Make an Ideal Impression
Parameters to consider when selecting an impression material
John O. Burgess, DDS, MS | Nathaniel C. Lawson, DMD, PhD
A recent survey reported that dentists use digital impression systems for 9% of their crown impressions.1 Although digital impression systems have advanced in accuracy and ease of use, there will always be a use for traditional impression materials. For crown preparations with subgingival margins, the ability of an impression material to physically penetrate between the gingival tissue and tooth structure cannot be replicated with optical scanning techniques. And the efficiency of a dual-arch impression tray with a fast-setting impression material will rival even the most time-effective digital impression processes. Currently, the most commonly used materials for final impressions are polyvinyl siloxane (PVS) (77%) and polyether (12%).1
Although the classes of impression materials have not changed in the past decade, there have been slight advances in the materials themselves. This article discusses several parameters that clinicians should consider when selecting an impression material.
Flow
Flow is dependent on the ability of the material to resist shear forces. An impression material must be able to penetrate the narrow subgingival sulcus and tight interproximal areas. To achieve this, it must be able to resist the shear forces as it is pushed between the teeth and gingival walls. To maximize flow, the hydrodynamic impression technique is commonly used. In this manner, a high viscosity material is used in the tray to push a lower viscosity material into the sulcus. Most clinicians reported using a heavy body (57%) or medium body (34%) material in the impression tray and a light (77%) or ultra-light viscosity (12%) material to inject into the sulcus.1
Not only is the initial flowability of an impression material important, but its flow rate is also significant. An ideal material would flow easily during the working time and then immediately cease to flow once it is positioned in the patient’s mouth. This property is important because a rapid reduction in flow can help prevent the material from deforming if the impression is disturbed while it is setting in the patient’s mouth.
A study of impression material flow rates reported that two regular-setting PVS materials began to decrease in flow within 30 seconds of mixing and did not cease to flow until 3 minutes after mixing.2 A regular-setting, ultra-light viscosity material maintained high flow for the first 1:30 minutes after mixing, but flow ceased by 3:30 minutes. The regular-setting polyether material maintained high flow for the first 2:00 minutes and ceased to flow by 3:30 minutes. Perhaps the best flow rate was demonstrated by a fast-setting polyether material, which maintained high flow for the first 1:30 minutes after mixing and ceased to flow by 2:30 minutes. This study demonstrates the importance of stabilizing a material in the patient’s mouth while the impression is setting.
There have been advances in the formulations of some PVS materials that expedite the setting rate when the material is exposed to an increase in temperature. This allows the material to achieve a favorable working time, but then rapidly cease to flow when it is introduced to the oral temperature. A faster setting time also affords a clear advantage with respect to the time efficiency of the practice and the convenience of the patient.
Tear Strength
Tear strength is the ability of a material to resist tearing under a tensile stress. Impression materials are subjected to tensile stresses when they are removed from the oral cavity and from stone models. Arguably, the most important detail in an impression is the thin flash of material captured to define the crown margin. It is important that this flash of material does not tear when being removed from the mouth. Ideally, the flash will also be able to survive the first pour of a dental model in case a second pour is desired.
Tear strength develops over time. A laboratory study reported that a commonly used PVS material only achieved 50% of its final tear strength at 30 seconds prior to its recommended mouth removal time.3 The results also indicated that the same material experienced a 30% further increase in its tear strength if it was allowed to set for an additional minute following its recommended set time. In addition, the study reported that the tear strength of a polyether material was about 30% of the strength of the PVS materials and that this material gained half of its strength in the final minute of setting. These observations highlight the importance of leaving an impression in the patient’s mouth until the complete setting time has elapsed.
Hydrophilicity
Hydrophilicity refers to a material’s attraction to water-based substances. All PVS impression materials are inherently hydrophobic (water fearing), so the best impressions are captured if the preparation and surrounding tooth surfaces are thoroughly dried prior to impressing. Bleeding from gingival tissue can ruin an impression, and dentists reported using hemostatic agents for 57% of final impressions.1 The use of retraction pastes and compression caps are another effective way to stop active bleeding at an impression site. Despite the dentist’s best intentions, there is always the possibility that some last-second moisture contamination could occur on the surface of the teeth being impressed, such as from gingival crevicular fluid. For this reason, surfactants have been added to PVS impression materials to increase their hydrophilicity. Surfactants have been present in PVS materials for more than 20 years; however, newer, more effective surfactants have been developed within the past 3 to 5 years.
