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J Dent Res 82(2): 146-150, 2003
© 2003 International and American Associations for Dental Research

RESEARCH REPORT
Biomaterials & Bioengineering

Effect of Smear Layer on Root Demineralization Adjacent to Resin-modified Glass Ionomer

A.S. AL-Helal1, S.R. Armstrong1, X.J. Xie2, and J.S. Wefel1,*

1 Dows Institute for Dental Research, College of Dentistry N413, and
2 Department of Biostatistics, College of Public Health, The University of Iowa, Iowa City, IA 52242-1010;

*corresponding author, james-wefel{at}uiowa.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The cariostatic effect of resin-modified glass ionomer (RMGI) on secondary root caries is well-documented. However, this beneficial effect may be dependent upon the mode of cavity surface treatment. To investigate this relationship, we studied 4 cavity surface treatments prior to the placement of RMGI: no treatment (None), polyacrylic acid (PAA), phosphoric acid (H3PO4), and Scotchbond Multi-Purpose adhesive (SMP) as a control. Specimens were aged for two weeks in synthetic saliva, thermocycled, and subjected to an artificial caries challenge (pH 4.4). Polarized light microscopy (PLM) and microradiography (MRG) showed significantly less demineralization with the H3PO4 cavity surface treatment as revealed by ANOVA and Tukey’s multiple comparisons (p <= 0.05). Dentin fluoride profiles determined by electron probe microanalysis (EPMA) supported PLM and MRG findings. It may be concluded that removal of the smear layer with phosphoric acid provides significantly enhanced resistance to secondary root caries formation adjacent to RMGI restorations.

KEY WORDS: smear layer • demineralization • root • fluoride • glass ionomer


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Dental instrumentation during cavity preparation results in the formation of a smear layer, which is composed of a mixture of denatured collagen and mineral (Pashley, 1992). The effects of this layer on microleakage and bond strengths of different restorative materials have been studied; however, its influence on the development of secondary caries is not completely understood.

Secondary caries is the most common reason for restoration replacement and is frequently found on the root surface (Kidd et al., 1992). It is usually composed of two components, the outer lesion and the wall lesion. The outer lesion represents the primary carious attack along the tooth surface adjacent to the restoration, whereas the wall lesion is the development of caries along the tooth/restoration interface in the cavosurface tooth structure.

A series of artificial caries experiments shows that the frequency and extent of secondary caries outer and wall lesions were reduced around glass-ionomer restorations. Further, several investigations demonstrated inhibition zones adjacent to glass-ionomer cement; whereas other restorative materials do not produce inhibition zones and often even demonstrated caries wall lesions ( ten Cate and van Duinen, 1995; Tam et al., 1997; Hsu et al., 1998). Caries resistance and inhibition zone formation appear to be associated with the amount of fluoride released from glass ionomers and subsequent fluoride uptake by adjacent cavosurfaces (Francci et al., 1999; Torii et al., 2001). Recently, two-dimensional mapping by electron probe microanalysis demonstrated that the teeth restored with conventional and resin-modified glass ionomers took up higher amounts of fluoride with penetration deeper into dentin than enamel (Yamamoto et al., 2000).

The objective was to investigate the potential role of the smear layer on the development and inhibition of secondary root caries adjacent to RMGI. The hypothesis to be tested was that, in roots restored with RMGI and subjected to an artificial caries challenge, pre-treatment of the smear layer would influence the root demineralization adjacent to the restoration.


   MATERIALS & METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Specimen Preparation and Grouping
Human molar use falls under the expedited review category of the Institutional Review Board of the University of Iowa. Fifty-four caries-free permanent molars were selected, mounted in dental stone, and prepared in a microspecimen-former device. Class V cavities (2x2x5 mm) were made on the buccal surface at the cemento-enamel junction (CEJ) by means of a straight fissured carbide bur (#57) in a high-speed handpiece under copious air-water spray. Prepared teeth were divided randomly into 4 experimental groups:

Scanning Electron Microscopy
We randomly selected 6 of the prepared teeth where the cavity surface was treated according to the first three treatment modalities to examine the dentin surface morphology produced by each cavity surface treatment. The SMP group had presumably few details to reveal except smooth resin and therefore was excluded from the SEM analysis. We then sectioned each tooth transversely through the cavity preparation to obtain a single section containing the gingival floor of the cavity preparation. Sections were sputter-coated with Au-Pd and observed with an SEM (AMRAY 1820, AMRAY, Inc., Bedford, MA, USA) at 20 kV and 80 µA.

