During increasing demands for esthetic and biocompatibility concerns,

During the recent decades, due to the increasing demands for esthetic and biocompatibility concerns, using prosthetic ceramic restorations has grown rapidly (1, 2). Esthetics and resistance to fracture are two of the fundamental determinants in clinical success of ceramic restorations; likewise any other dental restoration(3). Marginal adaptation is the third requirement and plays an important role in the long-term success of restorations (3, 4). According to literatures (5-7), deficiency in marginal adaptation causes serious complications, which may lead to failure of 2 to 6% of ceramic restorations with yttrium-stabilized zirconia copings after 3 to 5 years (8, 9). The marginal fit of restorations is influenced by various factors such as preparation, finish line design(10-12), material and methods used for fabrication of the restoration(13-15) and different conventional or digital impression techniques(16-18).

The introduction of computer-aided design (CAD) and computer aided manufacturing (CAM) technologies, provides an innovation in production of high performance ceramic restorations in a shorter period of time (19-21). In recent decades, all ceramic restorations have largely benefited from yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) copings due to its unique characteristics including excellent biocompatibility, low plaque accumulation and sophisticated high strength(21, 22). Advanced improvements in CAD-CAM technology helped in processing of zirconia, which includes a series of steps, such as scanning, software designing of the restoration, milling and sintering (16, 23, 24). This process chain starts with taking an impression of the clinical situation which is the most critical step (16, 23). Indirect laboratory digitizing starts with a conventional impression. Subsequently, dental stone can be poured into these impressions (20, 25-27). The impression or the resulting gypsum model can be digitized using a scanner (17, 23, 25, 28). An ever more widely applied alternative is direct intraoral digitization, which is the latest development in CAD/CAM dentistry and does not require the use of conventional impressions, trays and gypsum model fabrication (17, 23, 28, 29). Each of the above mentioned methods of data capturing have advantages and disadvantages.

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As mentioned, for indirect laboratory digitization, conventional impression is still needed. Therefore, extraoral digitization always encounter with the errors introduced by impression and gypsum model fabrication, besides the errors resulting from the digitization(29). conventional impression process has several disadvantages, including patient discomfort caused by gagging, pain or an unpleasant taste, distortion of the impression materials, limited suitability for storage, technique sensitivity, imprecision in detail reproduction such as pulls, tears, bubbles, voids, tray-to-tooth contact or separation from the impression tray, inaccurate pouring, limited working time, bacterial contamination, which requires disinfection of the impressions before casting the working models and the overall long process chain (16, 17, 20, 30).

In contrast, by direct digital intraoral impression techniques, such conventional impression drawbacks might be eliminated (20). intraoral scanners represents a logical direct access to dental CAD/CAM system and are more widely used in clinical dental practice, since their application simplifies the overall procedure and causes significant savings in time and materials (16, 31) and make it feasible to achieve an accurate representation of the soft and hard tissues (17).However, not all aspects related to intraoral scanning may be considered desirable (16). Intraoral digital scanners are encountered with several issues including, complication in scanning of subgingival finish lines, problems with the intraoral conditions such as the presence of moisture (saliva or blood), movements of the patient and confined space of the oral cavity specially in the molar region (16, 23, 29, 32). Because of the space restrictions, intraoral devices should have a smaller measuring tip than extraoral digitizers and several captures must be obtained and combined by matching process(23).Moreover, some intraoral scanners require the application of a powder on the tooth surface to reduce the translucency and reflections, which may produce some dimensional error(16, 29). All of the mentioned possible complications and errors may lower the precision compared to conventional impressions (25, 31, 33).
Extraoral digitizing eliminates many of the problems noted above and have been proved to be more steady and accurate(31) The digitization is possible by scanning of either the impression or the resulting gypsum model(23, 28). Impressions are normally made by using vinyl polysiloxane impression materials because of the high accuracy and stability (32, 34, 35). One benefit of directly digitizing the impression material is that it is no longer necessary to make a gypsum model, which is a time-consuming and error-prone method(29). It is even possible to digitize the impression directly with office intraoral scanner and It is obvious that ability to be digitized is one of the highly benefits of an impression material (32).

Several previous studies, compared the marginal and internal fit of restorations obtained using conventional or intraoral digital impression methods (18, 36-38). However, there are limited published data in dental literature regarding to the marginal fit of zirconia copings provided by direct intraoral digital scanners in comparison with different types of indirect digital techniques (scanning the vinyl polysiloxane impressions by in office intraoral scanner or scanning it by laboratory extraoral scanner or scanning the gypsum models by extraoral scanner). Therefore, the aim of this in vitro study was to evaluate and compare the marginal fit of zirconia copings fabricated by each of the above four mentioned techniques. The null hypothesis believed that there would be no difference in the marginal gap of copings fabricated by these four methods.

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