March 1, 2011
by Albert Mehl, DDS
The first step on the way to the “impression-free” dental practice was taken in 1985. The inventors of the CEREC system adopted the following approach: the intraoral measurement of the tooth cavity by means of a camera sensor operating on the phase shift principle; the processing of the acquired data by means of a dedicated design software; and the computation of a machinable CAD/CAM restoration. As a result, it was possible to subtractively mill a restoration out of a ceramic block directly at the chairside. A major breakthrough was the deployment of a US-made Fairchild video CCD, which had previously been used to acquire topographical satellite images and hence had been subject to military secrecy restrictions. Accompanied by the critical scrutiny of dental professionals, the intraoral and extraoral scanning of teeth and tooth models has become an established procedure all over the world. From the outset, CEREC decided in favour of single image sequences, which deliver optimum sharpness and depth of field. Today, these single-tooth impressions can be augmented by additional angled images in order to acquire impressions of entire quadrants.
The advantage of the CEREC chairside procedure lies in the fact that the dentist controls the entire fabrication process – i.e. the intraoral impression, the design process and the milling process. The dentist can also directly influence collaborative procedures (e.g. for bridge restorations) involving an in-house or external dental laboratory. There are no limiting factors on communication, the exchange of data or the selection of materials.
BLUE LIGHT – CROSSING THE RUBICON
A milestone was the introduction of the CEREC Bluecam, which emits short-wavelength blue light (470 nm). This led to a perceptible increase in precision. In addition, an aspherical lens system ensures that the light beam is aligned parallel to the CCD light sensor. This enhances the depth-of-field, as well as the light sensitivity. The exposure time for each 3D image has been reduced to approx. 100 milliseconds. The processing time for sequential images has likewise been reduced. Each image consists of approx. 500,000 pixels, thus ensuring the highly-detailed mapping of the tooth surface.
A new calibration method reduces distortions at the image margins. This virtually eliminates systematic errors when individual images are combined. In vitro experiments have established that the CEREC Bluecam achieves an accuracy of measurement of 19 µm, a figure comparable to that of high-resolution stationary reference scanners. In the case of quadrant images the average deviation is 35 µm.
A further new feature is the Automatic Capture function, which continuously monitors the image quality and triggers the exposure only when the required sharpness is guaranteed. This provides the basis for acquiring overlapping sequences of quadrants as well as entire jaw arches. The 3D image catalogue displays the 14x17mm images on the monitor. The software indicates and rejects substandard images and combines the remaining usable images in order to create a virtual model. Images that have been compromised by the tongue, rubber dam or cotton wool rolls, etc. are automatically replaced as soon as a better image becomes available. Alternatively, the images are cropped in order to eliminate those parts which are not usable. Inadequate images are detected and processed accordingly. The assumption that the superimposition of several different images would lead to greater inaccuracies in the model has not been confirmed in scientific investigations. By means of additional angled images it is possible to increase the number of measuring points on steeply inclined surfaces and to detect areas beneath the equator. In particular, the user is able to visualize the preparation margins more accurately in the proximal area and hence create better contact surfaces in relation to the adjacent teeth. It is possible to acquire images from all directions. This means that there are virtually no restrictions with regard to undercuts. In the case of quadrant restorations it is possible to rotate the images in order to allow for different insertion angles. This does not result in any data losses at the preparation margin or within the preparation itself.
THE IMPRESSION-FREE PRACTICE HAS BECOME A PRACTICAL REALITY
The precision and size of the individual images as well as the extent of the overlaps are determining factors. The “dense” data quantity in each primary image and an overlap of approx. 30 percent are the basis for creating a virtual jaw model. Large-sized partial-jaw and whole-jaw images are important, above all for the CEREC Connect procedure. In this case the data is transmitted to an external production centre, which then creates a resin model by means of a stereolithography process. To this end, the dentist scans the antagonists/opposing jaw and registers the habitual terminal occlusal. This data is then relayed to the dental lab together with all the other information required in order to complete the restoration (i.e. the preparation margin, the tooth shade and the specified ceramic material). To this extent, “impression-free” dentistry has been transformed into a practical reality.
