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Recent Advances in Digital Impressions
When it comes to data acquisition of the intraoral condition, dentistry is undergoing a paradigm shift from the analog world to the digital one. The gold standard for capturing this intraoral information has been creating a stone model from an intraoral impression, which is usually created from an elastomeric material loaded into an impression tray. It has been proven that current digital impression devices have similar accuracy when compared to the gold standard.1 Excellent clinical results have been shown when proper technique is used.2 In vitro studies have shown that digital impressions are equally accurate when compared to stone models made from polyether impressions. This applies for trueness (accuracy compared to subject model) as well as precision (consistency between subsequent impressions, comparatively).3 As a result, clinicians can be confident that adopting this modality allows consistent if not superior care for patients while being more efficient for dentists and their staffs.4 In addition, patient comfort and perception is superior with digital impressions compared to conventional impressions.3,5 Digital impressions have been available for more than 30 years, and now there is a very active growth phase for this technology. This growth phase has brought new products to the market and, more importantly, advances the technology.
Steps in the Digital Process
Data acquisition is the initial step in the digital process and is most accurately done directly in the mouth using a digital impression system (Figure 1 through Figure 3), although it also can be accomplished using a stone model or impression with a tabletop scanner. Scanners attempt to address the inaccuracies that can be found in traditional techniques, including elastomeric silicone inaccuracy, gypsum model inaccuracy, and margin tearing.6 Ideally, digital data would be captured directly from the source rather than producing and then scanning a model with inaccuracies. Intraoral scanners use light and sensors to replicate the topography of the area of interest. The information is captured on either a stand-alone desktop computer or a laptop, and can enter a traditional workflow via a printed or milled dental model. One distinct advantage is that these models can be mounted via a scanned bite taken of the patient in occlusion, which removes human error out of the mounting process. It can also be used digitally via a stereolithography (.stl) computer file. Every computer file has an extension, and digital scanners save files with the extension .stl) This .stl file is a standardized file that can be read and modified. Once in .stl format, the data is very portable via the Internet and it can be used to create dental restorations (Figure 4).
Computer-aided design (CAD) is the next step in the process. Design software is used to design an ever-increasing array of restorative options that include crowns, fixed partial dentures, inlays, onlays, veneers, provisionals, implant custom abutments, splints, partial frameworks, dentures, implant surgical guides, zirconia hybrid prostheses, and cobalt-chromium frameworks. The design software allows the clinician to import the .stl file that was captured of the intraoral condition and design custom-fabricated restorations. Currently, design software is widely used in laboratories to create restorations more efficiently. This often takes the place of diagnostic wax-ups and allows for detailed fine-tuning of contacts and occlusion, which can be standardized. The design software produces a file that can be used to fabricate restorations in milling or printing devices. Some examples of design software include Dental System (3Shape Inc), DWOS (Dental Wings), and exocad® DentalCAD (exocad GmbH).
Computer-aided manufacturing (CAM) is the final stage in fabricating a designed restoration. CAM uses the design software to control the machinery that builds the restoration or the coping for a restoration. There are a variety of options for fabrication, but the most common is milling in a wet environment using standardized material blocks and precision-cutting instruments. It has been shown that CAD/CAM frameworks fit better than conventionally fabricated frameworks in implant-supported prostheses.7 Although currently there are no approved printed products available for long-term use in the mouth in the United States, 3D printing is being increasingly used. It can be used to fabricate wax patterns for crowns or surgical guides for implant placement or even metal can be printed. Milling machines and printers allow for operational efficiency by creating multiple restorations at one time.
Digital systems are typically divided into two main categories, systems designed primarily for data acquisition via digital impressions, and systems that not only acquire the digital impression but design and fabricate the restoration (CAD/CAM). For the purpose of this article, the focus will be on digital impressions but an understanding of system options is important. Digital impression systems capture the data and this data is sent to the laboratory for restoration design and fabrication. CAD/CAM systems are designed for data acquisition, design, and fabrication all within the dental office. This option eliminates the laboratory from the workflow and brings that responsibility into the dental office. Obviously the CAD/CAM workflow requires additional dentist and staff training to produce quality dental restorations.
Open vs Closed Architecture
Digital systems are typically divided into two types of software architecture: open and closed.
Open architecture allows for the interpretation and modification of files by a varied number of software programs using the .stl file. The data in the file is not encrypted and can be opened by any program that accepts .stl files. It basically allows people to access the data, so different brands of software and machines can talk to one another in a computer format and use the information in the file.
Closed architecture is a system in which technical specifications are not made public, thus modification by third-party software is restricted. The data is encrypted in such a way that brand-specific software must be used to open it. This significantly restricts the software and machines that can use files made in closed format. The information may still be stored as an .stl, but it is not readable by anything not designed to be compatible. Digital-impression–focused systems must be open because they require third-party software for design and fabrication. CAD/CAM systems are often closed because they accomplish all facets of the process within one system. This may prevent the ability to use scanners, software, and milling machines of different brands. As laboratories could potentially receive digital impression files from many brands of scanners, it makes sense to provide an open architecture (.stl) for reading the data, which could help to avoid the expense of redundant equipment.
