Considerations in Impressioning Methods and Material Selection

Impressions, whether digital or physical, will make or break a restorative case. I started 25 years ago with digital restorative dentistry and today utilize digital means for most of my restorative and appliance dependent cases. Does that mean that physical (analog) impression materials have diminished in relevance or are on their way to becoming obsolete? Are intraoral scanners superior to analog materials? Does economics favor one over the other? Is the hype greater than the reward? What are the preferences of dentists, laboratories, and patients? Can you be 100 percent digital, and will it make you a better clinician? Dental marketplace statistics indicate interest and adoption for intraoral scanners (IOS) accelerating when used for the sole intent of digital impression transmission to laboratories. Intraoral scanners are seeing more interest and rapid adoption than their progenitor, chairside CAD/CAM systems, because most dentists still prefer laboratory support despite the known advantages of the chairside CAD/CAM fabrication. Whether analog or digital, clinical success is primarily dependent on the quality of the data (physical or analog impression).

Choosing the right material for the job

I utilize two intraoral scanners, have and continue to perform chairside CAD/CAM (since 1995), and I have also had experience with most major IOS devices. Despite my digital legacy, I still use physical materials because they can pick up the slack when digital materials fall short. These materials have a long history of clinical success and can also be more efficient in certain cases. Rather than tell you to use one or another, I encourage you to focus on evidence and performance, whether it is with a device, material, or your own hands. The real question is: How do we optimize each material and where are they best applied? Beginning with physical materials, let’s identify the properties needed for a “universal” or ideal impression material. Typically, we need:

• All clinical indications
• Clinical accuracy (trueness, detail and precision, reproducibility)
• Wettability or hydrophilicity in fluid conditions (blood and saliva)
• Ease of dispensing with a consistent, void-free mix
• Setting times matched to technique
• Economical
• Pleasant odor, taste, color
• Ability to be disinfected
• Ease of removal from mouth and for separation of gypsum models
• Tear and breakage resistant
• Elastic properties that prevent deformation after strain
• Dimensional stability initially and over time
• Adequate shelf life

While not totally inclusive, no one such perfect material exists creating the need for multiple materials. With IOS the ideal equipment would command:

• All clinical indications
• Clinical accuracy (trueness, detail and precision, reproducibility)
• Economical

In this piece I will represent both digital and physical materials along with the comparative skillsets required by each.

Where polyvinylsiloxanes and polyethers shine

Current evidence in the literature cautions or contraindicates the following for IOS:

• Subgingival margins pose challenges for all IOS systems, discretion is advised. IOS cannot see through fluids or into undercuts or displace soft tissue like impression materials.
• Edentulous and partially edentulous cases are contraindicated at this time.

I also prefer physical materials for the following:

• Most anterior implants or any situation where soft tissue reproduction can be a make-or-break factor for the final prosthetics – most labs agree with me here.

When it comes to edentulous and partially edentulous patients, I would think most US dentists would use a VPS or polyether material, with VPS in the clear majority. I won’t argue that excellent results cannot be achieved using both, but I will state that polyether (PE) has distinct advantages, and, in my experience, VPS falls a bit short. This is why PE has been my go-to material for over 25 years where IOS is not indicated (edentulous and partially edentulous). PE is inherently hydrophilic and exhibits the lowest water contact angles of all elastomeric materials. VPS materials, by comparison, have surfactants added to them to lower the contact angles, but they are not equivalent to PE. I’ve tried many VPS materials over the years and always go back to PE, especially with today’s formulations which have by and large addressed tastes, odors, difficult removal, and long set times. As an inherently hydrophilic material, PE excels in terms of flow. It has the inherent wettability necessary for the reproduction of the finest details where success is measured in microns. And let’s be real, moisture forgiveness is a big deal! For these reasons, when physical materials are needed, PE satisfies all my needs for fixed and removable prosthetics. Laboratories that I have worked with over the years also generally report fewer remakes with PE as compared to VPS. Full arch cases and frameworks for fixed or removable prosthetics demonstrate precision fits. PE is available in varying viscosities and I prefer “soft” quick polyether materials with a syringeable light body, which is much easier to remove from undercuts. My ideal set time is 3:00 minutes (a regular set of 4:15 is also available). Soft, quick polyether materials are applicable for the largest and most routine of cases. In terms of delivery, I prefer the centralized, automated mixing system as opposed to the gun-cartridge system. I do use VPS in one situation – for gaggers and behavioral patients, where speed is paramount and clinically sufficient detail can be realized. Imprint 4 Super Quick (set time 1:30) is great for single units and a lifesaver for problematic patients needing partials and dentures (custom tray recommended).

Considerations for success

The literature supports the premise that dental appliances and fixed prosthetics can be successfully accomplished using digital or physical material techniques with no clinically significant differences in outcomes, though physical material has the highest degree of capability. Technique and application can be the doctor’s Achilles heel, as such, I offer these points for consideration:

• Crown lengthening is under-utilized. Don’t finalize impressions until soft tissues can accommodate a good impression.
• The drier the mouth the better – consider antihistamines.
• The “braided, impregnated two-cord technique” remains the “gold standard” for soft tissue retraction.
• Retraction pastes can be effective for physical and IOS impressions, and, in some cases, they can replace or work well with a single cord.
• IOS devices require a clear line of sight and cannot see through fluids or into undercuts or displace soft tissue like physical impression materials. Materials (see polyether discussion) have modest advantages in the presence of fluids and can displace soft tissues.
• The more hydrophilic your material, the better you can cope with fluids (see polyether discussion).
• Use lasers to trough and expose margins to prevent bleeding instead of stopping it.
• Custom trays still rule, secondly metal stock, and finally plastic stock.
• Triple trays, when used, should be metal – no exceptions! Dead soft is preferred because they lack memory, which can strain a set material.
• When using a triple tray, allow the material to fully set and then have the patient open. Use a finger to lift the tray, using the handle for support only. Removal by handle alone can bend the tray creating distortion.
• The more rigid the material the more reliable it is (prevents distortion), but rigid materials may prove more difficult to remove.

In conclusion

The literature, along with my own experience, does support the following claims for IOS:

• IOS impressions are more comfortable for the patient and take less time than physical impressions.
• Digital re-takes are easier and less costly than physical retakes.
• IOS impressions eliminate the need for plaster and labor.
• A digital workflow assists with patient and lab communications and allows for streamlined laboratory workflows.
• Scans can be used as part of the digital patient record for smile analysis/design, implant prosthetics/placement, or anything else you can do with a “virtual model” that is stored via local disk or in the cloud.

If you operate with physical materials only, you are already 100 percent capable, and your results can mirror that of any IOS device if you achieve retraction, can account for oral fluids, and address the challenges associated with physical materials vs. the challenges for a camera. If you already use or are contemplating using an IOS device, physical materials are also needed because digital impressions lack 100 percent capability. What does separate IOS devices is their overall functionality in today’s rapidly growing dental and digital workspace. Where a practice fits into this equation today or in the future is unique to each organization. When considering comparative costs, future enhancements, patient acceptance, overall efficiency, ease of use, and intangible benefits, these systems are getting more comparable; especially as capabilities expand.