pH neutralization and influence on mechanical strength in self-adhesive resin luting agents

Abstract

Objective

The aim of this study was to evaluate if pH-neutralization behavior of self-adhesive resin cements has an influence on their mechanical properties.

Methods

pH-neutralization, water sorption, solubility and flexural strength of G-Cem Automix (GCA), G CEM Capsules (GCC), Maxcem Elite (MCE), RelyX Unicem Clicker (RXC), RelyX Unicem Maxicap (RXM), RelyX Unicem 2 (RX2), and Speed-Cem (SPC) were tested in self-curing mode. Specimen’s pH-values were recorded up to 24 h with a pH-electrode. Water sorption (WS) and solubility (SO) were measured. Flexural strength (FS) was evaluated before and after thermocycling (TC) and fracture surfaces analyzed under SEM.

Results

RX2 (pH 24 5.89), RXC (pH 24 5.49) and SPC (pH 24 5.42) showed highest 24 h pH-values, followed by GCA (pH 24 5.34). Capsulated products and MCE (pH 24 3.90) reached lower pH-values. GCC (75.78 μg/mm 3 ) showed the highest WS followed by MX (69.64 μg/mm 3 ), RXM (64.76 μg/mm 3 ), GCA (25.86 μg/mm 3 ) and SPC (30.42 μg/mm 3 ). Capsulated products RXM (56.58 μg/mm 3 ) and GCC (30.94 μg/mm 3 ) presented the highest SO, GCA (4.06 μg/mm 3 ) and SPC (4.26 μg/mm 3 ) revealed the lowest. GCA (114.03 MPa) had the highest initial FS followed by SPC (79.81 MPa), RXM (41.61 MPa) the lowest. FS of all materials decreased significantly after TC except for RXC (44.65 MPa) and RX2 (65.92 MPa). FS of GCC (16.47 MPa) and MCE (28.21 MPa) decayed the most. A highly linear correlation was observed between percentage decrease of FS after TC and 24 h pH-values ( R 2 = 0.861).

Significance

Within the limits of this study pH-neutralization behavior has an influence on mechanical stability. When using self-adhesive resin luting agents, automix-syringe products with improved pH-neutralization behavior should be favored.

Introduction

Different luting agents have been used in dentistry for more than a century. Today these materials can be categorized in two main classes according to ISO-standards: dental water-based cements consisting of zinc phosphate cements, polycarboxylate cements, glass ionomer cements, resin-modified glass ionomer cements and dental polymer-based luting materials such as compomer and composite resin luting agents. The main difference between these two classes is their main curing mechanism which is water-dependent for cements and through polymerization for luting agents. In literature, the term resin-cements is often used for the latter material class.

Ferracane et al. defined this material class as filled polymeric materials designed to adhere to tooth structure without the requirement of a separate adhesive or etchant . This is the main difference to conventional luting resins, which require a pre-treatment of the tooth with either etch-and-rinse or self-etch adhesives prior to luting.

Self-adhesive luting agents have reached great popularity among practitioners because they accelerate and simplify the procedure of adhesive placement of indirect restorations . Compared to conventional multi-step adhesive techniques no pre-treatment of tooth tissue is needed, reducing technique sensitive application steps .

In order to promote adhesion between the luting agent and the tooth substrate acid-functionalized monomers, predominantly (meth)acrylate monomers with carboxylic or phosphoric acid-groups are utilized to achieve demineralization of enamel and dentin. The acidic groups attach to calcium in the hydroxyapatite of the demineralized smear layer, dentin or enamel creating a bonding via ionized phosphoric acid-methacrylates to the resin network . The adhesion mechanism of these materials is not in terms of hybrid layer and tag formation, only a partial smear layer/dentin demineralization or infiltration of dentin could be observed, being more similar to the adhesion mechanism of glass ionomer cements than to conventional resin-based luting agents .

Two different curing mechanisms take place simultaneously: on one side a free radical methacrylate polymerization, whether initiated by photo initiators or by acidic initiating redox systems, and on the other side the acid–base reactions with ionic crosslinks between acidic groups of acid functionalized monomers and ions of the acid-soluble glass fillers or the mineralized tooth tissue. The reaction mechanism is responsible for pH-neutralization in the cured material .

The concentration of acidic monomers plays a crucial in the curing process: it has to be high enough to guarantee proper demineralization and bonding to dentin and enamel and as low as possible to avoid excessive hydrophilicity in the cured material. As reported by Ferracane et al., a hydrophilic character due to a low pH-value in the cured material can compromise mechanical stability by excessive water sorption . In summary, an ideal so-called self-adhesive resin cement would be highly acidic and hydrophilic in the beginning, in order to reach demineralization for adhesion and adaption to the tooth surface, and become completely pH-neutral and hydrophobic after curing.

More than a dozen products are commercially available and clinical and mechanical properties have been investigated such as adhesion to tooth substrates , marginal adaption , wear resistance and esthetics . Setting pH-values and pH-over-time development were measured by Han et al. and Saskalauskaite et al. but until now no correlation to mechanical stability was performed.

