Endocrown luting procedures: analysis of dual luting cements conversion rate

Purpose/aim: The purpose of this study was to investigate the conversion of dual cements polymerized by LED curing light and used to lute endocrowns, made by lithium silicate with high and low translucency milled by a CAD/CAM device. Material and methods: A freshly extracted sound first maxillary premolar and a first maxillary molar were used and prepared - according to Bindl and Mörmann criteria - to receive an endocrown. The preparations were digitally scanned by an intraoral scanner (Cerec Bluecam, Software Version 4.2, Sirona Dental System) and 24 endocrowns were produced by a milling machine (Cerec MCXL Wounder, Sirona) using zirconia-reinforced lithium silicate (LS) (Calibra Duo, Dentsply) blocks. Half of the specimens (N = 12) were prepared for the molars and the remaining half were designed and produced for premolars. Each group - molar and premolar - was further divided into two subgroups: one in which the restorations were made by a high translucency LS block (N = 6) and the other was obtained by a low translucency LS block (N = 6). After applying an insulator (Separating Fluid, Ivoclar) to the tooth and the tested dual cement to the endocrown, we placed the endocrown on the tooth and cured for 60 s using a LED light curing unit (Valo, Ultradent) with irradiance of 1000mW/cm2 from the occlusal site. In addition, 10 blocks of dual cement, size 5 x 7mm and thickness of 1mm as a control group (GC) we prepared. They were polymerized for 60 s through a transparent Mylar acetate strip. The conversion rate of the dual cement was evaluated by measuring the Vickers Hardness of the material at the bottom of the preparation and compared it with the GC. Results: The highest mean Vickers hardness values were recorded in GC: 41.9 (3.7), followed by the molar group (M) in lithium disilicate (LS) with high translucency (HT) (M LS HT): 39.7 (6.8). The lowest values were recorded in the premolar group (P) in LS with low translucency (LT) (P LS LT): 16.8 (1.1). Conclusions: The conversion rate of the dual cement under LS endocrowns is strongly dependent on both the translucency of the material and the type of the tooth.