Evaluation of adding different reline materials on conventional, 3D printed and milled denture base materials

The aim of this in vitro study is to evaluate and compare the tensile bond strength of soft and hard reline materials bonded to denture base resins fabricated by three different techniques: conventional heat polymerization, subtractive CAD-CAM milling, and additive 3D printing. A total of nine experimental groups were created by combining three denture base fabrication methods—conventional PMMA, milled PMMA, and 3D-printed resin—with three types of lining materials: a laboratory-processed silicone-based soft liner (Molloplast-B), a chairside silicone-based soft liner (GC Reline Soft II), and a chairside hard liner (Ufi Gel Hard C). Tensile bond strength was measured using a universal testing machine and results were recorded in megapascals (MPa). Failure modes were evaluated and classified as adhesive, cohesive, or mixed. Data were analyzed using SPSS 28.0 software. Depending on the data distribution, statistical comparisons were made using one-way ANOVA, Kruskal-Wallis, and appropriate post hoc tests with a significance threshold of p<0.05. The results showed statistically significant differences in tensile bond strength among the nine groups (p<0.001). The highest bond strength was observed in the Ufi Gel Hard C–conventional PMMA group (12.56 MPa), while the lowest was in the GC Reline Soft II–3D printed group (1.28 MPa). All combinations exceeded the minimum clinical threshold of 0.44 MPa. Laboratory-processed reline materials demonstrated superior bonding performance compared to chairside-applied materials. Failure modes varied by material and method: adhesive failures were predominant in 3D-printed groups, whereas cohesive or mixed failures were more common in conventionally and subtractively fabricated bases. In conclusion, both the lining material and the fabrication technique significantly affect the tensile bond strength. 3D-printed denture bases demonstrated the weakest performance, suggesting the need for optimized surface treatments to improve bonding in additive manufacturing applications.