Accuracy of Resin-Based Crowns Fabricated by a Hybrid CAD-CAM Manufacturing Technique Compared to Subtractive and Additive Methods.

Statement of problem: To improve the surface finish of additively manufactured resin-based crowns and ease the fabrication process, a hybrid technique combining subtractive and additive manufacturing methods was recently investigated, where the restoration is milled with a chairside milling machine from an additively manufactured block. However, the accuracy of crowns fabricated by using this technique compared to other CAM techniques is unknown.

Purpose: The purpose of this in vitro study was to evaluate the trueness, precision, and intaglio fit of resin-based crowns fabricated by using a hybrid CAD-CAM manufacturing technique compared to subtractive and additive methods.

Materials and methods: A typodont maxillary first molar was prepared for a full coverage crown, scanned, and designed using the exocad software program. The reference design was used to fabricate resin-based crowns with 3 different techniques (N = 30): group S, subtractive manufacturing; group A, additive manufacturing, and group H, hybrid manufacturing. All the fabricated crowns were scanned, providing the test data as standard tessellation language (STL) files, which were imported with the reference data into a reverse engineering software program to evaluate the trueness and precision of each technique. To evaluate the intaglio fit, the triple scan method was used. One-way analysis of variance (ANOVA) and 2-way mixed model ANOVA tests were used to analyze the data (α = 0.05).

Results: For 3D trueness, root mean square (RMS) values representing the deviations between the scans of the fabricated crowns and the reference design varied significantly among different groups (p < 0.001), with the highest mean deviation found in group A (98.13 ± 2.33 μm), while the lowest mean deviation was in group H (60.68 ± 6 μm). For precision, significant deviations were found among test groups (p < 0.001), with the highest deviation in group H (61.76 ± 4.4 μm), while the lowest deviation was found in group A (17.88 ± 3.59 μm). For the intaglio fit, there was a significant difference among deviations measured at different areas in group S (p = 0.005), with the highest deviation measured occlusally (152.2 ± 5.3 μm). However, there was no significant difference among different areas of measurement in groups A (p = 0.818) and H (p = 0.192).

Conclusion: Resin-based crowns fabricated by a hybrid CAD-CAM manufacturing technique showed acceptable 3D trueness and fit compared to those fabricated solely by using a subtractive or an additive technique; however, the precision of the technique needs additional control over the variability introduced by the sequential nature of its process.

Clinical significance: In clinical practice, the ability to use additively manufactured blocks for the hybrid manufacturing technique can enhance the fabrication of accurate restorations by addressing the limitations of subtractive and additive manufacturing methods.