The null hypothesis tested in this study was that pH-neutralization behavior has no influence on long term mechanical stability under in vitro thermal loading.

Materials and methods

Materials used

Table 1 shows eight commercially available self-adhesive resin cements tested in this study and its main composition. All materials were processed according to manufacturer instructions and the self-cure mode was utilized for all specimen preparation.

Table 1
Materials tested in this study (manufacturer information).
Code Material Manufacturer Shade/lot Mixing Composition Setting time (self-cure)
GCA G-Cem Automix GC-Corp. Translucent/0909211 Automix 4-META, UDMA, dimethacrylate, water, phosphoric ester monomers, initiators, stabilizers, aluminosilicate glass fillers 4 min
GCC G-Cem Capsule GC-Corp. Translucent/1001191 Capsule 4-META, UDMA, dimethacrylate, water, phosphoric ester monomers, initiators, stabilizers, aluminosilicate glass fillers 4 min
MCE Maxcem Elite Kerr Corp. Clear/3441545 Automix Methacrylate ester monomers, inert mineral fillers, ytterbium fluoride, activators and stabilizers >4 min
RXC RelyX Unicem Clicker™ 3M ESPE Translucent/403121 Paste-paste, hand mix Methacrylated phosphoric esters, dimethacrylates, acetate, initiators, stabilizers, glass fillers, silica, calcium hydroxide 6 min
RXM RelyX Unicem Maxicap™ 3M ESPE Translucent/398868 Capsulated Methacrylated phosphoric esters, dimethacrylates, acetate, initiators, stabilizers, glass fillers, silica, calcium hydroxide 6 min
RX2 RelyX Unicem 2 3M ESPE Translucent/430451 Automix Methacrylated phosphoric esters, dimethacrylates, acetate, initiators, stabilizers, glass fillers, silica, calcium hydroxide 6 min
SPC Speed-Cem Ivoclar Vivadent Transparent/N16128 Automix Dimethacrylates, methacrylated phosphoric esters, barium glass, ytterbium trifluoride, copolymers, high dispersed silica, initiators, catalysts and stabilizers 6 min

pH-neutralization

To examine the pH-neutralization behavior over time, specimen discs ( n = 6 per material) of 15.0 ± 0.1 mm in diameter and 1.0 ± 0.1 mm in thickness were prepared. Therefore a metal mold was used. To prevent the formation of an air-inhibition layer while setting PMMA cover plates were placed on top and bottom of the mold. The mold was pre-heated mold at 37 °C, the material filled in and left to maturate for 10 min at this temperature in a chamber with 100% humidity. For all specimens the self-cure polymerization mode was selected. Capsule products were mixed in a standard mixing device (Capmix, 3M ESPE, Seefeld, Germany) for 10 s. After 10 min of setting the specimens were submitted to pH-measurement. Therefore each specimen was covered with 30 μL of 1 mmol/L NaCl solution and a flat pH-electrode (InLab Surface, Mettler Toledo, Germany) was placed on the probe. The measurements started exactly 15 min after the start of mixing the cements and lasted for 24 h with a pH-data acquisition rate each 20 s. The chamber was constructed to exclude any curing influence from environmental light (dark conditions) and to ensure a constant humidity of 100% over the whole measuring period with a constant temperature of 37 °C. After each measurement the pH electrode was recalibrated at pH 4 and pH 7. Statistical analysis of the final 24 h pH-values was conducted using IBM SPSS Statistics software 19.0 (IBM, Armonk, NY, USA). Means and standard deviations were calculated. Normal distribution was tested by the Kolmogoroff–Smirnoff test. Multiple comparisons were done with two-way ANOVA followed by Student–Newman–Keuls post hoc test (p < 0.05).

Water sorption (WS) and solubility (SO)

To examine the amount of absorbed water during an interval of water storage specimen discs ( n = 5 per material) of 15.0 ± 0.1 mm in diameter and 1.0 ± 0.1 mm in thickness were prepared. Therefore a metal mold was used. To prevent the formation of an air-inhibited layer while setting PMMA cover plates were placed on top and bottom of the mold. The mold was pre-heated at 37 °C, the material filled in and left to maturate in the mold for 10 min at this temperature in a chamber with 100% humidity, in order to simulate oral environment. For all specimens the self-cure polymerization mode was selected. Capsule products were mixed in a standard mixing device (Capmix, 3M ESPE, Seefeld, Germany) for 10 s.

