Salivary Fluoride Levels inOverdenture Wearers After

Topical Fluoride GelApplication

Timo O. Närhi, DDS, PhD"

Ronald L Ettinger, BDS, MDS, DDSc''

¡udy R. Heliman, BS'

fames S. Wefel, PhD''

This study describes salivary fluoride levels alter topical fluoride gelapplication on overdenture abutments. Fluoride ievels were evaluatedseparately for the subjects with normal unstimulated salivary flow rate(n = 16) and for those with a low flow rate (n = 8). One drop of fluoride gel(Karigel-N, Lorvic) was placed in two abutment depressions of the duplicatedoverdenture, after which unstimulated whole saliva was collected for 30minutes. Samples tor fluoride analysis were taken at S-minute intervals. Twoadditional samples were taken at 45 and 60 minutes. Fluoride concentrationat the a hutment-denture interface (remaining fluoride concentration) wasmeasured at the end of the study. Saiivary fluoride concentrations decreasedgradually in both groups of subjects, but after 1 hour they remained at ahigher level m subjects with low flow rates. Subjects' salivary flow ratescorrelated negatively with remaining fluoride concentration at the denture-tissue interface. Consequently, mean remaining fluoride concentration wassignificantly higher in subjects with low flow rate than in their normalcounterparts, inf ! Prostbodonf 1997:10:553-561.

In the last two decades a number of longitudinalciinical studies have been published describing

overdenture populations.^"*^ The primary problemsassociated with overdenture use have been cariesand periodontal disease of the abutment teeth. In1978 Toolson and Smith^ showed thaf brushingalone was not sufficient to prevent caries on theabutment teefh. Thus, successful therapy has reliedon a combination of good home care and the useof topical fluorides.''

Dentures create a favorable environment andprovide a good surface for the attachment andgrowth of different oral microorganisms.^ Therefore,

'Assistant Professor. Department of Prosthodontics, Universityof Helsinki Institute of Dentistry, Helsinki. Finland.

''Professor, Department of Prosthodontics, The University ofIowa College of Dentistry, Iowa City. Iowa.

'Dows Institute for Dental Research. The University of IowaCollege of Dentistry, Iowa City, Iowa.

''Professor. Dows Institute for Dental Research.The Universityof Iowa College of Dentistry, Iowa City, Iowa.

Reprint requests: Dr Timo O. NSrhi. Oral Function and Pros-thetic Dentistry, University of Nijmegen. PO Box 9101, bSOOHB Nijmegen, The Netherlands.

all the tooth surfaces, especially the roots, that arecovered by the denture are at increased risk fordental caries. This risk is even higher if saliva flowis prevented or significantly decreased.'*'-'' Thepresence of fluoride during the demineralizationand remineralization episode is of critical impor-tance in caries prevention.'- Stamm et al'^ havedemonstrated that life-long consumption of fluori-dated water has significantly reduced the preva-lence of root caries when compared with the pres-ence of caries in similar individuals living in anonfluoridafed community. Burt et al''* confirmedthese findings when investigating two communi-ties in New Mexico, and stated that their results"confirm that root caries experience is directly re-iated to the fluoride concentration in drinkingwater."

Different methods have been developed to in-corporate fluorides info fhe aqueous phase sur-rounding tooth surfaces. In their studies. Billings etal'^ and Zero et al"* showed that gels providedhigher levels and more prolonged retention of fluo-ride in whole saliva and dental piaque when com-pared with fluoride rinses or fluoride dentifrices.Even more prolonged clearance times and higher

Í10, N.,mbei6,199? 553 The Inlernalional Journal of Prosthodontics

Salivary Fluoride Lfvels After Fluoride Gel Appiitation

fluoride levels have been found after the use of flu-oride varnishes.'' However, fluoride concentra-tions in varnishes are very high, and they cannotbe used in patients' home care programs.

The use of daily topical fluoride gels on over-denture abutments seems to improve their resis-tance to caries,'" but it is not entirely clear howfluoride acts on the abutment surfaces. Althoughmost patients are asked to refrain from eating anddrinking for 30 minutes after application of the flu-oride, there is no information available on fluorideclearance rates after topical application on over-denture abutment teeth.

The purpose of this study was to measure sali-vary fluoride levels after fluoride gel application onoverdenture abutments, and to determine tbe fluo-ride concentration at the denture-tissue interface 1hour after fluoride application.

