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CEREC is a completely new system that combines 3dimensional scanning, computer-aided design, and numerically-controlled milling to make ceramic tooth fillings. It will make it possible for dentists to produce and insert ceramic fillings in a single session. Fast ceramic fillings for teeth THE ELECTRONIC A MALGAM, epoxy resins and gold are the standard materials for fillings. Now, however, ceramic fillings are possible, thanks to the CEREC system developed by Dr Werner H. Mormann and Dr Marco Brandestini in collaboration with Siemens. The search for an alternative to traditional metal fillings has been on for about 20 years. Ten years ago, about 95% of posterior restorations (ie, fillings for back teeth) were of amalgam (gold plays a minor role in this area). Today, amalgam is only used for about 50% of restorations, with tooth-col4 SILICON CHIP oured composite resin materials being used for the other 50% . The reason for this trend may, among other things, be that amalgam has suffered from a good deal of public debate regarding health-risks. The goal since 1980 has therefore been to find tooth-coloured restoration materials for posterior teeth. Such fillings have been in use since 1958, which was when new composite resin materials were being developed in the US. However, these materials have two serious disadvantages. The first of these is that they lose surface substance due to masticatory abrasion (ie, chewing). The second problem is that they shrink during polymerization and this can mean a reduction in volume of 2-3%. That's why large composite resin fillings never make a perfect seal, even from the .very beginning. This material is therefore unsuitable for restoring pos- Top of page: this photo shows a dentist using the CEREC optical scanner to scan a patient's tooth cavity. The impression is immediately displayed on the computer monitor. terior tooth cavities and for bearing the stresses of chewing. Since 1980, it has been found that both problems - abrasion and shrinkage - can be brought under control if the filling is processed as an inlay outside the oral cavity. This inlay can then be bonded into the tooth using the same composite resin material. That was the basic discovery on which the development of the CEREC system rests. In fact, the inlay method is the basis of gold fillings too. The big disadvantage of the conventional inlay technique is that an impression must be taken which the dental technician then uses to form the inlay. The dentist has to seal the cavity and remove the temporary filling again in the next session. Only then can the gold inlay be inserted. The big advantage of amalgam and traditional composite resin fillings is that they can be completed in a single session. So that became the goal: to be able to produce a tooth-coloured inlay at the dentist's office in a single sitting. In 1980, Dr Mormann tested all the possible ways of producing such inlays using the conventional method of impression, cast and counter cast. However, because these required the dentist to function as a dental technician, it quickly became clear that an insufficient resolution. But the problem intrigued him. Shortly after, he came to the conclusion that only an optical scanning method could do the task. Their first experiment was conducted in 1982. It proved that it is possible to measure dental cavities precisely using a special video camera; that is, to take an optical impression. What evolved from this was an active triangulation method in which a striped pattern is projected onto the cavity and a recording is made from which a computer can calculate the depth of each pixel. So, the basic principle, using the inlay technique in conjunction with an optical scanning method, was taken care of. But the story didn't end there. There were a number of goals that still had to be met. For instance, the dentist should be able to use the system directly. The system should make it easy and simple to scan a cavity in just fractions of a second. The data should be immediately available, and the dentist should be able to process it directly. Finally, by using a numerically-controlled milling machine, the system should make it possible for the dentist to make inlays in just a few minutes. Developing the milling machine wasn't as easy as originally anticipated. At first, they simply enlarged and strengthened a dental drill and made it 3-dimensionally mobile using a complex motor drive system. This was not entirely successful. After that, the machine was drastically simplified, limiting it to three degrees of freedom, and utilising just one milling disc, which creates the completed inlay from a ceramic block in just a few minutes. The idea of confining the milling head to just three degrees of freedom might sound limiting but it works in most cases. Dental decay usually follows a typical pattern. This leads to certain standard types of cavity preparation for which inlays can be easily produced by the CEREC system. Preparation does not differ in prin- DENTIST improved production method would be necessary. It was then realised that the main obstacle was the lack of a cavity scanning system. Optical scanning Dr Mormann at first thought about scanning with ultrasound. He discussed this with the co-inventor of the CEREC system, Dr Brandestini, who at that time was working intensively at Advanced Technologies Laboratories Inc in Seattle, Washington, on ultrasound scanners for medial diagnostics. He explained that ultrasound would not be feasible because of its This monitor photograph shows the optical scan of a patient's tooth and the area to be restored by a ceramic inlay. Using the information gathered from the optical scan, the computer controlled milling machine manufactures a ceramic inlay to restore the tooth. It does this in only a few minutes. ciple from conventional inlay preparation. However, the machine can't work hollow parts on the inlay, Therefore, during cavity preparation, the dentist simply has to keep CEREC's design rules and operation in mind . . The tooth can then be prepared with as little loss of substance as with other methods. A dentist skilled in this method can restore the full range of inlay and onlay cavities in his surgery. Inlay cavities are always designed so that the filling can be inserted from MARCH 1992 5 I ins were not durable enough - and, although they are much improved since first introduced, they are still not as strong as ceramics. · Ceramics have a lot of advantages. They can be produced to perfectly match tooth colour and they are nonsoluble, durable and stable in the oral cavity. Although ceramics are nonelastic and can break when subjected to overload, they are in this respect no different from dental enamel. Ceramics also stand up to machineprocessing very well. Generally, because factory produced material has fewer flaws than individually fired dental porcelain, it is easier control its quality than it is with blocks produced in dental labs. Other materials Dr Werner H. Mormann, of the Dental Institute of Zurich, in Switzerland, is a co-developer of the CEREC ceramic inlay method of tooth restoration. Intended mainly to replace amalgam fillings, it is hope that the method may eventually be used for full crown restorations. one direction and so that all the cavity margins and surfaces can be seen from a viewing angle which coincides with the insertion axis of the cavity. This makes it possible to record the shape with a single scan. The idea is that if a dentist is capable of preparing a cavity in this way, he is also capable of holding a camera and man- Compared with amalgam fillings (below), the new CEREC ceramic inlay fillings (right) are virtually invisible. These fillings had been in place for two years when photographed. 6 SILICON CHIP ipulating it just like a normal examination mirror. In Switzerland, a CEREC restoration costs two to three times as much as an amalgam filling. But on the other hand, if the dentist makes an impression of the area to be restored, and sends it out to a lab so that an inlay can be made, then the total cost is five to eight times as high as an amalgam filling. So CEREC-produced fillings often offer substantial savings, and then save patients the trouble of having to come in a second time. Ceramic material Dr Mormann discovered during the development stage that composite res- In principle, composite resins are suitable provided they can be industrially manufactured in suitably sized blocks. Glass ceramics can also be used and have the advantage of having almost exactly the same degree of hardness as dental enamel. In addition, their physical properties are pretty similar to those of dental enamel. The one disadvantage of this material, namely that it was transparent, has been solved: tooth-coloured, natural-looking glass ceramics are now available. Today, CEREC can produce inlays, onlays, and veneers very simply and quickly for anterior applications. These can be used as aesthetic half crowns to restore the labial surfaces of anterior teeth. Onlays and veneers are a step in the direction of full crowns, which is the ultimate aim. Acknowledgement: our thanks to Siemens Ltd for photos and permission to reproduce material from Siemens Review, Volume 57, March/April 1990. SC