Influence of temperature on volumetric shrinkage and contraction stress of dental composites.

OBJECTIVES: To test the influence of temperature on contraction stress and volumetric shrinkage of Clearfil AP-X, Venus Diamond, Premise and Filtek Z250. METHODS: Volumetric shrinkage measurements were carried out using mercury dilatometry, while a constraint tensilometer set-up was used for the measurement of contraction stress. Measurements were carried out with a composite temperature of 23, 30, 37, and 44°C. RESULTS: Volumetric shrinkage increases with higher temperature. Premise and Venus Diamond show lower volumetric shrinkage than Clearfil AP-X and Filtek Z250. Clearfil AP-X shows the highest contraction stress which slightly increases with higher temperatures. The other composites only show an increase in contraction stress between 23 and 30°C. SIGNIFICANCE: Heating of dental composites results in a higher volumetric shrinkage. However, the contraction stress does not change significantly due to increased temperature above 30°C.

Shear bond strength of three dual-cured resin cements to dentin analyzed by finite element analysis.

OBJECTIVES: To determine the shear bond strength to bovine dentin of dual-cured resin cements cured in different circumstances, the contraction stress and volumetric shrinkage in both polymerization modes, and to review the failure stress distribution at the cement-tooth interface with finite element analysis. METHODS: The volumetric shrinkage of RelyX Unicem, Panavia F 2.0 and DC Core Automix was determined by mercury dilatometry. Polymerization contraction stress was determined using a constraint tensilometer set-up. For the shear bond strength test, cement discs on bovine root dentin (self-cured and dual-cured), composite discs cemented to dentin (self-cured and dual-cured), and dentin cemented to dentin (self-cured) specimens were fabricated. Specimens were stored in water for 24h (37°C, 100% humidity) and tested (crosshead speed 1mmmin(-1)). FE modeling of the specimens was carried out in order to calculate the maximum shear stresses in the cement-dentin interface. Differences between groups were determined using two-way ANOVA with Tukey post hoc tests, and paired samples t-tests (α<0.05). RESULTS: Panavia F2.0 showed significantly lower volumetric shrinkage than the other cements. Dual-curing lead to higher contraction stresses for all tested cements compared to self-curing. RelyX Unicem showed higher volumetric shrinkage when dual-cured. Shear bond strength and maximum shear stress was positively influenced by dual-curing. DC Core Automix performed best and Panavia F2.0 worst in terms of shear bond strength and maximum shear stress. SIGNIFICANCE: Curing mode may play an important role in the final bond strength to dentin of indirect restorations, depending on the material used.

Influence of polymerization mode and C-factor on cohesive strength of dual-cured resin cements.

OBJECTIVES: 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. METHODS: 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 20mm long with an inner diameter of 1.6 or 1.8mm. 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 1mm, stored in water (37°C, 24h), and subjected to microtensile testing (1mmmin(-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). RESULTS: 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. SIGNIFICANCE: 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.

Reduced contraction stress formation obtained by a two-step cementation procedure for fiber posts.

OBJECTIVES: In a previous study, a 60% increase in push-out strength was obtained in vitro with a two-step cementation of fiber posts, a procedure equivalent to the layering technique of composite restorations. The aim of this study is to find the rationale for this increase in push-out strength with finite element analysis (FEA). METHODS: FEA models were created of the push-out test set-up of fiber posts cemented according to a one-step and two-step procedure and of the complete root with post. The failure loads of glass-fiber posts cemented with RelyX Unicem as obtained in a previous study were used as the load in the push-out FEA models. For the complete root model, a load of 100N was used. The stresses due to the shrinkage of the cement layer and the applied load were determined for the one-step and two-step procedure of the push-out test specimens and for the one-step procedure of the complete root. RESULTS: Even though the load in the two-step push-out model was 60% higher compared to the one-step model, the combined stresses were comparable. The stresses due to shrinkage alone in the complete root approached or exceeded the bond strength of resin cements to dentin in the coronal and apical areas. SIGNIFICANCE: FEA of this test set-up explains the results of the in vitro study. Two-step cementation of fiber posts leads to a decrease in internal stresses in the restoration which results in higher failure loads and possibly in less microleakage.

Influence of surface pretreatment of fiber posts on cement delamination.

OBJECTIVES: To evaluate the influence of post surface pretreatment on the delamination strength of different cements from a prefabricated FRC post tested in a three-point bending test. METHODS: Three cements were tested; RelyX Unicem, DC Core Automix, and Panavia F2.0. Per cement, 40 posts (D.T. Light Post Illusion size 3) were divided into four groups; no pretreatment (control), sandblasting, silanization, and sandblasting followed by silanization. A cement layer was applied to the posts using a standardized poly-oxy-methacrylate mold. The specimens were subjected to a three-point bending test recording the initial and catastrophic failure loads. Two-way ANOVA and Tukey post-hoc tests were used to analyze the differences between the variables. RESULTS: At the initial failure load, all specimens demonstrated delamination of the cement layer, therefore initial failure load was defined as delamination strength. With RelyX Unicem, none of the pretreatments showed significant differences. When using Panavia F2.0, silanization (735+/-51 MPa) resulted in higher initial failure values than sandblasting (600+/-118 MPa). When DC Core Automix was used, silanization (732+/-144 MPa) produced significantly higher initial failure values than the no pretreatment group (518+/-115 MPa) and the combined sandblasting and silanization group (560+/-223 MPa). Two failure types were observed; cohesive and adhesive failure. In the silanization groups, more cohesive failures were observed for all cements tested. SIGNIFICANCE: Especially when non self-adhesive cements are used, silanization of fiber posts has a beneficial effect on cement delamination strength and failure type.

Benefits of a two-step cementation procedure for prefabricated fiber posts.

PURPOSE: To determine whether two-step cementation of prefabricated fiber posts leads to higher bond strengths. MATERIALS AND METHODS: Forty-eight human canine teeth were divided into six groups and fiber posts were cemented with DC Core, RelyX Unicem and Panavia F2.0, according to a one-step or two-step procedure. Per root, four cross-sections were prepared. The differences in push-out strength between procedure, cement and location within the root were determined. RESULTS: For all cements, the push-out strengths for the two-step procedure were significantly higher than for the one-step procedure (p < 0.001). Differences between the cements for both the one-step and two-step procedure were not statistically significant (p = 0.05). CONCLUSION: The reduction of the C-factor by means of a two-step cementation, a procedure equivalent to the layering technique of composite restorations, resulted in significantly higher bond strengths. The increase of 60% in bond strength may be beneficial to the retention of post and core restorations.