Bond strength comparison of chemically activated hard reline materials on CAD-CAM milled and conventional heat-polymerized PMMA denture bases

Statement of problem. Comprehensive data are needed on the performance of chemically activated, chairside hard reline materials when used with computer-aided design and computer-aided manufacturing (CAD-CAM) milled polymethyl methacrylate (PMMA) denture bases and conventionally processed bases. This lack of data affects decisions regarding the chairside reline material to be used for improving the fit and retention of relined complete dentures. Purpose. The purpose of this in vitro study was to evaluate and compare the shear bond strength (SBS) of 3 chemically activated, chairside hard reline materials on CAD-CAM milled and conventional heat-polymerized PMMA denture bases. Material and methods. A total of 120 cylindrical specimens were prepared and divided into 2 groups: 60 specimens fabricated from CAD-CAM milled PMMA and 60 specimens fabricated from conventional heat-polymerized PMMA. Following thermocycling to simulate aging, the specimens were divided into 3 subgroups (n=20) and relined with 1 of 3 chemically activated reline materials: Ufi Gel, Rebase II, and Unifast. After 24 hours of storage at 37 degrees C, the SBS was determined using a universal testing machine. Adhesive, cohesive, and mixed failures were examined using a stereomicroscope and scanning electron microscope (SEM). Statistical analysis was conducted using the generalized linear model (GLM) with post hoc comparisons using the Holm-Bonferroni correction. Chi-squared tests evaluated failure mode differences, and Weibull analysis determined bond strength reliability (alpha=.05). Results. Ufi Gel and Unifast exhibited significantly higher SBS on CAD-CAM milled PMMA than conventional heat-polymerized PMMA (P<.001). Rebase II also showed a significant difference between both denture base types (P=.001), but its overall SBS was lower. Ufi Gel achieved the highest SBS, while Rebase II had the lowest across both denture base types. Weibull analysis revealed that CAD-CAM milled PMMA with Ufi Gel had the highest bond strength and reliability. Conversely, conventional heat-polymerized PMMA with Rebase II had the weakest performance, marked by the lowest bond strength and increasing failure rates. SEM analysis showed more cohesive and mixed failures with CAD-CAM milled PMMA, while adhesive failures were more prevalent in conventional heat-polymerized PMMA. No significant difference in failure types was observed between the 2 denture base materials (P=.079). Conclusions. This study demonstrated that CAD-CAM milled PMMA provided better bond strength and reliability compared with conventionally processed PMMA, particularly when combined with Ufi Gel. Ufi Gel exhibited the highest bond strength and reliability, making it suitable for long-term clinical use. Unifast also performed well but had slightly lower bond strength, while Rebase II showed the weakest bond strength and more adhesive failures, indicating limited durability. These findings emphasize the importance of selecting effective reline materials to enhance denture retention, patient satisfaction, and longevity.