Digital Facebow: A Review

“Digital transformations”, “digitized workflows” and “technical developments”, these terms describe some of the game changers of the 21st century, both in social life as well as in dental medicine. The use of mobile devices, tablets and smartphones, and the easy access to technology and the World Wide Web, have changed the cultural habits of our society in general. It is not surprising that more advanced technologies and easy of its availability and accessibility are finding increasing application in daily life.¹

In dentistry, digital workflows are becoming more popular and more accurate in terms of any technique or providing accurate findings of research. However, some procedures are still being developed and not all the steps have been digitalized. Nevertheless, virtual environments constitute the present and future of the field to reduce the work overload of the dental professional and provide the best of the services.¹

Over the past few decades, the mechanical articulators used to simulate mandibular movements have now been replaced or supplemented with dental computer-aided design/computer-aided manufacturing (CAD/CAM) systems. These digital systems, consisting of 3-dimensional (3-D) CAD of dental casts, have improved design by introducing new materials, automating or reducing manual labor and providing higher profitability with better control of quality. Currently, several CAD/CAM systems provide a virtual articulator simulation. However, the main problem with these virtual articulators is transferring data from the patient to the simulation. Ultimately, the aim of this review been that very few methods are available in the literature to have a better understanding to the new technology avail for the transfer of the maxilla-mandibular relation in a virtual format, a cumulative information is to be provided.²

The first virtual articulator was developed by Szentpetery at the Martin-Luther University of Halle, Germany. The technique was based on a mathematical simulation of the mandibular movements that take place in an articulator. He described a technique for locating the maxillary and mandibular casts on the virtual articulator by using CAD/CAM technology.²

The second version was developed by Kordass et al, Gaertner and Kordass at Greifswald University in Germany. That articulator was designed to record the exact movement paths of the mandible by using an electronic jaw movement registration system called Jaw Motion Analyser (Zebris) and then to move digitized dental arches along those paths in the computer. With these tools, static and kinematic occlusal collisions could be calculated and visualized.²

The primary purpose of this article is to give a cumulative review about various techniques or methods stated till now regarding the digitalization of the facebow. Since there are very few methods given in literature, this article mentions the overview of those methods.

Review:

Eneko Solaberrieta et al gave a technique in which a single-camera photogrammetry method based on the structure-from-motion (SFM) photogrammetric scanning technology (SFM method) was used. This technique was divided into 2 phases. The phase one consist of obtaining photographs and transferring data. Here the maxillary and mandibular dental arches of the patient were scanned with an intraoral dental scanner (3Shape TRIOS; 3Shape A/S) to obtain digital casts (Figure 1). He used 3 adhesive targets onto the patient’s head. The first 2 points were marked next to the temporomandibular joints and the third point onto the infraorbital point (Figure 2). He used scannable elastomeric impression material placed on plastic colored facebow fork. He made 8 to 10 photographs by using a digital camera (Nikon D3200) and reverse engineering software to obtain the 3D spatial relationship of the shape of the head with target points related to the facebow (Figure 3). The images were uploaded into the software and 3-D geometry of the patient’s face was built with targets positioned on the facebow fork (Figure 4). The impression was scanned (Figure 5). By using reverse engineering software (Rapidform CADv2006;INUS Technology), the facebow fork 3D geometry was uploaded, and it was aligned to the maxillary digital cast by using the best-fit command (Figure 6).²

Phase two consists of alignment of 3-D facebow fork and impression-facebow fork. The software blended the different surfaces of the scanned maxillary digital cast into a single virtual cast, eliminated surface abnormalities, remeshed the organization of the triangulated mesh of points, and filled in the surface gaps that remained after data elaboration. The cranial coordinate system was created by using the 2 temporomandibular points and the infraorbital point, locating the maxillary digital cast on this reference system (Figure 7). The maxillary digital cast was transferred to the virtual articulator software, bringing the cranial coordinate system to coincidence with the virtual articulator’s coordinate system. The mandibular digital cast was located by scanning the virtual interocclusal record with an intraoral scanner in centric occlusion from 3 directions (left, right, and front). These scans were matched with the maxillary and mandibular digital casts by positioning the mandibular digital cast towards the maxillary digital cast in the virtual articulator in maximum intercuspation (Figure 8).²

Alexandru Petre et al also gave another technique using standardized extraoral photographs for the transfer of the digitized maxillary cast in exocad (exocad GmbH). The application exocad is one of the most widely used dental CAD-CAM applications including a virtual articulator module used by this technique. The patient was made to sit in upright position with the back straight and minimally or not supported. The patient was told to provide an exaggerated smile revealing as much as possible of the maxillary arch with slight interarch separation. The maxillary teeth were to be used as reference for indexing with the digital casts.³

The photo was cropped and was transferred to the DentalCAD application in such a way that only dentition was seen. The photo was cropped from the upper border of the upper lip to the lower border of the lower lip in vertical dimension and from corner of the mouth from both the sides in horizontal dimension. The virtual casts were obtained from a TRIOS 3 intraoral scan and were imported, at the same time as the standardized photographs were made as standard tessellation language (STL) files. The maxilla was resized so that the dimensions of the teeth are as close as possible to those of the background image.³

