Fracture resistance of cement-retained and screw-cement-retained milled posterior crowns with screw-access hole preparations before and after firing: An in vitro study.

STATEMENT OF PROBLEM: When a cement-retained lithium disilicate crown needs to be removed, perforation is needed to reach the abutment screw. How this process affects its strength is unclear. PURPOSE: The purpose of this in vitro study was to evaluate the fracture strength of milled lithium disilicate implant crowns with screw-access holes created before and after the firing stages. MATERIAL AND METHODS: Nine implant replicas with stock abutments were embedded in acrylic resin. Forty-five lithium disilicate (IPS e.max CAD) crowns were designed, milled, and divided into groups. The control group had no screw access, the blue state group had the screw access created in the intermediate phase of the ceramic, and the postfiring group had the screw access created after firing. The crowns were cemented with a self-adhesive resin cement, placed in an incubator for 24 hours and then thermocycled. The screw-access holes were sealed with composite resin. The crowns were loaded to failure with a universal testing machine using a 1- mm-diameter ball at a crosshead speed of 0.5 mm/min and a 5-kN cell load. Data were analyzed by ANOVA and the Tukey post hoc test (α=.05). RESULTS: Screw-access holes created before and after firing stages had a significant effect on the fracture resistance of lithium disilicate crowns (P=.002). No difference in fracture resistance was observed between the control group and the blue state group (P>.05), while the fracture resistance of the postfiring group was significantly lower than that of the control group and the blue state group (P<.05). CONCLUSIONS: The fracture strength of lithium disilicate crowns was not affected by a screw-access hole created during the blue state of the ceramic followed by repair with composite resin. Creating a screw-access hole after firing the ceramic reduced fracture strength.

Grading Class I Preparations in Preclinical Dental Education: E4D Compare Software vs. the Traditional Standard.

The aim of this study was to compare the effectiveness of a novel assessment software system with the traditional grading protocol used in the University of Texas School of Dentistry at Houston operative dentistry preclinical curriculum. In the study, conducted in 2016, 98 Class I preparations were evaluated both traditionally and digitally by two teams of calibrated preclinical faculty members (two evaluators for each team). Scores from each faculty pair were averaged for the traditional and the digital grading systems, and the scores for the two grading systems were compared. The analysis found no significant difference between the two grading systems with respect to isthmus width (p=0.073) and remaining marginal ridge (p=0.5841), but there was a significant difference with respect to pulpal floor depth assessment (p<0.0001). The data suggested that both grading techniques can be used with repeatable confidence for two out of three grading criteria: isthmus width and remaining marginal ridge. In addition, the software offers a self-assessment tool for students to perfect their psychomotor skills while promoting independence and immediate feedback.