Influence of Material Type and Specimen Geometry on Polymerization Shrinkage of Resin-Based Cements: A 3D-DIC Multifactorial Study

Background/Objectives: Polymerization shrinkage of resin-based cements critically affects the integrity and longevity of indirect dental restorations, yet the combined influence of material composition, specimen geometry, and measurement strategy remains insufficiently quantified. The aim of this study was to evaluate the individual and interaction effects of these parameters on shrinkage strain using a multifactorial approach. Methods: Shrinkage strain was evaluated using three-dimensional digital image correlation (3D-DIC) coupled with three-way ANOVA. Four commercially available dual-cured self-adhesive resin-based cements were analyzed in a 4 × 2 × 3 multifactorial design, with specimen thicknesses of 1 and 2 mm and specimen diameters of 3, 4 and 5 mm (n = 3 per group, 72 specimens). Strain was assessed in both circular sections at the material–mold interface and orthogonal linear sections. Results: Material type (p < 0.001) and its interaction with specimen thickness (p = 0.002) were identified as dominant factors governing strain, whereas specimen diameter showed no statistically significant effect (p = 0.460). Increasing thickness from 1 to 2 mm resulted in a rise in strain values (from 2.273% to 3.001%), with the highest strain observed for one of the cements at 2 mm thickness (4.107%). The circular section exhibited the highest sensitivity to thickness-dependent variations, while linear sections showed consistent but less pronounced trends. Post hoc analysis separated the cements into high-strain and low-strain subsets, and the grouping depended on the measurement section. Conclusions: Polymerization shrinkage is primarily governed by material properties and their interaction with layer thickness, rather than specimen size within the investigated range. A specimen thickness of 2 mm and circular section analysis are recommended for improved sensitivity and comparability in experimental studies. Clinically, minimizing cement layer thickness and selecting appropriate materials are essential for reducing shrinkage-induced stresses and improving the durability of dental restorations.