Testing condition during mechanical cycling: Does it matter and how does it compare to thermocycled and uncycled composite restoration?

Abstract

This study aimed to evaluate the μTBS and failure mode of resin composite restorations bonded to dentin and aged through different testing conditions during mechanical cycling in comparison to thermocycled and uncycled conditions. Fifty sound third molars were sectioned, restored with resin composite (Filtek Z350 XT, 3M/Solventum), and embedded in polyester resin and low viscosity vinyl polysiloxane (Panasil, Ultradent Products). Fifty teeth were allocated into the following five groups (n = 10): MCDI - mechanically cycled in distilled water, MCAS - mechanically cycled in artificial saliva, MCDC - mechanically cycled under dry conditions, TC - thermocycled and CO - control (left uncycled). Mechanical cycling was performed using 500,000 fatigue cycles under a 130 N load at 2 Hz in an electromechanical testing machine (MSFM, ElQuip), while thermal cycling (MCT, Elquip) was conducted for 5000 times. All samples were then sectioned into beams (1 mm²) and subjected to μTBS testing in a universal testing machine (EZ Test, Shimadzu). Subsequently, failure modes were evaluated under a stereomicroscope and classified as adhesive (AD), cohesive within dentin (CD), cohesive within resin (CR), or mixed (MX). Data were analyzed using one-way analysis of variance, Tukey's test and chi-square test (α = 0.05). MCDI, MCAS and MCDC groups did not differ in μTBS, but showed significantly lower values than CO group (p = 0.009). TC did not differ from any of the other four groups. Groups differed in failure mode (p = 0.004), with AD ruptures being predominant in the groups MCDI and MCAS. MCDC group had significantly lower frequency of AD failures. CR failures was the most common type failure in the TC group. In the MCDI group, the proportion of CR failure was 2.5 times higher than CD failure, but reversed in the MCAS group. It was concluded that as far as the occurrence of aging of resin composite-dentin interface is a goal, regardless of the testing condition, mechanical cycling would be preferable over thermal cycling, as the latter did not cause discernible reduction in bond strength in comparison to uncycled counterparts.