The effect of atlas anterior translation and rotation on axis canal size. A computer-assisted anatomic study.

Eighteen adult paired atlas and axis specimens were used in conjunction with computer analysis to determine the average transverse canal area of the axis after different degrees of anterior translation and three types of rotation of the atlas. After each displacement, the remaining transverse canal area of the axis was calculated. The data showed that the mean transverse canal area of the axis was 351.5 +/- 8.8 mm2 for males and 276.9 +/- 8.6 mm2 for females, respectively. After a 3-, 4-, and 5-mm anterior translation of the atlas, the mean transverse canal area of the axis decreased to 300.4 +/- 5.1 mm2 (85.5%) for males and 238.0 +/- 2.1 mm2 (85.9%) for females, 283.9 +/- 5.9 mm2 (80.8%) for males and 228.5 +/- 3.1 mm2 (82.5%) for females, and 260.4 +/- 4.2 mm2 (74.1%) for males and 216.4 +/- 2.8 mm2 (78.2%) for females, respectively. After a 40 degree rotation, 40 degree rotation with 4-mm anterior translation, and 40 degree rotation with 8-mm anterior translation of the atlas, the mean transverse canal area of the axis for both sexes decreased approximately to 58%, 42%, and 23%, respectively. This study confirmed that the size of the transverse canal area of the axis was consistently correlated with the degrees of the anterior translation and rotation of the atlas.