To improve orthodontic treatment, evaluations of the factors that may impair bracket/molar tube adhesion are carried out. Laboratory tests are often used to evaluate the performance of adhesive systems before long-term clinical trials to determine the clinical efficacy of improved adhesive systems in the oral cavity . The adhesion between the enamel and the bracket/molar tubes weakens due to the three different phenomena: mechanical, chemical, and thermal changes. Although in vitro studies cannot accurately reproduce in vivo conditions, when properly prepared and exposed, we can simulate different phenomena that occur in the oral cavity . Thus, thermal cycling is fully accepted and widely used by the scientific community in experimental studies to create conditions similar to those in the oral cavity . Even though some differences in temperature changes measured in the mouth and different tolerances for extreme temperatures have been reported, researchers agree that in laboratory tests on thermal cycle samples, the temperature should be from 5°C to 55°C . These temperatures also comply with the technical specification of ISO TS 11405 for testing the adhesion to the tooth structure . The number of cycles in laboratory testing has not been based on scientific data .
The number of samples, the bracket/molar tube model, the time spent in the water baths, the transfer time, the number of cycles, and the removal techniques are chosen by the researchers, and the result of the adhesion of the brackets/molar tubes to the tooth surface can vary greatly depending on the method used. For this reason, Fritz with other authors proposed a separate control for each study .
It is important to mention that previous studies have shown that molar tubes debonded more often than brackets of premolars and anterior teeth (canines and incisors) [4-7]. In a year-long study of bracket/molar tube debonding frequency, Maijer and Smith showed that debonding rates of incisor, canine, premolar, and molar brackets/molar tubes were, respectively, 3.6%, 1.6%, 4.8%, and 11.6%, and it was reported that there was no significant difference of molar tube debonding rates between the first and the second molars . For this reason, first molar teeth were used in this study.
In our study, while taking into account other studies [1, 22-26] the teeth of the experimental group were dipped 2,000 times in 5oC and 55oC saline. The obtained results showed that the debonding force in the experimental group was, on average, by 20.75N, or by 1.35 times lower than that in the control group. The statistically significant difference in forces of the composite (HIGH-Q-BOND BRACKET) between the control and the experimental groups was confirmed (p>alpha, p=0.0345, alpha=0.05).
In studies where the conventional bracket/molar tube bonding method was used , the mean values of bracket/molar tube adhesion to the tooth surface after 2,000 and 5,000 thermal cycles decreased slightly. These reductions were not statistically significant. However, Daub et al.  reported that the average adhesion of the brackets/molar tubes, using the conventional method of attaching the bracket/molar tube to human premolars and using Transbond XT adhesive, was significantly reduced by 16.7% after 500 thermal cycles. Saito et al.  observed a significant decrease in bracket/molar tube adhesion after 2,000 and 5,000 thermal cycles using the conventional bracket/molar tube attachment method.
Also in our study, using computer equipment, by debonding the molar tubes in the vertical direction, minimal displacement of the molar tubes until its complete detachment from the tooth surface was recorded. Such research has not been done in other studies. Comparing the correlation between the molar tubes displacement and force during debonding in both groups, we found that in the experimental group to achieve the same force as in the control group, the molar tubes displacement during debonding is 1.75 greater.
Based on the results of our and other studies [25-26], an additional recommendation of orthodontist to the patient before orthodontic treatment should be included: to change dietary habits and not to cause sudden changes in the temperature in the oral cavity as this weakens the adhesion and increases the chance of debonding of the brackets/molar tubes.
Regarding the causes of the deterioration of the adhesion of the brackets/molar tubes, it is important to mention that during the thermal cycle test, the samples undergo sudden temperature changes and are exposed to water. Differences in the coefficient of the thermal expansion of metal brackets/molar tubes, the adhesive, and the tooth result in repetitive shrinkage/expansion stresses [12, 29]. The resin expands and contracts because its coefficient of thermal expansion is higher than that of the teeth; the higher the coefficient of thermal expansion of the resin, the worse it would be for the adhesive bond, as the volumetric changes in the resin will be greater. In addition, the water in this procedure causes hygroscopic expansion as well as chemical decomposition of the resinous components, which is called plastification [30-31].
It has been suggested that the minimum bracket/molar tube adhesion force should be 6–10MPa to achieve an acceptable clinical result . From 1975, a standard procedure in the orthodontic practice is to etch the tooth surface with 35-40% phosphoric acid before bonding the brackets/molar tubes to ensure the micro-porosity on the tooth surface to allow the resin monomers to penetrate and mechanically bond and to increase the adhesion strength of the brackets/molar tubes from 9 to 35 MPa . Our results far exceeded these limits. The difference could be due to the choice of different protocols and molar tube removal methods .
In vitro studies such as this have some limitations. Saliva and patients’ oral hygiene, diseases, and habits may affect the results, but in vitro, we cannot accurately simulate a multifactorial oral environment, thus the results of our study show the effect of only one factor on the debonding force of orthodontic brackets/molar tubes. In addition, some other factors, such as the type of the adhesive used, the mechanical and chemical surface preparations, and the number of thermal cycles may also affect the adhesion of the brackets/molar tubes. Thus, in order to obtain more accurate results and to evaluate and compare the effect of the thermal cycles on different types of adhesives, a greater number of subjects and studies are required. The effect of the thermal cycles on the adhesion strength of ceramic brackets/molar tubes to the tooth surface should also be evaluated.