Objectives: The proper thickness of die spacer for porcelain laminate veneer (PLV) is not clear. This study evaluated the effect of die spacer thickness on shear bond strength (SBS) of PLVs. Methods: Eighty rectangular-shaped PLV specimens (5mmx3mmx1mm) were made on the surface of blocs with four different number of die spacer coatings (0, 2, 4, 6 coatings). All PLV specimens were pretreated with sandblasting (50um, Al₂O₃) and ultrasonic cleaning (5min). A vertical planar enamel-bonding surface on the buccal or labial enamel of 80 freshly extracted sound human teeth was prepared with low speed diamond saw. The PLV specimens were cemented to the etched enamel with the composite resin cement (Rely X veneer, 3M ESPE). Forty enamel-ceramic specimens were kept in deionized water at room temperature for 1 week after cementation and the other specimens were thermocycled (2500 cycles, 5^oC to 60^oC). SBS measurement was performed with an Instron. Results: Results are shown below, die spacer thickness (um), ave(s.d.); shear bond strength (MPa), ave(s.d.). The results were compared by Tukey multiple comparisons test (p=0.05). There were no significant differences in SBS between the experimental groups (p>0.05). Thermocycling treatment resulted in a distinct decrease in SBS with no significant difference between die spacer thicknesses.

Treatments	No-thermocycled				Thermocycled (2500)
Die spacer(#)	0	2	4	6	0	2	4	6
Thickness(um)	-	12.8(2.6)a	26.8(3.9)b	38.1(4.2)c	-	12.8(2.6)a	26.8(3.9)b	38.1(4.2)c
SBS (MPa)	8.7(2.6)d	12.6(4.8)d	10.7(2.9)d	9.6(2.8)d	4.1(2.0)e	7.1(3.4)e	6.8(2.1)e	6.1(2.4)e

Note: Means within rows with the same superscript were not statistically different (p>0.05).

Conclusion: Within the limitations of this study, effect of different die spacer thickness on shear bond strengths of composite-bonded porcelain laminate veneers is not significant, but double coatings of die spacer would be recommended. This study demonstrated the reduction in SBS of PLV by thermocycling.