The aim of this study is to determine the influence of the C-factor and the mode of polymerization on the cohesive strength of various dual-cure resin cements.
Three curing conditions were tested; chemical curing with free shrinkage conditions ( C = 0), and constraint shrinkage conditions ( C = 25), and dual-curing with free shrinkage conditions ( C = 0). Opaque polyethylene, brass (pretreated with Clearfil SE bond), and transparent polyethylene tubes respectively, were filled with the different cements. The tubes were 20 mm long with an inner diameter of 1.6 or 1.8 mm. Five cements, DC Core Automix, Panavia F 2.0, Maxcem, Multilink, and RelyX Unicem, were tested with ten specimens per group. The specimens were trimmed to an hour-glass shape with a neck diameter of 1 mm, stored in water (37 °C, 24 h), and subjected to microtensile testing (1 mm min −1 ). SEM analysis was carried out on chemically cured samples of DC Core Automix C = 0 and C = 25. Data were statistically analyzed (Two-way ANOVA, Tukey’s post hoc test, p < 0.05).
Most cements showed no significant differences between the curing modes. A high C-factor negatively influences the cohesive strength of some cements. SEM analysis shows that chemical curing of DC Core Automix in a high C-factor environment leads to more and larger microvoids in the cement.
Constraint shrinkage conditions, i.e. a high C-factor, can negatively influence the physical properties of a dual-cured resin cement, which would clinically be the case in the confined space of a root canal or post space preparation.
The ideal luting cement has to meet high standards. An important requirement is an adequate cohesive strength of the cement, which first and foremost depends on its composition . However, it is often overlooked that clinically not always the maximum strength that any given cement can achieve will be achieved . For instance, the degree of conversion and the mode of polymerization, i.e. chemical, light, or dual-curing, of a material can affect its mechanical properties . It has been shown that dual-curing of dual-cured resin cements results in better mechanical properties and higher degrees of conversion than the same cements, which are solely chemically cured.
At the same time, the polymerization shrinkage causes contraction stresses, which can result in a tensile pre-load of the cement, the bond, and the adjacent structures, which could potentially reduce the functional strength. The magnitude of the resulting stresses highly depends on the C-factor, which is defined as the ratio between bonded and unbonded surface area of the cavity in which the material sets . The material is assumed to shrink toward the fixed surfaces, i.e. cavity walls. As long as the setting reaction of the material allows this, the shrinkage stresses can be, partially, compensated for by viscous flow from the free surface areas of the cavity . Contraction stresses tend to be higher in cavities with a higher C-factor, in other words with a higher relative amount of bonded surface area . Examples of this are deep and narrow cavities and cavities with narrow access openings, for instance deep Class I and V cavities and endodontic post spaces, which in many ways can be regarded as deep Class I cavities.
From clinical studies it is known that debonding is the most common mode of failure with fiber posts, especially when little natural tooth tissue remains . These studies indicate that bond strengths between the root canal dentin and the post are not very high inside intact canals. The C-factor in the post space preparation has been calculated by Bouillaguet and Jongsma and may reach values in excess of 200, which will result in high contraction stresses, and may have a detrimental effect on the integrity of the bond to the root canal walls. It has been shown that in constraint conditions, contraction stress values can exceed the bond strength or even the cohesive strength of the resin composite itself. Even if an apparently adequate bond is maintained after curing, it is questionable whether the cohesive strength of the cement is not compromised compared to a free shrinkage situation.
Clinical failure is reduced significantly when sufficient coronal dentin is available, because the restoration does not rely as much on the bond between post and root dentin . Apart from the deteriorating effect on bond strength, it would be interesting to know if the high C-factor inside the root canal has an effect on the cohesive strength of the cement.
The aim of this study is to determine by microtensile testing whether there is a difference in cohesive strength between dual-cured cements that are chemically cured or dual-cured within a constraint shrinkage situation with a C-factor of 25, or a free shrinkage situation with a C-factor of 0. The term cohesive strength is used instead of microtensile strength because the term microtensile strength could easily be mistaken for the bond strength. The hypothesis is that dual-curing of cements will result in higher cohesive strengths than self-curing, and that a high C-factor will have a negative effect on cohesive strength.
