Brazilian Journal of Oral Sciences Piracicaba Dental School - UNICAMP EISSN: 1677-3225 Vol. 6, Num. 20, 2007, pp. 1295 - 1300

Brazilian Journal of Oral Sciences, Vol. 6, No. 20, January - March 2007, pp. 1295 - 1300

Effect of superficial treatment on the tensile bond strength of two reliners and a thermopolymerizable resin

Cristiane Maria Brasil Leal¹ , Carlos Alberto Muzilli² , Milton Edson Miranda² , José Renato Ribeiro Pinto³ , José Antônio Nunes de Mello⁴

¹MS, Professor at the State University of Amazonas, UEA, Brazil
²PhD, Professor at the CPO São Leopoldo Mandic, Campinas, SP, Brazil
³PhD, Professor at the Pontifícia Universidade Católica de Campinas, SP, Brazil
⁴PhD, Professor at the State University of Amazonas, UEA, Brazil
Correspondence to: Cristiane Maria Brasil Leal Rua Virolas, 137, Cj Kissia, D. Pedro CEP 69040-360 Manaus, Amazonas, Brazil Phone: +55 92 3238-3176 Fax: +55 92 3656-2132 E-mail: cbleal@uol.com.br

Received for publication: December 05, 2006
Accepted: March 10, 2007

Code Number: os07010

Abstract

The study aims to evaluate the tensile bond strength of two direct hard reliners, Kooliner and Tokuyama Rebase II and a thermopolymerizable acrylic resin, Vipi Cril, following mechanical superficial treatment. Forty specimens were prepared and separated in four groups according to the relining material and the surface treatment of the thermopolymerizable resin: group I - Kooliner joined to the surface of the resin without treatment, group II - Kooliner joined to the surface of the resin with treatment, group III - Tokuyama joined to the surface of the resin without treatment and group IV - Tokuyama joined to the surface of the resin with treatment. Obtained values were statistically compared by the Bartlett's test and variance analysis to 5%. The results are presented in Kgf: Group 1, 74.35 ± 15.89; Group 2, 104.95 ± 13.97; Group 3, 86.91 ± 14.63; Group 4, 100.42 ± 14.93. No significant difference was found between the reliners. The mechanical superficial treatment on the thermopolymerizable resin for denture base has significantly increased its tensile bond strength with the studied reliners.

Key Words: complete denture, denture reliners, dental materials

Introduction

The stability and retention of removable prosthesis is essential to provide the patient with comfort and adequate functioning. To achieve this, the adaptation of the bases of removable prosthesis, partial or totally, on the ridge mucosa is necessary and ensures the uniform distribution of the oclusal force.

The adaptation between the removable prosthesis and the underlying tissue is achieved by several techniques including specific molding techniques. However, the continuous use of the prosthesis can result in the reabsorption of the residual ridge which alters the initial adaptation and causes maladjustment of the prosthesis base with consequent loss of oclusal equilibrium, non-uniform distribution of the chewing load, accumulation of food residues between the base and the ridge and discomfort to the patient¹. There are also other factors that can result in loss of adaptation like the contraction of resin following polymerization and its expansion following water absorption in the mouth cavity^2-3.

Thus, periodic controls after installation of the prosthesis to verify the need for readjusting the base is inevitable. Readjusting can be done through a laboratory procedure or using direct hard reliners material. The latter reduces the costs, is quicker and does not require the patient to remain without the prosthesis during laboratorial phases^1,4 but it has some clinical limitations as the applied acrylic resin has an unpleasant smell and taste. Moreover the released monomer and the high temperature during polymerization can irritate the mouth soft tissues^1,5.

To minimize these disadvantages, some modifications have been suggested in the composition of the liquid part of these products like the addition of the cross-linking agents, and/or the substitution of the metilmethacrylate by isobutylmethacrylate³ or betamethacryloil oxiethylpropionate⁶.

Based on these considerations and to identify a material with appropriate adhesion to the acrylic resin of the denture base, we intended to compare the tensile bond strength between two hard materials for direct relining of denture base, Kooliner and Tokuyama Rebase II, with an thermopolymerizable acrylic resin, Vipi Cril (VIPI), either with a mechanical treatment on resin's surface or without treatment.

