The Current State of Digital Impressions
Four chairside systems can scan a prepared tooth and produce a 3D image.
If a picture is worth a thousand words, then is a scanned image of a prepared tooth worth more than an impression? Will digital imaging become the standard and replace impressions all together? With the rapid advances in technology, these questions may be answered more readily today than just a few years ago. According to one recent article,1 much progress has been made in computerized dental technology, such as digital model scanners, intraoral digital impression-capture devices, cone beam computed tomography, rapid-prototyping 3D printers, laser sintering units, and milling machines, to name just a few. The progress seems to have increased exponentially and there are even more exciting technologies on the horizon. But first, where are we today?
Chairside Digital Impressions
On his instructional videotape, “The Perfect Impression,” Gordon Christensen, DDS, MSD, PhD, states that out of the 40 million impressions taken each year, 90% do not have all of the margins captured, which equates into 36 million crowns that do not have all of their margins captured every year.2,3 That is a sad commentary on our profession. The quality of an impression determines the quality of the indirect restoration. The type of restorative material is only secondary to the absolute representation of the prepared tooth.
The history of the dental impression goes all the way back to using beeswax as the medium.4 Over the years, there have been numerous materials developed to improve the quality and accuracy of the impression, but still, the reproduction of the entire tooth is paramount in producing a superior restoration. It is difficult for the clinician to visualize every detail of the impression until it is poured and produces a positive model. On the other hand, having the capability of taking a picture of the prepared tooth and seeing it on a computer monitor eliminates the “guesswork” associated with conventional impressions. The operator will see exactly what they are getting. Upgrades to computer software have increased the ability to design and manufacture more complex clinical scenarios. Accuracy of the scan versus the fit has also improved greatly.5 The clinical performance of a system is based on the marginal fit and the integrity of the fabricated coping or framework, and should be the basis for the selection of a particular system. For closed systems, the entire scanning device, design module, milling material, and milling unit depends on the sophistication of the CAD/CAM unit.
Comparable studies on fit were first established in 1989 by Holmes et al6 by instituting the terms marginal gap, absolute marginal gap, vertical and horizontal marginal gap, overextension, and underextension (Figure 1 and Figure 2). Since then other studies have compared the fit of milled zirconia restorations. In 2006, at the European division of the International Association of Dental Research, Piowowarczyk and Lauer7 presented a thorough analysis of fit (marginal and absolute marginal gap) of a 4-unit fixed partial denture (FPD) framework fabricated by three different manufacturers. The lowest marginal and absolute marginal gap was fabricated by Lava™ (3M™ ESPE™, www.3mespe.com) as compared to Cercon® (DENTSPLY Ceramco, www.ceramco.com) and Digitizing Computer System (DCS Precident™, Popp Dental, www.poppdental.com). In 2007, Shannon et al8 found in their comparison study of vertical marginal gap of zirconia copings fabricated by five manufacturers that Lava had no significant difference in fit to the control. The vertical marginal gap of zirconia copings fabricated by Everest® (KaVo Dental, www.kavo.com), Procera® (Nobel Biocare, www.nobelbiocare.com), Zeno (Wieland Dental, www.wieland-dental-systems.com) and InLab® (Sirona Dental Systems, www.sirona.com) ranged from 10 µm to 27 µm when compared to control cast copings.
Open-System and Closed-System CAD/CAM Software Architecture
The definition of open-system software architecture is where data is presented in an industry-standard format. The most common is the STL-data format, and can be read by various equipment modules from different vendors.6 In other words, scanned data for the fabrication of a 3-unit FPD could be interpreted by a variety of different designing systems and would not be dependent on the manufacturer. Although it may seem very sensible for a laboratory to get involved with an open system, it is more complicated than it first appears. A laboratory would require someone capable of integrating the received data with the in-house designing unit and be able to analyze and modify that data to complete a desired design. Not all open systems provide the obligatory information to proceed forward with the design phase. It also would take an on-site information technician to interpret various data sources sent to the laboratory. The open system data must be error-free, accurate, and oriented properly in order for everything to work.6
On the other hand, closed-system software architecture collects, interprets, and manipulates data by modules of the same manufacturer. For example, data obtained by a Sirona scanner can only be used by a Sirona CAD/CAM unit. The closed system limits the use to the proprietary data format but, then again, the laboratory does not have to hire an information technology staff member to interpret various incoming data. It is advantageous, however, to choose a technician that can visualize in 3D so that traditional laboratory skills can be applied in a digital format.
There are four chairside systems that are capable of scanning a prepared tooth, digitizing the information, and producing a 3D image. Two of these four systems can fabricate single-unit restorations at the chair (CEREC® by Sirona Dental Systems and E4D by D4D Technologies, LLC, www.d4dtech.com) while the other two (iTero™ by Cadent [www.cadentinc.com] and Lava™ C.O.S. by 3M ESPE) outsource the digital data to a clearing house or a laboratory, via the Internet, which in turn creates a working model used by the dental technician.
The original CEREC and its subsequent second- and third-version acquisition cameras used infrared technology. Sirona introduced the laboratory version in 2001 (CEREC InLab) which used a red laser for scanning. This same red laser technology is used in the E4D acquisition camera. One problem with red laser technology is the lack of complete scanning of an image. A speckled image is the result of incomplete laser light reflection. Therefore, multiple images are required in order to have the entire tooth imaged. A combination of confocal imaging and green laser technology is used in the iTero camera.9 This combination digitally captures the surface contours of the tooth and surrounding gingival tissue. The scanner captures 100,000 points of laser light in perfect focus at 300 focal depths, which are approximately 50 µm apart.10 The Lava C.O.S. System uses a proprietary “3D-in-Motion” technology. This technology captures the image in a continuous 3D video. The file is usually very large (gigabyte level) and requires an overnight download/upload to the central facility.
