Evaluation of Elastomeric Impression Materials' Hydrophilicity: An in vitro Study.

INTRODUCTION: Hydrophilicity of dental impression materials is crucial for obtaining an accurate impression and necessary for the production of a well-fitting cast restoration. The most common technique for evaluation of hydrophilicity is a contact angle measurement. The aim of the present in vitro study was to compare the water contact angles of four groups of elastomeric impression materials, before and during setting.
MATERIAL AND METHODS: Flattened specimens (n=10) of tested impression materials were prepared by the use of a Teflon mold with specific dimensions. A 5μl droplet of deionized water fell on the specimen, and photos were taken using a Nikon D3200 DSLR camera and a 105 mm macro lens (Nikorr, Nikon) in specific time points.
RESULTS: The CAD/CAM material showed the highest contact angle measurements. The light body polyvinylsiloxane (PVS) material 1, polyether and vinylsiloxanether material showed comparable contact angle measurements especially at the initial time point. A statistically significant reduction of contact angles was reported during setup time for all PVS, PE and vinylsiloxanether materials, while the most expressed reduction of contact angle measurements, and thus the most significant increase of hydrophilicity were reported for light wash PVS material 2.
CONCLUSIONS: The CAD/CAM impression material showed the most hydrophobic behavior. PVS materials showed excellent hydrophilicity. Polyether and polyvinyloxanether impression materials presented lower contact angle measurements, and thus superior hydrophilicity, compared with other tested materials initially and during setting. All tested impression materials presented a stepwise development of hydrophilicity during the setting stage.

Introduction

Accuracy is the key word for an impression material to be considered clinically successful so that all the supragingival and subgingival prepared tooth details can be impressed and an accurate stone cast can be produced. Thus, accurate impression is necessary for the production of a well-fitting cast restoration (, ). Over the years, a variety of impression materials have been introduced in the field of prosthetic dentistry. Reversible hydrocolloids, alginate materials, polysulfides, condensation polysiloxanes, addition polyvinylsiloxanes (PVS) and polyethers (PE) are representative examples, each presenting advantages and drawbacks (). Among the elastomeric impression materials, PVS and PE are the most commonly used materials in dental practice due to their favorable clinical properties and minimal dimensional change (, ).

Simplicity of use, high dimensional stability, and superior elastic recovery from undercuts, low viscoelastic properties, high flow characteristics and short setting time are some of the advantages of the PVS impression materials (, ). These materials show hydrophobic behavior. As a result, moisture may negatively affect the accuracy of the definite impression (, ). Incorporation of several surfactants in the PVS matrix provides hydrophilic characteristics (). PE impression materials containing copolymer tetrahydrofuran and ethylene oxide have a purely hydrophilic behavior, with the ability to wet the tooth and gingiva surface in presence of saliva ().

A new impression material that combines the properties of PE and PVS, vinylsiloxanether or vinyl polyether siloxane has been introduced in the dental market since 2009 (Identium, Kettenbach Co, Eschenburg, Germany) (). This material has been reported to combine easy removal of a PVS material with hydrophilic wetting properties of a polyether (), which makes it a promising material for demanding prosthetic conditions in which both easy removal and moisture control are necessary, such as with narrow and deep gingival crevices ().

The introduction of digital dentistry in recent years has led to the development of CAD/CAM systems that use an intraoral scanner for digital impression procedures and for patient comfort. This progress is followed by use of new impression materials, instead of conventional materials, which can be easily digitally scanned for impression making and a digital workflow ().

Since accuracy of dental impressions depends on flowing and wetting properties of the applied impression materials, hydrophilicity is regarded as a major influencing factor in the outcome of an impression (). Several studies investigated wettability of the already set impression materials, showing no statistically significant differences between PVS and PE materials (). However, wettability of an impression material during its setting time proved to be a field that needs further investigation (, ).

There are several methods for determining wettability of impression materials (). Dynamic contact angle sessile drop goniometry and dynamic Wilhelmy tensiometry are commonly used (, ). Contact angle measurement was proved to be the most clinically relevant technique. Using this method, the investigator measures the contact angle of a distilled water droplet on a flat surface of a solid specimen of an impression material. The contact angle value may be affected by the drop volume that may be decreased due to evaporation (). The lower the contact angle, the more increased is wettability and the greater is hydrophilicity ().

The aim of the present in vitro study was to compare water contact angles of different impression materials, including one PE, two light wash PVS materials and a CAD/CAM scannable impression material, initially and during setting, in an effort to determine their surface wettability.

