Steels rule of thirds
At the level of the first cervical vertebra (C1), the anteroposterior diameter is divided into thirds, allowing one-third for the dens, one-third for the spinal cord, and one-third for free space. Because there is significant free space at this level, small degrees of C1 to C2 subluxation (3–7 mm) usually do not compromise the cord. However, when the anterior atlantodens interval (measured from the posterior part of the anterior arch of C1 to the anterior aspect of odontoid) becomes >10 to 12 mm, all the atlantoaxial ligamentous complex has usually been destroyed, and the space available for the spinal cord is usually compromised. Likewise, when the posterior atlantodens interval (measured from posterior aspect of the odontoid to the anterior aspect of the posterior arch of C1) is <14 mm, the spinal cord is usually compressed.