Objective. To evaluate the effect of different hydrofluoridric acid (HF) etching times on the bond strength of adhesively bonded resin cement to two zirconia reinforced lithium silicate (ZLS) glass ceramics and one lithium disilicate glass ceramic. Materials and methods. 200 rectangular ceramic discs (6 x 1.5 × 6 mm) of each material Celtra Duo, Dentsply-Sirona Suprinity by Vita and IPS e.max CAD by Ivoclar-Vivadent) were obtained from CAD/CAM blocks using a low speed rotary saw. Group 1 (IPS e.max CAD) and group 2 (Vita Suprinity) were crystallized according to manufacturer recommendations. Celtra Duo was divided into two groups: group 3 (unfired samples), and group 4 (subjected to glaze firing cycle). The samples were embedded in acrylic resin and the bonding surfaces were finished with sandpaper (600 grits). Samples from each group were further subdivided into 5 subgroups (n = 10) and treated with 9% hydrofluoric acid, as follows: 0, 10, 20, 30 and 60 seconds (subgroups A, B, C, D, respectively). A custom-made silicon putty fixture was attached to the samples containing a cylindrical hole (4 mm height and 2 mm in diameter), and resin cement G-CEM LinkForce was applied and light cured for 40 seconds from 4 different sides. The specimens were then thermocycled for 5,000 cycles, at 5°C and 55°C, with a dwell time of 30 seconds and a transfer time of 15 seconds, then subjected to a shear bond strength test using a universal testing machine with a cross head speed of 1 mm/min, until failure. Data was analyzed using analysis of variance, and multiple comparisons were performed using Tukey post hoc test. Results and discussion. For the IPS e.max CAD group, the etching time of 20 seconds showed the highest shear bond strength compared to the other subgroups (p< 0.05). For Celtra Duo’s non-fired group, there was no significant difference in shear bond strength among the three etching times tested (10, 20 and 60 seconds), while the 30 second subgroup exhibited the highest shear bond strength (p<0.05). For the Celtra Duo fired group, the control subgroup had a significantly lower shear bond strength than the other subgroups, while there was no significant difference in shear bond strength among the 10, 20, 30 and 60 seconds subgroups (p>0.05). Lastly, the Vita Suprinity group showed significant differences among all subgroups (p<0.05). Conclusions. Extending the etch time of ZLS ceramics negatively influenced Vita Suprinity more than it did on Celtra Duo, in both the fired and un-fired groups. An overall better bond strength is to be expected from firing of Celtra Duo ceramics. Moreover, etching ZLS ceramics with 9% HF acid for 20-30 seconds seems to provide overall best bond strengths, compared to other etching times. Clinical significance. This research will provide the clinician with an estimate of the adequate hydrofluoric acid etching time of zirconia lithium silicate glass ceramics, and show how various etching times will affect their bond strength. This research can also provide guidance to clinicians regarding the effects of different etching times on the final bond strength of ZLS materials using 9% HF.
Keywords:- hydrofluoric acid
- lithium disilicate glass ceramic
- zirconia reinforced lithium silicate