Time dependence of enamel fluoride acquisition from APF gels. II. In vivo study.

In vivo enamel fluoride (F) uptake as a function of time after a single application of a new topical APF gel, MinuteGel TM (gel A) , was compared to a control APF gel, Nupro ® (gel B). The retained F in enamel after different post-treatment intervals also was assessed. Forty orthodontic patients aged 10-16 with four premolars scheduled for extraction were divided into two subgroups according to post-treatment intervals of 30 min or 24 hr. Enamel F was assessed using an acid-etch biopsy technique as for the in vitro study. The results confirmed the findings of the in vitro study in that: (1) uptake by dental enamel is time dependent and 4 min of application time resulted in significantly greater F uptake than the 1-minute application; (2) the F retained in enamel after 24 hr was slightly lower than after 30 min, but the difference was not statistically significant; (3) gel B produced a higher enamel F uptake compared to gel A in the same time periods; and (4) the results support the current recommendation of the Council on Dental Therapeutics of the American Dental Association and that 4 min should be used for professional topical F applications. A new topical APF gel (Minute-Gel" -Oral-B Laboratories; Redwood City, CA) was introduced in 1985. It was claimed that a 1-minute application time of MinuteGel increased the enamel F concentration up to 77.4% (12,000 ppm) of that obtained after a 4-minute application (15,500 ppm). The implication of the 1-minute application of Minute-Gel is that dentists can save 3 min of clinical chairside time without sacrificing the efficacy of the agent. In an in vitro study, Wei and Hattab (1987, .1988) found that shortening the application time of Minute-Gel and another conventional APF gel resulted in a significant decrease (2.5-fold) in the uptake of compared to the 4-minute treatment. Because of the scarcity of scientific data of in vivo enamel F uptake as a function of application times, the present study was carried out to elucidate this relationship. The aims of this study were to evaluate enamel F uptake after a single application of Minute-Gel and a conventional APF gel in vivo. The primary goal was to assess the F uptake as a function of application time. Additionally, the retained F in enamel after different post-treatment intervals also was assessed. The surface morphology of the enamel surfaces after such topical F treatment was studied by scanning electron microscopy (SEM), but will be the subject of a separate report (Lau 1987). Materials and Methods The participants of the study were 40 orthodontic patients aged 10-16 years with life-long exposure to fluoridated water (0.7 ppm) and with no history topical F application for at least 3 months prior to the study. All patients had 4 premolars scheduled for extraction. The selected teeth were caries free, with no visible cracks, hypoplastic or other defects. The patients were randomly assigned to receive either gel A (MinuteGel, an APF gel containing 1.23% F from sodium fluoride and hydrogen fluoride at pH 3.5, batch no. XHDT), or 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 0.1M phosphoric acid at pH 3.0-3.5, batch no. 5M5823). The study was blind and the 2 products were identified by code, which was not revealed before completion of the study. The 20 patients of each gel group were divided into 2 subgroups according to the post-treatment intervals of 30 min or 24 hr. Prior to topical F application, the teeth were cleaned with a rubber cup and aqueous slurry of pumice. One of the 4 premolars was selected randomly as the control while the 3 remaining teeth were topically treated for either 1, 2, or 4 min on a random basis. After the predetermined application time, the teeth were washed thoroughly with compressed water/air via a triplet syringe for 60 sec. The F-treated tooth was isolated carefully with cotton rolls so that there would be no transfer of the applied F to the remaining teeth. During the first visit, the control Pediatric Dentistry: September, 1988 Volume 10, Number 3 173 tooth for each patient was extracted. For patients in subgroup 1, F-treated teeth were extracted after 30 min. Patients in subgroup 2 were instructed to retain their normal daily routine of brushing and eating until the next appointment at 24 hr when the remaining teeth were extracted. The Acid-Etch Biopsy Procedure The enamel biopsy technique was the same as for the in vitro study (Wei and Hattab 1988). Each tooth was examined under a stereo-dissecting microscope at 30x to assure that the surfaces selected were free of defects before enamel biopsy. The F concentration in the solutions containing the sampled enamel were determined using combination F-ion electrodes (Orion model 960900 -Orion, MA). The phosphate concentration was 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 mean F concentrations were adjusted to standardized depths of 3, 5, and 13.5 ~tm. An analysis of variance was used to evaluate the differences in enamel F concentrations between treated and controls with respect to the following factors: (1) topical F agents (gel A vs. gel B); (2) application times (1, 2, and min); and (3) extraction times (30 rain vs. 24 hr). The data were further analyzed by Scheffe’s test to determine the level of significance. A and B, at 30-minute extraction time. For gel A, the F concentration at 3 ~tm after 4 min was 4013 compared to the 1-minute treatment at 3780. For gel B, the F concentration after 4 min was 5433 compared to 4285 ppm after I min. There are significant differences between the two F gel treatments at a level of P < 0.05 (Table 1). Teeth that were extracted after 4-minute applications of gel B (3504 ppm) gave a higher F uptake than gel A (1520 ppm) under the same conditions. Similarly, at 24 hr after topical F application (Table 2), gel B (3065 ppm) gave significantly higher F uptake than gel A at 2 min (670 ppm) and 4 min (668 ppm) treatments (P < 0.05). Table 3 shows the effect of F application time on enamel F uptake at the 3 standardized depths for both gels combined. The F uptake after a 1-minute application was significantly lower than after 4 min for layer 1 (29 vs. 54%). The total F uptake after 1-minute (1197 ppm) and 4-minute (2440 ppm) applications was also TABLE 1. Fluoride Concentrations at the Three Enamel Depths (Standardized for Comparison) of Control (APF-gel Treatment) Enamel Surfaces After 1, 2, or 4 Min Exposure to Topical F Agents Topical F Agent F Concentration (ppm) Acquired F and Application Standardized Depths in the Outer 13.5 Time (Extraction at (Means +_ SD) Thick Enamel 30 Min) (N) 3 ~m 8 I~m 13.5 i~m ppm Gel A Control 1