Accuracy of double-layer and conventional injection guides for direct composite veneers across different restoration spans: an in vitro study

Abstract Background Direct composite resin veneers are widely used for esthetic restorations, but the accuracy of the injection guide is critical for successful clinical outcomes. This study compared the trueness and precision of a double-layer (DL) injection guide and a conventional polyvinyl siloxane (PVS) guide across different restoration spans. Methods Twelve independently fabricated specimens were evaluated with each guide type at each 2-, 4-, 6-, and 8-tooth span. Trueness was assessed from the root mean square (RMS) deviation of the restored labial surfaces from the original CAD wax-up after registration on nonrestored arch surfaces. Precision was assessed using RMS deviations for all 66 unique specimen pairs within each group over the same labial region. Trueness was analyzed using span-specific Welch t tests with Bonferroni correction and within-guide Welch ANOVA with Games-Howell comparisons; precision was summarized descriptively. Results Mean specimen-to-CAD RMS values were 107.0–121.5 μm with DL and 170.6–194.6 μm with PVS. The DL values were lower at all four spans in both the full restoration and shared central-incisor regions (all adjusted p < 0.001). For both guides, the 2-tooth span had lower RMS than the longer spans; within DL, the 4- and 8-tooth spans also differed ( p = 0.007). Mean within-group precision RMS values were 119.2–124.3 μm with DL and 125.3–133.7 μm with PVS. Conclusions The DL guide achieved higher trueness than the PVS guide at all tested restoration spans. The DL groups also showed numerically lower RMS in the within-group precision assessment at every span.