Improving Impressions: Go Digital!

Dentists have tried to perfect the ultimate way to reproduce the intraoral condition extraorally since the early days of the profession. One only needs to study the development of articulators to gain appreciation for the different theories, and means employed to reproduce mandibular movement on a laboratory bench.1


Central to recreating the intraoral condition extraorally is to have an accurate cast of the dentition and corresponding soft tissue (or strictly the edentulous ridges). Until recently, the only available means to accomplish this was by taking a physical intraoral impression, which enabled the dentist or technician to make a stone model of the teeth, gingival, and/or edentulous areas. Our predecessors used impression plaster, compound in copper bands, reversible and irreversible hydrocolloid, and polysulfide rubber base. At times they even fabricated wax patterns directly in the tooth preparation. Today, the most commonly used impression materials in the world are polyether and vinyl polysiloxane (VPS).

 

ERRORS IN PHYSICAL IMPRESSIONS CAN BE COSTLY
Inherent errors exist in the making of an analog impression as well as errors introduced in making the corresponding cast. Dimensional changes, in both the impression material and the stone with which the cast is made, can produce inaccuracies. Even in the best practitioner’s and dental technician’s hands, air bubbles, voids, tears, blood/saliva/other contamination, and distortion due to a variety of causes, can render a hard fought impression unusable. Pinning the dies can make unstable models. Any of these problems can result in extensive adjustments to get the proper occlusal or interproximal fit, or in the worst case, may make the impression worthless. Compounding the problem are issues that may not be readily apparent, only to be discovered in the process of remaking a restoration or appliance.

ADVANCES IN INTRAORAL SCANNING TECHNOLOGIES
Technological alternatives have existed for many years in the form of digital impression systems. The first to market in 1987 was the CEREC 1 (Siemens), which used a 3-dimensional (3-D) scanner and optical powder on the teeth to create a virtual model. The development of the infrared camera (CEREC 1) was one of the first steps in providing the profession with a digital practice experience. For nearly 20 years, CEREC was the only system capable of direct intraoral digital impression making. In addition, with the CEREC system, the practitioner could use CAD/CAM technology to fabricate one visit inlays, onlays, and crowns. Over the years, software and hardware improvements, as well as restorative material improvements have made it easier for the practitioner to make durable and aesthetic one visit CEREC restorations.


Until the recent introduction of CEREC AC powered by BlueCam (Sirona), in January of 2009, the imaging device remained unchanged. According to the manufacturer’s Web site, the latest “CEREC BlueCam captures highly detailed images using a powerful (short wavelength) blue light-emitting diode.” This technology is 2x faster than infrared scanning, and it can now be done in a few minutes.1 This new camera system takes exceptionally accurate images, and improves further on the previous version offered by allowing imaging of an entire arch. However, it does continue to require an optical powder medium to properly image the desired area.

More recently, a number of competitive technologies have entered the market. The E4D Dentist System, launched in 2008, also creates a finished restoration in one visit and unlike CEREC, does not require the use of a powder in most instances.

In the field of dedicated 3-D impression scanners, where final restorations are produced at the dental laboratory, there are several systems on the market: 

CEREC Connect (Sirona) is a Web-based communication platform designed exclusively for use by CEREC dentists and Sirona inLab laboratories. This allows CEREC dentists to electronically transmit a digitally-scanned impression to the inLab laboratory of their choice.

Lava Chairside Oral Scanner (C.O.S.) (3M ESPE) was launched in 2008. This system produces a digital impression from a recorded video sequence and requires a light powdering. Upon completion, the impression is forwarded to 3M ESPE, where a model is created using stereolithography. 

The iTero Digital Impression System (Cadent) was launched in 2006. It uses parallel confocal imaging, which utilizes laser and optical scanning to digitally capture the surface and contours of the tooth and gingival structures. The iTero scanner captures 100,000 points of laser light and has perfect focus images of more than 300 focal depths of the tooth structure. This approach, which differs from the technology employed by CEREC and 3M ESPE, does not require the need for scanning powder. The iTero images are converted by Cadent into a working model that is sent to the dental laboratory of the doctor’s choice.

 

OUR PRACTICE EXPERIENCE: INTRAORAL SCANNING

Figure 1. The iTero (Cadent) digital scanner.

Prior to acquiring the iTero system in February 2008, I used CEREC for virtually all single-unit restorations that were delivered in our practice. However, I continued to take VPS impressions for multiple unit restorations and multiple unit cases. The fact that I was an experienced CEREC user, made the integration of the iTero scanner into my practice easy to accomplish.


