Influence of the Solubility Parameter of Intermediary Resin on the Effectiveness of the Gluma Bonding System E. ASMUSSEN, E.K. HANSEN, and A. PEUTZFELDT Institute of Dental Materials and Technology, Royal Dental College, Norre Alli 20, DK-2200 Copenhagen N. Denmark

The aim of the present study was to investigate the effect of the solubility parameter of the intermediary resin in the Gluma system on the bonding to dentin. The solubility parameter of the resins was varied between 18.8 x 103 and 21.1 x 103 J1"2/m312 by varying the composition of the resin. The efficacy of the bonding system was determined by measurements of marginal gaps formed by polymerization contraction of a restorative resin in dentin cavities treated with the bonding system. The bonding system had maximum efficacy at a solubility parameter of the intermediary resin of 8 = 20.0 x 103 J/2/m312. This finding corroborates a concept of bonding to dentin that involves a mechanical interlocking by interpenetrating resins. J Dent Res 70(9):1290-1293, September, 1991

Introduction. Ideally, a dentin adhesive for bonding of restorative resins to dentin should cause the filling to be retained without undercuts and give rise to a significant reduction of marginal gaps (Hansen and Asmussen, 1989). With regard to the mechanism of bonding, dentin adhesives may react with and bond to dentin in several ways (Asmussen and Munksgaard, 1985). The modes of bonding may be of a chemical nature, and reactions with the Ca2+-ions of the inorganic part and/or the collagen of the organic part of the dentin are conceivable (Bowen, 1965). The bond may also have a mechanical component. Enamel-bonding agents, i.e., low-viscosity, unfilled resins, have been shown to penetrate deeply into the tubules of acid-etched dentin (Qvist et al., 1977). However, the strength of a bond relying solely on this type of penetration is low. Another type of mechanical bonding is possible: Nakabayashi et al. (1982) and Nakabayashi (1985) have identified a mechanical adhesion based on "interpenetration". This term denotes the superficial penetration of resin monomers into intertubular dentin where polymerization takes place, locking the polymerized material in the dentinal surface structures. Interpenetration is reported to occur with certain monomers that have both hydrophilic and hydrophobic moieties. From a physical point of view, the interpenetration describes the ability of the resin to soften or solubilize the surface of the dentin, and may thus be related to the solubility parameter of the adhesive resin (Gardon, 1965). This implies that the resins used in the dentin-bonding agents should have a composition corresponding to a certain solubility parameter at which the bonding system has optimum efficacy. It was the aim of the present work to investigate this possibility in connection with the Gluma bonding system by using intermediary resins of different solubility parameters.

Received for publication January 11, 1991 Accepted for publication April 16, 1991 1290

Materials and methods. The efficacy of the various resins was assessed by measurements of marginal gaps. The gaps were formed during polymerization of a resin composite applied in dentin cavities treated with the Gluma bonding system. Cavity test.-This part of the study was carried out on human teeth of the permanent dentition. After extraction, the teeth were cleaned mechanically and stored in tap water at room temperature for periods of time ranging from one day to two months. At the start of the investigation, one of the root surfaces was ground flat on wet carborundum paper No. 220 (Struers A/S, Copenhagen, Denmark), and a cylindrical buttjoint cavity (diameter, 4.0 mm; depth, ca. 1.5 mm) was prepared in the ground dentin surface. After cavity preparation, the cavity wall and the surrounding dentin were rubbed for ten s with a cotton pellet soaked in Gluma Cleanser, which is a 0.5-molIL ethylenediaminetetraacetic acid (EDTA) solution adjusted to pH 7.4 with NaOH (Munksgaard and Asmussen, 1984). The cavity was then rinsed and dried with a stream of air, both for ten s. The cleaned dentin surfaces were treated with Gluma for ten s, and excess was blown out of the cavity until the dentin appeared to be completely dry. Gluma is an aqueous solution, 5% w/w glutaraldehyde and 35% w/w HEMA (2-hydroxyethyl methacrylate). Following this, the primed dentin was covered with a low-viscosity resin (the composition of this intermediate layer will be described later). The cavity was then filled with a restorative resin (Silux Plus, 3M, St. Paul, MN) by means of a syringe system (Hawe-Neos, Gentilino, Switzerland). The free surface of the filling was covered with a matrix (HaweNeos), and the restorative resin was irradiated for 20 s with a visible-light-curing unit (Visilux, 3M), with close contact maintained between the exit window of the lamp and the matrix. The tooth was then stored in tap water. Ten min after the polymerization, a standardized method of gentle wet-grinding and polishing was used for removal of approximately 0.1 mm of the dentin and the filling (Hansen, 1982). The width of the maximum marginal contraction gap (MG) was measured by use of a light microscope (Reichert MeF Universal Microscope, Vienna, Austria; 8 x 63) with a measuring ocular. In most cases, the gap was located only along part of the circular filling margin observed in the microscope. The extent of the gap (GP) was measured and expressed as a percentage of the total filling periphery. All procedures except cavity preparation, mixing of the various low-viscosity resins, and handling of the dentin-bonding agent and composite restorative were carried out in a room maintained at 36.5 ± 0.5TC. Statistics.-In a previous study (Hansen and Asmussen, 1988), it was found that when dentin-bonding agents were used, there was a positive correlation between the maximum gap width (MG) and the extent of the gap in percent of the total filling periphery (GP). The statistical analyses were therefore based on MG x GP/100, referred to in the following as the marginal index (MI). This index is an approximation of the calculation of the free surface area of the contraction gap (Hansen and Asmussen, 1990). As an example, a 10-,um-wide