Materials and Methods

Study Population

The subjects for the study were recruited accordingto the following criteria: Candidates should (1) beambulatory; (2) be able to give informed consent;(3) be physically able to clean their teeth indepen-dently; (4) be using a mandibular overdenture withtwo abutment teeth opposing a complete maxillarydenture; (5) have healthy tissues with periodontalpockets no deeper than 3 mm.

All 310 patients who had received overdenturesin the Department of Prosthodontics and were par-ticipating in a longitudinal study were identified.Of these subjects, 54 fulfilled the inclusion criteria.Two subjects had died since their last recall visit,and two other subjects had lost all of their abut-ment teeth, leaving an available population of 50overdenture wearers. However, 13 of these wouldnot respond to telephone and mail contacts and 10persons declined participation. Of the subjectswho did not participate, eight were working andwould not arrange an appointment for the study,while the others were too ill to do so. Two subjectswere dropped from the study after the initial ap-pointment because their salivary fiow rates wereextremely low; parotid gland surgery had been per-formed on one of these subjects and the other sub-ject was using multiple medications with possibiexerogenic side effects. The final study populationthus consisted of 25 overdenture wearers, 20 menand five women (mean age 70.4 ± 9,6 years) andrepresented 50% of all the available subjects. Themean age of the subjects' existing overdentureswas 8.5 years (range 1,1 to 20 years).

Saliva Collection

At the first visit the study protocol was explained,and after signing the informed consent, the patient'sbaseline unstimulated salivary flow rate was mea-sured by the draining method,^^ This method in-structed subjects to swallow and tben let their un-stimulated saliva flow out of their mouth into apreweighed collection vessel for 5 minutes. The col-lected saliva was then weighed on an electronicscale (Ohaus, Model GT 210, Ohaus) to determineflow rate. A small sample of saliva (0,5 mL| was thentaken for fluoride analysis. After saliva collection, pa-tients' overdenture abutment teeth were scaled andpolished, and their current overdenture was dupli-cated. Denture duplicates were made using clear or-thodontic resin. Detailed information about the du-plication technique has been reported elsewhere,^"At the end of the appointment, patients were askedto refrain from using their home fluoride for 1 week.

After 1 week a baseline II saliva sample was taken,A well-polished duplicated overdenture was given toa subject, and to accommodate to it the subject worethe denture for 15 to 30 minutes before saliva collec-tion was started. This time was considered sufficient,as the duplicated denture was an exact copy of sub-ject's own overdenture. Subjects were divided intotwo groups according to baseline salivary flow rates:those with a normal flow rate (> 0,1 g per minute;n - ^7), and those with a low flow rate (< 0,1 g perminute; n = 8),^' Subjects were placed into the lowflow rate group if one of the baseline measurementswas less than 0,1 g per minute.

The dentures were then rinsed and dried care-fully. One drop (0,1 mL) of commercially availablephosphate fluoride gel (Karigel-N: 5,000 ppm,Lorvic) was placed into each of the abutment de-pressions of the duplicated denture. Application wasperformed using a 1,0 mL tuberculin syringe to stan-dardize the size of the drop. The dentures wereplaced into the subjects' mouth, after which unstim-ulated saliva was coiiected continuously for 30 min-utes, Saiivary flow rates were measured and sampiesfor fiuoride analysis were taken once every 5 min-utes for a totai of six samples. Subjects were not ai-iowed to speak or swallow during the initiai 30-minute coiiection period. Two additional sampieswere taken at 45 and 60 minutes after the fiuorideapplication; coiiection time for these sampies was 5minutes. In subjects with low fiow rates (< 0,1 g perminute) the coiiection period was proionged to 15minutes, and oniy four sampies were taken duringthe 1-hour coiiection period. After 1 hour the den-tures were removed and the tissue surface of theoverdenture was rinsed with deionized water (20

The Inlernationai iournai of ?iosth¡>donú< 554 Voiume TO, Numbers, 1997

Fiimride Levels After Fluoride Gel AppiJcation

ml) into a preweighed collection vessel. The water-saliva mixture was weighed and a sample (0.5 mL]was taken for assay of the fluoride concentration.

Fluoride Analysis

Immediately after the collection, 0.5 mL of salivawas removed and 0.5 mL of Total Ionic StrengthAdjustment Buffer (TISAB, Orion Research) wasadded to standardize the ionic strength and pH,The samples were frozen at -4°C until they couldbe assayed for fluoride concentration. The remain-ing saliva was stored frozen at -4°C.

