Fluoride is needed for the normal development of bone and teeth. At high levels, however, fluoride affects the development of teeth and bone. Specifically, DF is caused by the ingestion of excess fluoride mainly through the drinking of water [17, 18]. Additionally, the use of fluoride is related to the exacerbation of DF. Water fluorination and the use of fluoride products, such as fluoride toothpaste, specific food/beverages, and fluoride supplements, are considered to be risk factors for the occurrence of DF [1].
In fluorosed teeth, the highly mineralized enamel surface layer comprises a mixture of many large and extremely small crystals, whereas the hypomineralized subsurface area comprises fairly sparsely arranged large crystals with a few small crystals. Some of the crystals in the subsurface hypomineralized layer exhibit defects such as perforations [19]. In more severe cases, the hypomineralization is extensive, and the outer well-mineralized surface layer is rather brittle. Thus, chewing forces may result in the formation of surface enamel defects, which appear as single pits along the perikymata. Alternatively, the surface enamel may be chipped away, corresponding to incisal edges or cuspal tips. Particularly, the occlusal surfaces are rapidly worn, often to such an extent that the hypomineralized porous layer is abraded away [20].
Presently, with regard to dental enamel bonding theory, various scholars hold that a mechanical chimera, physical adsorption, and chemical bonding are the main means of dental enamel bonding [21]. Because the retention force between the adhesive and the enamel mainly depends on mechanical fitting, the surface morphology of the enamel has a certain effect on the bonding strength. Most studies on the adhesive strength of teeth have considered healthy teeth. However, the DF enamel is different from the crystal enamel structure of normal teeth [17]. This study demonstrated that the adhesive and dental enamel are firmly held, mainly by the mechanical fitting of the micropores produced by enamel acid etching, as observed using CLSM. Hence, the relationship between the physical state of the enamel surface and the bond strength after acid etching is very important.
By observing the fracture interface of a mildly fluorosed enamel using SEM, it was found that the distinct dissolution of the outer surface exhibited many obvious surface irregularities such as shallow fissures and other defects. For moderately and severely fluorinated enamels, the loss of the external microscopic surface was visible and some dentin tubules had been exposed. These results are consistent with the results obtained by various previous studies. Although fluorapatite may be more acid-resistant, other structural changes, such as the fluorotic enamel’s outer hypermineralized layer with a porous hypomineralized subsurface and low surface energy, impair the surface wetting, which results in the decrease in bond strength [22].
In this study, the interface fracture was observed using SEM, and cohesive failure was the most common mode of failure. Moreover, the fractured specimens were observed using CLSM, and cohesive failure was the most prevalent mode of failure in moderate and severely fluorosed teeth that had previously undergone phosphoric acid etching. This observation should be considered because the self-etching primer adhesive system produces a predominant adhesive failure [23]. Notably, various studies have demonstrated that most specimens (98%) exhibit adhesive/mixed failures, with 1% exhibiting premature failure and 1% exhibiting cohesive failure exclusively within the enamel or cohesive failure exclusively within the resin composite in DF [6].
By comparing the sound and fluorotic enamel, the lower bond strengths were obtained for the fluorotic enamel, regardless of the application technique. These results should be investigated further because the fluorotic enamel is more resistant to acidic dissolution owing to the presence of fluorapatite, which adversely affects the adhesive performance [10]. Various studies [24] have reported that the enamel of DF produces micropores owing to the effect of acid etching and decalcification. As the fluorosis severity increases, the subsurface enamel along the tooth becomes increasingly porous (hypomineralized), and the lesion extends toward the inner enamel [25].
From the viewpoint of mechanical bonding [24], it is inferred that, if the glazed column of the glazed surface is perpendicular to the bonding surface after acid treatment, it can produce an ideal honeycomb structure with maximum adhesion. Additionally, if the partial glaze column is parallel to the bonding surface after acid treatment, it can produce minimum adhesion. As has been confirmed, the abnormal enamel morphology, whether it is the collapse of the enamel development or the disorder of the glaze column, affects the bonding strength. Moreover, it has been reported that the enamel surface of fluorosed teeth appears highly uneven and rough, and exhibits cracks and fissures. The enamel surface exhibits pits with variable dimensions in the discolored area of the teeth, which appear as punched lesions on the enamel surface and thus expose the underlying porous enamel [26]. A recent study on the ground sections of fluorosed teeth reported crescent-shaped hypomineralized areas in the enamel and increased interglobular dentin spaces [27].
This study found that, in tensile and shear testing, the all-etching bonding system for teeth with mild fluorosis had higher bonding strength compared with the self-etching bonding system, which is consistent with the results obtained by many previous studies. Owing to the particular enamel structure of the DF, the micromechanical fitting after acid etching is not satisfactory, and this affects the bonding effect between the adhesive and the enamel [12, 13]. Self-etching adhesive systems use weak acidic monomers to completely condition the enamel/dentin substrate. Various previous studies [23] have demonstrated that K-etchant gel (37% phosphoric acid) produces typical etching patterns consistent with phosphoric acid, regardless of the fluorosis severity. The etching pattern observed by a self-etching primer is shallower than that of the K-etchant, which uses MDP as the etching agent. This is attributed to a shift in the crystal orientation or the enlargement of laminar pores, which are considered as more important diffusion pathways, compared with the interprismatic enamel, during the acid attack [28]. Many studies have also reported that the two-step etch-and-rinse adhesive system used in the present study, namely, Single-Bond, uses 35% phosphoric acid with a pH of approximately 0.6. Notably, 35% phosphoric acid is strong acid. The two-step self-etch adhesive, SE Bond, is classified as a mild self-etch adhesive with a pH of 2.0. Because self-etch adhesives are less acidic than phosphoric acid, they do not demineralize the enamel to the same extent. Thus, they yield a less micro-retentive surface and, consequently, lower bonding strength [29].
Shallow fissures with other irregularities have been observed in mildly fluorosed enamel [30]. The all-acid etching bonding system uses phosphoric acid to etch the tooth enamel before demineralization, and removes the stained layer through acid etching. Compared with the self-etching bonding system, the all-acid etching bonding can form a wider penetration area on the DF surface, and the formed resin protrusions are more uniform, which improves the effect of micromechanical fitting [12, 31, 32]. Various studies have reported that self-etching bonding systems are inferior to all-etching bonding systems when bonding is performed for moderately and severely fluorosed enamel. However, this is inconsistent with our results [11, 32]. A more detailed examination of the fluorotic enamel results reveals that, although the fluorotic enamel is more acid-resistant, the prolonged application mode improves the demineralization, which appears as an increase in micro-irregularities and prism porosities in the microscopy analysis. Further, the mode also improves the interaction of acidic monomers with prismatic and interprismatic areas [6].
However, there was no significant difference between the all-etching bond system and the self-etching bond system for moderate and severe fluorosis, possibly because the degree of demineralization of the enamel in the moderate and severe fluorosis was low, and the enamel was more fragile and easier to break [30]. Various studies have reported that the prolonged application time of universal adhesives in SE mode to a fluorotic enamel enhances the enamel-etching pattern and promotes the similarity of the results in terms of adhesive enamel bond strength [6].