New Technologies for Easier and More Accurate Impressions
Thomas Berry; Gary Radz
Impressions are a part of every day for general practitioners and for many specialists. Because models are required for extraoral diagnostic procedures, analysis of occlusal functions, and fabrication of indirect restorations and appliances, some reliable method of making those models is required.
Historically, many different materials have been used. Dental impression compound and other waxes as well as plaster and zinc oxide-eugenol have served as impression materials. They had obvious disadvantages, including distortion (waxes) and an inability to flex over and around undercuts with breakage (compound, plaster, and zinc oxide-eugenol). The introduction of reversible and irreversible hydrocolloid materials offered significantly im-proved accuracy. Their disadvantages were a relatively low tear-resistance and a lack of dimensional stability over time, even when carefully stored.
The introduction of polysulfide rubber base provided accuracy, reasonably good tear-resistance, and better dimensional stability. While its introduction was a step forward in impression-making, polysulfide rubber base also possesses undesirable characteristics. It is more difficult to mix, has a bad odor, stains clothing, and undergoes some dimensional changes so the timing of pouring the cast is of concern.
CURRENT TECHNIQUES AND MATERIALS
The introduction of polyethers and condensation and the addition of silicone impression materials such as polyvinyl siloxane (PVS) minimized or eliminated some of the problems inherent in previous impression materials. Although these materials are similar to each other in many ways, there are some significant and important differences. Condensation silicone possesses some desirable characteristics but it has poor wetting ability because it is very hydrophobic and is not entirely stable over time. Polyether materials have a short setting time and are very stable dimensionally if stored dry. They tend to provide greater sulcular penetration than do other materials. They also exhibit slight hydrophilic properties that make them less affected by the presence of moisture. But they have drawbacks. They set rather rigidly so they are somewhat difficult to remove from the mouth, especially if there are anatomic undercuts or severe gingival recession on other teeth. Removal of casts from the impression is difficult without fracturing teeth on the cast. Lastly, patients will complain about the unpleasant taste.
PVS materials have an acceptable taste and odor, good elastic recovery, and good dimensional stability. They are also easier to remove from the mouth after setting. However, some brands are negatively affected by ferric sulfate hemostatic agents and by some latex gloves.1,2 Despite manufacturers’ advertisements to the contrary, there remains the question of whether they are truly hydrophilic. Surfactants have been added to them to improve wetting properties but studies have not shown that this has been particularly effective in wetting tooth surfaces.3,4 They are only truly accurate with a relatively dry field.5 Temperature also affects PVS materials’ working time.6 Although rare, a few cases of allergic reaction to the material have been reported.7
Both polyether and PVS materials demonstrate excellent accuracy. Currently available materials demonstrate sufficient dimensional stability to be packaged and mailed to the laboratory. Materials are currently packaged to give the clinician another advantage. Impression "guns" mix the catalyst and base materials as they are expressed from the tubes. This conveniently mixes the material in correct proportions and with few air bubbles. There are shortcomings, however. Not all of the polymerization conversion of the material is absolutely complete by the time the impression is removed from the mouth. This is not a problem while the impression is in the mouth because the tooth surfaces restrain the shrinkage. Once removed, the impression material continues to shrink slightly. Fillers placed in the material minimize this shrinkage so materials are reasonably stable.
These materials are a distinct improvement over past materials available in the category. Nevertheless, they still have limits. There is no "intelligent" material that recognizes exactly where it needs to be. No materials are absolutely successful in completely avoiding the inclusion of air bubbles or dealing with excessive moisture. Moisture and bleeding are real deterrents to successful impressions with PVS. Tissue management is a critical concern. It must be dis-placed to reveal all the finish lines to allow ingress of the impression material. All of these factors combine to make impression-making a time-consuming, technique-sensitive, and not always predictable process. Additionally, the patient is put through a time-consuming, less-than-pleasant process (Figure 1). It is estimated that achieving adequate retraction, injecting the light-bodied material, seating the full impression tray, and letting the material set sufficiently consumes at least 10 to 15 minutes. Because it has been shown that the use of a custom-made tray improves the accuracy of these elastomer impression materials,8 the time required to fabricate the tray must also be considered. The success of that effort is still not absolutely guaranteed.
