Effect of Connector Size and Luting Protocols on the Fracture Resistance of 3D-Printed Resin-Based Fixed Dental Prostheses: An In Vitro Study

Objectives: Limited information is available on how connector size and luting protocols influence the fracture resistance of 3D-printed resin-based fixed dental prostheses (FDPs). This in vitro study evaluated the effect of connector size and luting agent type on fracture load. Methods: Eighty 3-unit posterior FDPs were 3D-printed (GC Temp PRINT, GC Corp.) and assigned to eight groups (n = 10) by connector size (GroupA 5 × 5 mm or Group B 3 × 3 mm) and luting protocol (1: no cement; 2: TempBond temporary cement; 3: Ketac Cem glass ionomer; 4: G-Cem One self-adhesive resin cement). Specimens were seated on standardized metal abutments and loaded to failure (Instron 5567, 1 mm/min). Data analyzed by Shapiro–Wilk normality test, Mann–Whitney U (connector size), ANOVA/Kruskal–Wallis (luting within size; α = 0.05). Results: Connector size significantly affected fracture resistance (Mann–Whitney U, p < 0.001): 5 × 5 mm groups showed ~3× higher loads (1468–1638 N) than 3 × 3 mm groups (266–384 N). In 5 × 5 mm groups, luting protocol had no significant effect (ANOVA, p > 0.05). In 3 × 3 mm groups, resin cement (343 N) and temporary cement (384 N) showed higher loads than no-cement controls (266 N; Kruskal–Wallis p = 0.022, exploratory U p < 0.05); glass ionomer showed no significant difference. Conclusions: Within the limitations of this in vitro study, larger connectors substantially increased 3D-printed FDP fracture resistance. Resin-based luting agents increased loads in smaller-connector FDPs.