Original Paper Published online: April 28, 2015
Caries Res 2015;49:291–296 DOI: 10.1159/000381192
Oral Fluoride Levels 1 h after Use of a Sodium Fluoride Rinse: Effect of Sodium Lauryl Sulfate Gerald L. Vogel a Gary E. Schumacher a Laurence C. Chow a Livia M.A. Tenuta b
b
American Dental Association Foundation, Dr. Anthony Volpe Research Center, Gaithersburg, Md., USA; Piracicaba Dental School, University of Campinas, Piracicaba, Brazil
Key Words Fluoride · Plaque · Plaque fluid · Saliva · Sodium lauryl sulfate
Abstract Increasing the concentration of free fluoride in oral fluids is an important goal in the use of topical fluoride agents. Although sodium lauryl sulfate (SLS) is a common dentifrice ingredient, the influence of this ion on plaque fluid and salivary fluid fluoride has not been examined. The purpose of this study was to investigate the effect of SLS on these parameters and to examine the effect of this ion on total (or whole) plaque fluoride, an important source of plaque fluid fluoride after a sufficient interval following fluoride administration, and on total salivary fluoride, a parameter often used as a surrogate measure of salivary fluid fluoride. Ten subjects accumulated plaque for 48 h before rinsing with a 12 mmol/l NaF (228 μg/g F) rinse containing or not containing 0.5% (w/w) SLS. SLS had no statistically significant effect on total plaque and total saliva fluoride but significantly increased salivary fluid and plaque fluid fluoride (by 147 and 205%, respectively). These results suggest that the nonfluoride components of topical agents can be manipulated to improve the fluoride release characteristics from oral fluoride reservoirs and that statistically significant change may be ob-
© 2015 S. Karger AG, Basel 0008–6568/15/0493–0291$39.50/0 E-Mail [email protected] www.karger.com/cre
served in plaque fluid and salivary fluid fluoride concentrations that may not be observed in total plaque and total saliva fluoride concentrations. © 2015 S. Karger AG, Basel
Based on thermodynamic and kinetic considerations [Arends and Christoffersen, 1990; Margolis and Moreno, 1990], in vitro studies [ten Cate, 1990, 1997], and human and animal studies [Featherstone, 2000], the ability to maintain an increased concentration of free fluoride in the oral fluids surrounding the teeth appears to be one of the major factors in predicting the cariostatic effect of a topical fluoride treatment. The apparent importance of oral fluid fluoride (plaque fluid and saliva fluid) concentration has in turn focused attention on the moderately soluble oral fluoride reservoirs responsible for elevating these fluid fluoride levels over an extended period of time.
Certain commercial materials and equipment are identified in this paper to specify the experimental procedure. In no instance does such identification imply recommendation or endorsement by the National Institute of Standards and Technology or the ADA Foundation or that the material or the equipment identified is necessarily the best available for the purpose.
Gerald Vogel American Dental Association Foundation, Dr. Anthony Volpe Research Center National Institute of Standards and Technology 100 Bureau Drive Stop 8546, Gaithersburg, MD 20899–8546 (USA) E-Mail jvogel @ nist.gov
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a
Material and Methods Subjects All procedures were done with the informed written consent of the study participants following protocols approved by the Institutional Review Board of the American Dental Association and
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Caries Res 2015;49:291–296 DOI: 10.1159/000381192
reviewed by the Institutional Review Board of the National Institute of Standards and Technology (NIST). The subjects were chosen from adult volunteers employed by NIST or Dr. Anthony Volpe Research Center (located on the NIST campus in Gaithersburg, Md., USA). They were screened before inclusion to ensure good oral health and normal salivary gland function (>0.2 ml/min, assessed by measuring unstimulated salivary flow in each potential participant). All study participants lived in Montgomery and Fredrick counties in Maryland, which are areas with fluoridated public water supplies (yearly average fluoride = 0.94 μg/g, range 0.42– 1.30 μg/g, from the municipal water authority of these counties), and used their normal oral hygiene procedures between periods of plaque accumulation (i.e. no special tooth cleaning procedures were performed). The number of subjects (n = 12) was based on an 80% power to detect a difference of 50% with a p < 0.05. The data used in this calculation were taken from similar studies found in the literature [Vogel et al., 2006b, 2008b] and previous studies performed in the laboratory [Vogel and Schumacher, 2011]. One subject withdrew from the study, and 1 subject provided too little plaque fluid for analysis so that 10 volunteers completed all parts of the study. Their average age was 46 years (range 34–60 years, 2 women, 8 men). The same set of subjects was used throughout the study. Test Rinses and Study Design The study, which was performed in late 2009 and early 2010, employed two aqueous 12 mmol/l fluoride solutions (228 μg/g as NaF), with and without 0.5% (w/w) SLS. Before each experiment the participants were asked to brush and