An In Vitro Analysis of Surface Treatment Effects on the Tensile Bond Strength Between Heat-Cured Denture Base and Heat-Polymerized Polymethyl Methacrylate Liner.

INTRODUCTION: Successful relining of complete dentures requires strong interfacial adhesion between the existing heat-cured polymethyl methacrylate (PMMA) denture base and the newly applied heat-polymerized PMMA liner to prevent delamination under functional stress. This in vitro investigation examined the efficacy of different surface conditioning methods in enhancing bond integrity and microstructural characteristics at the resin-resin interface.

MATERIALS AND METHODS: 48 rectangular specimens were prepared from heat-cured PMMA resin and subjected to standardized long-cycle polymerization. Each specimen was sectioned at the midline to expose a uniform bonding surface and randomly allocated to six experimental groups (n = 8): no treatment (control), methyl methacrylate monomer application, 36% phosphoric acid etching, 50 µm alumina particle sandblasting, erbium, chromium, yttrium-scandium-gallium-garnet (Er,Cr:YSGG) laser irradiation followed by phosphoric acid etching, and laser irradiation followed by alumina sandblasting. All procedures used calibrated equipment, digital timing, and custom positioning jigs to ensure reproducibility. The liner was applied using an identical resin from the same batch, trial-packed under controlled pressure, and polymerized using the same technique. The specimens were stored in distilled water for 30 days before testing. The tensile bond strength was determined by three-point bending using a universal testing machine. The selected samples underwent gold sputter coating and examination using field-emission scanning electron microscopy.

RESULTS: The tensile bond strength varied significantly across groups (p < 0.001). The untreated control recorded the lowest mean tensile strength at 0.92 ± 0.06 megapascals (MPa). Monomer application achieved the highest value of 1.89 ± 0.04 MPa, followed by laser plus phosphoric acid at 1.74 ± 0.03 MPa, laser plus sandblasting at 1.41 ± 0.04 MPa, sandblasting alone at 1.24 ± 0.03 MPa, and phosphoric acid alone at 1.15 ± 0.02 MPa. Post-hoc comparisons confirmed the statistical superiority of the monomer group over all other modalities, and positioned the laser-phosphoric acid combination as the next most effective approach. Scanning electron microscopy revealed smooth, non-retentive control surfaces with adhesive failure, whereas monomer-treated interfaces displayed polymer interdiffusion and cohesive failure. Laser-phosphoric acid samples exhibited deep micro-undercuts with extensive resin penetration, whereas sandblasting produced superficial grooves with incomplete infiltration.

CONCLUSION: Monomer application was found to be the most reliable and clinically practical method for maximizing liner adhesion through chemical interpenetration. The laser phosphoric acid protocol provides a viable alternative when advanced equipment is available, combining precise ablation with enhanced wettability.