Three-dimensional Frankfort horizontal plane for 3D cephalometry: a comparative assessment of conventional versus novel landmarks and horizontal planes

Three-dimensional Frankfort horizontal plane for 3D cephalometry: a comparative assessment of... Summary Background To assess the reproducibility of landmarks in three dimensions that determine the Frankfort horizontal plane (FH) as well as two new landmarks, and to evaluate the angular differences of newly introduced planes to the FH. Methods Three-dimensional (3D) surface models were created from CBCT scans of 26 dry human skulls. Porion (Po), orbitale (Or), internal acoustic foramen (IAF), and zygomatico-maxillary suture (ZyMS) were indicated in the software by three observers twice with a 4-week interval. Angles between two FHs (FH 1: Or-R, Or-L, mid-Po; FH 2: Po-R, Po-L, mid-Or) and between FHs and new planes (Plane 1–6) were measured. Coordinates were exported to a spreadsheet. A statistical analysis was performed to define the landmark reproducibility and 3D angles. Results Intra- and inter-observer landmark reproducibility showed mean difference more than 1 mm for x-coordinates of all landmarks except IAF. IAF showed significantly better reproducibility than other landmarks (P < 0.0018). The mean angular difference between FH 1 and FH 2 was 0.7 degrees. Plane 3, connecting Or-R, Or-L and mid-IAF, and Plane 4, connecting Po-R, Po-L and mid-ZyMS, both showed an angular difference of less than 1 degree when compared to FHs. Conclusions This study revealed poor reproducibility of the traditional FH landmarks on the x-axis and good reproducibility of a new landmark tested to replace Po, the IAF. Yet, Or showed superior results compared to ZyMS. The potential of using new horizontal planes was demonstrated. Future studies should focus on identification of a valid alternative for Or and ZyMS and on clinical implementation of the findings. Introduction A cephalometric analysis is an essential part of orthodontic treatment planning. The technique has been used for several decades after its introduction by Hofrath in Germany and Broadbent in the USA (1, 2). It is traditionally performed on a lateral and a posteroanterior cephalogram. One of the important elements in performing a cephalometric analysis is the horizontal reference plane. The Frankfort horizontal plane (FH; also called the auriculo-orbital plane) was established at the World Congress of Anthropology, in Frankfort, Germany in 1882. First introduced by Von Ihering, in 1872, this plane used to pass through the centre of the external auditory meatus to the lowest point of the inferior margin of each orbit. The Frankfort agreement then modified this definition, so that the plane would pass through the upper borders of each ear canal or external auditory meatus (Porion/Po), and through the inferior border of the orbital rim (Orbitale/Or). The Frankfort plane was employed for orientation of the patient and was chosen as the best anatomic indicator of the true horizontal line. It is also closely related to the natural head position (NHP) (3, 4). Although the reliability and validity of the FH for cephalometric analysis was questioned by several authors (5–9), it is still widely accepted in contemporary cephalometric use (4). Recently, three-dimensional (3D) imaging modalities, especially cone-beam computed tomography (CBCT), have become essential for diagnosis and treatment planning in the oral and maxillofacial region (10). In orthodontics, CBCT images allow 3D visualization of the craniofacial structures without superimposition (11–14). With its relatively lower radiation doses than multi-slice computed tomography (MSCT) (15), this modality proves to be useful for various orthodontic indications requiring advanced imaging and diagnosis such as canine impaction, root resorption, sleep disorders, and orthognathic surgery (10–14). Cephalometric analysis, traditionally done on two-dimensional (2D) radiographs (16), is also moving towards the direction of 3D imaging (17). In 3D cephalometry, cephalometric landmarks are identified in three orthogonal planes or on 3D models with the aid of a 3D image viewing software (18, 19). The original 2D planes or lines were transformed and used in 3D cephalometric analyses. Some publications have shown the reproducibility and accuracy of cephalometric landmark identification in 3D, including Po and Or which are used to form the Frankfort plane (17–27). In a few studies, it was found that reproducibility and reliability of these two landmarks in 3D was not optimal (25–27) with only one study conducted on a 3D surface model (23). Only one publication evaluated the accuracy and reliability of the FH using 3D magnetic resonance imaging (MRI) (28). Novel landmarks and horizontal planes have not been in the focus of interest while assessing the use of 3D cephalometry. Nevertheless, these might have some impact for diagnostics and treatment planning in orthodontics. Therefore, the aim of this study was twofold: (1) to assess the reproducibility of existing landmarks in three dimensions that determine the FH including two new landmarks and (2) to compare the angular differences of newly introduced planes to the FH. Materials and methods Sample Twenty-six dry human skulls with upper and lower first incisors and first molars present were selected from the Department of Anatomy of the University of Hasselt, Belgium. No age and gender information of the skulls was available. The dentition ranged from a mixed dentition to a permanent dentition. Mandibles were fixed to the skulls using surgical tapes. The tape was wrapped around the skull, starting from the temporal area of one side, crossing the lower border of the mandible to the temporal area of the other side. The occlusion was fixed at maximum intercuspation. The study protocol was approved by the local Medical Ethics Committee (reference number: ML6960, BE322201010078). All procedures were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki Declaration of 1975, as revised in 2008. Imaging modalities CBCT scans of the samples were taken using the largest field of view (FOV; diameter 170 × height 120 mm) of the 3D Accuitomo® 170 (J. Morita, Kyoto, Japan), using the High-Fidelity mode: 90 kVp, 154 mAs, voxel size 0.25 mm. A 1.7-mm-thick copper filter was attached in front of the X-ray source of the CBCT device during image acquisition to simulate soft tissue attenuation. CBCT data were exported to DICOM and then imported to Maxilim® software version 2.3.0.3 (Medicim NV, Sint-Niklaas, Belgium). Three-dimensional surface models were created for all samples based on previous published methods (29, 30). The full CBCT volume was included with 0.5 mm sub-sampling of voxels. The threshold was set at 276 to segment the hard tissues for the 3D models (29, 30). FH analysis A preset for the FH evaluation was created in the Maxilim® software. The analysis was composed of four operator-indicated landmarks (eight landmarks when counting both sides) and four mid-landmarks calculated by the software (Table 1). In 3D, a plane can be made from three landmarks/points, thus two possibilities of FH were created: FH 1 and FH 2 (Figure 1; Table 1). FH 1 was drawn using Or left (Or-L) and right (Or-R) and a landmark in the middle of the two Porions (mid-Po). On the other hand, FH 2 was created using Porion left (Po-L) and right (Po-R) and a landmark in the middle of the two Orbitales (mid-Or). Table 1. Definition of landmarks and planes. Name Definition Landmark  Orbitale right and left (Or-R, Or-L) The lowest point on the inferior margin of the orbit  Porion right and left (Po-R, Po-L) The most superior midpoint of the external auditory meatus (anatomic Po)  Internal acoustic foramen right and left (IAF-R, IAF-L) The most lateral point of the internal auditory meatus at the skull base  Zygomatico-maxillary suture right and left (ZyMS-R, ZyMS-L) The zygomatico-maxillary suture line crossing at the lower orbital rim. The point is located on the inferior margin of the orbit.  Mid-Or A point in the middle between right and left Or indicated by the software  Mid-Po A point in the middle between right and left Po indicated by the software  Mid-IAF A point in the middle between right and left IAF indicated by the software  Mid-ZyMS A point in the middle between right and left ZyMS indicated by the software Plane  Frankfort horizontal plane 1 (FH 1) FH by connecting mid-Po, Or-R, and Or-L  Frankfort horizontal plane 2 (FH 2) FH by connecting mid-Or, Po-R, and Po-L  Plane 1 A plane connecting mid-Or, IAF-R, and IAF-L  Plane 2 A plane connecting mid-Po, ZyMS-R and ZyMS-L  Plane 3 A plane connecting Or-R, Or-L, and mid-IAF  Plane 4 A plane connecting Po-R, Po-L, and mid-ZyMS  Plane 5 A plane connecting ZyMS-R, ZyMS-L, and mid-IAF  Plane 6 A plane connecting IAF-R, IAF-L, and mid-ZyMS Name Definition Landmark  Orbitale right and left (Or-R, Or-L) The lowest point on the inferior margin of the orbit  Porion right and left (Po-R, Po-L) The most superior midpoint of the external auditory meatus (anatomic Po)  Internal acoustic foramen right and left (IAF-R, IAF-L) The most lateral point of the internal auditory meatus at the skull base  Zygomatico-maxillary suture right and left (ZyMS-R, ZyMS-L) The zygomatico-maxillary suture line crossing at the lower orbital rim. The point is located on the inferior margin of the orbit.  Mid-Or A point in the middle between right and left Or indicated by the software  Mid-Po A point in the middle between right and left Po indicated by the software  Mid-IAF A point in the middle between right and left IAF indicated by the software  Mid-ZyMS A point in the middle between right and left ZyMS indicated by the software Plane  Frankfort horizontal plane 1 (FH 1) FH by connecting mid-Po, Or-R, and Or-L  Frankfort horizontal plane 2 (FH 2) FH by connecting mid-Or, Po-R, and Po-L  Plane 1 A plane connecting mid-Or, IAF-R, and IAF-L  Plane 2 A plane connecting mid-Po, ZyMS-R and ZyMS-L  Plane 3 A plane connecting Or-R, Or-L, and mid-IAF  Plane 4 A plane connecting Po-R, Po-L, and mid-ZyMS  Plane 5 A plane connecting ZyMS-R, ZyMS-L, and mid-IAF  Plane 6 A plane connecting IAF-R, IAF-L, and mid-ZyMS View Large Table 1. Definition of landmarks and planes. Name Definition Landmark  Orbitale right and left (Or-R, Or-L) The lowest point on the inferior margin of the orbit  Porion right and left (Po-R, Po-L) The most superior midpoint of the external auditory meatus (anatomic Po)  Internal acoustic foramen right and left (IAF-R, IAF-L) The most lateral point of the internal auditory meatus at the skull base  Zygomatico-maxillary suture right and left (ZyMS-R, ZyMS-L) The zygomatico-maxillary suture line crossing at the lower orbital rim. The point is located on the inferior margin of the orbit.  Mid-Or A point in the middle between right and left Or indicated by the software  Mid-Po A point in the middle between right and left Po indicated by the software  Mid-IAF A point in the middle between right and left IAF indicated by the software  Mid-ZyMS A point in the middle between right and left ZyMS indicated by the software Plane  Frankfort horizontal plane 1 (FH 1) FH by connecting mid-Po, Or-R, and Or-L  Frankfort horizontal plane 2 (FH 2) FH by connecting mid-Or, Po-R, and Po-L  Plane 1 A plane connecting mid-Or, IAF-R, and IAF-L  Plane 2 A plane connecting mid-Po, ZyMS-R and ZyMS-L  Plane 3 A plane connecting Or-R, Or-L, and mid-IAF  Plane 4 A plane connecting Po-R, Po-L, and mid-ZyMS  Plane 5 A plane connecting ZyMS-R, ZyMS-L, and mid-IAF  Plane 6 A plane connecting IAF-R, IAF-L, and mid-ZyMS Name Definition Landmark  Orbitale right and left (Or-R, Or-L) The lowest point on the inferior margin of the orbit  Porion right and left (Po-R, Po-L) The most superior midpoint of the external auditory meatus (anatomic Po)  Internal acoustic foramen right and left (IAF-R, IAF-L) The most lateral point of the internal auditory meatus at the skull base  Zygomatico-maxillary suture right and left (ZyMS-R, ZyMS-L) The zygomatico-maxillary suture line crossing at the lower orbital rim. The point is located on the inferior margin of the orbit.  