Abstract
For many years, mechanical and laser-based scanning technology has been used with computer-aided design and computer-aided manufacturing applications in the field of dentistry, but most have been limited to the dental laboratory. For the past 20 years, only 1 intraoral scanning device has been available to the dentist for in-office use. Recently, a new kind of intraoral scanning technology was introduced to the dental market. This technology, based on a laser scanning protocol called "parallel confocal," allows the dentist to take electronic impressions intraorally. This technology is coupled with traditional laboratory protocol for the construction of fixed dental restorations, providing the dentist with an accurate and efficient system to produce high-quality fixed dental restorations of all types. In a blind study, crowns developed using this technology were preferred over crowns generated using conventional impressions and criteria of marginal fit, contacts, occlusion, and time of adjustment in nearly 70% of cases. This article introduces scanning technology including a discussion of its clinical applications and an overview of the benefits.

Since its introduction to the dental arena, computer-aided design and computer-aided manufacturing (CAD/CAM) technology has been largely limited to the realm of the dental technician. Many systems have been used for the design and fabrication of fixed dental prostheses by the dental technician, but for the past 2 decades, only 1 system that is capable of direct intraoral impression taking (CEREC 3D^a) has been available to the dental practitioner.¹ Recently, another system (iTero^b) has been introduced. The 2 systems are very divergent in their development. CEREC emphasizes 1 visit, in-office scanning and milling of ceramic prostheses from prefabricated ceramic monoblocks; whereas, iTero has been developed as an office-based intraoral scanning system, connected by the internet to a centralized milling center and to the traditional dental laboratory technician.² In this article, the author will introduce the reader to this new scanning technology, discuss its use and clinical applications, and provide an overview of the benefits of such a system in the clinical setting.

The fabrication of a high-quality fixed dental prosthesis requires the expertise of the dentist all along the clinical pathway. The dentist must provide adequate axial and coronal reduction, establish proper draw to create a path of insertion for the prosthesis, establish a properly designed finish line, manage soft tissue, and record the results of these efforts in a dimensionally accurate and stable impression and bite registration, which is then presented to the dental technician.^3,4 The technician depends on the dental impression to accurately duplicate the dentition and occlusal relationships. If the occlusal relationship is not accurate, all subsequent efforts on the part of the technician are rendered moot.

Figure 1—The iTero Electronic Impression Device.

Unfortunately, a tour of virtually any commercial dental laboratory will reveal that most dental impressions presented to the technician are inadequate, missing areas of the finish lines, and containing pulls and voids. In 2005 Christensen stated that 50% of conventional dental impressions do not show the entire preparation margin to fabricate an indirect dental prosthesis, and surveys of laboratory technicians have indicated that as high as 90% of conventional dental impressions have incomplete registration of finish lines.^5,6 Without the accurate duplication of finish lines, the prosthesis becomes little more than an approximation of the properly fitted prosthesis.

Clearly, there was a need in the dental profession to improve on this questionable record, which was recognized over 2 decades ago by some of the pioneers who began the search for a better way to record the dental impression. Duret and colleagues, Mormann and colleagues, and others began working on electronic impression taking in the mid 1980s.^7,8 Their work evolved into the modern day CEREC 3D system. In the past decade, Taub, Kopelman, Babayoff, and other engineers at the Tel Aviv engineering laboratory of Cadent began laying the foundation for the newest electronic impression device, Cadent iTero.

Development of the Electronic Impression Device

First attempts at developing an electronic impression device (EIDs) began over 2 decades ago. Despite multiple advances in CAD/CAM technology during that time and active development efforts by several research groups, 1 intraoral device, the CEREC 3D, was successfully brought to market.⁹ In 1996, engineers at Cadent developed a scanning system called OrthoCAD^b. This digitizing system is for conventionally taken and poured models for various orthodontic applications such as digital model fabrication, virtual tooth setups, and indirect bonding and bracket-placement applications.¹⁰

Figure 2—iTero scanner disposable sleeve.

The company realized that OrthoCAD technology might have application in the area of prosthodontics. As a result, Cadent began the development of an intraoral EID.¹¹ Initial prototypes were cumbersome and unwieldy, suspended from a cable system, and were large and heavy. However, they were able to register accurate digital optical impressions from which fixed dental prostheses could be produced. After 18 months of initial testing and analysis of the suspended system, a handheld version was developed, and in the fall of 2004, this author took possession of the first Cadent handheld scanner. The author and his staff began a double-blind clinical evaluation in a private office setting in Horsham, Pennsylvania, of crowns made on EID scans and compared them with crowns made on conventional quadrant impressions of the very same preparations. This evaluation continued to the summer of 2006, when the data was tabulated and analyzed.

