Efficacy of silane after thermal-fatigue on bond-strengths degradation to zirconia

Objectives : To evaluate the influence of artificial aging by thermocycling on deterioration of microtensile bond-strength (μTBS) to zirconia.

Methods: Pre-sintered zirconia blocks (e.max ZirCAD, Ivoclar) were cut (14.5x15.5x7 mm) and fully sintered according to manufacturer's instruction. After air-abrasion with Al₂O₃ particles, three different silanes were tested: 1. Clearfil Ceramic Primer (CLE; Kuraray); 2. Monobond Plus (MB; Ivoclar); 3. Z Prime Plus (ZP; Bisco), and control group (CO) without silane application. A dual-cure resin cement (Clearfil Esthetic, Kuraray) was standardized for all experimental groups. Each group was assigned into three subgroups subjected to different thermal fatigue as: 5,000 thermal cycles (5K), 20,000 thermal cycles (20K), or no thermal cycles (NA) of artificial aging. Thermocycling was performed at 5°C and 55°C (SD-Mechatronik). All specimens were sectioned with a precision saw and 0.64mm² sticks obtained.  The sticks were tested for μTBS, using a universal testing machine (Instron) at a crosshead speed of 0.5 mm/min. The data were submitted to Kruskal-Wallis and Mann-Whitney Tests (α=0.05).

Results: (Pre-testing failure/number of specimens) Mean±SD in MPa

	Artificial aging
	No aging	5,000 cycles	20,000 cycles
CLE	(0/68) 52.47±11.1aA	(0/66) 24.54±5.3bA	(0/71) 24.19±4.9bA
MB	(0/68) 43.94±13.9aB	(0/70) 26.43±6.4bA	(0/71) 22.82±6.3cA
ZP	(0/72) 20.13±7.1aC	(12/69) 4.88±3.6bB	(52/71) 1.00±2.0cB
CO	(70/70) 0.00±0.0aD	(69/69) 0.00±0.0aC	(67/67) 0.00±0.0aB

Means with same lower-case letters, in the same row, are not statistically different. Means with same upper-case letters, in same column, are not statistically different.

Conclusions: Thermocycling significantly reduces μTBS to zirconia. CLE and MB were able to withstand intense thermal fatigue, without pre-testing failure. Silane is essential for stabile long-term bond-strength to zirconia.