Influenza della traslucenza dell’endocrown sul grado di conversione del cemento duale = Influence of endocrown translucency on the degree of conversion of the luting dual resin cement

OBJECTIVES There are several parameters that influence the degree of conversion of dual cement, such as: the type of light source used for the polymerization, the intensity of the light source, the thickness of the restoration and the translucency of the material used for the restoration. The purpose of this study was to investigate the effect of translucency of ceramic material on the degree of conversion of a dual- cure resin cement used to lute endocrowns. MATERIALS AND METHODS A freshly extracted first maxillary molar, without any dental caries, was used and prepared to receive an endocrown, according to Bindl and Mörmann criteria. The cavity preparation was acquired by an intraoral scanner (Cerec Omnicam, Dentsply Sirona, Charlotte, North Carolina) with the aim of executing the entire workflow digitally, from the design to the milling of endocrowns. In this way, 12 endocrowns, with 4.5 mm thickness, were produced by a milling machine (Cerec MCXL Wounder, Dentsply Sirona, Charlotte, North Carolina). The tested material was zirconia- reinforced lithium silicate with a high translucency (HT, n = 6) (Celtra Duo, Dentsply Sirona, Charlotte, North Carolina) and with a low translucency (LT, n = 6) (Celtra Duo, Dentsply Sirona, Charlotte, North Carolina). After applying an insulator (Separating Fluid, Ivoclar, Schaan, Liechtenstein) to the tooth, the dual cement (Calibra Dual Resin Cement, Dentsply Sirona, Charlotte, North Carolina) was applied to the endocrown. The endocrowns were placed on the tooth one by one and cured for 60 s using a LED light-curing unit (Valo, Ultradent, South Jordan, Utah) with a light intensity of 1000 mW/cm2. The excesses of dual cement were removed by using a probe. Additionally, six samples of dual cement, sized 5.0×7.0 mm, were prepared with a thickness of 1.0 mm, serving as control group (Control) and polymerized for 60 s through a Mylar strip. The microhardness (Vickers) of the dual cement was evaluated at the bottom of the luted endocrowns and the controls. The results were statistically analyzed with one-way ANOVA and Fisher’s test (p <0.05). RESULTS Significantly highest mean microhardness values were recorded in the control group (Control): 41.9 (± sd 1.8). Endocrowns with low translucency (LT) produced the significantly lowest values: 29.6 (± sd 0.8) (p <0.05 compared to the control). Control and HT groups were not significantly different (p >0,05). CONCLUSIONS The microhardness of the dual cement is strongly dependent on both the translucency of the material with which endocrowns are produced. A low translucency leads to a significantly lower degree of conversion, which cannot guarantee a long-lasting restoration. CLINICAL SIGNIFICANCE Dual cements have been designed to ensure an optimal degree of conversion, regardless of the presence or absence of a light source and its intensity. This does not apply in the case of their use for endocrown luting procedures. In fact, the degree of conversion of dual cement depends not only on the type of light source and its intensity, but also and above all on the translucency of the restoration. Different levels of translucency affect the microhardness of the luting resin cement, potentially affecting its adhesive strength and, ultimately, the longevity of the chosen prosthodontic treatment.