Materials and methods
In this study the cohesive strength of DC Core Automix, Panavia F 2.0, Maxcem, Multilink, and Rely X Unicem was evaluated ( Table 1 ). Three curing conditions were tested; chemical curing with free ( C = 0) and constraint ( C = 25) shrinkage conditions, and dual-curing with free shrinkage conditions ( C = 0).
|Clearfil DC Core Automix
|Kuraray||00052A||Bis-GMA, TEGDMA, silanated colloidal silica, barium glass, N,N-diethanol p-toluidine|
|Clearfil DC Core Automix
|Kuraray||00052A||Bis-GMA, TEGDMA, silanated colloidal silica, barium glass, d , l -camphorquinone, benzoyl peroxide|
|RelyX Unicem Aplicap
|Glass fillers, silica, calcium hydroxide, self-cure initiators, pigments, light-cure initiators|
|RelyX Unicem Aplicap
|Methacrylated phosphoric esters, dimethacrylates, acetate, stabilisers, self-cure initiators|
|Panavia F 2.0
|Kuraray||00308A||Hydrophobic aromatic dimethacrylate, hydrophobic aliphatic dimethacrylate, hydrophilic dimethacrylate, sodium aromatic sulfinate (TPBSS), N, N-diethanol-p-toluidine, surface treated (functionalized) sodium fluoride, silanated barium glass|
|Panavia F 2.0
|Kuraray||0052B||MDP, hydrophobic aromatic dimethacrylate, hydrophobic aliphatic dimethacrylate, hydrophilic dimethacrylate, silanated silica, photoinitiator, dibenzoylperoxide|
|Kerr||2895677||Uretanedimethacrylate, camphorquinone photo-initiator, Fluoroaluminosilicate glass filler, fumed silica filler, stabilizer|
|Kerr||2895677||Bis-GMA, TEGDMA, glyceroldimethacrylate dihydrogen phosphate, self-cure redox initiatior, barium glass filler, stabilizer|
|Ivoclar Vivadent||K49940||HEMA, dimethacrylates, barium glass fillers, ytterbium trifluoride, silicon dioxide fillers, catalysts and stabilizers, pigments, t-amine|
|Ivoclar Vivadent||K49940||HEMA, dimethacrylates, barium glass fillers, ytterbium trifluoride, silicon dioxide fillers, catalysts and stabilizers, pigments, dibenzoyl peroxide|
|Clearfil SE bond bonding agent||Kuraray||01166A||MDP, HEMA, hydrophobic dimethacrylates, microfiller, photoinitiator, accelerator|
The materials were cured in 20 mm long tubes. Free shrinkage conditions with C = 0 were created with polyethylene (PE) tubes with an inner diameter of 1.8 mm. In the absence of a bond between the cements and PE, a C-factor of 0 was achieved. Opaque tubes were used for the chemically cured, and transparent tubes for the dual-cured specimens. Constraint shrinkage with C = 25 specimens were created in brass tubes with an inner diameter of 1.6 mm and sufficient rigidity to withstand the shrinkage stresses. The specimen dimensions were similar to a post space preparation. In order to bond the tested cements to the brass tube walls, these were pretreated with Clearfil SE Bond bonding agent (Kuraray medical Inc., Okoyama, Japan), without the use of alloy primer, and according to manufacturers’ instructions, which was light-cured from both ends for 20s (Elipar Freelight 2, 3M ESPE, Seefeld, Germany).
The cements were mixed according to manufacturers’ instructions, and applied with needle tubes (Accudose Needle Tubes, Centrix Inc. U.S.A.). The chemically cured specimens were stored in the dark. The dual-cured specimens were light-activated directly after filling by placing the light-tip over the center of the specimen and curing for 20 s. Subsequently, the adjacent sections with a length equal to the diameter of the light tip were irradiated for 20 s each. After two hours at 37 °C the specimens were trimmed to an hour-glass shape with a neck diameter of 1 mm (1.02 ± 0.11 mm) using a customized Proxxon MF 70 micro-mill (Conrad Electronic Benelux BV, Oldenzaal, Netherlands) The exact neck diameter of the hour-glass shape was measured with a digital laser scan micrometer (Mitutoyo LSM-503/6000 Mitutoyo Corp., Kawasaki, Japan). The polyethylene tubes were removed from the specimens. The specimens were kept at 37 °C in water for 24 h before testing.
The specimens were cleaned with ethanol, and ground flat on one side. The neck part of the hour-glass was not affected by this grinding. The specimens were attached to a microtensile testing device using Clearfil SE Bond bonding agent. The microtensile testing device was specially designed at ACTA to facilitate alignment of the specimens with the applied force during testing. The specimens were loaded until failure with a Hounsfield H109KM universal testing machine (Hounsfield, Redhill, UK) at a crosshead speed of 1 mm min −1 . The cohesive strength was calculated according to the following equation:
Cohesive strength ( MPa ) = F ( 1 / 4 ) π d 2