Dental Impressions: Maturity of Imaging Technologies Driving Shift From Physical to Digital

The physical impression workflow involves selecting an appropriate material to create an impression, along with a proper carrier or tray to deliver the material and achieve the intended outcomes. The resulting "impression" is then converted into a physical model, typically with a variety of dental plasters and stones. These models are then used to fabricate the restoration or appliance.

The physical workflow essentially consists of: material selection, tray selection, proper isolation, impression, creation of a physical model (in-office or laboratory), and, finally, fabrication of the restoration or appliance (laboratory). Historically, impression materials comprise those listed in Table 1. Irreversible hydrocolloid (alginate) is among the most widely used dental impression materials.¹ Though statistical data is not readily available, anecdotally, based on numerous discussions the author has had with dental laboratories across the United States, polyether and polyvinyl siloxane (PVS) addition silicone materials are the most popular synthetic elastomers for physical impressions.

Alginate is commonly used for the fabrication of study models; working casts; simple dental appliances such as mouthguards, retainers, and bite plates; orthodontic appliances; and full and partial denture prostheses. Based on more than 30 years of clinical experience and personal observation, the author considers advantages of alginate to include: relative patient comfort (with regard to taste and consistency) with fast setting, simple to mix, flexible impressions when set, moderate to good accuracy, hydrophilicity, relatively low cost, and minimal need for equipment. Conversely, disadvantages of alginate are that it is easily distorted, it possesses poor dimensional stability and tear strength, and is highly technique sensitive (ie, affected by excessive hydration or desiccation).²

Polyether materials are widely used and known for their accuracy and excellent working properties. Available in a range of viscosities, they are supplied as an accelerator and a base either in tubes or may be auto-dispensed, set relatively quickly and are dimensionally stable, but are hydrophilic and do absorb water and distort over time when exposed to moisture due to imbibition; however, their hydrophilicity is an advantage when impressioning in environments that lack complete moisture control. They are extremely rigid when set and resist tearing, though the rigidity may be uncomfortable for patients and pose difficulty in handling.³ The taste of polyether materials may be unpleasant, and, in the author's experience, some patients may exhibit allergic reactions to the material.

PVS materials, also known as addition reaction silicone materials, are supplied as a base and an accelerator, come in various viscosities, and are commonly delivered with plastic trays and polyvinyl adhesive, or with triple trays. PVS materials are supplied in tubes, automix cartridges, and Pentamix cartridges (3M ESPE, 3m.com) depending on the manufacturer.   Highly accurate with properties such as excellent working time and elastic recovery with low deformation and high tear strength, PVS materials have good dimensional stability over time⁴ and are hydrophobic. If polyvinyl impressions are taken preoperatively to create provisionals, the material should be thoroughly rinsed to remove acrylic residue prior to applying a wash material for the final impression. Nitrile or non-latex gloves should be used when handling PVS materials, and impression cord should not be handled with latex gloves.

Dental impressions are no longer restricted to physical materials. The advent of acquiring impressions of dental structures digitally in the late 1980s has evolved into what is now a mature technology, and digital dental impressions have gained a stable and rapidly growing foothold in clinical dentistry. Obtaining dental impressions digitally using optical scanners or lasers allows for accurate reproduction of intraoral hard and soft tissues. Data is acquired via the scanner, stored on a computer, and displayed on a monitor, allowing the clinician to either manipulate the images to create chairside milled or 3D-printed restorations or send the files to a laboratory to produce the desired restoration.

Digital impressions are useful in many dental disciplines, including simple restorations for inlays and crowns, small fixed bridgework, large fixed prosthetic cases, partial and full removable appliances, and a complete range of implant applications such as surgical guides, implant abutments, and restorations of all types. Using a scanner (eg, Planmeca FIT^®, Planmeca CAD/CAM, planmecausa.com; CEREC^® Omnicam, Dentsply Sirona, dentsplysirona.com), dentists have the option to fabricate simple restorations chairside or send files to a commercial laboratory for the fabrication of virtually any type of dental prosthesis. Simply stated, a digital scanner used to acquire dental impressions is similar to an intraoral camera. It is a handheld wand or handpiece that allows computer software to convert the object of the scan (eg, a tooth preparation) into data that can be used to create a physical restoration such as a crown, bridge, implant abutment, or denture. Once the data is acquired it can be used to design and mill a restoration onsite, or it can be sent to a dental laboratory to produce the restoration. Single-unit restorations may be milled and completed with or without a digitally printed physical model.

Technology commercially available today allows for digital acquisition of all surfaces that traditional "physical" impression materials capture. Working with digital impressioning for more than 20 years, the author has found that study models captured with alginate and poured in dental stone can be acquired digitally in the same amount of time or less without the added step of having to pour a stone model, and then either manipulated digitally for the intended study or printed by a digital printer. Also, onlay and full-coverage preparations can be digitally captured in far less time than with physical materials, and restorations may be created chairside that rival laboratory-produced restorations in every way at a fraction of the cost in dollars and time.

Patient experience with digital impressioning is favorable compared with physical impressioning techniques.⁵ Additionally, the digital workflow allows practitioners to digitally image a physical impression from their material of choice to create a virtual model and subsequent restoration without the need to actually pour a model, ie, digital impressions can be taken of a physical impression to create a model.

Digital impressions help facilitate implant dentistry, allowing for simple and immediate acquisition of abutment scan bodies for the production of abutments and crowns, as well as complex implant-borne restorative prostheses.Appliances, including orthodontic aligners, occlusal guards, and surgical implant guides, and complete dentures can all be cost-effectively designed and manufactured from digitally acquired data, and much of this is now available to individual dental facilities through affordable and cost-effective printing technologies.

When using a digital impression workflow, several options and variations are possible. One workflow option is: intraoral digital scan of the intended site, digital design by the dentist or staff, in-office fabrication of prosthesis or digital transfer of files to commercial laboratory for fabrication of prosthesis. A second workflow option is: physical impression with tray and material of choice, model poured either in-office or by laboratory, scan of physical models, digital transfer of files to commercial laboratory for fabrication of prosthesis.

In the author's experience, the advantages of digital impressioning include:highly accurate and reproducible impressions without degradation over time, indefinite storage capabilities of digital impressions without consuming physical office space, time savings compared to conventional impressioning, minimal considerations with regard to infection control, improved patient comfort and patient acceptance, increased operator control over processes and outcome, eco-friendly workflow that minimizes the use of disposable trays and impression materials, and the ability to be used for a full range of dental applications. Additionally, digital impressioning can have positive marketing implications due to its patient friendliness and accurate results. Staff participation can also be increased because of more automated and less technique-sensitive procedures, and greater profitability is possible.⁶

With regard to the aforementioned second digital impression workflow option, some disadvantages include: a high learning curve to master digital impressioning and technologies, a need to learn related computer software, the complexities of incorporating such technology into a practice location, staff training and education considerations, maintenance costs, and relatively high capital investment cost.

Dentists today are afforded a wide range of options with respect to the clinical impressioning of dental and oral structures. Beyond physical impression modalities, the maturation of digital impression scanners and techniques has elevated the options available in clinical dentistry. The dental laboratory in 2018 is largely a digitally driven workshop, and the digital technologies available to dentists allow for the cost-effective production of fully functional and highly esthetic restorations in ways never before available to the profession.

Eugene Antenucci, DDS
Private Practice, Bay Shore, New York. Dr. Antenucci lectures internationally covering technologies such as digital impressioning, intra- and extraoral imaging, digital radiography, CBCT imaging, office automation, and incorporation of social media and Internet into dental marketing.