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Effect of Mixing and Impression Techniques Using Vinyl Polysiloxane (VPS) on the Accuracy of Fixed Partial Dentures

Department of Oral Rehabilitation, The Maurice and Gabriela Goldschleger School of Dental Medicine, Tel Aviv University, Klazkin St. 34, Tel Aviv 39040, Israel
Departments of Pedodontology, School of Dental Medicine, Tel Aviv University, Tel Aviv 39040, Israel
Author to whom correspondence should be addressed.
Equally contributed to the study.
Appl. Sci. 2021, 11(17), 7845;
Submission received: 17 June 2021 / Revised: 11 August 2021 / Accepted: 23 August 2021 / Published: 26 August 2021
(This article belongs to the Special Issue Predictable Restorative Dentistry)


Two critical factors that influence the accuracy of an impression include the proper manipulation of the impression materials and the technique used to make the impression. The purpose of this study was to clinically evaluate the effect of different mixing techniques on the accuracy of vinyl polysiloxane (VPS) impressions by assessing metal framework fit of fixed partial restorations. The study included 92 consecutive patients diagnosed with partial edentulism and treated with fixed partial denture restorations. The mixing technique was one of the two following mixing methods: hand mixing technique (45 patients), with the putty material mixed according to the manufacturer’s instructions; or mechanical mixing technique (47 patients), with the putty material mixed by a Pentamix device. Under both mixing methods, vinyl polysiloxane was used as the impression material. Two impression techniques were randomly used by the operators (One/Two-stage putty –wash impression techniques). The accuracy of the metal framework restorations was tested clinically and radiologically, resulting in significant statistical difference (p = 0.04) between different mixing techniques. The mechanical mixing produced more accurate restorations (metal framework misfit only in 14.9% of patients vs. hand mixing 31.1%). Regarding the impression techniques, the two-stage impression technique was found to be significantly more accurate (p = 0.04), resulting in 14.6% ill-fitted metal frameworks vs. 31.8%, in the one-stage technique. It can be concluded that mechanical mixing yields more accurate impressions leading to more accurate restorations, especially when combined with two-stage impression technique.

1. Introduction

Impression accuracy significantly affects the quality of fixed partial denture marginal adaption, internal fit, interproximal and occlusal contacts [1,2]. Restoration fit is an important predictor that determines the survival of fixed partial dentures due to its influence on plaque retention, caries rate and periodontal and pulpal health of the prepared teeth [3,4,5,6,7]. The impression phase is predominantly controlled by the clinician and may influence the restoration fit accuracy [8,9,10,11,12]. The impression accuracy is mainly subjected to the effect of proper handling of the impression materials [13,14,15,16,17,18,19] and impression techniques applied [20,21,22,23,24].
In spite of technology advancement and increased usage of digital impressions, Vinyl polysiloxane (VPS) is still extensively used as a preferable impression material for fixed restoration production because of the high level of feature imitation, dimensional constancy and low price [17,18].
Previous studies have shown that different mixing techniques have a substantial effect on the quality of impressions and castings obtained [19,25]. The working time of impression material can also be affected by the mixing technique [26]. Generally, the mixing techniques can be divided into two major categories: mechanical mixing and hand mixing techniques. One of the major problems with using hand mixing is that the air bubbles incorporated into the impression material cause void formation on the impression surface. Although it is impossible to achieve bubble-free mixing, the frequency and size of bubbles and voids can be reduced by automatic mixing. Additionally, due to the more uniform blending of the base with the catalyst, machine mixing enhances the polymerization reaction and generates homogeneity of the impression material with superior physical properties, especially when using high viscosity materials [27,28,29,30]. Automatic mixing also allows for the maintenance of hygiene and avoidance of contamination of the impression material [31,32,33,34,35]. Likewise, there is a clear preference for automatic mixing among medical staff (clinicians and assistants) due to the convenience of the machine method [36]. Furthermore, automatic mixing is considered to be more economical compared to hand mixing thanks to minor waste of the impression material [37,38,39,40].
Various impression techniques have been proposed to improve the accuracy of VPS impressions. The putty-wash impression can be made using two principal techniques: the putty-wash one-step technique and putty-wash two-step technique. Some laboratory studies have suggested that the impression accuracy does not depend on the impression technique, Refs. [41,42] while many other in vitro studies concluded that the impression technique is an important factor affecting impression accuracy, mainly because it controls the wash bulk thickness (1–2 mm), which is considered to be the major determining factor [13,14,15,16,19,20,21]. However, there is little information regarding the association between different mixing techniques, impression techniques and restoration marginal fit accuracy. The novelty of the present in vivo study is due to the influence of the intraoral environment, while most of the previously reported data comes from in vitro studies.
The purpose of the present retrospective study was to evaluate, in vivo, the effect of two mixing techniques (hand and machine mixing) using two different VPS impression techniques on the marginal fit accuracy of fixed partial denture metal frameworks. The working hypothesis was that the mechanical mixing combined with the two-step impression technique would yield more accurate impressions, leading to more accurate metal frameworks.

