CAD/CAM dentures are the fabrication of crowns, bridges or implant accessories using computer-aided technology. Both the design (CAD: Computer Aided Design) and the manufacture (CAM: Computer Aided Manufacturing) are carried out with the aid of intelligent software programs and by milling units networked with them. The prerequisite for this was the rapid developments in computer technology over the last few decades, which have made it possible to couple complicated program controls with milling machines with expanded movement capabilities. Initially developed for the aerospace and automotive industries, the technology was eventually adopted in modern dental technology. CAD/CAM technology can encompass all steps from surface acquisition of the prepared (milled) tooth to milling of the workpiece. First, the preparation must be transferred three-dimensionally. Then the workpiece is designed, taking into account the positional relationship with the neighboring teeth and the teeth of the opposing jaw. Finally, the design is converted into the workpiece by a milling robot. While the fabrication of dentures in the posterior region is already frequently monolithic (from one piece), crowns and bridges in the more esthetically demanding anterior region are generally fabricated by first fabricating a CAD/CAM framework and then veneering it with ceramic materials. This veneer is still applied by hand by an experienced dental technician in several layers of color and then fired. CAD/CAM technology has advanced the use of high-quality, biocompatible ceramic materials (feldspar, glass ceramic, lithium disilicate, zirconium dioxide). However, cobalt-chromium alloys, plastics and biocompatible titanium can also be processed with CAD/CAM technology.
Indications (areas of application)
- Inlays
- Onlays
- Partial crowns
- Veneers
- Crowns / frameworks
- Bridges/frameworks
- Implant accessories
- Implant superstructures (dentures on implants)
- Bars
- Attachment
- For ceramics: incompatibility against metal alloys.
Contraindications
- In cases of bruxism (teeth grinding), the use of ceramics should be carefully considered, even though monolithic zirconia (e.g. BruxZir) is now available for this indication.
- Veneering of CAD/CAM frameworks in bruxism due to the risk of chipping (shearing of the veneer from the framework during grinding).
- Monolithic ceramics in the anterior region – Monolithically fabricated anterior crowns do not meet high esthetic standards. Here, the experienced dental technician should resort to the individual, hand-laid ceramic veneer of a CAD/CAM framework.
- Hypersensitivity to adhesive resin luting material – Here, the choice of dental restorative material is limited to materials (zirconia) that can be used with conventional cements (zinc phosphate, glass ionomer, carboxylate).
The process
I. Chairside procedure
Optical scanning of the preparation is performed intraorally (in the mouth) in the dental office (chairside: at the dental chair) by a camera with a small head, which allows 3D images to be taken throughout the mouth. Camera systems are available which require the teeth to be powdered prior to scanning (e.g. CEREC Bluecam) in order to eliminate reflections, as well as powder-free cameras (e.g. CEREC Omnicam). An anti-shake function ensures that the image is automatically triggered only when the camera is steady. Modern programs deliver true-to-life modelled proposals for occlusal surface design which still have to be individualized by the dentist (CAD). The finished design is transferred to a milling unit located in the dental practice (e.g. CEREC MC X), which machines the complete workpiece from a blank – usually a ceramic monoblock (CAM). The milling process for a crown, for example, takes less than a quarter of an hour. Afterwards, however, the workpiece has to be polished by hand. The advantages of the chairside procedure are, on the one hand, that no impression of the prepared tooth has to be taken for transfer to the dental laboratory and, on the other hand, that the patient can be given a definitive restoration quickly in one treatment session. The classic indication for the chairside technique is the restoration of individual teeth.However, the fabrication of small bridges is also possible. II. labside procedure
II.1 Dentist
For CAD/CAM workpieces produced in the laboratory (labside), impressions of both jaws are taken in the dental practice after preparation (grinding) of the teeth to be restored. In addition, a bite registration is taken to bring the upper and lower jaws into position in relation to each other. II.2 Laboratory
II.2.1 Model fabrication
In the laboratory, plaster models – a working model (jaw model with the prepared teeth) and an opposing jaw model – are first produced conventionally by casting the impressions. II.2.2 Scanning
The jaw models are transferred to the CAD/CAM program by a scanning process. There are various options for this, depending on the system. Digitization can be performed using a camera or scanning with a laser. II.2.3 Computer-aided design (CAD)
The scanning unit transfers the acquired data into a three-dimensional graphic representation. The design of the workpiece is the responsibility of the experienced dental technician, who is assisted in the modeling by the software archive, but must still meet functional criteria such as preparation margin, positional relationship to neighboring teeth and bite situation, and also take esthetic considerations into account. II.2.4 Computer-aided milling (CAM)
The design data are transferred either to an in-house milling unit or to an off-site production center. The milling unit creates the workpiece fully automatically from the CAD model by moving the workpiece three-dimensionally to the milling unit or by moving both the milling unit and the workpiece relative to each other. The milling process not only takes into account the complex geometry of the three-dimensional model itself: If zirconia blanks are milled, which have the advantage of a soft, chalky consistency and only undergo the final sintering firing (sintering: heating under increased pressure, thereby solidifying and hardening) after milling, the volume shrinkage of about 30 percent that occurs during this process must also be included in the program. II.2.5 Veneering of frameworks
If the CAD/CAM workpiece is not an all-ceramic dental restoration, but initially only a crown or bridge framework, it is veneered in the conventional sintering process after the milling process: Individualized ceramic masses are applied by hand in several layers and then fired on, whereby the veneer undergoes volume shrinkage, which the dental technician takes into account beforehand during application. Subsequent veneers have the esthetic advantage of enamel-like translucency (light transmission comparable to natural tooth enamel). II.3 Dentist
- Control of the completed dental prosthesis
- Cleaning the prepared teeth
- Trying in the denture
- Preparation of teeth for cementation – If adhesive cementation is planned, the enamel margins are conditioned with 35% phophoric acid gel for about 30 sec; dentin etching for a maximum of 15 sec, then application of a dentin bonding agent to the dentin, which is only carefully dried or slightly moistened again.
- Preparation of the denture – etching of the inside of the crown with hydrofluoric acid (not for zirconium oxide), thoroughly spray off and silanize
- Insertion of the crown in adhesive technique – with a dual-curing (both light-initiated and chemically curing) and high-viscosity luting composite (resin); excess cement is removed before light curing; sufficient polymerization time (time during which the monomeric basic building blocks of the material chemically combine to form a polymer), during which the crown is exposed from all sides, must be observed.
- Control and correction of occlusion (final bite and chewing movements).
- Finishing the margins with ultra-fine grit polishing diamonds and rubber polishers.
- Fluoridation – for inlays, onlays and partial crowns to improve the surface structure of the remaining enamel.
Possible complications
- Complications can arise from the multitude of technique-sensitive steps and ultimately lead to inaccuracies in terms of fit or bite.
- In veneered frameworks, risk of chipping: shearing of the veneering ceramic from the framework under load.
- Fracture (fracture)
- Tooth sensitivities (hypersensitivities) due to errors in fastening
- Crown margin caries – due to inadequate oral hygiene or washout of luting material from the adhesive joint.
