PRISMLESS ENAMEL IN HUMAN NON-ERUPTED DECIDUOUS MOLAR TEETH: A SCANNING ELECTRON MICROSCOPIC STUDY

ESMALTE APRISMÁTICO EM MOLARES DECÍDUOS HUMANOS NÃO ERUPCIONADOS: ESTUDO AO MICROSCÓPIO ELETRÔNICO DE VARREDURA

Abstracts

The frequency, structure and thickness of the prismless enamel layer in the buccal and lingual surfaces of non-erupted deciduous molar teeth were described. The teeth were extracted, kept in a 70% ethanol solution, dried, coated with gold and examined in a scanning electron microscope JEOL, JSM-6.100. The aprismatic layer was observed in the occlusal, middle and cervical thirds of all buccal and lingual surfaces. The hydroxyapatite crystals were arranged parallel to each other and perpendicular to the enamel surface. No statistically significant differences were observed between the occlusal, middle and cervical thirds, which had 7.257 <FONT FACE="Symbol">m</FONT> m of average thickness.

Prismless layer; Dental enamel; Microscopy, electron, scanning

PRISMLESS ENAMEL IN HUMAN NON-ERUPTED DECIDUOUS MOLAR TEETH: A SCANNING ELECTRON MICROSCOPIC STUDY^*

ESMALTE APRISMÁTICO EM MOLARES DECÍDUOS HUMANOS NÃO ERUPCIONADOS: ESTUDO AO MICROSCÓPIO ELETRÔNICO DE VARREDURA

Marcelo FAVA ^**

Ii-Sei WATANABE ^***

Flávio FAVA-DE-MORAES ^***

Luciane Ribeiro de Rezende Sucasas da COSTA ^****

FAVA, M. et al. Prismless enamel in human non-erupted deciduous molar teeth: a scanning electron microscopic study. Rev Odontol Univ São Paulo, v. 11, n. 4, p. 239-243, out./dez. 1997.

The frequency, structure and thickness of the prismless enamel layer in the buccal and lingual surfaces of non-erupted deciduous molar teeth were described. The teeth were extracted, kept in a 70% ethanol solution, dried, coated with gold and examined in a scanning electron microscope JEOL, JSM-6.100. The aprismatic layer was observed in the occlusal, middle and cervical thirds of all buccal and lingual surfaces. The hydroxyapatite crystals were arranged parallel to each other and perpendicular to the enamel surface. No statistically significant differences were observed between the occlusal, middle and cervical thirds, which had 7.257 m m of average thickness.

UNITERMS: Prismless layer; Dental enamel; Microscopy, electron, scanning.

INTRODUCTION

Several studies about the prismless enamel layer have been done either with deciduous (RIPA et al.¹¹, 1966; MIYOSHI et al.⁹, 1972; CARRASSI et al.², 1986; KODAKA et al.⁸, 1989) or permanent teeth (RIPA et al.¹¹, 1966; WHITTAKER¹³, 1982; FAVA et al.⁶, 1993). It represents a regular structural component of the tooth enamel surface. These studies described the presence of hydroxyapatite crystals disposed parallel to each other and perpendicular to the enamel surface on erupted teeth. They also demonstrated a high variety of thickness of the prismless enamel.

The aim of this study was to verify the frequency, structure, and thickness of the prismless enamel in the buccal and lingual surfaces of non-erupted deciduous molar teeth, using scanning electron microscopy methods.

MATERIALS AND METHODS

Thirty-five non-erupted human deciduous molars, having completely formed crowns, were obtained from deceased children with ages varying from 12 to 18 months, whose autopsies were conducted at the obituary service of the School of Medicine of the University of São Paulo.

The buccal and lingual surfaces were subdivided in three regions: cervical, middle and occlusal, and the teeth were preserved in 70% ethanol according to BOYDE's¹ method (1976). Then, all teeth were fractured in buccolingual directions using a vise. Subsequently, all fractured teeth were rinsed with distilled water and dehydrated by means of an increasing series of ethanols. After air drying, the specimens were mounted on metal stubs and coated with gold using a Balzers "Ions Sputter" SCD-040. The specimens were examined under the scanning electron microscope, JEOL, JSM-6.100, at the Institute of Biomedical Sciences of the University of São Paulo.

Length measures of the thickness of the three regions (cervical, middle and occlusal) were obtained through 71 selected electron microscopic figures using a digital caliper with 0.01 mm precision (MITUTOYO, Japan). For each figure, five measurements of the prismless layer thickness were performed.

Subsequently, all values were transformed in micrometers and tabulated to proceed the statistical study, using one-way analysis of variance (KEMPTHORNE; FOLKS⁷, 1971).

RESULTS

The prismless layer of non-erupted deciduous molar teeth was observed in the buccal and lingual surfaces of all samples.

In the occlusal region, the prismless layer showed a uniform thickness (Figure 1). With greater microscopic magnification, the three-dimensional aspects of the aprismatic layer were observed, constituted of hydroxyapatite crystals arranged parallel to each other and perpendicularly to the enamel surface (Figure 2).

