Three-Dimensional Cephalometric Landmark Annotation Demonstration on Human Cone Beam Computed Tomography Scans

The aim of this project is to provide a detailed methodology for the conduction of 3D cephalometric analysis with the use of a validated set of 3D skeletal landmarks. These landmarks cover the areas of the cranial base, face maxilla and mandible, and can be used in clinical as well as research studies. There is a learning curve for those who annotate the landmarks for the first time on the 3D volume, and a lot of practice is strongly recommended to acquire more experience and consistency before applying these landmarks on clinical and research studies.

The 3D cephalometric analysis offers more accurate measures in comparison to 2D cephalometric analysis. Because of the distortion and magnification of the anatomical structures in conventional x-ray films and 2D digital formats. 2D cephalometric analysis can lead to inaccurate tracings and interpretations.

Moreover, the coordinates of the 3D landmarks can be exported and used for the conduction of geometric morphometric analysis to study shape. The use of the 3D landmark data analysis can be part of an in-depth quantitative phenotypic approach"in the case of craniofacial deformities. In addition, the integration of morphometric analysis with functional analysis and genetic information will provide new knowledge in the case of craniofacial development in healthy as well as disease cohorts.

To begin, open the software Invivo6 and click on browse file"from this device, select the scan to be analyzed and click open. Go to the 3D analysis module and click on the save information floppy disk icon. Then select load a configuration"and browse for the configuration file.

Click on the reorientation icon. In the window that opens, select the by picking landmarks option. This allows the user to orientate all the scans in the same way.

For this protocol, nasion is selected as the original landmark. Then three point definition with landmarks, right orbitale, right porion, left porion, and define anteroposterior axis midsagittal plane with landmarks, nasion and bayesian are selected. Adjust the brightness and contrast to reduce the CBCT scan image noise from the menu on the left side of the screen.

Zoom in and out by holding down the control key and left clicking simultaneously and sliding across the screen. Move the image in a bodily manner by holding down the shift key and left clicking simultaneously, and sliding across the screen. Enable clipping from the settings menu to create sectional views in all planes of space.

To add new landmarks in the settings menu, click on landmarks"to reveal a list of available landmark options, and then select the landmark of choice. To set a default view for landmarks, select tracing task. Click on edit"choose landmark.

Set the view as desired, and click on use current view settings. Repeat these steps to change the default view for any additional landmark. To annotate 3D anatomical landmarks, select create tracing"from the top of the menu and click start.

Start annotating the 3D landmarks by left clicking directly on the 3D volume at the location where the landmark should be placed based on its definition. Adjust the location of the landmark using the section views on the right of the screen. In case they cannot be seen, from the layout selection menu select slice locator.

To confirm the placement of a landmark click stop"and select the desired view to visualize the landmark, then proceed to place the remaining landmarks. To change the position of the landmark using the 3D volume, stop the analysis by clicking on stop"at the bottom of the tracing tasks menu, click on the landmark point to be moved and drag it to the new desired position. Re annotate a landmark by double clicking on the ticked square next to the landmark, and then answering yes"to the follow-up question.

Start with the annotation of the preselected origin landmarks. The midpoint on the anterior border of the anterior curvature of the foramen magnum is called bayesian. To annotate bayesian, for the axial section, locate the deepest end of the curvature of the section of the foramen magnum.

For the sagittal section, look for the most posterior point of the midsection of the foramen magnum. For the coronal section, look for the inferior midpoint of the curvature of the foramen magnum. The most superior, posterior and external point located at the upper margin of each ear canal is called porion.

To annotate porion for the axial section, look for the edge of the margin of the external auditory meatus. For the sagittal section, look for the point of intersection of the Eustachian tube with the bony canal. For the coronal section, look for the midpoint on the inferior border of the superior curvature.

The vertical line through the point roughly bisects the ear canal. The landmark nasion is located at the intersection of the suture between the frontal bone and the nasal bones. For the axial section, look for the midpoint at the height of curvature of the suture.

For the sagittal section, look for the anterior point of the suture where the frontal and nasal bones meet. For the coronal section, look for the center of the frontal nasal suture. The vertical line through it roughly bisects the nose.

Orbitale is the most anteroinferior point on the inferior orbital rim. To annotate this, set the frontal view or bony window as default and clip axially from inferior to superior until the curvature of the lower margin of the orbit is reached, to locate the inferior most point of the lower curvature of the orbit. Use 2D views to confirm that the landmarks on the bone adjust the sagittal and coronal sections to reflect the anterior positioning of orbitale.

Ensure that the landmark is anterior in position just to the point where the orbital room starts to curve out. The most superior and anterior point of the superior orbital rim is called supraorbitale. To annotate it set the frontal view as default in the 3D volume, and gradually clip axially from superior to inferior until the curvature of the upper margin of the orbit is reached, to locate the most superior point of the upper border of the orbit.

Adjust the sagittal and coronal sections to reflect the anterior positioning of the landmark. Ensure that the landmark is anterior in position just to the point where the orbital room starts to curve out. For the cellar midpoint, look for the center of the sella turcica or hypophyseal fossa, which is a saddle shaped depression in the body of the sphenoid bone, where the pituitary gland or hypothesis is positioned.

Adjust the landmark to the center of the sella turcica in all planes. For the sagittal section, place the landmark in the center of the sella turcica. For the axial and coronal sections, adjust the views accordingly.

Next, trace the right mandibular profile with a series of points. Then do the same for the left mandibular profile. After tracing the mandibular profile, the landmarks condylar, gonion and antegonion are annotated automatically.

Identify the condylar as the most posterior and superior point on the condyle and the gonion as the most outward point on the angle formed by the junction of the ramus and the body of the mandible. Then identify the antegonion as the highest point of the concavity of the lower border of the ramus, where it joins the body of the mandible. The frontal and threequarter views of a full head CBCT scan of a human skull with all the annotated 3D landmarks included in the current configuration are presented in this figure.