Objective: The aim of this study was to investigate the effect of repressing IPS e.max Press with different concentrations on the marginal gap and fracture strength of ceramic crowns.
Materials and methods: Twenty-five IPS e.max Press crowns were fabricated. They were divided into 5 groups (5 samples in each group) according to the Wt.% of the new ceramic ingots to the repressed ceramic. Group I is 100 % new ceramics. Group II is 75 % new and 25% repressed ceramics. Group III is 50 % new and 50 % repressed ceramics. Group IV is 25% new and 75% repressed ceramics. Group V is 100 % repressed ceramics.
Each crown was seated on the copper mold and the marginal gap was measured at 16 predetermined points using a computer connected stereomicroscope. For fracture strength test the crowns were luted using resin cement on epoxy dies and subjected to fracture strength test.
Results: Group 1 (100 % new) recorded the least marginal gap (32.7 µm), while group 5 (100% repressed) recorded the highest marginal gap (120.91 µm). No statistical significant difference between marginal gap of the first three groups (100 % new, 75% new + 25% repressed, 50% new + 50% repressed). A statistically significant difference was recorded between these 3 tested groups and group 4 (25% new + 75% repressed). There was also a significant difference between group 4 and group 5 (100% repressed). Fracture strength results showed that Group I (100 % new) recorded the highest fracture strength (929.724 N), while group III (50% new) recorded the least fracture strength (819.366N). A statistical significant difference was found between the first four groups and a siginficant difference between GroupV (100% repressed) and Group I,II,III. While, there was no significant difference between Group IV and Group V.
Conclusions: It was found that various weight percentages of repressed ceramics affected the marginal gap and had a significant effect when it exceeded 50 %, although all tested groups were clinically accepted. Also, homogeneity of the ceramic structure had a positive effect on fracture strength, as the least fracture strength was recorded for the 50% new + 50% repressed ceramic group and it increased significantly as this relation increased towards either the new ingot ceramics or the repressed ceramic: