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MANUSCRIPT TITLE – DIGITAL IMPRESSIONS IN PROSTHODONTICS – A REVIEW ARTICLE

ABSTRACT

Digital imprint and computer-aided design/computer-aided manufacture (CAD/CAM) systems offer several benefits compared to traditional techniques.Digital technologies have revolutionized clinical prosthodontics, extending diagnostic, treatment and follow-up possibilities. The concept of intraoral digital impressions was put forward in the early 1980s.Intraoral scanners are used to create a digital image of the patient’s teeth,eliminating the need for traditional impression materials, as well as increasing patient comfort and decreasing anxiety. It has drawn comprehensive attention from dentists and has been used for dental prosthesis fabrication in a number of cases.Recently, Intraoral scanners have been successfully used in the fabrication of removable dental prostheses. This new digital impression technique is expected to bring about absolute digitization to the mode of prosthodontics. This article discusses the various digital impression systems available in the market, to provide the clinician complete information and knowledge of application of the technology.

INTRODUCTION
Digital impressions represent innovative methods that enable dentists to construct a virtual, computer-generated copy of the hard and soft tissues of the oral cavity, with the use of lasers and other optical scanning machines. The digital method captures impression data with great accuracy.1 The accuracy of Intra oral scanner consists of trueness and precision. In dentistry CAD/CAM technology has been used since the 1980s to produce inlay and onlay fillings, crowns, laminates, fixed dental prostheses, implants, and is increasingly used by dentists and technicians. The brand name “CEREC” was the first CAD/CAM system used in dentists’ offices and came on the market in 1987. It was initially designed for the manufacture of esthetic ceramic restorations.2

CAD/CAM systems are composed of three major parts:
(1) Data acquisition unit
(2) Software for designing virtual  restorations
(3) A computerized milling device.

Advantages of digital    impression
•    Accuracy of impressions
•     Opportunity to view, adjust and rescan impressions
•     Saves time and one visit for in-office systems
•     Opportunity to view occlusion
•     Accurate restorations created on digital Models
•    Accurate, wear and chip-resistant physical CAD/CAM derived models
•    Cross-infection control4
•    Register excessively movable tissue in a passive state to achieve a mucostatic impression
•    Reduces gag reflex and sensitivity to impression material

Disadvantages of digital  impression
•    Lack of familiarity among dentists.
•    The cost of the equipment is high, but depreciation it become cheaper than the conventional technique.1
•    The scan needs to emphasize the finish line and precisely duplicate the surrounding and occlusive teeth.
•    Digital scanning requires the same type of soft-tissue management, retraction, moisture control, and hemostasis that is important for conventional impressions.

Digital intraoral scanner
Digital intraoral scanners are considered Class I medical electrical devices, designed    and constructed in accordance with the standards of ANSI/IEC 60601-1.

Every scanner has three major components: a wireless mobile workstation to support data entry; a computer monitor to enter prescriptions, approve scans, and review digital files; and a handheld camera wand to collect the scan data in the patient’s mouth.

Basically there are two different scanning possibilities:
Optical scanners  and mechanical scanners.

Optical scanner
The basis of this type of scanner is the collection of three-dimensional structures in a so-called ‘triangulation procedure’. Here, the source of light (eg laser) and the receptor unit are in a definite angle in their relationship to one another. The following can be named as examples of optical scanners in the dental market:
Lava Scan ST (3M ESPE, white light projections)
Everest Scan (KaVo, white light  projections)
es1 (etkon, laser beam).

Mechanical scanner
In this scanner variant, the master cast is read mechanically line-by-line by means of a ruby ball and the three-dimensional structure measured. The Procera Scanner from Nobel Biocare  is the only example for mechanical scanners in dentistry. This type of scanner is distinguished by a high scanning accuracy. The drawbacks of this data measurement technique the inordinately complicated mechanics, which make it very expensive with long processing times compared to optical systems.

Various digital impression system
The main intraoral digital impression systems currently available in the market include CEREC, Lava C.O.S. system, iTero, E4D, and TRIOS. They vary from each other in terms of key features such as working principle, light source, the necessity of powder coat spraying, operative process, and output file format.

CEREC system
The CEREC 1 system was brought to market in 1987 as the first intraoral digital impression. This system is designed with the concept of “triangulation of light,” in which the intersection of three linear light beams is focused on a certain point in 3D space. Currently, the most prevalent CEREC system is its fourth generation product, known as CEREC AC Bluecam. It captures images using blue LED diode as its light source. The CEREC AC Bluecam can capture one quadrant of the digital impression within 1 minute and the antagonist in a few seconds. Other system involve CEREC AC Omnicam, was brought to market in 2012. Omnicam can be used for a single tooth, quadrant, or full arch. This type of intraoral scanner can be used for single crowns, veneers, inlays, onlays, and implant-supported FDPs.4 The automatic “crown settling,” “cusp settling” and “virtual grinding” functions provide the dentist with a predictable method of controlling the vertical dimension of the restoration.

