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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