CAD-CAM IN DENTISTRY
Contents • Introduction • Definition • History • Uses • General principles of CAD/CAM • Commercially available systems • Benefits and shortcomings of CAD/CAM • Future developments
Introduction Traditional methods for the production of indirect restorations usually include impression making, die or model making and final manufacture. Inaccuracies can occur at any stage, and if introduced early on, may have an exaggerated effect on the final restoration.
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Costs can be high, in terms of clinical and laboratory time and expertise.
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The costs of producing remake restorations must be borne either by the dentist or the laboratory.
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Extra chairside time and visits are unwelcome to the patient, who may find them inconvenient and stressful. One visit treatment for indirect restorations has been made
possible by technological development which enable distilization and replication of the complex topography of the tooth surface using computer-aided design / computer-aided manufacture (CAD/CAM). Several techniques of varying degrees of sophistication, have been introduced. The concept common to all the systems is that spatial information relating to soft or hard tissues may be processed as numerical data and manipulated by computer. Additionally the machining or manufacturing system is not material specific.
Definition: CAD/CAM systems that can produce dental prosthesis with robots or computers. The first report on CAD/CAM, published in 1973, made it clear that the
optical impression technique,
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or dental CAD/CAM,
encompasses, all methods of analysis including diagnosis and treatment, working in space, ad developing means of measurement that preferably optical. CAD/CAM means manufacturing the prosthesis directly with the data taken from the patient’s mouth and at the utmost, it means staying analogical. In short, it is proceeding directly from paste to crown. The most topical example of a CAD/CAM system in the non-dental world is copying a drawing or sculpture with a pantograph.
History CAD/CAM technologies introduced to the dental profession in 1971. In 1979, Heitlinger and Rodder milled the equivalent of the stone model used by a dental technician to make the crown, inlay or pontic. In 1980 Moermann and Brandestini took a single picture and milled only the internal surface of the inlay. During the next 5 years, little was heard. The first dental CAD/CAM prototype was presented at the Ganaciene Conference (France) in 1983 and the first crown was publicly milled and installed in a mouth without any laboratory involvement in 1985. Though 1985 was a decisive year for computer aided dentistry there was still a long way to go. Several engineers took 2 hours to 3
operate the first usable system in a dental office. Nevertheless this demonstration French Congress indicated principles established 14 years earlier. Two new names appeared at this time, the Aoki team in Japan and Diane Rekown at the university of Minnesota. Dr. Rekow chose a photogrammetric method to acquire the third dimension and used the principle of the critical tooth for her second and third steps. It should also be mentioned that Raggie Candill (1988) at the university of Alabana, started a project aimed in the same direction.
Uses a) All CAD/CAM systems permit the production of restorations at the chairside. b) Eliminate potential inaccuracies associated with the traditional, multistage production of indirect restoration. c) Their use also minimizes cross infection.
General principles of CAD/CAM All true CAD/CAM systems exhibit there computer-linked functional components, although the degree of sophistication may differ. a) A means of data acquisition. b) Restoration design. 4
c) Restorative production. The first stage requires digital data to be collected from the patient’s mouth this is equivalent to traditional impression taking. The information may be stored in the form of three-dimensional coordinates of poils, on the surface of the oral tissues obtained with the aid of an intraoral camera on physical sensor, resolution depends on the design of the individual system, but it follow, that a laser beam offers greater accuracy than a contacting probe, whether in relation to soft tissue, tooth surface or cavity preparation. Restoration design even when aided by computer, still relies to a large extent on the operators clinical knowledge but may be aided by the availability of digital information from the unprepared dentition and a library of performs. Future development of artificial intelligence systems may offer further assistance.