Methods and materials

Four groups of dental impression materials were used in laboratory conditions 230±10C, 50%±5% relative humidity) in this in vitro study. The groups were as follows:

Two light wash materials - PVS 1: polyvinyl siloxane (Variotime, Heraus Kulzer Michui Chemicals) and PVS 2: polyvinyl siloxane (Detaseal lite, hydroflow impression silicone, Detax)

A soft polyether impression material PE (Impregum, 3M ESPE)

A CAD/CAM scannable polyvinylsiloxane CAD

Hybrid vinylsiloxanether impression material ID (Identium, Kettenbach Co, Eschenburg, Germany)

All materials except PE were provided in cartridges together with their mixing tips. The soft base and catalyst were set in the Pentamix TM 3 Automatic Mixing Unit and specific mixing tips were used ().

Hydrophilicity was evaluated by water contact angle measurement for each material before and during setting. Flattened specimens were prepared by the use of a Teflon mold with specific dimensions. A mixing tip was always embedded in the mold in order to avoid air entrapment and subsequent bubble formation. The Teflon molds were overfilled and a flattened surface of each impression material was obtained by means a glass slab that slid over the impression material after its initial infusion in the mold (Fig. 1). Ten () specimens for each impression material were created, which made 50 specimens in total.

Figure 1

The Teflon mold used in this experiment is shown. It is stabilized on a metal base, and a glass slab is used to achieve flattened surface. The hole at the center denoted with an arrow was designed as an escape route for excess impression material to avoid air entrapment and subsequent bubble formation.

A 5μl droplet of deionized water was collected in a calibrated micropipette and positioned above the flattened specimen surface (Figure 2) (, ). The droplet fell on the specimen and photos in specific time intervals were taken. (Figure 3)

Figure 2

Flattened impression materials specimens were prepared and a specific value drop fell on the surface.

Figure 3

Photos were taken after the drop felt on the specimen’s surface.

The imaging of the droplet was standardized for all impression materials. Photos were taken using a Nikon D3200 DSLR camera and a 105 mm macro lens (Nikorr, Nikon). In order for the camera and the specimen to be aligned, the DSLR camera was set on a 3 mm height basis and the specimen on a 6 mm height basis. The distance between the edge of the lens and the edge of the specimen was set at 12.5 mm by the use of a digital caliper. The photography parameters were the following: f=29, shutter of 1/125 sec, ISO 100 film, TTL ring flash with a shutter of ¼ sec.

Digital pictures were taken at two time points for all specimens; immediately after mixing of each impression material (t1) and at 50% of the suggested working time according to the manufacturer’s instructions for each impression material (t2).

For contact angle calculation, drop analysis program plugged in Image j software was used (, ).

The Wilcoxon matched-pair test was applied in order to explore the relation between two different time periods of the same material. The distribution of the materials in the same time period was compared and assessed using non-parametric tests, that is the Mann-Whitney and Kruskal-Wallis tests, in order to detect differences in distribution of sample populations. The significance level was set at 0.05 throughout the analysis. Statistical analysis was performed by using IBM SPSS 25.

Results

Contact angle values are presented in Table 1. A comparison of the contact angle measurements of the impression materials initially, after mixing, revealed statistically significant differences (p<.05). All groups presented statistically significant lower contact angles initially comparing to the CAD material. Although the PE material showed superior behavior concerning hydrophilicity among all the tested groups, a comparison of contact angles t1 between the PE and PVS1 material did not reveal any significant differences (Table 2).

Table 1

Mean values and standard deviation of contact angles measurements for all tested materials

Case summaries
Tested material		Time point_t1	Time point _t2
PVS 1	Mean	56.0158	49.0618
	Std. Deviation	1.88897	1.57554
PE	Mean	59.5220	41.3034
	Std. Deviation	9.12284	8.60896
PVS 2	Mean	70.8020	45.8863
	Std. Deviation	5.30755	4.82622
ID	Mean	56.5320	50.9244
	Std. Deviation	5.89244	5.33345
CAD	Mean	102.5576	99.8886
	Std. Deviation	14.15034	15.82237

Table 2

Statistically significant differences between contact angle measurements (significance level is (a<0.05)

Tested material	Statistical analysis	Significance level (a<0.05)
		t1	t2
PE- PVS1	Independent-sample Mann-Whitney U test	0.721	0.038
PE- PVS2	Independent-sample Mann-Whitney U test	0.004	0.568
PE-ID	Independent-sample Mann-Whitney U test	0.622	0.065

The contact angles measured during setting were significantly lower compared with those measured at initial time points for all the tested groups. Moreover, all tested impression materials presented a stepwise development of hydrophilicity in the setting stage, which was not observed at the initial time point t1. The PE presented lower measured contact angle values both at t1 and t2 examined time points. At t2, the PVS 2 showed the most pronounced reduction in contact angle measurements, and thus the most significant increase of hydrophilicity. As a result, no statistically significant difference was reported between the contact angle values of PVS2 and PE at t2 (during setting) (Table 2).

