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

To evaluate the accuracy of direct composite veneers thickness using three different types of injection guides fabricated with varied materials and techniques.

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).

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.

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.

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.