Effect of Injection Guide Design on the Accuracy of Direct Composite Veneers: An In Vitro Study

OBJECTIVE: To evaluate the accuracy of direct composite veneers thickness using three different types of injection guides fabricated with varied materials and techniques. METHODS: A standard maxillary dentition model was scanned to generate a digital model, from which five preparation designs for tooth #11 with varying labial veneer thicknesses (uniform: 0.1, 0.3, 0.5, 0.7 mm; variable cervical-incisal: 0.3-0.5-0.7 mm) were designed using software. Three types of injection guides were fabricated: a 6 mm transparent silicone guide (Group S), a 1.5 mm rigid 3D-printed guide (Group R), and a 2 mm hybrid guide with soft inner and rigid outer layers (Group H). Groups R and H were further subdivided into three subgroups (CI, MS, SG) based on injection strategy, each with 15 specimens as well as for Group S (N = 15). Veneers were fabricated using flowable composite resin via the guides, then scanned and compared to the reference model via best-fit alignment using analysis software. Accuracy was evaluated by the mean RMS ± SD. Statistical analysis included two-way ANOVA and Tukey's HSD test (α = 0.05). RESULTS: The veneer thickness design had no significant effect on the mean RMS values (p > 0.05). Significant differences in mean RMS values were observed among guide types across different regions (p < 0.05). In the cervical third, Group S and H showed comparable mean RMS values, both significantly lower than those of Group R. All guides performed similarly in the middle third. In the incisal region, Group S exhibited the lowest mean RMS values (0.070 ± 0.016 mm), significantly surpassing most subgroups of Groups R and H. On proximal surfaces, the MS (0.181 ± 0.026 mm) and SG (0.193 ± 0.023 mm) subgroups of Group R demonstrated significantly lower mean RMS values than Group S. CONCLUSIONS: Accuracy of direct resin veneers was determined primarily by guide type and restoration site, not veneer thickness design. Clinically, guide selection should be based on defect location: silicone guides for labial surfaces and rigid guides with matrix slots or stamp-type designs for proximal surfaces. CLINICAL SIGNIFICANCE: This study provides clinically relevant evidence for optimizing injection guide selection in anterior esthetic restorations. By recommending specific guide types for direct composite veneers, this approach offers a more predictable and efficient method for achieving morphologically accurate veneers, reduces technical sensitivity, and may improve long-term esthetic outcomes.