The new Bluecam technology from Sirona (CEREC® AC) uses a blue light-emitting diode (LED) that increases the speed in which images are captured. The manufacturer suggests that half of an arch can be imaged in 40 seconds while a full arch can be completed in about 2 minutes. The blue LED light is a narrow, shorter wavelength and does not penetrate human tissue as much, making it more reflective from the surface. This characteristic provides better contrast and, therefore, better precision.
This CAD/CAM technology has the longest track record of all the presently available units. The CEREC AC digital impression system with Blucam technology has a number of improved technologies including an automatic image-capturing system that actually determines the focus of the subject and instantly saves the image, eliminating the need for the operator to “click” a button or pedal. Included in the auto-capture camera is an anti-shake function with a broad depth of field.
One of the negatives that previous CEREC owners have shared is the sensitivity of the powdering step with reflective titanium dioxide. Uneven powdering has always yielded inappropriate images and increased the difficulty in using this technology. The CEREC AC is less sensitive to under-powdered areas and allows the operator to capture images faster.
The CEREC Connect Internet portal, which Sirona introduced 2 years ago, allows the dentist to send either a single or multi-unit file to a laboratory that has the laboratory version (InLab). The laboratory, in turn, then downloads the file and proceeds to create models.
The E4D unit acquires an image by means of laser technology. The E4D software, called the Dentalogic™, allows clinicians a 3D image of the real oral environment. They can view the actual enamel on the margins as it really is as opposed to the CEREC software that provides an animated model or one that is covered by a reflection powder. Both systems can produce feldspathic, leucite-reinforced, lithium-disilicate ceramic, and industrial composite resin restorations. The acquisition of an imaged model requires a minimum of nine scans because of the type of laser used. Although it is advertised as a powder-free system, an accent liquid can be used in situations where the adjacent tooth has a gold restoration or thin enamel remains after a partial crown preparation.
The tooth-designing portion of the software is also different from Sirona’s in that it features a system called autogenesis. When the software is developing a proposed restoration, it takes into account the adjacent and opposing teeth. The final proposal is based on what fits the patient as opposed to a pre-designed tooth.
The E4D can design up to 16 restorations on one screen simultaneously, whereas the CEREC can only create one restoration per screen. The advantage of using one screen makes a difference in completing multiple anterior restorations, when spatial requirements and interproximal canting become critical.
The iTero™ system produces a digitized representation of a prepared tooth, and that information is sent via e-mail to the Cadent manufacturing center. The manufacturing center then mills, from a solid plastic block, a working model which is then sent to the prescribed dental laboratory. This system has expanded its technology to include software that is a versatile open architecture. This means that the data generated can be interpreted by a number of different CAD/CAM systems. The digital information (STL file) can be exported into different milling software programs, such as Dental Wings, inLab, and any 3Shape-supported system (3Shape Dental Systems, www.3shape.com).11 The advantage is that the laboratory technician can view the file and make suggestions if needed before the dentist sends to file to the Cadent milling center.
Recently, 3M ESPE entered the marketplace with the Lava™ Chairside Oral Scanner (C.O.S.). This system is able to scan a prepared tooth and electronically transfer the data to a manufacturing center where rapid manufacturing methodology is used to fabricate models of individual teeth and arches made from a plastic material (Accura® SL Material).12 The unit uses the 3D Systems’ “Viper” Pro SLA® technology for the digital production of laboratory working models. The actual models are created by converting the Accura® SL Material into solid cross-sections, layer by layer, until the 3D working model is completed. The Accuras material is highly durable and reveals superior detail and resolution. It has mechanical properties comparable to those of tough engineering plastics such as ABS, which makes the resulting models much more durable than normal dental stones.13 Overall, this process provides 10 times the resolution of subtractive processes such as milling because of the size of high-definition laser beam as compared to a milling tool.13 Sorensen et al14 found that the optical impression/virtual die/digitally produced substructure crowns had excellent marginal fit similar to conventionally produced porcelain-to-metal crowns.
The State of the CAD/CAM Economy
Up until 2008, Sirona had the entire market share of chairside, in-office, same-day restorations. According to iData Research, Inc, the introduction of the D4D in 2008 saw the market for chairside CAD/CAM systems increase by 11.1%.15 Sirona is still the only system with a complimentary laboratory scanner (inEos™) and milling unit (inLab) but, as the technology advances, the other CAD/CAM systems could enter that marketplace as well. By 2015, the number of CAD/CAM restorations, which includes crowns, bridges, veneers, and inlays, is expected to be greater than 25% of the total units produced.15
Sometime in the near future, the art of impression taking will be nonessential. Just as children today have grown up always knowing the Internet, there will be a new generation of dentists that will learn to practice dentistry only using intraoral scanners.
The author would like to thank Ruth Egli for her editorial contribution.
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13. Data on file. Available at: http://www.3dsystems.com/products/datafiles/datasheets/SLA/DS_Accura_25_US.pdf. Accessed July 21, 2009.
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About the Author
Gregg A. Helvey, DDS
Adjunct Associate Professor
School of Dentistry
Virginia Commonwealth University