Comparative evaluation of retentive strength of polyvinyl siloxane im- pression materials to custom tray using different tray adhesives: An in vitro study

The reten(ve strength of the impression material to the impression tray is an important factor that can affect the quality of the final impression. Separa(on of the impression from the tray or tearing of the impression while removing from the mouth could be one of the reasons for distor(on. It is impera(ve to know the most effec(ve tray adhesive which can be used with different brands of rubber-based impression material. So, this study was directed to compare the different tray adhesives that can be effec(vely used in our day-to-day clinical prac(ce.


Introduction
The bond strength of an impression material to tray is essential and in case of rubber-based impression materials the retention is mainly achieved via chemical adhesion. To obtain an accurate impression, the impression material must be securely attached to the tray. If the material is not secured to tray firmly while retrieving from the mouth, it will result in improper impression and therefore cause distorted die, wax pattern and casting. [1,2] The polyvinyl siloxane impression materials are addition reaction silicone elastomers which were first introduced in the 1970s. [3] In the past decade, these materials have occupied a large share of the impression material market. They possess good physical properties and handling characteristics. They have achieved a high level of dentist and patient acceptance as they are clean, odorless and tasteless. [3] An effective adhesive is especially indicated when the impression material has a high tear resistance, so that it can effectively be removed from undercuts. The chemistry of the adhesives usually are polydimethyl siloxane and ethylsilicate. The adhesive reacts with the surface of the tray material and forms a chemical bond to the tray and to the impression material. It is usually recommended to wait http://dentistry3000.pitt.edu for ten to fifteen minutes after application of the adhesive before making the impression. This allows time for the solvent to react with the tray material. Sulong and Setchell [4] demonstrated that roughening the surface of the impression tray will significantly improve the effectiveness of polyvinyl siloxane adhesives.
Impression tray adhesives are applied as a spray, or are manually applied using a brush. Prior to tray loading, the adhesive requires a lengthy drying time. Suggested drying times for adhesives vary widely among manufacturers. Also, adhesive strength of different tray adhesive varies considerably and hence in our clinical practice the knowledge of adhesive strength of various impression materials with certain adhesive is of utmost important for the success of the impression and final result.
Permanent distortion occurs when the impression material does not adhere to the tray. Ideally, the impression material tears instead of undergoing considerable distortion before it is released from the undercut. If the material is not adequately retained in the impression tray when it is removed from the mouth, it may be a potential source of error. The most consistently accurate impression is obtained with the adhesive lined resin custom tray. Each class of elastomeric impression material has its own specific adhesive. Metal and plastic stock trays are used routinely for dental impressions, especially with the putty-wash systems. However, heatacrylic cured trays with an adhesive is recommended for additional silicones [4].
The purpose of this study is to compare the retentive bond strength of the three commonly available brands of poly vinyl siloxane impression materials (Aquasil, Virtual and Express TM ) to acrylic tray using universal tray adhesive (Zhermarck) and adhesives supplied by the respective manufactures using the universal testing machine. Brand specific tray adhesives:

Methods
The polyvinyl siloxane heavy body impression materials selected were Aquasil, Virtual, Express TM and these formed the groups I, II and III.
Group I: Aquasil impression material.
Group II: Virtual impression material.
Group III: Express TM impression material.
The tray adhesives selected were universal tray adhesives (Zhermarck) & adhesives of respective impression materials. The main groups were further subdivided based on the adhesives used. Total sample size used were 135.
The procedures were divided into five steps: Step 1: Preparation of the acrylic discs.
Step 2: Application of adhesives.
Step 3: Placement of disc on universal testing machine.
Step 4: Injection of heavy body material to acrylic disc.

Step 1: Preparation of acrylic disc
For preparing the acrylic disc a wax pattern of diameter of 64mm and height 9mm was made using modeling wax. Simultaneously another cylindrical wax pattern of diameter 28mm height and 12mm diameter was made for perpendicular shaft. The wax pattern was invested and acrylisation was done using compression molding technique. Then heat cured acrylic disc and shaft thus obtained were finished and smoothed using 320-grit silicone-carbide paper to standardize surface roughness. The heat cured acrylic cylinder shaft was attached to center of the heat cure acrylic disc using self-cure acrylic. Three pairs of same kind were fabricated for convenience purpose. For each time after the loading, ethanol was used for cleaning the acrylic disc as recommended by the manufacturers of tray adhesives.
Step 2: Application of adhesives on disc Based on the adhesives used the main group was divided into subdivisions A, B and C [ Table 1].
Subdivision A of main Groups I, II and III was kept as control group. The adhesive was not applied to all the samples tested in this subdivision.
In subdivision B of Group I, Caulk tray adhesives [brand specific] was applied on the disc. The adhesive was applied using a brush as thin as possible [ Fig.2]. Only a single coat was applied and it allowed to dry for 5 minutes*.
In Group II subdivision B, Virtual tray adhesives was applied using brush as thin as possible and allowed to dry for 3 minutes*.
In Group III subdivison B, 3M ESPE tray adhesive was applied as thin as possible and allowed to dry for 5 minutes*.
In subdivision C of all Groups, the universal tray adhesive (Zhermarck) was applied on the acrylic disc in the same manner. It was allowed to dry for 2 minutes*.

