Braz Dent J (1998) 9(2): 95-100 ISSN 0103-6440
| Introduction | Material and Methods | Results | Discussion | References |
Acid etching with phosphoric acid on the prismless layer of human dental enamel may produce different patterns of dissolution. Consequently, the presence of several patterns of enamel acid conditioning may indicate better or worse formation of resin tags. In this study, the heterogeneous patterns of 35% phosphoric acid conditioning of human non-erupted deciduous teeth were reported. SEM images demonstrated type I, type II and type III aspects reported by Silverstone et al. (Caries Research 1975;9:373-387), in spite of the etching time used and the surface area (buccal incisal, middle or cervical thirds). These characteristic features on the enamel surface may indicate the convenient use of this technique in deciduous teeth.
Key Words: etched enamel, non-erupted deciduous teeth.
Acid etching is the most useful choice for enamel retention of sealants and composite restorations due to its simplicity and efficiency. However, there is controversy regarding its actual effectiveness in deciduous tooth enamel. Whereas 15 to 30 seconds of acid application is recommended by some authors (Nordenvall et al., 1980; Carrassi et al., 1985; Hosoya, 1991; Gwinnett and Garcia-Godoy, 1992), others have reported that prolonged times, up to 4 minutes, would be necessary when primary teeth are considered (Gourley, 1975; Eidelman et al., 1976; Smutka et al., 1978; Bozalis et al., 1979; Redford et al., 1986).
This disagreement has been mostly attributed to the presence of an aprismatic zone on the enamel surface of deciduous teeth. Consequently, mechanical removal of this layer prior to the use of acid has been suggested as an alternative to longer etching periods (Sheykholeslam and Buonocore, 1972; Conniff and Hamby, 1976; Fuks et al., 1977; Bozalis et al., 1979; Meola and Papaccio, 1986).
Using scanning electron microscopy, the present study verified the acid conditioning effects on the prismless enamel of non-erupted deciduous teeth, using different etching times.
Material and Methods
Twelve non-erupted deciduous incisors having completely formed crowns were extracted from recently dead children, whose autopsies were performed at the Faculty of Medicine, University of São Paulo, Brazil.
These teeth were fixed in a modified Karnovsky solution containing 2% paraformaldehyde and 2.5% glutaraldehyde in a 0.1 M sodium phosphate buffer (pH 7.4) (Karnovsky, 1965). They were then stored in 0.1 M sodium phosphate buffer (pH 7.4) at 4ºC.
Subsequently, the buccal surface of nine teeth were conditioned with 35% phosphoric acid gel (Esticid®, Kulzer). The acid was applied with a brush for 15, 30 or 45 seconds (three teeth for each group). The surfaces were then washed with water spray for 30 s and dried with air spray for 5 s. Additionally, three teeth were selected for the control group.
The teeth were dehydrated in graded alcohol, critical point dried (Balzers CPD-010), mounted on metal stubs and coated with gold in an "Ions Sputter" Balzers, SCD-040. The samples were examined using a scanning electron microscope (Jeol JSM - T530A), at 15 kV.
The typical buccal surface of non-erupted anterior deciduous teeth (Figure 1A) showed a characteristic feature of smooth enamel (prismless zone) with round depressions of unpredictable depth. The acid conditioning of enamel surfaces using 35% phosphoric acid for different times exhibited heterogeneous dissolution patterns.
Fifteen-second acid exposure caused clear modifications on the external region, including a general but superficial loss of the enamel thickness (Figure 1B). Type III dissolution aspects (Silverstone et al., 1975) were shown by either nonprismatic etching (Figure 1B), or preferential waste of prismatic or interprismatic enamel (Figure 1C). At high magnification, details of the random distribution of type III pattern after fifteen-second enamel conditioning are shown (Figure 1D).
Three-dimensional structural appearance was also produced by the 30-second acid etching. Type I pattern (Silverstone et al., 1975), showing core prism dissolution, predominated in some areas (Figure 2A). Type II pattern, with interprismatic enamel predominantly etched, was found in other regions (Figure 2B), whereas type III pattern was rarely found.
Buccal enamel surface conditioned for 45 seconds showed varying aspects, i.e., nonprismatic, preferential loss of prisms with larger diameter hollows (Figure 2C), or predominant dissolution of the interprismatic structure (Figure 2D).
Acid conditioning of enamel buccal surface of non-erupted deciduous teeth, using 35% phosphoric acid gel for 15, 30 or 45 s, showed type I, II or III patterns as reported by Silverstone et al. (1975).
These characteristics, observed in the experimental groups, showed random distribution in all samples examined by SEM. Similar aspects were also verified in erupted deciduous teeth etched for 15 to 120 s by Tinanoff and Mueller (1977), Nordenvall et al. (1980), Garcia-Godoy and Gwinnett (1991), and also in agreement with Mayer and Schierle (1980), who used 6-min etching time.
These observations can be associated to a previous finding (Costa et al., 1994), which showed a thin layer of prismless enamel in various external regions of non-erupted deciduous incisors and canines.
Our results demonstrated that there was a tendency to predominating nonprismatic or type III pattern (Silverstone et al., 1975) with 15-s etching time. Silverstone and Dogon (1976), who studied the enamel etched for 60 to 90 s, also reported these characteristics. Hosoya (1991) observed that the type II pattern was the most prevalent in deciduous teeth, despite the time employed, the region etched and the depth of the enamel. He also speculated that 10- and 20-s acid conditioning would produce unsatisfactory alterations. On the other hand, Carrassi et al. (1985) reported that type I pattern was frequent with 15- and 30-s treatment.
In our study, patterns induced by acid became more pronounced as application time increased. This feature was also reported by Redford et al. (1986), Garcia-Godoy and Gwinnett (1991), and Hosoya (1991).
Bond strength tests reported by other authors (Redford et al., 1986; Gwinnett and Garcia-Godoy, 1992) did not show any significant difference among acid conditioning times varying from 15 to 120 s, and their results were similar to those reported for permanent teeth.
The need to reduce the conditioning time to 15 s, using 35% phosphoric acid, was advocated by Nordenvall et al. (1980), Carrassi et al. (1985), Garcia-Godoy and Gwinnett (1991) and Gwinnett and Garcia-Godoy (1992), and Hosoya (1991) recommended a 30-s period. In addition, our results obtained by SEM suggest that acid application for longer times in deciduous teeth would not be necessary. However, further laboratory studies are required in order to verify the actual efficiency of the acid etching technique, using periods shorter than 30 s in various pediatric dental clinic situations.
We would like to express thanks to the staff of the Obit Verification Department, Faculty of Medicine, University of São Paulo, for permission to obtain the teeth used in this study. This study was supported by CAPES and CNPq.
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Correspondence: Prof. Dr. Ii-sei Watanabe, Departamento de Anatomia, Instituto de Ciências Biomédicas, Universidade de São Paulo, Av. Prof. Lineu Prestes, 2.415, Cidade Universitária, 05508-900 São Paulo, SP, Brasil.
Accepted November 7, 1997
Electronic publication: March, 1999