All specimens were measured according to ISO 4049. After setting, the specimens were transferred to a desiccator, maintained at 37 ± 1 °C. The silica gel was freshly dried for 5 h at 130 °C and replaced with freshly dried gel after each weighing sequence. After 22 h, the specimens were removed, stored in a second desiccator, maintained at 23 ± 1 °C for further 2 h. The dimensions were measured to an accuracy of ±0.001 mm and weighed to an equilibrated accuracy of ±0.1 mg. An analytical balance (±0.05 mg, Sartorius, Germany) and a digital caliper (±0.001 mm, Mitutoyo, Japan) served as measuring tools. This cycle was repeated until the mass loss of each specimen was not more than ±0.1 mg in a 24 h period. The volume was calculated from the specimen dimensions. After drying, the specimens were immersed in water of 37 ± 1 °C for 7 d. After 7 d the specimens were removed, washed with water, gently dried until free from visible moisture and weighed 1 min after removal from the water. After weighing, the specimens were reconditioned to constant mass in the desiccators using the cycle described above. The values for water sorption and for solubility (in [μg/mm 3 ]) were calculated for each of the five specimens. Statistical analysis was conducted using IBM SPSS Statistics software 19.0 (IBM, Armonk, NY, USA). Means and standard deviations were calculated. Normal distribution was tested by the Kolmogoroff–Smirnoff-test. Multiple comparisons were done with one-way ANOVA followed by Student–Newman–Keuls post hoc test (p < 0.05).

Flexural strength (FS)

To assess flexural strength specimens (n = 30 per material) were prepared in special mold (2 mm × 2 mm × 25 mm). The mold was pre-heated at 37 °C and the material was filled in and left to maturate in the mold for 10 min at this temperature in a chamber with 100% humidity, in order to simulate the oral environment. The self-cure polymerization modus was selected. The capsule products were mixed in a standard mixing device (Capmix, 3M ESPE, Seefeld, Germany) for 10 s. To finish the specimens flanges SiC-paper was used at a roughness of 1200 grit. The specimens were stored in water for 24 h at a temperature of 37 °C. All specimens were measured according to ISO 4049. Half of the specimens of each group ( n = 15) were measured before and the other half ( n = 15) after 10,000 cycles thermocycling between alternating temperatures of 5 °C (for 30 s) and 55 °C (for 30 s) in an automatic thermocycler (Willitec, Munich, Germany). The bars were fixed in a 3-point-bending test rig (fin distance 20 mm) of a universal testing machine (Zwick Z2.5, Zwick, Ulm, Germany) and loaded until fracture with a crosshead speed of 0.75 mm/min. Statistical analysis was conducted using IBM SPSS Statistics software 19.0 (IBM, Armonk, NY, USA). Means and standard deviations were calculated. Normal distribution was tested by the Kolmogoroff–Smirnoff test. Multiple comparisons were done with two-way ANOVA followed by Student–Newman–Keuls post hoc test (p < 0.05).

Scanning electron microscope (SEM) evaluation

After initial flexural strength test representative fracture surfaces of specimens were pre-selected under light microscope and representative specimens were mounted on aluminum cylinders, sputter-coated with gold (120 s at 30 mA, Balzers SCD 40, Balzers, Liechtenstein) and examined with a SEM (acceleration voltage 20 kV, beam current 90 mA, full vacuum, Leitz ISI 50, Akashi, Tokyo, Japan).

Materials and methods

Materials used

Table 1 shows eight commercially available self-adhesive resin cements tested in this study and its main composition. All materials were processed according to manufacturer instructions and the self-cure mode was utilized for all specimen preparation.

Table 1
Materials tested in this study (manufacturer information).
Code Material Manufacturer Shade/lot Mixing Composition Setting time (self-cure)
GCA G-Cem Automix GC-Corp. Translucent/0909211 Automix 4-META, UDMA, dimethacrylate, water, phosphoric ester monomers, initiators, stabilizers, aluminosilicate glass fillers 4 min
GCC G-Cem Capsule GC-Corp. Translucent/1001191 Capsule 4-META, UDMA, dimethacrylate, water, phosphoric ester monomers, initiators, stabilizers, aluminosilicate glass fillers 4 min
MCE Maxcem Elite Kerr Corp. Clear/3441545 Automix Methacrylate ester monomers, inert mineral fillers, ytterbium fluoride, activators and stabilizers >4 min
RXC RelyX Unicem Clicker™ 3M ESPE Translucent/403121 Paste-paste, hand mix Methacrylated phosphoric esters, dimethacrylates, acetate, initiators, stabilizers, glass fillers, silica, calcium hydroxide 6 min
RXM RelyX Unicem Maxicap™ 3M ESPE Translucent/398868 Capsulated Methacrylated phosphoric esters, dimethacrylates, acetate, initiators, stabilizers, glass fillers, silica, calcium hydroxide 6 min
RX2 RelyX Unicem 2 3M ESPE Translucent/430451 Automix Methacrylated phosphoric esters, dimethacrylates, acetate, initiators, stabilizers, glass fillers, silica, calcium hydroxide 6 min
SPC Speed-Cem Ivoclar Vivadent Transparent/N16128 Automix Dimethacrylates, methacrylated phosphoric esters, barium glass, ytterbium trifluoride, copolymers, high dispersed silica, initiators, catalysts and stabilizers 6 min
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Nov 28, 2017 | Posted by in Dental Materials | Comments Off on pH neutralization and influence on mechanical strength in self-adhesive resin luting agents
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