Diagnostic Criteria of Basilar Impression

The Relationship Between Dens Height and Alar Ligament Orientation: A Radiologic Study

“…In healthy individuals, the inferior margin of the occipital bone should lie at or below this line. 17 As a comparison, a modified bimastoid line, defined by a line extending from the tips of the left and right mastoid processes in coronal view, was used to define the dens height relative to the occiput (Fig 2A, B). 17 These measures provide a baseline indication of the position of the dens relative to the occipital condyles.…”

“…17 As a comparison, a modified bimastoid line, defined by a line extending from the tips of the left and right mastoid processes in coronal view, was used to define the dens height relative to the occiput (Fig 2A, B). 17 These measures provide a baseline indication of the position of the dens relative to the occipital condyles. 17 The orientation of the alar ligament was measured in the coronal plane according to the following clearly defined protocol.…”

“…17 These measures provide a baseline indication of the position of the dens relative to the occipital condyles. 17 The orientation of the alar ligament was measured in the coronal plane according to the following clearly defined protocol. The relative orientation of the midsubstance of each alar ligament was calculated in 3 ways with respect to 1) a line running along the vertical axis of the dens process; 2) a line positioned orthogonally to the digastric line and passing through the tip of the dens ( Fig 3A); 3) a line orthogonal to a line drawn between the most lateral aspects of the inferior articular processes of the atlas (Fig 4).…”

“…The relative orientation of the midsubstance of each alar ligament was calculated in 3 ways with respect to 1) a line running along the vertical axis of the dens process; 2) a line positioned orthogonally to the digastric line and passing through the tip of the dens ( Fig 3A); 3) a line orthogonal to a line drawn between the most lateral aspects of the inferior articular processes of the atlas (Fig 4). 17 The initial measurement of alar ligament orientation was taken from an image aligned in coronal section along the breadth of the atlas defined by the anterior most point of the left and right transverse foramina. A coronal image at this point, including the left and right alar ligaments, was selected for measurement.…”

“…The digastric line connects the superior points of the digastric grooves located medial to the base of the mastoid process. 17,18 Once the digastric line was defined in coronal section, this line was maintained as a plane, whereas the CT reconstruction was followed anteriorly to define the dens (Fig 3A). A line perpendicular to the digastric line and bisecting the tip of the dens provided a measure of vertical from which the alar ligaments were measured (Fig 3A, B).…”