Mid-Or A point in the middle between right and left Or indicated by the software  Mid-Po A point in the middle between right and left Po indicated by the software  Mid-IAF A point in the middle between right and left IAF indicated by the software  Mid-ZyMS A point in the middle between right and left ZyMS indicated by the software Plane  Frankfort horizontal plane 1 (FH 1) FH by connecting mid-Po, Or-R, and Or-L  Frankfort horizontal plane 2 (FH 2) FH by connecting mid-Or, Po-R, and Po-L  Plane 1 A plane connecting mid-Or, IAF-R, and IAF-L  Plane 2 A plane connecting mid-Po, ZyMS-R and ZyMS-L  Plane 3 A plane connecting Or-R, Or-L, and mid-IAF  Plane 4 A plane connecting Po-R, Po-L, and mid-ZyMS  Plane 5 A plane connecting ZyMS-R, ZyMS-L, and mid-IAF  Plane 6 A plane connecting IAF-R, IAF-L, and mid-ZyMS View Large Figure 1. View largeDownload slide Landmarks indicated in this study: the frontal view (a) shows Orbitale (Or-R and Or-L) and the zygomatico-maxillary suture (ZyMS-R and ZyMS-L) located on the inferior orbital rim. The lateral view (b) shows the right Porion (Po-R), and the posterior view (c) shows the internal acoustic foramen (IAF-R and IAF-L) located at the lateroposterior point of the opening of the auditory canal inside the cranial cavity. The oblique view (d) shows the Frankfort horizontal plane 1 (FH 1), connecting mid- porion (Po), orbitale (Or)-R, and Or-L, in white and the Frankfort horizontal plane 2 (FH 2), mid-Or, Po-R and Po-Lmid-Or, Po-R and Po-L, in black. L, left; R, right. Figure 1. View largeDownload slide Landmarks indicated in this study: the frontal view (a) shows Orbitale (Or-R and Or-L) and the zygomatico-maxillary suture (ZyMS-R and ZyMS-L) located on the inferior orbital rim. The lateral view (b) shows the right Porion (Po-R), and the posterior view (c) shows the internal acoustic foramen (IAF-R and IAF-L) located at the lateroposterior point of the opening of the auditory canal inside the cranial cavity. The oblique view (d) shows the Frankfort horizontal plane 1 (FH 1), connecting mid- porion (Po), orbitale (Or)-R, and Or-L, in white and the Frankfort horizontal plane 2 (FH 2), mid-Or, Po-R and Po-Lmid-Or, Po-R and Po-L, in black. L, left; R, right. Two new landmarks: internal acoustic foramen (IAF; also known as internal acoustic meatus) and zygomatico-maxillary suture (ZyMS) were chosen. These two landmarks are well known in surgical anatomy (31, 32), but were selected in this study for the first time in the context of 3D cephalometry. These landmarks were chosen because their locations were close to the original FH landmarks and their definitions were clearer and possibly easier to identify than the original ones (Table 1). The new landmarks could be indicated without having to locate the lowest or the most superior point of an anatomical structure, which might cause an error. Four planes were drawn by connecting these new landmarks in combination with the traditional FH landmarks (Or and Po), thus creating Plane 1–4. In addition, these two landmarks were used alone to create two planes closely parallel to the FH (Planes 5 and 6; Table 1). In this study, the absolute angle differences of the six new planes (Plane 1–6) from the FH 1 and FH 2 were recorded. Three observers (two dentomaxillofacial radiologists with more than 5 years of experience, and one oral and maxillofacial surgeon with more than 10 years of experience) underwent an initial calibration session. Detailed instructions about landmark definition and software manipulation were given. The observers were given a few cases for training and calibration with the principal investigator prior to the observation. Thereafter, each observer completed the set of observations twice with a 4-week interval. Statistical analysis Reproducibility of landmarks Coordinates (x, y, z) of all landmarks and the angular values were exported to a spreadsheet (Microsoft Excel, Washington, USA). In this study of the 3D coordinate systems, the x-, y-, and z-axis were defined as the transverse direction, the sagittal direction, and the vertical direction, respectively. First, the intra- and inter-observer reproducibility of each landmark was calculated for x-, y-, and z-coordinates. Then, the mean Euclidean distance for a specific landmark was measured for evaluation of intra- and inter-observer reproducibility. Finally, the Euclidean distances were categorized into three levels: <0.5 mm, <1 mm, and ≥1 mm. The intra-observer and inter-observer reproducibility of landmarks was also visualized by using Bland–Altman plots. Angular measurements between planes Absolute angular differences were calculated between all pairs of planes and Bland–Altman plots were generated. Multiple paired t-tests with Bonferroni corrections were performed to compare the intra-observer Euclidean distance between observers. Comparisons of inter-observer Euclidean distance between landmarks were also done using a multiple paired t-test with Bonferroni correction. All data were analyzed in Medcalc® (MedCalc Software bvba, Ostend, Belgium) and SAS® 10.0.0 (SAS Institute Inc., Cary, North Carolina, USA). Results All data showed a normal distribution (Shapiro–Wilk test). Intra-observer reproducibility of landmarks Table 2 shows the intra-observer difference of each landmark on x-, y-, and z-axis. The intra-observer mean differences of IAF were less than 1 mm on all axes and for all observers. Intra-observer differences of greater than 1 mm were only found on x-coordinates. Supplementary Table 1 shows the intra-observer mean Euclidean distance of each landmark and each observer. Comparison of the intra-observer mean Euclidean distance between three observers for each landmark showed significant differences for Po-R and Po-L (P < 0.0167). The mean Euclidean distance of the IAF was significantly lower than those of other landmarks (P < 0.0018). Supplementary Table 2 shows the intra-observer reproducibility expressed as a percentage (%). The landmark reproducibility of Po and Or was greater than >1 mm for more than 50 per cent of the measurements in two observers. The best reproducibility was observed for IAF (>90% of below 1 mm). The Bland–Altman plots were done for all landmarks using the x-, y-, and z-axis separately. Figure 2 shows an example of the plots for intra-observer analysis of observer 1 and the landmarks on the right side. More errors were observed on x-coordinates for all landmarks except the IAF-R. ZyMS showed similar reproducibility on x- and y-axis when compared to Or, but a poorer reproducibility for ZyMS was observed on the z-axis. Table 2. Intra-observer absolute difference of x-, y-, and z-coordinates of the landmarks in mm. IAF, internal acoustic foramen; L, left; Or, orbitale; Po, porion; R, right; SD, standard deviation; ZyMS-R, zygomatico-maxillary suture. Landmark Observer 1 Observer 2 Observer 3 x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Or-R 0.96 1.15 0.29 0.28 0.08 0.08 1.48* 1.24 0.51 0.29 0.15 0.12 1.32* 1.41 0.62 0.60 0.24 0.19 Or-L 0.99 1.24 0.38 0.49 0.08 0.07 1.03* 0.87 0.39 0.26 0.16 0.14 1.51* 1.72 0.58 0.69 0.22 0.20 Po-R 1.14* 1.02 0.32 0.27 0.20 0.20 1.38* 0.96 0.43 0.42 0.23 0.24 2.71* 1.83 0.62 0.46 0.48 0.44 Po-L 0.81 0.93 0.46 0.50 0.17 0.13 0.78 0.64 0.45 0.37 0.13 0.13 2.57* 1.96 0.80 0.43 0.86 0.69 IAF-R 0.12 0.11 0.19 0.19 0.16 0.12 0.13 0.11 0.25 0.19 0.33 0.25 0.16 0.16 0.22 0.17 0.29 0.23 IAF-L 0.16 0.24 0.18 0.14 0.30 0.29 0.11 0.08 0.32 0.29 0.25 0.18 0.16 0.23 0.24 0.16 0.34 0.26 ZyMS-R 0.80 1.16 0.38 0.31 0.25 0.36 1.32* 1.45 0.34 0.44 0.22 0.16 1.54* 2.16 0.49 0.58 0.48 0.50 ZyMS-L 0.86 1.23 0.32 0.31 0.28 0.45 1.33* 1.63 0.46 0.46 0.26 0.32 1.65* 1.80 0.43 0.45 0.46 0.54 Landmark Observer 1 Observer 2 Observer 3 x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Or-R 0.96 1.15 0.29 0.28 0.08 0.08 1.48* 1.24 0.51 0.29 0.15 0.12 1.32* 1.41 0.62 0.60 0.24 0.19 Or-L 0.99 1.24 0.38 0.49 0.08 0.07 1.03* 0.87 0.39 0.26 0.16 0.14 1.51* 1.72 0.58 0.69 0.22 0.20 Po-R 1.14* 1.02 0.32 0.27 0.20 0.20 1.38* 0.96 0.43 0.42 0.23 0.24 2.71* 1.83 0.62 0.46 0.48 0.44 Po-L 0.81 0.93 0.46 0.50 0.17 0.13 0.78 0.64 0.45 0.37 0.13 0.13 2.57* 1.96 0.80 0.43 0.86 0.69 IAF-R 0.12 0.11 0.19 0.19 0.16 0.12 0.13 0.11 0.25 0.19 0.33 0.25 0.16 0.16 0.22 0.17 0.29 0.23 IAF-L 0.16 0.24 0.18 0.14 0.30 0.29 0.11 0.08 0.32 0.29 0.25 0.18 0.16 0.23 0.24 0.16 0.34 0.26 ZyMS-R 0.80 1.16 0.38 0.31 0.25 0.36 1.32* 1.45 0.34 0.44 0.22 0.16 1.54* 2.16 0.49 0.58 0.48 0.50 ZyMS-L 0.86 1.23 0.32 0.31 0.28 0.45 1.33* 1.63 0.46 0.46 0.26 0.32 1.65* 1.80 0.43 0.45 0.46 0.54 *Greater than 1 mm. View Large Table 2. Intra-observer absolute difference of x-, y-, and z-coordinates of the landmarks in mm. IAF, internal acoustic foramen; L, left; Or, orbitale; Po, porion; R, right; SD, standard deviation; ZyMS-R, zygomatico-maxillary suture. Landmark Observer 1 Observer 2 Observer 3 x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Or-R 0.96 1.15 0.29 0.28 0.08 0.08 1.48* 1.24 0.51 0.29 0.15 0.12 1.32* 1.41 0.62 0.60 0.24 0.19 Or-L 0.99 1.24 0.38 0.49 0.08 0.07 1.03* 0.87 0.39 0.26 0.16 0.14 1.51* 1.72 0.58 0.69 0.22 0.20 Po-R 1.14* 1.02 0.32 0.27 0.20 0.20 1.38* 0.96 0.43 0.42 0.23 0.24 2.71* 1.83 0.62 0.46 0.48 0.44 Po-L 0.81 0.93 0.46 0.50 0.17 0.13 0.78 0.64 0.45 0.37 0.13 0.13 2.57* 1.96 0.80 0.43 0.86 0.69 IAF-R 0.12 0.11 0.19 0.19 0.16 0.12 0.13 0.11 0.25 0.19 0.33 0.25 0.16 0.16 0.22 0.17 0.29 0.23 IAF-L 0.16 0.24 0.18 0.14 0.30 0.29 0.11 0.08 0.32 0.29 0.25 0.18 0.16 0.23 0.24 0.16 0.34 0.26 ZyMS-R 0.80 1.16 0.38 0.31 0.25 0.36 1.32* 1.45 0.34 0.44 0.22 0.16 1.54* 2.16 0.49 0.58 0.48 0.50 ZyMS-L 0.86 1.23 0.32 0.31 0.28 0.45 1.33* 1.63 0.46 0.46 0.26 0.32 1.65* 1.80 0.43 0.45 0.46 0.54 Landmark Observer 1 Observer 2 Observer 3 x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Or-R 0.96 1.15 0.29 0.28 0.08 0.08 1.48* 1.24 0.51 0.29 0.15 0.12 1.32* 1.41 0.62 0.60 0.24 0.19 Or-L 0.99 1.24 0.38 0.49 0.08 0.07 1.03* 0.87 0.39 0.26 0.16 0.14 1.51* 1.72 0.58 0.69 0.22 0.20 Po-R 1.14* 1.02 0.32 0.27 0.20 0.20 1.38* 0.96 0.43 0.42 0.23 0.24 2.71* 1.83 0.62 0.46 0.48 0.44 Po-L 0.81 0.93 0.46 0.50 0.17 0.13 0.78 0.64 0.45 0.37 0.13 0.13 2.57* 1.96 0.80 0.43 0.86 0.69 IAF-R 0.12 0.11 0.19 0.19 0.16 0.12 0.13 0.11 0.25 0.19 0.33 0.25 0.16 0.16 0.22 0.17 0.29 0.23 IAF-L 0.16 0.24 0.18 0.14 0.30 0.29 0.11 0.08 0.32 0.29 0.25 0.18 0.16 0.23 0.24 0.16 0.34 0.26 ZyMS-R 0.80 1.16 0.38 0.31 0.25 0.36 1.32* 1.45 0.34 0.44 0.22 0.16 1.54* 2.16 0.49 0.58 0.48 0.50 ZyMS-L 0.86 1.23 0.32 0.31 0.28 0.45 1.33* 1.63 0.46 0.46 0.26 0.32 1.65* 1.80 0.43 0.45 0.46 0.54 *Greater than 1 mm. View Large Figure 2. View largeDownload slide Bland–Altman plots of the intra-observer analysis of observer 1 and the landmarks on the right side. The errors are shown in millimetre. Plots show the mean and ± 1.96 SD reference lines. Higher error values were observed on the x-coordinates for all landmarks except IAF-R. IAF-R, internal acoustic foramen right; Or-R, orbitale right; SD, standard deviation; Po-R, porion right; ZyMS-R, zygomatico-maxillary suture right. Figure 2. View largeDownload slide Bland–Altman plots of the intra-observer analysis of observer 1 and the landmarks on the right side. The errors are shown in millimetre. Plots show the mean and ± 1.96 SD reference lines. Higher error values were observed on the x-coordinates for all