2. Materials and Methods

2.1. Enrollment

Ninety-two (42 women and 50 men) consecutive individuals were included. Mean age 49 ± 16 years.
Inclusion criteria:
  • Partially edentulous
  • Short span fixed partial denture restorations
  • Posterior jaw region (45% mandible, 55% maxilla)
The abutments were prepared by prosthodontists, residents in oral rehabilitation, or under their supervision at the Department of Oral Rehabilitation, School of Dentistry, Tel-Aviv University. All the patients had provided signed informed consent and the study protocol was approved by the Ethics Committee of Tel Aviv University.

2.2. Mixing Techniques

All the impressions were made with PVS (Express, 3M-ESPE) impression material in putty and light body (Express wash—Regular set light body hydrophilic ISO 4823 type 3) viscosities. The wash was mixed and applied with an automatic mixing syringe. The putty mixing techniques was chosen randomly according to the operator’s preference and was one of the two following mixing methods:
  • Hand mixing technique—the putty material (Express STD) was mixed in standardized proportions according to the manufacturer’s instructions. Mixing was performed without using hand gloves in order to avoid the negative effect of latex on vinyl poly siloxane [17].
  • Mechanical mixing technique—the putty (Express 2 Penta putty) material was mixed in a Pentamix device (3M, ESPE).

2.3. Impression Techniques

All the impressions were taken with perforated metal trays (Medesy) evenly coated with adhesive supplied by the manufacturer. All materials were mixed in standardized proportions and were measured and used according to the manufacturer’s recommendations.
The impression techniques were also randomly selected by the operator preference, and were one of the two following techniques:
  • One-stage impression technique—the wash and the putty were mixed simultaneously and placed on the impression tray, while the wash was injected on the prepared teeth area (Figure 1).
  • Two-stage impression technique—the preliminary impressions were taken with the putty material. The putty was allowed to set according to the manufacturer recommendation time. After tray removal, the operator performed controlled putty removal from prepared abutment sites using a round low-speed bur number 6 with a diameter of 1.8 mm. (Dentsply DeTrey Gmbh) in order to create a wash space of up to 2 mm for the wash material [13,14] (Figure 2).
The final impressions were taken with wash material that was injected onto the abutment teeth and inside the impression tray using an automatic mixing syringe. The impression was then reseated in the patient’s mouth and allowed to set.
After setting, all the impressions were disinfected and stored at room temperature (25 °C) for 1 h before pouring. The improved die stone (type ΙV, Supra stone; Kerr, Sybrone, Orange, CA, USA) was first mixed by hand and then mechanically mixed under vacuum for 15 s. The stone was vibrated into impressions and allowed to set for 1 h before the tray separation. The metal framework was produced using Argelite 60+ alloy (59.9% Pd, 26.3% Ag. ARGEN Corporation, San Diego, CA, USA). All the restorations were fabricated by a single laboratory (Shenhav Laboratory Ltd. Tel-Aviv, Israel).

2.4. Metal Framework Fit Verification

A total of 92 FPD metal frameworks were examined. The accuracy of the FPD restorations was tested clinically and radiologically. According to the prosthodontic department guidelines, three common accuracy assessment methods were used to validate the fit of all the metal frameworks:
  • Probe test (using a dental probe to confirm vertical and horizontal restoration marginal fitting at the buccal and lingual/palatal aspect)
  • Tactile sense test (“rocking test” for verifying the metal framework stability at the restoration’s masio-distal aspects)
  • Radiographic test (performing bitewing radiograph to verify restoration’s interproximal adaptation) [43,44].
Misfit of the metal framework was diagnosed when one of the three accuracy assessment methods was found to be defective. The restoration was demarcated as inaccurate, leading to redoing the impression.
According to the study design (Table 1), the data of the current study was recovered from patients’ files (retrospective study) and subjected to statistical analysis.