floss thoroughly, and cease all oral hygiene (including the use of dentifrices, mouth rinses, and chewing gums) for 48 h to allow for plaque accumulation. The subjects then fasted overnight before use of the topical rinses (without supervision) the following morning. The test rinses were 20 ml in volume and were used for 1 min. Samples were then collected 1 h later, after which the subjects were provided breakfast. The participants waited at least 2 weeks between each regimen. The agents were given in a random order (chosen using a computergenerated random number) and the participants were not aware of the identity of the test materials. However, it should be noted that many of the subjects noted an unpleasant taste in the case of the SLS-containing rinse. The analysts were not aware of the identity of the recovered samples. Sample Collection The sample collection procedures have been extensively described [Vogel et al., 1992, 2000, 2008b; Tenuta et al., 2006] and are only summarized here: (1) A 0.2-mm-thick plastic strip (size about 2 × 6 mm) and mineral oil-filled microcentrifuge tube (a 10-μl pipette tip with a heat-sealed tip) were weighed together. (2) The strip, grasped in a hemostat, was used to collect pooled plaque from the easily accessible buccal surfaces of all first and second molars and premolar teeth. (3) The strip and plaque sample were transferred into the oil-filled/sealed pipette tip and reweighed to determine the plaque mass. (4) The microcentrifuge tube and contents were centrifuged (5 min, 2 ° C, 14,000 rpm or ≈1,466 rad/s) and a sample (less than 0.1 μl) of the fluid recovered with a capillary micropipette for the determination of plaque fluid fluoride. However, in a departure from previous procedures, any plaque fluid samples showing traces of plaque particulates were recentrifuged in a new oil-filled microcentrifuge tube and the clar
Vogel/Schumacher/Chow/Tenuta
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Sodium lauryl sulfate (SLS) is a detergent commonly used in dentifrices at a concentration ranging from 0.25 to 2% w/w [Barkvoll et al., 1988; Robinson et al., 2006]. In spite of the near-ubiquitous presence of SLS in dentifrices and its potential impact on the cariostatic effect of fluoride, there seems to be only one study on the effect of SLS on the fluoride content of oral fluids. This study, which employed dentifrices as test agents, suggested that SLS could increase salivary fluid fluoride [Bruun et al., 1987]. Unfortunately such dentifrice components as abrasives, flavors, and thickening agent complicate the interpretation of the effect of SLS. Furthermore, there appear to be no studies on the effect of SLS on plaque fluid, the fluid phase associated with the site of the developing lesion, or on total (or whole) plaque fluoride, an important source of plaque fluid fluoride (especially during a cariogenic challenge), or on total (whole) salivary fluoride, a parameter commonly used as a measure of salivary fluid fluoride. SLS may interfere with the formation of labile oral fluoride reservoirs or induce their disintegration, due to the ability of this ion to bind the calcium ions [Iranan and Callis, 1960; Barkvoll et al., 1988] thought to be integral to their formation. Such negative effects can occur both in plaque and the oral mucosa [Pessan et al., 2006; Vogel et al., 2006b; Vogel, 2011] or on enamel and within artificial enamel lesions [Barkvoll et al., 1988; Barkvoll, 1991]. SLS may also, by altering and removing structural components of the mucosa or plaque [Herlofson and Barkvoll, 1993; Robinson et al., 2006], increase the formations of fluoride deposits within these substrates as well as the rate of fluoride transport from these substrates into salivary fluid. The purpose of this study was to measure the effect of SLS on total saliva/plaque fluoride and on salivary/ plaque fluid fluoride after use of an aqueous NaF solution. We believed that the ability of SLS to extract calcium from oral fluoride-calcium reservoirs during rinse administration would be the dominant factor. Hence we hypothesized that SLS would cause total plaque and saliva fluoride to decrease and that these changes would be reflected in decreases in plaque fluid and saliva fluid fluoride.
g. di
60
nc re ffe e
40
nc re ffe e
Fluid F (μmol/l)
With SLS
(1.7)
Si
20 0
(2.7)
[9]
[10]
(2.5)
No difference
Sig. difference
No difference
Total saliva
Salivary fluid
Salivary particulates
Fig. 1. Total saliva and salivary fluid fluoride given as the geometric
mean and standard error factor (in parentheses). The salivary particulates, which is the difference between total salivary fluoride concentration and the fluoride concentration in the salivary fluid, is given as arithmetic mean and standard error (in square brackets). Sold arrows show the difference between the no-SLS and SLS groups within each type of sample (total or salivary fluid fluoride, and salivary particulates). Dashed arrows show the statistical difference between total and salivary fluid fluoride within the SLS and no-SLS groups.
Table 1. Salivary flow and plaque mass
No SLS With SLS
Salivary flow, g/min
Plaque mass, mg
1.10 (0.12) 1.06 (0.13)
4.54 (0.45) 4.47 (0.50)
Data expressed as mean with standard error given in parentheses. There are no differences in each group.