The iTero Electronic Impression Device

Before discussing the results of the evaluation, a discussion of the iTero EID technology and its use are in order, as this is a unique and novel technology. The goal of an electronic scan is the same as the conventional impression: the dentist wants to accurately record the tooth preparation, emphasizing the finish line, the remaining teeth in that quadrant, the opposing dentition, and the occlusal relationship between them.

Figure 3—Triangulation-based device scan with opaque powder.

The iTero device consists of a mobile cart 29.33 inches tall, 26.13 inches wide, and 15.6 inches deep (Figure 1). It is mounted on caster type wheels for movement from operatory to operatory. A handheld scanner is attached to the cart by way of a proprietary data cable. The cart houses a personal computer optimized for the processing of video data and a 19-inch liquid crystal display (LCD) monitor. There is an uninterruptible power supply, which allows the unit to be unplugged for short periods and relocated from operatory to operatory. A small air compressor and regulator are incorporated that provide a flow of air over the lens of the scanner to prevent fogging and provide some cooling.

The device uses a wireless Internet connection for transmission of scanned data to Cadent for processing. Wireless mouse technology and a sealed keyboard in the top of the cart aid in data entry and the maintenance of asepsis for the unit. The scanning process is controlled by a wireless foot pedal so that once the initial patient data is entered into the system, the clinician need not touch anything other than the scanner head and the foot pedals while recording an electronic impression.

Figure 4—Triangulation-based device scan without opaque powder.

The scanner head houses sensors, light sources, focusing motors, analog-to-digital converters, and video cameras for aiming and recording a scan. In early beta testing, the scanner head was 3x the size, much heavier, and more difficult to control than the current unit. Miniaturization of many of the components was a technical challenge but resulted in a scanner that is better balanced, easier for the operator to handle, and more comfortable to the patient. Also, it provides easier access to second molar regions that are more difficult to reach. The scanner head uses disposable sleeves that are changed between patients to prevent cross contamination (Figure 2).

Much of the scanner technology is proprietary, but this much is known— in many optical scanning devices, a theory known as "the triangulation of light," the intersection of 3 linear beams of light, is used to locate a given point in 3-dimensional space. This theory has been used in a variety of industrial measuring devices, but surfaces that disperse light irregularly and do not reflect it evenly (eg, curved surfaces) and surfaces that are not continuous adversely affect the accuracy of scans based on triangulation.¹² The CEREC 3D intraoral scanning device currently available is based on triangulation. It uses a thin coating of an opaque powder to provide uniform light dispersion to enhance the accuracy of its scans.¹³ Figures 3 and 4 show scans taken with a scanning device based on triangulation both with and without opaque powder application. Note the difference in quality of the scan when powder is not applied.

Figure 5—Illustration of a confocal dual disk microscope.

While the principle of triangulation is widely used in industry and technology, the iTero scanner is based on an entirely different concept, a range finder technique by focus finding. It is described by Cadent engineers as "parallel confocal," making use of the confocal light theory found in microscopy (Figure 5). Confocal roughly translates as "having the same focal point." Briefly described, a light source is "filtered" by passing it through a small filtering pinhole. That light passes through the optics of the system, focuses on the target object, then reflects off of the object. Only an object at the proper focal length will reflect light back through the filtering device. Points above or below the confocal plane will direct light along a path that will not pass through the pinhole but will be blocked instead. Only those rays that are in focus will return through the filtering device.

Confocal microscopes have significant advantages over conventional types including notably better imaging by rejecting out-of-focus information and the ability to control depth of field.^14,15 The iTero device expands on this concept by projecting 100,000 beams of parallel red laser light in a 14 x 18 mm² pattern, providing a 13.5-mm scan depth, and converting the reflected light into digital data through the use of analog-to-digital converters, all of which happens in one third of a second. The registration of the surfaces of the oral structures is accurate to within 15 µm. These rays of laser light act as tiny optical probes, surfing the surface of the oral structures and recording the anatomic surfaces by detecting the confocal points.

Figure 6—Treatment information screen of the iTero system. View image in a larger size.