landmarks except IAF-R. IAF-R, internal acoustic foramen right; Or-R, orbitale right; SD, standard deviation; Po-R, porion right; ZyMS-R, zygomatico-maxillary suture right. Inter-observer reproducibility of landmarks Table 3 shows the inter-observer difference in x-, y-, and z-coordinates of each landmark. The inter-observer mean differences of IAF were less than 1 mm for x-, y-, and z-coordinates for all pairs of observers. Other landmarks showed differences of more than 1 mm mostly on x-coordinates. The inter-observer mean Euclidean distance for each landmark regardless of observers is shown in Supplementary Table 3. The inter-observer comparison revealed that IAF-R had a significantly lower mean Euclidean distance than all the other landmarks (P < 0.0018). Regarding inter-observer reproducibility, Po-R and Po-L showed 74 to 97 per cent of Euclidean distances more than 1 mm (Supplementary Table 4). The best reproducibility was observed for IAF, showing 59–95 per cent of Euclidean distances less than 1 mm. Table 3. Inter-observer absolute difference of x-, y-, and z-coordinates of the landmarks in mm. IAF, internal acoustic foramen; L, left; Or, orbitale; Po, porion; R, right; SD, standard deviation; ZyMS-R, zygomatico-maxillary suture. Landmark Observer 1/2 Observer 1/3 Observer 2/3 x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Or-R 0.83 1.47 0.22 0.42 0.22 0.15 0.02 1.25 0.46 0.57 0.21 0.18 0.84 1.54 0.24 0.55 0.01 0.14 Or-L 0.74 1.16 0.31 0.41 0.30 0.18 0.37 1.14 0.59 0.55 0.22 0.20 1.10* 1.42 0.28 0.53 0.08 0.19 Po-R 3.20* 1.66 0.23 0.53 0.07 0.23 1.45* 1.54 0.63 0.53 0.22 0.34 1.75* 1.55 0.85 0.70 0.15 0.37 Po-L 3.67* 1.85 0.90 0.63 0.20 0.32 0.98 1.68 0.33 0.62 0.25 0.61 2.69 1.94 1.23 0.82 0.45 0.67 IAF-R 0.09 0.13 0.58 0.26 0.08 0.21 0.01 0.14 0.17 0.22 0.11 0.20 0.08 0.16 0.74 0.39 0.04 0.20 IAF-L 0.03 0.20 0.55 0.31 0.02 0.27 0.05 0.20 0.04 0.21 0.02 0.24 0.08 0.21 0.50 0.34 0.00 0.28 ZyMS-R 2.22* 2.07 0.38 0.40 0.81 0.55 0.48 1.95 0.42 0.59 0.34 0.49 1.74* 2.01 0.04 0.39 0.47 0.43 ZyMS-L 3.16* 2.31 0.03 0.42 1.01* 0.64 0.37 1.58 0.49 0.40 0.23 0.33 2.79* 2.52 0.46 0.58 0.78 0.64 Landmark Observer 1/2 Observer 1/3 Observer 2/3 x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Or-R 0.83 1.47 0.22 0.42 0.22 0.15 0.02 1.25 0.46 0.57 0.21 0.18 0.84 1.54 0.24 0.55 0.01 0.14 Or-L 0.74 1.16 0.31 0.41 0.30 0.18 0.37 1.14 0.59 0.55 0.22 0.20 1.10* 1.42 0.28 0.53 0.08 0.19 Po-R 3.20* 1.66 0.23 0.53 0.07 0.23 1.45* 1.54 0.63 0.53 0.22 0.34 1.75* 1.55 0.85 0.70 0.15 0.37 Po-L 3.67* 1.85 0.90 0.63 0.20 0.32 0.98 1.68 0.33 0.62 0.25 0.61 2.69 1.94 1.23 0.82 0.45 0.67 IAF-R 0.09 0.13 0.58 0.26 0.08 0.21 0.01 0.14 0.17 0.22 0.11 0.20 0.08 0.16 0.74 0.39 0.04 0.20 IAF-L 0.03 0.20 0.55 0.31 0.02 0.27 0.05 0.20 0.04 0.21 0.02 0.24 0.08 0.21 0.50 0.34 0.00 0.28 ZyMS-R 2.22* 2.07 0.38 0.40 0.81 0.55 0.48 1.95 0.42 0.59 0.34 0.49 1.74* 2.01 0.04 0.39 0.47 0.43 ZyMS-L 3.16* 2.31 0.03 0.42 1.01* 0.64 0.37 1.58 0.49 0.40 0.23 0.33 2.79* 2.52 0.46 0.58 0.78 0.64 *Greater than 1 mm. View Large Table 3. Inter-observer absolute difference of x-, y-, and z-coordinates of the landmarks in mm. IAF, internal acoustic foramen; L, left; Or, orbitale; Po, porion; R, right; SD, standard deviation; ZyMS-R, zygomatico-maxillary suture. Landmark Observer 1/2 Observer 1/3 Observer 2/3 x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Or-R 0.83 1.47 0.22 0.42 0.22 0.15 0.02 1.25 0.46 0.57 0.21 0.18 0.84 1.54 0.24 0.55 0.01 0.14 Or-L 0.74 1.16 0.31 0.41 0.30 0.18 0.37 1.14 0.59 0.55 0.22 0.20 1.10* 1.42 0.28 0.53 0.08 0.19 Po-R 3.20* 1.66 0.23 0.53 0.07 0.23 1.45* 1.54 0.63 0.53 0.22 0.34 1.75* 1.55 0.85 0.70 0.15 0.37 Po-L 3.67* 1.85 0.90 0.63 0.20 0.32 0.98 1.68 0.33 0.62 0.25 0.61 2.69 1.94 1.23 0.82 0.45 0.67 IAF-R 0.09 0.13 0.58 0.26 0.08 0.21 0.01 0.14 0.17 0.22 0.11 0.20 0.08 0.16 0.74 0.39 0.04 0.20 IAF-L 0.03 0.20 0.55 0.31 0.02 0.27 0.05 0.20 0.04 0.21 0.02 0.24 0.08 0.21 0.50 0.34 0.00 0.28 ZyMS-R 2.22* 2.07 0.38 0.40 0.81 0.55 0.48 1.95 0.42 0.59 0.34 0.49 1.74* 2.01 0.04 0.39 0.47 0.43 ZyMS-L 3.16* 2.31 0.03 0.42 1.01* 0.64 0.37 1.58 0.49 0.40 0.23 0.33 2.79* 2.52 0.46 0.58 0.78 0.64 Landmark Observer 1/2 Observer 1/3 Observer 2/3 x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Or-R 0.83 1.47 0.22 0.42 0.22 0.15 0.02 1.25 0.46 0.57 0.21 0.18 0.84 1.54 0.24 0.55 0.01 0.14 Or-L 0.74 1.16 0.31 0.41 0.30 0.18 0.37 1.14 0.59 0.55 0.22 0.20 1.10* 1.42 0.28 0.53 0.08 0.19 Po-R 3.20* 1.66 0.23 0.53 0.07 0.23 1.45* 1.54 0.63 0.53 0.22 0.34 1.75* 1.55 0.85 0.70 0.15 0.37 Po-L 3.67* 1.85 0.90 0.63 0.20 0.32 0.98 1.68 0.33 0.62 0.25 0.61 2.69 1.94 1.23 0.82 0.45 0.67 IAF-R 0.09 0.13 0.58 0.26 0.08 0.21 0.01 0.14 0.17 0.22 0.11 0.20 0.08 0.16 0.74 0.39 0.04 0.20 IAF-L 0.03 0.20 0.55 0.31 0.02 0.27 0.05 0.20 0.04 0.21 0.02 0.24 0.08 0.21 0.50 0.34 0.00 0.28 ZyMS-R 2.22* 2.07 0.38 0.40 0.81 0.55 0.48 1.95 0.42 0.59 0.34 0.49 1.74* 2.01 0.04 0.39 0.47 0.43 ZyMS-L 3.16* 2.31 0.03 0.42 1.01* 0.64 0.37 1.58 0.49 0.40 0.23 0.33 2.79* 2.52 0.46 0.58 0.78 0.64 *Greater than 1 mm. View Large The same trend as for intra-observer reproducibility was observed when plotting the Bland–Altman as more errors were seen on x-coordinates for all landmarks except the IAF-R (Figure 3). ZyMS showed poorer inter-observer reproducibility when compared to Or on the x- and z-axis. Figure 3. View largeDownload slide Bland–Altman plots of the inter-observer analysis comparing observer 1 to 2 of the right landmarks. The errors are shown in millimetre. Plots show the mean and ± 1.96 SD reference lines. Higher error values were observed on x-coordinates for all landmarks except IAF-R. IAF-R, internal acoustic foramen right; Or-R, orbitale right; Po-R, porion right; SD, standard deviation; ZyMS-R, zygomatico-maxillary suture right. Figure 3. View largeDownload slide Bland–Altman plots of the inter-observer analysis comparing observer 1 to 2 of the right landmarks. The errors are shown in millimetre. Plots show the mean and ± 1.96 SD reference lines. Higher error values were observed on x-coordinates for all landmarks except IAF-R. IAF-R, internal acoustic foramen right; Or-R, orbitale right; Po-R, porion right; SD, standard deviation; ZyMS-R, zygomatico-maxillary suture right. Angular measurements between planes A summary of absolute angular difference between each pair of planes is shown in Table 4. FH 1 and FH 2 exhibited 0.7 degrees overall angular difference (standard deviation [SD] of 0.58 degrees). Planes 3 and 4, both showed a mean angular difference less than 1 degree from FH 1 and FH 2, respectively. Furthermore, of Bland–Altman plots of the angular differences between each pair of planes are shown in Figure 4. Table 4. Mean absolute angular measurements with standard deviation (SD) between between FH 1 and FH2, FH 1 and Plane 1–6, FH 2 and Plane 1–6. FH, Frankfort horizontal plane. Plane (n = 8) Angular measurement Mean SD FH 1–FH 2 0.70* 0.58 FH 1–Plane 1 1.87 1.15 FH 1–Plane 2 2.50 1.13 FH 1–Plane 3 0.53* 0.37 FH 1–Plane 4 0.82* 0.54 FH 1–Plane 5 2.46 1.18 FH 1–Plane 6 1.95 1.10 FH 2–Plane 1 2.13 1.05 FH 2–Plane 2 2.21 1.11 FH 2–Plane 3 0.91* 0.56 FH 2–Plane 4 0.28* 0.27 FH 2–Plane 5 2.17 1.16 FH 2–Plane 6 2.16 1.06 Plane (n = 8) Angular measurement Mean SD FH 1–FH 2 0.70* 0.58 FH 1–Plane 1 1.87 1.15 FH 1–Plane 2 2.50 1.13 FH 1–Plane 3 0.53* 0.37 FH 1–Plane 4 0.82* 0.54 FH 1–Plane 5 2.46 1.18 FH 1–Plane 6 1.95 1.10 FH 2–Plane 1 2.13 1.05 FH 2–Plane 2 2.21 1.11 FH 2–Plane 3 0.91* 0.56 FH 2–Plane 4 0.28* 0.27 FH 2–Plane 5 2.17 1.16 FH 2–Plane 6 2.16 1.06 *The absolute angular difference less than 1 degree. View Large Table 4. Mean absolute angular measurements with standard deviation (SD) between between FH 1 and FH2, FH 1 and Plane 1–6, FH 2 and Plane 1–6. FH, Frankfort horizontal plane. Plane (n = 8) Angular measurement Mean SD FH 1–FH 2 0.70* 0.58 FH 1–Plane 1 1.87 1.15 FH 1–Plane 2 2.50 1.13 FH 1–Plane 3 0.53* 0.37 FH 1–Plane 4 0.82* 0.54 FH 1–Plane 5 2.46 1.18 FH 1–Plane 6 1.95 1.10 FH 2–Plane 1 2.13 1.05 FH 2–Plane 2 2.21 1.11 FH 2–Plane 3 0.91* 0.56 FH 2–Plane 4 0.28* 0.27 FH 2–Plane 5 2.17 1.16 FH 2–Plane 6 2.16 1.06 Plane (n = 8) Angular measurement Mean SD FH 1–FH 2 0.70* 0.58 FH 1–Plane 1 1.87 1.15 FH 1–Plane 2 2.50 1.13 FH 1–Plane 3 0.53* 0.37 FH 1–Plane 4 0.82* 0.54 FH 1–Plane 5 2.46 1.18 FH 1–Plane 6 1.95 1.10 FH 2–Plane 1 2.13 1.05 FH 2–Plane 2 2.21 1.11 FH 2–Plane 3 0.91* 0.56 FH 2–Plane 4 0.28* 0.27 FH 2–Plane 5 2.17 1.16 FH 2–Plane 6 2.16 1.06 *The absolute angular difference less than 1 degree. View Large Figure 4. View largeDownload slide Bland–Altman plots of angular differences between each pair of planes. The errors are shown in degrees. Plots show the mean and ±1.96 SD reference lines. Planes 3 and 4 resulted in the least differences among all pairs of new planes including the FH planes. FH, Frankfort horizontal plane; SD, standard deviation. Figure 4. View largeDownload slide Bland–Altman plots of angular differences between each pair of planes. The errors are shown in degrees. Plots show the mean and ±1.96 SD reference lines. Planes 3 and 4 resulted in the least differences among all pairs of new planes including the FH planes. FH, Frankfort horizontal plane; SD, standard deviation. Discussion The FH has been used as a reference plane that closely links to the NHP for decades (8). As 3D imaging modalities have been emerging in dental medicine and orthodontics over the last decade (10–13), still no research has been done to test its reproducibility by different clinicians in three dimensions. One study on the reproducibility of the FH plane on MRI images revealed excellent intra- and inter-observer reproducibility of the FH plane through 3D landmark identification (28). This finding is certainly of interest, but in clinical dental practice, MRI images are still not widely used for orthodontic treatment planning purposes. The methodology of this study was based on two previously published articles (29, 30). Although the dry skull samples were used without known age and gender, landmark identification in this study was not affected by the sample variability because landmark identification was unrelated to the dental status. The segmentation process of this study was based on previous published studies. Sub-sampling was done to prepare data to be appropriate for the software processing as recommended by the software (29, 30). The density threshold of 276 was used for all CBCT datasets. For scans acquired from other CBCT scanners, different thresholds may be applied as density numbers varies among different CBCT machines lacking standardization via Hounsfield units (33, 34). In the present study, new 3D landmarks that form planes intended to be parallel to the original FH were introduced and tested. Interestingly, the results of the present investigation showed poorer intra- and inter-observer reproducibility of the traditional FH landmarks (Po, Or) on the x-axis, and exhibited good intra- and inter-observer reproducibility of the new IAF landmark. On the other hand, Or showed superior results compared to the new ZyMS. Traditionally, the FH plane is defined on a 2D lateral skull or a lateral cephalometric radiograph by placing the Po and Or landmarks. There are two types of Po in 2D. One is the ‘machine Po’ when the landmark is defined by a radiopaque marker in the ear rod as part of the cephalometric head positioning device. The other is called the ‘anatomic Po’ which refers to the upper edge of the shadow of the auditory canal that can be seen on cephalometric films, (usually located slightly above and posterior to the ‘machine Po’) (4). The Or is defined as the inferior border of the orbital rim. In this study, the anatomic Po was used because no positioning device was placed in the external auditory