There are several sources of fluoride, an agent that is well-known to influence the process of remineralization. Bacteria on teeth metabolize carbohydrates and produce acid. As a result, the remineralization process begins when fluoride is released from the surrounding dental plaque in response to lowered pH at the tooth-plaque interface.³ The released fluoride and the fluoride present in saliva then are adsorbed, along with salivary calcium and phosphate, by demineralized enamel to establish an improved enamel crystalline structure.⁴ The apatite crystals get larger and the surface becomes harder (Figure 1). This improved structure is more acid-resistant, contains more fluoride and has less carbonate than the original structure.

View larger version (73K): [in this window] [in a new window]   Figure 1. Demonstration of demineralization and remineralization processes. Remineralization of enamel leaves a surface with mineral crystals that are rich in fluoride and lower in solubility. Figure reproduced with permission of Dr. Martin Giniger, Martin Giniger & Company, New York.

Examination of teeth exposed to a lifetime of fluoride dentifrice has shown that the outer few micrometers of enamel can have fluoride levels as high as 1,000 to 2,000 parts per million, or ppm, but typically less interproximally.⁸ This may be because interproximal areas between teeth only rarely are affected by the mechanical action of toothbrushing owing to bristle size in relation to the tight contact area; in addition, it may be because the foam of standard toothpaste is too viscous and therefore unable to penetrate completely between teeth that are in tight contact with one another. Hence, bacteria tend to accumulate on interproximal surfaces and can demineralize them, further reducing effective fluoride concentration.⁸

Because of this tendency, and the fact that most people rarely floss,⁸ new dentifrice technologies with lighter foaming properties and finer, more abundant bubbles need to be developed to carry fluoride to interproximal and other hard-to-reach areas, enhancing the acid resistance of vulnerable enamel. New commercially available "liquid" dentifrice formulations that recently have been introduced by several manufacturers may be able to fill this role. (The term "liquid" is placed in quotes because while the new formulations are not as runny as a true liquid such as water, they are less viscous and pastelike than traditional dentifrices.) This is because in comparison with standard pastes, liquid formulations with lighter, thinner foam should be able to penetrate tight spaces better and thereby more efficiently deliver a local fluoride dose.

Consequently, we undertook a study to assess whether a new liquid dentifrice—Colgate Total Liquid Dentifrice (Colgate-Palmolive, New York), containing 1,100 ppm fluoride, 0.3 percent triclosan and 2 percent copolymer in a mildly foaming silica base—demonstrated enhanced performance at interproximal tooth surfaces compared with a standard, paste-style positive control dentifrice that held the American Dental Association Seal of Acceptance. We determined potential anticaries activity in situ by evaluating surface microhardness and enamel fluoride uptake in partially demineralized enamel specimens.

The use of an in situ model also permitted direct evaluation of the interproximal areas, which could not be performed with an in vivo examination. We believed that if we found significant differences in surface mineralization, fluoride uptake or both as a result of use of the new dentifrice, it would indicate that further large-scale clinical trials would be warranted. It also would indicate that the experimental toothpaste might be useful for patients who are prone to developing interproximal carious lesions.

	SUBJECTS, MATERIALS AND METHODS

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Subjects. We collected a database of potential volunteers from people responding to fliers posted at the Universidade Federal de Alagoas, Maceió, Brazil. We chose 19 study subjects who fit the study’s exclusion/inclusion criteria to participate. They all lived in Maceió, where there is no community water fluoridation (the concentration of fluoride ion, or F-, in tap water is < 0.1 ppm). The medical histories and the oral soft tissue health status of the subjects all were good, and none of the subjects had significant periodontal disease.

All subjects were required to have a history of more than five carious lesions in the past, but they also were required to have no more than four currently active lesions to enter the study. Informed consent also was required. The key inclusion criteria required each subject to be missing both mandibular second premolars and all mandibular molars; have a history of inter-proximal dental caries; have a measured salivary flow rate within normal limits (parafilm stimulated > 1.0 milliliter per minute; unstimulated, > 0.2 mL/minute); and wear a Kennedy Class II bilateral mandibular partial denture.

In comparison with standard toothpastes, liquid formulations with lighter, thinner foam should be able to penetrate tight spaces better and thereby more efficiently deliver a local fluoride dose.