Results
There were no differences between the salivary flow rates or plaque mass as a result of the addition of SLS (table 1). When SLS was included in the rinse, none of the changes in fluoride concentration in total (i.e. whole) saliva, saliva particulates (fig. 1) and total plaque (fig. 2) were statistically significant (p = 0.475, 0.564, and 0.945, respectively). In contrast (fig. 1, 2), SLS induced a much larger and statistically significant increase in both plaque fluid and salivary fluid fluoride (p = 0.022 and p = 0.046, respectively). In regard to the two methods of measuring salivary fluoride, total salivary fluoride levels were significantly larger than salivary fluid fluoride levels for both the non-SLS and SLS-containing rinses (p < 0.001 and p = 0.002, respectively, fig. 1). Caries Res 2015;49:291–296 DOI: 10.1159/000381192
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Oral Fluoride after SLS
80
di
Statistical Analysis and Data Presentation A significance level of p < 0.05 was used in all statistical tests, which were performed using SigmaStat software (Systat Software Inc., San Jose, Calif., USA). A Student’s t or paired t test was used to examine the effect of SLS on fluoride concentration in the different types of samples. In the case of the normally distributed data (plaque mass, salivary flow) the data are presented as mean and standard error. However, the fluoride data, with the exception of the salivary particulate concentrations, required logarithmic transformation [Zero et al., 1992a; Vogel et al., 2006a, b, 2008a, b; Pessan, 2010] in order to satisfy the requirement of variance homogenicity or normality required for the use of parametric tests. Consistent with the log transformation, the fluoride data are presented as the geometric mean, i.e. the antilog of the average of the log values, and standard deviation factor, the antilog of the average of the standard deviations of the log values. This method of presentation, which has frequently been used in reporting this type of data [Zero et al., 1992a; Vogel et al., 2006a, b, 2008a, b], has the advantage of suppressing the effect of large outliers that are common in such studies. The salivary particulate concentrations, which are the calculated difference between the total and salivary fluid values, are presented as the mean and standard error.
No SLS
(2.4)
g.
Analytical Procedure The fluoride in all samples was analyzed, after dilution of 9 parts sample with 1 part of TISAB III (Thermo Electron Corp., Waltham, Mass., USA), using an inverted fluoride electrode apparatus [Vogel et al., 1990, 2000]: (1) The total saliva or total plaque acid mixtures were extracted overnight into 1 M HClO4 after which equal volumes of these fluids and a mixture of 1 M NaOH with 20% TISAB III (v/v) were added, centrifuged, and the clarified fluid recovered. (2) Plaque fluid was diluted on the surface of the inverted electrode using micropipettes [Vogel et al., 1990] while the dilution of the salivary fluid was done with conventional mechanical pipettes. Both plaque fluid and salivary fluid fluoride were TISAB III-diluted as soon as possible after collection (about 1 h). It should be noted that, due to the low level of fluoride binding in these fluids (mainly from 1 to 3 mmol/l of calcium), the fluoride measured by this procedure is nearly identical to the free F (calculated using Chemist, Micromath, Saint Louis, Mo., USA).
100
Si
ified fluid recovered. (5) The microcentrifuge tube containing the plaque residue was cut at the heat-sealed tip with a razor blade, pressed into a hole in the cap of a 600-μl plastic centrifuge tube, and the residue expelled into the bottom of the tube by momentary centrifugation. (6) Perchloric acid (1 M) was then added (50 μl/mg of plaque) to the tube for the determination of total plaque F. It should be noted that the amount of fluoride contained in the recovered plaque fluid is too small to influence the determination of the total fluoride in plaque [Vogel et al., 2008b]. (7) Immediately after plaque collection, saliva was collected for 2 min by expectoration into preweighed 50-ml tubes. (8) The saliva was weighed for the determination of salivary flow, homogenized by momentarily vortexing for about 5 s, and then 90 μl of the fluid was dispersed into 10 μl of 10 M perchloric acid for the extraction of fluoride and the determination of total saliva fluoride. (9) The remaining saliva was then centrifuged for 5 min and an aliquot of the clear salivary fluid recovered for determination of salivary fluid F.
(3.3)
With SLS
(3.2)
1.5 40 1.0 (2.0)
0.5
0
No difference
Sig. difference
Total plaque
Plaque fluid
20
Plaque fluid F (μmol/l)
Total plaque F (μmol/g)
(1.8)
60
0
Fig. 2. Total and plaque fluid fluoride given as the geometric mean and standard error factor (in parentheses). Sold arrows show the difference between the no-SLS and SLS groups within each type of sample (total or plaque fluid fluoride).
