BackgroundRepair of composite restorations is a conservative method that can increase the longevity and durability of restorations while preserving the tooth structure. Achieving a suitable bond between the old and new composite is difficult. To overcome this problem, some methods have been recommended to increase the repair bond strength of composite.This study aimed to assess the effect of aging by thermocycling (5,000 and 10,000 cycles) and mechanical surface treatments (Er,Cr:YSGG laser and bur) on repair shear bond strength of composite resin.Material and MethodsTotally, 120 composite blocks measuring 6x4x4 mm were fabricated of Filtek Z250 composite and were randomly divided into three groups (n=40) based on initial aging protocol: (a) no aging: storage in distilled water at 37°C for 24 hours, (b) 5,000 thermal cycles, (c) 10,000 thermal cycles. Each group was then randomly divided into two subgroups (n=20) based on mechanical surface treatment (laser and bur). The laser and bur-prepared surfaces were silanized and Adper Single Bond 2 was then applied. The repair composite was bonded to surfaces. Half of the samples in each subgroup (n=10) were subjected to 5,000 thermal cycles to assess durability of bond. The remaining half were stored in distilled water at 37°C for 24 hours and all samples were then subjected to shear bond strength testing in a universal testing machine with a crosshead speed of 1mm/min. Data (in megapascals) were subjected to one-way ANOVA and Tukey’s test (P=0.05). Mode of failure was determined under a stereomicroscope.ResultsBur preparation significantly improved the bond strength compared to laser (P<0.001). Aging by 10,000 thermal cycles significantly decreased the repair bond strength of composite (P<0.001). No significant difference was noted in this regard between distilled water and 5,000 thermal cycles groups (P=0.699). Primary bond strength and bond strength after 5,000 thermal cycles in the same subgroups were not significantly different either (P=0.342).ConclusionsAging by 10,000 thermal cycles significantly decreases the repair bond strength of composite and surface preparation by bur provides a higher bond strength compared to laser. Key words:Thermocycling, Composite, Repair, Laser.