digital dentistry _ CAD/CAM I This is particularly beneficial in the case of inlays, onlays, partial crowns and seriously weakened cusps. Prof Mörmann’s goal was to deploy CAD/CAM technology to create immediate all-ceramic restorations chairside without the need for temporaries. This goal derived from his experience that temporarily restored inlay cavities have a significantly negative influence on the integrity of the enamel. In many cases, the non-adhesively bonded temporary was positioned like a wedge in the cavity and transmitted the chewing forces to the weakened residual tooth. The applied forces also deformed weakly protected cusp walls. This resulted in cracks in the oral and vestibular enamel surfaces. A second goal was to make use of high-strength oxide ceramics, such as aluminium oxide (Al2O3) and zirconium oxide (ZrO2), and computer-controlled milling machines in order to create crown-andbridge frameworks and hence pave the way for metal-free prosthetic treatment. Another recent development is the online transmission of intra-oral and extra-oral digital impressions and restoration design data to external dental laboratories, which then perform the milling tasks. As a result, the dental laboratories are now more closely integrated into the work flow of dental practices. The probability of survival was significantly higher than that of layered laboratory-produced ceramic inlays and was approximately equivalent to that of alternative long-term restorations, such as cast-gold inlays, which have a survival rate of 87 % after 20 years and an annual failure rate of 0.7 %.3 Dr Otto established that 166 of the CEREC inlays (of an original basis of 200 restorations in 1991) were clinically intact. This is equivalent to a success rate of 83 % after an average service time of 15 years. The survival rate was superior _Clinically proven All-ceramic chairside restorations number amongst the most intensively researched dental treatment procedures. Numerous studies confirm that the clinical performance of inlays and onlays is at least comparable with that of cast-gold restorations. Durability is one of the most important evaluation criteria for ceramic materials. This underlines the importance of the study published in 2008 by Dr Tobias Otto (Aarau, Switzerland) that presented long-term clinical data going as far back as 17 years.2 Since 1989, Dr Otto (one of the first CEREC users in Switzerland) has monitored 200 inlays and onlays produced using the CEREC 1 system and feldspar ceramic (VITA Mark I). These restorations were placed in 108 patients in his dental practice between period 1989 and 1991. He evaluated his findings on the basis of the modified USPHS criteria and summarised his clinical observations after 10 years and 17 years, respectively. According to Dr Otto, 187 of the 200 restorations were still in place after 17 years. This was a survival rate of 88.7% after an average service time of 15 years (Figs. 3–5). In other words, the annual failure rate was 0.75%. Failures with Charlie and Delta ratings (USPHS) occurred between the 6th and 13th year. In most cases, these failures were attributable to ceramic fractures. Fig. 2 to that established by Smales4 for cast inlays after 15 years (loss rate: 1.5 %). It also compares favourably with the 1.3 % annual failure rate established for allceramic, non-CAD/CAM ceramic inlays.5 A contributing factor is that chairside ceramic inlays can be placed immediately in a single appointment, thus eliminating the need for temporaries. In the case of conventional labside restorations, a temporary is unavoidable. Prof Roland Frankenberger established that under the influence of chewing forces, the tooth is subjected to torsional stress due to the low elastic modulus of the temporary composite material. These forces can deform inadequately protected cusp walls, cause partial fractures and incipient enamel cracking, and weaken dentine adhesion. By contrast, the immediate placement of the ceramic inlay facilitates a contamination-free adhesive bond with the hard tooth tissue and stabilises weakened cusps. The stabilising effect on the residual tooth and the existence of an adhesive bond obviously offset the consequences of wider adhesive gaps, as evidenced by long-term clinical findings.6 Fig. 2_Finite element measurement with the exertion of chewing forces: the ceramic inlay bears the chewing load; the tooth substance remains stress free (inlay is hidden). (Illustration: Prof Albert Mehl) cosmetic dentistry 4 _ 2009 I 35