Measurement Techniques, Vol. 53, No. 3, 2010
MEDICAL AND BIOLOGICAL MEASUREMENTS MODERN DENTAL CAD/CAM SYSTEMS WITH INTRAORAL 3D PROFILOMETERS
G. G. Levin,1 G. N. Vishnyakov,1 K. E. Loshchilov,1 T. I. Ibragimov,2 I. Yu. Lebedenko,2 and N. A. Tsalikova2
UDC 53.082.54
Modern CAD/CAM dental systems that incorporate an intraoral 3D profilometer are considered. The operating principle of the device and its functionality features are described. The first domestically produced intraoral 3D profilometer for a CAD/CAM dental system, which was designed at the All-Russia Research Institute of Optophysical Measurements, is presented. Key words: intraoral 3D profilometer, CAD/CAM dental system, Fourier transform profilometry, fringe pattern projection.
With the development of computer technologies, CAD/CAM (Computer-Aided Design/Computer-Aided Manufacturing) systems are now finding ever greater use in dental practice and have become popular tools among dentists. This has been due to the objective advantages gained with the use of such systems as compared with traditional therapy. The process of producing dental restorations with the use of a CAD/CAM dental system may be divided into three stages: 1) measurement of the form of the three-dimensional surface of the prepared tooth; 2) computer simulation of the repair process on the basis of the results of measurments; and 3) computer-aided manufacture of the dental restorations on a numerically-controlled machine tool. In dentistry, CAD/CAM systems are of two types, distinguished by the method used to measure the 3D profile of the surface of the prepared tooth: • photographing performed within the patient’s tooth cavity by means of an intraoral scanner; • measurement of the surface by means of a stationary (tabletop) scanner with gypsum model of the prepared tooth, preliminarily manufactured by hand. Below, we will consider CAD/CAM dental systems that use an intraoral 3D scanner. The first such system for computer-aided dental restoration, which has since entered into widespread use, is the CEREC system. The system was developed by Mörmann and Brandestini at the firm of Siemens AG, since 1997 the Sirona Dental System GmbH. The CEREC system constitutes an automated dentistry work station for restorative dentistry. It consists of the following components: 1) intraoral measurement camera, enabling contactless precision optical measurements of the three-dimensional surface of the tooth directly in the patient’s mouth; 1 2
All-Russia Research Institute of Optophysical Measurements (VNIIOFI), Moscow, Russia; e-mail:
[email protected]. Moscow State Dentistry University, Moscow, Russia; e-mail:
[email protected]. Translated from Izmeritel’naya Tekhnika, No. 3, pp. 52–54, March, 2010. Original article submitted December 29,
2009. 0543-1972/10/5303-0321 ©2010 Springer Science+Business Media, Inc.
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Fig.1. Intraoral 3D profilometer for a CAD/CAM dental system.
2) software complex for computer simulation of repairs on the basis of the results of measurements performed in several frames; and 3) a small-scale processing center for precison micro-milling of a complex form made of special ceramic material that has been modeled on a computer. The CEREC system may be used to manufacture dental work right in the dentist’s office in the patient’s presence. The time needed to measure the three-dimensional profile of the tooth in the patient’s mouth amounts to several seconds, while the time needed to process the ceramic blank on the latest model of the MC XL milling machine takes at most 6 min. The CEREC prototype series also incorporates a CEREC InLab stationary optical scanner for dental laboratories and three modifications of the system with an intraoral camera (CEREC-1, CEREC-2, and CEREC-3). The latest innovation consists of a computer program that enables computer simulation of repairs in 3D space. Projection of bands is used to measure the form of the tooth’s 3D surface in the CEREC intraordal camera [1]. Blue light from a light diode is used in the latest modification of the CEREC Bluecam camera instead of infrared light. To obtain a good contrast of the bands, the tooth which is to be measured must be tinted with a special diffusely reflecting, finely dispersed white powerder. Reconstruction of the 3D surface is performed by means of a phase step algorithm, hence at least four images of the tooth must be recorded. A shift in the bands occurs due to the movement of the photographic transparency by means of a piezoelectric drive. A half-tone image is obtained once the tooth is illuminated through this transparency, though with rapid to-and-fro motion opf the transparency. In 2004, information appeared to the effect that the firm of D4D Technologies, L.P. (United States) had begun development of its own CAD/CAM system. The Evolution 4D Dentist system that has now been created consists of an intraoral 3D scanner [2], a stationary optical scanner, computer software for 3D simulation of dental reparations, and a numerically controlled machine tool for manufacturing dental work. A method of pointwise triangulation by means of a fine laser beam is implemented in this firm’s intraoral 3D scanner. A double-coordinate optical scanner created according to the MEMS technology is used to enable passage of the beam along the entire surface of the of the prepared tooth. The firm emphasizes that its high-speed laser technology for creating 3D images of hard and soft tooth tissues does not require preliminary tinting. In 2006, the Israeli firm of Cadent Ltd. began work on the creation and improvement of its own CAD/CAM system, iTERO, which also comprises a 3D camera [3]. The iTero system is designed to measure the profile of the surface of a tooth by a dentist in the clinic and transmission of a digital 3D model by means of the Internet to the dental technician in a central treatment center. The method of confocal microscopy using the Nipkov rotating disk is implemented in the iTero intraoral 322
Fig. 2. Doubly foreshortened illumination diagram: 1) lattice; 2) recorder; 3) optical axis of recording system; 4) optical axis of projecting systems; 5) object.