Materials and methods. Abrasion depth and mineral density. We prepared enamel blocks or slabs (2.2 x 2 x 2 millimeters) from bovine molars stored in 2 percent formaldehyde solution, pH 7.0, for at least a month. When ready for preparation, we weighed these slabs and then abraded 200 µm from their exterior-most surfaces to remove the high-fluoride external layer. During the study, the abraded side of the slab would serve as the "exposed" surface in the oral cavity, situated interproximally between two posterior teeth.

We then measured the width and depth of each polished slab using a micrometer to calculate the final reference surface area of the exposed surface in square millimeters. To check that approximately the same amount of enamel had been abraded from the external surface of all slabs, we calculated the exact baseline abrasion depth, or AD, using the following equation (as formulated by Iijima and Koulourides⁹):

We then used the AD to calculate the mineral density, or MD, of each slab. Using the slab’s weighed mass and its surface area, we used the following equation (again, as formulated by Iijima and Koulourides⁹):

Fluoride content of enamel slabs. A controlled demineralization process of the abraded enamel surfaces initiated the calculation of fluoride content, or FC, of each enamel slab. First, we painted the nonexposed sides of each enamel block with clear nail polish. After the polish had dried, we exposed the remaining abraded surface to 0.25 mL of 0.25-molar perchloric acid solution for 15 seconds. After acid exposure, we removed the enamel blocks and neutralized the remaining acid solution supernatant with 250 microliters of Total Ionic Strength Adjustment Buffer (Sigma-Aldrich Brasil, São Paulo, Brazil) containing 10 grams of sodium hydroxide per liter. We then determined the supernatant’s fluoride concentration with an Orion 96-09 Fluoride (F-) Sure-Flow combination electrode (Thermo Electron, Beverley, Mass.), which the manufacturer states is accurate within 1 ppm. We then dried, weighed and saved the resulting demineralized enamel blocks for testing of surface micro-hardness (see below).

Since we knew the volume of the supernatant and fluoride atomic mass, we could calculate the fluoride mass, or FM (in micrograms), of the supernatant from its measured concentration. Ultimately, we calculated the FC of the slabs using the following formula:

Microhardness testing. After each determination of FC, we repolished the blocks for 10 seconds with 1-micrometer aluminum oxide particles in solution and assessed them for baseline surface microhardness, or BMH, with a Knoop indenter and a 50-g load.

We made indentations using a microhardness tester with a Knoop diamond indenter (HMV-2000 Microhardness Measurement Unit, Shimadzu, Kyoto, Japan) set at a 50-g load for 30 seconds. We made six indentations at 10, 20, 30, 50, 70 and 90 µm, successively, from the outer enamel surface in three different regions: above, below or directly at the center region of the enamel block. We then averaged the mean values of all regional measuring points. We calculated the areas under the curves (Knoop hardness number, or KHN x µm) using a trapezoidal rule.

We next subjected these baseline pretreatment blocks to a six-hour demineralization procedure using an acetate-caustic potash buffer of pH 5.0, and then reassessed them for demineralized microhardness, or DMH. We subsequently sterilized and stored these slabs until placing them in the subject’s partial dentures.

Exposure of slabs to test products. The goal of our study was to measure fluoride uptake and mineral uptake in the interproximal area between mandibular molar and premolar denture teeth bilaterally as influenced by two different dentifrices. We compared the effects of an experimental triclosan/copolymer/fluoride liquid dentifrice, or LD) to a commercially available, clinically proven ADA–accepted toothpaste, or TP, containing 1,100 ppm fluoride as a positive control (Table 1). The investigator involved with measuring fluoride and mineral uptake (M.F.D.S.) was blinded to this variable.

View larger version (60K): [in this window] [in a new window]   Figure 2. Mandibular Kennedy Class I distal extension prosthesis with an embedded, demineralized enamel block on the mesial surface of tooth 30. Inset: Mesial surface of tooth no. 30 showing the in situ enamel block in place after 10 days of contact with distal aspect of tooth no. 29. Figure reproduced with permission of Dr. Milton Fernando de A. Silva, Universidade Federal de Alagoas, Maceió, Brazil.

We conducted this study in two 10-day phases during a three-week period. After a washout period that followed the first phase, we assigned each subject to use the other dentifrice for the second phase. After completion of each phase, we recovered the enamel blocks, again assessed them for treatment microhardness, or TMH, and further analyzed FC using the methods described above.

