Effect of Various Acid Pretreatments on the Bonding Performance of Methyl Methacrylate-Based Resin Cement to Cobalt-Chromium Alloys

ObjectiveThree-dimensional (3D)-printed cobalt-chromium (Co-Cr) alloys are increasingly used in dentistry, but durable bonding to resin cements remains challenging.The purpose of this study was to evaluate the bonding performance of a methyl methacrylate (MMA)-based resin cement to additively manufactured and cast Co-Cr alloys after different acid surface treatments. Materials and methodsCo-Cr alloy specimens were fabricated by additive manufacturing or conventional casting, polished, and airborne-particle abraded.For each fabrication method, the specimens were assigned to five surface treatment groups: control (no acid treatment), S-Clean S-24 (SC), sulfuric acid (H₂SO₄), hydrochloric acid (HCl), and a hydrofluoric acid-containing porcelain etchant (BP).For shear bond strength (SBS) evaluation, each surface treatment group was further divided into four storage period subgroups: immediate, one day, seven days, and 28 days.An MMA-based resin cement was used for bonding.SBS was measured immediately after bonding and after one, seven, and 28 days of storage in deionized water at 37°C.Wettability was assessed by contact-angle measurements using distilled water and diiodomethane.Surface elemental composition and oxide-film thickness were evaluated by X-ray photoelectron spectroscopy (XPS).After SBS testing, failure modes were assessed under a stereomicroscope.Statistical analyses were performed using R Version 4.5.1 (R Foundation for Statistical Computing, Vienna, Austria).SBS data were analyzed using three-way analysis of variance (ANOVA), and contact-angle data were analyzed using two-way ANOVA, with Tukey's honestly significant difference used for post hoc comparisons.Statistical significance was set at p<0.05. ResultsSurface treatment significantly affected SBS.The H₂SO₄-and HCl-treated groups generally exhibited higher SBS than the other treatment groups during the first seven days of water storage; however, SBS decreased in all groups by 28 days, with no significant differences among surface treatments at that time point.XPS revealed oxide-film thicknesses of approximately 1.7-5.1 nm, with specimens analyzed 24 hours after acid treatment tending to have thicker oxide films than those analyzed immediately after treatment.Interfacial failure predominated in both fabrication groups, whereas mixed failure was observed in only a small number of specimens, and no cohesive failure within the resin cement was observed. ConclusionsH₂SO₄ and HCl surface treatments improved the initial bonding performance of MMA-based resin cement to cast and additively manufactured Co-Cr alloys.However, SBS decreased during water storage, and no significant differences among surface treatments remained after 28 days.These findings suggest that acid treatment can improve initial bonding but may be insufficient to ensure long-term bond durability.