J TJ Albert

Indexed on: 23 Apr '15Published on: 23 Apr '15Published in: Journal of spinal disorders & techniques



Abstract

A radiographic analysis of the anatomy of the C1 lateral mass using CT scans and Mimics software.To define the anatomy of the C1 lateral mass and make recommendations for optimal entry point and trajectory for anterior C1 lateral mass screws.Although various posterior insertion angles and entry points for screw insertion have been proposed for posterior C1 lateral mass screws, no large series have been performed to assess the ideal entry point and optimal trajectory for anterior C1 lateral mass screw placement.The C1 lateral mass was evaluated using CT scans and a 3D imaging application (Mimics software). Measuring the space available for the anterior C1 lateral mass screw (SAS) at different camber angles from 0° to 30° (5° intervals) was performed to identify the ideal camber angle of insertion. Measuring the range of sagittal angles was performed to calculate the ideal sagittal angle. Other measurements involving the height of the C1 lateral mass were also made.The optimal screw entry point was found to be located on the anterior surface of the atlas 12.88 mm (±1.10 mm) lateral to the center of the anterior tubercle. This optimal entry point was found to be 6.81 mm (±0.59 mm) superior to the anterior edge of the atlas inferior articulating process. The mean ideal camber angle was 20.92° laterally and the mean ideal sagittal angle was 5.80° downward.These measurements define the optimal entry point and trajectory for anterior C1 lateral mass screws and facilitate anterior C1 lateral mass screw placement. A thorough understanding of the local anatomy may decrease the risk of injury to the spinal cord, vertebral artery, and internal carotid artery. Delineating the anatomy in each case with preoperative 3-dimensional CT evaluation is recommended.