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CAD/CAM was just the beginning

Fig. 7: Automatic reconstruction of inlay cavities. Top: undamaged original tooth; centre: cavity; bottom: occlusal surface automatically reconstructed given only the remaining tooth substance (centre). (Image: Mehl)
Manfred Kern, Germany

Manfred Kern, Germany

Fri. 8. May 2009


Today, practising dentistry without digital technology and CAD/CAM procedures is unimaginable. Intra- and extra-oral imaging, scanning of antagonists and impressions, on-screen 3-D designing, the use of innumerable tooth shapes from the tooth database, the design of anatomic occlusal surfaces, functional articulation on virtual models, subtractive processing of high-performance ceramics—none of this would be possible without the use of computers.

The groundwork for this quantum leap in dental technology was laid in 1985. Using a Fairchild video sensor (which at the time was only used for military purposes and for which special permission was required for use in dentistry), for the first time it was possible for a preparation—made visible intraorally
with a triangulation camera—to be measured multidimensionally and transferred onto a screen. Then, with the help of a PC, imaging software, and a connected CNC grinding unit, the first inlay of silicate ceramic was produced at the University of Zurich.

In those days, only a few could imagine the new technologies and revolutionary treatment possibilities awaiting dentistry thanks to this development. Since then, more than 28 million all-ceramic restorations have been produced worldwide using CAD/CAM technology, both chairside and in the dental laboratory. Computerised milling machines have made subtractive processing of glass- and oxide ceramics possible from which to fabricate aesthetic, high-quality restorations with a reproducible, consistent material quality at a reduced cost.

Relatively recently, discussion was centred on accuracy of fit, cost-effectiveness, and userfriendliness.
The quality of CAD/CAM restorations was viewed critically, and only a few leaders in the field investigated this technology with scientific rigour. Currently, the initially hesitant, and even sceptical, attitude towards computer-manufactured dental prostheses has been replaced by one of approval, and this technology has become a standard procedure.

From a technical point of view, the development of 3-D image capture was propelled not only by more powerful microprocessors, but also by CCD image sensors with high-resolution photodiodes, as well as optical and tactile scanners that help read and upload preparations and models to the software. Laser scanners provide an impulse capacity for reproducing tooth surfaces at a rate of thousands of measured points per second. Upgraded CAD software with 3-D graphics applications receives the digital signals and recreates the clinical surface needing restoration.

Using ‘occlusal settling’ with preformed occlusal surfaces from the tooth databank, the software then virtually rebuilds the tooth surface. The cusps of the occlusal surface are ‘settled’ into their occlusal position. An articulation programme takes the occlusal characteristics of antagonists and the adjacent teeth’s occlusal surfaces and creates a contact-point pattern that fulfils the criteria of the individual function. An acquired, regional functional generated path registry detects sites that interfere with the gliding space and reduces them automatically (Fig. 1).

The impetus for this development in dental technology stems from two sources. The first was protagonists of computerised chairside restoration desiring to process an industrially fabricated ceramic with defined physical properties directly at the treatment unit (chairside) and provide the patient with the definitive restoration (omitting temporaries) in one appointment. The second was the idea of employing oxide ceramics, like ZrO2, for crownand bridge frameworks, by using CAD/CAM technology or digitally controlled milling techniques.

Other ceramics, such as lithium disilicate, also exhibit better properties after mechanical processing, as the blanks used are industrially manufactured under optimal conditions. In addition, the technology of CAD/CAM systems has been substantially improved. In the 1990s, computers became more powerful and measurement methods more effective, making it possible to adapt 3-D data acquisition systems to the needs of dentistry and simplify equipment handling. The evolution of CAD software enabled the development of a variety of construction possibilities and improved the quality of grinding/milling units (Fig. 2). Cost-effectiveness and high-quality restorations are the defining characteristics of CAD/CAM technology. Dentist and dental technician alike profit from this through standardised and controlled treatment and manufacturing processes—and so does the patient. Today, approximately 82 per cent of all-ceramic restorations in Germany are made using computer technology, which indicates that CAD/CAM technology is establishing itself in dental offices and laboratories. The next step in its evolution is now anticipated.

Figs. 1–4: Virtual automatic reconstruction: the scan data of the antagonist, the functional movement, the adjacent teeth, and the preparation can be taken into consideration in their entirety to design an occlusal surface that fits according to all requirements (Fig. 1; Image: Mehl). CAD construction of a widespanning ZrO2 bridge framework. The system examines the connectors for minimum thickness and loadbearing capacity (Fig. 2; Image: Mehl). Individual intra-oral images are anatomically correct, as they are compiled in a virtual quadrant model (Fig. 3; Photo: Sirona). The intra-oral camera scanner enables an optical impression of the entire maxilla or mandible, leading the way for the impression-free practice (Fig. 4; Image: Wiedhahn).