FIGURE 1 - General view of the occlusal region showing a uniform thickness of prismless layer (arrows) (X2,250). FIGURE 2 - Higher magnification shows hydroxyapatite crystals arranged parallel to each other (arrows) (X4,500).

In the middle third of the prismless layer, the fractured surface shows a clear limit between the prismless layer and the prismatic enamel (Figure 3). At high magnification the parallel disposition of hydroxyapatite crystals is clearly observed. The external surface of the enamel is smooth with some round depressions (Figure 4).

FIGURE 3 - Middle third. The prismless layer can be observed (small arrows), and in the lower area the prismatic enamel is clearly seen (large arrows) (X2,250). FIGURE 4 - At the high magnification, at the middle third, the hydroxyapatite crystals (small arrow) and round depressions (large arrow) can be clearly observed (X4,500).

In the cervical region, the prismless enamel showed a homogeneous layer with a thin and well defined structure (Figure 5). These data can be observed in Figure 6, at high magnification.

FIGURE 5 - Cervical third shows a prismless layer (small arrows) and the prismless enamel (large arrows) (X2,250). FIGURE 6 - Details of the Figure 5 at high magnification. Prismless layer (small arrow). Prismatic enamel (large arrow) (X4,500)

The results regarding the thickness of the prismless enamel obtained in the three regions (occlusal, middle and cervical) were statistically evaluated through the one-way analysis of variance: ANOVA (Table 1).

TABLE 1
- Prismless layer. Length of thickness of the prismless layer in non-erupted molars deciduous teeth, in three regions: occlusal, middle and cervical.

Regions Sources of variation	Sample size	Average micrometers	Standard deviation	Standard error
Occlusal	37	7.312	2.193	0.365
Middle	20	7.191	1.551	0.357
Cervical	14	7.270	1.955	0.542

A p = 98% was obtained in the one-way ANOVA which means that there is no significant difference in the prismless layer thickness between the occlusal, middle and cervical thirds of non-erupted deciduous molar teeth. The overall average of the prismless layer thickness was 7.257 micrometers.

DISCUSSION

Several studies have already concluded that the prismless layer is a component of the surface structure of the enamel. The prismless layer is seen in deciduous and permanent teeth, anterior and posterior, upper and lower, as well as in all of their regions: incisal/occlusal, middle, cervical, buccal, lingual, mesial and distal (GWINNETT⁴, 1966; GWINNETT⁵, 1967). Its presence was also observed in pits and fissures (WATANABE et al.¹², 1995).

There is also consensus that the prismless layer is predominantly constituted of hydroxyapatite crystals arranged parallel to each other and perpendicularly to the enamel surface (GWINNETT⁴, 1966; KODAKA et al.⁸, 1989; FAVA et al.⁶, 1993; COSTA et al.³, 1996).

On the other hand, there are controversies about the thickness of this layer when deciduous or permanent, erupted or non-erupted, or different surfaces or regions of the teeth are studied. This is true even if different methods of measurement are used.

As a general rule, the measures in erupted teeth always undergo modifications in the original layer of the prismless enamel due to abrasion. Therefore, the measures taken in non-erupted but completely formed teeth are more reliable. In this instance the unique difference, already described, is in the thickness of the prismless layer in the non-erupted molars between permanent and deciduous teeth.

While the non-erupted third permanent molar shows an increase of the prismless layer from the occlusal region (3.63 µm) to the medium (5.28 µm) and cervical (8.11 µm) ones in any surface (FAVA et al.⁶, 1993), for the deciduous molar the average thickness is of 7.257 m m for all surfaces and regions. The same result was obtained by CARRASSI et al.² (1986) and COSTA et al.³ (1996) in the anterior deciduous teeth even though the average thickness of the prismless layer was 4.67 µm.

Are these structural results relevant for preventive actions or restorations in pediatric practice? There is no doubt about that, especially if we consider clinical acts which are dependent on acid etching, such as: the application of sealants and the adhesion of orthodontic brackets.

The dissolution of the prismless layer through acid etching is important so that tags can be obtained in the lower prism layer for a good retention. This result is dependent on the acid etching, maturation of the enamel and, obviously, on the thickness of the prismless layer.

REDFORD et al.¹⁰ (1986) demonstrated that the acid etching with phosphoric acid at 37% during 15 seconds creates micropores with depths of 9 m m in the surface of the enamel of deciduous molars. They also observed that this depth was sufficient for an adequate retention of sealants.

Therefore, the acid etching during 15 seconds seems sufficient to cross the 7.257 µm of the prismless layer of deciduous molars or 4.670 m m of deciduous anterior teeth (COSTA et al.³, 1996), maintaining the ability of retention of adhesive materials.

The information contained herein must be taken into clinical consideration. However, studies on the prismless layer should continue to be carried out.

CONCLUSIONS

1. The prismless enamel was always present on non-erupted deciduous molar teeth, showing a thin and well-defined layer.

2. The prismless layer structure of the non-erupted deciduous molar teeth was constituted of hydroxyapatite crystals arranged parallel to each other and perpendicularly to the enamel surface.