Material and Methods

Ten "T" shaped metallic dies of stainless steel were built with the purpose of providing and standardizing the space for the insertion of the thermopolymerizable acrylic resin. These dies presented the following dimensions: the "T" base was a 10×10mm square, its height was 35mm and its upper part was 30mm long, 10mm thick and 10mm high. The area in touch with the reliners material was the base of the "T", a 10x10mm square^7-9. Two screws in stainless steel with metallic thread of 12mm diameter, compatible with the machine of analyses, and 50 mm of length were also built and soldered to two supports for adaptation of the "T" shaped specimens during the tensile test.

A rectangular bronze flask was built specially for this research, being 22 cm long, 11 cm wide and 7 cm high. In the inside, a gutter was milled to place a removable aluminum space making bar, 210 mm long, 12 mm high and 3 mm thick. On the outside, it was placed 2 guiding pines with the purpose of guiding the counter flask in an only axis of insertion, besides from stabilizing it so that there were no movements.

The internal surface of the flask was isolated with solid vaseline and the base filled with Durone plaster of the IV type (Dentsply, Petrópolis, Rio de Janeiro, Brazil) in the proportion of 100g of powder to 19 ml of water, vacuum spatulated and poured onto the base under constant vibration, avoiding the occurrence of porosities, mainly in the surface of the plaster. The space making bar that is inserted into the gutter of the counter-flask was placed over the plaster of the base. After the crystallization of the plaster, the surface was planned with sandpaper of aluminum trioxide of 100 granulation (3M do Brasil, Ribeirão Preto, SP). Five "T" shaped metallic dies were aligned, in parallels, on each side of the space making bar of the flask in a way that their bases touched it (Figure 1). In this position, the dies were fixed to the plaster of the base with cyanocrilate adhesive, Super-bond gel (Loctite, Itaperi, SP, Brazil).

Dies bonded and positioned on the base of the flask were covered with silicone of Zetalabor enclosure (Zhermack S.P.A., Rovigo, Italy). The manipulation of the silicone was carried out following the manufacturer's instructions. The mixture prepared was adapted over the set metallic dies / space making bar, under digital pressure, obtaining a layer of silicone approximately 10 mm thick. In the surface of the silicone, it was made retentions for imbrication of the plaster that was subsequent pouring over it. Solid vaseline was spread over the surface of the plaster on the base and in the interior of the counter flask. The counter flask was put into position and the final filling of the counter flask was performed with plaster of the IV type, vacuum spatulated and poured under constant vibration. Next, the counter flask was closed; taken to the bench hydraulic press (Delta Máquinas Especiais, Vinhedo, SP, Brazil) and submitted to the pressure of 1.25 ton for 1 hour. After detachment of the base of the flask / counter flask, the "T" shaped metallic dies were removed from the silicone, leaving the printed mold (Figure 2). The space making bar remained in position in the mold, and the surfaces were isolated with insulation for acrylic resin (Cel-Lac, SS White, Rio de Janeiro, Brazil).

Resin used in this experiment was Vipi Cril (Vipi Ind. Com. Exp. Imp. de Prod. Odont. Ltd, São Paulo, Brazil) a thermically activated acrylic resins. The material was manipulated in a phial for paladon (Jon Com. Prod. Odont. Ltd, São Paulo, Brazil) in the proportion of 14 g of polymer to 6.5 ml of monomer. Once reached the plastic phase, the resin was settled with aid of a spatula 36 in the interior of the molds, in the flask. The flask was closed and two plates of iron with screws on the edges were put over it and on its base for pressing. After obtaining the pressure of 1.25 ton, the screws were tightened and the flask taken for polymerizing in water on 75^oC ± 2^oC during 9 hours. Afterwards, the flask stood on the bench for slowly chilling until reaching room temperature.

Next, the flask was opened and the samples removed for the final touch with a maxi cut drill (H251E, Komet, Brasseler. Gebr., Germany) and aluminum trioxide sandpapers of 100 and 400 granulation.

Forty specimens were produced; 20 received treatment onto the surfaces that touched the hard reliner material and 20 did not. The treatment on the surfaces included the preparation of grooves with steel / metal sandpaper of 60 granulation (3M do Brasil, Ribeirão Preto, SP) which was divided into sections 14 cm long and 3 cm wide. The base of the acrylic resin sample was positioned perpendicular to the sandpaper and rubbed over the section, traversing 4 times the length of the section (Figure 3). Only one sample was rubbed on each section and this procedure was performed by the same person.

