Impression-Taking: Conventional Methods Remain Steadfast as Digital Technology Progresses

Capturing an accurate impression is one of the most important aspects for a successful, lucrative dental practice. Dentists typically employ one of two ways to take impressions: conventional with the use of polyvinylsiloxane (PVS) or polyether materials or digital systems. Although criticisms remain regarding the use of traditional means in that they may be prone to error and increase the need for retake—and can be uncomfortable for patients—most dentists continue to use this tried-and-true method. Manufacturers of traditional materials have responded to market demand by producing materials to help remedy specific problems inherent in conventional impression-taking, such as addressing the presence of blood, saliva, and crevicular fluid. These more sophisticated materials have helped to enhance the conventional impression process.

While the benefits of switching to digital may include fewer procedural steps and reduced chances for error, costs and trepidation regarding new technology remain factors for many established providers. Traditional impressioning methods and materials consist about 85% to 90% of the market share, according to various sources. This is expected to change in the coming years in favor of digital impressioning. In the meantime, however, conventional methods remain a steadfast preference among many practitioners.

Success in impression-taking involves the following factors: moisture control, atraumatic tissue retraction, cleanliness of the tooth preparation and associated tissues, avoidance of air entrapment, avoidance of material tearing or distortion upon removal, and adequate inspection of the impression after removal to ensure complete accuracy.¹ Clinician skill and experience also influence the success of conventional techniques.

The advantages of PVS impression materials include²:

• flexible setting times and short intraoral setting times
• excellent dimensional stability
• good flowability and wettability
• high tear strength,³ and the best elastic recovery of all available materials
• extremely accurate detail reproduction that meets American Dental Association specifications for impression materials capable of recording fine detail of 25 μm or lessresistance to slumping
• compatibility with various die materials, such as gypsum
• ability to work well with electroplating; and preferred when using refractory materials

Periodontal tissue management is an important aspect for a successful impression procedure. Once the tissues are healthy, the practitioner must take care to minimize any tissue trauma during the preparation and impression phases. Protecting the gingiva will enhance the tissue environment by reducing tissue hemorrhaging.²

Clinicians have a number of tools at their disposal when challenges of moisture and bleeding control as well as working time begin to become a concern in complex cases.² The use of antisialogogues can help with saliva inhibition. Propantheline is given 30 minutes before the impression appointment and works for up to 3 hours. However, this medication has some precautions such as in patients who are pregnant or have glaucoma.

Another effective strategy may be the use of 0.12% chlorhexidine gluconate prior to the procedure. This has been shown to significantly reduce plaque levels and associated gingivitis, improves gingival health, and results in reduced bleeding during preparation and the impression procedure.^4,5

For single- or quadrant-die impressions, the use of an isolating aid can help with moisture management.² Components include a bite block, a flap to isolate the tongue and cheeks, and suction devices to remove moisture at the working field.

Although conventional impressions remain the more common practice, digital impressioning offers significant advantages to clinicians and patients, including reducing the need for remakes. If defects or inadequate preparation reduction are observed, the area can be rescanned as opposed to having to do a new impression. Also, because digital impression devices measure the occlusal clearance between the prepared and opposing teeth, the dentist is able to make changes before sending the impression to the laboratory.

Digital impressions also allow the dentist greater control in that the imaging process can be halted to enable the dentist to remove blood and saliva. Another benefit is the accuracy and durability of the resin model, which is more abrasion resistant and precise than a gypsum model typically used in conventional impressions.

Another advantage with digital devices is the elimination of the uncomfortable, messy impression-taking process, which causes some patients to gag. The digital approach also tends to improve delivery time because records are communicated electronically.

However, cost can be an inhibitor to many practitioners. Digital impression-taking requires significant financial investment, and a steep learning curve is likely to be needed. While there is less consumption of materials, processing costs of the digital impression are also a consideration. Another potential drawback with digital systems is that the wand size can present difficulties for patients with restricted openings.

Researchers have examined the accuracy of digital versus conventional impressions. One study⁶ compared copings made by digital impressions and PVS conventional impressions. The researchers observed that the marginal accuracy of the zirconia crowns produced from the digital system demonstrated no statistically significant difference compared with crowns from conventional impressions.

However, in another evaluation⁷ the researchers observed that crowns fabricated from digital impressions had better marginal fit and proximal contact and required equal occlusal adjustment. In yet another study,⁸ the authors reported that digital impressions were equally or more accurate and precise than plaster models. More data are needed before any conclusions can be made regarding the accuracy of digital impressions.

Whether digital is faster is debatable.⁹ A digital scan may take 3 to 5 minutes, which is equitable to the time it takes for traditional impression materials to set. However, with conventional impressions, the dental team must select a tray, apply tray adhesive, clean the trays, assemble the materials, and disinfect.

Conventional techniques are certain to be the more popular method of impression-taking for several years to come, however digital impressioning is expected to continue to make inroads. Manufacturers and clinicians must work together to develop and create better products to overcome challenges brought by moisture and other clinical demands. Digital techniques offer several advantages, but cost and learning curves may continue to challenge some practitioners as they consider converting to new technology. Ultimately, it is up to the practitioner to make the decision regarding what best suits his or her clinical needs.

Gerard Kugel, DMD, MS, PhD
Professor, Associate Dean for Dental Research, Tufts University School of Dental Medicine, Boston, Massachusetts

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