A novel testing method was created to check the hydrophilicity of impression materials while they are in their unset state (ie, the state they are in when they are impressing the tooth).4 A recent study used this testing method to show that polyether materials and PVS materials modified with new surfactants demonstrated a significantly improved hydrophilicity when compared with conventional impression materials.5
Another study determined that hydrophilic impression material produced less voids while impressing a wet tooth than a more hydrophobic material.6 Although this study should not motivate the clinician to impress in a wet field, these new materials may be more forgiving if occasional moisture contamination occurs.
Detail Reproduction
Detail reproduction refers to the ability of an impression material to capture fine surface features. This property may be particularly important when the laboratory is reading the margins of a final impression. In dry conditions, both polyether and PVS impression materials demonstrate acceptable detail reproduction as evaluated by their ability to record a 20 micron inscribed line on a metal die.7 In moist conditions, detail reproduction has been shown to be related to the hydrophobicity of the impression material.8 Therefore, the new surfactants added to PVS materials may help with capturing fine details on moist tooth structure.
Elastic Recovery
Elastic deformation is the change in dimensional shape of (impression) material following an applied stress, and elastic recovery refers to its ability to return to its original dimensions. Impression materials are deformed under tension when they are pulled over undercuts, sharp line angles, and interproximal spaces. The accuracy of an impression is partially related to its ability to recover from this deformation.
One study found that permanent elastic deformation from tensile stretching of several PVS impression materials ranged from 0.03% to 1.33%.9 Greater permanent deformation of two polyether materials was also recorded, ranging from 5.91% to 6.23%. When the polyether materials were allowed to set an additional 1:30 minutes past their recommended mouth removal time, the permanent deformation was reduced to 4.30% to 5.01%. No additional benefit in reduction of deformation was found when the PVS impression materials were allowed to set beyond their recommended mouth removal time. To improve elastic recovery, the authors recommend increasing the setting time of polyether materials. They emphasize that it is crucial to keep these materials in the mouth until their final set time to preserve impression accuracy.
Conclusions
In summary, new impression materials with faster setting rates may show a reduction in distortion caused by impression movement during setting. However, when multiple preparations are impressed, slower setting impression materials may be more advantageous. Impression materials with new surfactants may reduce voids and improve detail reproduction if accidental contamination occurs. To achieve an ideal impression, proper clinical practice should be maintained, including elimination of blood and saliva prior to impressing, stabilization of the impression while setting, and adherence to the manufacturer’s recommended mouth removal times.
References
1. McCracken MS, Louis DR, Litaker MS, et al. Impression techniques used for single-unit crowns: findings from the national dental practice-based research network. J Prosthodont. 2017 Jan 11. doi: 10.1111/jopr.12577. Epub ahead of print.
2. Lawson NC, Cakir D, Ramp L, Burgess JO. Flow profile of regular and fast-setting elastomeric impression materials using a shark fin testing device. J Esthet Restor Dent. 2011;23(3):171-176.
3. Lawson NC, Burgess JO. Are you removing your PVS impression materials too soon? Alpha Omegan. 2013;106(1-2):38-39.
4. Kugel G, Klettke T, Goldberg JA, Benchimol J, Perry RD, Sharma S. Investigation of a new approach to measuring contact angles for hydrophilic impression materials. J Prosthodont. 2007;16(2):84-92.
5. Menees TS, Radhakrishnan R, Ramp LC, Burgess JO, Lawson NC. Contact angle of unset elastomeric impression materials. J Prosthet Dent. 2015;114(4):536-542.
6. Michalakis KX, Bakopoulou A, Hirayama H, Garefis DP, Garefis PD. Pre- and post-set hydrophilicity of elastomeric impression materials. J Prosthodont. 2007;16(4):238-248.
7. Vadapalli SB, Atluri K, Putcha MS, Kondreddi S, Kumar NS, Tadi DP. Evaluation of surface detail reproduction, dimensional stability and gypsum compatibility of monophase polyvinyl-siloxane and polyether elastomeric impression materials under dry and moist conditions. J Int Soc Prev Community Dent. 2016;6(4):302-308.
8. German MJ, Carrick TE, McCabe JF. Surface detail reproduction of elastomeric impression materials related to rheological properties. Dent Mater. 2008;24(7):951-956.
9. Balkenhol M, Haunschild S, Erbe C, Wöstmann B. Influence of prolonged setting time on permanent deformation of elastomeric impression materials. J Prosthet Dent. 2010;103(5):288-294.
About the Author
John O. Burgess, DDS, MS
Assistant Dean for Clinical Research
University of Alabama at Birmingham
Birmingham, Alabama
Nathaniel C. Lawson, DMD, PhD
Assistant Professor and Division Director of Biomaterials
University of Alabama at Birmingham
Birmingham, Alabama
