Influence of Different Alumina Particle Sizes and Artificial Aging on the Shear Bond Strength of Orthodontic Brackets to Milled and 3D-Printed Provisional Restorations

Background: Computer-aided diagnosis and computer-aided machining (CADCAM) dentistry have resulted in both additive (printing) and subtractive (milling) processes, with each having their own specific provisional restorative materials. Scarce studies investigate the shear bond strength (SBS) of orthodontic brackets for milled and 3D-printed provisional restorative materials. Purpose: This in vitro study ascertained the impact of three distinct alumina particle sizes (25 μm, 50 μm, and 100 μm) on the SBS between two distinct provisional crowns (milled and 3D printed) and stainless steel orthodontic brackets following the artificial aging of 2200 cycles, or 18 months of clinical time. Materials and methods: Eighty specimens [disc 10 mm diameter/15 mm height] were fabricated with two provisional crown materials: milled (CopraTemp) [group (GP) M] and three-dimensional printed (Asiga DentaTooth) (GP P) and divided into eight subgroups based on alumina oxide particle size surface treatments of 25 μm [P25, M25], 50 μm [P50, M50], and 100 μm [P100, M100], with no surface treatment specimens serving as control [PC, MC]. After thermocycling (2200 cycles), the SBS and Adhesive Remnant Index (ARI) were measured. Statistical analytic tests included one-way analysis of variance (ANOVA) using the Kruskal-Wallis test, followed by post hoc analytical tests [Tukey HSD, Dunn’s test], with the probability ‘p’ value being considered significant at a predefined value of less than 0.05. Results: Without surface treatment, the 3D-printed provisional crown had the lowest SBS [median (IQR); 12.8 (2.74)]. The highest SBS was found in both milled and 3D-printed PMs with 50-micron particle sizes [Milled = 23.10 (2.3); Printed = 20.72 (2.31)], followed by 100-micron [Milled = 20 (2.36); Printed = 17.99 (3.45)] and 25-micron [Milled = 16.13 (2.71); Printed = 15.08 (1.55)]. A majority of cohesive failures were seen in milled subgroups, while all subgroups of 3D-printed provisional material had adhesive bond failures. Conclusions: Sandblasting, irrespective of particle size, enhances SBS in both milled and 3D-printed provisional restorations; however, 50-micron alumina particles are recommended, since they enhance SBS substantially. 3D-printed provisional restorative materials leave more adhesive behind, which is clinically unfavorable.