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Commercially available systems: 1) CELAY introduced in 1990 Manually controlled, rather than computer-controlled, the Celay system (developed by Mikrona Technologie, Spreitenbach, Switzerland) has two main features. 1. A hand-operated contacting probe that traces the external contour of an acrylic or wax inlay, previously fabricated directly in the mouth. 2. A milling arm, following the probe by means of a pantographic arm with eight degrees of freedom, that cuts a copy of the ‘pro-inlay’ from a porcelain or glass-ceramic block. Both inlays and onlays may be produced using this method. Manufacturing details: A wax or resin model of the required restoration is produced by the clinician at the chairside. The topography of this ‘pro-inlay’ is traced by a disc-shaped contacting sensor and the information simultaneously transmitted, via the pantographic arm, to a diamondcoated wheel which mills a copy restoration from a pre-manufactured ceramic block. The initial cut is carried out using a relatively coarse 6
(126µm) diamond-tipped wheel under liquid coolant. The cut is repeated with a liner wheel (64µm) to smooth the surface. Fine anatomical detail can be achieved using conical and cylindrical diamond points. Points: a. This is a relatively simple technique for the production of inlays. b. The accuracy of the final restoration depends on fit of the resin prototype at the First International Celay Symposium, it was reported that marginal fit ranged from 50 to 80µm. The system was first introduced in 1992 in Europe. A technique for manufacturing crowns using the Celay system, which allows replication of fitting surfaces, was introduced in 1993: •
A light-cured replica of the crown core is produced, scanned and milled from a Vita-Celay alumina blank.
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This core is strengthened by the addition of a colour-matched glass, applied in the form of an aqueous slurry.
2) PROCERA introduced in 1987, designed by Anderson and developed by Bobelpharma. The Procera system (Nobelpharma Inc. Goteborg, Sweden) combined pantographic reproduction with electrical 7
discharge (spark-erosion) machining. It allows the production of titanium copings, which are subsequently veneered with a compatible porcelain (Ti-Ceram) or composite to form crowns or bridges, the latter requiring laser welding of the individual titanium units. Manufacturing details: A traditional die is produced from a conventional impression of the prepared tooth and is placed under the reading head of a pantograph. A copy milling device produces several replicas, one from titanium and two or three more from graphite cylinders. These dies are used as electrodes in an electro-erosion apparatus to precisely manufacture the interior fitting contour of a titanium coping. Points: •
This is a lengthy, expensive procedure, the accuracy of which is dependent on the accuracy of the conventionally produced stone die and the precision of the copy milling and spark erosion apparatus.
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The outer ceramic contour is formed manually by traditional porcelain fused-to-metal techniques.
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3) SOPHA This sophisticated French system, invented by Duret, allows for the production of inlays, anterior and posterior crowns and bridges. It also permits consideration of both static and dynamic occlusal factors. An impression of the tooth is taken using a lager imaging system and holography. Information from the light source is then digitized by the camera, presented on a video display and transferred to a CAD/CAM program that creates a model of the preparation. The complex software allows the dentist to access a library of theoretical teeth, which may be adapted and modified according to the requirements of the oral situation. Manufacturing details: The laser probe scans the tooth and the signal is relayed to the computer. A final view is taken with the teeth in a selected occlusal position of reference. The complete set of pictures is displayed on a high-resolution video screen, and the restoration designed in a series of stages starting with the fitting surface, then going on to the external and occlusal surfaces. The CAD system is also linked to a proprietary articulator (the Access articulator), which provides data related to dynamic jaw movements.
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Points: •
This procedure is lengthy and very expensive. It is technique sensitive.
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Marginal accuracy has been reported to range from 0 to 60µm.
