- Tensile bonding strength(TBS)
The all-etching adhesive had significant differences in the tensile bonding strength of the mild, moderate and severe dental fluorosis (P <0.05).The self-etching adhesive had no significant difference in the tensile strength of the mild, moderate and severe dental fluorosis (P >0.05). By comparing the tensile strength of the same degree of dental fluorosis with different adhesives, it can be seen that the tensile strength of the mild dental fluorosis with all-etching adhesive was significantly higher than that of the self-etching adhesive (P <0.05) and that moderate and severe dental fluorosis with all-etching adhesive and self-etching adhesive had no significant difference (P >0.05). (Table 3, Fig 5)
- Shear bonding strength
All-etching adhesive had significant differences in the shear bonding strength of the mild, moderate and severe dental fluorosis (P <0.05).The self-etching adhesive had no significant difference in the shear strength of the mild, moderate and severe dental fluorosis (P >0.05). By comparing the shear strength of the same degree of dental fluorosis with different adhesives, it can be seen that the shear bonding strength of the mild dental fluorosis with all-etching adhesive was significantly higher than that of the self-etching adhesive (P <0.05), and that moderate and severe dental fluorosis with all-etching adhesive and self-etching adhesive had no significant difference (P >0.05). (Table 4, Fig 6)
- Observing the fracture interface under the SEM
The SEM results of the study was found that the enamel of mild dental fluorosis was relatively uniform and dense. A small number of crystals were disorderly arranged, and the interspace of the crystals was slightly widened, and various amounts of adhesive residue were visible. It showed that the fracture occured between the resin and enamel surface under the influence of force, which was the interface fracture. The bonding strength of mild dental fluorosis was determined by the adhesive. The enamel on the surface of severe fluorosis was severely exfoliated, and even some of the dentin tubules were exposed. The enamel was relatively loose and non-uniform. (Fig7, 8) The fracture forms of the two adhesives are mainly cohesive-enamel, and there is no statistical difference between the two groups( =1.660, P >0.05).(Table 5)
It showed that the enamel strength of severe dental fluorosis was low, and the enamel was exfoliated occurs under the influence of force, which was enamel fracture. Thus, the bonding strength of severe dental fluorosis was determined by the strength of enamel. Moderate dental fluorosis was similar to severe fluorosis, and the enamel was also exfoliated under force, but the degree of enamel exfoliation was lighter than that of severe fluorosis. The electron microscope results were basically consistent with the results of mechanics property test.
- Observing the fracture interface under the CLSM
The results of the study was found that with the increase of the degree of dental fluorosis, the enamel crystal was loosely arranged. Moreover, the bonding interface after all-etching adhesive bonding process was rougher than the self-etching adhesive, and the bonding interface appeared more uneven. (Fig 9, 10)
Figure legend:
Fig.1 Preparation of experimental specimens. A Severe dental fluorosis. B Moderate dental fluorosis. C Mild dental fluorosis. D Normal healthy teeth.
Fig.2 Schematic diagram of TBS and SBS test.
Fig.3 Tensile experiment. A Tensile device. B Fixture. C Bonding interface.
Fig.4 Shear experiment. A Shear device. B Fixture. C Bonding interface.
Fig.5 Histogram analysis of tensile strength determination.
Fig.6 Histogram analysis of shear strength determination.
Fig.7 SEM results of mild, moderate, and severe dental fluorosis fracture interface after all-etching adhesive strength test. Original magnification, 5.00k×; scale bar=10.0μm. (a) Fracture interface of mild fluorosis. There is more adhesive residue in the bond interface. (b) Fracture interface of moderate fluorosis. A small amount of adhesive remained and part of the dentin tubules were exposed. (c) Fracture interface of severe fluorosis. The enamel was completely exfoliated. The dentin tubules were completely exposed, and the crystals were disordered. In addition, the interspace between the crystals was significantly widened, and the number of enamel columns was small. (d) Normal healthy enamel bonding group as a control group
Fig.8 SEM results of mild, moderate, and severe dental fluorosis fracture interface after self-etching adhesive strength test. Original magnification, 5.00k×; scale bar=10.0μm. (a) Fracture interface of mild fluorosis. The enamel of the bonding surface was relatively uniform and dense. A small number of crystals were disorderly arranged, and the interspace between the crystals was slightly widened, and a small amount of adhesive remains. (b) Fracture interface of moderate fluorosis. The enamel was exfoliated under tension, which was lighter than severe fluorosis. (c) Fracture interface of severe fluorosis. The surface enamel was severely exfoliated, and even some dentin tubules were exposed. (d) Normal healthy enamel bonding group as a control group
Fig.9 CLSM results of mild, moderate, and severe dental fluorosis fracture interface after all-etching adhesive strength test. Original magnification, 100×; scale bar=20μm.
(a) Fracture interface of mild fluorosis. (b) Fracture interface of moderate fluorosis. (c) Fracture interface of severe fluorosis. (d) Normal healthy enamel bonding group as a control group
Fig.10 CLSM results of mild, moderate, and severe dental fluorosis fracture interface after self-etching adhesive strength test. Original magnification, 100×;scale bar=20μm. (a) Fracture interface of mild fluorosis. (b) Fracture interface of moderate fluorosis. (c) Fracture interface of severe fluorosis. (d) Normal healthy enamel bonding group as a control group.