Artificial Root-surface Caries
After the appropriate cavity surface treatment, the teeth were restored with RMGI (Photac-Fil, ESPE, Seefeld, Germany) and light-cured for 60 sec. Restored teeth were placed in synthetic saliva (20 mmol/L NaHCO3, 3 mmol/L NaH2PO4.H2O, and 1 mmol/L CaCl2.2 H2O) for a two-week aging period. The aging medium was maintained at a pH of 7.0 and changed every 48 hrs to avoid saturation of the solution. Thermocycling followed in 5°C and 55°C distilled water for 1000 cycles with a dwell time of 30 sec and a transfer time of 15 sec. Teeth were then coated with an acid-resistant varnish, except for a window exposing the restoration and a 1-mm rim of tooth surface around the margins. Lesion formation occurred in a partially saturated buffer solution containing 2.2 mmol/L calcium (CaCl2), 2.2 mmol/L phosphate (NaH2PO4), and 50 mmol/L acetic acid at a pH of 4.4 for 5 days ( ten Cate and Duijsters, 1982).

Polarized Light Microscopy and Microradiography
Buccolingual tooth sections of 100 ± 20 µm were cut longitudinally in the midline of the restoration, and 3 sections per tooth were obtained. Sections were soaked in water and viewed with a polarized light microscope (PLM). The root demineralization area and depth at a distance of 100 µm from the gingival margin of the restoration, as well as the inhibition zone formed adjacent to the margin, were examined with PLM and traced by Image Pro Plus software (Media Cybernetics, Silver Spring, MD, USA). The inhibition zone was determined according to the secondary caries assessment model described previously (Hsu et al., 1998).

Tooth sections used for PLM evaluation were used again for microradiographic (MRG) analysis of mineral loss ({Delta}Z). Contact microradiographs of these sections were made and subjected to an image analysis program (BioQuant True color windows, R & M Biometric, Nashville, TN, USA) coupled with a computer program in the same manner as described previously by Heilman et al. (1997).

Electron Probe Microanalysis
Three specimens from each group were randomly selected. They were embedded in self-curing resin and subjected to serial polishing of the sectioned surface by means of 600- to 2400-grit silicon carbide papers and an aluminum oxide microabrasive system. These specimens were then coated with a thin layer of carbon in preparation for EPMA (Scanning Electron Microprobe, Bausch & Lomb, Sunland, CA, USA). Spot analysis for fluoride concentration was made at 10-µm intervals along a 100-µm transverse line from the gingival cavity wall into the adjacent sound dentin. A fluorapatite crystal containing 3.53 wt% fluoride was used as a standard. The operating conditions were 10 kV accelerating voltage, 40 nA probe current, and 30 sec counting time at each spot.

Statistical Analysis
All demineralization data were tested by one-way ANOVA, followed by Tukey’s multiple-comparison testing at p <= 0.05.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
SEM analysis revealed unique topographical appearances for each dentin substrate as a result of different cavity surface pre-treatment. In the group where no treatment was performed, the smear layer covered the dentin surface completely. It appeared as a uniform amorphous layer of cutting debris covering intertubular and peritubular dentin, with no dentinal tubules observed. In a cross-sectional view of a tooth section from this group, smear plugs were extended into the dentinal tubules for variable distances. In the group where the cavity surface was treated with PAA, a partial removal of the smear layer was noted. The smear layer was removed from intertubular dentin, but was maintained in the dentinal tubules, which were partially or completely occluded with the smear plugs (Fig. 1AGo). In the group that was treated with H3PO4 (Fig. 1BGo), both the smear layer and smear plugs were totally removed, and the dentinal tubules appeared widely opened and were funnel-shaped.



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  		Figure 1. SEM photomicrographs of dentin surface morphology resulting from different cavity surface treatments. (A) The smear layer is removed from intertubular dentin, but partially or completely occluded dentinal tubules are found in the PAA-treated group. (B) Total removal of the smear layer and smear plugs by H3PO4, left open, and funnel-shaped dentinal tubules.

 
Representative polarized light micrographs of tooth sections demonstrating different root demineralization patterns adjacent to RMGI restorations are shown in Figs. 2A, 2B, and 2CGo. The results of the PLM and MRG measurements are presented in the TableGo. Root demineralization was significantly less (p <= 0.05) in the H3PO4-treated group compared with the other groups, except for the PAA-treated group, for lesion area and lesion depth. The mineral loss ({Delta}Z) values were smaller in the H3PO4-treated group and significantly different from those in the other groups. The PAA-treated group was also significantly smaller (p <= 0.05) than the SMP-lined group for lesion depth and the untreated group for lesion area.



