3D morphometry of the transverse and alar ligaments in the occipito-atlanto-axial complex: an in vitro analysis.

This study verifies the three-dimensional anatomical features of the transverse and alar ligaments with reference to the axis using a direct in vitro approach. In 20 fresh spine specimens, metal markers were inserted on the cranium, atlas, and axis. After registration of the intact specimen, the bony segments were separated, and markers and anatomical landmarks were digitized. The length and the orientation of the ligaments with reference to the axis were derived from the relative position data. The transverse ligaments of the atlas have a mean estimated length of 21 mm and an absolute angle (i.e., independent of any reference frame) of 119 degrees +/- 17 degrees . The alar ligaments have a mean length of 9 +/- 2.5 mm, and the mean absolute angle between the ligaments is 117 degrees +/- 31 degrees . The plane of the alar ligaments shows a mean backward inclination of -10 degrees +/- 52 degrees . This plane has a mean inclination of 6 degrees +/- 4 degrees with reference to the sagittal plane indicating left-right symmetries. The transverse ligament arches around the dens and demonstrating its function as a stabilizer for the dens as well as guidance for axial rotation movements. A posterior inclination of the alar ligaments may induce a coupled extension in combination with a lateral bending during axial rotation. These detailed aspects of motion steering may be important to consider when attempting to reduce or restore movement.