3. Results

A total of 92 participants, aged 49 ± 16, were enrolled in this study. Of the 92 fixed partial denture metal frameworks that were manufactured and examined, 47 were for the upper jaw and 45 were for the lower jaw. Twenty-one of the 92 metal frameworks were found to be ill-fitting (22.8%). The remaining 71 metal frameworks were found to be accurate (77.2%). A total of 45 impressions were taken using the hand mixing technique of the putty material, and in the remaining 47 impressions, machined mixing of the putty material was performed. When using mechanical mixing, misfits were detected in 7/47 of the frameworks (14.9%). While using the hand mixing technique, misfits were found in 14/45 of the frameworks (31.1%) (Table 2).
The one-step mixing technique was used in 44 patients, and misfits were detected in 5/44 of the metal frameworks when machine mixing was performed (11.4%) and in 9/44 of the frameworks when hand mixing was applied (20.4%). the two-stage impression technique was used in 48 patients, and misfits were detected in 1/48 metal framework when the automatic mixing was applied (2.1%) and 6/48 metal frameworks when hand mixing was used (12.5%) (Table 3).
Fisher’s Exact test showed a statistically significant difference (p = 0.04) between the two mixing methods, with more accurate metal frameworks being obtained using the machine mixing technique.
Additionally, a statistically significant difference was found between the different impression techniques, resulting in a more accurate fit using the two-stage impression technique (p = 0.04).
Logistic regression statistical analysis showed that the combination of hand mixing and one-stage mixing technique was 3.858 OR [95% CI 1.189–12.519] prone to produce ill- fitted metal frameworks compared to other combinations of mixing and impression techniques.

4. Discussion

Two major factors affecting impression quality are appropriate handling of the impression materials and the impression technique. Influencing the marginal fit of the cast restoration may determine the restoration’s durability and abutment teeth survival [1,2,3,4,5,6,7].
Previous in vitro studies have shown that different mixing techniques have an effect on impression quality and machine mixing superiority [39,40]. These findings are consistent with our in vivo study results, which emphasize the fact that mechanical mixing produced significantly more accurate metal framework restorations than hand mixing (p = 0.04). The clinical outcome shows that hand mixing led to twice as many metal frameworks misfits compared to machine mixing (31.1% vs. 14.1%). The mechanical mixing allows for better control of the amounts and proportions of the impression material used, facilitates polymerization reaction, reduces the number and the size of the bubbles incorporated in the impression material and avert contamination. Moreover, mechanical mixing is preferred by dental staff due to the ease of use [29,32,34,38,40]. Additional factors that influence the operator’s decision to use the mixing technique for impression material is the accessibility of a mixing machine [36].
Similar to previous in vitro study results, the present study has shown a significant difference in PFD metal frameworks’ fit accuracy when compared two common impression techniques. While the one-step technique has the benefits of being faster, it has also led to fabrication of twice as many ill-fitted metal frameworks compared to the two-stage impression technique (31.8% vs. 14.6%). Previous studies have found an association between the impression technique used and the metal framework fit accuracy of fixed partial dentures [19,21,25]. In the one-step technique, the viscous putty material tends to push the light body material away from the abutment teeth, and thus they are recorded by the less precise putty material. Uncontrolled impression tray pressure during setting may also lead to a significant effect on precision. Tray rigidity may also affect the accuracy of polyvinyl siloxane impressions. Metal trays produce more accurate impressions than plastic trays [20,21,22,23,24,25]. Moreover, the uncontrolled wash bulk thickness causes dimensional changes (vertical and horizontal) due to impression material contraction towards the tray walls, resulting in distorted stone dies on the working models [45,46,47,48]. While with the two-step putty-wash technique, the wash layer thickness is minimal and uniform, the critical details are copied precisely, with minimal distortion [13,14], emphasizing the fact that the controlled wash bulk is the critical factor in fabricating accurate restorations [19]. The present in vivo results demonstrated the superiority of the two-step impression technique in terms of restoration accuracy while using a custom metal tray.
In addition, various combinations of mixing methods and impression techniques were examined, and it was found that a combination of a hand mixing method and the one-stage impression technique resulted in greater inaccuracy of metal frameworks, as much as four times, compared to the other combinations of mixing and impression techniques. While the combination of the mechanical mixing method and the two-stage impression technique showed superiority, leading to more accurate metal frameworks. The results support our working hypothesis (mechanical mixing combined with the two-step impression technique would yields more accurate impressions), which has been proven previously in laboratory studies and validates their results clinically in the intraoral environment.
Additional in vivo studies are needed to assess the influence of different mixing techniques on the impression and restoration accuracy in the intraoral setting.