The desire for a new and more efficient approach to impression-making is being realized through digital technology. This technology allows the creation of digital impressions of a patient’s teeth and surrounding tissues. It eliminates the need for impression materials and trays as well as the patient’s dread of the impression procedure. Computer-assisted design/computer-assisted manufacturing (CAD/CAM) technology has been available for some time, so the capability for recording impressions is not really new. Digital impressions are obtained using an intraoral camera and then sent to a computer design station. The digital camera develops a customized scanning sequence for the specific situation. As the camera passes over the teeth, the system ties together scans of prepared and adjacent teeth, adjacent soft tissues, and occlusal relationships into one digital model (Figure 2 ).
Most optical scanners have been based upon some form of triangulation. Systems had three elements in common: a light source, an illuminated subject, and a detector. This triangulation of light was compromised when scanning curved surfaces because the reflection angle reduced the viewing area. Accuracy was also compromised when surfaces reflected light unevenly (eg, amalgam vs natural tooth structure). Therefore, it was necessary to coat the target with an opaque, reflective material. If the coating was uneven, if there was saliva on the scanner probe, or if the tongue touched the surface, accuracy was compromised.
The answer to this problem came from a technologic leap. A parallel confocal system filters light passing through a small pinhole. Only the light reflected from the object at the proper focal distance will pass through that pinhole. A telecentric system maintains the same field of view and area being scanned regardless of distance from the object being imaged. There is no need to compensate for different magnification levels or to hover over the object. A coating is not needed. The probe can be placed directly on the object for greater stability.
Digital files can be transmitted onto CAD systems and onto CAM production systems. Steriolithography is an additive CAM method that builds up material. The digital impressions can interface with CAD and CAM systems.
One currently available system and one to be introduced to the market in early 2008 are presented in this article: the Cadent iTero™ System (Cadent, Carlstadt, NJ) (Figure 3) and the 3M ESPE Lava™ Chairside Oral Scanner (3M ESPE, St. Paul, MN) (Figure 4). Although some of the technology is basically similar, there are specific differences in the clinical application of the image capture and the process by which models are created and used. The units consist of mobile carts with a flat-screen monitor, keyboard, mouse, and handheld scanning unit. The units resemble a CEREC® unit (Sirona Dental Systems, Inc, Charlotte, NC).
No modifications in the normal preparation, design, or coating of the prepared teeth before the digital scan are required. The preparation should be relatively dry and clean, and all preparation finish lines clearly visible. Normal tissue retraction procedures can be used to reveal the finish lines.
The Cadent iTero system relies on the parallel confocal method. It simultaneously projects 100,000 beams of parallel red light rays with each individual scan. After each scan, the image is immediately displayed in real time. After the scan sequence is completed, the system automatically registers all images. It displays a magnified 3-dimensional (3-D) digital model of the target area and opposing arch in occlusal contact on a flat-panel display with real-time analytical tools. This permits analysis of areas of the preparation needing adjustment (eg, occlusal clearance). This image can be rotated in every direction, magnified, finish lines examined, and additional scans taken if desired. If needed, the preparation can be modified and additional scans taken. Once the dentist determines the preparation and the scan are correctly done, he or she pushes the "send" icon on the computer. The dentist can then proceed with fabrication of the provisional restoration. The digital impression is compressed and transmitted to both Cadent and to a participating laboratory.
The laboratory uses CAD that is uploaded to the Cadent unit. Mounted models, removable dies, and copings can be produced at Cadent’s production center in New Jersey and delivered to the chosen dental laboratory for completion of the restorations. The models can then be used for any type of restorative system (Figure 5). The digital scan creates the digital impression and does not require a CAM system unless desired.
The 3M ESPE system combines hardware and software developed by research at Massachusetts Institute of Technology and at Brontes Technologies, Inc (Lexington, MA). It uses what it calls active wavefront sampling (AWS) for capturing target-area images. The method enables what the manufacturer refers to as "3-D video in motion" that captures broad oral anatomy. The clinician moves the wand-type scanner through the mouth to capture all surfaces of the dentition and tissues. This AWS greatly minimizes potential inaccuracies produced by extrapolation of data. Unlike triangulation and laser methodologies used by previous technology, AWS does not rely on warping of a laser or light pattern on an object to determine 3-D data. The traditional 3-D methodologies suffered from distortion and optical illusion, and were rather slow. AWS promises to capture the data in a video sequence and model it in real time. This results in greater accuracy than with previous generations of oral optical scanners.