Discussion
The salivary and plaque fluoride concentrations observed in this study in the absence of SLS are similar to previous 1-hour postrinse values obtained using similar methodologies [Vogel et al., 2000, 2001, 2006a, b, 2008a, b, 2010]. The average ratio of the 1-hour to baseline fluoride concentrations in these studies varied from 3.7× (plaque fluid) to 6.5× (total plaque), which suggests that variations in the baseline fluoride concentrations should have only a minimal influence on the 1-hour values obtained here. It should be noted, however, that in these previous studies 1-hour postrinse plaque fluid fluoride concentrations were obtained which exceeded the salivary fluid fluoride concentrations, while in the current study these values were similar. Although the reason for this divergence is not apparent, improvements in technique, specifically the double centrifugation of plaque fluid samples showing traces of plaque solids, may partially explain this result. After salivary flow has cleared the applied agent, the primary source of plaque fluid and salivary fluid fluoride are the labile reservoirs of this ion in the plaque and oral mucosa [Duckworth and Morgan 1991; Zero et al., 1992b; Vogel et al., 2006a, 2008b; Vogel, 2011]. Salivary particles may also be a significant source of fluoride particularly in the case of salivary fluid. It is these particles, which consist of fluoride-containing precipitates and oral mu294
Caries Res 2015;49:291–296 DOI: 10.1159/000381192
cosa fragments, that increased total salivary fluoride above salivary fluid fluoride both in the absence or presence of SLS (fig. 1). Perhaps more importantly, there was a clear, statistically significant increase in the 1-hour postrinse salivary fluid fluoride concentration but a smaller nonsignificant increase in total salivary fluoride concentration (fig. 1). It is noteworthy in this regard that, since the contribution of salivary particulates to the total salivary fluoride is similar in the SLS and non-SLS samples (fig. 1), the entire increase in total salivary fluoride concentration between the SLS and non-SLS rinses can be attributed to an increase in the fluoride concentration of the salivary fluid. Even more pronounced than its effect on salivary total/fluid fluoride concentration, SLS produced only a negligible increase in the total plaque fluoride concentration, but tripled plaque fluid fluoride concentration (fig. 2). Given that plaque fluid fluoride makes only a very small contribution to total plaque fluoride [Vogel et al., 2000, 2008b, 2010], it can be concluded that, as in the case of salivary particles, the inclusion of SLS in the rinse had only a small effect on the uptake of fluoride by plaque. It is fluoride solubilized into the fluid phase of saliva that can interact with the tooth surface to exert a cariostatic effect. However, total salivary fluoride measurements can include large contributions from nonsolubilized fluoride-containing particulates which can, as observed in this study, eliminate statistical differences seen in salivary fluid fluoride measurements. Hence the rationale for the use of total saliva fluoride concentrations as an indicator of the potential therapeutic effect of a topical treatment is questionable [Vogel, 2011]. This does not, however, obviate the value of total plaque fluoride measurements as an indicator of the size of the plaque fluoride reservoir that may release additional fluoride into plaque fluid during the critical low pH period following a cariogenic attack [Arends and Christoffersen, 1990; Margolis and Moreno, 1990; Vogel, 2011]. SLS has been shown to disrupt or open up the structure of both the mucosa [Herlofson and Barkvoll, 1993] and plaque [Robinson et al., 2006], a process that might be expected to increase the deposition of fluoride into oral reservoirs and also increase the subsequent rate of fluoride loss from them. However, in this study it was observed that, 1 h after administration of the fluoride rinse, the inclusion of SLS had little effect on the fluoride content of the total plaque or of the salivary particulates (which are derived mostly from the mucosa) but significantly increased the concentration of plaque fluid and salivary fluid fluoride (fig. 1, 2). Although any Vogel/Schumacher/Chow/Tenuta
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No SLS
2.0
mechanism for these results is speculative, the fact that the fluoride ion diffuses about 7 times faster than the lauryl sulphate ion [Deng et al., 1996; Stewart, 2005] provides a plausible explanation: the rapidly diffusing fluoride ion may be fixed into these substrates before the slower diffusing lauryl sulfate ion penetrates. Furthermore, since oral fluoride binding in the plaque and mucosa appears to be calcium-mediated [Rose et al., 1996; Whitford et al., 2002; Vogel et al., 2006a, 2008b, 2010; Vogel, 2011], the subsequent fluoride release from these deposits into the fluid phase of plaque or saliva should, in accordance with the results observed here, be increased by their subsequent exposure to a relatively high concentration of a calcium-binding (i.e. calciumextracting) ion such as lauryl sulfate [Iranan and Callis, 1960; Barkvoll et al., 1988]. The SLS disruption of the mucosa should also increase salivary fluid fluoride, which in view of the possible salivary and plaque fluid fluoride equilibrium noted above should lead to an increase in plaque fluid fluoride. In conclusion, the results of this study show that, contrary to the hypothesis of the Introduction, SLS had little effect on total plaque fluoride, induced a small, not statistically significant increase in total saliva fluoride, but significantly increased plaque fluid and salivary fluid fluoride. Such a result suggests that the nonfluoride components of fluoride dentifrices and rinses, in particular surfactants such as SLS, can be manipulated to improve the fluoride release characteristics from oral fluoride res-
ervoirs following conventional topical fluoride treatments. The characterization of the effect of these components on these reservoirs, perhaps using a recently described extraction technique [Vogel et al., 2010, 2014], combined with recent procedures shown to greatly increase the amount of these reservoirs [Vogel et al., 2006a, b, 2008a, b], may lead to the development of topical agents that induce a much larger and more sustained increase in oral fluid fluoride. Treatments based on this approach could potentially have a much larger cariostatic effect than can be obtained with current fluoride topical agents with the same fluoride dose.
Acknowledgments This study was supported by a grant from the ADA Foundation and is part of the dental research program conducted at the National Institute of Standards and Technology in cooperation with the ADA Foundation. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. The roles of the authors in this study were as follows: G.L.V. conceived and designed the experiments. G.E.S. performed the clinical examination and collected samples. G.L.V. performed the analysis and analyzed the data. G.L.V., G.E.S., L.C.C., L.M.A.T. wrote the manuscript.
Disclosure Statement All authors declared that there are no conflicts of interest.