5. Conclusions

Within the limitations of the present clinical study (one center, one impression material), it can be concluded that the combination of the mechanical mixing method and the two-stage impression technique produces more accurate metal frameworks when using VPS impression materials.

Author Contributions

Conceptualization, J.N., O.R., H.Z. and S.M.; data curation, S.N., H.A., I.M., A.G., G.M., G.B.-I. and J.N.; formal analysis, S.M., O.R. and H.Z.; methodology, S.M., O.R., S.N., H.A., I.M., H.Z. and J.N.; writing—original draft, H.Z., S.N., O.R., S.M. and J.N.; writing—review and editing, S.M., S.N., O.R., H.Z., H.A., I.M., A.G., G.M., G.B.-I. and J.N. All authors have read and agreed to the published version of the manuscript.


This research received no external funding.

Informed Consent Statement

All the patients had provided signed informed consent and the study protocol was approved by the Ethics Committee of Tel Aviv University.

Conflicts of Interest

The authors declare no conflict of interest.


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Figure 1. One-step impression technique.
Figure 1. One-step impression technique.
Applsci 11 07845 g001
Figure 2. Two-step impression technique.
Figure 2. Two-step impression technique.
Applsci 11 07845 g002
Table 1. Study groups.
Table 1. Study groups.
Automatic MixingHand Mixing
One-stage impressionOne-stage impression
Two-stage impressionTwo- stage impression
Table 2. Clinical result for mixing technique.
Table 2. Clinical result for mixing technique.
Mixing TechniqueNMisfitFit
Automatic mixing477 (14.9%)40 (85.1%)
Hand mixing4514 (31.1%)31 (68.9%)
Total92 (100%)21 (22.8%)71 (77.2%)
Table 3. Clinical result for combination of impression and mixing techniques.
Table 3. Clinical result for combination of impression and mixing techniques.
Impression TechniqueMixing MethodMisfitFitTotal
One stageAutomatic5 (11.4%)17 (38.6%)22
Conventional9 (20.4%)13 (29.5%)22
Two stageAutomatic1 (2.1%)22 (45.8%)25
Conventional6 (12.5%)19 (43.2%)23
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Zelikman, H.; Rosner, O.; Naishlos, S.; Azem, H.; Meinster, I.; Glikman, A.; Melamed, G.; Ben-Izhack, G.; Nissan, J.; Matalon, S. Effect of Mixing and Impression Techniques Using Vinyl Polysiloxane (VPS) on the Accuracy of Fixed Partial Dentures. Appl. Sci. 2021, 11, 7845.

AMA Style

Zelikman H, Rosner O, Naishlos S, Azem H, Meinster I, Glikman A, Melamed G, Ben-Izhack G, Nissan J, Matalon S. Effect of Mixing and Impression Techniques Using Vinyl Polysiloxane (VPS) on the Accuracy of Fixed Partial Dentures. Applied Sciences. 2021; 11(17):7845.

Chicago/Turabian Style

Zelikman, Helena, Ofir Rosner, Sarit Naishlos, Hanaa Azem, Isabelle Meinster, Ari Glikman, Guy Melamed, Gil Ben-Izhack, Joseph Nissan, and Shlomo Matalon. 2021. "Effect of Mixing and Impression Techniques Using Vinyl Polysiloxane (VPS) on the Accuracy of Fixed Partial Dentures" Applied Sciences 11, no. 17: 7845.

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