Median Lingual Foramen, a new midmandibular cephalometric landmark

Abstract Purpose In asymmetrical mandibles, it is often challenging to identify the mandibular midline. The median lingual foramen (MLF) is located at the midline of the anterior mandible. The purpose of this study is to evaluate the reproducibility of identifying the MLF compared to conventional landmarks on cone beam computed tomography's (CBCT's) to mark the mandibular midline. Material and Methods Ten symmetrical class II, 10 symmetrical class III, ten asymmetrical class II and 10 asymmetrical class III patients were included. On CBCTs, the cephalometric landmarks menton, pogonion, genial tubercle and MLF were identified twice by two observers. Results A high intra‐ and interobserver reproducibility was found for all landmarks, the highest being the MLF. The gain in accuracy is 0.998 mm, 0.824 mm and 0.361 mm compared to pogonion, genial tubercle and menton, respectively (P‐value <.05). Conclusion MLF is a reliable and reproducible landmark to indicate the midline of the mandible, particularly in Class II asymmetric mandibles.


| INTRODUC TI ON
Surgical correction of mandibular asymmetry is challenging in the field of orthognathic surgery. The aberrant shape as well as the malposition of the mandible results in multiplanar facial asymmetries. 1 The emergence of three-dimensional (3D) cone beam computed tomography (CBCT) has enabled orthognathic surgeons to diagnose maxillofacial deformities and to allow a more accurate pre-operative planning. [1][2][3][4][5][6] Whilst planning an operation, cephalometric landmarks are used to determine the morphology of the facial skeleton. The traditional landmarks to indicate the midline of the mandible include pogonion, mention and genial tubercle. The reliability and reproducibility of conventional cephalometric landmarks were validated on patients without a clinically significant asymmetry of the mandible. 3,[7][8][9][10] In asymmetrical cases, the landmarks are more difficult to identify.
Mirroring is frequently used to superimpose one hemimandible upon the other in order to analyse the severity of mandibular asymmetries and calculate the difference in volume. It provides accurate guidance throughout the surgery, minimalizing the residual asymmetry. 11,12 The most challenging issue in mirroring is the selection of the mirroring plane, or the midline of the mandible. 4,13 Whilst analysing CBCT data in our daily practice, a distinct anatomical structure was noticed in the mandibular midline of the symphysis region, which seemed to be universally present. Upon reviewing literature, this anatomical landmark was named the median (or midline) lingual canal (MLC), which had previously been describes. [14][15][16][17] Inferior to the apices of the lower central incisors and in some cases just superior to the genial tubercles, a neurovascular bundle perforates the lingual cortex of the mandible, called the medial lingual foramen (MLF). Even though the vertical location of the MLF varies, it is always located at the midline of the mandible on the transverse plane. Therefore, the MLF has the potential to be used as a landmark for patients with asymmetrical mandibles. 18 The aim of this study is to evaluate the reproducible and reliability of MLF as a cephalometric landmark compared to traditional landmarks.

| Imaging methods and set-up of reference frame
Of all patients, a cone beam CT (CBCT) of the entire mandible was available, acquired using standardized CBCT scanning protocols, First a reference frame was set-up using the landmarks sella and nasion to reconstruct the horizontal, median and vertical reference planes. 10 Also, three axes were set-up, having the X-axis directed from left to right, the Y-axis from front to back and the Z-axis from cranial to caudal.

| Landmark identification
The traditional cephalometric landmarks pogonion, menton and genial tubercle were identified by the observer. The definition of the landmarks and cephalometric planes used are shown in Table 1.
The new landmark MLF was subsequently identified according to a well-defined three-step procedure.
MLF is visualized by scrolling through the axial slices in a cranio-caudal direction. In the region below the apices of the inferior incisors, a small radiolucent canal is present perforating the lingual cortex ( Figure 1A). When two or more canals were present, the more superior canal (MLFsu) was used. The MLF landmark was plotted on the most cranial slice, on which the lingual cortex showed an irregular form ( Figure 1B).
The sagittal slice that dissected the plotted MLF was selected to maintain its position in the medio-lateral direction, that is, the x-axis.
The MLF landmark is replotted at the intersection between the lingual cortical bone of the mandible and the most cranial bone surrounding the radiolucent canal. In this way, the position of the MLF on the y-and z-axis can be determined. To aid the identification of MLF in this final step, a line can be drawn connecting the lingual cortical bone cranially and caudally to the radiolucent canal ( Figure 1C).
Finally, the MLF landmark is checked on the corresponding coronal slice ( Figure 1D). In case the correct position was questioned, the three-step procedure was repeated.

| Statistical analysis
The statistical data analysis was carried out with the SPSS software program, version 22 for windows (SPSS Inc) by a professional statistician. The discrepancy between the observers in plotting landmarks was calculated using the 3D Euclidean distance, which represents the distance between two points in space (three dimensions). The Euclidean distance between two landmarks for example A1 (Xa1, Ya1, Za1) and A2 (Xa2, Ya2, Za2) was calculated with the

| RE SULTS
An intra-and interobserver reliability of 0.978 or more was calculated for all cephalometric landmarks, indicating a steady and predictable way of plotting by the observers.   In one patient, the MLC was visible on the axial slice, but not on the sagittal slice, due to small diameter of the foramen. In another F I G U R E 2 Scatterplots showing the interobserver agreement in the laterolateral dimension (X-axis) for Median Lingual Foramen, Menton, Pogonion and Genial Tubercle. The closer the dots situated to the diagonal, the higher the agreement patient, multiple exostoses on the lingual side of the mandible, in combination with dense cancellous bone, caused difficulties in identifying the location of the neurovascular bundle. Despite these inconveniences, we were able to plot MLF in every patient by using two different CBCT scanners and scanning protocols.

| D ISCUSS I ON
The interobserver discrepancy was smallest for MLF compared to the other traditional landmarks. This increase in accuracy is believed to be explained by the use of multiplanar CBCT slices instead of using a 3D rendered surface model. 9 In addition, MLF is based on a small and distinct anatomical structure instead of a broader surface, its identification is less influenced by the view angle and the magnitude of mandibular asymmetry.
A greater increase in accuracy in the identification of MLF was found among patients with mandibular asymmetry. As the location of a midmandibular landmark in the x-axis is the most clinically relevant among patients with mandibular asymmetry, the use of MLF would ease the pre-operative planning and post-operative evaluation of orthognathic surgery, as it is able to indicate the true anatomical centre of an (asymmetric) mandible. 1,13,[18][19][20] Further studies might be required in order to assess the correlation between MLF and different jaw deformities. In addition, the MLF might be helpful in the classification of different mandibular asymmetries.

| CON CLUS ION
The present study demonstrates that the new anatomical cephalometric landmark MLF can be identified in a more accurate, easier and reproducible way compared to conventional midline cephalometric landmarks.

ACK N OWLED G EM ENTS
A special thanks to Ewald Bronkhorst for his contribution to the statistical analysis of data.

CO N FLI C T O F I NTE R E S T
The authors have no conflict of interest to declare.