Effect of margin thickness and cement gap on the fabrication and fit accuracy of additively manufactured crowns in definitive resins with different filler content.

PURPOSE

To evaluate how commercially available composite resins in varying filler content affect the fabrication and fit accuracy (trueness and precision) of additively manufactured (AM) definitive crowns with different margin thickness and cement gaps by comparing them to subtractively manufactured (SM) polymer-infiltrated ceramic crowns.

MATERIALS AND METHODS

Two identical unprepared mandibular right first molars were prepared by an experienced prosthodontist for crowns with either 0.5 mm or 1.0 mm chamfer margins, verified using silicone indexes and a periodontal probe, and then digitized with an industrial-grade scanner. These scan files were used to design reference crowns with either 50-µm or 80-µm cement gaps in standard tessellation language (STL) format. These reference STLs were used to manufacture a total of 112 crowns, 28 crowns for each crown margin thickness-cement gap pair, using three additively manufactured resins with different filler ratios (AM-20-40%, AM-50%, AM-60%) and one SM polymer-infiltrated ceramic network (n = 7). After fabrication, an intraoral scanner was used to obtain STL files of each crown, the prepared typodont teeth, and each crown seated on its corresponding typodont. All STLs were imported into a metrology-grade 3-dimensional (3D) analysis software program to quantify the surface deviations (overall, external, intaglio, and marginal) of the crowns using the root mean square method and their fit with the average gap measurements. Precision was defined as the average of how much each measurement differed from the overall mean. All data were analyzed using generalized linear model analysis and Bonferroni-corrected post-hoc tests (Minitab Software, Version 17) with a significance level set at α = 0.05.

RESULTS

The accuracy of external surface deviations and the precision of the average gaps were affected by the interaction among all main factors. The interaction between material type and margin thickness influenced the accuracy of intaglio surface deviations, while the interaction between material type and cement gap affected the accuracy of marginal surface deviations; both interactions impacted the average gaps. In addition, the accuracy of overall deviations was affected by the interaction between margin thickness and cement gap (p ≤ 0.040). AM-20-40% crowns mostly had higher overall accuracy and SM crowns mostly had higher external and intaglio surface accuracy (p ≤ 0.033). AM-20-40% crowns mostly had higher fit accuracy, while the crowns with 0.5 mm-thick margins or 50-µm cement gap had lower average gaps (p ≤ 0.044).

CONCLUSIONS

AM crowns in resin with low filler ratio (AM-20-40%) and SM crowns mostly had higher fabrication accuracy, which may enable fewer clinical adjustments and more reliable fabrication than AM crowns in resins with higher filler ratios (AM-50% and AM-60%). Only AM-20-40% and SM crowns with 0.5 mm-thick margins or 50-µm cement gap had average gap values similar to previously reported clinical thresholds.