A Guide to Selecting Impression Material

Making a good impression is essential to the success of your indirect restorations. When it comes to conventional impressions, good ones start, at least in part, with excellent materials. In other words, the impression materials you choose could make it or break it for you - sometimes literally. 

Dental professionals have high expectations for impression materials. The ideal material has all the following characteristics:

• Produces an accurate impression by adapting in great detail to the patient’s teeth and gums.

• Retains a consistency that resists tearing when removed but doesn’t cause the patient discomfort during the process.

• Is biocompatible and hypoallergenic.

• Has an inexpensive price point per use.

• Provides a decent patient experience regarding taste and set time while a patient holds it in their mouth (and fights their gag reflex).

Dental labs have high expectations of impression materials, too. Lab technicians expect it to retain dimensional accuracy when disinfected. Moreover, the materials should remain dimensionally stable so the technician can pour multiple casts. 

Today’s market contains a plethora of choices of impression materials. The most common types have been used since the middle of the 20th century and include alginates, polyethers (PEs) and vinyl polysiloxane (VPS). The most recent addition, which joined the group about 10 years ago, is vinyl polyether siloxane (VPES), a hybrid of PE and VPS that exhibits qualities of each. 

“Most of the impression materials in use today are VPS or polyethers,” says Sam Simos, DDS, a dentist in private practice in Ottawa, Illinois, and a national lecturer in cosmetic and restorative dentistry. “Clinicians fall into either camp: They are either a VPS clinician or a polyether clinician. And honestly, clinically, materials have gotten so good that there is not much difference.”

Each type has its pros and cons - situations where the material provides excellent performance and others that leave less than optimal results. 

“There are so many great impression materials,” Dr. Simos says. “It’s a matter of preference for most clinicians.”

Characteristics of Impression Materials
Comparing the attributes of materials is helpful when choosing one for a given case. Impression materials are often subject to the following criteria¹:

• Dimensional accuracy: how closely the impression material reproduces the details of the patient’s mouth. 

• Dimensional stability: how long the material maintains accuracy after an impression is taken. Some materials have a longer time, with a poured cast remaining accurate up to two weeks, whereas others last just two hours or less. It is an essential consideration depending on how soon a cast will poured. 

• Hydrophilic properties: the ability to tolerate moisture and produce an accurate impression. The chemistry dictates whether a material can work in wet environments. 

• Wettability: how well the material flows into small areas. More wettability means that the material displaces moisture well and results in fewer voids in the impression.  

• Elastic recovery: ability to resist distortion once set and removed from the patient’s mouth. In other words, the material should return to the original dimensions. 

• Flexibility: ease of removal from the mouth after setting. If the material is stiff, it is considered less flexible. 

• Ease of handling: the “how it works in my hands” characteristic inherent in how clinicians choose dental materials. Features that affect ease of handling include viscosity and the working time/set time ratio. 

• Tear strength: likelihood of tearing when removed from the patient’s mouth. Materials with high tear strength are essential for an impression to capture a subgingival margin. 

• Contact angle: how easily the material reproduces detail. The lower the contact angle, the less technique sensitive the impression material is. A material with a high contact angle requires more care in pouring and extra attention to proper form in the impression-taking process. 
  
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Dr. Simos describes himself as a “VPS guy.” He says his go-to impression material is forgiving and has great dimensional stability and excellent tear strength. 

“I always reach for VPS because you will see a really good margin. If you prepped a really good margin and have excellent retraction, you will get a good result,” Dr. Simos says.

Many factors must be considered when reaching for an impression material for use in the operatory. This overview will facilitate the selection of an impression material that will facilitate success. 

Alginates
Alginates, or irreversible hydrocolloids, are common impression materials in dental practices today. They first appeared in dental operatories in 1947 and were in widespread use by the 1950s.

