A study on a method to identify the border between mandibular corpus and ramus on lateral cephalometric radiograph.

The mandible has two major parts, the corpus and the ramus. These two parts must be considered separately because each has its own functional counterpart with the maxilla and the posterior cranial base. Since the mandible is a single bone, the border of the corpus and the ramus can not be visualized on the conventional two-dimensional lateral cephalometric radiograph. The purpose of this study is to propose a reliable method to distinguish the border between the mandibular corpus and the ramus on the lateral cephalometric radiograph. In this study, 64 dried skulls in different developmental stages were examined. Metallic markers were placed at the diverging point (point T) on both sides of the dried skulls. From the occlusal view of the mandible, point T was considered to be the lingual border of the corpus and the ramus. Then, the lateral cephalometric radiographs of the dried skulls were taken. On each lateral cephalometric radiograph, a vertical line L was drawn from point Ra (the intersection of the occlusal plane and the anterior border of the ramus) to the mandibular plane. Thus, the distance from the midpoint of point T on each side to the vertical line L was measured. As a result, over 80% of the midpoints T were found on the vertical line L. Therefore, the vertical line L from point Ra to the mandibular plane can be considered to play the role of a reasonable reference line for distinguishing between the corpus and the ramus on the lateral cephalometric radiograph.