The findings of this study support the null hypothesis (H₀), indicating that exposing orthodontic bonding systems to different temperatures does not significantly affect their adhesion to enamel. This suggests that temperature variations within the tested range have no meaningful impact on the bonding performance of these systems.

The minimum shear bond strength required for orthodontic bracket bonding is not universally defined and can differ based on various studies and specific clinical needs. As per Reynolds, the minimum SBS required in orthodontic treatment is between 5.9 and 7.8 MPa [21]. An ideal orthodontic biomaterial should provide sufficient adhesion to withstand chewing loads of 5–10 MPa, while ensuring that the adhesion is not excessively strong to avoid damage to the enamel during debonding (40–50 MPa) [22]. The results of the present study demonstrated that the two adhesive systems maintained adequate bond strength within the range of 5.27–21.51 MPa when applied at different temperatures. However, there were seven cases where SBS values were lower than the minimum required SBS, indicating bond failure and insufficient attachment of the molar tubes to the teeth; therefore, these cases were excluded from this study.

The mean SBS values of Transbond XT were greater than those of the GC Ortho Connect resin; however, the difference was not significant. Iglesias et al. also did not find a statistically significant difference between conventional and self-adhesive systems [23]. In contrast, other studies have shown significantly lower SBS of the self-adhesive resins in comparison to the conventional etch-and-rinse adhesive system [24, 25].

Some studies have indicated that temperature has a minimal impact on shear bond strength [15, 26], while other studies have demonstrated that temperature variations can indeed affect SBS values and potentially influence the adhesion of orthodontic biomaterials to enamel [13, 20, 27]. In the present investigation, the two adhesive systems yielded different outcomes, although the results were not statistically significant. In the Transbond XT group, the minimum bond strength values occurred at 20 °C. However, quite a few studies have reported the lowest SBS values at 4 °C, using either dental composites for restorations [27, 28] or adhesive systems in orthodontics [13]. Lower temperatures typically result in higher viscosity of dental adhesives, making them thicker and less capable of penetrating into the irregularities of the tooth surface and bracket base mesh, thereby creating worse adaptation and weaker composite-tooth and composite-bracket interactions [29, 30]. A decrease in temperature also results in decreased mobility of monomer molecules within the resin matrix of the resin-based composite and more constrained radical formation, which ultimately leads to weaker bonding [31].

The maximum SBS in the Transbond XT group was recorded at 55 °C. This outcome aligns with the findings of Akarsu et al. [27], who reported that adhesive systems obtained the highest SBS values when heated to 55 °C. Further studies revealed that adhesives pre-heated to 60 °C presented the highest SBS values [15, 20]. Heating the composite can enhance the polymerization reaction [32] and increase monomer conversion, leading to improved physical characteristics of the composite: higher surface hardness, greater flexural strength, enhanced mechanical strength and wear resistance [18, 33]. Raising the polymerization temperature of a resin composite lowers the viscosity of the material, leading to increased flow due to the enhanced movement of monomer molecules [17].

In the GC Ortho Connect group, the lowest bond strength values were obtained at 37 °C. This orthodontic adhesive system differs from Transbond XT because it is used without a separate binder or primer resin, which could have influenced the study’s results by increasing the risk of microleakage at the enamel-adhesive interface [34]. Transbond XT and GC Ortho Connect orthodontic adhesives also differ in their chemical compositions, which can affect their performance and properties. Both adhesive systems are resin-based composite materials that normally contain a combination of Bis-GMA (bisphenol A-glycidyl methacrylate) and TEGDMA (triethylene glycol dimethacrylate), as well as filler particles such as silica [35]. However, GC Ortho Connect also incorporates a phosphoric ester monomer and 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP), which ensures a stable bond, eliminating the need for a separate primer.

