Since TJ-OGS induces the position change and bone remodeling in the skeletal structures and produces the metallic images of the OB, SP-S, and FR, the accuracy and reliability of cephalometric landmark identification in serial lateral cephalograms are important for assessment of treatment outcomes.16 As total landmarks exhibited a good mean error value and a high degree of accuracy (1.17 mm and 74.2%, respectively, Table 1) without significant difference among the four time-points (P > 0.05, Table 2), accuracy of the AI-assisted digitization was not significantly affected by the presence of OB, SP-S, FR, and bone remodeling change during orthodontic treatment and TJ-OGS. Regardless of the degree of accuracy of each landmark (Table 1), none of the five cranial base landmarks exhibited a significant difference in the mean errors among the four time-points (T0, T1, T2 and T3) and between the two time-points [(T0, T1) vs. (T2, T3)] (Table 2). Accuracy of the cranial base landmarks can be regarded as baseline for comparison of serial lateral cephalograms because the positions of these cranial base landmarks are not affected by TJ-OGS.
Three error patterns were found in the maxillary skeletal landmarks. First, the mean errors of ANS were different among the four time-points (T0, 1.07 mm; T1, 1.22 mm; T2, 1.78 mm; T3, 1.49 mm, P < 0.01; Table 2) and presented an increased error value after TJ-OGS than before TJ-OGS [(T0, T1) vs. (T2, T3), P < 0.01; Table 2], which suggested that the metal image of the SP-S adjacent to ANS as well as surgical shape modification of ANS 17,18 (Fig. 1) could affect the accuracy of AI-assisted landmark detection. Second, although the error of A point was not significantly different among the four time-points (T0, 1.27 mm; T1, 1.28 mm, T2, 1.50 mm, T3, 1.59 mm, Table 2), it presented an increase in the mean error value after TJ-OGS than before TJ-OGS [(T0, T1) vs. (T2, T3), P < 0.05; Table 2]. This occurred because A point might be less affected by the metal image of the SP-S installed at the maxilla and have a lower chance for surgical shape modification, compared to ANS (Fig. 1). Third, in case of posterior impaction and/or anteroposterior movement of the maxilla, the position of PNS had to be changed. However, for PNS, no significant difference was found either among the four time-points (T0, 1.16 mm; T1, 1.14 mm, T2, 1.29 mm, T3, 1.17 mm; P > 0.05, Table 2) or between the two time-points [(T0, T1) vs. (T2, T3), P > 0.05; Table 2]. This might be due to (1) absence of the metal image of the SP-S within the ROI of PNS and (2) the end point of the hard palate can still be easily defined.
There are three explanations of the errors in the mandibular skeletal landmarks. First, since there were no metal images within the ROI of Articulare and Menton, their mean errors were not significantly different among the four time-points and between the two time-points (all P > 0.05, Table 2). Second, the mean error of Pogonion was not significantly different among the four time-points and between the two time-points (P > 0.05; Table 2), which suggests that the metal image of the SP-S adjacent to Pognion (Fig. 1) might not affect the accuracy of AI-assisted landmark detection. Third, although the mean errors of B point did not differ among the four time-points (T0, 1.00 mm; T1, 1.01 mm; T2, 1.29 mm; T3, 1.31 mm, P > 0.05; Table 2), comparison of the two time-points revealed an increase in error after TJ-OGS than before TJ-OGS [(T0, T1) vs. (T2, T3), P < 0.01; Table 2]. These findings suggest that the metal image of the SP-S adjacent to the B point (Fig. 1) might affect the accuracy of AI-assisted landmark detection.
There are two sources of errors in the dental landmarks. First, regardless of the degree of accuracy in the dental landmarks (Table 1), Mx1C, Md1C, Mx1R, Md1R, Mx6R, and Md6R did not exhibit significant difference in the mean errors among the four time-points and between the two time-points (all P > 0.05; Table 2). Second, the mean errors of Mx6D and Md6D were significantly different among the four time-points (Mx6D: T0, 1.66 mm; T1, 1.63 mm, T2, 1.20 mm, T3, 1.23 mm; Md6D, T0, 2.15 mm; T1, 1.71 mm, T2, 1.51 mm, T3, 1.33 mm; all P < 0.01, Table 2) and presented decreased mean error values after TJ-OGS than before TJ-OGS [(T0, T1) vs. (T2, T3), all P < 0.01; Table 2]. Possible reasons might be as follows: (1) Horizontal and vertical overlapping of the right and left maxillary and mandibular first molars made it difficult to accurately locate the Mx6D and Mn6D at T0 lateral cephalogram; and (2) Orthodontic treatment and TJ-OGS improved the alignment of the maxillary and mandibular dentition and corrected the cant, shift and yaw of the maxilla and mandible, reducing the double images of the maxillary and mandibular first molars.
No significant difference was found in the mean errors in the landmarks adjacent to the genioplasty area including B point, Pogonion, Menton, Md1C, and Md1R (all P > 0.05, Table 3). Possible reasons might be as follows: (1) Menton and Md1C were located relatively far from the SP-S installed at the symphysis and their shapes were not affected by orthognathic surgery; (2) Since Pogonion and B point are the most forward and deepest points on the anterior surface of the symphysis, respectively, they can be easily identified despite the presence of the metal image of the SP-S; and (3) Although Md1R had a fair mean error value and a low degree of accuracy (1.57 mm and 58.2%, respectively), these patterns were not aggravated at T2 and T3 despite the presence of the metal image of the SP-S.
Although this study might provide meaningful results about the accuracy of AI-assisted landmark identification of the hard tissue landmarks in serial lateral cephalograms, further studies are needed to investigate the accuracy of soft tissue landmark identification in serial lateral cephalograms.