Afterwards, the samples were stored in distilled water on 37^oC in a bacteriological greenhouse (ECB 1.3 digital, Odontobrás Ind. de Equip. Méd. Odont., Ribeirão Preto, SP, Brazil) for 48 hours. Next, they were divided in 4 groups with 10 specimens each: Group 1, surfaces without treatment and relined with Kooliner resin (GC America Inc., Alsip, IL, U.S.A. GC); Group 2, treated surfaces and relined with Kooliner resin; Group 3, surface without treatment and relined with Tokuyama Rebase II Fast (Tokuyama Co., Ltd., Tokyo, Japan); Group 4, treated surfaces and relined with Tokuyama Rebase II Fast.

The mold previously employed for enclosure of the acrylic resin was used to joined the samples to the hard reliner material. The space making bar of 3 mm was removed and the samples in acrylic resin were positioned in the mold. The space of 3 mm between them was filled with the hard reliner (Figure 4). The acrylic resin area in touch with the reliner corresponded to a square of 10 mm per 10 mm^7-8. The hard reliner resin was manipulated according to the manufacturer's instructions. The material was manipulated in a glass pot, always with saturation of the monomer by the polymer and inserted into the space of the mold, bonding two samples, the flask was closed and taken to the hydraulic press. When 3 minutes had elapsed, the flask was open and the excesses of reliner were trimmed with a scalpel blade. Next, the flask once again was closed and taken to the press. After 20 minutes, the flask was opened, the specimens were secluded and submitted to the final touch with maxi cut drill and sandpaper of 100 and 400 granulations. The specimens were encoded according to their group and stored in distilled water on 37^oC for 48 hours, in a bacteriological greenhouse.

For performing the tensile strength test the machine of universal experiment Emic, model DL 2000, with computerized control system LG brand, and software Tesc / VirMaq (EMIC Equip. e Sistemas de Ensaio, Ltd., São José dos Pinhais, PR, Brazil) was used. The load cell was on 200 Kgf and the speed was 5 mm/min. A screw with fix support was threaded to the load cell of the equipment and another screw was threaded to the base of the equipment, in this way, the specimens were adapted to the support of the screw, perpendicular to the horizontal plan (Figure 5). The machine was set in motion and the tensile was maintained up to the rupture of the union (Figure 6).

The Kolmogorov-Smirnov normality test was carried out and, to evaluate the homocedasticity between the groups the test of Bartlett was used. Moreover, the variance analysis to two criteria, fix model, level of significance of 5% (p<0,05), software Statistic for Windows version 5.1 (StatSoft Inc., Tulsa, USES) was used to verify the effect of the material used and the type of surface (treated or untreated).

Results

The values of resistance, in Kgf, obtained in the mechanical experiment of tensile of the union between the direct hard reliners, Kooliner and Tokuyama, and the acrylic resin thermically activated, Vipi Cril, are described at Table 1.

The variance analysis has shown significant statistical difference only due to the surface criterion (p<0,001), not being significant the difference between materials (p=0,399) neither the interaction between the effects (p=0,077).

Discussion

The tensile bond strength of the Kooliner and the thermopolymerizable acrylic resin for denture base is weak^2,10-11. Aydin et al.¹² have concluded that the tensile bond strenght of the Kooliner to the thermopolymerizable resin (Paladent 20) diminished after storage in water. Deficiencis in the interface of the union may create a surface favorable to the proliferation of bacteria or result in the complete dissociation between the reliner material and the resin for denture base^11,13.

Brevilieri et al.¹⁴ concluded that the Tokuso Rebase has qualities that provide the relining with a smaller peak of heat liberation, bigger stability of color, smaller porosity and a bigger adhesion to the denture base due to the presence of an adhesive⁶. Tokuyama Dental has developed and marketed a resin for direct relining named Tokuyama Rebase II to solve and improve the still unsatisfactory peculiarities of the resin Tokuso Rebase and by the introduction of the Resin Hardener in its composition, a great superficial smoothness was observed in addition to the previous qualities of the Tokuso Rebase.

Having in mind that through the tensile test a better understanding of the distribution of the stress could be obtained, this kind of test has been used by several authors to evaluate the tensile bond strength between resilient materials and resins for denture base^8-9 and the bond between hard reliners and these resins^4,10. Therefore, the tensile test was selected for this study.

