EFFECT OF MARGIN DESIGNS AND PORCELAIN VENEERING ON ACCURACY OF BI LAYERED ZIRCONIA CROWN

Statement of problem: Degree of marginal discrepancy determines effectively success and failure of fixed restorations.
Purpose: To evaluate the effect of porcelain veneering techniques on the marginal fit of bi- layered zirconia crown using four marginal designs.
Materials and Methods: A 40 Plaster replicas were made from two master dies and divided into two main groups (20 for each) according to type of finish line. Each group subdivided into two subgroups (10 for each) to obtain the following groups {A1} shoulder without cervical collar {A2} shoulder with 1 mm cervical collar. {B1} chamfer without cervical collar {B2} chamfer with 1 mm cervical collar. Zirconia blanks were milled using CAD/CAM system to produce 40 zirconia copings. Then, Veneering of the copings was done. Vertical gap distances were measured at 18 predetermined points.
Results: Veneering with porcelain exhibited statistical significant difference in mean values of marginal gap in both group {A1} and {B1}, while veneering with porcelain did not exhibit significant difference in both group {A2} and {B2} in comparison with the pre veneering state. Correlations based on the finish line design exhibited significant difference in mean values of marginal gap between all tested groups. Correlations based on presence or absence of cervical zirconia collar did not exhibited statistical significant difference between tested groups. On other hand, there were significant differences in mean values of marginal gap between tested groups in post veneering state, at the level of P value < 0.05.
Conclusions: Marginal fit of the final crowns with chamfer finish lines were better than that with shoulder, however, veneering with porcelain significantly raised marginal gap distance of crowns with collarless shoulder and chamfer preparations, while in that designs with cervical collar, veneering was not significantly affected on the marginal gap distance for both finish line designs