camera. The firm claims that the iTero 3D camera does not require tinting of the measurement object in order to obtain photographs, hence it is suitable for photographing articles made of any type of material whatsoever. In 2008, the firm of Hint-ELs GmbH (Germany) declared it had created its own intraoral 3D scanner, called directScan, which it had developed with scientists from the Fraunhofer Institute of Applied Optics and Precision Engineering (IOF, Germany). A prototype of the camera is described in [4]. The method of phase correlation in combination with stereophotogrammetry and the band projection method are implemented in the 3D camera. The camera’s optical circuit contains a single projecting and two reflecting stereo channels. The steps of forming and shifting of the bands in the projecting channel are performed by means of an LCOS (liquid-crystal-on-silicon) digital space-time light modulators illuminated by the radiation from a blue light diode. Images of the tooth in two different foreshortenings in two halves of the same CCD array. A total of eight images, created over a rather lengthy interval of time, aournd 240 msec, is recorded. The first domestically produced measurement intraoral 3D camera (Fig. 1) for a CAD/CAM dental system has been created at the Holography and Optical Tomography Research Division of the All-Russia Research Institute of Optophysical Measurements (VNIIOFI). The basic specifications for the development of the device are as follows: • error in measurement of the form of the surface of the object at most 40 μm; • overall dimensions to assure free access of the profilometer inside the cavity of the patient’s mouth; • short measurement time, which must preclude any influence of tremor of the dentist’s hand; and • high ergonomic characrteristic. A method of quadruply foreshortened projection of parallel bands in conical beams is used in the intraoral 3D camera. This makes it possible to simplify the optical circuit and maintain low overall dimensions with the use of multiply foreshortened illumination of the object. Multiply foreshortened illumination and the combined processing of the images obtained by a novel method of Fourier synthesis [5, 6] assure a higher spatial resolution of the measured surface of the tooth than is possible with the use of the singly foreshortened method [7] based on a Fourier transformation, as well as making it possible to automatically discover regions in the image of the tooth with shadows and low band contrast. The optical circuit of the intraoral 3D camera consists of four identical projecting (illuminating) channels and a single recording channel. As an example, the circuit used in the illumination of an object with two projecting channels and a single recording channel is presented in Fig. 2. The optical axis of all the channels converge in the optimal focusing plane. The projecting channels are situated symmetrically relative to the recording channel. The angle between the optical axes of the projecting and recording channels amounts to 5°. The illuminating channel comprises a light source, i.e., high-brightness light diode (working wavelength, 850 nm), condensing lens, diffusor, transparency lattice, and projecting objective. The radiation of the light diode is focused by the con323
densing lens in the inlet pupil of the objective in such a way as to assure uniform illumination of the transparency lattice. The projecting objective constructs images of the lattice lines on the surface of the tooth which is being measured. The recording channel incoroporates a narrow-band light filter to eliminate external illumination of the projecting objective, which constructs an image of the tooth being measured in the plane of the CCD camera’s sensing element. Four additional light diodes without any projecting optical element are also used in the 3D intraoral camera to illuminate the tooth withn the patient’s oral cavity in the course of focusing on and searching for the optimal position of the profilometer. The camera’s field of vision is 14 × 19 mm, depth of focusing 14 mm, overall dimensions of the camera 50 × 65 × × 240 mm, and weight 350 g. In the measurement process, the intraoral 3D camera records four images of the tooth with projecting bands from which a reconstruction of the 3D surface of the tooth is performed. The exposure time amounts to 160 msec. The standard deviation of the measured value of the form of the surface does not exceed 40 μm. Measurement of the tooth surface is performed by the dentist directly in the cavity of the patient’s mouth after preliminary preparation of the tooth and deposition on the tooth surface of a special diffusely reflecting, finely dispersed powder on its surface for the purpose of increasing the contrast of the projecting bands. The intraoral 3D camera is turned on by pressing a pedal and setting it in a depressed position, after which a video signal begins to enter the video capture board in the computer; an image from the CCD camera may then be observed in real time on the monitor screen. Four special light diodes are switched on to illuminate the teeth during this period. By manipulating the intraoral 3D camera and viewing the image on the monitor screen, the dentist is able to determine the best position at which the measured tooth is present in the optimal focusing plane. Once the pedal has been released, the power of the four light diodes generating the auxiliary illumination is switched off and the light diodes in the projecting optical channels are sequentially connected. Four images of the measured tooth with projecting bands are stored in the computer, and reconstruction of the form of the tooth surface is realized in automatic mode. The intraoral 3D camera has been used to measure the form of the surface of a tooth that has been prepared for a crown and filling. The multiply foreshortened illumination of the object used in the 3D camera and photographing of a tooth from different sides with successive “glueing together” of the 3D surface makes it possible to measure complex tooth surfaces with high gradients and shaded sectors.
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