The heights of the C1 pedicle, the posterior arch under the groove and the posterior lamina at the screw entry point are the major determinants for the possibility of placing pedicle screws in C1 of a given patient. This study indicates that it is feasible to place a 3.5-mm pedicle screw safely in C1 in most patients, and the lateral mass of C2 is a reliable anatomic landmark that can be easily identified to help the surgeon determine the optimal screw entry portal conveniently during surgery.
The width and height of the atlas lateral mass were larger than that of the C2 pedicle, and there was enough space to insert a 3.5-mm diameter screw in the atlas lateral mass over the C2 nerve. The pullout force of the screw on the lateral mass of the atlas was the same as that of the C2 pedicle screw. It is possible toinsert a 3.5-mm screw in the lateral mass of the atlas. The direction of the screw should be about 20 degrees anterosuperior in the vertical plane and 15 degrees inward in the horizontal plane. The suitable length of the screw should be approximately 22 mm inside the lateral mass.
A large percentages of C2 laminae are of sufficient size to safely accommodate a bicortical 3.5-mm diameter screw. The thickness of the lamina and the height of the spinous process are the 2 limiting factors for safe translaminar screws placement. Using a bicortical technique confirms the position of the screw and thereby helps to decrease the risk of neurologic injury from screw penetration of the inner cortex of the lamina.
C1 pedicle screws are stiffer than C1 lateral mass screws. Unicortical C1 pedicle screw provided the same pullout resistance and three-dimensional stability as bicortical C1 lateral mass fixation. Although lateral mass screw placement into C1 requires bicortical purchase, pedicle screw insertion into the atlas only requires unicortical fixation.
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