References
Oral Fluoride after SLS
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Caries Res 2015;49:291–296 DOI: 10.1159/000381192
Oral Fluoride Levels 1 h after Use of a Sodium Fluoride Rinse: Effect of Sodium Lauryl Sulfate Gerald L. Vogel a Gary E. Schumacher a Laurence C. Chow a Livia M.A. Tenuta b
b
American Dental Association Foundation, Dr. Anthony Volpe Research Center, Gaithersburg, Md., USA; Piracicaba Dental School, University of Campinas, Piracicaba, Brazil
Key Words Fluoride · Plaque · Plaque fluid · Saliva · Sodium lauryl sulfate
Abstract Increasing the concentration of free fluoride in oral fluids is an important goal in the use of topical fluoride agents. Although sodium lauryl sulfate (SLS) is a common dentifrice ingredient, the influence of this ion on plaque fluid and salivary fluid fluoride has not been examined. The purpose of this study was to investigate the effect of SLS on these parameters and to examine the effect of this ion on total (or whole) plaque fluoride, an important source of plaque fluid fluoride after a sufficient interval following fluoride administration, and on total salivary fluoride, a parameter often used as a surrogate measure of salivary fluid fluoride. Ten subjects accumulated plaque for 48 h before rinsing with a 12 mmol/l NaF (228 μg/g F) rinse containing or not containing 0.5% (w/w) SLS. SLS had no statistically significant effect on total plaque and total saliva fluoride but significantly increased salivary fluid and plaque fluid fluoride (by 147 and 205%, respectively). These results suggest that the nonfluoride components of topical agents can be manipulated to improve the fluoride release characteristics from oral fluoride reservoirs and that statistically significant change may be ob-
© 2015 S. Karger AG, Basel 0008–6568/15/0493–0291$39.50/0 E-Mail [email protected] www.karger.com/cre
served in plaque fluid and salivary fluid fluoride concentrations that may not be observed in total plaque and total saliva fluoride concentrations. © 2015 S. Karger AG, Basel
Based on thermodynamic and kinetic considerations [Arends and Christoffersen, 1990; Margolis and Moreno, 1990], in vitro studies [ten Cate, 1990, 1997], and human and animal studies [Featherstone, 2000], the ability to maintain an increased concentration of free fluoride in the oral fluids surrounding the teeth appears to be one of the major factors in predicting the cariostatic effect of a topical fluoride treatment. The apparent importance of oral fluid fluoride (plaque fluid and saliva fluid) concentration has in turn focused attention on the moderately soluble oral fluoride reservoirs responsible for elevating these fluid fluoride levels over an extended period of time.
Certain commercial materials and equipment are identified in this paper to specify the experimental procedure. In no instance does such identification imply recommendation or endorsement by the National Institute of Standards and Technology or the ADA Foundation or that the material or the equipment identified is necessarily the best available for the purpose.
Gerald Vogel American Dental Association Foundation, Dr. Anthony Volpe Research Center National Institute of Standards and Technology 100 Bureau Drive Stop 8546, Gaithersburg, MD 20899–8546 (USA) E-Mail jvogel @ nist.gov
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a
Material and Methods Subjects All procedures were done with the informed written consent of the study participants following protocols approved by the Institutional Review Board of the American Dental Association and
292
Caries Res 2015;49:291–296 DOI: 10.1159/000381192
reviewed by the Institutional Review Board of the National Institute of Standards and Technology (NIST). The subjects were chosen from adult volunteers employed by NIST or Dr. Anthony Volpe Research Center (located on the NIST campus in Gaithersburg, Md., USA). They were screened before inclusion to ensure good oral health and normal salivary gland function (>0.2 ml/min, assessed by measuring unstimulated salivary flow in each potential participant). All study participants lived in Montgomery and Fredrick counties in Maryland, which are areas with fluoridated public water supplies (yearly average fluoride = 0.94 μg/g, range 0.42– 1.30 μg/g, from the municipal water authority of these counties), and used their normal oral hygiene procedures between periods of plaque accumulation (i.e. no special tooth cleaning procedures were performed). The number of subjects (n = 12) was based on an 80% power to detect a difference of 50% with a p < 0.05. The data used in this calculation were taken from similar studies found in the literature [Vogel et al., 2006b, 2008b] and previous studies performed in the laboratory [Vogel and Schumacher, 2011]. One subject withdrew from the study, and 1 subject provided too little plaque fluid for analysis so that 10 volunteers completed all parts of the study. Their average age was 46 years (range 34–60 years, 2 women, 8 men). The same set of subjects was used throughout the study. Test Rinses and Study Design The study, which was performed in late 2009 and early 2010, employed two aqueous 12 mmol/l fluoride solutions (228 μg/g as NaF), with and without 0.5% (w/w) SLS. Before each experiment the participants were asked