Superficial treatment of the thermopolymerizable resin, whether mechanical or chemical is an attempt to increase the union between the reliners and the denture bases. Ohkubo et al.¹⁵ made the surface of the thermopolymerizable acrylic resin rough to promote asperity and alterations in the morphology and improve the tensile bond strength with the Kooliner direct relining resin. The use of bonding agents increase the tensile bond strength between resilient⁷ reliners and hard^2,11 reliners. As the use of the reliners material was done according to the manufacture's instructions, the surfaces that were united with the Tokuyama Rebase II in this essay have received an application of the adhesive that accompanies the product.

The results showed that the mechanical superficial treatment increase the rupture force. For the Kooliner material, the average increment was 41.2% and for the Tokuyama Rebase II it was 15.5%.

These results are in accordance to the work of Ohkubo et al.¹⁵ that has verified that the time of immersion of the resins for base and the pressure put during the relining did not influence in the adhesive resistance between the reliners and the resin of base, however, the presence of roughness in the surface of union increased the adhesive resistance of the Rebaron and Kooliner material.

The results of this research have diverged from the conclusions of the work of Moradians et al.¹⁶, who have used conventional autopolymerizable acrylic resin (SOS) and a fluid resin (Palapress), both for prosthesis repair, to evaluate the tensile bond strength of these resins with a thermopolymerizable resin for denture base. The surfaces of union were submitted to the following conditions: without treatment, application of the monomer of the resin used for repairing, preparation of grooves and pumice-stone polishing. The authors have observed that the best results were those presented by the surfaces polished with pumice-stone and have justified this result with the theories of adhesion, in which the smooth surfaces of union are necessary for the adequate moistening and drainage of the repairing material. Thus, for the authors, although the preparation in the form of groove provides bigger union surface area, the retention of air in possible irregularities present in this kind of preparation may diminish the obtained resistance.

Shen et al.¹⁷ studied the effect of the surface treatment in the tensile bond strength between two thermopolymerizable resins and an autopolymerizable resin for prostheses repairs. The authors came to the conclusion that, in general, it is accepted the concept where the resistance of the repair can be increased by making the surfaces of union rough. However, the microscope analyses have indicated that this procedure can result in surface being characterized by particles and small grooves along thin furrows. Such characteristics may stop the drainage of the repairing material and reduce the attraction forces, fundamental for increasing the tensile bond strength.

The need for increasing the adhesive resistance of direct relining material led manufacturers to supplying substances used as primers that modify the surface of the base resin previously to relining¹. Arima et al.¹³ have concluded that such substances are, in general, organic solvents (dichloromethane or acetone), methacrylate monomers or the association between methacrylate polymer and monomer. These products differ among themselves as regards the capacity of dissolving the polymethyl methacrylate structure. According to Arena et al.², the use of the adhesive increases the polymolecular chains available for the cross-linking and the union between the materials. The adhesive agent of the Tokuyama Rebase II is composed of acetone and ethyl acetate, and it has probably been by the use of the adhesive agent that in the surface without treatment of the thermopolymerizable acrylic resin the average of rupturing force was bigger for the Tokuyama (86,91 Kgf) than for the Kooliner (74,35 Kgf). The low union resistance between the Kooliner and the thermopolymerizable resin may be related to the relatively low capacity of the isobutyl methacrylate monomer of dissolving the surface of the base resin (polymethyl methacrylate)^1,4,13.

However, when the Tokuyama Rebase II was used on the surface of the resin with treatment (rough), the rupturing force average was lower than that of the Kooliner, despite this difference not being statistically significant, this result may be explained by the higher fluidity of the Kooliner that has allowed its bigger penetration into the grooves.

So being, the making of roughness, as those performed in this work, on the surface of the thermopolymerizable acrylic resin for denture base (Vipi Crill) has shown to be an option for making the union with the direct hard reliners (Kooliner and Tokuyama Rebase II) more effective. However, others studies are necessary for evaluating other characteristics such as superficial hardness, solubility, sortion, micro-organisms adhesion, beyond of color, flavor and smell, and also longitudinal clinical studies in order to verify the clinical reality for the use of each material.

Based on obtained results and according to employed methodology, it is possible to conclude that Kooliner and Tokuyama Rebase II have similar resistance values to the tensile bond strenght with the resin for denture base and the mechanical superficial treatment on the thermopolymerizable acrylic resin has increased the tensile bond strength of the two direct hard reliners that were studied.

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