Abstract
There are several trajectories described for the insertion of screws in the lateral facets of the cervical spine (Magerl, An, Anderson, based on spinous processes,..), they all rely on milimeters and precise angles; but anatomical variety (osteophytes, mass anatomy, spinous process deformities...) might render them not fully reproducible. It is proposed to consider the lateral mass as an slanted cube rotated to present a crest stright posterior, and define a trajectory as close to the 3D diagonal as possible from inferomedial corner at dorsal-medial facet to the anterosuperior cornar of the antero-lateral facet of such a cube.
Hypothesis
That trajectory is free of biass and represents the longest posible one in a cubic body, so obtaining the strongest anchor possible.
Material and Methods
10 cervical spines (dry bone) from thew Anatomy Department of Santiago de Compostela University were gathered and 4 different trajectories were developed in each lateral mass from C3 to C6 (both sides) so getting 320 different trajectories. A Kirschner Wire 0.9 mm in diameter with an “ad hoc” guide were used to avoid destruction of the specimen. Spines were fixed to a table by a sargent clamp with the dorsal surface upwards, resembling an OR situation. The 4 trajectories tested were after the techniques originally published by Magerl, An and Anderson, and then, the new one (N), based on the spatial isolation of the lateral mass for the 3D diagonal. Variables: a) the length of each trajectory, b) violations of facet joint, c) potential injuries to vertebral artery (defined by a 3 mm wire passed through the tranverse processes) and d) potential injuries to the root (defined by the same kind of wire set exiting through the foramen). Then the results in each variable was compared among all trajectories to define the longest one and the safest one. Statistics: Linear model of mixed effects, REML adjusted for “length” and adjusted to the major probability (Laplace) for all lesions.
Results
a) Length: N longer than any and against each one. 16 mm vs <15 mm (p < 0.0001) at C6 the longest one with any trajectory (p < 0.001). b) facet violations: none. c) potential vertebral artery injury: 8/320, all right sided. N technique: 0; An technique:0. d) potential root injury: N the safest nad against each one (p < 0.02), followed by An technique right side had a higher risk p = 0.001.
Conclusions
A new technique (N) is presented that allows for a longer screw insertion. Besides being longer, N technique is safer than any other relating to potential root injuries and also safer than Magerl and Anderson for potential vertebral artery injury. Right side seems more prone to both potential vertebral artery and root injury