Digital Impression Systems
There are a number of digital impression systems currently on the market, all with unique characteristics. Following is a very brief description of several such systems and how they work. Clinicians should investigate any system in which they are interested more thoroughly with the system’s manufacturer.
CS 3500 by Carestream Dental
This is is a powder-free intraoral scanner that provides color 2D and 3D images by taking a fast series of photographs which are then joined together by a computer algorithm to speed image acquisition. The acquired color-rendered impression can be sent to the laboratory via open .stl file, or it can be used to design and fabricate a restoration with Carestream’s design software and milling machine.
CEREC AC with Omnicam by Dentsply Sirona
This system uses continuous photo-stitching to produce a 3D color-rendered image without powder. Typically it is used as a CAD/CAM system where restorations are designed and milled using the accompanying software and milling unit. It uses closed architecture; therefore, Sirona’s software is necessary for data manipulation.
iTero® Element™ by Align Technology, Inc
This system uses parallel confocal imaging technology to capture a color-rendered 3D image. It allows for continuous stitching of the images that are acquired to produce a color image without the use of powder. Files can be exported in open .stl format, or they can be used to fabricate polyurethane dental models.
Planmeca PlanScan™ by E4D Technologies
This is a powder-free digital impression system with optional color rendering that uses blue laser streaming technology to produce 3D images. The digital impressions can be exported via open .stl or used to fabricate restorations with E4D’s proprietary design software and milling unit.
TRIOS® 3 by 3Shape
This system is a powder-free, color-rendering impression device using ultrafast optical scanning technology. It automatically scans the color of the adjacent teeth, so it is capable of shade-matching suggestions. The images can be exported via open .stl or processed using design software, including Dental System by 3Shape.
3M™ True Definition Scanner by 3M
This scanner, with the smallest wand currently available on the market, uses blue LED light and video imaging to produce a real-time video model of the intraoral condition, capturing 20 3D images per second. It requires a light dusting of titanium-oxide powder, which improves image-capture speed. Once the image is captured it can be visualized on screen in 3D with the provided glasses. It produces an open .stl file, or printed models can be fabricated.
One big trend is the presence of digital impression systems. According to a newer report by Transparency Market Research, the standalone scanner market is expected to grow from $54.4 million in 2013 to a predicted $178.9 million by 2020, expanding at a compound annual growth rate of 17.1%.8. Dental laboratories are providing incentives to dentists who use this technology because it allows them to be more efficient and reduces human error.
Improvements in the size of the scanner wind and the focal trough distance are two trends that have been evolving over time. The smallest scanner wand currently available has a tip length and width of 15 mm, making it easily maneuverable in the oral cavity. Focal trough, which is the distance the scanner wand must be from the tooth for the image to be in focus, has also significantly improved. This allows the wand to be touching the tooth in many cases and also improves access in the mouth. While further improvement should be expected in these areas, the systems currently on the market already do allow for reaching areas that were previously found difficult, such as the distal aspect of the second molars.
Workflows are being created around digital scanners to improve the quality of care. Recording the preoperative condition of a patient could be very helpful in creating restorations that allow for the replication of tooth-wear patterns, thus improving the results of care.
Color scanning is an obvious trend in digital impressions. Some of the reported benefits include improved margin visualization for easier margin marking. While the image is in color, it is a computerized color rendering. Currently, it would not be accurate to say that color scanners are as accurate as traditional digital photography.
Scanning without powder is a trend also on the rise. Powder application speeds image-acquisition rate and reduce reflection.9 Production of open .stl files is found in the majority of scanners described earlier.
Orthodontic offices are using digital scanners for removable aligner cases and data collection in lieu of traditional model-making. Digital impressions are broadening orthodontic applications to improve patient outcomes and predictability. Surgical dentistry is combining digital scans with cone-beam computed tomography to improve surgical outcomes and workflows for implant placement by using precise surgical guides.
Future trends seem to be centered around software advances to broaden use of the scanners. At least three companies that currently make scanners are developing software to be able to evaluate tooth wear and erosion over time. Scans would be taken at different times and evaluated for change. The ease of data collection and storage allows for continued advancement and comparison over time to improve outcomes of patient treatment. Recording and archiving the condition of the mouth digitally over time could lead to a greater understanding of how and why things change in the mouth over a lifetime. It also would lead to precise analysis of how much change is happening, which puts credible data into treatment planning.
