Innovative 3D Dental Measurement for Tooth Model Restoration

This research develops an innovative free-form surface scanning system using laser triangulation for 3D dental data required for crown reconstruction. This novel design employs double laser diodes to produce two 45° structured-light lines projecting onto the plaster tooth models and three CCD cameras to capture deformed fringes to achieve fast and accurate 3D surface measurement of plaster tooth models. Effective strategies were implemented to overcome problems such as potential measurement occlusion and data registration inaccuracy, commonly encountered by other data scanning methods. The developed system has distinctive features such as laser projecting angles for complete surface measurement coverage, digitizing accuracy, and compact scanner volume for potential applications on 3D surface digitization of tiny industrial components. Experimental results verified that the proposed system achieves a 20μm digitizing accuracy and possesses fast scanning capability. Maximum one minute is used for a single-tooth model and 30 minutes are used for scanning the whole jaw. Introduction In current dental practice, full-crown or partial tooth restoration process is not only complex and time-consuming, but also inaccurate and error-prone. This is mainly due to the lack of an efficient scanning technology for rapid acquisition of accurate 3D dental geometry. The conventional practice is that the dentist first copies jaw impressions from the treated shape of the decayed tooth, which are then sent to a dental technician for casting a plaster model as a topological copy for 3D reconstruction of the tooth. In the last decade, some computer-aided design/computer-aided manufacturing (CAD/CAM) systems have been introduced to create 3D tooth models for direct computer numerical control (CNC) machining. A dental scanner for performing accurate 3D data acquisition has become extremely important for the dental industry. Figure 1 displays the traditional crown fabrication process. First, the dentist makes impressions of both the lower and upper jaws after preparing the decayed tooth. These impressions are then sent to a dental technician for casting a plaster model of both jaws. These models serve as the basis for fabricating wax models, which are then used in precision casting of the tooth crown made of Titanium or other materials. After the green part of the casting is polished, a crown is finally fabricated and ready for a dental clinical restoration. However, due to the error-prone fabrication process, the dentist has to make some final changes to ensure an adequate fit. This unavoidably increases potential risks of failure and incurs great cost. In the last decade, some CAD / CAM systems have been introduced for crown production. Shown in Fig. 2, the new generation CAD/CAM crown fabrication process deploys a 3D scanning technique for topological data. Tooth plaster models can be scanned by various measurement strategies. The CAD model is created from the measured data and is then deployed for tool path generation in CAM machining. Advances in technology have enhanced the overall efficiency of tooth restoration. However, the shortcomings of the conventional procedures such as time-consuming scanning, inaccurate measurement and extensive model preparation are still Key Engineering Materials Vols. 295-296 (2005) pp 145-150 online at http://www.scientific.net © (2005) Trans Tech Publications, Switzerland Online available since 2005/Oct/15 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 130.203.133.33-17/04/08,19:07:03) restricting the full development of the CAD/CAM crown fabrication. However, many of these drawbacks may be overcome by a promising 3D dental scanner. Fig. 1 Traditional crown fabrication process Fig. 2 New generation CAD/CAM crown fabrication process deploying 3D scanning technique for model topological data Literature Review The measurement principles of 3D dental scanners mainly include laser holography, laser triangulation, passive stereo detection, mechanical contact sensing, structure lighting for phase shifting and X-ray computer topography. Altschuler first developed a 3D dental scanner using holographic imaging in 1975 [1]. Two orthogonal raster patterns were generated by two laser light sources and produced a 2D interferometric fringe to be calculated into the depth information with a spatial resolution of 25 μm. Following this, in 1988, Duret employed a similar principle, laser holography, to detect tooth shape [2]. To increase the accuracy of shape measurement within the occluded region, the multi-individual views were recorded and correlated to generate 3D profiles. However, due to its complex alignment procedure, the implemented system was not widely utilized for dental restoration. Apart from using laser holography, stereo depth detection was employed by Rekow in 1988 [3]. The stereo-paired images acquired by a high-resolution camera were matched to determine the 3D information. This method provided a good alternative to the other approaches since only inexpensive digitizing equipment was required and its measurement was of intra-oral type. However, due to the difficulty in detecting the occluded surface sections of tooth shape, it was not adopted for commercial use. Following the improvement in digitizing accuracy, mechanical tactile probes were deployed for dental surface digitization [4]. The measurement accuracy could reach few micrometers, but the scanning efficiency could be reduced due to tedious mechanical contacts. To overcome this limitation, various kinds of laser triangulation scanning, including different light-pattern projection of points, lines and spatially encoded diagrams, were developed to increase the scanning efficiency while maintaining acceptable measurement accuracy [5]. In these approaches, multi-laser heads and scanning strategies were used to avoid encountering any potential blind region when digitizing an occluded surface section. However, no scanning system is yet capable of delivering a convincing Impression Taking Model Casting Scanning 3D Plaster Model Data Preparation Crown Machining (CAM) Plaster model Crown Model Reconstruction (CAD) Impression Taking Model Casting Coping Fabrication Crown wax model Crowns Plaster model Casting 146 Measurement Technology and Intelligent Instruments VI