Virtual Endodontics: Three-Dimensional ToothVolume Representations and their Pulp CavityAccess

Kleoniki Lyroudia, DDS, PhD, Georgios Mikrogeorgis, DDS, Panagiota Bakaloudi, DDS,Eleutherios Kechagias, DDS, Nikolaos Nikolaidis, DDS, PhD, and Ioannis Pitas, DDS, PhD

The purpose of this study was the application ofboth digital three-dimensional image processingand virtual reality techniques in endodontics.Three-dimensional volume representations of 2teeth from each tooth category, 16 teeth in total,have been reconstructed. All teeth were embeddedin polyester resin, and serial cross-sections0.75-mm thick were taken from each tooth by us-ing a special microtome. Each section was studiedunder a stereoscopic microscope, and its micro-scopic image was directly digitized using a video-camera. The surfaces of hard dental tissues weresegmented from each section. Semiautomaticalignment and frame interpolation were performedon the sequence of tooth sections by using appro-priate digital image processing techniques. Three-dimensional volume representations from eachtooth were achieved in this project to produce thefinal three-dimensional teeth models, on which vir-tual accesses of pulp cavities have been per-formed. Three-dimensional teeth volume repre-sentations and virtual tooth “drilling” could serveas perfect educational tools under certaincircumstances.

There is a great variety of tooth malformations, or dental anomaliesin general, in number, size, and form of teeth. Thus, the study ofinternal and external tooth morphology becomes of great interest,especially for an endodontist. Many different methods have beenused for the study of the tooth morphology. These methods includecasts preparation of the root canals with Wood’s metal (1), cellu-loid (2), polyester resin (3), decalcification of the teeth and dyeinjection (4), sectioning of extracted teeth (5), and roentgeno-graphic studies (6). All these methods have unavoidable pitfalls,and many difficulties can be encountered during their application.

The method of three-dimensional surface representation wasalso used, combining histological evaluation with computer

graphic technologies (7–9). The main advantage of this methodwas that one could study in detail the internal anatomy of the teethfrom different view angles.

This paper presents a new method for the study of tooth mor-phology based on three-dimensional, computer-aided volume re-construction. By using computer methods, three-dimensional vol-ume representations of 2 teeth from each tooth category, 16 teethin total, were made. On these volume reconstructions, virtualaccess of their pulp cavities was achieved.

MATERIALS AND METHODS

Two teeth from each tooth category were used in this study—16teeth in total. All these teeth were put in 3% NaOCl solution aftertheir extraction and were washed under running water and air-dried. They were then embedded in a two-phase polyester resin,and serial cross-sections were taken from each tooth by using aspecial microtome (Isomet, Buehler, Lake Bluff, IL). The thicknessof each section was 0.75 mm. Each section was studied under astereoscopic microscope (Stemy SV8, Zeiss, Germany), and itsmicroscopic image was directly digitized by using a videocamera.The surfaces of hard dental tissues (enamel, cementum, dentine)were segmented from each section. Semiautomatic alignment andframe interpolation were performed on the sequence of each toothsections by using appropriate digital image processing techniques.Three-dimensional volume representations were achieved to pro-duce the final three-dimensional teeth models, on which virtualaccesses of their pulp cavities were performed. The three-dimen-sional tooth models can be rotated in any direction in space.

Virtual dental bur simulators were created to achieve the pulpchamber virtual accesses. The user can produce a new, spherical,cylindrical, or cylinder-conical bur tool with determination of itsshape parameters. Alternatively, one can select a virtual bur toolshape from the ones already available commercially. The shapeand the size of the virtual burs were arranged according to the realcommercial dental burs that are available from Maillefer(Dentsply-Maillefer, Ballaigues, Switzerland). The bur tool wasthen applied to the appropriate point of the three-dimensional toothmodel by clicking on it with the mouse. Using this procedure, ahole having the shape of the bur tool was created in the three-dimensional tooth model and displayed on screen. The virtual tooth

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“drilling” is implemented as a series of successive morphologicalerosions of the three-dimensional tooth volume. The mouse posi-tion and move determines the drilling position and direction. Theprocedure is very fast and gives the visual impression of a realtooth drilling on screen. During the progress of the procedure of thevirtual tooth drilling, a red sign appears and an acoustic signalsound at the time the bur reaches the pulp cavity.

If a user (i.e. an inexperienced student) makes an error (e.g. pulpcavity penetration), he can undo his last drilling steps and continuein the right direction. This is impossible when drilling on real teethphantoms and has significant educational value. Furthermore, theuser can undo part of his work and continue with a differentdrilling approach or can store a partial virtual drilling job andcontinue another time.