ADVANTAGES AND DISADVANTAGES OF THE SYSTEMS
Both systems have been tested by the companies and have proven their accuracy to the respective company’s satisfaction. The Cadent iTero system is now being used in offices and laboratories. So far, the results appear very good.9 The 3M ESPE device is not yet on the market but the company plans to introduce it soon.
The systems allow identification of preparation problems so that improvements can be made immediately rather than waiting until a model has been poured. Specifically, the system will be able to indicate if there is inadequate reduction (Figure 6). The digital impression will be viewed on a large computer monitor, allowing the dentist to have confidence that all of the margins are adequately captured in the image before the digital impression is sent to the model fabrication facility. No physical occlusal bite registration material is needed. Both systems save time. The many steps involved in impression-making, bite registration, and model creation requires significant time. Every step in the process requires dentist, auxiliary, or technician time. Additionally, each step has some potential for error. As a result of cumulative small errors, the whole process may develop an error great enough to necessitate a remake.
There is significant cost to the systems. Each has a learning curve, although dentists have stated that the technique is not difficult to learn. After a few uses, the process goes rather quickly. Cadent estimates that an experienced user can perform the scans needed in 2 minutes or less and that the total process—from start until the desired final images have been examined and accepted—is approximately 4 minutes.9 Patients with small mouths or limited openings may present a problem inserting the scanning wand into the mouth, so scans of second and third molars may be compromised. The 3M ESPE wand, as now designed, is significantly smaller than the iTero wand, so it may minimize this problem.
Perhaps in reality, the biggest advantage may not be procedural time saved or the patient’s appreciation for not having the impression tray in the mouth for several minutes. It may well be the reassurance that an accurate impression of a correctly prepared tooth has been captured before it is sent to the laboratory.
Accurate computerized images are now available for dental impressions. Two new systems used this modified technology to make highly accurate impressions that can be manipulated and viewed from many angles to assure accuracy and that the relationships of prepared teeth to adjacent and opposing teeth are correct. These new systems offer a real promise of faster and more accurate impressions.
1. Reitz CD, Clark NP. The setting of vinyl polysiloxane and condensation silicone putties when mixed with gloved hands. J Am Dent Assoc.1998;116(3):371-375.
2. Roberson, TM, Heymann HO, Swift EJ. Sturdevant’s Art & Science of Operative Dentistry. 4th ed. St. Louis: Mosby; 2002:845.
3. Mandikos MN. Polyvinyl siloxane impression materials: an update on clinical use. Aust Dent J. 1998;43(6):428-434.
4. Takahashi H, Finger WJ. Dentin surface reproduction with hydrophilic and hydrophobic impression materials. Dent Mater. 1991;7(3): 197-201.
5. Boening KW, Walter MH, Schuette U. Clinical significance of surface activation of silicone impression materials. J Dent. 1998;26(5-6):447-452.
6. Setz J, Lin W, Geis-Gerstorfer J. Profilometric studies on the surface reproduction of dental impression materials. Dtsch Zahnarztl Z. 1989;44(8):587-589.
7. Sivers JE, Johnson GK. Adverse soft tissue response to impression procedures: report of case. J Am Dent Assoc. 1988;116(1):58-60.
8. Millstein P, Maya A, Segura C. Determining the accuracy of stock and custom tray impressions/casts. J Oral Rehabil. 1998;25(8):645-648.
9. Data on file, Cadent, Carldstadt, NJ.
|Figure 1 Patients had to endure the flow of impression material into the back of the throat without gagging.||Figure 2 The "virtual" model can be viewed and manipulated to review the preparation.|
|Figure 3 The Cadent iTero digital impression system.||Figure 4 The 3M ESPE Lava Chairside Oral Scanner.|
|Figure 5 A physical model can be fabricated so that any indirect restoration system can be made. The model is made of a very durable, chip- and abrasion-resistant material.||Figure 6 The virtual model enables easy viewing of the occlusal clearance between the preparation and the opposing teeth, including the difficult-toview lingual cusps.|
|About the Author|
Thomas Berry, DDS, MA