All samples were assayed for fluoride using aModel 96-09 combination fluoride electrode(Orion Research). Electrodes were used in con-junction with an Orion Model EA 920 expandable10 N Analyzer and Orion CLP Printer.

The electrodes were calibrated with fluoridestandards and mixed 1:1 (50/50) with TISAB buffer.The electrodes were rinsed well with deionizedwater and dried before analyzing the samples. Thefrozen saliva samples with buffer (1 mL) werethawed at room temperature and placed under theelectrodes, and the relative mi l l ivo l ts wererecorded after 5 minutes. This method has beenshown to be valid for the fluoride analysis of storedsaliva samples.-^ The millivolt readings were con-verted to ppm using logarithmic regression analy-sis. A previously read set of standards was used inthis determination.

One subject's flow rate and fluoride concentra-tion recordings were omitted before the statisticalanalysis. His baseline II fluoride level (0.45 ppm)was clearly out of the normal salivary fluoride con-centration range suggested for persons living inhigh fluoride areas."-"' It was suspected that hehad followed his normal oral hygiene routines andapplied fluoride gel in his denture before his sec-ond appointment, resulting in a very high fluorideconcentration in the baseline II sample.

Statistical Analysis

Statistical analysis was performed using theStatView-HGraphics program (BrainPower). The sig-nificance of difference in salivary flow rates andfluoride concentrations between two groups ofsubjects was studied by an unpaired two-tailedf test. The difference between tbe means of two re-peated measurements was studied with a pairedtwo-tailed ttest. Pairwise comparisons among sev-eral means were made by Fisher's PLSD methodafter the F test for repeated measurements froman analysis of variance lANOVA) was found to be

Table 1 Salivary Flow Rate and Fluoride Concentra-tion in Subjects With Normal Flow Rates (n = 16)

Flow rate Fluonde(g/min) concentration (ppm)

mean i SD mean ± SD

Baseline IBaseline II0-5 mm5-10 min10-15 min15-20 min20-25 mm25-30 mm45-50 min60-65 minRemaining fluoride"

0.28-0.170.30 ±0.210.26 ± 0.220.24 ±0.130.22 ±0,100.25 ± 0.090,23 ±0.120.25 ±0.130.36 ± 0.2040.35 ±0.144

0 16± 0140 08± 0.03-

33 04 ± 56.3938.51 ±40.77t22.95 ± 20.7721.51 ±20.4014.59 ± 14.62*13.47 ± 12.43*4.22 ± 3.08*2.09 ± 1.59*0.27 ± 0.52

"Fiuoride concentration significantiy lower than In 8aseiine i sample.Paired two-tai i ed /lest, PeO.05.

'Fiuoride concentrations in 5-to-10-niinjte saiiva samples cor'eiatednegatively with the remaining fl jorde Ieveis |f=-0.3S).

'Fluoride concentrations signiticantiy iowerthan in S-tc-IO-minulesampie. ANOVA, repeated measures. F=4.4QQ, P= 0.0003. Differ-enoe between means: P< 0.05 ¡Fisher's PLSD¡.

^Saiivary tlow rates signifcantly higher than flow rafes measuredbefween 5 and 30 minutes. AhJOVA, repeafed measures: F= 5.72a.P = 0.0001. Drtlerence between means: P < 0 05 (Fisfier's PLSD).

"Fiuonde concentration in fhe miifure oí saiiva and disfilled water[20 mU, which was used for nnsing fne danlures afte: saliva collection.

Table 2 Salivary Flow Rate and Fluoride Concentra-tion in the Subjects with Low Flow Rates (n - 8)

Base i i ne 1Baseiine II0-15 mm15-30 min45-60 mmRemaining fluoride*

Flow rate(g/min)

mean ± SD

0,12 + 0.050.05 + 0.040.04 -f 0.020.05 + 0.040.09 + 0,05*

Fiuondeconcentration ¡ppm]

rtiean ± S D

0.33 ± 0.440.18 ± 0.13

90,22 ± 55.57*64.18 ±68 .2623.56 + 18.16

0.59 ± 0.84

•Fiuonde conce nf rations in 0-15 min saliva samples con-eiafed nega-tively with fhe remaining f joride levels | r= -0.44).

