November/December 2009, Volume 30, Issue 9
Published by AEGIS Communications
Beyond First Impressions
Gerard Kugel, DMD, MS, PhD
When I started practicing dentistry nearly 25 years ago, taking a good impression was no simple chore. Whether the issue was dimensional stability, hydrophilicity, inadequate working time, or difficulties with mixing base and catalyst components, all of the materials presented some challenges for capturing an accurate impression. Throughout the years, in both my private practice and my work at the university, I have had ample opportunity to try virtually all of the different materials and mixing systems that have been introduced. I have witnessed through my own use how various improvements in both materials and their delivery systems have made it easier to obtain accurate impressions. Despite these improvements, laboratories continue to have significant problems with the impressions they receive. In a paper that my colleagues and I published in the Journal of the American Dental Association in June 2000, it was very evident that inaccurate and unclear margins was, by far, the major issue that laboratories reported with poor dental impressions. We did a recent follow-up to this survey and found little change in the data.
Although few clinicians hand mix impression materials today, this was not the case more than 15 years ago when dental companies introduced automixing equipment. Those of us old enough to remember the days of hand mixing realize this was not a trivial improvement in impression making: This greatly simplified the impression-taking process. These automix systems produce less material waste and a better mix (no voids or air pockets), resulting in a smooth, even color. Whether using polyether or vinyl polysiloxane (VPS) impression materials, we are less likely to have a bad impression with an accurate, even mix.
In addition to offering innovative delivery systems, manufacturers have improved the chemical properties of impression materials. One notable area is the hydrophilicity of polyethers and VPS—the ability of the materials to handle moisture. Using these improvements, dentists are better able to “keep the field clean,” helping eliminate moisture, bleeding, and sulcular fluids.
An ideal impression material should have some common qualities. It should be dimensionally stable both before and after mixing and disinfecting. It should be easy to read; have good tear strength, long working time, and short setting time; and allow for multiple pours. An ideal impression material also should have a taste and odor that does not offend the patient. Despite these improvements, we still do not have an ideal material.
The three most commonly used materials for final impressions are polyether, VPS, and, more recently, the vinyl-polyether hybrids. The polyether impression materials are supplied as a base and catalyst with a polymer formed during the cationic polymerization and opening of the imine rings. Polyethers are more rigid and will help prevent a nonrigid (triple) tray from distorting. Using a more rigid impression material also can be beneficial when working with pick-up or implant impressions. In addition, polyether impression material possesses a low contact angle, which means that the material flows well when it hits the surface, even in smaller areas. One situation, however, that can occur with rigid impression materials is that other restorations may be pulled out with the material. When this happened, I viewed it as a blessing in disguise—a sign that the original restorations were not properly secured.
The VPS materials were introduced in the 1970s as a base-and-catalyst system. When mixed, an addition reaction occurs (addition reaction silicones) between the silane and the vinyl groups, resulting in a cross-linked silicone rubber. VPS materials are less rigid than polyethers. In a dental practice, there are sometimes situations in which we want to avoid using a rigid impression material: when there are restorations, such as crowns and bridges, that we definitely do not want pulled out, or for high-quality diagnostic impressions when we do not want to disturb anything in the mouth. Also, laboratory technicians can often do only one pour with rigid polyethers and may, in fact, want to do multiple pours for bleaching trays and study models. For these instances, the polyether manufacturers have introduced a “soft” polyether. This material offers the same hydrophilicity and contact angle as a polyether but with a consistency more akin to a VPS impression material.
Some companies have introduced a new class of impression material referred to as vinyl-polyether hybrids. These two-paste systems contain a polymer with polyether and siloxane groups, which, in theory, should combine the benefits of both groups. The polyether groups contribute the hydrophilic material without using a surfactant, and the siloxane groups contribute to both the dimensional stability and the ability to resist deformation. Manufacturers have promoted this class of material as being hydrophilic during setting and after polymerization, and as being a blend of hydrophilicity and hydrophobicity. It is my opinion that these claims need more research in the peer-reviewed literature.
Although most dentists are still using conventional impression materials, there has been a recent explosion in the field of digital dentistry. The area of computerized dental technology is becoming readily available to both the dentist and technician. We have seen the introduction of digital model scanners, cone beam computed tomography, intraoral digital impression using an array of capture devices, rapid prototyping 3D printers, and CAD/CAM in-office systems. The use of these technologies presents a paradigm shift in the way dental impressions are taken. I have had the opportunity to use almost all the capture devices on the market. As I indicated with conventional impression material, no system is ideal. I have seen great improvement over time in the ease of use of many digital systems. Since I was first exposed to the field of digital image capture in 1989, I have grown confident in the belief that this is the future of impression making. There are now four chairside systems that are capable of scanning the prepared teeth, and then digitizing the information and producing a 3D image. Two systems outsource the digital information and allow the laboratory to fabricate a model for crown production, or, in some cases, a coping or crown can be milled directly from the digital file. Two other systems can fabricate single-unit restorations chairside. Although not the recommended technique, the chairside system can be used for outsourcing crowns if the dentist does not wish to invest the time or money in a milling center for his or her practice. Having used all four systems, I believe the dentist must be willing to spend the time required to train. All four systems represent a new age in impression making. In a recent article in the Journal of Dental Technology, it was estimated that 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.
It must be remembered that no matter how we capture the information needed to fabricate a restoration, good preparation, design, and tissue management are critical. The most common management techniques include a single or double cord, soft-tissue laser, electrosurgery, ferrous sulfate, and aluminum chloride. In the future, I believe we will have digital systems that will allow us to view the hard tissue only, thus minimizing our need for retraction.
Impression taking certainly has come a long way since I started practicing dentistry. Whether using polyether, VPS impression materials, vinyl-polyether hybrids, or digital capture, significant improvements have been made in impressioning. These improvements, combined with proper use of the materials and techniques, will certainly help us make more accurate impressions.
For many professionals, the choice of impressioning is a matter of personal preference. Regardless of which impression material or digital system we choose, it is important to realize that continued success is possible only through proper clinical technique and training.
About the Author
Gerard Kugel, DMD, MS, PhD
Research and Professor of Prosthodontics and Operative Dentistry
Tufts University School of Dental Medicine