From the “articular jaw correction” window, the position of the virtual cast in manual mode was translated until the incisal edges of the maxillary teeth matched as accurately as possible to those of the background image and the virtual articulator was aligned with the background image. The hinge axis was marked on the skin using one of the standard methods with an appropriate pencil. A lateral profile photo was made with the smile in exaggerated form. The profile images serve for the antero-posterior (sagittal) positioning of the maxillary cast relative to the terminal hinge axis of the patient. All the photos were merged and were rotated to see that they coincided with the virtual facebow software. After the assurance with the proper orientation of the maxillary cast, the mandibular cast was oriented using intraoral scans in centric occlusion.³

Eneko Solaberrieta et al used one more technique to obtain the maxillo-mandibular relation in virtual format. Here he used an extraoral scanning device to locate the three extraoral points and three intraoral points of references. These points were transferred to the software. The intraoral points of reference were marked by using an articulating paper placed on the bite fork. The most prominent cusps were marked and were used as the intraoral points of references. The fork was then attached to the extraoral scanning device and the scan was made. With reverse engineering software (Rapidform CAD, v2006; INUS Technology, Inc, Seoul, Korea), the modelled geometry of the pointer was loaded and aligned with the partial scanned surface of the pointer in each position. With these 6 points (three extraoral and three intraoral) the patient’s cranial coordinate system was created. By scanning the 2 dental arches with an intraoral scanner in centric occlusion from 3 directions (left, right and front), the mandibular cast was located in relation to the maxillary digital cast on the software. ⁴

Junying Li et al investigated the trueness and precision of virtual facebow records using a smartphone as a three-dimensional face scanner (Figure 9). They made twenty repeated virtual facebow records on two subjects using a smartphone as a 3-D face scanner (Figure 10). For each subject, a virtual facebow was attached to maxillary arch and face scans were performed using a smartphone with a 3-D scan application (Figure 11). The subject’s maxillary arch intraoral scan was aligned to the face scan by the virtual facebow fork and the procedure was repeated 10 times for each subject (Figure 12). To investigate if the maxillary scan is located at the right position to the face, these virtual facebow records were superimposed to a cone-beam computed tomography (CBCT) head scan from the same subject by matching the face scan to the 3D face reconstruction from CBCT images (Figure 13). The location of maxillary arch in virtual facebow records was compared with its position in CBCT.⁵

Discussion

The technique presented by Eneko Solaberrieta et al earlier described the attempt at digitizing this process was based in reverse engineering techniques using an optical GOM industrial 3-D scanner, and all the steps were integrated into the digital workflow. Since its development, this procedure has been improved to minimize deviations. The primary advantage of this technique is that it works with any type of virtual articulator, thus generating a universal virtual facebow with patient’s information which can be transferred to any machining or sintering center in the world, resulting in greater flexibility and autonomy. In addition, this technique provides a digital copy of the patient’s face that is available throughout the diagnostic, planning and treatment phases. But because of its advanced machinery and software’s which are required, it becomes difficult to attain a proper orientation and co-relation and to make it available to mass population.²

The later technique given by them described a technique for locating the maxillary and mandibular casts on the virtual articulator by using CAD/CAM technology. By using this procedure, it is no longer necessary to obtain the measurements with the physical facebow, making an occlusion record and mounting the casts on the physical articulator.⁴

Alexandru Petre et al made use of 2-D images for digital smile design in dental CAD-CAM applications. The virtual facebow transfer was made practically possible in DentalCAD application based on standardized frontal and lateral photographs of the patient. The technique made the correct alignment of the digital maxillary cast to the virtual articulator with respect to the patient’s planes and the skin markings of the condylar axis. However, this method requires more of the research since 2-D photographs were used to orient the maxilla-mandibular relation in a 3-D format but ensured a cost- effective method too.³

Another technique given by Junying Li et al made use of smartphone 3-D scanner in virtual facebow transfer. The study showed that the maxillary position can be reproducibly recorded with an average trueness of 1.14 mm and precision of 1.09 mm suggesting that use of a smartphone 3-D scanner in the dental clinic is promising. Significant differences in several measurement between the two subjects were found, suggesting there were subject-dependent factors influencing the accuracy this workflow. However, the study showed some of the limitations. Only one smartphone (iPhone 11 pro) and 3-D scanner application (Hege 3D scanner) were tested. Secondly, the factors influencing accuracy were not explored by the researcher. Further studies should investigate the impact of different smartphones and 3-D scanner applications on the accuracy of virtual facebow transfer as well as directly comparing smartphones 3-D scanners with conventional facebows and industrial scanners.⁵

Conclusion

The virtual patient also known as a simulation created by superimposing varied three-dimensional images from an actual patient, is becoming a progressively popular aid in the field of digital dentistry by rendering scans possible of the face, teeth, oral soft tissue and even bony structures. One of the most crucial steps in the fabrication of a virtual patient includes pairing the maxillary cast alignment to the facial scan through the use of a virtual facebow technique.

However, the virtual patient technology currently available can only make use of a limited amount of actual patient data. In the future, a system, still to be fully developed, will need to integrate data on movement registration, occlusal records, digitalization, cast location, and 3-D face geometry into the 3-D virtual patient application. Furthermore research is required to ensure accuracy, trueness and precision of the maxillary and mandibular relation on a digital platform.