to brush and floss thoroughly, and cease all oral hygiene (including the use of dentifrices, mouth rinses, and chewing gums) for 48 h to allow for plaque accumulation. The subjects then fasted overnight before use of the topical rinses (without supervision) the following morning. The test rinses were 20 ml in volume and were used for 1 min. Samples were then collected 1 h later, after which the subjects were provided breakfast. The participants waited at least 2 weeks between each regimen. The agents were given in a random order (chosen using a computergenerated random number) and the participants were not aware of the identity of the test materials. However, it should be noted that many of the subjects noted an unpleasant taste in the case of the SLS-containing rinse. The analysts were not aware of the identity of the recovered samples. Sample Collection The sample collection procedures have been extensively described [Vogel et al., 1992, 2000, 2008b; Tenuta et al., 2006] and are only summarized here: (1) A 0.2-mm-thick plastic strip (size about 2 × 6 mm) and mineral oil-filled microcentrifuge tube (a 10-μl pipette tip with a heat-sealed tip) were weighed together. (2) The strip, grasped in a hemostat, was used to collect pooled plaque from the easily accessible buccal surfaces of all first and second molars and premolar teeth. (3) The strip and plaque sample were transferred into the oil-filled/sealed pipette tip and reweighed to determine the plaque mass. (4) The microcentrifuge tube and contents were centrifuged (5 min, 2 ° C, 14,000 rpm or ≈1,466 rad/s) and a sample (less than 0.1 μl) of the fluid recovered with a capillary micropipette for the determination of plaque fluid fluoride. However, in a departure from previous procedures, any plaque fluid samples showing traces of plaque particulates were recentrifuged in a new oil-filled microcentrifuge tube and the clar
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Sodium lauryl sulfate (SLS) is a detergent commonly used in dentifrices at a concentration ranging from 0.25 to 2% w/w [Barkvoll et al., 1988; Robinson et al., 2006]. In spite of the near-ubiquitous presence of SLS in dentifrices and its potential impact on the cariostatic effect of fluoride, there seems to be only one study on the effect of SLS on the fluoride content of oral fluids. This study, which employed dentifrices as test agents, suggested that SLS could increase salivary fluid fluoride [Bruun et al., 1987]. Unfortunately such dentifrice components as abrasives, flavors, and thickening agent complicate the interpretation of the effect of SLS. Furthermore, there appear to be no studies on the effect of SLS on plaque fluid, the fluid phase associated with the site of the developing lesion, or on total (or whole) plaque fluoride, an important source of plaque fluid fluoride (especially during a cariogenic challenge), or on total (whole) salivary fluoride, a parameter commonly used as a measure of salivary fluid fluoride. SLS may interfere with the formation of labile oral fluoride reservoirs or induce their disintegration, due to the ability of this ion to bind the calcium ions [Iranan and Callis, 1960; Barkvoll et al., 1988] thought to be integral to their formation. Such negative effects can occur both in plaque and the oral mucosa [Pessan et al., 2006; Vogel et al., 2006b; Vogel, 2011] or on enamel and within artificial enamel lesions [Barkvoll et al., 1988; Barkvoll, 1991]. SLS may also, by altering and removing structural components of the mucosa or plaque [Herlofson and Barkvoll, 1993; Robinson et al., 2006], increase the formations of fluoride deposits within these substrates as well as the rate of fluoride transport from these substrates into salivary fluid. The purpose of this study was to measure the effect of SLS on total saliva/plaque fluoride and on salivary/ plaque fluid fluoride after use of an aqueous NaF solution. We believed that the ability of SLS to extract calcium from oral fluoride-calcium reservoirs during rinse administration would be the dominant factor. Hence we hypothesized that SLS would cause total plaque and saliva fluoride to decrease and that these changes would be reflected in decreases in plaque fluid and saliva fluid fluoride.
g. di
60
nc re ffe e
40
nc re ffe e
Fluid F (μmol/l)
With SLS
(1.7)
Si
20 0
(2.7)
[9]
[10]
(2.5)
No difference
Sig. difference
No difference
Total saliva
Salivary fluid
Salivary particulates
Fig. 1. Total saliva and salivary fluid fluoride given as the geometric
mean and standard error factor (in parentheses). The salivary particulates, which is the difference between total salivary fluoride concentration and the fluoride concentration in the salivary fluid, is given as arithmetic mean and standard error (in square brackets). Sold arrows show the difference between the no-SLS and SLS groups within each type of sample (total or salivary fluid fluoride, and salivary particulates). Dashed arrows show the statistical difference between total and salivary fluid fluoride within the SLS and no-SLS groups.
Table 1. Salivary flow and plaque mass
No SLS With SLS
Salivary flow, g/min
Plaque mass, mg
1.10 (0.12) 1.06 (0.13)
4.54 (0.45) 4.47 (0.50)
Data expressed as mean with standard error given in parentheses. There are no differences in each group.
