The test samples consisted of 90 recently extracted (<6 months) bovine permanent mandibular incisors. Bovine mandibular incisors are considered a viable option in bonding studies as they are readily available, inexpensive, and similar to human teeth. In addition, they have larger crystal grains and more lattice defects than human teeth, resulting in lower critical surface tension, probably related to their slightly lower bonding values than human teeth. Studies have demonstrated that bonding strength increases in older teeth as opposed to recently extracted teeth [20, 21].
These recently extracted incisors were obtained from the Animal Technologies, Inc. (Tyler, TX, USA). In order to standardize the study, we controlled for the variability of results by using one specific sample type; thus, deciduous mandibular incisors were excluded from the study. As reported by Oesterle et al., there are differences in bond strength between bovine deciduous incisors (21%) and permanent incisors (35%) . The teeth were only obtained from The United States Department of Agriculture or equivalent inspected facilities, where animals received ante- and post-mortem inspection, and were free of contagious diseases. The substrate was collected from the animals < 30 months from the same lot. The bovine teeth were extracted from a different lot, representing different extraction times, to standardize and control for the variability of results, as reported by Nakamichi et al. who established that bonding strength increases as teeth age .
Immediately after extraction, the teeth were washed in running water, and all blood and adherent tissue were removed. The teeth were then placed in distilled water and stored at 37°C. The 90 teeth were divided into three groups of 30 specimens; the three groups represented three time points: T1, T2, and T3. A standard reproducible flat surface was utilized on each tooth, where two brackets (precoated and conventionally bonded) were placed on each facial surface. The tooth surfaces were kept wet at all times. The enamel was cleaned with pumice. The enamel was etched with 35% phosphoric acid gel (3 M Unitek, Monrovia, CA, USA) for 20 s, rinsed under running water for 20 s, and then dried with oil- and moisture-free compressed air. The teeth were mounted in a custom-made baseholder and then bonded (Transbond™ XT, 3 M Unitek) and light cured using Ortholux LED (3 M Unitek) at a wavelength of 460 nm for a total of 10 s (5 s on the mesial and 5 s on the distal aspects) on selected brackets (maxillary left incisors). This was performed in a standardized manner, utilizing height gauge with an identical amount of pressure applied to each bracket, namely, 30 g of force using a force gauge (Dontrix gauge, Invecta®, GAC, Bohemia, NY, USA).
The following bracket systems were used: type A was Smart Clip MBT High TQ (3 M Unitek) self-ligating metal brackets, and type B was APC™ II Adhesive Coated Appliance Smart Clip MBT High TQ (3 M Unitek) self-ligating metal brackets. Each bracket system was tested at three different time points: (1) Very short-term (T1): 15 min after bonding, (2) short-term (T2): 24 h after bonding, and (3) after thermocycling (T3): 1,000 cycles in water between 5°C and 55°C after 24 h of storage in water at 37°C. Each cycle was at least 20 s, with a transfer time between baths for 5 to 10 s.
Testing of shear bond strength
The specimens were stored in distilled water prior to testing at (37°C ± 2°C) and tested immediately after removal from water. An MTS machine (MTS Insight 1, MTS Systems Corporation, Eden Prairie, MN, USA) was used to evaluate the force applied to debond the brackets. Debonding was performed with an MTS Insight 1 machine with a blade design, pin under an occlusogingival load at a crosshead speed of 0.5 mm/min, beginning at 2 mm from the bracket to the metal pin of the MTS unit that recorded the test results. The results were recorded in MPa by a computer connected to the machine. Each tooth was oriented so that its facial surface was parallel to the direction of force during the shear testing. The shear force application was directly applied to the bracket-tooth interface, near the base.
Testing the adhesive remnant index
After bracket failure, the enamel surface was examined under optical magnification (×10), and the amount of adhesive remaining on the tooth was recorded using the ARI. The criteria for ARI scoring were as follows: 0, no adhesive on the tooth; 1, less than 50% adhesive on the tooth; 2, more than 50% adhesive on the tooth; and 3, all adhesive remained on the tooth.
Student's t test analysis was used to determine whether there was a significant difference in the shear bond strength between the two test groups. One-factor analysis of variance (ANOVA) was used to analyze the difference and to compare each bracket performance within itself at T1, T2, and T3. Mann–Whitney non-parametric statistical analysis was used to compare the ARI between the two test groups, and Kruskal-Wallis non-parametric statistical analysis was used to compare each bracket performance within itself at T1, T2, and T3. All the statistical analyses were performed using the SPSS software (version 18, SPSS, Inc., Chicago, IL, USA). P values less than 0.05 were considered significant.