Introduction

CAD/CAM techniques are increasingly used in dentistry for the design and fabrication of teeth restorations. The functionality of existing systems [DBD88] [Hof90] [Rek91] [vdZ93] usually fulfills the requirements for designing and manufacturing onlays, inlays, crowns, bridges and partial dentures (i.e., restorations), but efficient use of these systems involves extensive training and technical knowledge not related to dentistry. This distracts the dentist from the treatment of the patient. Hiring a trained operator helps, but significantly increases the cost of treatment.

Virtual reality technology is already applied in medical areas where treatment costs are high or where it enables to do new treatments and more safe operations (e.g., neurosurgical planning). Virtual reality extends traditional user interfaces because it simplifies navigation in 3-D space, as is often required in medical applications. In the past, the high costs of virtual reality systems could not be justified for use in dentistry, where the costs of a single treatment are relatively low. However, increasingly affordable VR systems encourage their development as a standard user interface technique in dental systems and other medical applications.

In traditional dentistry, mechanical articulators are used for design of dental restorations and articulation diagnostics. Most computer-based dental care systems [Hof90] [Rek91] poorly model the occlusal surface of restorations, because jaw articulation is ignored. In other systems an analysis of contacts/collisions between jaws is simplified, and speed of calculations is far from interactive. The quality of reconstructed occlusal surfaces is important because restoration materials used in modern dentistry are very hard (e.g., porcelain) and manual corrections of the restoration are difficult.

In this paper we present a new virtual reality user interface for simulating jaw articulation and editing of the occlusal surface of teeth, developed as a part of the ongoing project ``Intelligent Dental Care System''. The interaction with the dentist is based on metaphors familiar from everyday practice.

This paper also presents a method for detecting interference between existing teeth and restoration by articulation simulation [KHM94]. Collisions and contacts can be detected staticly for jaw positions chosen by the dentist and dynamically for lower jaw movements. Also, the load distribution on the teeth is calculated [KMO95], providing the dentist with information not available with traditional techniques. The interferences detected during articulation simulation and the load distribution on the teeth are used for the automatic adjustment of the shape of the occlusal surface of restoration. Articulation simulation and resulting shape adjustments are repeated in a loop until collisions are eliminated and the load on restored and opponent teeth meets the imposed requirements. The dentist can break this design loop at any stage, interactively edit the occlusal surface, and resume interference checking.

Motion of the mandibular jaw is simulated using 3-D measurement data, obtained using the MM-JI-E system developed at Tokushima Bunri University and Tokushima University. At present we use a mechanical scanner to measure the surfaces of the upper and lower jaws based on a plaster model.

The paper is organized as follows. In the next section we discuss our algorithm for collision detection and automatic removal of interferences between teeth and restorations. Then we present our virtual reality approaches for interactively editing the occlusal surfaces of teeth. Section 4 describes our visualization techniques of jaw articulation, motion trajectories and teeth load during grinding. In Section 5 we propose a treatment report generator which produces hypertext including virtual reality models. Section 6 gives some implementation details. Finally, we conclude and propose some directions for further research.