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Of the five cements tested, the most retrievable CAD/CAM restorations were luted with Temp Bond NE and Improv Temporary Cement. Resin-modified glass ionomer retentive forces were closer to those of the "temporary cements" than those of the permanent adhesive-resin cements. The abutment surface area became less important when using adhesive-resin cements. Retention of CAD/CAM all-ceramic restorations to prefabricated abutments has not been reported in the literature. This in vitro study demonstrated clinically significant variation among the selected cements used to retain all-ceramic CAD/CAM restorations to implant abutments. In addition, abutment size influenced the retention of all-ceramic CAD/CAM restorations.

Objective: This study evaluated the marginal fit of CAD/CAM copings milled from Hybrid ceramic (Vita Enamic) blocks and lithium-disilicate (IPS e-max CAD-Ivoclar Vivadent) blocks. It also evaluated the effect of crystallization firing on the marginal fit of lithium-disilicate copings. Method and Material:A standardized metal die with a 1mm wide shoulder finish line was imaged using the CEREC® AC Bluecam (Sirona). The coping was designed utilizing CEREC 3 software. The design was used to fabricate 15 lithium-disilicate and 15 hybrid ceramic copings. Design and milling were accomplished by one operator. The copings were seated on the metal die utilizing a pressure clamp with a uniform pressure of 5.5 Ibs. A Macroview Microscope was used for direct viewing of the marginal gap.Four areas were imaged on each coping (buccal, distal, lingual and mesial). Image analysis software was used to measure the marginal gaps in micrometers in 15 randomly selected points on each of the four surfaces. A total of sixty measurements were made per sample. For lithium-disilicate copings the measurements for marginal gap were made before and after firing. Data was analyzed by paired t-test. Results:The overall mean difference in marginal gap between the hybrid ceramic and crystallized lithium-disilicate copings was statistically significant (p<0.01). The overall mean difference in marginal gap before and after firing (pre-crystallized and crystallized lithium-disilicate copings) showed an average of 62 microns increase in marginal gap after firing. This difference was also significant (p<0.01).v Conclusion: This study concluded that crystallization firing can result in a significant increase in the marginal gap of the lithium-disilicate CAM/CAM crowns. In addition a significant difference exists in the marginal gap discrepancy when comparing hybrid ceramic and lithium disilicate CAD/CAM crowns.

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