Chairside Repair of Provisional Restorative Materials: Effects of Material Type, Repair Protocol, and Thermocycling on Shear Bond Strength

(1) Background: Repairing fractured provisional restorations may avoid refabrication, but success depends on substrate chemistry, repair protocol, and aging. This study evaluated shear bond strength (SBS) and failure behavior after chairside repair of PEMA-, PMMA-, and bis-acryl-based provisional materials. (2) Methods: In total, 380 specimens were prepared. Repairs used the substrate material, a dedicated repair system, or flowable composite without pretreatment or after a methacrylate repair primer, an MMA/Bis-GMA primer, or a multifunctional methacrylate coating. Half underwent 5000 thermocycles (5–55 °C). SBS was determined using a notched-edge test based on ISO 29022:2013, and failure modes were evaluated. An HC3-robust three-factor model was followed by Holm-adjusted Welch comparisons. (3) Results: A material × protocol × aging interaction occurred (p < 0.001). Homologous repair yielded 17.88–20.35 MPa for PEMA and 25.09–25.30 MPa for PMMA. Pretreatment improved composite repair across substrates. Pre-test failures affected 40.0% of PEMA and 13.3% of PMMA specimens but no bis-acryl specimens. Unprimed flowable composite produced complete pre-test failure with PEMA, low SBS with PMMA, and moderate SBS with bis-acryl. Thermocycling effects were protocol-dependent. (4) Conclusions: Conventional acrylics should be repaired homologously or after chemical conditioning, whereas flowable composite alone may be suitable for minor, non-load-bearing bis-acryl corrections.