Scan strategy is the path the scanner wand takes while the user captures the intraoral condition. This path has been shown to affect the trueness of the acquired data.10 Scan strategy can vary between manufacturers and technology, thus it is crucial to learn the best scan strategy for individual scanners. Ender and Mehl showed that not following the manufacturer’s scan strategy caused significant changes in trueness for a scanned object.10 When scanning objects, the scanner looks for familiar shapes to know how to build an image. In the mouth, the buccal and lingual surfaces have similar shapes that make it hard for the scanners to orient themselves. One thing that powder does is apply a unique topography to the surfaces of the teeth, making it easier for scanners to understand where in space they are, thus speeding image acquisition. One of the most common scan strategies is to start from the occlusal, return on the palatal, and go back on the buccal. This allows the scanner to see parts of all three surfaces on the first path and build the image by adding the data acquired on the palatal and facial. Studies have shown this to be the most accurate.11 Other scanners may have a strategy that additionally dictates pulsating near and far from the surface to be scanned. Scanning a full arch also has specific scan strategies; scanning a sextant at a time and then moving to the next with the same scan strategy on each sextant. Although these concepts are not challenging, it is important to realize how to get the most accurate information out of each scanner. This is a newer concept so it is important to check with the manufacturer to understand their recommendations.
Accurate scanning ultimately relies on proper isolation to capture the intraoral condition without the influence of blood, saliva, tongue, cheek, or a dental device. Most accuracy studies are performed in vitro and have shown intraoral scanners to provide incredible accuracy compared to conventional techniques.2,9,12 In the mouth, with the variety of factors that challenge everyday dentists, proper isolation is crucial to maintaining that accuracy. It is not only important to isolate the section of the mouth to be imaged but also the area of interest to be scanned. To isolate the mouth, retractors are very helpful with the addition of dry angles to block the parotid glands. In the mandible, cotton roll isolation around the tongue is helpful to absorb saliva during scanning.
Tooth isolation principles are similar to traditional isolation techniques for elastomeric intraoral impressions. One advantage of elastomerics is that they are capable of capturing undercut surfaces, especially interproximally. Digital impression systems need a direct line of sight to capture data. They can only record what they can see. When retracting the gingiva around restorative margins, it is crucial to retract horizontally so that the scanner can differentiate tooth from gingiva. This is often done with two retraction cords or a diode laser. In the author’s experience, the most challenging areas to scan tend to be the interproximal restorative margins, subgingival margins, and interproximal surfaces of adjacent teeth.
All digital impression systems are capable of capturing information and providing high-quality single-tooth and full-mouth impressions comparable to, if not superior to, traditional dental workflows.13 A recent study in the ADA Professional Product Review evaluated the in-vitro accuracy of six intraoral scanning devices.14 The article compared trueness (accuracy compared to a reference dataset) and precision (accuracy between subsequent data sets produced by the scanning device) in replicating a single molar tooth in vitro. The article concluded that all of the scanning devices studied were capable of producing accurate single-tooth scans. A comparison of 3-unit fixed partial dentures created in a CAD/CAM approach reported the greatest accuracy when using intraoral scanners compared to other modalities. A marginal gap of 26 µm was reported with one of the scanners, which is dramatically better than the acceptable standard of 120 µm.15 Another study measured accuracy by scanning implant abutments in different areas of a dental arch.16 Three polyetheretherketone cylinders were scanned about the arch and compared against measurements made on a high-accuracy 3D scan. The results revealed that all of the scanners studied were capable of producing single- and multi-unit restorations similar to traditional techniques. Studies of full-arch models used in orthodontics have shown reliable accuracy, reliability, and reproducibility compared to plaster models with greater efficiency.17 It is the operator’s technique that has the greatest influence on treatment outcomes.
At this point digital impression systems have been shown to be as accurate if not superior to traditional intraoral impressions.18 It has also been proven that patients prefer digital impressions rather than traditional impressions.19 Clinicians are more time-efficient when using digital impression techniques vs traditional techniques.20 Yuzbasioglu showed an average treatment time for traditional impressions to be 605.38 seconds vs 248.48 seconds for digital impressions.21 This study compared the time it took to only take the opposing full-arch polyether impressions with bite registration versus it took to complete patient data entry, fill out the lab prescription, and take the opposing full-arch digital impressions with bite scan of the occlusion. It is obvious that this technology is where dentistry is headed and is quickly going to become the standard for capturing the intraoral condition. Linking digital scanners to in-office 3D printers and increasingly sophisticated milling units will be the next logical step for the advancement of this technology. It is very logical that once printed materials are approved for use in the mouth, dentistry will find expansion of the varied use of digital scanners.
The author believes it is quite possible that the next evolution in technology will lead to the use of ultrasound technology. Advantages might include the ability to penetrate tissue, saliva, and blood without retraction. This panacea could simplify the digital capture of the intraoral condition.23 There are no ultrasound devices currently available but there are companies seeking FDA approval for this type of technology, so it is evident the future of intraoral data capture is digital.
Looking to the future, complex orthodontic devices using 3D printers and wire-bending robots are already being used outside of the United States.22 The orthodontic appliances are made in a totally digital format and exhibit excellent fit. Ultimately, the quality of the end product for dental patients should always be the primary concern. As workflows for a rapidly changing digital world are being redefined, we should always have the advancement of results as our primary concern.
Dr. Hodges received an honorarium from Updates in Clinical Dentistry for the preparation of this manuscript.
ABOUT THE AUTHOR
Dr. Hodges is a general dentist in Asheville, North Carolina.
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