Stress-guided determination of screw trajectory for C1 lateral mass fixation: A finite element approach

This study aimed to determine the proper positioning for a C1 (atlas) lateral mass screw by a stress analysis on a 3D model of the C1 vertebra based on computerized tomography scans.

A 3D model of the C1 vertebra was constructed using computed tomography images from 80 patients who presented to our hospital with neck pain but no cervical pathology. Using a geometrical approach for screw placement in the C1 lateral mass, the screw trajectory was traced, and a suitable entry point, screw angulations, and screw length were calculated. These values were determined through finite element analysis based on stress distribution resulting from screw pull-out forces.

The developed geometrical method can be utilized to determine the screw path by using radiologic images for patient specific surgical planning. Additionally, the average distance from the midline to the screw entry site was 20.27 ± 1.48 mm, an acceptable medial angle of 20.75 ± 3.84°. There is only minor effect of mean superior angle on the stress distribution. Based on these entry point and angulation values, the favorable screw length was determined as 21.16 ± 2.37 mm.

There is no consensus on the most optimum screw entry and angle values for C1 screw stabilization in terms of structural integrity. This study offers strong evidence supporting the applicability of the C1 lateral mass screw technique for cervical instability by providing favorable entry site and angulation values.