Discussion

It is known that water interaction with elastomeric dental impression materials may compromise the quality and accuracy of impressions (). Contact angle measurements have been suggested for hydrophilicity evaluation of elastomeric impression materials. Drop analysis used in this in vitro experiment has been reported as an alternative method for contact angles measurements and analysis of hydrophilic properties of unset PE, ID, CAD and PVS1 and PVS 2 impression materials (). Impression material samples of 2mm thickness were chosen because it was recommended by most manufacturers (). It has been speculated that not only lower but also higher thickness of impression material could compromise the final result as well (, ).

Contact angles values measured in this experimental study were lower compared with those reported in other in vitro experiments (, ). More specifically, the tested PVS materials presented contact angle values higher than 700 at initial time points tested (, , ). Light consistency of the tested PVS materials presented the values lower than 600, while other studies reported contact angle values higher than 800 for similar materials (, ). These differences could be attributed to the consistency of the tested PVS impression materials and to differences in the experimental protocols ().

The theory argues that surfactants in PVS materials reduce contact angles by either migrating to the PVS surface and increasing its wettability (), or releasing from the PVS surface and reducing the surface tension of the wetting liquid (). An experiment by Balkenhol et al. () discovered that PVS surfactants were present within the wetting liquids exposed to set PVS materials. In the current study, surfactants of PVS materials were released from impression materials surface interacting with the dropped water droplet causing a downregulation at contact angle values, and consequently increasing wettability and hydrophilicity of impression material. Unlike water, saliva contains dissolved proteins and salts. Saliva also contains mucins, which cause strong adhesiveness and increase its viscosity. If surfactants could dissolve in saliva, they might not be able to overcome the surface tension produced by adhesive forces of mucins (). Protocols using drops of saliva may report higher contact angle values, as PVS materials with surfactants may not achieve low contact angles when they come in contact with saliva. Low contact angles measured in this in vitro study – using water for hydrophilicity measurements - could not be directly correlated with the clinical condition.

The superiority of PE could be attributed to the intrinsic hydrophilicity of PE impressions. Other studies () showed that PE favored moist surfaces producing precise reproductions despite the presence of moisture. Also, Shah et al. () concluded that PE has a significantly better accuracy than polyvinyl siloxane. A 3D laser scanner was used to measure plaster models obtained with a double-phase 1-step impression in the absence of moisture. In contrast, in the present in vitro study, the main goal was to evaluate hydrophilicity and therefore impression accuracy in the presence of moisture. This difference could provide explanation for the absence of statistically significant differences among PE, ID and PVS 2 materials in this study.

There is a wide range of scientific papers concerning contact angle values that are reported in the literature (, , ). Different experimental protocols do not allow the comparison between contact angle values. The volume of droplet, the choice of saliva or distilled water and the time points at which photo images are taken affects the contact angles values. Although they differ when experimental design differs, it is supported that ID and PE usually presented lower contact angle values compared to other impression materials. This comparable and superior hydrophilicity was established in many studies and is more pronounced in case of using saliva instead of water droplets (, , ). More specifically, the hybrid impression material which has been introduced in dental market as Identium, and characterized as vinylsiloxanether material, combines the ease of removal of PVS with the hydrophilicity wetting properties of polyether ().

In line with the findings of Menees et al. (), PE showed the smallest deviations after setting, according to contact angle measurements and thus the best hydrophilic behavior. PVS 2 indicated statistically significant differences of contact angle measurements between the two examined time points, presenting consequently a statistically significant improvement of hydrophilicity, which is important for the accuracy of impression when moisture is present. This result could be attributed to the composition of this elastomeric impression material (PVS 2 Detaseal), containing polydimethylsiloxane with functional groups and fillers and pigments additionally, while the catalyst additionally contains platinum complex compound.

Several factors, including the hydrophilicity measured with contact angle values, material thickness and the impression materials types are of great importance and are related with accuracy of impressions either at every day clinical practice (fixed or removable prosthodontics) () or at innovative protocols in maxillofacial prosthetics (). The limitation of this study was that specimens of impression materials were flat. Furthermore, deionized water was used instead of saliva, which is in agreement with other similar studies (, ). Artificial saliva does not represent clinical conditions, as saliva viscosity and composition varies among different persons (, ). Attachment of saliva drops to the tip of a calibrated pipette could affect contact angle measurements negatively. Higher variability and contact angle values were recorded in studies using saliva.

Conclusion

Within the limitations of this in vitro study, the following conclusions can be made: PE and ID showed superior behavior concerning hydrophilicity both initially and during setting; The PVS materials showed excellent hydrophilicity. PVS 1 presented comparable hydrophilicity with PE and ID initially, while PVS 2 showed the most pronounced contact angle value reduction; All impression materials presented statistically significant lower contact angles initially comparing to CAD; All impression materials developed a stepwise hydrophilicity.