What is Steels rule of thirds

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Steel noted that the area of the spinal canal at the C1 level in a normal person could be divided into equal thirds with one third occupied by the odontoid process, one third by the spinal cord, and one third as empty space (Fig. 37-2). The empty space serves as a safe zone into which displacement can occur without neurologic impingement. In the presence of atlantoaxial instability, the safe zone may decrease resulting in spinal cord compression.

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Figure 37-1. Method of measuring atlantooccipital instability according to Wiesel and Rothman. These lines are drawn on flexion and extension lateral radiographs, and translation should be no more than 1 mm. Atlantal line joins points 1 and 2. Line drawn perpendicular to atlantal line at posterior margin of anterior arch of atlas. Point 3 is basion. Distance from point 3 to perpendicular line (represented by X) is measured in flexion and extension. Difference represents anteroposterior translation. (From Gabriel KR, Mason DE, Carango P. Occipitoatlantal translation in Down's syndrome. Spine 1990;15:997.)

Figure 37-1. Method of measuring atlantooccipital instability according to Wiesel and Rothman. These lines are drawn on flexion and extension lateral radiographs, and translation should be no more than 1 mm. Atlantal line joins points 1 and 2. Line drawn perpendicular to atlantal line at posterior margin of anterior arch of atlas. Point 3 is basion. Distance from point 3 to perpendicular line (represented by X) is measured in flexion and extension. Difference represents anteroposterior translation. (From Gabriel KR, Mason DE, Carango P. Occipitoatlantal translation in Down's syndrome. Spine 1990;15:997.)

Figure 37-2. Steel's rule. (From Moskovich R. Atlanto-axial instability. Spine State Art Rev 1994;8:533, with permission.)

Figure 37-2. Steel's rule. (From Moskovich R. Atlanto-axial instability. Spine State Art Rev 1994;8:533, with permission.)

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