الفهرس 
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Abstract
This study evaluated the effect of different concentrations of hydrofluoric acid and etching times on the retention strength of lithium disilicate glass ceramics luted to adhesive resin cement after artificial aging(thermocycling) using crown pull-off test.
Forty-five sound human upper central incisors freshly extracted were collected ultrasonically cleaned and then stored at distilled water at room temperature. at the roots of the teeth, 1mm depth round holes were prepared as a retention mean. Teeth were set in the acrylic resin mold on the long axis of the teeth.
A silicon index was made for each tooth before starting the preparation to assure uniform reduction. The preparations criteria were 1.5mm axial reduction, 1.5 mm incisal reduction, 1mm thickness shoulder finish line, about 15-20 ° taper which were done by the same operator. Teeth were stored in water at room temperature till cementation.
Scanning of the prepared teeth was done using Cerec intra oral laser scanner and each crown was designed with two arms at the incisal one third to work as a holding mean in universal testing machine. Forty-five crowns were constructed from e.max blocks and were divided into three main groups: group (1) using 4.9% hydrofluoric acid, group (2) using 9.6% hydrofluoric acid and group (3) using 11% hydrofluoric acid.
Each main group was then subdivided into three subgroups A, B and C according to acid application time (n=5): -
Groups 1A,1B and 1C: 4.9% hydrofluoric acid for 20 sec ,35 sec and 60 sec respectively.
Groups 2A,2B and 2C: 9.6% hydrofluoric acid for 20 sec ,35 sec and 60 sec respectively.
Groups 3A,3B and 3C: 11% hydrofluoric acid for 20 sec ,35 sec and 60 sec respectively.
Acid etching was applied to fitting surface of the crown then a silane coupling agent. each specimen then cemented to its prepared tooth according to manufacturer’s instructions using duo-link universal resin cement. Artificial aging was done through thermocycling in 5000 cycle. Crown tooth assembly was then subjected to crown pull-off test using universal testing machine with cross head of 0.5 mm/minutes until failure. Maximum pull-off forces were collected in N. data were collected tabulated and statistically analyzed using SPSS software.
Regarding HF concentrations Statistical analysis ANOVA test was used to compare the mean difference between groups. Post-hoc test was used for pairwise comparison with Tukey’s correction revealed that there was statistically significant difference between the groups regarding mean pull-off test results (p < 0.001). Broadly, mean pull-off test results for group I (4.9% hydrofluoric acid) was significantly lower (206.5 ± 13.1) compared with group II (9.5% hydrofluoric acid) (244.1 ± 25.4, p < 0.001) and group III (11% hydrofluoric acid) (284.4 ± 13.5, p < 0.001). Also, mean pull-off test results for group II (9.5% hydrofluoric acid) was significantly (p < 0.001) lower compared with group III (11% hydrofluoric acid)
In etching time point of view there was non-statistically significant difference between the groups regarding mean pull-off test results (p = 0.349). In other words, mean pull-off test results for group I (4.9% hydrofluoric acid) was insignificantly lower (236.6 ± 36.7) compared with group II (9.5% hydrofluoric acid) (244.7 ± 35.2, p = 0.327) and group III (11% hydrofluoric acid) (253.8 ± 38.8, p = 0.209). Likely, mean pull-off test results for group II (9.5% hydrofluoric acid) was non-significantly (p = 0.302) lower compared with group III (11% hydrofluoric acid).
Repeated Measure ANOVA test was used to compare the mean difference between groups over time so, respecting the difference in each group; for group I, there was significant (p=0.021) steady increase in the mean pull-off test results from 20s to 60s. On the other hand, for reported from 20s to 60s.
Conclusion
Within the limitation of this study, the following could be concluded: -
1. The retention of lithium disilicate was affected by the concentration of hydrofluoric acid and 11% hydrofluoric acid concentration showed higher significant mean values than 9.6% and 4.9% concentrations
2. For 9.6% and 11% HF acid concentrations, etching durations used in this study do not have a significant effect on the retention of lithium disilicate glass crowns. However, for 4.9% HF the retention means significantly increased with elevating the etching duration.
Recommendation
1.As HF acid is a biohazardous material, 20 seconds as a least application time for 9.6% and 11% HF concentrations must be used as higher application times do not produce significant higher retention strengths. However, for 4.9% HF more time is needed (60 seconds) to produce acceptable results.
2.Clinical trials with a controlled standardized study design are recommended to illustrate the clinical long-term performance of the tested material and surface treatment.