This technology is able to capture all of the materials found in the mouth in their native form. The more diffusive dentin, the more translucent enamel, amalgam, gold, ceramics, resins, and soft tissue are recorded with equal accuracy and without the need for a coating to produce uniform light dispersion. All finish line designs such as feather edges, chamfers, shoulders, or bevels can be recorded. As long as proper tissue management allows the scanner to visualize the finish line, both supragingival and subgingival positions can be recorded with accuracy. Having a telecentric (the rays of light are parallel to the optical axis) aperture is important because this maintains a constant field of view and the clinician does not need to be concerned with distance from the cusp tip. This property and the absence of a powdered surface allow the clinician to place the device directly on the teeth, which enhances stability and allows for the image capture of steeper slopes. This in turn results in fewer incomplete scans because of undercuts. Before each individual scan, the system provides the clinician with a true color video image, magnified 50x, with an overlaid crosshair for aiming. After the one third of a second individual scan period, a 3-dimensional colored model of a single scan is presented on the LCD screen (Babayoff, Noam, Cadent, personal communication, June 12, 2006).

Taking an Electronic Impression

Figure 7—iTero active scanning screen view.

The EID is user friendly despite the previous discussion of technology because most of the sophisticated processing occurs in the background, transparent to the user. To demonstrate the device's ease-of-use, the following is a simple posterior single unit crown case from start to finish. The ensuing discussion assumes that the iTero unit has been installed along with a wireless router in the dentist's office and that the dentist has chosen and been paired with 1 or more of the several iTero-certified dental laboratories around the United States. The laboratory and dentist information will have been preinstalled into the database of the iTero software.

After the patient is seated in the dental chair, local anesthesia is administered in the clinician's usual manner, and while waiting for anesthesia to take effect, shade selection is completed. The first step in the electronic impression process is to fill in the necessary data on the initial screen, an electronic counterpart of the dental laboratory instruction sheet (Figure 6).

Figure 8—Screen view of articulated virtual models.

Information such as type of restoration, material to be used, finish-line design, shade chosen, and any special instructions are entered using either the keyboard or by clicking drop-down boxes with the wireless mouse. Missing teeth are marked as missing, including whether an edentulous space remains. The abutment tooth or teeth are marked as such. This allows the iTero software to correctly provide visual and audio prompts to the clinician during the scanning process, which requires a few minutes to accomplish.

Although the software allows the clinician the option of scanning both arches and the bite registration after completion of preparations, it is the author's preference to use the "scan opposite arch first" option in most cases. There are several advantages to this approach. First, the time between administration of anesthesia and the start of dental preparations is effectively used. The patient is given an opportunity to get used to what is likely to be a new experience and at the same time, it allows the software to initially process digital renditions of the opposing arch while tooth preparations are completed. In some locations, the dentist may be able to delegate this responsibility to the dental assistant.

The flat panel display has 3 windows (Figure 7). The upper left window is a visual prompting window, showing the proper position of the scanner for the current scan. The bottom left window displays the results of the last individual scan taken a few seconds after it is recorded, which allows the clinician to evaluate it for completeness in near real time. The largest window on the right side of the screen is in real time and provides an aiming crosshair graphic to assist the clinician in proper positioning of the scanner. There also is an audio prompt to provide additional assistance.

Figure 9—iTero laboratory technician working with an iTero scan.

A typical series of scans can range from 15 to 30 scans. Generally more scans are needed in the anterior because esthetic needs require the recording of data from the contralateral and the ipsilateral side. Typically, fewer scans are required in the posterior.

Preparations are completed in the manner preferred by the clinician. The scanner is capable of recording virtually any dental preparation; therefore, veneers, crowns, bridges, inlays, onlays (essentially any fixed prosthesis that can be recorded by conventional impression techniques) can be recorded electronically. However, it should be mentioned that electronic impressioning will not compensate for inadequate preparation and tissue-management techniques. As conventional impression material must be able to "see" the entire margin of a preparation, so too must the electronic impression device. In the clinical data presented here, a double-corded technique was used, but any technique including expansive materials, laser or electrosurgical troughing, curettage diamonds, and others preferred by the individual clinician may be used.

Figure 10—CNC 5-axis milling machine.

