Comparative Analysis of Dimensional Accuracy in the Integration of Different Setting Times of Impression Materials

Dental impressions are conventionally obtained using specific materials and employed to replicate the teeth, gums and surrounding oral tissues to reproduce accurate prostheses. Numerous studies have been conducted on impression materials; however, these studies have focused on the utility of elastomeric impression materials [1]. Although rubber-based materials have been developed, elastomeric substances are often used when precision is of utmost importance. The commonly used elastomeric substances include polysulfide, condensation silicone, polyether (PE) and addition silicone polyvinyl siloxane

(PVS). To date, no impression substance has achieved absolute accuracy [3]. Moreover, impression accuracy is influenced by various factors, including dentist experience, impression technique, material manufacturer, tray selection, material working time and patient compliance [4]. All impressions should be carefully evaluated for accuracy, particularly in the finishing line area [5].

Elastomeric impression materials are the preferred options for fixed prosthodontics owing to their unique characteristics, including the capacity to diminish marginal voids and distortions [6] (Table 1) for elastomers materials properties. Until recently, the options for elastomers were primarily dependent on two reliable choices: Either PE or polyvinyl siloxane (PVS). Addition silicone, commonly referred to as PVS, is divided into four categories based on its consistency: Light, medium, heavy and putty. PVS is versatile and one of the most widely used impression materials in the dental field, offering predictable and accurate results as well as ultimate patient satisfaction [7]. It offers notable benefits including remarkable dimensional stability, minimal polymerization shrinkage, high tear strength and outstanding elastic recovery [8]. Alternatively, PEs offer moderate stability and good accuracy [9], making PE impression materials hydrophilic, which improves material flow and tooth structure reproduction [10]. However, PE has the following limitations: it is expensive; has low tear strength, making it difficult to remove from the patient’s mouth; and may cause allergic reactions [8,11].

As no single impression material fulfills all desired requirements, researchers have attempted to combine commonly used materials [12]. To this end, the mechanical properties of a recently developed elastomeric impression material, which is a combination of vinyl polysiloxane (VPS) and PE, called polyvinyl ether silicone (PVES) were evaluated. The new material was evaluated by measuring the dimensional changes in the cast and comparing them with the reference model. In addition, two balls were added to the lingual and occlusal surfaces to measure any changes in distance. The new material showed results similar to those of regular impression materials [10]. Despite the limitations of the study, PVES exhibited high flexibility and tens ile strength, suggesting that this material is particularly suitable for undercut areas because it facilitates the removal of impressions without causing tears or distortions [6].

Another major challenge in impression-making is the long intraoral setting time, ranging from 5 to 7 min for most impression materials [10]. Fast-setting elastomeric impression materials have been developed to reduce setting time and enhance patient experience. These materials allow quicker and more efficient procedures and show improved flowability and low viscosity, which facilitates the accurate capture of fine details [13]. Singer et al. [14] conducted an analysis in which they employed a variety of tools to assess dimensional alterations. To ensure accuracy, each sample was examined under a stereomicroscope (Leica Microsystems, Wetzlar, Germany). The primary objective of this examination was to verify the consistent presence of a 75-μm line in every specimen. Dimensions were measured using a Carl Zeiss stereomicroscope at a magnification of 12×. The impressions materials were divided into fast and normal based on their setting times. Therefore, dentists have several choices of impression materials depending on preference or availability of the material in the clinic or market. According to accessibility and availability, some dentists attempt to make impressions using different setting times, which may affect the dimensional accuracy and marginal fit of the prosthesis.

Previous studies focused on comparing different types of elastomeric impression materials, such as combining PVS and PE; however, the effect of combining impression materials with different setting times has not been established in the literature. This represents a clear gap in the literature, especially since such combinations could be useful in everyday dental practice, including those with limited resources. This in vitro study aimed to investigate the impact of combining different setting times on impression accuracy.

Objectives

This study aimed to assess the impact of different setting times of impression materials on accuracy and consistency in clinical applications. The null hypothesis posits that varying setting times do not influence the accuracy of impression materials, while the alternative hypothesis suggests that differences in setting times lead to variations in accuracy.