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  		Figure 2. PLM photomicrographs of sections soaked in water (L, lesion; D, dentin; R, restoration; IZ, inhibition zone). (A) Section from a tooth that received no cavity surface treatment. Note that no inhibition zone or wall lesion formation was found on the root adjacent to the gingival margin of the restoration. (B) A relatively small inhibition zone formed on the root adjacent to the margin in a tooth treated with PAA. (C) Large inhibition zone formed on the root adjacent to the margin in a tooth treated with H3PO4.

 

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  		Table. Demineralization Area, Depth, and Mineral Loss ({Delta}Z) Measured 100 µm from Gingival Margin
 
The H3PO4-treated group revealed a high percentage of specimens (83%) with inhibition zone formation. Similar prevalence was also found with the PAA-treated group (83%); however, the size of the inhibition area (1460 ± 463 µm2) was significantly smaller (p <= 0.0187) than that of the H3PO4-treated group (1961 ± 401 µm2). In contrast, only a low percentage of specimens with inhibition zone formation was found in the untreated group (33%) and the SMP-lined group (8%). Fluoride profiles (Fig. 3Go) were not subjected to statistical analysis due to small sample size. Nevertheless, they demonstrated a trend of more dentin fluoride uptake with the H3PO4 cavity surface treatment for 30-50 µm from the cavity wall, which may support PLM and MRG findings.



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  		Figure 3. Dentin fluoride profiles obtained at 10-µm intervals perpendicularly from the cavity gingival wall. Each spot represents the mean of three measurements of fluoride concentration ± SD (error bar). More fluoride uptake with phosphoric acid cavity surface treatment compared with other cavity surface treatments can be noted.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The results of the reduced lesion area, lesion depth, and mineral loss with a greater area of inhibition zones with phosphoric acid cavity surface treatment may be partially explained by the SEM findings. Unique topographical appearances were observed which are in agreement with previous SEM studies that showed complete smear layer removal and patent tubules from acid-etching with phosphoric acid and only partial smear layer removal with polyacrylic acid dentin-conditioning (Pashley, 1992; Eliades et al., 1997; Ayad, 2001). Phosphoric acid removed the smear layer and smear plugs, opened the dentinal tubules, and increased intertubular dentin surface porosities. Therefore, dentin surface permeability and the effective surface area for fluoride ion absorption and ionic exchange with RMGI were presumably higher compared with other surface treatment modalities.

There is a compositional difference between RMGI and conventional glass ionomer, due to the addition of a small quantity of light-curing resins. Hence, the adhesion mechanism may be different, as indicated by the higher dentin bond strengths with RMGI as compared with conventional glass ionomers (Lin et al., 1992; Friedl et al., 1995). Because of resin inclusion (HEMA) in RMGI, total removal of the smear layer not only facilitates the dynamic ion exchange process, but also provides micromechanical bonding, as described for resin-based adhesives. Therefore, the conception of avoiding strong conditioners because they may unduly demineralize the tooth and reduce the efficiency of the ion-exchange adhesion may not hold with RMGI, in contrast to conventional glass ionomer. Acid-etching with RMGI will enhance the micromechanical adhesion through the hybrid-like layer and intratubular tag formation, which were not frequently encountered with conventional glass ionomer. Some investigations have demonstrated these adhesion potentials with RMGI, showing dentinal tubules infiltrated with resin tags containing glass particles, as well as the formation of thin hybrid-like and resin-rich layers, at the dentin interface (Pereira et al., 1997; Gladys et al., 1998). This micromechanical adhesion may in turn increase the effective surface area for physicochemical interaction, including the dynamic ion exchange process between the dentin and glass ionomer.

Using an acid with a consistency, concentration, and application time similar to those used in the current study, Perdigão et al. (1996) demonstrated that demineralization is limited to the uppermost 1.6 µm of the dentin surface layer and results in a zone of collagen fibers with scattered hydroxyapatite inclusions. The exposure of subsurface collagen fibers, as a result of the acid-etching, may provide some degree of chemical adhesion between the carboxyl groups of the glass ionomer and collagen (Mount, 1999). Recently, a hydrogen bond has been suggested between the ester carbonyl group in HEMA, which usually forms most of the resin component in RMGI, and the carboxylic acid group in dentinal collagen (Nishiyama et al., 2002). Furthermore, it has been found that when the dentin was treated with acids that remove the smear layer, demineralize the dentin, and expose subsurface collagen fibers, dentin tensile and shear bond strength to conventional and resin-modified glass ionomer were either the same as or even higher than polyacrylic acid treatment or no treatment (Smith et al., 1988; Hinoura et al., 1991; Prati et al., 1992). Thus, bonding by various mechanisms–including chemical interaction, micromechanical interlocking to etched surfaces, and penetration into dentinal tubules–is favored by acid-etching and contributes, in part, to the caries-inhibition process.