+Salivary flow rate signiflcanfly higher tfian flow lates after 15 and 30m I ngf es. ANOVA. repeated measures: Ffesf ^ 9.539, P = 0 02. Differ-ence between means: P •; 0.05 (Fisher's PLSD)

*Fluoride concenfrafion in the mlxfure of saliva and distilled water(20 mL), wfiich was used for rinsing the dentjres after saliva coiiecfion.

significant at the 5% level. Salivary flow rates andfluoride concentrations were correlated by meansof correlation coefficient analysis.

Results

The mean unstimulated salivary flow rates and cor-responding fluoride concentrations are presentedseparately ioi subjects with normal flow rates(n = 16) and subjects with low flow rates (n = 8) inTables 1 and 2. There was a significant differencein baseline salivary flow rates between these twogroups (Baseline 1, P = 0,01 and Baseline II,P= 0.003).

O, Number6, 1997 555 The International louinal of Prosthodontii

Saliv.iry Fluoride Levels After FiuurideCc

Fig 1 Salivary fluoride leyels in the subjeots with normal unstimulated saliva flov^ rate {> 0.1 g per minute) after the application oftwo drops of b I gti-con centrat i on fluoride gel. Data are presented using a logarithmic scale

In all 25 subjects combined, the salivary fluorideconcentrations were higher at the baseline I sam-ples (0.21 ± 0,23 ppml compared with the baselineII samples (0,11 ± 0,09 ppm) that were taken priorto the .ÎO-minute collection period. However, thedifference was statistically significant only for sub-jects with normal salivary flow rate. Fluoride con-centrations decreased gradually during collectionin both groups of subjects, but at the end of thecollection period they remained at a higher level inthe subjects with low flow rates compared withthose with normal saliva flow.

The subjects' mean saiivary flow rates correlatednegatively with their remaining fluoride concentra-tion (r = -0,33], The remaining fluoride concentra-tion was higher in the subjects with low flow rate(0.59 ± 0,84 ppml than in their normal counter-parts (0.27 ± 0,52 ppm); however this differencewas not statistically significant.

In the subjects with normal flow rates, the highestmean fluoride concentration (38,51 ± 40,77 ppm)was recorded after 10 minutes of saliva collection(see Table 1]. The concentrations recorded at thispoint correlated negatively with the subjects' re-maining fluoride levels {r - -0.35), The SLibjects

i ol Prosthodont i ( 556 Í to. Number 6, 1997

Salivary Fluoride Level; After FiLoridp Gel Applicatii

Fig 2 Salivary fluoride levels in the subjecfs wifh low unsfmulafed saliva Row rafe (< 0.1 g per minute) affer fhe appiicafion of twodrops of high-concenfrafion fluoride gel. Dafa are presenfed using a logarithmic scale.

whose recording was equal to or greater than 38.51ppm (n = 5) had lower remaining fluoride levels(0,04 ± 0-02 ppm) than those whose fluoride con-centration was less than 38.51 ppm in = 11; 0.37 ±0.60 ppm) after 10 minutes. However, this differ-ence was not statisticaily significant.

In the subjects with low flow rates, the greatestmean fluoride concentration (90.22 ± 55.57 ppmlwas recorded after 15 minutes of saliva collection(see Table 2). The fluoride concentration in the 15-minute sample correlated negatively with the sub-jects' remaining fluoride levels (r = -0.44]. Thesubjects whose fluoride concentration was greater

than or equal to 90.22 ppm (n = 5) had lower re-maining fluoride levels (039 ± 0-50 ppm) thanthose whose fluoride concentration was less than90.22 ppm (n = 3; 0-92 ± 1.31 ppm). The differ-ence, however, was not statistically significant.

Fluoride clearance in the subjects with normalsaliva flow is presented by logarithmic model inFig 1, and for subjects with low flow rates in Fig 2.

There was no correlation between the age of theoverdentures and the fluoride clearance rates ineither group of subjects.

t^Volume 10, Number 6, 1997 557 The International tournai of Prosthodontii

Saiivary Fluoride Leueis Afler FiLoride Cei AiJ[)in atii

Discussion

The present study group is part of an overdenturepopulation that has been treated in the Departmentof Prosthodontics, University of Iowa, during the lasttw,'o decades. The subjects recruited for this studywere divided into two groups representing thosewith normal saiivary flow rates (flow rate > 0.1 g perminute) and those with low salivary flow rales (flowrate < 0.1 g per minute) as suggested hy Sreebny etal.-' In the low-flow-rate group, the saliva collectionperiod had to be prolonged up to 15 minutes to ob-tain sufficient saliva for analysis. Therefore, fewersamples were collected from the subjects with lowflow rates than from subjects with normal flow rates.There were more women in fhe low-flow-rate groupthan in the normal group, but no other differencesbetween the groups were observed. According toprevious studies, the unstimulated whole saliva flowrate is lower in women than in men.^''-'' This gen-der-related difference was also observed in thesesubjects, although the difference in flow rates wasnot statisticaily significant. No differences in thenumber of daily medications between the twogroups were found.