Once the preparation is completed, scanning of the arch containing the abutment preparations can proceed. The process is initiated by a simple actuation of the foot-pedal control. The software will then prompt for a series of 5 scans per abutment that target the abutment, followed by additional scans for recording the remainder of the quadrant. The final 2 scans are for bite registration. The patient is guided into a fully closed position, and 2 scans are taken at 90º to the long axis of the teeth at the level of the occlusal plane.

When all scans have been completed, another press of the foot pedal begins an assimilation process, and in less than 1 minute, 3-dimensional magnified virtual models of both arches appear on the flat screen. These models will be virtually articulated by the software, which finds overlapping points in common between the final bite registration scans and the arch modeling scans (Figure 8).

The models can be rotated and manipulated in virtual space to any desired viewing position. Utilities included in the software allow the finish line to be marked and evaluated and occlusal clearance to be judged. If anything is found to be unacceptable, additional scans can be taken or the preparation can be modified and rescanned. If the clinician should move excessively during a scan, the software will reject the scan and prompt the clinician to rescan. It should be noted that the visual representation presented on the flat screen is not a full resolution final view, but it is a preliminary view that allows the clinician to evaluate his or her work while further processing of the scans occurs in the background.

Figure 11—Dies and models are being milled from polyurethane blocks.

When the clinician is satisfied with the results, a mouse click on the "send" icon starts a process of packing and organizing scan data for transmission to Cadent's servers. The case is digitally stamped with the name, date, and time of scan, so even if the same tooth is scanned a second time, one can identify distinct files. Provisional restorations can then be fabricated for the patient in the clinician's preferred manner, and the patient is dismissed.

The completed scan file will then appear in a second software program, the iTero case manager. This can be thought of as a digital laboratory work progress log. All cases transmitted will appear in the case manager, and as the case progresses, the dentist can look at the case manager to determine the production sequence of every scan.

The Cadent Processing Sequence

After completing a successful scan sequence, the clinician clicks the "send" icon, and the data files are forwarded to Cadent's servers. On arrival, the data files undergo a process called "modeling." The scans will receive a "cleanup" process where artifacts and nonessential structures are eliminated. After modeling, the virtual models seen on the computer screen will look very much like the final physical models generated from the data file. The modeled files are then transmitted to a highly trained iTero dental technician who will do the initial design of the case, identify finish lines, and determine the path of insertions not only for the prosthesis, but also for the abutment dies in relationship to the arch models.

Figure 12—Models and dies are assembled on proprietary iTero articulator.

The data file is then transmitted to the iTero-certified dental laboratory with which the practitioner is paired. The laboratory technician can approve of the design as is or make corrections or modifications before transmitting the case back to Cadent for model milling (Figure 9). The model-milling process is accomplished on a computer numerical control (CNC) 5-axis milling machine at Cadent (Figure 10). The patient's iTero data file is translated into "g code," a language the milling machines understand. The upper model, lower model, and dies are milled from 3 separate blocks of a proprietary polyurethane material made in Germany to Cadent's specifications for this purpose (Figure 11).

Cadent has developed a proprietary die system that allows easy removal of a die for finish work by the technician, while holding the die with no discernable movement when placed into the model. These milling machines are accurate to within 2 µm. The upper and lower models are milled with special modifications that allow the models to be quickly and easily articulated on a proprietary simple-hinge articulator, which maintains the occlusal relationship between the arches that were captured in the scanning procedure (Figure 12).

The finished models and dies are physically shipped to the clinician's partner laboratory for fabrication of the prosthesis. If a metal coping or full-cast metal crown is requested, a wax pattern can be milled from a special proprietary plasticized burnout wax and sent to the laboratory with the models and dies. The laboratory then proceeds with the case as it would to create a conventional poured impression case, and the final prosthesis is returned to the dentist for insertion.

Figure 13—iTero study evaluation form. View image in a larger size.

It is assumed that this process would result in longer fabrication time than conventional techniques, but it may make the entire process more efficient. In preparing this article, the author visited and toured the Cadent facility and a partner laboratory located within a 2-hour drive from his office. During the visit to Cadent, a scan was transmitted in the morning. By the time the author arrived at the laboratory that afternoon, modeling and the design phase by Cadent technicians had been completed, and the laboratory had received the electronic transmission. At that point, the laboratory technician and the author were able to make a few modifications to the design and transmitted the case back to Cadent. The next morning, the author visited Cadent headquarters and within 30 minutes of his early-morning arrival, he was presented with the physical dies and models. Less than 24 hours had passed since the initial scan was transmitted and dies and models were on their way to the laboratory. (Note: These results are not typical. Standard processing time for Cadent milled models is 3-5 days.)

