digital dentistry _ CAD/CAM I Within the framework of a meta-analysis, the clinical survival probability of high-quality conservative restoration types with the respective production costs was investigated. Gold inlays and CEREC inlays had the highest success rates. The CEREC restorations perform better in terms of cost effectiveness versus durability. The higher production costs of cast-gold inlays are a disadvantage here.12 _Biogeneric occlusal surfaces The design of functional occlusal surfaces poses a challenge to rehabilitating the chewing function. In this area, too, CEREC has exploited advances in digital technology. IT provides valuable assistance with recreating lost tooth tissue in such a way that the restoration harmonises well with the existing dentition in terms of its structural and functional characteristics. With the aid of biogeneric modelling software, Prof Albert Mehl et al. succeeded in automatically creating patient-specific occlusal surfaces for inlays, onlays and partial crowns.13–15 In this case, the residual occlusal tooth tissue was compared with several thousand digital scans of natural occlusal surfaces contained in the CEREC tooth library (Fig. 8). The software identifies matching morphological characteristics (fissures, cusps, marginal ridges, gliding contact angle) and then inserts corresponding cusps, fossae, fissures and contact surfaces into the virtual model of the restoration. On the basis of the contact point distribution, the cusp apexes and the proximal contacts, the software is capable of creating a well-matched tooth and detecting possible collisions with the bite registration. This biogeneric modelling process creates natural, individual and functional occlusal surfaces. The extension of the CEREC indications spectrum to chairside crowns and multiple-unit labside bridges (both temporary and permanent) has placed increased demands on the intra-oral measuring process. The recently introduced CEREC AC system deploys a short-wavelength blue LED light source. In combination with the built-in anti-shake system this blue light source reduces the measurement tolerance to 19 µm in comparison with a stationary reference laser scanner.16 The preparation is optoelectronically scanned from various angles in the patient’s mouth. The individual images are then combined to create complete quadrants (Fig. 9). Inadequate images are automatically detected. With a scan of the antagonists, the digital impression of the partial arch/quadrant is transmitted via a wireless link to the in-house dental laboratory. Alternatively, the data can be sent via the CEREC Connect web portal to an external dental laboratory or to an external milling centre equipped with a stationary Fig. 6 Fig. 7 Fig. 8 CAD system. This is followed by the virtual design of the restoration. If required, a 3-D working model can be created using a special stereolithography process (SLA). This model provides the basis for the fine tuning of the CAD/CAM-milled crown or bridge framework. ‘Impression-free’ dentistry offers numerous advantages. The patient does not have to endure the discomfort of a conventional impression (such as gag reflex). In addition, dental laboratories can reduce their production times and achieve significant productivity gains. Fig. 6_Survival rate of CEREC inlays and onlays: 84.4 % overall after 18 years. (Source: Dr Bernd Reiss) Fig. 7_Survival rate: 90 % with dental adhesive; 80 % without dental adhesive. (Source: Dr Bernd Reiss) Fig. 8_Examples of the tooth surfaces contained in the library (here: 6th molar of the upper jaw); at present, approximately 400 tooth surfaces are available for each posterior tooth type. (Illustration: Prof Albert Mehl) cosmetic dentistry 4 _ 2009 I 37