After the data has been downloaded by the dental laboratory, the dentist and the dental technician generally consult with each other in order to specify the wall thicknesses, occlusal surface design, the contact points and the characterization of the restoration. The models are mounted on an articulator in order to simulate terminal occlusal. This ensures that faulty contacts can be detected and eliminated immediately. After the design of the restoration has been finalized, the milling process is initiated. Using the same set of data the dentist can simultaneously mill a temporary chairside restoration (crowns; bridges with up to four units) out of a polymer material. More complex temporary bridges can be delegated to the dental laboratory. Practical experience has shown that a digital model is sufficient for conservative restorations such as inlays, onlays, partial crowns and monolithic crowns. In the case of veneered crowns and bridges, a sawcut model is necessary in order to design the occlusal surfaces, fine-tune the contacts with the adjacent teeth and antagonists, to adjust the occlusion bit and function and – lastly – to evaluate the final fit.
In terms of workflow, impression-free dentistry delivers efficiency gains for the dentist, the dental technician and the patient. Time-consuming tasks have been eliminated. For example, it is no longer necessary to create casts and stone models, trim the jaw segments, reveal the preparation margins, create separate sawcuts and wax-up the restoration. In addition, the modern CAD/CAM process saves material and labour costs and streamlines documentation and archiving procedures.
NATURAL OCCLUSAL SURFACES
A further milestone in the development of CEREC is the biogeneric reconstruction of occlusal surfaces. The first step was the analysis of thousands of intact tooth surfaces and the derivation of mathematical formulae capable of describing natural dental morphologies. This new approach encompasses all previous occlusal design concepts. The next step was to describe naturally occurring occlusal surfaces on the basis of a small number of parameters and traits. As a result an adaptive software is in a position to detect various physical characteristics such as cusps, fissures, marginal ridges, cusp slopes and larger surface areas and compare these characteristics with a universal set of human teeth. A typical representative of a specific tooth type is then computed. An occlusal surface emerges which contains frequently recurring characteristics, whereas the more variable characteristics are cancelled out. The combination of typical representatives and individual deviations is designated the “Biogeneric Tooth Model”. For the first time it is possible to mathematicall
y describe a large proportion of naturally occurring occlusal surfaces.
As a result it is now a feasible proposition to reconstruct a missing tooth by analyzing the morphology of the adjacent tooth or the antagonist. Scientific studies have demonstrated that the morphology of the first lower molar can be accurately deduced from the morphology of the first upper molar. The deviations in the occlusal surfaces are in the region of 180 µm-i.e. are less than in the case of wax-ups created by experienced dental technicians. Unlike other occlusal design concepts, the Biogeneric Tooth Model facilitates the metric (i.e. computer-compatible) determination of the missing tooth surface. There is a high degree of probability that the reconstruction will match the patient’s individual dentition harmoniously and functionally. In principle the design of the missing tooth can be deduced from any other intact reference tooth in the patient’s mouth. However, the greater the distance between the reference tooth and the restoration, the smaller the degree of correlation. For example, tooth 4 in the upper jaw will supply only a limited amount of information for tooth 7 in the lower jaw. However, the decisive factor is whether the biogeneric software identifies the reference tooth as typical and then generates a suitable design proposal. This enhances the reliability of the reconstruction proposal and at the same time facilitates a high degree of automation. To this extent the Biogeneric model offers decisive advantages compared with other non-knowledge-based concepts and the exclusive reliance on dental databases. OH
Prof. Dr. Albert Mehl studied dentistry at the Friedrich-Alexander University in Erlangen Nuremberg and qualified as a dentist in 1989. Parallel to this he obtained a degree in physics. In 1992 he received his doctorate in dental medicine and transferred to Munich University. In 1998 he was appointed to the post of Assistant Professor of Restorative Dentistry, Periodontology and Paediatric Dentistry. In 2002 he became Full Professor for Restorative Dentistry. In 2003 he obtained a doctorate in Human Biology.
His research activities focus on the physical and mechanical properties of restorative materials with special reference to ceramics and composites. On the basis of his knowledge of physics he developed scanners and software for the computer-aided manufacture of all-ceramic restorations. Albert Mehl’s research work led to the optimization of industrially produced scanners. Within the framework of various CAD/CAM projects he investigated the functional and biological morphology of natural teeth and developed an algorithmic model for the computation of patient-specific occlusal surfaces.
Since 2008, Mehl has been employed in the Dept. for Computerized Restorations at the Zurich University Clinic for Preventive Dentistry, Periodontology and Cariology.
Oral Health welcomes this original article.