meatus. In 3D, the anatomical structures were used instead of the 2D shadows. As described in the literature, the reproducibility or reliability of both landmarks (Po/Or) is limited (25–27). In this study, two new landmarks were proposed: IAF and ZyMS. The IAF is regularly used to identify the facial nerve in CT and MRI images (35). This landmark was chosen because of its good visualization, and because it relates with the Po as it refers to the other end of the auditory canal. The upper edge of the oval shape was not selected, but instead the lateral end was chosen and defined as a landmark because of its corner-like location. In CBCT scans of patients, this landmark is well visible, but it must be noted that this landmark will only be included when the diameter of the FOV of the CBCT is big enough to cover the cranial cavity. Therefore, a cylindrical FOV with 17 cm diameter was used in the present study. The ZyMS was selected as it is close to the traditional Or but might be more defined in location. A thin line or a notch on the inferior orbital rim is usually seen on the 3D model and can be identified as ZyMS, thus the observer did not have to identify the lowest point of the orbital rim, which is prone to subjectivity on 3D images. However, the short distance of between the ZyMS-R and ZyMS-L, about one third closer than Or-R and Or-L, may not appropriate to create a reliable reference plane. The results showed that the intra- and inter-observer reproducibility of IAF were significantly better than other landmarks, while Po showed poorer reproducibility. These findings were in agreement with previous publications (25–27). Ludlow et al. (25) compared the precision of cephalometric landmarks on lateral cephalograms and multi-planar reformatted (MPR) images of CBCT scans. It was reported that Po showed poorer precision on MPR images than other landmarks (25). In 2012, Schlicher et al. reported that the Po was the most inconsistent landmark and the Or was the most imprecise landmark from the study conducted on MPR images (26). In another study done by Hassan et al. (27), in which the precision of cephalometric landmark identification from CBCT data on both MPR and 3D models of 10 patients was evaluated, the authors reported that the poorest precision was that of the Po. The precision of Or was considered as moderate (27). In the present study, ZyMS showed poorer results than expected. This might have been caused by the process of creating 3D models. When the 3D surface model was smoothened, the ZyMS, which only appears as a notch, may not be visible for identification. The previous study done by Hassan et al. suggested using both, MPR images and 3D surface models to help locate the landmarks and thus to strengthen the reproducibility of the landmarks (27). This methodology should be further evaluated in future studies. The results of the present study also showed that most landmarks except IAF exhibited poorer reproducibility for x-coordinates (i.e. transverse plane). The results corresponded to results published by Titiz et al., and Hofmann et al. (36, 37). Titiz et al. have stated that for Po and Or, identification in the x-axis was difficult and Po also showed difficulty in the y-axis. In x-axis, Po-L and Or-L showed higher SD for the total reproducibility with 1.36 and 3.27 mm, respectively (36). As all landmarks are located on the hard tissue surface, the anatomy and morphology of the area of interest could be one of the key factors. To improve and minimize this variability, 3D definitions of landmarks must be clear and concise, especially landmarks that are located on a curvy surface such as Po, Or, and ZyMS. Two-dimensional definitions are based on shadows on conventional radiographs but when transforming to 3D, another dimension must be accounted for. In 3D cephalometry, it is recommended to modify 2D definition to a more robust location of the respective landmark. It was shown in a study by de Oliveira et al. that good definitions of landmarks could help improve observer performance and if a good operator training and calibration is followed (24). Using both MPR images and 3D surface models to help locate the landmarks should further strengthen the reproducibility of the landmarks (27). Another method to reduce the position variance of FH landmarks was published by Hofmann et al. (37). The authors investigated 3D reliability of the FH landmarks and used the nearby anatomical structures to help enhance their reliability. It was found that the presented adjacent landmarks improved the SD of Po and Or significantly (37). When trying to transform a 2D plane to a 3D plane, there are four landmarks involved because of the right and left side. Maxilim® software cannot connect all four landmarks together but can only connect three landmarks to generate a 3D plane; thus, a mid-landmark of one of the landmarks had to be calculated. By performing this method, two possibilities of a FH plane could be generated. FH 1 was drawn using Or-L, Or-R, and mid-Po. FH 2 was created using Po-L, Po-R, and mid-Or. The results showed that FH 1 − FH 2 presented a small angular difference (mean 0.7°, SD 0.58°). From the results of landmark reproducibility, the z-axis (i.e. vertical direction) showed low errors (Figures 2 and 3). This may imply that the high variability seen in the x-axis (and to a lesser extent in the y-axis) probably posed little influence on the angulation of the horizontal plane. Among the newly introduced planes, the plane that showed angles closest to the original FHs were Plane 3, connecting Or-R, Or-L, and mid-IAF, and Plane 4, connecting Po-R, Po-L, and mid-ZyMS (mean angular difference <1° from FHs). For Planes 3 and 4, only one of the three landmarks used to form these planes was the new mid-landmark (mid-IAF for Plane 3 and mid-ZyMS for Plane 4). Since Planes 3 and 4 share common points with the FH planes, it is self-evident that they have a higher possibility to deviate less from the original FH planes. It was observed that FH 2 showed slightly more deviation from Plane 3 than Plane 4 (Figure 4). The reason might be the difference in the composition of landmarks. FH 2 and Plane 4 shared to two common landmarks: Po-R and Po-L. The other landmark used in FH 2 was mid-Or and the other landmark used in Plane 4 was mid-ZyMS. Or and ZyMS were located close to each other, which posed a very small difference in distance and angulation. On the other hand, when comparing FH 2 and Plane 3, a slightly bigger difference was seen (Figure 4). The similarity between FH 2 and Plane 3 was Or (FH 2 used mid-Or, while Plane 3 used Or-R and Or-L). The other landmarks used in FH 2 were two Po’s, and the other landmark used in Plane 3 was mid-IAF. Po was located further from IAF, thus might give a slightly bigger difference in location and angulation. From the findings, the distance between landmarks is another crucial factor that may create errors in angulation. Landmarks with a short distance between each other may create more error when generating a plane. Although Plane 4 showed good results, the short distance between ZyMS-R and ZyMS-L might hamper the generation of a reliable horizontal reference plane. Plane 3 despite using the mid-IAF in place of the mid-Po was promising because of the superior reproducibility of IAF in z-axis when compared to Po. Therefore, the new horizontal Plane 3 might have potential to replace the traditional FH, but more evidence supporting these findings is necessary. An important critical concern regarding 3D cephalometry is the radiation dose. This technique requires a big FOV of the CBCT scan that includes the maxilla, mandible, and the skull base. Studies showed that the radiation dose was strongly related to the size of the FOV size, the specific CBCT machine used, and the scanning parameters (e.g. resolution) (15, 38–41). A small voxel size is usually not necessary for 3D surface models; thus, the radiation dose can be limited by choosing standard resolution parameters (e.g. voxel size of 0.4 mm). Nevertheless, case selection and good justification must always be applied according to published national and international guidelines and recommendations on the use of CBCT (42–46). Study limitations As this is an in-vitro study, the samples did not represent real patients. Absence of soft tissues may alter the images and effect segmentation results. The method used in this study only facilitated 3D surface models. On curved anatomical area, landmarks might be difficult to identify. Thus, in future studies 3D surface models should be used in combination with MPR images. This can increase the visibility of landmarks and increase the reliability of identification. Furthermore, 3D cephalometry depends heavily on software functions. The software used in this study allowed to record coordinate information, but did not offer any information about the angular differences between planes, nor the direction of the planes. Thus, only the angular values between planes were presented in the results. One further limiting factor might be the clinical background of the observers. In this study, two dentomaxillofacial radiologists and one oral and maxillofacial surgeon were assessing the images. It might be that measurements by trained orthodontists might have resulted in slightly different findings. Nevertheless, the radiologists and the surgeon all underwent a calibration session, and should be familiar with novel imaging modalities and cephalometric analysis. Clinical relevance Many studies have been conducted on cephalometric landmarks identification and the accuracy of this procedure. Studies on measurement errors and landmark identification generally set the clinical significant level at 1 mm, which means that errors below 1 mm should not influence clinical treatment (13, 17). According to the results presented, IAF has shown its potential as a novel landmark. Although this study evaluated the 3D reproducibility of the FH landmarks, the reproducibility is only one of many factors that contribute to a validation of new planes to replace the well-established FH. The FH resembles closely the NHP and FH can be identified directly on patients or photos without any radiation exposure, both clearly depicting the soft tissues, which is the most clinically relevant tissue of our patients. The results of this study have shown that angular differences between FH planes and newly proposed planes were averagely less than 1 degree, which may not be clinically significant. However, the relationship between newly proposed planes, which were based only on hard tissue landmarks, and NHP should thoroughly be studied before these planes can be implemented for daily clinical use. Conclusions The present study investigated the reproducibility of landmarks incorporated in a FH in three dimensions. The results revealed poor reproducibility of the traditional FH landmarks on the x-axis and good reproducibility of a new landmark tested to replace Po, the IAF. On the other hand, Or showed superior results compared to the new ZyMS. The potential of using new horizontal planes was demonstrated. Future studies should focus on the identification of a valid alternative for Or and ZyMS and on clinical implementation of the findings. Supplementary material Supplementary data are available at European Journal of Orthodontics online. Funding This work was supported by a doctoral scholarship in the framework of the Interfaculty Council for Development Co-operation (IRO). Conflict of interest None to declare. Acknowledgements The authors would like to thank Dr B. Mowafey and Dr A. Sobhy from OMFS-IMPATH research group, University of Leuven for their contributions in the observations. References 1. Hofrath , H . 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Oral Surgery, Oral Medicine, Oral Patholology and Oral Radiology , 116 , 238 – 57 . CrossRef Search ADS © The Author(s) 2017. Published by Oxford University Press on behalf of the European Orthodontic Society. All rights reserved. For permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png The European Journal of Orthodontics Oxford University Press