3. In general, the thickness of the prismless layer is 7.257 micrometers and it is similar in the occlusal, middle and cervical regions.

ACKNOWLEDGEMENT

We gratefully acknowledge the assistance of Mrs. Laura Fusaro in the preparation of the manuscript, and the Service of Obituary Verification of the School of Medicine of the University of São Paulo for the supply of teeth. This work was supported by CNPq (Process 150033-94-0 (nv)).

FAVA, M. et al. Esmalte aprismático em molares humanos decíduos não erupcionados: um estudo ao microscópio eletrônico de varredura. Rev Odontol Univ São Paulo, v. 11, n. 4, p. 239-243, out./dez. 1997.

Foram descritas a frequência, a estrutura e a espessura da camada de esmalte aprismático nas superfícies vestibular e lingual de molares decíduos não erupcionados. Os dentes foram extraídos e mantidos em solução de etanol a 70% e, em seguida, desidratados, cobertos com ouro e examinados em um microscópio eletrônico de varredura JEOL, JSM-6.100. A camada aprismática foi observada nos terços oclusal, médio e cervical de todas as superfícies vestibular e lingual. Os cristais de hidroxiapatita seguiram um arranjo paralelo entre si e perpendicular à superfície do esmalte. Nenhuma diferença estatisticamente significante foi observada entre os terços oclusal, médio e cervical, que apresentaram uma espessura média de 7,257 µm.

UNITERMOS: Camada aprismática; Esmalte dentário; Microscopia eletrônica de varredura.

REFERENCES

1. BOYDE, A. Enamel structure and cavity margins. Operat Dent, v. 1, n. 1, p. 13-28, Winter 1976.

2. CARRASSI, A. et al. Morfologia dello smalto e distribuzione dello strato aprismatico nei denti decidui. Dent Cadm, v. 54, n. 2, p. 51-59, Feb. 1986.

3. COSTA, L. R. R. S. et al. A SEM study of prismless enamel in non-erupted human deciduous teeth, Braz J Morphol Sci, v. 13, n. 2, p. 219-223, Jul./Dec. 1996.

4. GWINNETT, A. J. The ultrastructure of "prismless" enamel of deciduous teeth. Arch Oral Biol, v. 11, n. 11, p. 1109-1115, Nov. 1966.

5. GWINNETT, A. J. The ultrastructure of the "prismless" enamel of permanent human teeth. Arch Oral Biol, v. 12, n. 3, p. 381-387, Mar. 1967.

6. FAVA, M. et al. Fine structure and histometry of the enamel prismless layer of unerupted third molar teeth. Rev Chil Anat, v. 11, n. 1, p. 19-24, Jan./July 1993.

7. KEMPTHORNE, O.; FOLKS, L. Probability, statistics and data analysis. Iowa : University Press, 1971. p. 555.

8. KODAKA, T. et al. Structure of the so-"prismless" enamel in human deciduous teeth. Caries Res, v. 23, n. 5, p. 290-296, Sept./Oct. 1989.

9. MIYOSHI, S. et al. Scanning electron microscopy of prismless enamel in human teeth. Arch Oral Biol, v. 17, n. 2, p. 359-362, Feb. 1972.

10. REDFORD, D. A. et al. The effect of different etching times on the sealant bond strength, etch depth and pattern in primary teeth. Pediatr Dent, v. 8, n. 1, p. 11-15, Mar. 1986.

11. RIPA, I. W. et al. The "prismless" outer layer of deciduous and permanent enamel. Arch Oral Biol, v. 11, n. 1, p. 41-48, Jan. 1966.

12. WATANABE, I. S. et al. Three-dimensional characteristics of enamel" prismless" layer of the human premolar fissurae: a scanning electron microscopy study. Braz J Morphol Sci, v. 12, n. 1, p. 65-70, Jan./Jun. 1995.

13. WHITTAKER, D. K. Structural variations in the surface zone of human tooth enamel observed by scanning electron microscopy. Arch Oral Biol, v. 27, n. 5, p. 383-392, May 1982.

Recebido para publicação em 24/04/97

Aceito para publicação em 05/06/97

* Part of Post-Doctorate Research done at the School of Dentistry of the University of São Paulo, supported by CNPq (Process 150033-94-0 (nv)).

Part of Post-Doctorate Research done at the School of Dentistry of the University of São Paulo, supported by CNPq (Process 150033-94-0 (nv)).

** Department of Pediatric Dentistry, School of Dentistry - UNESP - São José dos Campos - SP - Brazil.

*** Department of Anatomy/Histology, Institute of Biomedical Sciences - USP - São Paulo - SP - Brazil.

**** Department of Pediatric Dentistry, School of Dentistry - Federal University of Goiás - GO - Brazil.

Publication Dates

• Publication in this collection
  23 Nov 1998
• Date of issue
  Oct 1997

History

• Received
  24 Apr 1997
• Accepted
  05 June 1997