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Dental Tribune Middle East & Africa Edition

10 Dental Tribune Middle East & Africa Edition | September-October 2014clinical > Page 12 Fig 1. Scnannig the preparation Fig 2. Drawing the limit line Fig 3. Designed molar restorations using dental designer software. Lingual view 1 - Chairside technique: The development of Computer Aided Design-Computer Aided Machining (CAD-CAM) tech- nologies for dental applica- tions has enabled clinicians to prepare and indirectly restore tooth tissue with an esthetic all-ceramic restoration, manu- factured at the chairside in a single patient visit. Chairside CAD-CAM tech- niques offer advantages to the patient including eliminating the laboratory procedure and the requirement for intra-visit temporization of the prepared tooth structure:8 It eliminates several cumber- some dental office tasks, such as selecting trays, preparing and using materials, disinfect- ing and sending impressions to the laboratory. It also removes a source of discomfort and gagging. Moreover, it enables the clinician to take a digital impression as well as design and mill the restoration in-of- fice, and to fabricate cosmetic crowns, onlays and veneers, With full management over contours and tooth shade and finally it enhances the accura- cy of adaptation of the restora- tion to the preparation.9 In summary, with these sys- tems, final restorations are pro- duced in models created from digitally scanned data instead of plaster models made from physical impressions. There are three main sequenc- es to this workflow. The first se- quence is to capture or record the intra-oral condition to the computer. This involves the use of a scanner or intra-oral cam- era. During scanning , the clinician must ensure that all margins of the cavity are captured by the scan and visualized. The accuracy of CAD/CAM restora- tions depends on the scanner’s ability to visualize the margin. A true laser scanner/digitizer takes precise digital images of the preparation, including the margin, the undercuts, the contours, the adjacent denti- tion, and the gingiva. It cap- tures hundreds of thousands of points of reference with each image, and then utilizes a mil- lion data points to create an ex- act replica of the prepared tooth and neighboring dentition. Depending on the system, a light and rapid dusting of an opacifier may be required prior to capturing the digital scans of the preparation arch, opposing arch, and buccal bite registra- tion. Once the data has been recorded to the computer, a software program is used to complete the custom design of the restoration, the preparation is shown on the monitor and can be viewed from every angle to focus or magnify areas of the preparation. Inadequate imag- es are automatically detected. The “die” is virtually cut on the virtual model, and the finish line is delineated by the dentist directly on the image of the die on the monitor screen. Then, a CAD system, called “biogene- ric”, provides a proposal of an idealized restoration and the dentist can make adjustments to the proposed design using a number of simple and intuitive on-screen tools. The software identifies match- ing morphological characteris- tics (fissures, cusps, marginal ridges, gliding contact angle) and then inserts corresponding cusps, fossae, fissures, contacts surface into the virtual model of the restoration. On the basis of the contact point distribu- tion, the cusp apexes and the proximal contacts, the soft- ware is capable of creating a well-matched tooth and detect- ing possible collisions with the bite registration. This biogeneric modelling pro- cess creates natural, individual and functional occlusal sur- faces. A pre-manufactured block is inserted into the machine and is milled using diamonds. The final sequence requires a milling device to fabricate the actual restoration from the de- sign data in the CAD program. Digital systems The Cerec Bluecam, E4D in- traoral digitizer, and i Tero scanner are considered single- image cameras. They capture a series of individual digital im- ages that overlap one another. The overlapping images are “stitched” together by the com- puter software program to pro- cess a single 3-D virtual model. CEREC AC ® system powered by BlueCam: A LED camera projects a changing pattern of blue light onto the object us- ing projection grids that have a transmittance random distri- bution and which are formed by sub regions containing trans- parent and opaque structures. Thus, the intensity of light de- tected by each sensor element is a direct measure of the dis- tance between the scan head and a corresponding point on the target object. As a disadvan- tage of the system, the triangu- lation technique requires a uni- form reflective surface since different materials (as dentin, amalgam, resins, gums) reflect light differently. It means that it is necessary to coat the teeth with opportune powders before the scanning stage to provide uniformity in the reflectivity of the surfaces to be modeled. TheearlierversionsofCEREC® employed an acquisition cam- era with an infrared laser light source. The latest version employs blue light-emitting diodes (LEDs); the shorter- wavelength intense blue light projected by the blue LEDs al- lows for greater precision of the output virtual model. The E4D Dentist system was introduced in early 2008. It consists of a cart containing the design center (computer and monitor) and laser scanner head , and a separate milling unit. The IntraOral Digitizer is a single image camera with red laser light. It also works by re- cording reflected data from the hard and soft tissues.10 The Cadent iTero digital im- pression system by Cadent LTD, IL came into the market in early 2007. iTero system em- ploys a parallel confocal white and red laser light camera to record series of single images to create 3D model. The scan- ner emits a beam of light that is reflected off the tooth sur- face. Only data reflected back through the filtering device at the correct focal distance is re- corded.11 Using this technique iTero cap- tures all structures and materi- als found in the mouth without the need to apply any reflective coating to the patient’s teeth. 