4) CICERO The Cicero (Computer-integrated crown reconstruction) system makes use of optical scanning, near net-shaped metal and ceramic sintering and computer-aided fabrication techniques. Manufacturing details: Information is digitized from a gypsum cast, using a fast laserstripe scanning method. Up to 100,000 surface points may be recorded per minute. An appropriate tooth is selected from a collection of generic forms of theoretical teeth in the programme’s library. Mesial and distal contacts of the proposed restoration are outlined by the operator on a video screen and the margin of the new crown adjusted to the preparation. Working, balancing and protrusive pathways for the new restoration are also computed at this stage. After interior and exterior tooth surfaces have been defined, the interfaces between cement and metal, and between dentine and
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incisal porcelains, are delineated. Built into the software is accommodation for the overall thickness of cement and metal substructure. Milling of a pre-formed refractory block is followed by sintering a thin layer of a gold alloy powder onto the milled surface. The appropriate dentine shade of fine-grained, leucite reinforced ceramic in the form of a colloidal vacuum-kneaded paste is cold pressed onto the metal-covered refractory shape and vacuum fired. The dentine porcelain is automatically ground back. Enamel porcelain is then added and the restoration re-fired before final milling of the external contour. The last step includes individual staining and glazing 5) CEREC Invented by Mormann and Brandestini and developed by Siemens, Cerec was introduced to the dental profession in 1987. It is the most widely investigated and reported CAD/CAM system to data. The main features comprise an intra-oral camera, an imageprocessing unit linked to a viewing screen and a micro milling machine. The production and placement of inlays and veneers, milled either from blocks of feldspathic porcelain or from ceramic, may be accomplished in one visit.
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Manufacturing details: The cavity is prepared according to precise criteria (e.g. 90째 axio/floor line angle no cavo/surface bevel) determined by the mechanical limitations of the machining process. The preparation and abutment teeth are coated with a reflecting powder and a noncontacting scan head, incorporating a light-emitting diode and lens system, is positioned over the tooth surfaces. Information relating to three teeth may be stored at an one time. This system also incorporates a monitor that displays the preparation data being processed, permitting visual verification. All pulpal and cervical borders are manually outlined on the video screen determines the occlusal and proximal cavosurface margins and the marginal ridge position. Inlay fabrication is accomplished with a three axis N/C milling machine. A liquid-cooled, high-speed turbine powers a diamond bur that is rigidly attached to a y axis platform. The restoration tried into the prepared cavity and assessed for accuracy of fit. Once clinically acceptable the proximal surface is polished and the fitting surface etched and treated with a silane coupling agent.
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Points: •
Pulpal floors must be as flat as possible because any contour irregularities on the pulpal or gingival floors cannot be reproduced.
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Simultaneous milling by both a milling disc and a cylindrical diamond bur takes place.
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The additional use of a cylindrical diamond bur allows the production of restorations that could not have been machined using the milling disc alone. Therefore, a wider range of restorations, such as inlays, onlays, partial crowns and veneers, may be produced at the chairside in this way.
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The process of tracing and recording the preparation details is relatively easily learned, and could be delegated to the trained dental surgery assistant.
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One of the main advantages of this system is that restorations may be produced at the chairside. However, initial financial outlay may well be prohibitive. Proposed solutions include leasing or sharing equipment.
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6) DUX The Dux system, also known as the Titan system (DCS, Dental Allschwill, Switzerland) consists of a miniature contact digitizer, a central computer and a milling unit. The manually operated tracing unit, which is said to have an average accuracy of 3µm, consists of a table that shifts a die or model beneath a contact stylus. The central computer converts the three-dimensional model data into a CNC milling programme. Titanium crown and bridge substructures may be produced using this technique. Points: •
Accuracy of the Dux system is claimed to be in the order of 30 (±20)µm and the milling process takes about 10 minutes.
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This system is also applicable to quality control and model and tool manufacturing.
