Working area, safety zones, and angles of approach for posterior C-1 lateral mass screw placement: a quantitative anatomical and morphometric evaluation.

Object: The authors measured relevant quantitative anatomical parameters to define safety zones for the placement of C-1 posterior screws.
Methods: Nineteen linear, two angular, and four surface parameters of 20 dried atlantal specimens were evaluated. The Optotrak 3020 system was used to define the working area. Ideal angles for screw positioning were measured using digital radiographs and a free image-processing program. Six silicone-injected cadaveric heads were dissected bilaterally to study related neurovascular anatomy. The depth (range 5.2-9.4 mm, mean 7.2 +/- 1.1 mm) and width (range 5.2-8.1 mm, mean 6.5 +/- 0.9 mm) of the transverse foramen varied considerably among specimens. The mean posterior working area was 43.3 mm2. All specimens accommodated 3.5-mm-diameter screws, and 93% accepted 4-mm-diameter screws. In 10 specimens (50%), partial removal of the posterior arch was necessary to accommodate a 4-mm screw. The mean maximum angle of medialization was 16.7 +/- 1.3 degrees; the mean maximum superior angulation was 21.7 +/- 4.7 degrees.
Conclusions: The anatomical configuration of the atlas and vertebral artery (VA) varied considerably among the cadaveric specimens. The heights of the C-1 pedicle, posterior arch, and posterior lamina determine the posterior working area available for screw placement. The inferior insertion of the posterior arch may have to be drilled to increase this working area, but doing so risks injury to the VA. A dense venous plexus with multiple anastomoses may cover the screw entry site, potentially obscuring the operative view and increasing the risk of hemorrhage.