Lava C.O.S. system
LavaTM C.O.S. (Lava Chairside Oral Scann) is an intraoral digital impression device invented in 2006 and brought to market in 2008. It works under the principle of active wavefront sampling. This principle refers to obtaining 3D data from a single-lens imaging system. Three sensors can capture clinical images from diverse angles simultaneously and generate surface patches with infocus and out-of-focus data by proprietary image-processing algorithms. Twenty 3D datasets can be captured per second, embodying over 10,000 data points in each scan. The Lava C.O.S. has the smallest scanner tip—only 13.2mm wide. The scanner sends out pulsating visible blue light as light source and works with a mobile host computer and a touch-screen display.  With real-time visibility, dentists can immediately see if they are receiving enough information from the preparation, capturing the preparation scan. If the display shows a critical missing or blurry area the dentist simply needs to rescan the specific area, and the software will be amended automatically.

iTero system
iTero was introduced to the market in 2007. The iTero system captures intraoral surfaces and contours by laser and optical scanning based on the principle of parallel confocal imaging. A total of 100,000 points of laser light at 300 focal depths of the tooth structure can be obtained during one scan. These focal depth images are separated at the level of approximately 50 μm, allowing the camera to acquire precise data of tooth surfaces. Parallel confocal scanning with the iTero system can capture all structures and materials in the mouth without coating teeth with scanning powders. This system uses red laser as a light source. iTero is an open system in the treatment of crowns, FPDs, veneers, implants, aligners, and retainers.

E4D system
The E4D system work under the principle of optical coherence tomography and confocal microscopy. It uses red laser as a light source and micromirrors to vibrate 20,000 cycles per second. E4D’s high-speed laser formulates a digital impression of the prepared and proximal teeth to create an interactive 3D image. The laser technology traps images from every angle. The software builds a library of images. The image library can wrap around a precise virtual model in seconds. This system also functions as a powder-free intraoral scanning device. Like the CEREC AC Bluecam and Omnican systems, the E4D system can work with a chairside-milling device. That means this system can also function as a “single-visit treatment” and provide high-strength ceramic prostheses or composite even for minimally prepared teeth.

TRIOS system
In 2010, 3Shape launched a new type of intraoral digital impression system, TRIOS, presented to market in 2011. This system works under the principle of ultrafast optical sectioning and confocal microscopy. The system recognizes variations in the focus plane of the pattern over a range of focus plane positions while maintaining a fixed spatial relation of the scanner and the object being scanned. By analyzing a large number of pictures obtained, the system can create a final digital 3D model instantly to reflect the real configuration of teeth and gingival color. TRIOS intraoral scanner is a powder-free device in the scanning process. The TRIOS system boasts an essential trait, “the variation of the focal plane without moving the scanner in relation to the object being scanned.” According to Logozzo et al “The focal plane should be continuously varied in a periodic fashion with a predefined frequency, while the pattern generation means the camera, the optical system, and the object being scanned are fixed in relation to each other. TRIOS includes two parts: TRIOSR Cart and TRIOSR Pod.

Digital impressions in removable partial dentures
Limited information is available regarding the application of digital technology and CAM software in the fabrication of partial removable dental prostheses or frameworks. Williams et al published an early report on the application of CAD/CAM technology in the fabrication of removable partial dentures. This preliminary report provides proof of concept regarding the use of an intraoral scanner routinely in prosthodontics to accurately capture both hard and soft tissue images. However, the intraoral technique of capturing soft tissue morphology, although accurate, has disadvantages that may limit its application. The intraoral scanner does not capture appropriate extensions of movable tissue that would normally be captured in a conventional manner. Even so, the concept of capturing a digital impression and then using CAD/CAM framework design and printing the framework pattern in resin for conventional casting proved to be an effective alternative to conventional framework fabrication. The technique worked well in Kennedy Class III clinical situation, in which capturing border molded extensions of the soft tissue was not as critical.

Digital impression in complete dentures
The making of an edentulous impression is more difficult than that of a dentulous impression because the object is a soft tissue and the impression must include the basal seat within the limits of the function of supporting and limiting tissues.11 But Several reports have focused on the feasibility and accuracy of intraoral digital impressions for complete arches.12 Conventional complete denture prosthetics require several appointments to register the maxillomandibular relationship and evaluate the esthetics. The fabrication of milled complete dental prostheses with digital scanning technology may decrease the number of appointments. The digital system facilitates the completion of dentures in 2 visits. Impressions, occlusal relation records, and an orientation record are made at the first visit and the dentures inserted at the second. This significantly reduces the time the patient spends in the dental office.  A repository of the digital record is stored, and an exact duplicate denture can be reproduced at any time. The denture can be designed according to the dentist’s specifications. Although initial results are promising, the technique has some disadvantages. The central bearing tracing device can be a challenging method of recording jaw relationships.

Digital impression in fixed partial dentture
Marginal and internal fitness are important criteria for the success of FDPs like ceramic restorations. To obtain a precise restoration, a high level of impression accuracy is important. Syrek et al conducted an in vivo experiment to compare the fitness of zirconia single crowns made from an intraoral digital impression with that from a conventional silicone impression. The study concluded that ceramic crowns fabricated from a digital impression had a better fit than conventional impressions. The interproximal contact was better for digital impressions than for the conventional impressions.

Digital impression in implant dentistry
Digital impressions were proposed as viable alternative to make impressions for tooth- and implant supported restorations. Intraoral scanner system are clinically viable tools for the taking of implant impression demonstrating comparable accuracy and improved efficiency as compared to conventional methods.

Conclusion
The intraoral impression technique has been used in prosthodontics to aids the CAD/CAM process. As a relatively new technique, dental products fabricated with intraoral digital impressions have presented accuracy as compared with conventional impressions but there is a repeatability of the intraoral digital impression which needs to be solved. The dentist must provide a high quality dental care to all patients, in order to maintain and restore oral functions. Therefore, applying novel methods and materials, such as digital impressions  is of utmost importance for future oral health protocols.

References
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