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Quality Impressions Are Essential
Eric C. Bailey, DMD; and Philip S. Baker, BS, DDS, FACP
As Shillingburg et al1 wrote in the Fundamentals of Fixed Prosthodontics, “It is critical to have accurate casts because it is critical to have well-fitting restorations. The second cannot exist without the first.” These words ring true for as long as dentists continue fabricating fixed dental prostheses. In the digital realm, having an accurate duplication of dental arches is essential. The purpose of this article is to examine the present and future trends in dental-impression procedures.
In the past half century, significant progress has transpired in the accuracy and ease of use in dental impression materials. Alginate (irreversible hydrocolloid) and polyvinyl siloxane (PVS) impression materials have retained the highest sales volume in the dental market for many years. Where dimensional stability and accuracy are required, PVS continues to be among the most popular impression materials in use. The authors’ search of dental-supply catalogs helps support this claim, with a large supply of PVS offerings from various manufacturers. Due to the many advantages and few disadvantages, PVS has remained the standard of comparison for full-mouth impression-making. However, digital dentistry has had numerous advances since the 1980s, with digital impression-making among them. Computer-aided design and computer-aided manufacturing dentistry has matured and continues to see improvements as evidenced by the literature.2-4
The Industry Standard
Silicone-based impression materials have been available for nearly 50 years in dentistry. The earliest formulations were condensation-reaction silicones, which differ in their properties from addition-reaction silicones. Specifically, condensation-reaction silicones are less dimensionally stable than polyether or addition-reaction silicones.5 For the purpose of this article, PVS materials refer to addition-reaction silicones and their associated properties. Manufacturers have continued to perfect the formula of PVS impression materials to meet the needs of the clinician. Earlier PVS formulations would often produce voids on the surface of stone casts due to the release of hydrogen gas as a byproduct of the setting reaction if poured within 24 hours. The subsequent addition of a palladium scavenger in modern formulations has rectified this.5 Another initial issue with PVS materials—and to some extent still exists—is the inherent hydrophobicity. Most formulations today contain surfactant to increase this property. One of the greatest attributes of a PVS material is not only its dimensional accuracy, but also its dimensional stability. Among impression materials, PVS provides the greatest combination of replication accuracy and dimensional stability: the accuracy of a PVS impression remains unchanged long after removal from the mouth and remains precise even after multiple pours.6 Individual cartridges facilitate ease of delivery and are available in contrasting colors for different viscosities. This is certainly not groundbreaking, but the point is that the improvements to PVS materials in more recent years have been directed mainly toward ease of use and delivery rather than enhancements to the impression material itself. Because no statistically significant differences in the accuracy of polyether and PVS materials have been observed, the continued use of polyether materials may be due to the clinician’s personal preference and comfort with handling. That said, PVS materials continue to lead the market in sales volume and remain the standard of comparison for the development of new impression materials and techniques.
Error and Comfort
PVS materials have limitations. The required setting time during the reaction and setting of a PVS material may be a source of error if the tray moves inadvertently. Packing cord and preparing for a multi-unit impression takes time and patience from the clinician, patient, and assistant. The need to repeat that process in case of failure to properly capture a margin or other anatomy can be problematic and frustrating. Limitations include: setting time, inherent hydrophobicity, and tear strength in areas where the material is thin. Further, many patients prefer a method of impression-making that can be accomplished without a mouthful of material. While it is not known what percent of the population avoids dental care because of a hypersensitive gag reflex, every dentist at some point will probably encounter this issue.7 Leder’s study8 of 69 patients revealed 3% had a hypersensitive gag reflex. The comfort of this population may be enhanced through the use of intraoral scanners. Even a normal gag reflex may be triggered during impression-making, and these patients might also find additional comfort with an intraoral scanner when compared to a conventional impression technique.
Two Steps Forward, One Step Back
As a profession, we are trained to practice evidence-based dentistry. We apply this same standard to digital technology and the associated techniques. In 2016, a systematic review by Tsirogiannis et al9 concluded no significant difference was observed regarding the marginal discrepancy of single-unit ceramic restorations fabricated after digital or conventional impressions were taken. A study conducted at Harvard School of Dental Medicine10 determined second-year students were more efficient with digital impression-making than traditional techniques and also preferred the process to a conventional procedure. Surveys involving patients have shown higher satisfaction in their experiences with digital impressions versus conventional methods.11 Though more research is needed, one apparent limitation of digital impression-making is the inconsistency of some intraoral scanners when scanning the complete arch.12 Kuhr et al13 recently concluded conventional impressions remained significantly more accurate than full-arch scans with intraoral scanners in a clinical setting. Ender and Mehl14 compared PVS, polyether, and digital scans with point-to-point measurements of the models. They concluded digital impression systems revealed higher local deviations from the full-arch model. Further, digital impression systems did not show superior accuracy compared to highly accurate conventional impressions, but digital scans did provide excellent clinical results within their indications. The limitations and indications of specific digital scanners must be understood.
An increasing demand for digital impression-making alongside improvements in use and expanding scope is likely. Hardware costs may remain a limiting factor for the foreseeable future. Many private practitioners may find it difficult to justify such a large investment in technology that may not meet all of their needs or could quickly become technologically outdated. As previously stated, improvements to PVS are primarily focused on ease of use and delivery rather than enhancements to the impression material itself. In the authors’ opinion, the literature suggests digital impression systems have not quite reached that level of technological maturity. The costs of intraoral scanners may be prohibitive for some clinicians; however, as with any new technology, investing may soon be at hand.
About the Authors
Eric C. Bailey, DMD
Advanced Education Program in Prosthodontics
Dental College of Georgia
Philip S. Baker, BS, DDS, FACP
Advanced Education Program in Prosthodontics
Dental College of Georgia
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