Results
There were no differences between the salivary flow rates or plaque mass as a result of the addition of SLS (table 1). When SLS was included in the rinse, none of the changes in fluoride concentration in total (i.e. whole) saliva, saliva particulates (fig. 1) and total plaque (fig. 2) were statistically significant (p = 0.475, 0.564, and 0.945, respectively). In contrast (fig. 1, 2), SLS induced a much larger and statistically significant increase in both plaque fluid and salivary fluid fluoride (p = 0.022 and p = 0.046, respectively). In regard to the two methods of measuring salivary fluoride, total salivary fluoride levels were significantly larger than salivary fluid fluoride levels for both the non-SLS and SLS-containing rinses (p < 0.001 and p = 0.002, respectively, fig. 1). Caries Res 2015;49:291–296 DOI: 10.1159/000381192
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Oral Fluoride after SLS
80
di
Statistical Analysis and Data Presentation A significance level of p < 0.05 was used in all statistical tests, which were performed using SigmaStat software (Systat Software Inc., San Jose, Calif., USA). A Student’s t or paired t test was used to examine the effect of SLS on fluoride concentration in the different types of samples. In the case of the normally distributed data (plaque mass, salivary flow) the data are presented as mean and standard error. However, the fluoride data, with the exception of the salivary particulate concentrations, required logarithmic transformation [Zero et al., 1992a; Vogel et al., 2006a, b, 2008a, b; Pessan, 2010] in order to satisfy the requirement of variance homogenicity or normality required for the use of parametric tests. Consistent with the log transformation, the fluoride data are presented as the geometric mean, i.e. the antilog of the average of the log values, and standard deviation factor, the antilog of the average of the standard deviations of the log values. This method of presentation, which has frequently been used in reporting this type of data [Zero et al., 1992a; Vogel et al., 2006a, b, 2008a, b], has the advantage of suppressing the effect of large outliers that are common in such studies. The salivary particulate concentrations, which are the calculated difference between the total and salivary fluid values, are presented as the mean and standard error.
No SLS
(2.4)
g.
Analytical Procedure The fluoride in all samples was analyzed, after dilution of 9 parts sample with 1 part of TISAB III (Thermo Electron Corp., Waltham, Mass., USA), using an inverted fluoride electrode apparatus [Vogel et al., 1990, 2000]: (1) The total saliva or total plaque acid mixtures were extracted overnight into 1 M HClO4 after which equal volumes of these fluids and a mixture of 1 M NaOH with 20% TISAB III (v/v) were added, centrifuged, and the clarified fluid recovered. (2) Plaque fluid was diluted on the surface of the inverted electrode using micropipettes [Vogel et al., 1990] while the dilution of the salivary fluid was done with conventional mechanical pipettes. Both plaque fluid and salivary fluid fluoride were TISAB III-diluted as soon as possible after collection (about 1 h). It should be noted that, due to the low level of fluoride binding in these fluids (mainly from 1 to 3 mmol/l of calcium), the fluoride measured by this procedure is nearly identical to the free F (calculated using Chemist, Micromath, Saint Louis, Mo., USA).
100
Si
ified fluid recovered. (5) The microcentrifuge tube containing the plaque residue was cut at the heat-sealed tip with a razor blade, pressed into a hole in the cap of a 600-μl plastic centrifuge tube, and the residue expelled into the bottom of the tube by momentary centrifugation. (6) Perchloric acid (1 M) was then added (50 μl/mg of plaque) to the tube for the determination of total plaque F. It should be noted that the amount of fluoride contained in the recovered plaque fluid is too small to influence the determination of the total fluoride in plaque [Vogel et al., 2008b]. (7) Immediately after plaque collection, saliva was collected for 2 min by expectoration into preweighed 50-ml tubes. (8) The saliva was weighed for the determination of salivary flow, homogenized by momentarily vortexing for about 5 s, and then 90 μl of the fluid was dispersed into 10 μl of 10 M perchloric acid for the extraction of fluoride and the determination of total saliva fluoride. (9) The remaining saliva was then centrifuged for 5 min and an aliquot of the clear salivary fluid recovered for determination of salivary fluid F.
(3.3)
With SLS
(3.2)
1.5 40 1.0 (2.0)
0.5
0
No difference
Sig. difference
Total plaque
Plaque fluid
20
Plaque fluid F (μmol/l)
Total plaque F (μmol/g)
(1.8)
60
0
Fig. 2. Total and plaque fluid fluoride given as the geometric mean and standard error factor (in parentheses). Sold arrows show the difference between the no-SLS and SLS groups within each type of sample (total or plaque fluid fluoride).
