Effects of Relining Materials and Surface Treatments on Shear Bond Strength of 3D-Printed Dentures

Abstract This study aimed to evaluate the effects of relining material type and surface treatment protocol on shear bond strength within conventional heat-polymerized polymethyl methacrylate (PMMA), stereolithography (SLA), and digital light processing (DLP) denture bases. Denture base specimens were fabricated from conventional heat-polymerized PMMA (n = 96), SLA resin (n = 128), and DLP resin (n = 128), in accordance with ISO 9653:1998. The number of specimens differed because the conventional group included three relining material subgroups, whereas the SLA and DLP groups each included an additional subgroup relined with the corresponding three-dimensional (3D)-printed resin. Following 5,000 thermocycles, the specimens were assigned to one of four surface treatment groups: untreated control, air-particle abrasion, monomer treatment, or a combination treatment. Relining was carried out using auto-polymerizing PMMA (Unifast Trad Pink), PEMA (polyethyl methacrylate)-based materials (Kooliner, Tokuyama Rebase II), and the corresponding 3D-printed resin. Each subgroup consisted of eight specimens (n = 8). Shear bond strength was determined using a universal testing machine, and the failure modes were evaluated using a stereomicroscope. Data were analyzed using two-way ANOVA (analysis of variance) and Tukey's post hoc test (α = 0.05). Surface treatment, relining material, and their interaction significantly influenced shear bond strength in all groups (p < 0.05). Air-particle abrasion generally produced the highest bond strength, whereas the untreated control showed the lowest values. The highest values were observed with Unifast Trad Pink in the conventional group and with the corresponding printed resins in the SLA and DLP groups. Mixed failure predominated, and scanning electron microscopy showed greater surface alteration after air-particle abrasion and combination treatment compared with the untreated control. Air-particle abrasion generally produced the highest bond strength, particularly in 3D-printed denture base groups. The corresponding printed resins performed best in the SLA and DLP groups, whereas the PMMA-based relining material performed best in the conventional group.