Fluoride level in solutions is most frequently ana-lyzed using the ion-specific electrode, althoughsome Other methods are available. The electrodecan be used in a straightforward manner, but its lowconcentration limit is usually extended by the use ofa microdiffusion method, which has been recom-mended for all samples in which the fluoride ion {F")concentration is expected to be below 0.1 ppm.-''Microdiffusion is also preferable when electrode-in-terfering molecules, such as salivary proteins, arepresent. In this study, salivary fluoride concentra-tions were analyzed directly from the undiffusedsamples. This method was chosen because it wasexpected that after the use of a high-concentrationfluoride product the salivary fluoride levels wouldremain relatively high for the entire collectionperiod. The possible interference of salivary proteinscould have limited the electrode sensitivity in thisstudy, although the high concentration of fluoridewouid tend to negate their influence.

Baseline salivary fluoride concentration was de-termined two times for all participants. No instruc-tions regarding the use of home fluorides were givenbefore the first appointment. Therefore, baseline Isalivary fluoride concentrations were high, and inmost subjects they were clearly out of the normalsalivary fiuoride levels (0,01 to 0.05 ppm),^^ Salivaryfluoride concentration is known to be sensitive towater intake.^^ In their study, Oliveby et aP"showed that fluoride concentration in unstimulated

whole saliva was three times greater in childrenwho lived in an area with 1.2-ppm fluoride in thedrinking water than in children who resided in alow-fluoride community (0.2 ppm). In the presentstudy, no attempts were made to restrict the sub-jects' water intake after the first appointment. In sub-jects with a normal salivary flow rate, after refrainingfrom using home fluoride products for 1 week (Base-line II), fluoride levels dropped into a range (0.04 to0,16 ppm) that could be considered normal for per-sons living in a high-fluoride community. However,in one subject the fluoride level was still very high(0.45 ppm), and fhese recordings were droppedfrom the final statistical analyses.

Fluoride concentration in saliva is relatively in-dependent of flow rate, but higher concentrationsmay be found in persons with a low flow rate.^"Also, for the subjects in this study, the baseline flu-oride concentration was significantly greater insubjects having low flow rate than in their normalcounterparts.

The frequency of swallowing saliva is an impor-tant factor in models of oral clearance.^' To elimi-nate this factor the authors chose to collect salivacontinuously for 30 minutes after the application offluoride gel. This collection time was chosen be-cause several studies have shown that salivary flu-oride clearance is most pronounced during the first30 minutes after application.^^"^"'

During a 30-minute collection period, six salivasamples were taken from the subjects with normalflow rates, after which there was a 15-minutebreak. During that break, the subjects were allowedto speak and to swallow. The 45-minute samplealso was taken using a 5-minute collection period.This was followed by a 10-minute break, and thefinal 5-minute collection was made 1 hour after flu-oride application. Interestingly, unstimulated sali-vary flow rates were significantly higher after thesebreaks than the flow rates during the 30-minutecontinuous collection period. The same trend wasalso noted for persons with low flow rates. No dif-ferences were observed in salivary flow rates duringthe 30-minute collection period. It seems apparentthat the stimulation in flow rates was caused by in-creased activity of the masticatory and oropharyn-geal muscles during the breaks. These data confirmthe regimen'^ that subjects should be allowed toaccommodate for a few minutes before collectingunstimulated whole saliva, since saliva flow may beactivated after speaking or swallowing.

