Precision Matters: The Evolution of Impression Systems and Materials

The use of physical impression trays and materials dates back to the 19th century and remained largely unchanged throughout the 20th century.¹ Reversible hydrocolloids were introduced in 1925, followed by irreversible hydrocolloids (alginate) in 1941. Their main disadvantage is shrinkage over time during storage caused by loss of water, leading to inaccuracies. In the late 1960s, polyether-based impression materials were proposed due to their excellent accuracy, improved mechanical properties, and low shrinkage. During the 1970s, polyvinyl siloxane (PVS) entered the market and became popular because of its very high dimensional stability and elastic recovery over time and various temperatures, even in a moist environment.^2,3 The development and improvement of elastomeric impression materials enhanced the quality and predictability of dental impressions. They are still considered a gold standard for impression-making of soft and mobile tissue, for example, to fabricate removable partial and complete dentures, as well for implants supporting full-mouth fixed reconstructions or overdentures.

A rapid movement in digital technologies spread across various industries worldwide in the middle of the 20th century, including the healthcare field. In the early 1980s, Professor Werner Mörmann designed and patented a handheld intraoral scanner (IOS), which was part of the first-generation "chairside economical restoration of esthetic ceramics" (CEREC) scanning and milling system. Some early restorations fabricated with this system were less than optimal, but the fact that it could fabricate an indirect ceramic restoration in the same day using an optical scanner and a milling machine in the dental office was revolutionary.^4,5 In today's contemporary era of dentistry digitization, digital impressions have become mainstream.¹

Impression-making, whether in an analog or digital manner, is the first step of the indirect restorative workflow and can be described as intraoral information gathering or data acquisition. For analog impressions, dental practitioners must obtain a negative form of the teeth and tissues into which a die material can be processed to create the working analogs.² The most popular types of impression materials for removable, fixed, and implant prosthodontics are irreversible hydrocolloids, polyethers, and PVS. PVS is one of the most preferred impression materials in dentistry because of its excellent properties of great detail reproduction and dimensional stability and its ability to be poured with stone multiple times due to its high tear strength and high elastic recovery.⁶ A significant disadvantage of these types of impressions, however, lies precisely in their nature: they must be physically made by the operator and sent to the laboratory. Given that many laboratories currently operate with a digital workflow, this process tends to be less efficient, as it requires more time to have the cast model digitized, and amid other steps, there is a risk of information and accuracy loss.

Intraoral scanners in dentistry are devices for capturing direct optical impressions. They project a light source onto the object to be scanned and produce images that are processed by a software, creating 3D models.⁵ Digital impressions offer speed, efficiency, and the ability to store information indefinitely, enabling the transferal of digital images and other critical information between the dental office and the laboratory. Offering great utility, IOSs are applied in various fields of dentistry, from diagnosis to fabricating restorations or custom devices for surgery and orthodontics. Current evidence suggests that with the use of correct technique, IOS technologies are as accurate and precise as conventional impressions for single or short-span multi-unit restorations.^6-13 For larger spans, angled implants, and full-arch fixed partial dentures, however, conventional impression techniques still demonstrate superior accuracy.^8,12,13

There are several advantages of digital impression and scanning systems over their analog counterparts, such as better patient acceptance, improved workflow, reduced chairside time, immediate 3D pre-visualization of tooth preparations, tooth shade analysis, and potential cost- and time-effectiveness.^6,7,9,10 The IOS is more comfortable and less invasive for patients, especially those with sensitive gag reflex or profuse salivation. Also, the digital impression can be inspected immediately, and areas that have not been captured adequately can simply be rescanned without having to remake the entire impression.^4,10,11,14

Despite their benefits and tremendous growth, especially over the past few years, CAD/CAM technologies are yet to be fully embraced by the dental community. The main reasons for some hesitations are related to the initial investment to acquire the intraoral equipment and software maintenance fees.^1,4 Also, the learning curve is steep and can be difficult for some individuals. However, dental students exhibit a preference for digital techniques, indicating that the use of digital impression technique is likely to continue expanding significantly.^9,14

Studies show that IOSs cannot capture subgingival margins well, especially if covered with blood, saliva, or tissue.^8,9 Excellent contamination and soft-tissue control are mandatory for optical scans. Undercuts also present a particular challenge. Conventional impressions allow the impression material to flow into the undercuts and may better reproduce finish lines of subgingival preparations due to the applied physical pressure.^8,11

The integration of CAD/CAM technologies is undoubtedly becoming essential in routine dental procedures. These technologies offer a more efficient approach to treatment planning and communication, leading to reduced chairtime. Just like analog protocols and techniques, the treatment outcomes of CAD/CAM methods vary and are directly related to the experience and skill level of the clinician and/or technician involved. However, as key steps are automated, accuracy and predictability improves and becomes more standardized. Alternative scan methods like photogrammetry provide superior accuracy for precise 3D intraoral capturing of dental implants. Several IOSs are equipped with tools for tooth shade analyses and even caries detection, employing fluorescence technologies or near-infrared imaging to identify carious lesions, further assisting with current concepts of minimally invasive dentistry. IOSs are quite instrumental in monitoring treatment and disease progress over time and can be applied for quantitative monitoring of various parameters such as tooth wear and soft-tissue dimensions.

Digital technologies are here to stay. Today, open-platform interfaces allow other imaging modalities, like cone-beam computed tomography and facial scans, to be combined and used for diagnosis, smile design, and treatment planning, often powered and supported by artificial intelligence. Other techniques like ultrasonography and optical coherence tomography, which are radiation-free methods used widely in medicine, have also shown promise in the dental field.² Next-generation scanning devices are expected to be able to differentiate between hard and soft tissues, blood and saliva, and take fast, accurate, and stress-free images, enhancing overall efficiency. Indeed, impression techniques and technologies, especially digital ones, are constantly evolving and are geared toward providing a better experience and outcomes for the clinician, laboratory technician, and even more so, the patient.

Macarena Rivera, DDS, MSc
Assistant Professor, Department of Prosthodontics, University of Chile, Santiago, Chile; Adjunct Professor, Department of Preventive and Restorative Sciences, University of Pennsylvania School of Dental Medicine,Philadelphia, Pennsylvania; Private Practice, Santiago, Chile

Markus B. Blatz, DMD, PhD
Professor of Restorative Dentistry, Chair, Department of Preventive and Restorative Sciences, and Assistant Dean, Digital Innovation and Professional Development, University of Pennsylvania School of Dental Medicine, Philadelphia, Pennsylvania