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CAD/CAM Magazine

_Digital impressions are more accurate Atthe12th annualmeetingoftheInternationalSo- ciety of Computerized Dentistry, Prof Gerwin Arnetzl, University of Graz, compared the accuracy of digital- ly generated impressions with that of conventional elastic impressions. When conventional impressions demonstrate an elastic recovery of 98.5 % after de- formation, a fitting accuracy of 35 to 75 µm for an in- lay cavity can be expected. For cast pieces, additional tolerances of up to 46.5 µm accumulate,4 so that in- directly manufactured crowns can attain deviations ofupto114µm.5 Different elastomeric impression techniques can cause considerable deviations. For instance, in ana- logue impression-taking using different impression materials and trays, dimensional changes compared with the reference (a cast metal control) varied be- tween 0.32 and 1.17 %. A deviation of 49 µm was foundforstandardand122µmforcontrolimpression- taking.6 As a rule, however, the studies on analogue impression-taking techniques were performed using 2-D measurements; the new studies on the imaging accuracy of optical methods were conducted with 3-Dvolumedifferenceanalyses. Digitally or optically produced images by different operators exhibited a measurement accuracy of 11 µm.7 Withtheanalogueimpression-takingtechnique,thede- viationsforawholequadrantrangedfrom72to101µm, while the measurement error tolerance of digital im- ages is only about 35 µm, thanks also to the enhanced accuracy made possible by angled images. Potential sources of error in the digital impression-taking tech- nique are scanner adjustment, magnetic interference fields during image processing, image noise and the software.AccordingtoProfArnetzl,theseresultsprove that given the correct use of a camera or scanner, digi- tally generated data exhibits fewer errors and greater accuracy than the conventional impression-taking techniquewithelastomericimpressionmaterials.8 A virtual model of the maxilla/mandible is com- puted from the scans of the quadrants or complete dental arch with the antagonist dentition. Via the Internet, the dentist sends the datasets from C.O.S. Lava or iTero to the manufacturer, where they are checked before being used to produce a resin model (Figs. 9 & 10). After CAD construction of the restora- tion, the dental technician can either mill the frame- work in his/her own laboratory or have it done at the milling centre. The resin model is needed to layer on the veneers and perform articulation. CEREC AC also computes a virtual model (Fig. 11). Framework- free crowns and short-span FDPs can be milled im- mediately, directly from the dataset, in the practice’s laboratory or in another dental laboratory with an onlineconnectiontothepractice.Forveneeredcrowns and multi-unit bridges, a stereolithographically pro- duced resin model (SLA) is necessary, which is provid- ed by InfiniDent (Sirona) and makes veneering the frameworkandarticulationpossible(Figs.12–14). Optoelectronic impression-taking systems are ex- tremelypromising.Owingtotheofferedadvantagesin standardisation,qualityassuranceandpatientcomfort, digital intraoral impression-taking systems have great potentialforthefuture.Inthecomingyears,theywillbe seen in ever-increasing numbers in daily dental prac- tice.Thedatasetstheycreate,thankstotheexchangeof information online, simplify communication between the dentist and the dental technician, regardless of distance. Supplemental facial photos, information on tooth colour, individualisation, material, occlusal con- cept,etc.canalsobeattached.Allofthishappenswith- outconventionalimpression-takingandtheassociated gagreflex,waxcheck-biteandstonemodel._ Editorial note: A complete list of references is available fromtheauthoratkern.ag-keramik@t-online.de. 40 I I trends _ digital datasets CAD/CAM 2_2011 Manfred Kern German Society of Computerized Dentistry – International Society of Computerized Dentistry secretariat@dgcz.org www.dgcz.org CAD/CAM_contact Fig. 12_SLA model (acrylic) for trying in the framework. Fig. 13_Trying in the ZrO2 framework. Fig. 14_Veneering and articulation. (Figs. 11–14 courtesy of Baltzer) Fig. 12 Fig. 14Fig. 13