2 - Integrated chairside-labo- ratory procedure An integrated chairside—labo- ratory technique requires two visits. The clinician either can scan the preparation directly and then send the scan to the labo- ratory, or can take a traditional impression, after which a stone model is poured and the labo- ratory scans the stone model. The digitalization of the dies was performed by a laser scan- ner (Cercon eye, DeguDent®, Hanau, Germany) and the substructures were designed on the CAD program of the system. Digital impression sys- tems are designed to electroni- cally transmit the recorded data file to the dental labora- tory for restoration fabrication. Efficient chairside assistants will increase the overall pro- duction of dental practices by aiding dentists in completing their procedures more quickly and more effectively. Other systems are also used by laboratories to create copings, substructures, and abutments by CAM, after which hand fab- rication of any required ceram- ics and finishing is conducted either by the same laboratory or by the laboratory that scanned and referred the case for mill- ing of the substructure. Ceram- ic blocks for laboratory-milled restorations are available as zirconia (zirconium oxide) and lithium disilicate glass blocks. Zirconium oxide can be used to create accurate and strong cop- ings and bridge substructures. After milling, the unit can be adjusted using an external lin- er (Zirliner, Ivoclar Vivadent) that enables characterization before the outer ceramic su- prastructure is created. The external ceramic layer can be created either using press ce- ramics (in the same manner as for a traditional bridge) or layering ceramic material onto the substructure using a fine brush and powder/liquid. Advantages of a laboratory CAD/CAM milled restoration include reduced chairside time and increased accuracy. Since a stone model is not used, stone pouring errors are eliminated as well as errors associated with abrasion of the adjacent and opposing teeth due to ma- nipulation of the models during fabrication that could result in over-contouring, tight contacts, and excessive occlusal height. In addition, reduced time is required for fabrication of the substructure. Materials CAD/CAM restorative mate- rials are currently available in number of sizes in many shades and translucencies, in- cluding multiple shades within one dense gradated restorative block. The material used de- pends on functional and es- thetic demands and on whether a chairside or laboratory CAD/ CAM restoration is fabricated.13 A range of dental ceramic sub- strates have been developed for chairside machining and are represented as prefabricated blocks, manufactured using processing routes identified to reproducibly control the resul- tant ceramic composition and microstructure.15,15,16 For chairside CAD/CAM res- torations, an esthetic, strong material requiring minimal post-milling esthetic adjust- ment to minimize chairside time is needed.17,18,19 Leucite- reinforced glass ceramics (IPS Empress CAD, Ivoclar Viva- dent; Paradigm C, 3M ESPE) and lithium disilicate glass ceramics (IPS e.max, Ivoclar Vivadent) can be used for chair- side and laboratory CAD/CAM single restorations. Leucite-re- inforced material is designed to match the dentition for strength and surface smoothness and to offer esthetic results by scatter- ing light in a manner similar to enamel.20 A study has been done to evalu- ate and compare the marginal gap, internal fit, and fracture load of resin-bonded, leucite- reinforced glass ceramic mesio-occlusal-distal (MOD) inlays fabricated by computer- aided design/manufacturing (CAD/CAM) or hot pressing: as a result, they provided clini- cally acceptable marginal and internal fit with comparable fracture loads after luting.21 Ceramic blocks for laboratory- milled restorations are avail- able as zirconia (zirconium ox- ide) and lithium disilicate glass blocks. Zirconium oxide (IPS e.max ZirCAD, Ivoclar Viva- dent; Cercon, Dentsply Ceram- co) can be used to create ac- curate and strong copings and bridge substructures. Zirconia offers some significant physi- cal properties that are advan- tageous for dental restorations besides its high strength. It has a similar color to natural teeth, which reduces the need to opaque it or mask it as would be done for a metal substructure. Zirconia also has good opac- ity. This may be an advantage when trying to block out under- lying discolored teeth or restor- ative materials. It may also be a disadvantage when trying to develop a more translucent ap- pearance to the crown. Some manufacturers can color the zirconia substructure to simu- late dentin shades to improve the desired esthetic result.22 After milling, the unit can be adjusted using an external lin- er (Zirliner, Ivoclar Vivadent) that enables characterization before the outer ceramic su- prastructure is created. The external ceramic layer can be created either using press ce- ramics (in the same manner as for a traditional bridge) or layering ceramic material onto the substructure using a fine brush and powder/liquid. Composite resin blocks are also available for CAD/ CAM resto- rations.23 Another option is the use of a new resin nano-ceram- ic block that consists of ceramic clusters within a highly cross- linked resin matrix. The result- ing block is homogenous, and the restoration can be CAD/ CAM-milled chairside or in the laboratory. Discussion Marginal adaptation is an im- portant factor affecting the lon- gevity of all-ceramic restora- tions.24 Considerable research hasbeeninvestedinthemargin fit and internal adaptation of CAD-CAM restorations.25,26,27,28 Software limitations as well as accuracy of milling devic- es may affect the fit of CAD/ CAM restorations. Most clini- cians agreed that marginal gap should not be greater than 100 μm. It has been reported in the literature that restorations pro- duced by CAD/CAM systems can have marginal gaps of 10- 50 μm which is considered to be within the acceptable range.29 Giannetopoulos S and Al in- vestigated and compared the marginal integrity of ceramic copings constructed with the CEREC3 and the EVEREST system employing three dif- ferent margin angle designs. They explored to what extent these CAD/CAM machines can produce acute marginal angles creating restorations with ac- ceptable margins. They found < Page 9 Fig 4. Designed molar restorations using dental designer software. Occlusal view

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