Measurement of plane, long axis of the tooth, alveolar process and basal bone

E-3D Medical Software V16.19 (Central South University, Changsha, China) was utilized to load the CBCT imaging data. The criteria to record the measurement of plane, long axis of the tooth, alveolar process, basal bone and angle measurement method of calculation were referenced from previous studies [9, 20–22].

The measurement plane for the tooth position was determined by adjusting the coordinate axis. The details of the adjustments were as follows. (1) The tooth horizontal plane was adjusted to the neck of the tested tooth; (2) the cross-sectional plane passed through the midpoint of the tested tooth mesiodistally; (3) the improved cross-sectional plane aligned along the long axis of the tooth until the plane showed complete root in the single-rooted molar, or the mesial and distal root bifurcations displayed complete mesial root (Fig. 1).

Determination of the measurement plane. a The horizontal plane was adjusted to the neck of the lower first molar and the cross-sectional plane passed through the midpoint of the tested tooth mesiodistally. b The improved cross-sectional plane aligned along the long axis of the tooth. c The selected measurement plane paralleled the improved cross-sectional plane when the plane showed complete root in the single-rooted molar or the mesial and distal root bifurcations displayed complete mesial root

The long axis of the tooth was determined by connecting the apex of the mesial root (point A) and the midpoint of a line drawn from the abrupt point of the buccal crown surface (point B) to the lingual counterpart (point C) (Fig. 2a).

Data measurement. a The long axis of the tooth was determined by connecting the apex of the mesial root (point A) and the midpoint of a line drawn from the abrupt point of the buccal counterpart (point B) to the lingual counterpart (point C). b The long axis of the alveolar process (line 2) was marked by bisecting the buccal line of the alveolar process (line Le) and lingual line of the alveolar process (line Lf). The upper internal angle a was formed by L1 and L2. The width GH was between the buccal and lingual alveolar plates at the lowest point of the alveolar bone and perpendicular to L2. KL was between the midpoint of EF and GH. c The long axis of the basal bone (line 3) was marked by bisecting the buccal basal bone surface (line Lg) and lingual basal bone surface (line Lh). The upper internal angle b was formed by L2 and L3. The width IJ was between the buccal and lingual basal plates at the highest point of the basal bone and perpendicular to L3.Distance EF was between the buccal and lingual alveolar crest

On the selected measurement plane, both the buccal line (line Le) and the lingual line (line Lf) were marked by a line best-fit to the buccal alveolar surface and the lingual alveolar surface, respectively. The alveolar line (line L2) was marked by bisecting the buccal and lingual lines, which indicated the angulation of the alveolar process on the selected measurement plane (Fig. 2b).

On the selected measurement plane, both the buccal line (Lg) and the lingual line (Lh) were marked by a line best-fit to the buccal basal bone surface and lingual basal bone surface. The basal line (line L3) was marked by bisecting the Lg and Lh. Line L3 indicated the angulation of the basal bone on the selected measurement plane (Fig. 2c).