Samples

This in vitro study was conducted from February 1, 2024, to March 28, 2024, at the Prosthetic Laboratory, Faculty of Dentistry, King Abdulaziz University. A power analysis was performed utilizing the most recent version of G*Power software (version 3.1.9). The parameters used for the calculation were alpha level of 0.05, statistical power of 80% and an expected effect size of f = 0.4. A total of 72 samples were divided into three groups, each containing 24 samples (Figure 1). The first group (Fast-Fast) used a light-body fast-set impression material in combination with a regular-body fast-set material. The second group (Fast-Normal) utilized a light-body fast-set impression material, along with a regular-body normal-set material. Finally, the third group (Normal-Fast) used a light-body normal-set impression material, together with regular-body fast-set material. All impressions were collected from a Nissin typodont using a standardized preparation on two teeth: Molar #16 and premolar #24. All impressions were taken and evaluated by two calibrated clinicians and were created using a conventional method, with stock trays and tray adhesives applied. The impression materials were mixed using a manual dispensing gun and light-body impression materials were carefully injected around the prepared tooth. The trays were then filled with the same amount of regular-body material and topped off with light-body material. Impressions were visually assessed for abutment clarity and the presence of voids or tears.

Figure 1: Experimental workflow showing the distribution of dental impression samples

Figure 2: Maxillary impression using a two-step technique with light and regular body silicone materials

All samples included in the study had a clear finish line without any defects, ensuring a usable impression (Figure 2). After visual assessment of the impressions, they were digitized using a Zirkonzahn scanner and primary Nissin typodont was chosen as the reference digital cast (Figure 3). Digital impressions were then superimposed to assess the accuracy of the impressions. Digital impressions were aligned and overlayed to impressions of the reference cast and alignment was performed using reference points or landmarks on the casts to ensure precise positioning. After superimposing the digital impressions, the overlapping areas were measured using digital calipers and specialized software tools in Exocad. We measured six specific parameters related to the height and width of dental impressions in millimeters (Figure 4). For height, we evaluated mesial, distal, buccal and palatal measurements. Furthermore, we assessed the mesiodistal and buccopalatal dimensions.

Figure 3: Reference cast of Nissin typodont used as a control for dimensional accuracy comparison

Statistical Analysis

Statistical analysis, particularly a one-way analysis of variance (ANOVA), was performed to determine any significant differences across multiple groups that represent different combinations of setting time impression materials. Prior to conducting ANOVA, the distribution of variables within each group was assessed using tests of normality (Kolmogorov–Smirnov and Shapiro–Wilk tests). The data exhibited normal distribution (p>0.05) for all groups, confirming the appropriateness of parametric testing. 

Figure 4: Digital superimposition and measurement of impression scan using Exocad software

Table 1: Elastomers' impression materials properties

Tamhane’s test was selected because it is a conservative method that does not assume equal variances between groups, making it suitable in cases where the assumption of homogeneity of variances may be violated. The purpose of these statistical assessments was to determine whether there were any statistically significant differences in the measured parameters, which in this case was the dimensional accuracy of the impressions between the control and experimental groups.

This study compared three groups of setting times of impression materials compared to the primary Nissin typodont as the reference digital cast. ANOVA revealed statistically significant differences between the groups for three variables: buccal (p = 0.0002), mesiodistal (p = 0.0002) and buccopalatal (p = 0.006) (Table 1). For buccal difference, the mean and 95% Confidence Interval (CI) were as follows: Fast-Fast group (0.0128, 95% CI:-0.0143, 0.0399), Fast-Normal group (0.0860, 95% CI: 0.0552, 0.1167) and Normal-Fast group (0.0791, 95% CI: 0.0560, 0.1022). For mesiodistal difference, the means and 95% CIs were as follows: Fast-Fast group (-0.0095, 95% CI:-0.0423, 0.0233), Fast-Normal group (0.0585, 95% CI: 0.0385, 0.0785) and Normal-Fast group (0.0546, 95% CI: 0.0323, 0.0769). For buccopalatal difference, the means and 95% CIs were as follows: Fast-Fast group (0.0618, 95% CI:-0.0009, 0.1246), Fast-Normal group (0.1693, 95% CI: 0.1311, 0.2075) and Normal-Fast group (0.1156, 95% CI: 0.0793, 0.1518) (Table 3). There were no statistically significant differences among the groups in terms of mesial (p = 0.638), distal (p = 0.597) or palatal differences (p = 0.785) (Table 2). This implies that the choice of impression material affects specific dimensional differences in dental impressions. Post-hoc assessments (Tamhane) revealed that the Fast-Normal group exhibited significantly different mean values compared to those of both the Fast-Fast and Normal-Fast groups (Table 4). The

Table 2: One-way ANOVA results comparing dimensional differences between impression groups for six measurement parameters

Table 3: Descriptive Statistics: Mean (SD), Standard Error and 95% CI for Setting Time Combinations

Fast-Normal group generally had larger mean values for buccal, mesiodistal and buccopalatal differences, suggesting that combining fast-set and normal-set materials resulted in greater dimensional differences compared to those with the other groups. The Kolmogorov-Smirnov and Shapiro-Wilk tests revealed some deviations from normality for variables such as mesial, palatal, mesiodistal and buccopalatal differences between the groups (Table 5).

