[Fluoride gels--local application with a tray or a toothbrush?]. 
Fluoride gels have been used since the 1960s.  Most gels are recommended for use in mouth-trays.  An alternative is the application on a toothbrush.  The present study compared the fluoride concentration in surface enamel and its acid resistance following the application in a tray or on a toothbrush in vitro.  In a first experiment 80 halves of molar crowns were used.  In group A 20 were coated with fluoride gel.  In group B 20 were brushed with gel.  20 each served as water control.  The surface fluoride concentration and acid resistance of all samples were assessed using an acid etch technique.  In a second experiment 30 halves of molar crowns were used.  10 were mounted in a plaster socket mimicking a dental arch for which a customized miniplast-tray was made.  This tray was loaded with 1 g of gel and applied for 3 min on 3 consecutive days.  10 samples were brushed for 3 min on 3 days using 1 g of gel and 10 served as water control.  Enamel that had been fluoridated with gel on a toothbrush showed a superior acid resistance.  The fluoride concentration in the outer 20 to 25 microns of enamel was higher after gel application on a toothbrush than after tray application.