Comparing Conventionally Fabricated Prostheses with EID-Generated Prostheses

In Fall 2004, the author received a prototype that would later evolve into the iTero. He was asked to critically evaluate the unit and conduct a comparative assessment of crowns fabricated on conventional quadrant impressions vs crowns fabricated only from iTero scans. If it were determined that iTero-produced prostheses were inferior to conventionally fabricated cases, there would be little need to develop the unit.

A protocol was developed where each clinical case was both electronically and physically impressed. Early on, the scanner was optimized for scanning single units of posterior teeth, so it was decided to take physical impressions using one piece triple trays (Polybite^c) and vinyl polysiloxane impression material (light and heavy body Correct VPS^d), providing models that would extend from second molars to canines in both instances. Eventually, the study was expanded to include multiple single units, bridges, and anterior crowns and veneers. The finished crowns were returned to the author from the dental laboratory labeled as "A" and "B," without dies or models. At the insertion visit, an evaluation form was filled out for each crown, assessing clinical parameters such as marginal fit, retention, contact points, occlusion, and adjustment time, if necessary, to make the prosthesis clinically acceptable (Figure 13).

A master list of the origin of each crown was maintained by one individual at Cadent headquarters, and only after insertion, was the author notified as to their origin.

Discussion

The evaluation involved 117 patients over 18 months. A few patients were dropped from the evaluation because conditions developed that adversely influenced the results for either type of prosthesis including fracture of core buildups and premature loss of provisional restorations.

After data tabulation, certain patterns emerged. The most notable of these is that in 68% of the cases, EID-processed crowns were identified as the crown of choice for insertion based on the clinical acceptability criteria. The second notable finding was 85% of EID-produced crowns were judged to be clinically acceptable compared with 74% of conventionally produced crowns. Also, the iTero-scanned crowns averaged 2.4 minutes to adjust to clinical acceptability compared with 3.2 minutes for the physically impressed cases. It should be noted that a second dental laboratory was introduced during the latter part of the study, and the results were essentially unchanged from the early data. This suggests that the advantages identified of EID impression taking were, to some degree, independent of the dental laboratory and laboratory technician.

Encouraged by these findings, additional scanners were made available to a larger group of dentists for their evaluation in a second phase of beta testing. Many of these dentists took impressions both digitally and conventionally for a short time and did their own comparisons, but because the cases were not returned blindly, their data is not included here. It is interesting to note, however, that in all instances, the preference for scanner-based crowns held true. As of this writing, 4691 restorations have been fabricated using iTero technology.

Through exit polling of patients and round-table discussions with participating dentists and participating laboratory technicians, some other interesting observations emerged:

1. Laboratory technicians commented that they perceived an improvement in the quality of the crown preparations of their partnered clinicians after electronic impression taking was introduced. They attributed the difference to the magnified image that the dentist had of his or her preparation, making deficient areas in a preparation scan much more obvious, along with the analytical tools built into the iTero software.
2. Patients found scanning more comfortable than conventional impression taking. Phobic patients commented that the ability to stop or rest between scans, which cannot be done with the conventional technique, was important to them.
3. Dentists using the EID commented that the chair-side time allotted to adjustments decreased significantly.

Conclusion

In this article, the development, use, and benefits of the EID have been discussed. Its ease of use, coupled with multiple benefits perceived by dentists, laboratory technicians, and patients, led the author to conclude that this technology is here to stay. It is the author's opinion that the application of such technology will rapidly increase over the next decade. The dental profession has seen a rapid growth in the areas of digital radiography and in laboratory-based scanning technologies in recent years. Intraoral digital recording is taking a logical next step. This technology has significant potential beyond the arena of fixed prosthetics. Envision the orthodontist using the EID for evaluation and appliance fabrication. Imagine capturing virtual models and using them to treatment plan cases more accurately, less invasively, and less work intensively than before. Visualize the results of preprosthetic surgery and esthetic alteration done in a noninvasive, virtual manner. Then, picture what can be accomplished when digital impressioning is coupled with other advanced technology currently being developed. It is a wonderful time to be involved in the field of dentistry.

Disclosure

Dr Henkel has actively consulted with Cadent during the development of the iTero Electronic Impression Device and has received honoraria from Cadent for his participation.

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