Three-dimensional Frankfort horizontal plane for 3D cephalometry: a comparative assessment of conventional versus novel landmarks and horizontal planes

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© The Author(s) 2017. Published by Oxford University Press on behalf of the European Orthodontic Society. All rights reserved. For permissions, please email: journals.permissions@oup.com
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Abstract

Summary Background To assess the reproducibility of landmarks in three dimensions that determine the Frankfort horizontal plane (FH) as well as two new landmarks, and to evaluate the angular differences of newly introduced planes to the FH. Methods Three-dimensional (3D) surface models were created from CBCT scans of 26 dry human skulls. Porion (Po), orbitale (Or), internal acoustic foramen (IAF), and zygomatico-maxillary suture (ZyMS) were indicated in the software by three observers twice with a 4-week interval. Angles between two FHs (FH 1: Or-R, Or-L, mid-Po; FH 2: Po-R, Po-L, mid-Or) and between FHs and new planes (Plane 1–6) were measured. Coordinates were exported to a spreadsheet. A statistical analysis was performed to define the landmark reproducibility and 3D angles. Results Intra- and inter-observer landmark reproducibility showed mean difference more than 1 mm for x-coordinates of all landmarks except IAF. IAF showed significantly better reproducibility than other landmarks (P < 0.0018). The mean angular difference between FH 1 and FH 2 was 0.7 degrees. Plane 3, connecting Or-R, Or-L and mid-IAF, and Plane 4, connecting Po-R, Po-L and mid-ZyMS, both showed an angular difference of less than 1 degree when compared to FHs. Conclusions This study revealed poor reproducibility of the traditional FH landmarks on the x-axis and good reproducibility of a new landmark tested to replace Po, the IAF. Yet, Or showed superior results compared to ZyMS. The potential of using new horizontal planes was demonstrated. Future studies should focus on identification of a valid alternative for Or and ZyMS and on clinical implementation of the findings. Introduction A cephalometric analysis is an essential part of orthodontic treatment planning. The technique has been used for several decades after its introduction by Hofrath in Germany and Broadbent in the USA (1, 2). It is traditionally performed on a lateral and a posteroanterior cephalogram. One of the important elements in performing a cephalometric analysis is the horizontal reference plane. The Frankfort horizontal plane (FH; also called the auriculo-orbital plane) was established at the World Congress of Anthropology, in Frankfort, Germany in 1882. First introduced by Von Ihering, in 1872, this plane used to pass through the centre of the external auditory meatus to the lowest point of the inferior margin of each orbit. The Frankfort agreement then modified this definition, so that the plane would pass through the upper borders of each ear canal or external auditory meatus (Porion/Po), and through the inferior border of the orbital rim (Orbitale/Or). The Frankfort plane was employed for orientation of the patient and was chosen as the best anatomic indicator of the true horizontal line. It is also closely related to the natural head position (NHP) (3, 4). Although the reliability and validity of the FH for cephalometric analysis was questioned by several authors (5–9), it is still widely accepted in contemporary cephalometric use (4). Recently, three-dimensional (3D) imaging modalities, especially cone-beam computed tomography (CBCT), have become essential for diagnosis and treatment planning in the oral and maxillofacial region (10). In orthodontics, CBCT images allow 3D visualization of the craniofacial structures without superimposition (11–14). With its relatively lower radiation doses than multi-slice computed tomography (MSCT) (15), this modality proves to be useful for various orthodontic indications requiring advanced imaging and diagnosis such as canine impaction, root resorption, sleep disorders, and orthognathic surgery (10–14). Cephalometric analysis, traditionally done on two-dimensional (2D) radiographs (16), is also moving towards the direction of 3D imaging (17). In 3D cephalometry, cephalometric landmarks are identified in three orthogonal planes or on 3D models with the aid of a 3D image viewing software (18, 19). The original 2D planes or lines were transformed and used in 3D cephalometric analyses. Some publications have shown the reproducibility and accuracy of cephalometric landmark identification in 3D, including Po and Or which are used to form the Frankfort plane (17–27). In a few studies, it was found that reproducibility and reliability of these two landmarks in 3D was not optimal (25–27) with only one study conducted on a 3D surface model (23). Only one publication evaluated the accuracy and reliability of the FH using 3D magnetic resonance imaging (MRI) (28). Novel landmarks and horizontal planes have not been in the focus of interest while assessing the use of 3D cephalometry. Nevertheless, these might have some impact for diagnostics and treatment planning in orthodontics. Therefore, the aim of this study was twofold: (1) to assess the reproducibility of existing landmarks in three dimensions that determine the FH including two new landmarks and (2) to compare the angular differences of newly introduced planes to the FH. Materials and methods Sample Twenty-six dry human skulls with upper and lower first incisors and first molars present were selected from the Department of Anatomy of the University of Hasselt, Belgium. No age and gender information of the skulls was available. The dentition ranged from a mixed dentition to a permanent dentition. Mandibles were fixed to the skulls using surgical tapes. The tape was wrapped around the skull, starting from the temporal area of one side, crossing the lower border of the mandible to the temporal area of the other side. The occlusion was fixed at maximum intercuspation. The study protocol was approved by the local Medical Ethics Committee (reference number: ML6960, BE322201010078). All procedures were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki Declaration of 1975, as revised in 2008. Imaging modalities CBCT scans of the samples were taken using the largest field of view (FOV; diameter 170 × height 120 mm) of the 3D Accuitomo® 170 (J. Morita, Kyoto, Japan), using the High-Fidelity mode: 90 kVp, 154 mAs, voxel size 0.25 mm. A 1.7-mm-thick copper filter was attached in front of the X-ray source of the CBCT device during image acquisition to simulate soft tissue attenuation. CBCT data were exported to DICOM and then imported to Maxilim® software version 2.3.0.3 (Medicim NV, Sint-Niklaas, Belgium). Three-dimensional surface models were created for all samples based on previous published methods (29, 30). The full CBCT volume was included with 0.5 mm sub-sampling of voxels. The threshold was set at 276 to segment the hard tissues for the 3D models (29, 30). FH analysis A preset for the FH evaluation was created in the Maxilim® software. The analysis was composed of four operator-indicated landmarks (eight landmarks when counting both sides) and four mid-landmarks calculated by the software (Table 1). In 3D, a plane can be made from three landmarks/points, thus two possibilities of FH were created: FH 1 and FH 2 (Figure 1; Table 1). FH 1 was drawn using Or left (Or-L) and right (Or-R) and a landmark in the middle of the two Porions (mid-Po). On the other hand, FH 2 was created using Porion left (Po-L) and right (Po-R) and a landmark in the middle of the two Orbitales (mid-Or). Table 1. Definition of landmarks and planes. Name Definition Landmark  Orbitale right and left (Or-R, Or-L) The lowest point on the inferior margin of the orbit  Porion right and left (Po-R, Po-L) The most superior midpoint of the external auditory meatus (anatomic Po)  Internal acoustic foramen right and left (IAF-R, IAF-L) The most lateral point of the internal auditory meatus at the skull base  Zygomatico-maxillary suture right and left (ZyMS-R, ZyMS-L) The zygomatico-maxillary suture line crossing at the lower orbital rim. The point is located on the inferior margin of the orbit.  Mid-Or A point in the middle between right and left Or indicated by the software  Mid-Po A point in the middle between right and left Po indicated by the software  Mid-IAF A point in the middle between right and left IAF indicated by the software  Mid-ZyMS A point in the middle between right and left ZyMS indicated by the software Plane  Frankfort horizontal plane 1 (FH 1) FH by connecting mid-Po, Or-R, and Or-L  Frankfort horizontal plane 2 (FH 2) FH by connecting mid-Or, Po-R, and Po-L  Plane 1 A plane connecting mid-Or, IAF-R, and IAF-L  Plane 2 A plane connecting mid-Po, ZyMS-R and ZyMS-L  Plane 3 A plane connecting Or-R, Or-L, and mid-IAF  Plane 4 A plane connecting Po-R, Po-L, and mid-ZyMS  Plane 5 A plane connecting ZyMS-R, ZyMS-L, and mid-IAF  Plane 6 A plane connecting IAF-R, IAF-L, and mid-ZyMS Name Definition Landmark  Orbitale right and left (Or-R, Or-L) The lowest point on the inferior margin of the orbit  Porion right and left (Po-R, Po-L) The most superior midpoint of the external auditory meatus (anatomic Po)  Internal acoustic foramen right and left (IAF-R, IAF-L) The most lateral point of the internal auditory meatus at the skull base  Zygomatico-maxillary suture right and left (ZyMS-R, ZyMS-L) The zygomatico-maxillary suture line crossing at the lower orbital rim. The point is located on the inferior margin of the orbit.  Mid-Or A point in the middle between right and left Or indicated by the software  Mid-Po A point in the middle between right and left Po indicated by the software  Mid-IAF A point in the middle between right and left IAF indicated by the software  Mid-ZyMS A point in the middle between right and left ZyMS indicated by the software Plane  Frankfort horizontal plane 1 (FH 1) FH by connecting mid-Po, Or-R, and Or-L  Frankfort horizontal plane 2 (FH 2) FH by connecting mid-Or, Po-R, and Po-L  Plane 1 A plane connecting mid-Or, IAF-R, and IAF-L  Plane 2 A plane connecting mid-Po, ZyMS-R and ZyMS-L  Plane 3 A plane connecting Or-R, Or-L, and mid-IAF  Plane 4 A plane connecting Po-R, Po-L, and mid-ZyMS  Plane 5 A plane connecting ZyMS-R, ZyMS-L, and mid-IAF  Plane 6 A plane connecting IAF-R, IAF-L, and mid-ZyMS View Large Table 1. Definition of landmarks and planes. Name Definition Landmark  Orbitale right and left (Or-R, Or-L) The lowest point on the inferior margin of the orbit  Porion right and left (Po-R, Po-L) The most superior midpoint of the external auditory meatus (anatomic Po)  Internal acoustic foramen right and left (IAF-R, IAF-L) The most lateral point of the internal auditory meatus at the skull base  Zygomatico-maxillary suture right and left (ZyMS-R, ZyMS-L) The zygomatico-maxillary suture line crossing at the lower orbital rim. The point is located on the inferior margin of the orbit.  