The ability of RMGI to reduce root demineralization and create an inhibition area appears to be associated with the amount of fluoride released by the material and then diffused into the adjacent tooth structure (Francci et al., 1999; Torii et al., 2001). Among true resin-modified glass ionomers, Photac-Fil, which was used in our investigation, has been suggested to release either amounts of fluoride significantly higher than or comparable with those released by other RMGI restorative materials, such as Fuji II LC (GC Corp., Tokyo, Japan) or Vitremer (3M ESPE Dental Products, St. Paul, MN, USA) (Diaz-Arnold et al., 1995; Forsten, 1995; Torii et al., 2001). The fluoride uptake from RMGI illustrated in the fluoride profiles may further explain the reduction in secondary caries. The presence of fluoride enhances normal calcium and phosphate precipitation rates during remineralization and catalyzes the transformation of acidic calcium phosphate phases to more stable apatitic forms (Wefel, 1990). Intimate margin adaptation and enhanced mechanical and chemical interaction with organic and inorganic dentinal matrix facilitated more fluoride uptake by the adjacent dentin after phosphoric acid surface treatment for approximately 30-50 µm from the cavity wall. The increased fluoride concentration indicates that ion exchange has been enhanced, probably due to a greater surface area available for the diffusion of fluoride and increased permeability. However, the fluoride concentration at all intervals remains considerably lower than that recorded by EPMA (Yamamoto et al., 2000) for side wall dentin of a class V cavity which was etched with 37% phosphoric acid and restored with a fluoride-releasing composite resin. The presence of the smear layer or additional use of an intermediary adhesive liner decreased dentin fluoride uptake. A similar effect has been noted by Tam et al. (1997), where prior application of an adhesive resin layer to resin-modified glass ionomer significantly reduced the depth of fluoride uptake into the adjacent dentin.

It may be concluded that removal of the smear layer by acid-etching reduces demineralization and enhances caries resistance in the root cavosurface adjacent to resin-modified glass-ionomer restoration.


   ACKNOWLEDGMENTS
 
This investigation was supported by USPHS Research Grant P30 DE-10126 from the National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, MD 20892.

Received May 10, 2002; Last revision October 9, 2002; Accepted November 6, 2002


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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Diaz-Arnold AM, Holmes DC, Wistrom DW, Swift EJ Jr (1995). Short-term fluoride release/uptake of glass ionomer restoratives. Dent Mater 11:96–101.[ISI][Medline]

Eliades G, Palaghias G, Vougiouklakis G (1997). Effect of acidic conditioners on dentin morphology, molecular composition and collagen conformation in situ. Dent Mater 13:24–33.[ISI][Medline]

Forsten L (1995). Resin-modified glass ionomer cements: fluoride release and uptake. Acta Odontol Scand 53:222–225.[ISI][Medline]

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Gladys S, Van Meerbeek B, Lambrechts P, Vanherle G (1998). Marginal adaptation and retention of a glass-ionomer, resin-modified glass-ionomers and a polyacid-modified resin compsite in cervical Class-V lesions. Dent Mater 14:294–306.[ISI][Medline]

Heilman JR, Jordan TH, Warwick R, Wefel JS (1997). Remineralization of root surfaces demineralized in solutions of different fluoride levels. Caries Res 31:423–428.[ISI][Medline]

Hinoura K, Miyazaki M, Onose H (1991). Dentin bond strength of light-cured glass-ionomer cements. J Dent Res 70:1542–1544.[Abstract/Free Full Text]

Hsu CY, Donly KJ, Drake DR, Wefel JS (1998). Effect of aged fluoride-containing restorative materials on recurrent root caries. J Dent Res 77:418–425.[Abstract/Free Full Text]

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Nishiyama N, Suzuki K, Komatsu K, Yasuda S, Nemoto K (2002). A 13C NMR study on the adsorption characteristics of HEMA to dentinal collagen. J Dent Res81:469–471.[Abstract/Free Full Text]

Pashley DH (1992). Smear layer: overview of structure and function. Proc Finn Dent Soc 88(Suppl 1):215–224.

Perdigão J, Lambrechts P, van Meerbeek B, Tome AR, Vanherle G, Lopes AB (1996). Morphological field emission-SEM study of the effect of six phosphoric acid etching agents on human dentin. Dent Mater 12:262–271.[ISI][Medline]

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