Influence of firing cycles on the margin distortion of 3 all-ceramic crown systems

Statement of problem. Although all-ceramic restorations arc widely used, there is a lack of information about how the fit is affected by fabrication procedures. The adequacy of the fit of all-ceramic restorations has been questioned. Purpose. This study examined the effect of porcelain and glaze firing cycles on the fit of 3 types of all-ceramic crowns. Material and methods. Ten standardized all-ceramic crowns were fabricated on a metal die from each of 3 systems: conventional In-Ceram, copy-milled In-Ceram, and copy-milled feldspathic crowns. Copings of the conventional and copy-milled In-Ceram crowns and nonglazed copy-milled feldspathic crowns served as the control. A device was used to apply a uniform load on specimens during measurement and to reposition the specimens on the measurement device after each manufacturing process. The specimens were not cemented and were measured on the metal die using a profile projector. Measurements were recorded at 18 points selected along horizontal and vertical planes. The crown systems were compared by use of the Student t test and 1-way analysis of variance (ANOVA). Data of measurements repeated at identical locations were analyzed with a multivariate repeated-measures ANOVA. The Bonferroni post hoc test was used for multiple comparisons (alpha=.05). Results. The conventional In-Ceram (57 +/- 24 mu m) and copy-milled In-Ceram (57 +/- 32 mu m) crowns demonstrated nearly identical marginal discrepancy values, followed by the copy-milled feldspathic crowns with a mean of 17 +/- 12 Am in the vertical plane. The copy-milled In-Ceram crowns had a mean horizontal discrepancy value of -12 +/- 4 mu m, followed by the copy-milled feldspathic crowns with a mean of -4 +/- 5 mu m and the conventional In-Ceram crowns with a mean of -6 +/- 4 mu m. Statistical analyses demonstrated no significant differences in the marginal discrepancy values among the 3 all-ceramic crown systems, except for the horizontal discrepancy values between the conventional and copy-milled In-Ceram crowns after the porcelain firing cycle. Results indicated that the addition of porcelain to the copings caused a significant change (P <.05) in the marginal fit of the crowns, except for the fit in the horizontal plane of the conventional In-Ceram crowns. However, no significant changes occurred in the fit of the 3 all-ceramic crowns after the glaze firing cycle. There were significant differences in the marginal discrepancy values among the measurement locations (P <.05), and the discrepancy value at each location was independent of the mean of the entire crown. Conclusions. Within the limitations of this study, it was concluded that the 3 all-ceramic crown systems demonstrated a comparable and acceptable marginal fit. The porcelain firing cycle affected the marginal fit of the all-ceramic crowns. However, the glaze firing cycle had no significant effect on fit. The conventional and copy-milled In-Ceram crowns demonstrated medial deformations at the labial and palatal Surfaces that might result in occlusal displacement of the crown.