Mid-Or A point in the middle between right and left Or indicated by the software  Mid-Po A point in the middle between right and left Po indicated by the software  Mid-IAF A point in the middle between right and left IAF indicated by the software  Mid-ZyMS A point in the middle between right and left ZyMS indicated by the software Plane  Frankfort horizontal plane 1 (FH 1) FH by connecting mid-Po, Or-R, and Or-L  Frankfort horizontal plane 2 (FH 2) FH by connecting mid-Or, Po-R, and Po-L  Plane 1 A plane connecting mid-Or, IAF-R, and IAF-L  Plane 2 A plane connecting mid-Po, ZyMS-R and ZyMS-L  Plane 3 A plane connecting Or-R, Or-L, and mid-IAF  Plane 4 A plane connecting Po-R, Po-L, and mid-ZyMS  Plane 5 A plane connecting ZyMS-R, ZyMS-L, and mid-IAF  Plane 6 A plane connecting IAF-R, IAF-L, and mid-ZyMS Name Definition Landmark  Orbitale right and left (Or-R, Or-L) The lowest point on the inferior margin of the orbit  Porion right and left (Po-R, Po-L) The most superior midpoint of the external auditory meatus (anatomic Po)  Internal acoustic foramen right and left (IAF-R, IAF-L) The most lateral point of the internal auditory meatus at the skull base  Zygomatico-maxillary suture right and left (ZyMS-R, ZyMS-L) The zygomatico-maxillary suture line crossing at the lower orbital rim. The point is located on the inferior margin of the orbit.  Mid-Or A point in the middle between right and left Or indicated by the software  Mid-Po A point in the middle between right and left Po indicated by the software  Mid-IAF A point in the middle between right and left IAF indicated by the software  Mid-ZyMS A point in the middle between right and left ZyMS indicated by the software Plane  Frankfort horizontal plane 1 (FH 1) FH by connecting mid-Po, Or-R, and Or-L  Frankfort horizontal plane 2 (FH 2) FH by connecting mid-Or, Po-R, and Po-L  Plane 1 A plane connecting mid-Or, IAF-R, and IAF-L  Plane 2 A plane connecting mid-Po, ZyMS-R and ZyMS-L  Plane 3 A plane connecting Or-R, Or-L, and mid-IAF  Plane 4 A plane connecting Po-R, Po-L, and mid-ZyMS  Plane 5 A plane connecting ZyMS-R, ZyMS-L, and mid-IAF  Plane 6 A plane connecting IAF-R, IAF-L, and mid-ZyMS View Large Figure 1. View largeDownload slide Landmarks indicated in this study: the frontal view (a) shows Orbitale (Or-R and Or-L) and the zygomatico-maxillary suture (ZyMS-R and ZyMS-L) located on the inferior orbital rim. The lateral view (b) shows the right Porion (Po-R), and the posterior view (c) shows the internal acoustic foramen (IAF-R and IAF-L) located at the lateroposterior point of the opening of the auditory canal inside the cranial cavity. The oblique view (d) shows the Frankfort horizontal plane 1 (FH 1), connecting mid- porion (Po), orbitale (Or)-R, and Or-L, in white and the Frankfort horizontal plane 2 (FH 2), mid-Or, Po-R and Po-Lmid-Or, Po-R and Po-L, in black. L, left; R, right. Figure 1. View largeDownload slide Landmarks indicated in this study: the frontal view (a) shows Orbitale (Or-R and Or-L) and the zygomatico-maxillary suture (ZyMS-R and ZyMS-L) located on the inferior orbital rim. The lateral view (b) shows the right Porion (Po-R), and the posterior view (c) shows the internal acoustic foramen (IAF-R and IAF-L) located at the lateroposterior point of the opening of the auditory canal inside the cranial cavity. The oblique view (d) shows the Frankfort horizontal plane 1 (FH 1), connecting mid- porion (Po), orbitale (Or)-R, and Or-L, in white and the Frankfort horizontal plane 2 (FH 2), mid-Or, Po-R and Po-Lmid-Or, Po-R and Po-L, in black. L, left; R, right. Two new landmarks: internal acoustic foramen (IAF; also known as internal acoustic meatus) and zygomatico-maxillary suture (ZyMS) were chosen. These two landmarks are well known in surgical anatomy (31, 32), but were selected in this study for the first time in the context of 3D cephalometry. These landmarks were chosen because their locations were close to the original FH landmarks and their definitions were clearer and possibly easier to identify than the original ones (Table 1). The new landmarks could be indicated without having to locate the lowest or the most superior point of an anatomical structure, which might cause an error. Four planes were drawn by connecting these new landmarks in combination with the traditional FH landmarks (Or and Po), thus creating Plane 1–4. In addition, these two landmarks were used alone to create two planes closely parallel to the FH (Planes 5 and 6; Table 1). In this study, the absolute angle differences of the six new planes (Plane 1–6) from the FH 1 and FH 2 were recorded. Three observers (two dentomaxillofacial radiologists with more than 5 years of experience, and one oral and maxillofacial surgeon with more than 10 years of experience) underwent an initial calibration session. Detailed instructions about landmark definition and software manipulation were given. The observers were given a few cases for training and calibration with the principal investigator prior to the observation. Thereafter, each observer completed the set of observations twice with a 4-week interval. Statistical analysis Reproducibility of landmarks Coordinates (x, y, z) of all landmarks and the angular values were exported to a spreadsheet (Microsoft Excel, Washington, USA). In this study of the 3D coordinate systems, the x-, y-, and z-axis were defined as the transverse direction, the sagittal direction, and the vertical direction, respectively. First, the intra- and inter-observer reproducibility of each landmark was calculated for x-, y-, and z-coordinates. Then, the mean Euclidean distance for a specific landmark was measured for evaluation of intra- and inter-observer reproducibility. Finally, the Euclidean distances were categorized into three levels: <0.5 mm, <1 mm, and ≥1 mm. The intra-observer and inter-observer reproducibility of landmarks was also visualized by using Bland–Altman plots. Angular measurements between planes Absolute angular differences were calculated between all pairs of planes and Bland–Altman plots were generated. Multiple paired t-tests with Bonferroni corrections were performed to compare the intra-observer Euclidean distance between observers. Comparisons of inter-observer Euclidean distance between landmarks were also done using a multiple paired t-test with Bonferroni correction. All data were analyzed in Medcalc® (MedCalc Software bvba, Ostend, Belgium) and SAS® 10.0.0 (SAS Institute Inc., Cary, North Carolina, USA). Results All data showed a normal distribution (Shapiro–Wilk test). Intra-observer reproducibility of landmarks Table 2 shows the intra-observer difference of each landmark on x-, y-, and z-axis. The intra-observer mean differences of IAF were less than 1 mm on all axes and for all observers. Intra-observer differences of greater than 1 mm were only found on x-coordinates. Supplementary Table 1 shows the intra-observer mean Euclidean distance of each landmark and each observer. Comparison of the intra-observer mean Euclidean distance between three observers for each landmark showed significant differences for Po-R and Po-L (P < 0.0167). The mean Euclidean distance of the IAF was significantly lower than those of other landmarks (P < 0.0018). Supplementary Table 2 shows the intra-observer reproducibility expressed as a percentage (%). The landmark reproducibility of Po and Or was greater than >1 mm for more than 50 per cent of the measurements in two observers. The best reproducibility was observed for IAF (>90% of below 1 mm). The Bland–Altman plots were done for all landmarks using the x-, y-, and z-axis separately. Figure 2 shows an example of the plots for intra-observer analysis of observer 1 and the landmarks on the right side. More errors were observed on x-coordinates for all landmarks except the IAF-R. ZyMS showed similar reproducibility on x- and y-axis when compared to Or, but a poorer reproducibility for ZyMS was observed on the z-axis. Table 2. Intra-observer absolute difference of x-, y-, and z-coordinates of the landmarks in mm. IAF, internal acoustic foramen; L, left; Or, orbitale; Po, porion; R, right; SD, standard deviation; ZyMS-R, zygomatico-maxillary suture. Landmark Observer 1 Observer 2 Observer 3 x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Or-R 0.96 1.15 0.29 0.28 0.08 0.08 1.48* 1.24 0.51 0.29 0.15 0.12 1.32* 1.41 0.62 0.60 0.24 0.19 Or-L 0.99 1.24 0.38 0.49 0.08 0.07 1.03* 0.87 0.39 0.26 0.16 0.14 1.51* 1.72 0.58 0.69 0.22 0.20 Po-R 1.14* 1.02 0.32 0.27 0.20 0.20 1.38* 0.96 0.43 0.42 0.23 0.24 2.71* 1.83 0.62 0.46 0.48 0.44 Po-L 0.81 0.93 0.46 0.50 0.17 0.13 0.78 0.64 0.45 0.37 0.13 0.13 2.57* 1.96 0.80 0.43 0.86 0.69 IAF-R 0.12 0.11 0.19 0.19 0.16 0.12 0.13 0.11 0.25 0.19 0.33 0.25 0.16 0.16 0.22 0.17 0.29 0.23 IAF-L 0.16 0.24 0.18 0.14 0.30 0.29 0.11 0.08 0.32 0.29 0.25 0.18 0.16 0.23 0.24 0.16 0.34 0.26 ZyMS-R 0.80 1.16 0.38 0.31 0.25 0.36 1.32* 1.45 0.34 0.44 0.22 0.16 1.54* 2.16 0.49 0.58 0.48 0.50 ZyMS-L 0.86 1.23 0.32 0.31 0.28 0.45 1.33* 1.63 0.46 0.46 0.26 0.32 1.65* 1.80 0.43 0.45 0.46 0.54 Landmark Observer 1 Observer 2 Observer 3 x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Or-R 0.96 1.15 0.29 0.28 0.08 0.08 1.48* 1.24 0.51 0.29 0.15 0.12 1.32* 1.41 0.62 0.60 0.24 0.19 Or-L 0.99 1.24 0.38 0.49 0.08 0.07 1.03* 0.87 0.39 0.26 0.16 0.14 1.51* 1.72 0.58 0.69 0.22 0.20 Po-R 1.14* 1.02 0.32 0.27 0.20 0.20 1.38* 0.96 0.43 0.42 0.23 0.24 2.71* 1.83 0.62 0.46 0.48 0.44 Po-L 0.81 0.93 0.46 0.50 0.17 0.13 0.78 0.64 0.45 0.37 0.13 0.13 2.57* 1.96 0.80 0.43 0.86 0.69 IAF-R 0.12 0.11 0.19 0.19 0.16 0.12 0.13 0.11 0.25 0.19 0.33 0.25 0.16 0.16 0.22 0.17 0.29 0.23 IAF-L 0.16 0.24 0.18 0.14 0.30 0.29 0.11 0.08 0.32 0.29 0.25 0.18 0.16 0.23 0.24 0.16 0.34 0.26 ZyMS-R 0.80 1.16 0.38 0.31 0.25 0.36 1.32* 1.45 0.34 0.44 0.22 0.16 1.54* 2.16 0.49 0.58 0.48 0.50 ZyMS-L 0.86 1.23 0.32 0.31 0.28 0.45 1.33* 1.63 0.46 0.46 0.26 0.32 1.65* 1.80 0.43 0.45 0.46 0.54 *Greater than 1 mm. View Large Table 2. Intra-observer absolute difference of x-, y-, and z-coordinates of the landmarks in mm. IAF, internal acoustic foramen; L, left; Or, orbitale; Po, porion; R, right; SD, standard deviation; ZyMS-R, zygomatico-maxillary suture. Landmark Observer 1 Observer 2 Observer 3 x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Or-R 0.96 1.15 0.29 0.28 0.08 0.08 1.48* 1.24 0.51 0.29 0.15 0.12 1.32* 1.41 0.62 0.60 0.24 0.19 Or-L 0.99 1.24 0.38 0.49 0.08 0.07 1.03* 0.87 0.39 0.26 0.16 0.14 1.51* 1.72 0.58 0.69 0.22 0.20 Po-R 1.14* 1.02 0.32 0.27 0.20 0.20 1.38* 0.96 0.43 0.42 0.23 0.24 2.71* 1.83 0.62 0.46 0.48 0.44 Po-L 0.81 0.93 0.46 0.50 0.17 0.13 0.78 0.64 0.45 0.37 0.13 0.13 2.57* 1.96 0.80 0.43 0.86 0.69 IAF-R 0.12 0.11 0.19 0.19 0.16 0.12 0.13 0.11 0.25 0.19 0.33 0.25 0.16 0.16 0.22 0.17 0.29 0.23 IAF-L 0.16 0.24 0.18 0.14 0.30 0.29 0.11 0.08 0.32 0.29 0.25 0.18 0.16 0.23 0.24 0.16 0.34 0.26 ZyMS-R 0.80 1.16 0.38 0.31 0.25 0.36 1.32* 1.45 0.34 0.44 0.22 0.16 1.54* 2.16 0.49 0.58 0.48 0.50 ZyMS-L 0.86 1.23 0.32 0.31 0.28 0.45 1.33* 1.63 0.46 0.46 0.26 0.32 1.65* 1.80 0.43 0.45 0.46 0.54 Landmark Observer 1 Observer 2 Observer 3 x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Or-R 0.96 1.15 0.29 0.28 0.08 0.08 1.48* 1.24 0.51 0.29 0.15 0.12 1.32* 1.41 0.62 0.60 0.24 0.19 Or-L 0.99 1.24 0.38 0.49 0.08 0.07 1.03* 0.87 0.39 0.26 0.16 0.14 1.51* 1.72 0.58 0.69 0.22 0.20 Po-R 1.14* 1.02 0.32 0.27 0.20 0.20 1.38* 0.96 0.43 0.42 0.23 0.24 2.71* 1.83 0.62 0.46 0.48 0.44 Po-L 0.81 0.93 0.46 0.50 0.17 0.13 0.78 0.64 0.45 0.37 0.13 0.13 2.57* 1.96 0.80 0.43 0.86 0.69 IAF-R 0.12 0.11 0.19 0.19 0.16 0.12 0.13 0.11 0.25 0.19 0.33 0.25 0.16 0.16 0.22 0.17 0.29 0.23 IAF-L 0.16 0.24 0.18 0.14 0.30 0.29 0.11 0.08 0.32 0.29 0.25 0.18 0.16 0.23 0.24 0.16 0.34 0.26 ZyMS-R 0.80 1.16 0.38 0.31 0.25 0.36 1.32* 1.45 0.34 0.44 0.22 0.16 1.54* 2.16 0.49 0.58 0.48 0.50 ZyMS-L 0.86 1.23 0.32 0.31 0.28 0.45 1.33* 1.63 0.46 0.46 0.26 0.32 1.65* 1.80 0.43 0.45 0.46 0.54 *Greater than 1 mm. View Large Figure 2. View largeDownload slide Bland–Altman plots of the intra-observer analysis of observer 1 and the landmarks on the right