Transient and Residual Stresses in a Pressable Glass–Ceramic Before and After Resin–Cement Coating Determined Using Profilometry.
Objective: The effect of heat-pressing and subsequent pre-cementation (acid-etching) and resin-cementation operative techniques on the development of transient and residual stresses in different thicknesses of a lithium disilicate glass–ceramic were characterised using profilometry prior to biaxial flexure strength (BFS) determination.
Methods: 60 IPS e.max Press discs were pressed and divested under controlled conditions. The discs were polished on one surface to thicknesses of 0.61 ± 0.05, 0.84 ± 0.08, and 1.06 ± 0.07 mm (Groups A–C, respectively). The mean of the maximum deflection (acid-etching and resin-coating was determined using high resolution profilometery prior to BFS testing. Paired sample t-tests were performed (p < 0.05) on the 20 individual samples in each group (Groups A–C) for each comparison. Differences between the baseline quantification and resin–cement coating deflection values and BFS values for Groups A–C were determined using a one-way ANOVA with post hoc Tukey tests (p < 0.05).
Results: Baseline quantification for Groups A–C identified no significant differences between the group means of the maximum deflection values (p = 0.341). Following HF acid-etching, a significant increase in deflection for all groups (p < 0.001) was identified compared with the baseline quantification. Additionally, resin–cement coating significantly increased deflection for Group A (p < 0.001), Group B (p < 0.001) and Group C (p = 0.001) specimens for the individual groups. The increased deflection from baseline quantification to resin–cement coating was significantly different (p < 0.001) for the three specimen thicknesses, although the BFS values were not.
Significance: The lower reported baseline quantification range of the mean of the maximum deflection for the IPS e.max® Press specimens was predominantly the result of specimen polishing regime inducing a tensile stress state across the surface defect integral which accounted for the observed surface convexity. Acid-etching and resin-cementation had a significant impact on the development and magnitude of the transient and residual stresses in the lithium disilicate glass–ceramic investigated.
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