Fluoride Concentrations in the Three Enamel Depths of the Control ( F - un - treated ) and Experimental ( APF - gel Treatment ) Enamel Surfaces After

The objectives of this study were to investigate the effect of application times on fluoride (F) uptake by sound enamel exposed in vitro to APF gels. Two groups of 12 teeth each were treated with one of the following 1.23% APF gels: MinuteGelTM (gel A) and Nupro® (gel B). The F treatment times were 1, 2, and 4 min and the F concentrations and enamel biopsy depths were determined using an acid-etch biopsy technique. There was a significant increase (P < 0.001) in F concentrations at the depths of 5 and 10 ~tm following 1, 2, or 4minute treatments with both gels as compared with their controls. For both tested gels, application time of 4 min significantly increased the F concentrations at all enamel depths as compared with the 1-minute treatment (P < 0.02 and P < 0.001). The net acquired F in the 15 ~tm-thick enamel exposed 4 min to gels A and B were 2.8and 2.4-fold, respectively, more than the 1-minute treatment. There were no significant differences in F concentrations of enamel treated with gels A and B. The results indicate that enamel F uptake is a diffusion-controlled process which is time dependent. Professionally applied topical fluoride is an important part of the entire program for preventing dental caries (Brown and Konig 1977; Clarkson and Wei 1982). One measure of the effectiveness of topical fluoride (F) therapy is the ability of the agent to deliver and incorporate F into enamel. There are many variables in the topical agents as well as in the therapeutic procedure that may affect the acquisition of F by enamel including the F concentration, pH, temperature, type of agent (gel, solution, varnish, etc.), application time, and frequency of use. Although the effect of most of these parameters on enamel F uptake are well documented (Brudevold et alo 1967; Mellberg and Loertscher 1972; Joyston-Bechal et al. 1973; Retief et al. 1980; Mellberg and Ripa 1983), there is little documentation in the literature on the optimal length of application time to provide a satisfactory F uptake by enamel. The 40th edition of the Accepted Dental Therapeutics (1984) listed 23 acidulated phosphate-fluoride (APF) gels accepted by the American Dental Association. Most of these gels contain 1.23% F, from sodium fluoride or hydrogen fluoride, and 1% phosphoric acid in an aqueous medium containing carboxymethyl cellulose as the gelling base. Concerning the mode of application, the Accepted Dental Therapeutics recommends that the APF gel should be applied to the patient’s dental arches by means of a tray for 4 min. A new topical F gel, Minute-GelTM, was introduced by Oral-B Laboratories, Inc. (Redwood City, CA). It was claimed that "Minute-Gel delivers 12 times the minimum fluoride uptake in just I minute." The minimum F uptake mentioned in the advertisement (Oral-B Lab 1985) was based on the hypothesis that a concentration of 1000 ppm in the enamel surface is needed to confer protection against dental caries (Muhlemann 1963). The reported F uptake following 1and 4-minute applications of Minute-Gel were 12,000 and 15,500 ppm, respectively (1985). The implication of the 1-minute application is that dentists can save 3 min of valuable clinical chairside time per F treatment. However, the full data documenting this claim is not published and it is difficult to assess its validity. In view of the great clinical importance to dentists to use the "optimal" treatment time for topically applied F gel, the present study was carried out to clarify this point. This study aims to test the hypothesis that MinuteGel produces as much enamel F uptake in 1 min as a conventional APF gel in 4 min. The F uptake by enamel as a function of application time also was evaluated. Materials and Methods Preparation of Teeth Human third molars extracted due to impaction were collected from individuals residing in Hong Kong, which is a fluoridated community with 0.7 ppm F in its water. Teeth with no visible cracks, hypoplastic areas, or 168 TIME DEPENDENCE OF ENAMEL FLUORIDE UPTAKE -PART I: Wei and Hattab caries on the buccal and lingual surfaces, when examined under a stereomicroscope at 20x magnification, were stored in deionized water containing thymol at 4 ° C. At the time of use, the teeth were lightly polished with a rotating rubber cup and an aqueous slurry of pumice followed by thorough washing in tap water. Two 4-ram round adhesive discs were placed on the buccal and lingual surfaces of the crown of each molar giving 4 delineated areas for each tooth. The teeth then were entirely covered with nail varnish and dried with air. Experimental Design and Treatment Procedure Two groups of 12 teeth each were treated with one of two gels: gel A (Minute-Gel, an APF gel containing 1.23% F from sodium fluoride and hydrogen fluoride at pH 3.5, batch no. XHDT); and gel B (Nupro®Johnson and Johnson Dental Products Co; East Windsor, NJ, an APF gel containing 1.23% F from sodium fluoride and hydrogen fluoride in 0.1 M phosphoric acid at pH 3.03.5, batch no. 5M5823). For each tooth, one of the areas was assigned as a control (F-untreated) while the other areas were designated for F gel treatment of 1, 2, or 4 min. In this way each tooth served as its own control. The disc covering the control area was removed leaving behind a varnish-free enamel surface "window" of 12.6 mm2, for acid-etch enamel biopsy. After the first biopsy on the control window, it was covered with nail varnish. Thereafter the second disc was removed and the F gel was applied to the exposed enamel surface as a layer of gel approximately 2 mm thick, using a cotton bud loaded with the gel. The gel was allowed to stay on the enamel surface for 1 min at room temperature in a relative humidity of 100%. Immediately after F treatment, the enamel surface was washed for 30 sec under running tap water and for 1 min in deionized water. The same treatment procedure was carried out for the third and fourth windows of each tooth except that the treatment time of the F gel on enamel was.2 min for window 3 and 4 min for window 4. Enamel Biopsy and Chemical Analysis Acid-etch enamel biopsies were carried out using Ffree cotton pellets. The pellet was saturated with 0.1 ml of 0.5 M HCIO4 and then held in forceps for 15, 30, and 30 sec consecutively. Immediately after each etching the solution was buffered by directly pipetting onto the enamel surface 0.4 ml of 0.5 M citrate buffer followed by 0.5 ml of deionized water. The residual solution left on the tooth surface was aspirated with a microsampling pipette and small pieces of filter paper and transferred to the original sample solution. The F concentration in the solution containing the sampled enamel was determined using combination Fion electrodes (Orion model 960900 w Orion, MA) coupled to a digital ionalyzer (Orion model 901 -Orion, MA). A calibration curve was constructed from sodium fluoride standards containing 0.05, 0.1, 0.5, 1.0, and 5 ppm F, prepared in citrate buffer. The phosphate concentrations in the samples were determined by a double-beam spectrophotometer (Shimadzu model UV-150-02; Tokyo, Japan) using the one-step malachite green method (Hattab and Linden 1984). The mass of enamel in each sample was calculated by assuming that human enamel contains 17.5% phosphorus. The thickness of the biopsied enamel layers was estimated from the following formula: Layer mass of enamel (~tg) thickness in ~tm = biopsy area (mm2) x density of enamel The density of enamel is assumed to be 2.95. Because the thicknesses of the biopsied layers are not totally controllable variables and because there is a steep F gradient in the outermost enamel, the mean F concentrations were adjusted to standardized depths of 5, 10, and 15 ~tm from enamel F profile curves. An analysis of variance was used to evaluate the differences in enamel F concentrations following the application of each gel at 1-, 2-, and 4-minute application times compared to their respective controls. Comparisons also were made between the two gels on their effect to increase the F uptake in enamel following different application times (1, 2, and 4 min).