side. The errors are shown in millimetre. Plots show the mean and ± 1.96 SD reference lines. Higher error values were observed on the x-coordinates for all landmarks except IAF-R. IAF-R, internal acoustic foramen right; Or-R, orbitale right; SD, standard deviation; Po-R, porion right; ZyMS-R, zygomatico-maxillary suture right. Figure 2. View largeDownload slide Bland–Altman plots of the intra-observer analysis of observer 1 and the landmarks on the right side. The errors are shown in millimetre. Plots show the mean and ± 1.96 SD reference lines. Higher error values were observed on the x-coordinates for all landmarks except IAF-R. IAF-R, internal acoustic foramen right; Or-R, orbitale right; SD, standard deviation; Po-R, porion right; ZyMS-R, zygomatico-maxillary suture right. Inter-observer reproducibility of landmarks Table 3 shows the inter-observer difference in x-, y-, and z-coordinates of each landmark. The inter-observer mean differences of IAF were less than 1 mm for x-, y-, and z-coordinates for all pairs of observers. Other landmarks showed differences of more than 1 mm mostly on x-coordinates. The inter-observer mean Euclidean distance for each landmark regardless of observers is shown in Supplementary Table 3. The inter-observer comparison revealed that IAF-R had a significantly lower mean Euclidean distance than all the other landmarks (P < 0.0018). Regarding inter-observer reproducibility, Po-R and Po-L showed 74 to 97 per cent of Euclidean distances more than 1 mm (Supplementary Table 4). The best reproducibility was observed for IAF, showing 59–95 per cent of Euclidean distances less than 1 mm. Table 3. Inter-observer absolute difference of x-, y-, and z-coordinates of the landmarks in mm. IAF, internal acoustic foramen; L, left; Or, orbitale; Po, porion; R, right; SD, standard deviation; ZyMS-R, zygomatico-maxillary suture. Landmark Observer 1/2 Observer 1/3 Observer 2/3 x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Or-R 0.83 1.47 0.22 0.42 0.22 0.15 0.02 1.25 0.46 0.57 0.21 0.18 0.84 1.54 0.24 0.55 0.01 0.14 Or-L 0.74 1.16 0.31 0.41 0.30 0.18 0.37 1.14 0.59 0.55 0.22 0.20 1.10* 1.42 0.28 0.53 0.08 0.19 Po-R 3.20* 1.66 0.23 0.53 0.07 0.23 1.45* 1.54 0.63 0.53 0.22 0.34 1.75* 1.55 0.85 0.70 0.15 0.37 Po-L 3.67* 1.85 0.90 0.63 0.20 0.32 0.98 1.68 0.33 0.62 0.25 0.61 2.69 1.94 1.23 0.82 0.45 0.67 IAF-R 0.09 0.13 0.58 0.26 0.08 0.21 0.01 0.14 0.17 0.22 0.11 0.20 0.08 0.16 0.74 0.39 0.04 0.20 IAF-L 0.03 0.20 0.55 0.31 0.02 0.27 0.05 0.20 0.04 0.21 0.02 0.24 0.08 0.21 0.50 0.34 0.00 0.28 ZyMS-R 2.22* 2.07 0.38 0.40 0.81 0.55 0.48 1.95 0.42 0.59 0.34 0.49 1.74* 2.01 0.04 0.39 0.47 0.43 ZyMS-L 3.16* 2.31 0.03 0.42 1.01* 0.64 0.37 1.58 0.49 0.40 0.23 0.33 2.79* 2.52 0.46 0.58 0.78 0.64 Landmark Observer 1/2 Observer 1/3 Observer 2/3 x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Or-R 0.83 1.47 0.22 0.42 0.22 0.15 0.02 1.25 0.46 0.57 0.21 0.18 0.84 1.54 0.24 0.55 0.01 0.14 Or-L 0.74 1.16 0.31 0.41 0.30 0.18 0.37 1.14 0.59 0.55 0.22 0.20 1.10* 1.42 0.28 0.53 0.08 0.19 Po-R 3.20* 1.66 0.23 0.53 0.07 0.23 1.45* 1.54 0.63 0.53 0.22 0.34 1.75* 1.55 0.85 0.70 0.15 0.37 Po-L 3.67* 1.85 0.90 0.63 0.20 0.32 0.98 1.68 0.33 0.62 0.25 0.61 2.69 1.94 1.23 0.82 0.45 0.67 IAF-R 0.09 0.13 0.58 0.26 0.08 0.21 0.01 0.14 0.17 0.22 0.11 0.20 0.08 0.16 0.74 0.39 0.04 0.20 IAF-L 0.03 0.20 0.55 0.31 0.02 0.27 0.05 0.20 0.04 0.21 0.02 0.24 0.08 0.21 0.50 0.34 0.00 0.28 ZyMS-R 2.22* 2.07 0.38 0.40 0.81 0.55 0.48 1.95 0.42 0.59 0.34 0.49 1.74* 2.01 0.04 0.39 0.47 0.43 ZyMS-L 3.16* 2.31 0.03 0.42 1.01* 0.64 0.37 1.58 0.49 0.40 0.23 0.33 2.79* 2.52 0.46 0.58 0.78 0.64 *Greater than 1 mm. View Large Table 3. Inter-observer absolute difference of x-, y-, and z-coordinates of the landmarks in mm. IAF, internal acoustic foramen; L, left; Or, orbitale; Po, porion; R, right; SD, standard deviation; ZyMS-R, zygomatico-maxillary suture. Landmark Observer 1/2 Observer 1/3 Observer 2/3 x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Or-R 0.83 1.47 0.22 0.42 0.22 0.15 0.02 1.25 0.46 0.57 0.21 0.18 0.84 1.54 0.24 0.55 0.01 0.14 Or-L 0.74 1.16 0.31 0.41 0.30 0.18 0.37 1.14 0.59 0.55 0.22 0.20 1.10* 1.42 0.28 0.53 0.08 0.19 Po-R 3.20* 1.66 0.23 0.53 0.07 0.23 1.45* 1.54 0.63 0.53 0.22 0.34 1.75* 1.55 0.85 0.70 0.15 0.37 Po-L 3.67* 1.85 0.90 0.63 0.20 0.32 0.98 1.68 0.33 0.62 0.25 0.61 2.69 1.94 1.23 0.82 0.45 0.67 IAF-R 0.09 0.13 0.58 0.26 0.08 0.21 0.01 0.14 0.17 0.22 0.11 0.20 0.08 0.16 0.74 0.39 0.04 0.20 IAF-L 0.03 0.20 0.55 0.31 0.02 0.27 0.05 0.20 0.04 0.21 0.02 0.24 0.08 0.21 0.50 0.34 0.00 0.28 ZyMS-R 2.22* 2.07 0.38 0.40 0.81 0.55 0.48 1.95 0.42 0.59 0.34 0.49 1.74* 2.01 0.04 0.39 0.47 0.43 ZyMS-L 3.16* 2.31 0.03 0.42 1.01* 0.64 0.37 1.58 0.49 0.40 0.23 0.33 2.79* 2.52 0.46 0.58 0.78 0.64 Landmark Observer 1/2 Observer 1/3 Observer 2/3 x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate x-coordinate y-coordinate z-coordinate Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Average SD Or-R 0.83 1.47 0.22 0.42 0.22 0.15 0.02 1.25 0.46 0.57 0.21 0.18 0.84 1.54 0.24 0.55 0.01 0.14 Or-L 0.74 1.16 0.31 0.41 0.30 0.18 0.37 1.14 0.59 0.55 0.22 0.20 1.10* 1.42 0.28 0.53 0.08 0.19 Po-R 3.20* 1.66 0.23 0.53 0.07 0.23 1.45* 1.54 0.63 0.53 0.22 0.34 1.75* 1.55 0.85 0.70 0.15 0.37 Po-L 3.67* 1.85 0.90 0.63 0.20 0.32 0.98 1.68 0.33 0.62 0.25 0.61 2.69 1.94 1.23 0.82 0.45 0.67 IAF-R 0.09 0.13 0.58 0.26 0.08 0.21 0.01 0.14 0.17 0.22 0.11 0.20 0.08 0.16 0.74 0.39 0.04 0.20 IAF-L 0.03 0.20 0.55 0.31 0.02 0.27 0.05 0.20 0.04 0.21 0.02 0.24 0.08 0.21 0.50 0.34 0.00 0.28 ZyMS-R 2.22* 2.07 0.38 0.40 0.81 0.55 0.48 1.95 0.42 0.59 0.34 0.49 1.74* 2.01 0.04 0.39 0.47 0.43 ZyMS-L 3.16* 2.31 0.03 0.42 1.01* 0.64 0.37 1.58 0.49 0.40 0.23 0.33 2.79* 2.52 0.46 0.58 0.78 0.64 *Greater than 1 mm. View Large The same trend as for intra-observer reproducibility was observed when plotting the Bland–Altman as more errors were seen on x-coordinates for all landmarks except the IAF-R (Figure 3). ZyMS showed poorer inter-observer reproducibility when compared to Or on the x- and z-axis. Figure 3. View largeDownload slide Bland–Altman plots of the inter-observer analysis comparing observer 1 to 2 of the right landmarks. The errors are shown in millimetre. Plots show the mean and ± 1.96 SD reference lines. Higher error values were observed on x-coordinates for all landmarks except IAF-R. IAF-R, internal acoustic foramen right; Or-R, orbitale right; Po-R, porion right; SD, standard deviation; ZyMS-R, zygomatico-maxillary suture right. Figure 3. View largeDownload slide Bland–Altman plots of the inter-observer analysis comparing observer 1 to 2 of the right landmarks. The errors are shown in millimetre. Plots show the mean and ± 1.96 SD reference lines. Higher error values were observed on x-coordinates for all landmarks except IAF-R. IAF-R, internal acoustic foramen right; Or-R, orbitale right; Po-R, porion right; SD, standard deviation; ZyMS-R, zygomatico-maxillary suture right. Angular measurements between planes A summary of absolute angular difference between each pair of planes is shown in Table 4. FH 1 and FH 2 exhibited 0.7 degrees overall angular difference (standard deviation [SD] of 0.58 degrees). Planes 3 and 4, both showed a mean angular difference less than 1 degree from FH 1 and FH 2, respectively. Furthermore, of Bland–Altman plots of the angular differences between each pair of planes are shown in Figure 4. Table 4. Mean absolute angular measurements with standard deviation (SD) between between FH 1 and FH2, FH 1 and Plane 1–6, FH 2 and Plane 1–6. FH, Frankfort horizontal plane. Plane (n = 8) Angular measurement Mean SD FH 1–FH 2 0.70* 0.58 FH 1–Plane 1 1.87 1.15 FH 1–Plane 2 2.50 1.13 FH 1–Plane 3 0.53* 0.37 FH 1–Plane 4 0.82* 0.54 FH 1–Plane 5 2.46 1.18 FH 1–Plane 6 1.95 1.10 FH 2–Plane 1 2.13 1.05 FH 2–Plane 2 2.21 1.11 FH 2–Plane 3 0.91* 0.56 FH 2–Plane 4 0.28* 0.27 FH 2–Plane 5 2.17 1.16 FH 2–Plane 6 2.16 1.06 Plane (n = 8) Angular measurement Mean SD FH 1–FH 2 0.70* 0.58 FH 1–Plane 1 1.87 1.15 FH 1–Plane 2 2.50 1.13 FH 1–Plane 3 0.53* 0.37 FH 1–Plane 4 0.82* 0.54 FH 1–Plane 5 2.46 1.18 FH 1–Plane 6 1.95 1.10 FH 2–Plane 1 2.13 1.05 FH 2–Plane 2 2.21 1.11 FH 2–Plane 3 0.91* 0.56 FH 2–Plane 4 0.28* 0.27 FH 2–Plane 5 2.17 1.16 FH 2–Plane 6 2.16 1.06 *The absolute angular difference less than 1 degree. View Large Table 4. Mean absolute angular measurements with standard deviation (SD) between between FH 1 and FH2, FH 1 and Plane 1–6, FH 2 and Plane 1–6. FH, Frankfort horizontal plane. Plane (n = 8) Angular measurement Mean SD FH 1–FH 2 0.70* 0.58 FH 1–Plane 1 1.87 1.15 FH 1–Plane 2 2.50 1.13 FH 1–Plane 3 0.53* 0.37 FH 1–Plane 4 0.82* 0.54 FH 1–Plane 5 2.46 1.18 FH 1–Plane 6 1.95 1.10 FH 2–Plane 1 2.13 1.05 FH 2–Plane 2 2.21 1.11 FH 2–Plane 3 0.91* 0.56 FH 2–Plane 4 0.28* 0.27 FH 2–Plane 5 2.17 1.16 FH 2–Plane 6 2.16 1.06 Plane (n = 8) Angular measurement Mean SD FH 1–FH 2 0.70* 0.58 FH 1–Plane 1 1.87 1.15 FH 1–Plane 2 2.50 1.13 FH 1–Plane 3 0.53* 0.37 FH 1–Plane 4 0.82* 0.54 FH 1–Plane 5 2.46 1.18 FH 1–Plane 6 1.95 1.10 FH 2–Plane 1 2.13 1.05 FH 2–Plane 2 2.21 1.11 FH 2–Plane 3 0.91* 0.56 FH 2–Plane 4 0.28* 0.27 FH 2–Plane 5 2.17 1.16 FH 2–Plane 6 2.16 1.06 *The absolute angular difference less than 1 degree. View Large Figure 4. View largeDownload slide Bland–Altman plots of angular differences between each pair of planes. The errors are shown in degrees. Plots show the mean and ±1.96 SD reference lines. Planes 3 and 4 resulted in the least differences among all pairs of new planes including the FH planes. FH, Frankfort horizontal plane; SD, standard deviation. Figure 4. View largeDownload slide Bland–Altman plots of angular differences between each pair of planes. The errors are shown in degrees. Plots show the mean and ±1.96 SD reference lines. Planes 3 and 4 resulted in the least differences among all pairs of new planes including the FH planes. FH, Frankfort horizontal plane; SD, standard deviation. Discussion The FH has been used as a reference plane that closely links to the NHP for decades (8). As 3D imaging modalities have been emerging in dental medicine and orthodontics over the last decade (10–13), still no research has been done to test its reproducibility by different clinicians in three dimensions. One study on the reproducibility of the FH plane on MRI images revealed excellent intra- and inter-observer reproducibility of the FH plane through 3D landmark identification (28). This finding is certainly of interest, but in clinical dental practice, MRI images are still not widely used for orthodontic treatment planning purposes. The methodology of this study was based on two previously published articles (29, 30). Although the dry skull samples were used without known age and gender, landmark identification in this study was not affected by the sample variability because landmark identification was unrelated to the dental status. The segmentation process of this study was based on previous published studies. Sub-sampling was done to prepare data to be appropriate for the software processing as recommended by the software (29, 30). The density threshold of 276 was used for all CBCT datasets. For scans acquired from other CBCT scanners, different thresholds may be applied as density numbers varies among different CBCT machines lacking standardization via Hounsfield units (33, 34). In the present study, new 3D landmarks that form planes intended to be parallel to the original FH were introduced and tested. Interestingly, the results of the present investigation showed poorer intra- and inter-observer reproducibility of the traditional FH landmarks (Po, Or) on the x-axis, and exhibited good intra- and inter-observer reproducibility of the new IAF landmark. On the other hand, Or showed superior results