All the above procedures (surface extraction, alignment, inter-polation, three-dimensional volume representation, and virtualtooth drilling) were performed by using EIKONA3D (Alpha TecLtd., Thessaloniki, Greece), a digital three-dimensional image pro-cessing package developed for Microsoft Windows (10). Thissoftware has a special module for virtual tooth drilling (11).

RESULTS

Three-dimensional volume reconstruction was achieved foreach tooth from each tooth category. Some results are shown inFigs. 1–4. Figures 1–3 show the three-dimensional volume repre-sentation of three maxillary teeth either from a lingual view (Fig.1A), palatal-incisal view (Fig. 2A), or distal angle (Fig. 3A). InFig. 4, the three-dimensional volume reconstructions of a mandib-ular tooth from different view angles is shown.

After that, a suitable bur was created (Fig. 5) and a virtual accessof the pulp cavity was achieved for each tooth. On these images(Figs. 1 to 4) one can observe in detail the entrances of the rootcanals of each tooth from different view points. This is a veryhelpful and interesting ability, especially for the study of the upperand lower molars. The computer software allows the user to selectthe bur he wishes to work with, either a spherical, cylindrical, orcylinder-conical shape. A drilling tool used for a pulp chamberaccessing can be seen in Fig. 5. Also, the user can view and savethe whole procedure during the virtual preparation through a framegallery, on which the virtual access of the pulp cavity is depicted

in detail (Fig. 6). Sample volumes of virtual teeth (original anddrilled ones) can be found in reference number 11.

DISCUSSION

A number of articles have been published on the application ofthe virtual reality technology in orthodontics (12, 13), restorativedentistry (14), orthognathic surgery (15), and implantology (16,17), with very encouraging results. This is the first effort to applythe virtual reality technology in endodontics.

One of the advantages of the method is that the external anat-omy of the tooth can be observed from different view angles and

FIG 1. (A) Three-dimensional volume representation of a centralmaxillary incisor. Lingual view. (B) Virtual access of the pulp cavityof the same tooth.

FIG 2. (A) Three-dimensional volume representation of a maxillarycanine. Palatal-incisal view. (B) Virtual access of the pulp cavity ofthe same tooth.

FIG 3. (A) Three-dimensional volume representation of a maxillarysecond molar. (B) Virtual access of the pulp cavity. The entrances ofthe mesial- and distal-buccal (C) and the palatal (D) root canals areseen.

FIG 4. (A) Three-dimensional volume representation of a mandibularfirst molar. (B) Virtual access of the pulp cavity. View of the distal (C)and the mesial (D) root canals’ entrances.

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each stage of accessing the pulp chamber can be followed. Thismakes the method a very interesting educational tool. It is quiteremarkable that the computer program used in this study gives theopportunity to the user to see each step of the tooth drilling andaccessing the pulp cavity. The few problems that have becomeobvious are focused mainly on the method used to take the toothsections. Unavoidably, some sections are deformed or destroyedand that makes more difficult the achievement of the exact volumerepresentation of the tooth. Nevertheless, the results aresatisfactory.

The virtual technique that was developed may eventually be-come a valuable tool for students, undergraduate and postgraduate,as well as for clinicians who desire to study in detail the first stepof an endodontic therapy. It is known that the right pulp cavityaccess of a tooth can contribute highly to a good endodontictherapy. Problems that have been established during virtual drillinghad to do with the lack of certain real senses. During the virtualpulp cavity access, the characteristic sense of fall that you feelduring the real pulp cavity accessing or the hemorrhage in case ofa pulpitis are lost, which makes the whole session more difficult.Nevertheless, the red sign that appears and the acoustic signal thatsounds when the virtual bur reaches the pulp cavity are helpful.

This work was funded by the Greek Ministry of Research and Technology,Research Grant PENED’99, No 1926.

Dr. Lyroudia is associate professor, Dr. Mikrogeorgis is PhD student, Dr.Bakaloudi is postgraduate student, Department of Endodontology, DentalSchool; Dr. Kechagias is electrical engineer and PhD student, Dr. Nikolaidis iselectrical engineer, and Dr. Pitas is professor, Department of Informatics,

FIG 5. View of the computer screen, where one can see a toothsection, the result of the virtual access of a pulp cavity, as well as thepossibilities that are available in this program for a spherical, cylin-drical, or conical tooth bur to achieve a virtual access of the pulpcavity.

FIG 6. Frame gallery of the virtual access of the pulp cavity of thetooth shown on Fig. 1.

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Aristotelian University of Thessaloniki, Greece. Address requests for reprintsto Kleoniki Lyroudia, 23, Papafi Str., 54638 Thessaloniki, Greece.

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