Alginates’ many favorable characteristics led to their popularity. They are easy to use, cost less and set quickly.¹ They are also nontoxic, nonirritating and hydrophilic, which makes them preferable in situations when isolation is challenging.¹ In addition, alginates have a low wetting angle, which is helpful when taking full arch impressions.¹ Moreover, for a patient with undercuts, all but the deepest alginates can be removed from the patient’s mouth without tearing.¹

However, alginates also pose disadvantages. They do not reproduce details as well as other elastomeric materials and have poor dimensional stability, which is troublesome for complex cases.¹ Also, just one plaster cast can be poured from these impressions. ¹ Moreover, although they can capture subgingival detail, they tear when removed.¹

PEs
When PEs emerged in the 1960s, they challenged alginates with their improved mechanical properties and reduced polymerization shrinkage, which popularized them over the next decade. 

PEs have many advantages. Known for moderate hydrophilicity, PEs are excellent for taking impressions and capturing detail in a moist environment.¹ They also have superior wetting properties, making them easier to use for full-arch impressions. PEs also do not need much pressure to flow into “critical areas” to capture excellent detail and have high dimensional stability, which means it’s possible to cast more than once with the impression, which is an improvement over alginates.¹ Although the earlier materials had a rigid quality, improvements made them more flexible and easier to remove from the patient’s mouth without tearing.³ Moreover, PEs can capture subgingival detail.¹ In addition, they have a short setting time, less setting shrinkage, high elastic recovery and moderate tear strength.¹

A few disadvantages of using PEs: They have a short working time.¹ They require extra care in the disinfection process so they do not absorb the chemicals and distort.¹ Also, they taste bitter, which can lead to patient discomfort.¹

VPS
About 10 years after the introduction of PEs, VPS entered the dental impression material mix. It improved on the PE properties in many ways and became a staple in many dental practice inventories. 

VPS offers numerous advantages. It reproduces excellent detail.³ The material has excellent tear strength and a high elastic recovery, leading to excellent dimensional stability.² VPS also has good wettability characteristics and a short setting time.² It is inert once set, so it is trimmable and amenable to any die material needed for casting.¹

VPS materials also have a high rigidity and so are easier to remove than PEs. VPS also has fewer problems with distortion than PEs, so it is easier to disinfect without caution, and it tastes better than other materials. 

However, the chemistry of VPS leads to a few disadvantages, perhaps most significantly a susceptibility to contamination, often from latex gloves or retraction cords. Also, storage next to other impression materials, such as polysulfide impression materials, can cause contamination. Newly cured resin materials’ surface layer can contaminate and inhibit a set of VPS impression materials, as can residue from previously used impression materials, such as PE and polysulfide.¹

Moreover, temperature is crucial to VPS. If it gets too warm, it sets faster. If it is cool, it is thicker and harder to handle. VPS requires storage in a cool place and time to reach room temperature before using.² 

VPS materials call for precise handling. To avoid using materials that have been exposed to the environment, clinicians must extrude at least a quarter inch of the material before placing the mixing tip. Also, during pouring, the material must be fully set to avoid hydrogen bubbles, which create pitted areas in the cast. Furthermore, the hydrophobic nature of VPS makes isolation imperative to get excellent impressions.²

VPS also has disadvantages compared with other impression materials. It has less tear strength than PEs, although higher than alginate. VPS also has a high contact angle, so it is not as easy to use for full arch impressions as other impression materials.²

VPES
The most recent entrant into the dental impression materials is VPES, which arrived in 2009. The material is designed to combine the advantages of PE and VPS materials.⁴

There are many suggested advantages of VPES. The material is as easy to remove as VPS, with the hydrophilicity and flowability of PE, which may make it a candidate for cases that require an impression of a narrow and deep gingival crevice, although these claims are not yet supported by the literature.⁵ It is designed for dimensional accuracy and recovery because of the optimized elastomeric properties and has excellent dimensional stability.¹ In addition, it works fine with the disinfectants on the market.⁵

A few other advantages of VPES: It has a lower fracture risk during casting. It doesn’t smell or have a bad taste, which is excellent for the patient experience.⁵

