Impact of Actuator Shape and Polishing on Biaxial Flexural Strength

Objectives: Objectives: The objectives were to evaluate the effect of actuator shape (flat-piston or round) and disc preparation (finished or polished) on biaxial flexural strength (BFS) of a bulk fill composite.
Methods: Methods: Disk-shaped (14 mm in diameter; 1mm in height) resin composite (Filtek One Bulk Fill Restorative, 3M ESPE) specimens were prepared for BFS-testing according to manufacturer instructions, stored at 37^oC for 24h, and divided into groups according to actuator shape and surface polishing technique (n=13). The flat-piston was 1.4 mm in diameter, the round actuator 1.4 mm in radius. Finished specimens were treated using silicon carbide paper (600-grit); polished specimens were polished using a sequence of diamond pastes. BFS testing was performed using the biaxial flexural strength method recommended by ISO 6872:2014 at a crosshead speed of 1.0 mm/min. Data from BFS was analyzed using 2-way ANOVA followed by Tukey’s test (α=0.05). 3D Finite Element Analysis (FEA) was used to simulate the in vitro experimental setup and to predict the biaxial stress distribution on the composite disks. The two models were loaded using a 0.1 mm of displacement, constrained at the three balls and frictional contacts among ball/disk/actuator. The results were analyzed using maximal principal stress criteria.
Results: Results: No statistical significant difference was found regarding the finishing/polishing technique applied (p>0.05). The flat actuator resulted in higher values of BFS when compared to the round actuator (p<0.05). The FEA suggests that when the round actuator is used there is a higher probability of failure. When the flat actuator is used a higher force appears to be necessary to reach the critical stress of the resin composite.
Conclusions: Conclusions: BFS values obtained with a small diameter flat actuator are not comparable to BFS values obtained with a round actuator. Differences in degree of finishing and polishing were not discernable.