For dentists new to digital impressions, the iTero intraoral scanner (Figure 1) has an extremely short learning curve. As a CEREC user, I was already comfortable retracting the gingiva to enable the camera to clearly see the margins. The technique and materials are the same as with conventional VPS materials. I can pack the retraction cord and either remove it prior to scanning or leave it in place with the preparation margins exposed. Other options that I can use include: troughing the gingiva with a diode laser to expose the preparation margins; electrosurgical troughing; and curettage diamonds. With the use of the iTero digital scanner (as with the other digital impression systems), one can see in real time if the margins are visible and if the scan is accurately recorded on the computer monitor.

 

iTERO PROCESS

Figures 2 to 4. The milled dies in place. Note the crispness and sharp details of the margins and internal surfaces.


Table. Physical Characteristics of Cadent Polyurethane Model Material 2-4
Density ISO 845 g/cm3 1.0
Flexural Strength ISO 178 Mpa 48
E-Modulus ISO 604 MPa 1800
Compressive Strength ISO 178 MPa 47 at 10% compressive strain
Impact Strength ISO 179Ue kJ/m2 13

The entire process—imaging the preparations and adjacent teeth (including opposing arch and teeth in occlusion)—can be finished in less time than that of a conventional impression. When the operator is satisfied that the required information has been obtained, the completed scans are sent wirelessly to a Cadent facility in Tel Aviv (Israel) where a technician reviews the scans and confirms the margins with the laboratory. From there, the digital file is transmitted to Cadent (New Jersey), where the model is milled from a proprietary blended resin, pinned, trimmed, and articulated; all based on the digital impression created by the clinician. Cadent employs 5-axis industrial milling machines to guarantee the accuracy of the milled models and dies. The models go through no dimensional changes since they are precisely milled from the finished material.


The iTero model is made of a stable polyurethane material which presents numerous advantages (models of the case presented in this article are shown in Figures 2 to 4). As a single model that functions as both a working model and a soft-tissue model, it supports accurate and efficient fabrication of all restorations. There is exceptional resistance to wear when used in the dental laboratory. It will not break or chip if accidently dropped, and because the models are milled from the polyurethane, there is no polymerization shrinkage. Additionally, the plaster-like color is similar to conventionally poured models (Table).2-4

The biggest benefit of using digital impression systems is the consistency and reliability of the restorations. The marginal integrity is excellent, with nominal or no adjustment required to the occlusal and/or interproximal contacts. Seating time has been significantly reduced so we now spend much less time seating restorations; and sometimes eschew a metal try-in on fixed partial dentures in favor of completing the process in 2 visits. Patients also prefer digital to conventional dental impressions due to the fact that there is no uncomfortable tray with runny unpleasant tasting material in their mouths.

Digital impression systems also offer numerous benefits to the dental laboratory, including increased productivity. The dental technician should be educated to make the restoration the way it should fit in the mouth, without having to waste precious time overbuilding contacts or occlusion. Laboratory technicians will also be happy with lower rejection and remake rates.

 

CASE REPORT

Figure 5. Full-mouth radiographs.

Figures 6 and 7. Preparations of teeth Nos. 28, 29, 31 prior to placement of retraction cord.

Warren is a typical dental avoider who never took the time to care for himself. He endured years of pain, and fracturing teeth in the pursuit of his career. Only because his anterior teeth had an adequate aesthetic appearance was he able to delay his much-needed treatment (Figure 5).


In need of a full-mouth reconstruction culminating with cosmetic treatment in the anterior, Warren finally determined that he would make the effort to have his mouth rehabilitated. His treatment was to be done in phases: disease control (caries, periodontal, endodontic), posterior reconstruction in quadrants, and anterior cosmetic treatment.

As was the case throughout Warren’s dental history, treatment was done in fits and starts. The maxillary posterior quadrants were restored prior to my obtaining the iTero scanner. By the time he came in for the restoration of the mandibular right side, the 2 maxillary posterior quadrants had been completed.
As previously stated, existing disease was controlled (periodontal and endodontic) and cast gold posts and cores were made and seated for teeth Nos. 28 and 29 prior to the final preparation appointment for the mandibular right quadrant. After anesthesia and tooth preparation (Figures 6 and 7) a retraction cord (Gingi-Plain Z-Twist) was placed. I used a black cord, which provides a nice contrast between the tooth and the tissue, enabling me to see the margins and the displacement of the gingiva.

 

Figure 8. Stock photo of the iTero scanner in place recording images on the patient’s lower right side.

Figure 9. The enhanced quadrant image.

