Influence of preparation angle on marginal and internal fit of CAD/CAM-fabricated zirconia crown copings

Objective: To evaluate the effect of different preparation angles on the marginal and internal fit of zirconia single-crown copings. Method and Materials: Three ivorine maxillary right first molars were prepared with preparation angles of 4, 8, and 12 degrees. Impressions were made of each abutment tooth to obtain 20 specimens per group. The copings were manufactured by a CAD/CAM system and milled from semi-sintered zirconia blanks. After sintering, 10 copings were randomly chosen from each group and adapted by a dental technician. All 60 copings were cemented with glass ionomer on the master casts. After embedding, all specimens were cross-sectioned. Both marginal and internal fit were evaluated under an optical microscope. A 1-way analysis of variance (ANOVA) and a nonparametric test (Mann-Whitney U) were used to compare data (α = .05). Results: The 4-degree copings showed a mean marginal gap of 91 µm (± 15) before and 67 µm (± 8) after adaptation; the 8-degree group exhibited a mean marginal gap of 82 µm (± 13) before and 67 µm (± 11) after adaptation; the 12-degree specimens showed 50 µm (± 6) before and 46 µm (± 6) after adaptation. Adaptation led to a significant improvement of specimens from the 4-degree and 8-degree groups, while no statistical difference could be observed among 12-degree specimens before and after adaptation. Conclusion: The highest marginal gaps were found in the 4- and 8-degree groups. In the group with 12-degree preparation angle, additional adaptation did not improve the fit and can be considered unnecessary under the conditions of this study. Keywords: CAD/CAM technology, crown, internal fit, marginal fit, preparation angle, zirconia