Optimizing Interfacial Bonding of Additive Manufactured Resin via Combined Surface Treatments: An In Vitro Study on...

Fixed lingual retainers fabricated from 3D-printed resin (3DR) offer the benefits of geometric freedom; however, conventional approaches frequently rely on encapsulation or oversized retention pads to compensate for limited adhesion, increasing bulk and undermining these advantages. This study evaluated an interface-only bonding protocol using combinations of mechanical and chemical surface treatments for 3DR. Specimens were assigned to five groups: control; airborne-particle abrasion (APA) followed by universal adhesive (UA); APA followed by resin-matrix-penetrating primer (RMPP); non-thermal handheld plasma (NTHP) followed by UA; and NTHP followed by RMPP. Surface roughness, wettability, and chemical composition were characterized using contact profilometry, water-contact angle measurements, and attenuated total reflectance Fourier-transform infrared spectroscopy, respectively. Shear bond strength (SBS) and failure modes were evaluated before and after 10,000 thermal cycles. APA-containing protocols significantly increased surface roughness, whereas NTHP-containing protocols improved wettability without detectable topographical changes. Distinct chemical signals associated with urethane dimethacrylate and 10-methacryloyloxydecyl dihydrogen phosphate were identified in the RMPP- and UA-treated groups, respectively. Pre-aging SBS was comparable among groups, whereas APA followed by RMPP produced the highest post-aging SBS, and UA-containing groups showed the lowest values. These findings suggest that the combination of APA and RMPP improves resistance to in vitro hydrothermal degradation, although intraoral durability of interface-only 3DR retainers requires further clinical validation.