Figure 10. The enhanced image of the arches in occlusion. Color mapping gives the operator an idea of how much occlusal reduction has been achieved.

I next began to scan the prepared arch (Figure 8). (I had chosen the option to scan the opposite arch first. This was done while waiting for the onset of anesthesia.) The system directs you through the process with a series of voice prompts and illustrations to indicate the view of the teeth that it requires. The first scan is always of an entire preparation. You are shown a preview of the scan to ensure that all margins are visible. If so, and you are satisfied with it, you then move on to other views of the preparation, and the adjacent teeth. The final scans are of the arches in occlusion.


A provisional fixed partial denture was made, and the patient was appointed to return for the metal try-in. Due to the fact that this was a 4 unit fixed partial denture with 3 abutments, I decided that it would be prudent to do this intermediate step. (You will see later that it was not necessary!) 

One of the nice features of the iTero scanner is that you can rest the scanner directly on the teeth. This aids in stabilization. In addition, the unit will only scan when there is no movement. You can activate the scanner, obtain your best view and the scanner will fire automatically when the field is stable. After completion of all of the scans, the computer constructs a lower resolution model of the arches and the occlusion. The model also tells you how much occlusal reduction exists via a color map. If you are not happy with that, you can adjust your preparations and rescan (Figures 9 and 10). 

One of the difficulties of doing multiple restorations with some systems is getting all of your images to knit together. Variations in the roll, pitch, and yaw of the camera, as well as the position of the image in the frame may make it impossible for the computer to put all of the images together. Much of the time learning to use other systems can be spent on being able to take proper images so that the system can combine them. I have not found this to be a problem with iTero. Because of the great amount of data that the laser obtains, the images are combined effortlessly. In addition, the iTero scanner projects a “viewfinder” onto the image facilitating the job of the operator in obtaining the correct image. 

 

Figures 11 and 12. The removable dies.

Upon successful completion of the scanning process, the file was wirelessly transmitted to iTero. They enhanced the image and verified the margins with the laboratory. A working model with trimmed, ditched removable dies (Figures 11 and 12) was delivered directly to our lab (the position of the margins and the ditching is done virtually on the computer). Our laboratory then fabricated the restorations in the usual manner and delivered them to our office.

Try-in and Delivery of the Final Restoration

Figure 13. Metal try-in (on the model).

Figure 14. Metal try-in (intraoral).

Figure 15. Verifying the occlusion clinically.

Figure 16. Verifying clinical occlusion on the model.

Figures 17 and 18. Completed restoration on the master cast.

Figures 19 to 21. The unadjusted try-in of the fixed partial denture prior to cementation (occlusal view, lingual view, and buccal view).

Two weeks later, Warren returned for his metal try-in. After removing the provisional fixed partial denture and cleaning the preparations, the metal framework was tried in the mouth. Figures 13 and 14 show the initial metal try-in. No adjustments were made to get this framework to go into place. It was effortless! I verified the occlusion with bite registration material (Figures 15 and 16), and then sent the framework back to the dental laboratory along with the proper shade prescription. One week later, the final restoration was ready for cementation (MaxCem Elite [Kerr]) with only modest corrections needed (Figures 17 to 21).

CONCLUSION
Improvements to digital scanning systems have occurred over the years, and newly created digital systems have been introduced that enable the dentist to choose various methods of reproducing the intraoral situation in the extraoral environment. Cadent’s iTero system is just one of the latest advances in the art and science of dentistry. The precision of optical scanning enables the operator and the dental technician to provide superior restorations. With digital impression techniques, the number of operator and material variables is reduced and the restorative process becomes more predictable and easier to accomplish.


References

  1. Starcke EN. The history of articulators: a perspective on the early years, Part I. J Prosthodont. 1999;8:209-211.
  2. Information provided by Cadent, Inc.
  3. For further explanation of the ISO 178 test, go to http://www.ides.com/property_descriptions/ISO178.asp. Accessed August 31, 2009.
  4. ISO is the International Organization for Standardization: www.iso.org.

Dr. Zweig received his DMD degree from Washington University School of Dental Medicine (St. Louis, Mo), and his certificate in Prosthodontics from the USC School of Dentistry. He has held faculty positions at both institutions. For the past 28 years, Dr. Zweig has been in private practice in Beverly Hills, Calif, concentrating on complicated restorative, cosmetic and implant restoration cases. Dr. Zweig can be reached via e-mail at drzweig@beverlyhillsdds.com.

 

Disclosure: Dr. Zweig reports no conflicts of interest.

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