A couple of disadvantages: Over time, the dimensional stability of VPES degrades a bit compared with an immediate cast.⁵ VPES is also more hydrophobic than VPS, which requires excellent isolation and can be problematic.⁶ In addition, the VPES has higher contact angle than VPS, meaning it has a higher technique sensitivity.⁷

Both PE and VPS are forgiving materials, and there is no significant difference in clinical technique between them, according to Dr. Simos. VPES has an inherent advantage in the material for tear strength, he says. He agrees that the material is designed to give clinicians the best of both PE and VPS, but says that clinically, VPES doesn’t handle much differently. In general, he sticks with VPS and changes his technique if the clinical situation dictates.

“If I'm doing a multi-unit implant case, let's say, and I need some time, I’ll still go with my polyvinyl and select a material with a longer set time,” Dr. Simos says. “If I am doing a single-unit, regular crown prep, it will still be a VPS, but it will be a quick set. So, I manipulate it that way.”

Dr. Simos adds that isolation and retraction are areas that cannot be overemphasized, but all the impression materials are forgiving in that area, save one. He thinks dentists should throw out their alginates because the new materials are so much better than alginates.

“For me, it makes so much more practical sense to use an alginate substitute. Patients tolerate it better. It sets much quicker. It's got more dimensional stability. You can repour it. It's just all-around better,” Dr. Simos says.

References

1. Impression materials: a comparative review of impression materials most commonly used in restorative dentistry. Pocket Dentistry website. pocketdentistry.com/impression-materials-a-comparative-review-of-impression-materials-most-commonly-used-in-restorative-dentistry/. Published June 15, 2016. Accessed March 6, 2020.

2. Nandini VV, Venkatesh KV, Nair KC. Alginate impressions: a practical perspective. J Conserv Dent. 2008;11(1):37-41. doi: 10.4103/0972-0707.43416.

3.  Punj, Amit & Bompolaki, Despoina & Garaicoa, Jorge. (2017). Dental Impression Materials and Techniques. Dental Clinics of North America. 61. 779-796. 10.1016/j.cden.2017.06.004. Accessed via web. 4 March 2020. <https://www.researchgate.net/publication/320046633_Dental_Impression_Materials_and_Techniques>.

4. Shetty RM, Bhandari GR, Mehta D. “Vinyl Polysiloxane Ether: A Breakthrough in Elastomeric Impression Material.” World J Dent 2014;5(2):134-137. Accessed via Web. 6 March 2020. <https://pdfs.semanticscholar.org/c37b/649d57f01c43a3ea5da688d943be34c2702e.pdf>. 

5.  Nassar, Usama & Oko, Andrea & Adeeb, Samer & El-Rich, Marwan & Flores-Mir, Carlos. (2013). An in vitro study on the dimensional stability of a vinyl polyether silicone impression material over a prolonged storage period. The Journal of prosthetic dentistry. 109. 172-8. 10.1016/S0022-3913(13)60038-4. Accessed via web. 9 March 2020. < https://www.researchgate.net/publication/236074170_An_in_vitro_study_on_the_dimensional_stability_of_a_vinyl_polyether_silicone_impression_material_over_a_prolonged_storage_period>

6. Usama Nassar, DDS, MS; Faraz Tavoossi, DDS; Yan Wen Pan; Nathan Milavong-Viravongsa; Giseon Heo, PhD; John A. Nychka, PhD, Peng. “Comparison of the contact angle of water on set elastomeric impression materials.” J Can Dent Assoc 2018;84:i6. Accessed via web. 9 March 2020. < https://jcda.ca/i6>. 

7. Karaaslan G, Malkoc M A, Yildirim G, Malkoc S. Comparison of time-dependent two-dimensional and three-dimensional stability with micro-computerized tomography and wettability of three impression materials. Niger J Clin Pract [serial online] 2018 [cited 2020 Mar 9];21:912-20. Available from: http://www.njcponline.com/text.asp?2018/21/7/912/236162