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SOLUBILITY PARAMETER OF RESINS

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30 40 BU 70 50 60 30 70 60 40 50 BI Fig. 1-Marginal index (MI) in relation to composition (mol%) of intermediary resin in the Gluma bonding system. BU = n-butyl methacrylate; BI = BISGMA. Bar = range. %

encircling 50% of the filling is ascribed a value for the marginal index of 10 x 50/100 = 5. The distribution of the marginal index was skewed, and the analyses were therefore made with non-parametric statistics, i.e., Kruskal-Wallis one-way analyses of variance and the MannWhitney U test (Siegel, 1956). Low-viscosity resins. -Two series of intermediary resins were used. In series A, the resins were mixtures of n-butyl methacrylate (BUMA) with bisphenol-A-glycidyl dimethacrylate (BISGMA). In series B, the resins contained 20 mol% of HEMA in addition to BUMA and BISGMA. The exact compositions of the various resins will be accounted for below. The compositions were chosen so that a large range of solubility parameters was encompassed (see below). The monomers contained catalysts for light-curing (0.2% w/w of camphorquinone and 0.2% w/w of N,N-cyanoethylmethylaniline). In addition to measurements involving the above series A and B, the marginal index was determined with Silux Enamel Bond (3M) used as intermediary resin. This resin is a mixture of BISGMA and TEGDMA (triethyleneglycol dimethacrylate). For each of the above resins, ten measurements of marginal index were performed. Solubility parameter. -Since no data on the solubility parameters of BISGMA and HEMA are available, we calculated the solubility parameters of all monomers involved. The calculations were based on Small's method, as described by Gardon (1965). By this method, the molar attraction constants F of the different groups in a monomer molecule are added to give the sum XF. The solubility parameter 8 is given as 8 = XF/V, where V is the molar volume. For V to be calculated, the densities of the monomers must be known. The densities of HEMA and BUMA were taken as d = 1.07 g/mL and d = 0.894 g/mL, given by the supplying chemical company. The densities of BISGMA and TEGDMA have been determined in a previous work dealing with the viscosities of various monomers (Asmussen, 1977) and were d = 1.168 g/mL and d = 1.073 g/mL, respectively. For a mixture of monomers having molar fractions x1, X2, X3 ..., the solubility parameter was calculated as (x1.YF1+ x2_ F2 + x3 F3 + . -)/ (x V1 + x2 V2 + x3 V3 + ..), where each subscript refers to one of the monomers in the mixture.

gap

BU 60 20 30 40 50 BI 20 60 40 30 50 Fig. 2-Marginal index (MI) in relation to composition (mol%) of intermediary resin in the Gluma bonding system. HEMA = 2-hydroxyethyl methacrylate; BU = n-butyl methacrylate; BI = BISGMA. Bar = range. %

Results. The main results of the measurements of marginal index, MI, are presented in Figs. 1 and 2. Fig. 1 shows median value and range of MI in relation to composition of intermediary resins based on BUMA and BISGMA. It can be seen that a monomer mixture of 50% BUMA and 50% BISGMA gave a median MI below 2, whereas other mixtures gave MIs between 4 and 6. The statistical analysis showed that the MI obtained with 50% BUMA/50% BISGMA was statistically smaller than the other values (p

Influence of the solubility parameter of intermediary resin on the effectiveness of the gluma bonding system.

The aim of the present study was to investigate the effect of the solubility parameter of the intermediary resin in the Gluma system on the bonding to...
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