Topographic relationship between root apex of mesially and horizontally impacted mandibular third molar and lingual plate: cross-sectional analysis using CBCT

Abstract

The present study was aimed to determine the topographic relationship between root apex of the mesially and horizontally impacted mandibular third molar and lingual plate of mandible. The original cone beam computed tomography (CBCT) data of 364 teeth from 223 patients were retrospectively collected and analyzed. The topographic relationship between root apex and lingual plate on cross-sectional CBCT images was classified as non-contact (99), contact (145) and perforation (120). The cross-sectional morphology of lingual plate at the level of root apex was defined as parallel (28), undercut (38), slanted (29) and round (4). The distribution of topographic relationship between root apex and lingual plate significantly associated with gender, impaction depth, root number and lingual plate morphology. Moreover, the average bone thickness of lingual cortex and distance between root apex and the outer surface of lingual plate were 1.02 and 1.39 mm, respectively. Furthermore, multivariate regression analyses identified impaction depth and lingual plate morphology as the risk factors for the contact and perforation subtypes between root apex and lingual plate. Collectively, our findings reveal the topographic proximity of root apex of impacted mandibular third molar to the lingual plate, which might be associated with intraoperative and postoperative complications during tooth extraction.

Figure 1. Patients screen and inclusion protocol.

Figure 2. Selection of cross-sectional CBCT images when root apex of impacted mandibular third molar was located at its mostly distal slice and subsequent relevant measurements.

(A) The root apex of impacted mandibular third molar which was localized most distally was initially identified on the axial CBCT image. (B–D) Starting from the (A) image, the CBCT slice was scrolled distally or mesially. When the root tip was enlarged (C) or disappeared (D) on the next slices after the image was adjusted mesially or distally (0.1 mm). the present image (B) on the cross-sectional plane was further selected for various measurements and spatial analyses. (E) Schematic illustration of the measurements. Point A was identified as the most lingual site of root apex. A horizontal line through point A was automatically generated which was intersected with the inner (Point B) and outer surface (Point C) of lingual plate, respectively. The measurements such as thickness of lingual plate (B–C) and distance between root apex and the outer surface of lingual plate (A–C) were performed using the digital ruler provided by the software Simplant.

Figure 3. Topographic relationship between root apex of impacted mandibular third molar and lingual plate.

(A) Type A (non-contact), there is space between the root surface and the inner boarder of lingual plate; (B) Type B (contact), the root apex contacts with lingual plate directly; (C) Type C (perforation), the root apex perforates beyond the outer boarder of lingual plate.

Figure 4. Morphology of lingual plate at the level of root apex identified on cross-sectional CBCT image.

(A) Type U, undercut on the lingual side; (B) Type P, parallel to the buccal plate; (C) Type S, slanted with buccolingual width reduced on the lingual side; (D) Type R, round shape on the lingual side.