CREATION OF A VIRTUAL DENTAL PATIENT
The Virtual Dental Patient (VDP) application was designed to aid dentists in getting acquainted with the teeth anatomy, the handling of drilling instruments and the challenges associated with the drilling procedure. The VDP simulator allows the user to view/manipulate a 3-D head and oral cavity model constructed using anatomical data, adapt the model to the characteristics of specific patient using either facial photographs or 3D data, animate it using an MPEG-compatible facial animation player and perform virtual tooth drilling within the oral cavity using a Phantom haptic device to control the drilling tool. Drilling is performed on 3D volumetric/surface models of teeth, obtained from cross sections of real teeth. Apart from being used as a training tool for students, the system can also assist experienced dentists in planning a real tooth drilling intervention by getting familiar with the individual patient anatomy, identifying landmarks, planning the approach and deciding upon the ideal target position of the actual drilling activity.
In order to achieve realism, a head/oral cavity model comprised of anatomically meaningful 3D points taken on the various head tissues was constructed by using manual modeling techniques at the cost of increased modeling time. We have created such a 3D surface model consisting of 1392 3D points and 2209 triangles distributed among 11 different entities (nodes) using the publicly available anatomical (cryosections) and CT data of a male cadaver originated from the Visible Human Project (National Institute of Health (NIH), USA). The model includes the external face area and gums, palate, teeth, tongue, lips, cheeks, larynx and uvula, represented as triangular meshes. For modeling the face, we have used the CANDIDE – 3D face model and adapted it to the NIH data. Essentially, the constructed model provides an MPEG-4 compliant extension of the CANDIDE-3 towards both the oral cavity’s internal tissue surfaces (which are important for dental applications) and the head - neck outer surface.
The 3D face and oral cavity model incorporated in VDP.
Within the developed application, the user can adapt the generic facial mesh model to the characteristics of a specific person (personalization), using a semi-automatic approach that relies on two photographs of the person taken from two perpendicular directions, i.e. a frontal one and a side one. In order to proceed in the adaptation of the mesh on the photographs, user interaction along with a real-time 2D FEM method is used. Essentially the model’s mesh is represented as a system of springs that can be deformed through user interaction. The adaptation is implemented by having the user moving (by using the mouse) a certain number vertices of the model’s mesh, to the corresponding points in the picture. Subsequently, the FEM moves the remaining vertices of the mesh (i.e. those that have not been displaced by the user) move too. An alternative procedure that adapts the model to 3D facial data has been also developed.
Model adaptation using two facial photographs.
In addition, the user can replace at the 3D Oral Cavity model the limited resolution prototype of a certain tooth where virtual drilling will be performed with a more detailed model. For this operation, the user provides the volumetric model of the tooth in the form of a series of slice images. A database of such volumetric models for all types of teeth (canines, molars, premolars etc) is available. This database has been constructed by digitizing and post-processing (alignment and segmentation) physical cross sections of extracted teeth, viewed through an optical microscope.
For animating the 3D head model, compatibility with the MPEG-4 is provided. More specifically, the application includes a FAP player that reads FAP files from disk and animates the model by moving the corresponding MPEG-4 FDPs.
Haptics Enabled Tooth Drilling Simulation
The Phantom Desktop (SensAble Technologies Inc.) force-feedback haptic device has been used for performing virtual tooth drilling within the face and oral cavity model. Using this haptic device, the user can proceed in drilling, sensing contact/resistance the stylus of the haptic device controls the position of the dental bur during the material removal operation, the application makes use of the internal dual surface/volumetric representation of the tooth. Removal of material during drilling is implemented as a series of morphological operations on the volumetric (voxel-based) representation of the tooth.
Virtual tooth drilling in the 3D face and oral cavity model.
Virtual tooth drilling using the Phantom Desktop haptic device.
Surface and volumetric representation of a tooth drilled using the virtual drilling procedure.
Other Applications – Future Work
Apart from dental drilling simulation, the generic head and oral cavity model that has been created can be used in a multitude of applications that include creation of functional, animated virtual patient heads for use in medical education and research (e.g. in telemedicine), model-based segmentation of head volumetric data, research and education related to speech articulation and its pathologies, creation of talking heads that incorporate oral cavity information (useful in close-ups and extreme close-ups) as well as for model based coding and transmission. In the future the head model will be extended to cover additional anatomical structures of the head and its use in some of the above applications will be pursued. Methods for realistic head model animation are also under investigation.
L. Papaleontiou, N. Nikolaidis, K. Lyroudia and
· Moschos G., Nikolaidis N. and Pitas I., “Anatomically-Based 3D Face and Oral Cavity Model for Creating Virtual Medical Patients” , IEEE ICME 2004, Taipei, Taiwan, June 2004.