Two- and Three-Dimensional Accuracy of Tooth Reduction Depths in Guided Versus Conventional Veneer Preparation: An In Vitro Study

This study compares the two (2D)- and three-dimensional (3D) accuracy of tooth reduction depths in porcelain laminate veneer prepared using conventional and 3D-printed guide techniques. Forty 3D-printed maxillary casts were divided into four groups: freehand (FH) (n = 10), silicone guide (SG) (n = 10), cross-shaped 3D-printed guide (3D_C) (n = 10), and stackable 3D-printed guides (3D_S) (n = 10). Butt-joint veneer preparation was performed on the left central incisor. Two-dimensional analysis was performed to assess trueness using mean absolute differences (MADs) from the planned depth at eight designated points, while precision was compared within groups. Three-dimensional analysis evaluated trueness by superimposing post-preparation scans with reference casts and precision via intra-group superimposition, with deviation errors measured using the Root Mean Square (RMS) method. One-way ANOVA and Bonferroni post hoc tests were used (α = 0.05). In 2D analysis, 3D_S exhibited a significantly lower MAD than FH at most of the measured points (p < 0.05), more accurate incisal reduction at mesial and distal points compared to 3D_C (p < 0.001), and more accurate mesial (p = 0.011) and distal (p = 0.001) cervical margin preparation than SG. In the 3D trueness assessment, 3D_S exhibited significantly lower deviation errors than FH (p < 0.001) and SG (p = 0.012) while also achieving the highest overall 3D precision with the lowest RMS (0.067 ± 0.013), followed by 3D_C (0.086 ± 0.019). Veneer preparation guided by a stackable 3D-printed guide resulted in more accurate tooth reduction depths compared to the other three techniques.