Where do we stand today?

New methods constantly change customary processes, and advancements simplify the workflow. This is reflected in the increased mention of construction models, articulation on a Windows interface, biogeneric occlusal surface design using intelligent software, rapid prototyping, and 3-D printing in the context of CAD/CAM in scientific publications. The impression-free practice is the latest step in this development. At IDS 2009, the use of intra-oral 3-D measurement to, in part, make the impression-free practice possible will be demonstrated (Figs. 3 & 4). With data from an intra-oral image sequence, e.g. of a quadrant, working models can be produced using a wax-processing 3-D printer in a rapid prototyping system, on which prostheses can be manufactured conventionally or with CAD/CAM.

Via internet portals, the dentist can send optical impressions from intra-oral scans to the dental technician, which are then fed into the stationary CAD system. The impression-free practice is much more comfortable for patients because impression-taking and its incident gag reflex are eliminated.  Additionally, production time can be cut and the dental technician’s productivity increased considerably.

What is the future of CAD/CAM?

Those long familiar with the field were able to predict early on that manufacturing centres would play a crucial role: high efficiency, specialised personnel, centralised material purchasing, and high quality standards for the ‘standard restoration’ enable an efficient output that in turn makes it possible to pay off investments in high-tech manufacturing machines, while increasing cost-effectiveness (Figs. 5 & 6). Mid-sized and smaller dental laboratories will make best use of their core competency in the computer-supported manufacture of high-quality aesthetic restorations and in the specialised production of partial and implant-supported prostheses.

Figs. 5 & 6: Milling centres have an ingenious quality-control system for processing ZrO2 ceramic for crown- and bridge frameworks (Fig. 5; Photo: Etkon–Straumann). Milling centres operate costeffective and according to standardised manufacturing procedures (Fig. 6; Photo: Heraeus Kulzer).

Another trend is the computerised fabrication of inlays, onlays, and partial and single crowns, either chairside or in the office’s own CAD/CAM-equipped laboratory. Biogeneric occlusal surface design enables the reconstruction of the missing occlusal surface with inlays, onlays, and partial crowns as naturally as possible (Fig. 7). The one-appointment treatment saves the patient time and removes the need for provisional restoration, which minimises the risk of cusp fracture, enamel-margin chipping, and weakening of the dentine bond.

CAD/CAM and all-ceramics are frequently mentioned together, which falsely implies that CAD/CAM is limited to all-ceramics. The enormous potential inherent in the milling and, most recently, the laser sintering of metals is often completely overlooked. The fabrication of metal restorations (e.g. nonprecious metals and titanium) will eventually become a domain of CAD/CAM technology.

In the field of implantology, it is already possible to create long-term provisional restorations, abutments, and crowns using computer-assisted methods, which also shorten treatment steps. Digital volume tomography (DVT) yields a 3-D image of the bone structure, thus enabling much higher quality diagnosis, including the exact localisation of the alveolar nerve. Particularly in dental arches bearing partial prostheses, the DVT image quality is better than that of CT images, and the X-ray dosage required is much lower. The DVT thus provides the basis for the surgical planning of the implant.

In the future, the implant site and adjacent teeth will be scanned with an intra-oral digital camera, and a virtual model will be calculated. The 3-D volume tomogram will be superimposed on this model and the crown will then be exactly positioned in the X-ray image (Fig. 8). The position of the endosseous abutment will be suggested in the centre of the crown’s basal surface and in its insertion pathway, and based on this the situation will be examined for its surgical feasibility. When selecting the implant system for a given case, the case will be able to be completely simulated in a three-dimensional radiograph. Using special software, it will soon be possible to construct a stereolithographically manufactured drilling template, which will guarantee that the holes drilled in the bone and the implants are exactly positioned (Fig. 9).

Figs. 8 & 9: DVT image with superimposed suprastructure to determine implant postion (Fig. 8; Image: Bindl/Sicat). Special software will help construct a stereolithographically manufactured drilling template for exact positioning of drilling holes and implants (Fig. 9; Image: Nobel Biocare/Geiselhöringer).

The demands of CAD/CAM technology have inspired topics in basic research and hence propelled progress in other areas of dentistry too. Universities and industry can collaborate and thereby promote and shape these exciting developments. Thus far, CAD/CAM or computerised dentistry has not been a central area of interest at universities. But as CAD/CAM technology is relatively new and its performance potential is significant, this is likely to change in the next few years. In turn, this development will influence dental education curricula and thereby influence treatment options in private practices to the benefit of our patients.

Editorial note: This article was originally published in Cosmetic Dentistry Vol. 3, Issue 1, 2009.

Contact info

Manfred Kern, Secretary Society for Dental Ceramic (SDC) can be contacted at info@ag-keramik.de.

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