7) DENTICAD The DentiCAD system (BEGO, Bremen, Germany and DentiCAD, Waltham Mass, USA) consists of a miniature robot arm digitizer, CAD/CAM software with a facility for fully automated design and a milling machine. The robot arm can be used either
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intra-orally or indirectly on conventional dies or models. The milling process is directly controlled by computer. Points: This is the most fully automated system, allowing production of inlays, copings, crowns and bridges. The user needs only to digitize the required teeth (preparation, opposing and abutment teeth) and the rest of the procedure is carried out automatically. 8) The Japanese system: Under the supervision of Professor Tsutsumy, this system has the three usual CAD/CAM components, i.e. the camera, the CAD and the NCMT. The system is only at the development stage, but its inventor, Dr. Fujita, has already produced some crowns with it. No clinical experience has been reported to date. 9) The Dens system: Developed by Rohleder and Kammer in Berlin, this system is still in its early stages of development. However, a prototype was introduced in Cologne this year. It comprises an optical sensor that is very fast and a NCMT that can work titanium and was developed to this end. It has no CAD yet. No clinical experience has been reported to date.
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10) The Krupp system: This system deserves to be mentioned because it fails between the traditional method and the robot. Made with a special wax, the prosthesis is used to mould special electrodes that will reproduce the external and internal surfaces, separated at the line of the most important contours. The two moulded electrodes are used, as with the Procera system, to process by electro-erosion non-and semi precious metals of the Dentitan or Endocast type. It is possible to produce all-metal pieces like crowns and bridges using this system. No clinical experience has been reported to date, but the precision recognized by all who tried it is in the order of 40Âľm.
Benefits and shortcomings of CAD/CAM By comparing modern CAD/CAM systems copying techniques with previous ceramic systems which have been used in a constructive-modelling way various benefits and shortcomings may be set against each other. Advantages and shortcomings of conventional inlays versus subtractive processing (e.g. CAD/CAM)
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Dental technician subtractive Preparation (demands)
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Choice of materials
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Quality of material
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Occlusal contouring
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Marginal fit
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(+)
Repeatability
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Appointments
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1 or 2
System price
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Inlay fees
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1. The main disadvantages of CAD/CAM systems specified repeatedly by practitioners are lower fitting accuracy of the inlays and insufficient creation of the occlusal surface with Cerec. Nevertheless investigations of Celay as well as of Cerec 2 show that with exact preparation a precision comparable to ceramic inlays (order of 50-100Âľm) can be produced by dental technicians. Should indistinct preparation margins be present the dental technician is superior to the CAD/CAM systems (especially with regard to Cerec). Occlusion may be produced by means of copying techniques, for example very well with Celay and also to a high degree with Cerec 2, whereas details like side fissures cannot be ground with Cerec 2.
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2. With CAD/CAM systems partial higher demands are made on the preparation. In order to let a software process run as trouble-free as possible, for example the automatic detection of the cavity margins with the edge finder algorithm, an accurate preparation border is necessary. With subtractive fabrication of restorations, possibly sharp internal angles of the restoration cannot be worked out in detail with grinding instruments of a certain size. 3. Furthermore there were particular limitations with Cerec 1 with respect to requirements during preparation to produce an inlay. The grinding wheel limited clinical application in several situations. Buccal extensions for instance must not become wider buccally as non-grindable undercuts would have otherwise occurred. Similar constraints applied to shoulder preparations at cusp substitution. These limitations no longer exist with Cerec 2 due to the additional cylindrical grinder. 4. Purchasing costs for subtractive systems generally exceed those for conventional ceramic systems. Laboratory costs for ceramic inlays made by subtractive techniques is, in Germany however, mostly slightly less than that for inlays fabricated conventionally by dental technicians. A Cerec users opinion poll conducted by
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the German Association for Restorative Computer-aided Dentistry revealed that 88 per cent enjoyed working with Cerec. Currently the enthusiasm of dentists for CAD/CAM plays an important role in the decision to buy such a unit.
Future Developments Contouring the occlusal surface In the last years our research team already presented different possibilities as to how to create occlusal surfaces via computer. Based on various situations three different possibilities have been suggested:
With an existing suitable occlusal surface (either natural or restored) a scan is taken from the occlusal surface before preparation and the pictures of the ‘initial state’ and ‘post-cavity preparation’ will be superimposed. The software necessary for that purpose has already been developed in the authors department. This option has also been incorporated into Cerec 2.