Phase two of the crossover design was preceded by a seven-day washout period using a fluoride-free placebo dentifrice. After the washout, the subjects again reported to the clinical facility and had two new predemineralized enamel blocks placed in their mandibular removable partial dentures. At this appointment, they were provided with the opposite, crossover dentifrice for use during the final 10-day cycle. At the end of the experimental period, the blocks were recovered and the subjects were dismissed.

Percentage of surface mineral recovery. There is a linear relationship (r = 0.9) between the KHN and mineral content in enamel.¹⁰ Therefore, determination of the KHN allows for determination of percentage of surface mineral recovery, or SMR, using a simple calculation (as established by Zhang and colleagues¹⁰). The blocks recovered after 10 days of exposure to the oral cavity were subject to one last assay for TMH using the method described above. Ultimately, we calculated percentage of SMR by using the method of Zhang and colleagues¹⁰:

Statistical analysis. We expressed results as means ± standard deviations, or SDs. We tested differences between the groups for significance using an unpaired Student t test. Differences were regarded as statistically significant when P < .05.

	RESULTS

TOP ABSTRACT SUBJECTS, MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
ADs and baseline FC. Two sets of enamel slabs were used on each subject. Phase 1 slabs were those placed at baseline in subjects of both groups; phase 2 slabs were the second set, placed after the washout period, just before dentifrice crossover. We determined the ADs of both sets to be extremely similar. The phase 1 slabs had a mean AD of 1.89 µm ± 0.06 in the LD group and 1.90 µm ± 0.04 in the TP group. Similarly, the phase 2 slab AD means of both groups were within .07 µm of each other. Because bovine enamel is known to be extremely consistent in FC at any given AD,⁹ the close ADs yielded enamel slabs with a baseline FC that was not statistically different (P > .05) between any of the groups or sets.

FC. We measured fluoride uptake caused by the treatment dentifrice as the difference between the baseline FC and the FC of the slabs after 10-day exposure to either the LD or the TP dentifrice. This is an important measure of the potential anticaries activity of a test product, because remineralization has been shown to have excellent correlation with clinical efficacy.¹⁰ Table 2 shows the FC of the enamel blocks (in ppm) after 10 days of treatment with either an experimental or a control dentifrice. Enamel exposed to the liquid toothpaste had a mean fluoride uptake nearly 100 ppm (13.1 percent) greater than that of the positive control. This observed trend in favor of enhanced fluoride uptake performance by the LD group, while not statistically significant at the preset level of P< .05, cannot be discounted because the ADs, and thus the initial FC of the baseline slabs, were extremely uniform.

	DISCUSSION

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Because most of today’s toothpastes contain similar levels of fluoride and other inactive ingredients, their properties tend to be equivalent. While many dentifrices make marketing claims to reach in between the teeth, few if any ever have been shown to have an enhanced activity in that crucial area. Because many dental carious lesions form interproximally, a dentifrice that has demonstrated activity at these sites would be expected to have the potential to decrease overall caries prevalence.

In that regard, the results of this study are promising, as they show a progressive increase in the fluoride and MD of the interproximal in vivo tooth enamel when exposed to an experimental liquid dentifrice (Colgate Total Liquid Toothpaste). Also, the changes in both FC and MD we saw with the experimental dentifrice were greater than the changes we measured for the positive control toothpaste.

There are several mechanisms that could provide a possible explanation for the results and that warrant further study. One credible explanation is related to the liquid toothpaste’s light foaming properties and lower surface tension. These attributes, in theory, could allow the liquid dentifrice to be carried into the depths of pits, fissures and interproximal spaces better than a traditional toothpaste. The very presence of dentifrice in these narrow caries-prone locations could be sufficient to enhance the dentifrice’s anticariogenic properties, especially because few people floss effectively.⁴

In the present study, Colgate Total Liquid Dentifrice produced a 13.1 percent higher fluoride uptake in the interproximal enamel. And, most importantly, the difference in percentage of SMR was sufficiently large to be statistically significant and is likely to be clinically relevant. Therefore, ours is the first study to show that a triclosan-containing liquid dentifrice may prevent interproximal dental caries somewhat more than a standard fluoride dentifrice.

	CONCLUSION

TOP ABSTRACT SUBJECTS, MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
On the basis of the above arguments, we believe our study provides preliminary evidence that there are likely clinical implications of using this new liquid dentifrice. At the very least, the results provide preliminary evidence supporting clinicians’ recommendation of the product to patients who are prone to developing interproximal carious lesions. We also believe the study provides sufficient evidence to justify further large-scale clinical trials of this product.