compared to the new ZyMS. Traditionally, the FH plane is defined on a 2D lateral skull or a lateral cephalometric radiograph by placing the Po and Or landmarks. There are two types of Po in 2D. One is the ‘machine Po’ when the landmark is defined by a radiopaque marker in the ear rod as part of the cephalometric head positioning device. The other is called the ‘anatomic Po’ which refers to the upper edge of the shadow of the auditory canal that can be seen on cephalometric films, (usually located slightly above and posterior to the ‘machine Po’) (4). The Or is defined as the inferior border of the orbital rim. In this study, the anatomic Po was used because no positioning device was placed in the external auditory meatus. In 3D, the anatomical structures were used instead of the 2D shadows. As described in the literature, the reproducibility or reliability of both landmarks (Po/Or) is limited (25–27). In this study, two new landmarks were proposed: IAF and ZyMS. The IAF is regularly used to identify the facial nerve in CT and MRI images (35). This landmark was chosen because of its good visualization, and because it relates with the Po as it refers to the other end of the auditory canal. The upper edge of the oval shape was not selected, but instead the lateral end was chosen and defined as a landmark because of its corner-like location. In CBCT scans of patients, this landmark is well visible, but it must be noted that this landmark will only be included when the diameter of the FOV of the CBCT is big enough to cover the cranial cavity. Therefore, a cylindrical FOV with 17 cm diameter was used in the present study. The ZyMS was selected as it is close to the traditional Or but might be more defined in location. A thin line or a notch on the inferior orbital rim is usually seen on the 3D model and can be identified as ZyMS, thus the observer did not have to identify the lowest point of the orbital rim, which is prone to subjectivity on 3D images. However, the short distance of between the ZyMS-R and ZyMS-L, about one third closer than Or-R and Or-L, may not appropriate to create a reliable reference plane. The results showed that the intra- and inter-observer reproducibility of IAF were significantly better than other landmarks, while Po showed poorer reproducibility. These findings were in agreement with previous publications (25–27). Ludlow et al. (25) compared the precision of cephalometric landmarks on lateral cephalograms and multi-planar reformatted (MPR) images of CBCT scans. It was reported that Po showed poorer precision on MPR images than other landmarks (25). In 2012, Schlicher et al. reported that the Po was the most inconsistent landmark and the Or was the most imprecise landmark from the study conducted on MPR images (26). In another study done by Hassan et al. (27), in which the precision of cephalometric landmark identification from CBCT data on both MPR and 3D models of 10 patients was evaluated, the authors reported that the poorest precision was that of the Po. The precision of Or was considered as moderate (27). In the present study, ZyMS showed poorer results than expected. This might have been caused by the process of creating 3D models. When the 3D surface model was smoothened, the ZyMS, which only appears as a notch, may not be visible for identification. The previous study done by Hassan et al. suggested using both, MPR images and 3D surface models to help locate the landmarks and thus to strengthen the reproducibility of the landmarks (27). This methodology should be further evaluated in future studies. The results of the present study also showed that most landmarks except IAF exhibited poorer reproducibility for x-coordinates (i.e. transverse plane). The results corresponded to results published by Titiz et al., and Hofmann et al. (36, 37). Titiz et al. have stated that for Po and Or, identification in the x-axis was difficult and Po also showed difficulty in the y-axis. In x-axis, Po-L and Or-L showed higher SD for the total reproducibility with 1.36 and 3.27 mm, respectively (36). As all landmarks are located on the hard tissue surface, the anatomy and morphology of the area of interest could be one of the key factors. To improve and minimize this variability, 3D definitions of landmarks must be clear and concise, especially landmarks that are located on a curvy surface such as Po, Or, and ZyMS. Two-dimensional definitions are based on shadows on conventional radiographs but when transforming to 3D, another dimension must be accounted for. In 3D cephalometry, it is recommended to modify 2D definition to a more robust location of the respective landmark. It was shown in a study by de Oliveira et al. that good definitions of landmarks could help improve observer performance and if a good operator training and calibration is followed (24). Using both MPR images and 3D surface models to help locate the landmarks should further strengthen the reproducibility of the landmarks (27). Another method to reduce the position variance of FH landmarks was published by Hofmann et al. (37). The authors investigated 3D reliability of the FH landmarks and used the nearby anatomical structures to help enhance their reliability. It was found that the presented adjacent landmarks improved the SD of Po and Or significantly (37). When trying to transform a 2D plane to a 3D plane, there are four landmarks involved because of the right and left side. Maxilim® software cannot connect all four landmarks together but can only connect three landmarks to generate a 3D plane; thus, a mid-landmark of one of the landmarks had to be calculated. By performing this method, two possibilities of a FH plane could be generated. FH 1 was drawn using Or-L, Or-R, and mid-Po. FH 2 was created using Po-L, Po-R, and mid-Or. The results showed that FH 1 − FH 2 presented a small angular difference (mean 0.7°, SD 0.58°). From the results of landmark reproducibility, the z-axis (i.e. vertical direction) showed low errors (Figures 2 and 3). This may imply that the high variability seen in the x-axis (and to a lesser extent in the y-axis) probably posed little influence on the angulation of the horizontal plane. Among the newly introduced planes, the plane that showed angles closest to the original FHs were Plane 3, connecting Or-R, Or-L, and mid-IAF, and Plane 4, connecting Po-R, Po-L, and mid-ZyMS (mean angular difference <1° from FHs). For Planes 3 and 4, only one of the three landmarks used to form these planes was the new mid-landmark (mid-IAF for Plane 3 and mid-ZyMS for Plane 4). Since Planes 3 and 4 share common points with the FH planes, it is self-evident that they have a higher possibility to deviate less from the original FH planes. It was observed that FH 2 showed slightly more deviation from Plane 3 than Plane 4 (Figure 4). The reason might be the difference in the composition of landmarks. FH 2 and Plane 4 shared to two common landmarks: Po-R and Po-L. The other landmark used in FH 2 was mid-Or and the other landmark used in Plane 4 was mid-ZyMS. Or and ZyMS were located close to each other, which posed a very small difference in distance and angulation. On the other hand, when comparing FH 2 and Plane 3, a slightly bigger difference was seen (Figure 4). The similarity between FH 2 and Plane 3 was Or (FH 2 used mid-Or, while Plane 3 used Or-R and Or-L). The other landmarks used in FH 2 were two Po’s, and the other landmark used in Plane 3 was mid-IAF. Po was located further from IAF, thus might give a slightly bigger difference in location and angulation. From the findings, the distance between landmarks is another crucial factor that may create errors in angulation. Landmarks with a short distance between each other may create more error when generating a plane. Although Plane 4 showed good results, the short distance between ZyMS-R and ZyMS-L might hamper the generation of a reliable horizontal reference plane. Plane 3 despite using the mid-IAF in place of the mid-Po was promising because of the superior reproducibility of IAF in z-axis when compared to Po. Therefore, the new horizontal Plane 3 might have potential to replace the traditional FH, but more evidence supporting these findings is necessary. An important critical concern regarding 3D cephalometry is the radiation dose. This technique requires a big FOV of the CBCT scan that includes the maxilla, mandible, and the skull base. Studies showed that the radiation dose was strongly related to the size of the FOV size, the specific CBCT machine used, and the scanning parameters (e.g. resolution) (15, 38–41). A small voxel size is usually not necessary for 3D surface models; thus, the radiation dose can be limited by choosing standard resolution parameters (e.g. voxel size of 0.4 mm). Nevertheless, case selection and good justification must always be applied according to published national and international guidelines and recommendations on the use of CBCT (42–46). Study limitations As this is an in-vitro study, the samples did not represent real patients. Absence of soft tissues may alter the images and effect segmentation results. The method used in this study only facilitated 3D surface models. On curved anatomical area, landmarks might be difficult to identify. Thus, in future studies 3D surface models should be used in combination with MPR images. This can increase the visibility of landmarks and increase the reliability of identification. Furthermore, 3D cephalometry depends heavily on software functions. The software used in this study allowed to record coordinate information, but did not offer any information about the angular differences between planes, nor the direction of the planes. Thus, only the angular values between planes were presented in the results. One further limiting factor might be the clinical background of the observers. In this study, two dentomaxillofacial radiologists and one oral and maxillofacial surgeon were assessing the images. It might be that measurements by trained orthodontists might have resulted in slightly different findings. Nevertheless, the radiologists and the surgeon all underwent a calibration session, and should be familiar with novel imaging modalities and cephalometric analysis. Clinical relevance Many studies have been conducted on cephalometric landmarks identification and the accuracy of this procedure. Studies on measurement errors and landmark identification generally set the clinical significant level at 1 mm, which means that errors below 1 mm should not influence clinical treatment (13, 17). According to the results presented, IAF has shown its potential as a novel landmark. Although this study evaluated the 3D reproducibility of the FH landmarks, the reproducibility is only one of many factors that contribute to a validation of new planes to replace the well-established FH. The FH resembles closely the NHP and FH can be identified directly on patients or photos without any radiation exposure, both clearly depicting the soft tissues, which is the most clinically relevant tissue of our patients. The results of this study have shown that angular differences between FH planes and newly proposed planes were averagely less than 1 degree, which may not be clinically significant. However, the relationship between newly proposed planes, which were based only on hard tissue landmarks, and NHP should thoroughly be studied before these planes can be implemented for daily clinical use. Conclusions The present study investigated the reproducibility of landmarks incorporated in a FH in three dimensions. 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The European Journal of OrthodonticsOxford University Press

Published: Sep 7, 2017

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