Steel's Rule states that the spinal cord and the dens of the axis are approximately 1 cm in diameter. Relative to the inner diameter of the atlas, this corresponds to 1/3 spinal cord, 1/3 dens, and 1/3 free space. With an average spinal canal diameter at the level of C1 of 3 cm, this results in 1 cm of "safe space"1.
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