With medium-sized defects with remaining cusp slopes there is the possibility of reconstructing further trends of cusp slopes including fissures by means of CAD (HermiteSplines) (meanwhile also included with Cerec 2).
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Extensive defects pose the most difficult problem. In this case reference to a data bank of different teeth with average values is required, and adaptation of the occlusal surface considering the antagonists by means of CAD/CAM needs-to take place. This technique, however, is still in a stage of development.
Quality control by error analysis Once the CAD-calculation of the grinding process and the geometrical data of the restorations are present, online error analyses can be conducted. This can provide quality control by warning of potential problems. For example, if a projected inlay falls below a specified minimum thickness (depending on the used material) the system is able to provide a warning that there may be fracture-sensitive marginal areas, thereby reducing handling errors.
Computer-aided preparation Another interesting development is at present being pursued at the University of Frankfurt / Main (Germany) with the CAM-system (Computer Aided Cavity). Uniquely, the system attempts to take a picture of the tooth after caries removal and rough cavity preparation and to plot the cavity borders approximately on the screen. Thereupon a program calculates the outer contouring of the cavity estimating that a 20
certain cylindrical stone is suitable to follow the contouring without problems. In the next step the preparation diamond with right-angle handpiece will be fixed in the mouth and the cavity will be completely prepared, controlled by computer. With respect to the now known coordinates an inlay may be ground out of ceramic. As many cavity designs recur in their extensions again and again it may be quite possible that a few basic designs of inlays can be present and that the computer prepares a cavity suitable for the inlay and corresponding to the given defect size. One immediate problem is that an additional loss of tooth structure may will sometimes be suffered.
Conclusion The introduction of CAD/CAM in dentistry will influence the direction of clinical practice and research at universities. Results achieved must be analyzed with caution, but the extraordinary speed of development of this technology in industry affirms that it will be rapidly and definitively accepted in the dental profession. Its future evolution could be spectacular considering its numerous possibilities. All these systems offer an alternative to the currently practiced impressions-die-lost wax-casting technique. Potentially each can save the dentists time by providing restorations that accurately fit the patient without requiring chairside adjustments. In addition, they can provide 21
more consistency in restorative design and open the possibility of a array of different restorative materials, including machinable toothcolored ceramics.
References: 1) Francois Deret et al : CAD/CAM in dentistry. Journal of American Dental Association, 1988; 117: 715-721. 2) Diame Rekow : Computer-aided design and manufacturing in dentistry. A review of the state of the art. Journal of Prosthetic Dentistry, 1987; 58: 512-516. 3) Cripsin B.J. : Computerised design and manufacturing of aesthetic dental restorations. Dent. Clin. North Amer., 1992; 36: 797-807.
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RETENTION AND RESISTANCE FORM IN FIXED PARTIAL DENTURES Introduction Retention form a. Magnitude of dislodging forces. b. Geometry of the tooth preparation. c. Materials being cemented. d. Type of luting agent. e. Film thickness of luting agent. PATH OF INSERTION Resistance form a. Magnitude and direction of dislodging forces. b. Geometry of tooth preparation. c. Physical properties of the entity agent. Enhancement of internal features a. Pens. b. Grooves. c. Box. 24
d. Dowel cores. Crown lengthening procedures. Cores – in case of attritional or fractured teeth. Introduction Teeth do not possess the regenerative ability found in most other tissues. Therefore, once enamel or dentin is lost as a result of caries, trauma, or wear, restorative materials must be used to reestablish form and function. Teeth require preparation to receive restoration and these preparations must be based on fundamental principles from which basic criteria can be developed that help predict the success of prosthodontic treatment. Retention are resistance form are a part of these principles of tooth preparation and are the property of today’s discussion.
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