Compressive Strength and Break Interface of Differently Laminated Composite Resin

Strength of laminated composite resins is controversial. Objectives: To compare compressive bond strength between differently laminated increments of composite resin and evaluate break interfaces using SEM. Materials: Z100 (3M/ESPE) was used as composite and ScotchBond-MultiPurpose (3M/ESPE) as adhesive. A prefabricated silicon putty mould with dimensions simulating a restoration on a molar tooth (3mm3) was used. Methods: Using this mould, 220 composite resin samples (20 in group1 and 40 in groups 2-6) were manufactured as follow: Group 1 - bulk packed (control); Group 2 – 90º laminated; Group 3 – 45º laminated; Group 4 - 30º laminated; Group 5 - 0º laminated; Group 6 - diagonally laminated. In 20 samples of each group (2-6) the composite was placed directly, and in 20 samples adhesive was used in between the layers. Each lamination increment was light cured for 40sec with a Coltolux 75-Coltene Whaledent curing light. Each layer of adhesive was LC for 10sec. Ten samples of each group were thermocycled for 250 cycles, between 5ºC and 55ºC with a dwell time of 5sec. Compressive strength of each sample was tested using a Zwick Z010 materials testing device at a crosshead speed of 0.5mm per minute. Samples were compressed to fracture on an irregular surface. Fracture surfaces of samples from all groups were examined using SEM. Results: Huge variation in the compressive break force within the different groups was found, with the mean varying between 368 +- 165 and 597 +-106 N. Conclusions: Statistical analysis using the student t-test indicated that the mean compressive break force values of the different groups were not significantly different (p>0.05) and no specific lamination, use of adhesive or thermocycling of samples had a specific effect on the compressive strength of laminated resin. It was not possible to distinguish clear break interfaces on lamination borders using SEM.