Dental Tribune Middle East & Africa Edition

Media CME DENTALTRIBUNE Middle East & Africa Edition 6 (mCME articles in Dental Tribune (always page 6) has been approved by HAAD as having educational content accept- able for (Category 1) CME credit hours. Term of approval covers issues published within one year from the distribution date (September, 2010). This (Volume/Issue) has been approved by HAAD for 2 CME credit hours. 2 Hours Intraoral impression-taking: Digital datasets soon to catch on everywhere _The annual meeting of the German Society of Dentistry and Oral Medicine (DGZMK), held in conjunction with the Society for Dental Ceramics (AG Keramik), the DGZMK’s professional society, is a major event that critically examines experiences with all- ceramics andCAD/CAMmethodsinclin- icsandprac-tices.Atthisyear’s meeting, the 10 th Annual Ce- ramic Symposium, Prof Bernd Wöstmann, Head of Prostho- dontics at the University of Gießen in Germany, fo- cused in his paper on the progress that has been made in the digi- tisation of intraoral impres- sion-taking. Naturally, perfectly fitting restorations that can be seated without further correction are every dentist’s wish. This re- quires exact impressions of the preparation and dental arch. Quite some time ago, digitisation madeinroadsintothisdiscipline, beginning in 1985 with the first digital impressions by Prof Werner Mörmann at the Univer- sityofZürich.ProfWöstmannex- plained that en route to an exact restoration, creating an image of the intraoral situation either as a real or a virtual model is a very crucial step—it is only possible to produce the final restoration in- directly, whether it is an inlay or amulti-unitfixeddentalprosthe- sis bridge (FDP). Owing to material and haptic conditions, it is still impossible to produce a “flawless” conven- tional (stone) model from classi- calimpressionswithelastomeric im- pression material. Every vir- tual model produced on the basis of a classical impression is inex- act, regardless of the accuracy of the scanning procedure itself. It thus makes sense to perform scanning directly in the oral cav- ity. Now that producing all-ce- ramic restorations with- out CAD/CAM has become almost unthinkable, thenextstephasal- ready been taken towards com- plete digitisation of the process from preparation to seating the prosthesis: optical scanning to createadigital,intraoralimpres- sion. In terms of clinical use, the de- vices—CEREC AC (Sirona), C.O.S. Lava (3M ESPE), iTero (Cadent-Straumann)—are simi- lar, but they function according to different principles. Techni- cally, the systems are similarly constructed, but the procedures foracquiringthe3-Ddatasetsdif- fer. The acquisition unit of CEREC AC uses short-wave blue light and functions according to the principle of structured-light projection (Fig. 1). The scanning procedure captures single im- ages; the angled imaging func- tion acquires tooth areas below the equator and thus increases accuracy. Through matching, several images are computed of aquadrant orwholearch(Fig.2), as are the antagonist dentition and bite record. The wavefront sampling of C.O.S. Lava captures the tooth shape by moving the video cam- era over the teeth. The distance to the camera can be calculated from the changing position of in- dividual pixels during filming, giving rise to a 3-D image of the dental arch (Figs. 3 & 4). The functioning of the iTero scanner is based on the principle of laser triangulation. The image captures the tooth and vertically scans300levels,each50µmdeep (Figs. 5 & 6). According to Prof Wöstmann, the scanning ac- curacy of CEREC AC and C.O.S. Lava cor- responds to a conventional hy- drocolloid or polyvinyl-siloxane im- pression. The differences werenotsignificant.1 Meas-ure- ments of crown copings fabri- catedwithC.O.S.Lavayieldedan average of 33 µm (± 16 µm) for all marginal gaps. Copings pro- duced using the conventional im- pression-taking technique had a mean marginal gap of 69 µm (± 25 µm). Syrek et al. found comparable results in a clinical study.2 The mean marginal gap of conven- tionally manufac- tured crowns was 71 µm, as com- pared with 49 µm for the C.O.S. Lava crowns. For CEREC 3D, the literature cites a tolerance of 40 µm (± 21 µm). Another advantage of digital impressions is that the scanned preparation can be checked di- rectly on the screen, where im- perfections can also be immedi- ately corrected (Figs. 7 & 8). For patients with an easily trig- gered gag reflex, these scanning methods greatly im- prove treat- ment comfort. Further benefits result from fewer working steps involved, especially in the prac- tice. Choosing an impression tray, mixing the elastic impres- sion compound, waiting during setting and disinfection, as well as producing a model are no longer necessary. Fewer treatment and work- ing steps also mean fewer sources of error and better stan- dardisation, which in turn can improve the predictability of treatment out- come. Prof Wöst- mann cautioned that with crown mar-ginsthatareclearlysubgin- gival, the optical systems reach their limits; thus, conventional impression-taking techniques are still used in such cases. _Digital impressions are more accurate At the 12th annual meeting of the International So- 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 conven- tional impressions demonstrate anelasticrecoveryof98.5%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 of up to 114 µm. Different elastomeric im- pression techniques can cause considerable deviations. For in- stance,inana-logueimpression- taking using different impres- sion materials and trays, dimen- sional changes compared with the reference (a cast metal con- trol) varied be- tween 0.32 and 1.17 %. A deviation of 49 µm was found for standard and 122 µm for control impression- taking.6 Asarule,however,thestudieson analogue impression-taking techniques were performed us- ing 2-D measurements; the new studies on the imaging accuracy of optical methods were con- ducted with 3-D volume differ- ence analyses. Digitally or optically pro- duced images by different opera- tors exhibited a measurement accuracy of 11 µm. With the analogue impres- sion-takingtechnique,thede-vi- ations for a whole quadrant ranged from 72 to 101 µm, while the measurement error toler- ance of digital im- ages is only about 35 µm, thanks also to the enhanced accuracy made possi- ble by angled images. Potential sources of error in the digital im- pression-taking tech- nique are scanner adjustment, magnetic interference fields during image processing, image noise and the software. According to Prof Ar- netzl, these results prove that given the correct use of a camera or scanner, digi- tally generated data exhibits fewer errors and greater Fig. 1 Fig. 3Fig. 2 Fig. 4 Fig. 6Fig. 5 Fig. 7a Fig. 8 Fig. 7b