Microbial sealing and physicochemical degradation of composite resin seals in implant screw access channels: an in vitro study

The screw access channel (SAC) is a known vulnerability in screw-retained implant restorations. Despite its clinical importance, there is no established consensus on the optimal protocol for achieving a predictable seal. This study aimed to investigate the effects of restorative material category (high-viscosity, packable, and flowable), composite thickness, and adhesive strategy on microbial leakage, bond strength, and physicochemical stability within simulated zirconia SACs. A total of 192 zirconia specimens with standardized SACs were randomly assigned to 12 experimental protocols ( n = 16). The experimental variables included three composite resins with distinct handling characteristics (High-viscosity universal: Magnafill Putty [MP]; Packable: Filtek Bulk Fill [BF]; Flowable: Filtek Supreme XTE Flowable [XTF]), two restorative thicknesses (2 mm and 3 mm), and two bonding agents (Scotchbond Universal and G-Bond). Sealing performance was quantified via Streptococcus mutans microleakage and push-out bond strength (POBS) across independent non-aged and thermocycled subgroups (10,000 thermocycles, 5–55 °C, n = 8/subgroup). Hydrolytic stability was evaluated through water sorption and solubility. Data were analyzed using three-way ANOVA and Tukey post-hoc tests (α = 0.05). Microbial leakage was significantly influenced by the composite category and restorative thickness ( P < 0.05), whereas the choice of bonding agent exerted a negligible influence ( P > 0.05). The high-viscosity universal resin (MP) applied at a 3 mm thickness exhibited the lowest microbial infiltration ( P < 0.05). Furthermore, water sorption and solubility were primarily material-dependent, with the high-viscosity resin demonstrating superior resistance to hydrolytic degradation. Thermocycling significantly compromised the POBS across all groups (decreasing from a range of 7.64–10.65 MPa to 3.27–4.96 MPa, P < 0.05). The interfacial sealing of SACs is predominantly governed by the bulk physicochemical properties of the restorative material and its composite thickness, rather than the adhesive protocol. A 3 mm thickness of high-viscosity universal composite is suggested to improve resistance against microbial leakage.