Universal testing machine
Universal testing machine (model LR5K plus) consists of upper compartment and lower compartment. It is an automatic computerized machine for high accuracy load measurements with LCD display [ Fig. 3]. It is also connected to a personal computer for programming and data collection. It is operated using software called Nexygen. Once loaded this machine itself will automatically align the disc for measurements.
After the application of tray adhesives, the acrylic disc was placed on lower compartment and another acrylic disc was placed on upper compartment. Then ensured the acrylic disc aligned properly.

Injection of heavy body material
Heavy body material was injected [ Fig. 4] on the lower disc and then upper disc lowered until a space of 4mm [Fig. 5]. Care was taken to ensure that the heavy body material completely occupied in this 4mm space between the acrylic discs [ Fig. 6]. Impression material used was different for each group [ Table 1].  In Group I the Aquasil impression material injected using mixing gun and the material was allowed to polymerize for 5 minutes*.
In Group II, Virtual impression material dispensed using mixing gun and allowed to set for 4.30 minutes*. In case of Group III for Express TM , pentamix dispenser [ Fig. 6] was used and allowed to set for 3.30 minutes* * Time as recommended by the manufacturer.

Checking the retentive strength
The test specimens were tested in tensile mode at a cross-head speed of 6mm/min until separation failure [ Fig. 7] occurred and maximum force of separation is automatically recorded and saved to the computer both numerically and graphically [ Fig.8].
The study was conducted on remaining samples in a similar manner and results were statistically analyzed using One Way ANOVA and Tukey Post-Hoc test.

Results
On comparison Aquasil with universal tray adhesive showed greater retentive strength than Aquasil with brand specific tray adhesive and Aquasil without tray adhesive [ Table 2].
On comparison Virtual with universal tray adhesive showed greater retentive strength than Virtual with brand specific tray adhesive and Virtual without tray adhesive [ Table 3].
On comparison Express TM with universal tray adhesive showed greater retentive strength than Express TM with brand specific tray adhesive and Express TM without tray adhesive [ Table 4].
On applying One Way ANOVA, it was found that there was significant difference [p-value<0.001] in mean retentive strength between the study subgroups. The mean value of the retentive strength is highest for Virtual with universal tray adhesive and lowest for Aquasil without tray adhesive [ Table 5]. Table 6 shows the summary of interactions of sub groups with each other. Comparing subdivisions A, B and C of Group I, II and III, the subdivision C showed the greatest retentive values than subdivision B and subdivision A. When comparing Group I, II, and III, the Group II showed the greatest retentive values than Group I and Group III.    reported that acrylic resin trays roughened with eighty grit silicone-carbide created highest strength and they also concluded that adhesives did not adhere well to chromium plated metal or plastic stock tray material. Samman [5] studied about impression tray adhesives and reported that tray adhesives definitely improve the bonding of impression material to the tray. Tjan and Sung [6] reported the significance of tray adhesives in case where the impressions were poured repeatedly, to minimize accidental separation of the impression from the tray.
Though manufacturers have come out with its own tray adhesives for particular rubber-based impression materials, previous literature [1,7] has reported that it might not be the best. Paint-on adhesive on medium body VPS is found to be effective. [5,7] Considering the above facts, the study was done to compare the effectiveness of the universal tray adhesive, Zhermarck with the three commonly available medium consistency VPS impression materials, Aquasil, Virtual and Express TM with their respective tray adhesives, Caulk, Virtual and 3M ESPE. and custom tray material combination is related to 1. The chemistry of the adhesive agents 2. The surface chemistry of the resin tray material. [8] The significantly greater bond strengths of Zhermarck universal tray adhesive were probably attributable to the higher adhesive capability of these adhesives. The adhesion of the tray adhesives is achieved because of chemical adhesion between the impression material and methyl methacrylate tray material. Upon application of the tray adhesive, the carrier solvent present in the adhesive causes swelling of the outermost surface of the tray, thereby allowing the adhesive to penetrate and interact intimately with the impression material. The solvent then evaporates, leaving the entire tray surface covered with the adhesive, which is retained within the molecular network of the impression material.
The retention of impression materials to acrylic resin tray material ultimately depends on the ability of the solvent in the adhesive to dissolve the resin tray material. Thus solvent evaporation is considered as the "setting" of the adhesive and is dependent on time, temperature and relative humidity [9]. It is generally recommended to wait for a few minutes after application of the adhesive before making the impression. This allows time for the solvent to react with the tray material.
Therefore, in this present study, the better adhesive bond strength of Zhermarck universal tray adhesive in comparison with brand specific tray adhesives may be attributed to the difference in the solubility of the tray material by the solvent present in the tray adhesive.
From the results obtained in this study, Zhermarck universal tray adhesive with addition silicone impression material could be recommended as an ideal tray adhesiveimpression material combination using acrylic resin tray material. When comparing among the heavy body impression materials selected, Aquasil, Virtual and Express TM , the Virtual heavy body impression material showed the greatest retentive strength to custom tray with both brand specific tray adhesive and universal tray adhesive.

Clinical Implications
From the results of this study, it can be concluded that the universal tray adhesive can be used in our clinical practice with most of the elastomeric impression materials to attain maximum results without the need of the adhesive supplied by the respective manufacturer.