The Effect of Die Spacer Thickness on Retentive Strength of all Zirconium Crowns (An In vitro Study)

Aim: The purpose of this study was to evaluate the effect of different die-spacer thicknesses on the retentive strength of all zirconium crowns using self-adhesive resin cement (Relyx Unicem). Materials and method: A total of 24 freshly extracted sound human maxillary first premolar teeth were mounted and then received a standardized preparation for full contour zirconia crown restorations with a deep chamfer finishing line, planner occlusal reduction, 6 degrees overall taper. The teeth were scanned directly using Sirona Omnicam digital intra-oral scanner. Preparations surface areas were measured using Auto CAD architecture software. Teeth were then randomly assigned into 3 main groups according to the die spacer thickness selected during the designing phase by in Lab SW 16.1 designing program (n=8): Group A: 80 mu m; Group B: 100 mu m and Group C: 120 mu m. Monolithic zirconia crowns were constructed using Sirona In-Lab MC X5 milling unit. Each group was cemented with Relyx Unicem self-adhesive luting cement. Statistical analyses: The data were statistically analyzed using 1-way ANOVA and Post-hoc Bonferroni test at 0.05 level of significance. Results: The statistical analysis showed that die spacer thickness had a significant effect on the crown retentive stress and the highest mean of retentive stresses was recorded for Group A (7.4 +/- 0.7 SD) while Group C recorded the lowest mean of retentive stresses (5.5 +/- 0.8 SD). In addition, the predominant mode of failure was type 3 (cement principally on ceramic). Conclusion: Within the limitations of this study, the use of 80 mu m spacer thickness resulted in significally higher retention values than 100 mu m and 120 mu m spacer. In contrast, there was no significant difference in retention between 100 mu m and 120 mu m.