Microbial Biofilm Reduces the Strength Reliability of 3D-Printed Appliance Resins

Background/Objectives: Three-dimensionally (3D) printed resins are increasingly used for removable orthodontic appliances, but how their mechanical reliability changes in the oral environment is unclear. This study evaluated the effect of bio-ageing environment and material type on the biaxial flexural strength and Weibull reliability of one conventional acrylic resin (Orthocryl, OC) and two 3D-printed photopolymer resins (BioMed Clear, BMC; and KeySplint Hard, KH). The Weibull modulus (m) reflects how consistent, or predictable, the strength is among specimens, independent of its average value. Methods: Disc specimens (n = 32) were aged for 24 h at 37 °C in pooled human saliva, a Streptococcus mutans biofilm, or water, and the biaxial flexural strength was measured by the piston-on-three-balls method (1 mm/min). Data were analysed with two- and three-parameter Weibull statistics to estimate the Weibull modulus (m), characteristic strength (σ0), strength at 5% failure probability (σ5%), and threshold strength (σμ), with model selection guided by the Akaike Information Criterion. Results: Biofilm exposure lowered m while σ0 was preserved: m fell from 9.96 (saliva) and 7.98 (water) to 1.95 for OC, and from 17.61 and 13.01 to 2.62 for BMC, whereas σ0 stayed near 55 MPa (OC) and 64 MPa (BMC). Under biofilm the modulus of all three converged to low values (1.4–2.6), abolishing the differences among materials; OC and BMC required a three-parameter model only after biofilm (σu ≈ 41 MPa), whereas KH required it in every environment (σu = 28.9–38.7 MPa). Conclusions: Biofilm ageing mainly reduced strength predictability, while characteristic strength was largely preserved. This indicates that flexural strength alone may be insufficient to qualify resins for removable appliances, and that printed resins studied here are not inherently superior or inferior to conventional acrylic.