Objectives . Polymerization contraction of bonded dental composite produces stress in the surrounding substrate. A simple method to calculate the local contraction stress using indentation crack extension was reported earlier . The objective of this study was to calculate the composite contraction stresses remote from and at the bonded interface using this method.
Materials and methods . Initial cracks were made adjacent to the perimeter of a hole (3.0-mm diameter, 2.0-mm depth) in a soda-lime glass ( K c = 0.61 MPa m 0.5 ) using a Vickers indenter at a force of 9.8 N for 15 s. The indentations were located at distances ( h ) of 200, 300, 400 or 500 μm from the edge of the hole. The lengths of the cracks running parallel to the edge of the hole were measured at 500× before composite filling. Adhesives and restorative composites used for the filling were Clearfil SE Bond Primer, Clearfil Porcelain Bond Activator (Kuraray Medical), Point 4 (Kerr) and Heliomolar (Ivoclar Vivadent). A single increment of the composite was irradiated with the same radiant exposure (24,300 mJ/cm 2 ) in one-step or two-steps. The c-factor (bonded/non-bonded area) of the hole was 1.3. The crack lengths were re-measured at 2 and 10 min after light irradiation. Contraction stresses around the crack ( σ crack ) were calculated and evaluated using four-way ANOVA and regression analysis. In addition, stresses at the bonded interface ( σ interface ) were estimated with the corresponding regression equations.
Results . Means of σ crack and estimated σ interface in MPa.
| Material | Point 4 | Heliomolar | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Time | 2 min | 10 min | 2 min | 10 min | |||||
| Irrad step | 1-step | 2-step | 1-step | 2-step | 1-step | 2-step | 1-step | 2-step | |
| h | 500 | 5.5 | 3.6 | N/A | 4.4 | 2.9 | 1.9 | 3.7 | 3.0 |
| 400 | 7.2 | 4.2 | N/A | 5.2 | 3.8 | 1.9 | 4.6 | 3.0 | |
| 300 | 7.9 | 6.0 | N/A | 7.9 | 4.5 | 3.1 | 5.3 | 4.6 | |
| 200 | 10.3 | 6.3 | N/A | 8.7 | 5.3 | 3.6 | 6.7 | 5.5 | |
| σ interface | 12.5 | 8.2 | N/A | 11.5 | 6.7 | 4.6 | 8.2 | 6.9 | |
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