Therefore, these two adhesive systems could have different reactions to temperature changes, affecting the film thickness and viscosity [36]. Pre-heating the GC Ortho Connect adhesive decreased its viscosity and made it overly fluid, resulting in sliding of the tubes and making it difficult to control during application. Additionally, after tube seating, the material tended to spread more readily along the tube borders. In fact, the downward flow of material due to gravity is a major drawback for bonding of tubes as it may lead to inadequate coverage, an uneven polymer network, and weaker adaptation of the composite to the tube base [24]. Hence, the highest SBS was obtained at 20 °C. This result is in agreement with Sharafeddin et al., who reported that pre-heating the materials had no significant effect on shear bond strength and that the highest values were obtained at room temperature [28].

In regard to bracket debonding at the end of orthodontic treatment, there is an increased risk of enamel fracture or even tear-out [37]. In the literature, a variety of risk factors are listed, including the type of instrument and force used for bracket debonding, the type of material and protocol used to bond brackets, the bond strength between the enamel and adhesive, and the type of bracket used for treatment [38,39,40,41]. In the present study, six cases of enamel fracture were observed, with no statistically significant difference when comparing either the materials or the temperatures. Rix et al. suggested that the increased number of enamel cracks may be due to tooth extraction forces and could be lower when tested in vivo [42].

The amount of adhesive remnant on the tooth surface after bracket debonding depends on several factors, including bracket base design and the qualities of the adhesive type [43]. In this study, the bracket base design should not have influenced the adhesive remnant index score, because identical brackets were used for every specimen tested. Usually, three types of adhesive systems are used to bond brackets to teeth: conventional multi-step, self-etching, and self-adhesive. In the present study, conventional multi-step (Transbond XT) and self-adhesive (GC Ortho Connect) resins were used. A comparison of these materials has shown a statistically significant difference, with higher ARI scores observed in the Transbond XT group. Bracket failure typically occurs at the weakest link in the adhesive junction. The amount of adhesive remaining on the tooth surface after debonding can be explained by the bond failure mode: adhesive-enamel, adhesive-bracket, and cohesive. Bracket bonding with the conventional multi-step adhesive used in this study tends to show cohesive or adhesive-bracket bond failure modes, indicating that a greater amount of material remains on the tooth surface. These results are in accordance with the literature [23, 25]. In contrast, bonding with self-adhesive resin results in bond failure at the adhesive-enamel interface, showcasing most of the adhesive residues on brackets rather than on the tooth surface. Brackets bonded with self-adhesive material tend to have lower SBS values, and a weaker bond between enamel and resin is observed, leading to bond failure at the adhesive-enamel interface [25]. Cohesive or adhesive-bracket bond failure is generally considered ‘safer’ rather than adhesive-enamel debonding because it leaves the enamel surface relatively intact; however, the removal of the residual adhesive increases the possibility of damaging the enamel surface during the cleaning process [25, 44]. Nevertheless, in more than 40% of the cases, adhesive-enamel bond failure was observed with both adhesive systems, which corresponds to the results of Lobato et al., where ARI scores of 0 or 1 were predominant [15]. While in Borges’s study, which compared composite restoratives to conventional orthodontic adhesives, the entire composite stayed on the tooth surface in most groups [26]. Bishara et al. and Iglesias et al. also reported that, for the Transbond XT group, the majority of the adhesive remained on the tooth after debonding [23, 25]. On the other hand, less adhesive residue on the enamel surface after bracket debonding may be desirable in clinical practice as it reduces the chairside time. However, enamel fractures can occur during the debonding procedure [44].

In this study, the distribution of ARI across different temperatures showed no significant difference. These findings are similar to those of other studies, which also revealed no significant differences in the ARI at low and high temperatures [13, 15, 20].

It is important to note that it might be challenging to compare the results of different studies because a number of variables could account for discrepancies in the results. For example, the type of teeth selected for the study, the use of orthodontic adhesive systems as well as brackets/tubes from the same manufacturer, the application of identical temperatures, and the operator’s influence all highlight the need for standardizing the methodology to enable more efficient comparison of results [45].