able 4: Multiple Comparison Analysis: Tamhane's Test for Setting Time Combinations

*The mean difference is significant at the 0.05 level

Table 5: Shapiro-Wilk and Kolmogorov-Smirnov Normality Tests for Different Combinations of the Setting Time of the Impression Material

*This is a lower bound of the true significance, aLilliefors Significance Correction

This study aimed to establish evidence-based recommendations to enhance the reliability of dental impression techniques by evaluating the effects of combining different setting times of impression materials on accuracy and reproducibility. Conducted in a controlled laboratory setting, it eliminated clinical variations to ensure precise results. Findings indicate that combining impression materials with different setting times significantly influenced the buccal, mesiodistal and buccopalatal dimensions of dental impressions, while mesial, distal and palatal differences showed no statistically significant variations. These results led to the rejection of the null hypothesis, suggesting that material selection impacts only specific dimensional aspects. While theoretically beneficial in certain cases, combining different setting times requires proper handling, technique and material compatibility, which manufacturers generally do not recommend due to potential risks to accuracy. Despite limited previous research on this approach, further investigations are needed to assess its feasibility. Traditionally, clinicians have avoided mismatched setting times due to concerns about distortion. However, this study suggests that such combinations may be viable, providing flexibility in material selection without compromising accuracy. This could help reduce material waste and costs associated with cartridges.

Despite these promising findings, the study's controlled laboratory conditions present limitations, such as the exclusion of biological factors like soft tissues and saliva. Future clinical research is necessary to validate these results and assess potential variations introduced by patient-related or operator-dependent factors. Overall, this study provides valuable insights into impression-material combinations and their clinical relevance.

This study investigated the effect of impression materials with different setting times on various dimensional aspects of dental impressions. The key findings clearly demonstrate that the setting time has a significant influence on certain dimensional measurements. Our findings have important clinical implications for dental professionals as the choice of impression material and setting time can directly influence the dimensional accuracy of the final impression. As such, careful consideration should be given to material selection, particularly when the precision in specific dimensions is critical for optimal fit and function. Overall, this study highlights the importance of understanding the impact of impression material properties, such as setting time, on the dimensional accuracy of dental impressions. Considering these factors, dental professionals can make informed decisions to improve the quality and fit of prosthetic restorations and ultimately enhance patient outcomes.

Limitations

• This in vitro study excludes biological factors such as soft tissues and saliva
• The study is performed in a controlled laboratory environment
• This research specifically investigated a particular type of impression material, which may restrict the generalizability of the findings
• This study focused on linear measurements and did not assess surface detail reproduction

Acknowledgement

Special thanks to the Deanship of Scientific Research (DSR) and the faculty of dentistry at King Abdulaziz University, Jeddah for supporting this project.

Conflicts of Interest

The authors declare that there are no conflicts of interest.

Ethical Approval

The research proposal was approved by the regional research and ethics committee of King Abdulaziz University with an Ethical Approval number (167-11-23).

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15. [1]*Corresponding author: Asso

16. ciate Professor and Consultant of Restorative and Digital Dentistry, Restorative Dentistry Department, Faculty of Dentistry, King Abdul-Aziz University, Jeddah, Saudi Arabia.ORCID#0000-0002-8007-4178, Email: Sabuzinadah1@kau.edu.sa

17. Abdulrahman J. Alhaddad2

18. [2] Associated Professor and Consultant of Prosthodontics, Oral and Maxillofacial Prosthodontics Department, Faculty of Dentistry, King Abdul-Aziz University, Jeddah, Saudi Arabia. ORCID#0000-0001-8187-8883, Email: aalhaddad@kau.edu.sa

19. [3]^,4,7,8,9,10 Faculty of Dentistry, King Abdul-Aziz University, Jeddah, Saudi Arabia

20. ^5,6 Assistant Professor and Consultant of Prosthodontics, Oral and Maxillofacial Prosthodontics Department, Faculty of Dentistry, King Abdul-Aziz University, Jeddah, Saudi Arabia

21. ¹¹Dental technician, Faculty of Dentistry, King Abdulaziz University Dental Hospital, Jeddah, Saudi Arabia.

22. ¹² Professor and Consultant in Prosthodontics, Oral & Maxillofacial Prosthodontic Department,

23. Faculty of Dentistry, King Abdulaziz University, Jeddah, Saudi Arabia.

24. [6]*Corresponding author: Asso

25. ciate Professor and Consultant of Restorative and Digital Dentistry, Restorative Dentistry Department, Faculty of Dentistry, King Abdul-Aziz University, Jeddah, Saudi Arabia.ORCID#0000-0002-8007-4178, Email: Sabuzinadah1@kau.edu.sa