Effect of die spacer thickness on shear bond strength of porcelain laminate veneers.

STATEMENT OF PROBLEM: The application of die spacer may affect the shear bond strength (SBS) of porcelain laminate veneer (PLV). However, there is no standard for the amount of die spacer necessary for the fabrication of PLV restorations.

PURPOSE: The purpose of this study was to evaluate the SBS differences between enamel and a feldspathic PLV as a function of die spacer thickness.

MATERIAL AND METHODS: Eighty rectangular (5 x 3 x 1 mm) PLV (Super Porcelain EX-3) specimens were made using stone blocks on which die spacer (Nice Fit) was applied in different thicknesses (0, 2, 4, or 6 coats) (n = 20). Before bonding, all PLV specimens were pretreated with airborne-particle abrasion (1-, 2-, 3-, or 4-bar pressure; 10 sec/10-mm distance or 5 sec/20-mm distance) and cleaned with an ultrasonic cleanser. A vertically flat surface on the labial enamel of 80 freshly extracted sound teeth was prepared with a low-speed diamond-coated saw. The PLV specimens were bonded to the etched enamel specimens using a composite resin cement (RelyX Veneer). Forty enamel-ceramic specimens (Group NTc) were maintained in deionized water at room temperature for 1 week after cementation, and the other 40 specimens (Group Tc) were subjected to thermal cycling treatment (2500 cycles, 5 degrees C to 60 degrees C, 15-second dwell time). The SBS measurement was performed with a universal testing machine. The results were compared with a 2-way analysis of variance (ANOVA), and mean values were compared with Fisher's Protected Least Significant Difference (PLSD) intervals (alpha = .05).

RESULTS: For Group NTc specimens, the SBS values (MPa) were 8.7 +/- 2.6 (no coat), 12.6 +/- 4.8 (2 coats), 10.7 +/- 2.9 (4 coats), and 9.6 +/- 2.8 (6 coats). For Group Tc specimens, the SBS values were 4.1 +/- 2.0 (no coat), 7.1 +/- 3.4 (2 coats), 6.8 +/- 2.1 (4 coats), and 6.1 +/- 2.4 (6 coats). Two-way ANOVA showed that the SBS was influenced by the thermal cycling and the number of coats, but there was no significant interaction between the 2 variables. The Fisher's PLSD interval for the SBS for comparisons among the number of coats was 0.95. The group with 2 coats of die spacer showed higher SBS values than the group with no coat, whereas groups with more than 2 coats of die spacer resulted in similar SBS values when compared to each other. The thermal cycling treatment resulted in a significant decrease of the SBS. Group Tc showed a trend similar to Group NTc.

CONCLUSION: Within the limitations of this study it was found that the appropriate application of die spacer exerts a favorable influence on the SBS of composite-bonded PLV. The 2-coat application of die spacer provides suitable space to accommodate the cement thickness.