Discussion
The salivary and plaque fluoride concentrations observed in this study in the absence of SLS are similar to previous 1-hour postrinse values obtained using similar methodologies [Vogel et al., 2000, 2001, 2006a, b, 2008a, b, 2010]. The average ratio of the 1-hour to baseline fluoride concentrations in these studies varied from 3.7× (plaque fluid) to 6.5× (total plaque), which suggests that variations in the baseline fluoride concentrations should have only a minimal influence on the 1-hour values obtained here. It should be noted, however, that in these previous studies 1-hour postrinse plaque fluid fluoride concentrations were obtained which exceeded the salivary fluid fluoride concentrations, while in the current study these values were similar. Although the reason for this divergence is not apparent, improvements in technique, specifically the double centrifugation of plaque fluid samples showing traces of plaque solids, may partially explain this result. After salivary flow has cleared the applied agent, the primary source of plaque fluid and salivary fluid fluoride are the labile reservoirs of this ion in the plaque and oral mucosa [Duckworth and Morgan 1991; Zero et al., 1992b; Vogel et al., 2006a, 2008b; Vogel, 2011]. Salivary particles may also be a significant source of fluoride particularly in the case of salivary fluid. It is these particles, which consist of fluoride-containing precipitates and oral mu294
Caries Res 2015;49:291–296 DOI: 10.1159/000381192
cosa fragments, that increased total salivary fluoride above salivary fluid fluoride both in the absence or presence of SLS (fig. 1). Perhaps more importantly, there was a clear, statistically significant increase in the 1-hour postrinse salivary fluid fluoride concentration but a smaller nonsignificant increase in total salivary fluoride concentration (fig. 1). It is noteworthy in this regard that, since the contribution of salivary particulates to the total salivary fluoride is similar in the SLS and non-SLS samples (fig. 1), the entire increase in total salivary fluoride concentration between the SLS and non-SLS rinses can be attributed to an increase in the fluoride concentration of the salivary fluid. Even more pronounced than its effect on salivary total/fluid fluoride concentration, SLS produced only a negligible increase in the total plaque fluoride concentration, but tripled plaque fluid fluoride concentration (fig. 2). Given that plaque fluid fluoride makes only a very small contribution to total plaque fluoride [Vogel et al., 2000, 2008b, 2010], it can be concluded that, as in the case of salivary particles, the inclusion of SLS in the rinse had only a small effect on the uptake of fluoride by plaque. It is fluoride solubilized into the fluid phase of saliva that can interact with the tooth surface to exert a cariostatic effect. However, total salivary fluoride measurements can include large contributions from nonsolubilized fluoride-containing particulates which can, as observed in this study, eliminate statistical differences seen in salivary fluid fluoride measurements. Hence the rationale for the use of total saliva fluoride concentrations as an indicator of the potential therapeutic effect of a topical treatment is questionable [Vogel, 2011]. This does not, however, obviate the value of total plaque fluoride measurements as an indicator of the size of the plaque fluoride reservoir that may release additional fluoride into plaque fluid during the critical low pH period following a cariogenic attack [Arends and Christoffersen, 1990; Margolis and Moreno, 1990; Vogel, 2011]. SLS has been shown to disrupt or open up the structure of both the mucosa [Herlofson and Barkvoll, 1993] and plaque [Robinson et al., 2006], a process that might be expected to increase the deposition of fluoride into oral reservoirs and also increase the subsequent rate of fluoride loss from them. However, in this study it was observed that, 1 h after administration of the fluoride rinse, the inclusion of SLS had little effect on the fluoride content of the total plaque or of the salivary particulates (which are derived mostly from the mucosa) but significantly increased the concentration of plaque fluid and salivary fluid fluoride (fig. 1, 2). Although any Vogel/Schumacher/Chow/Tenuta
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No SLS
2.0
mechanism for these results is speculative, the fact that the fluoride ion diffuses about 7 times faster than the lauryl sulphate ion [Deng et al., 1996; Stewart, 2005] provides a plausible explanation: the rapidly diffusing fluoride ion may be fixed into these substrates before the slower diffusing lauryl sulfate ion penetrates. Furthermore, since oral fluoride binding in the plaque and mucosa appears to be calcium-mediated [Rose et al., 1996; Whitford et al., 2002; Vogel et al., 2006a, 2008b, 2010; Vogel, 2011], the subsequent fluoride release from these deposits into the fluid phase of plaque or saliva should, in accordance with the results observed here, be increased by their subsequent exposure to a relatively high concentration of a calcium-binding (i.e. calciumextracting) ion such as lauryl sulfate [Iranan and Callis, 1960; Barkvoll et al., 1988]. The SLS disruption of the mucosa should also increase salivary fluid fluoride, which in view of the possible salivary and plaque fluid fluoride equilibrium noted above should lead to an increase in plaque fluid fluoride. In conclusion, the results of this study show that, contrary to the hypothesis of the Introduction, SLS had little effect on total plaque fluoride, induced a small, not statistically significant increase in total saliva fluoride, but significantly increased plaque fluid and salivary fluid fluoride. Such a result suggests that the nonfluoride components of fluoride dentifrices and rinses, in particular surfactants such as SLS, can be manipulated to improve the fluoride release characteristics from oral fluoride res-
ervoirs following conventional topical fluoride treatments. The characterization of the effect of these components on these reservoirs, perhaps using a recently described extraction technique [Vogel et al., 2010, 2014], combined with recent procedures shown to greatly increase the amount of these reservoirs [Vogel et al., 2006a, b, 2008a, b], may lead to the development of topical agents that induce a much larger and more sustained increase in oral fluid fluoride. Treatments based on this approach could potentially have a much larger cariostatic effect than can be obtained with current fluoride topical agents with the same fluoride dose.
Acknowledgments This study was supported by a grant from the ADA Foundation and is part of the dental research program conducted at the National Institute of Standards and Technology in cooperation with the ADA Foundation. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. The roles of the authors in this study were as follows: G.L.V. conceived and designed the experiments. G.E.S. performed the clinical examination and collected samples. G.L.V. performed the analysis and analyzed the data. G.L.V., G.E.S., L.C.C., L.M.A.T. wrote the manuscript.
Disclosure Statement All authors declared that there are no conflicts of interest.
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