After 30 minutes the mean salivary fluoride con-centration was almost six times greater for the sub-jects with low salivary flow rates than for subjectswith normal flow rates. This finding is in agreement

The Internationai lournai of Prosthodontics 558 Volume 10, Number 6, 1997

arv Fluoride Levels After Fiuoride Gei Applicatii

with that reported in a study by Biilings et ai'^ thatevaiuated the retention of topicai fiuoride in themouths of xerostomic subjects. The authors re-ported tbat after 30 minutes of fiuoride gel treat-ment, saiivary fiuoride ievels were more than 20times higher in xerostomic subjects (181 ± 72 pg/g)compared with normal participants' levels (8 ± 1ig/g). In the present study it was further noted that

after 1 hour, saiivary fiuoride concentrations re-mained at signitlcantiy greater levéis in the subjectswith low fiow rates compared to those of partici-pants with normai fiow rates. The remaining fluo-ride concentrations in the denture, measured fromthe water used to rinse the mucosai surfaces of thedentures after 1 hour, were aiso higher in the sub-jects with low sai iva flow rates than in subjects withnormal fiow rates. These findings show that fluorideciearance rates at the abutment-denture interfaceare cieariy related to the subjects' saiiva fiow rate, ithas aiso been shown by Rudney and Larson^^ thatswailowing intervals are longest for individuals withiow fiow rates, which may further proiong their oraifiuoride ciearance. The delay in dilution of fluoridemay be bénéficiai for the individuals with iow sali-vary flow rates, since they are known to be at ahigher risk for dentai caries.'""

Zero et al '^ anaiyzed fiuoride concentrationsusing a microdiffusion method in piaque, whoiesaiiva, and ductai saiiva after appiication of home-use topicai fiuorides. They conciuded that: "themethod of fluoride deiivery, the fiuoride concentra-tion of the agent, and the time of appiication areimportant factors infiuencing fiuoride ievels in themouth," They aiso reported that the use of fiuoridegei resuited in significantiy greater salivary fluorideconcentrations than either a fluoride rinse or a fiuo-ride dentifrice. Heintze and Petersson^^ studiedsalivary fiuoride ciearance rates after different topi-cal fiuoride treatments in 19 dentai nurses and useda fluoride electrode for the analysis. They alsofound that the concentrations after 30 minutes oftreatment were significantiy greater foiiowing theapplication of fiuoride varnish (32,0 ± 5,1 ppm)and fiuoride gei (8,9 ± 2,3 ppm) compared withthose after the treatment with 0,2% (2.1 ± 0.6 ppm)and 0,05% (0,5 ± 0.2 ppm) fluoride mouthrinse,

Saiivary fiuoride levéis have been reported to re-main high for a iong period of time after fiuoride geitreatments,'^'^^ After the application, some of thefiuoride ions may precipitate as calcium fiuoHde inthe piaque matrix.''' This fiuoride fraction is consid-ered to function as a fiuoride-ion reservoir that isgradualiy dissoived. Dawes and WeathereiP® sug-gested that this may expiain why the ciearance ratefrom saiiva is significantly prolonged after the use of

high-concentration fluoride products. Zero et al^^determined that orai soft tissues, especiaiiy thetongue and mandibuiar posterior vestibule, are themajor sites of fluoride retention in the mouth. Ac-cording to the Zero et ai study^'' it is possible thatthe orai soft tissues in the present study's subjectsmay have piayed a major roie as fiuoride retentionsites.

According to this study the fluoride clearance atthe abutment-denture interface follows the samepattern that has been observed in previous topicaifiuoride studies;' -^ ' ^-^^ however, the interindivid-ual differences in fiuoride ciearance rates werequite large. Direct comparison of saliva fiuorideconcentrations in different studies is difficuit, sincefluoride concentrations have often been analyzedwith different methods and resuits have been givenin dift'erent units, Heintze and Petersson" used afiuoride eiectrode for analysis, and they reportedthat after 30 minutes of topical treatment with 6mL of 0,9% fluoride gel, their subjects' mean saiivafiuoride level was 8,9 ± 2.3 ppm. Aithough theamount and the concentration of the fiuoride wasiower in the present study, it was found that after30 minutes of fluoride appiication saliva concen-tration was 13,1 ± 12.2 ppm in the subjects with anormai saliva flow rate. Zero et aP^ evaluated theapplication of home-use topical fiuorides in eden-tuious and compietely dentate subjects and foundsignificantly higher saiivary fiuoride ieveis in eden-tulous subjects after the use of a fiuoride dentifriceand fiuoride gel treatment. The pre.sent study popu-lation consisted of subjects who had maxiiiarycompiete dentures and two abutment teeth to sup-port their mandibular overdenture. Greater fluorideieveis detected in these subjects compared withthose reported by Heintze and Petersson^' maypartiy be expiained by the differences in the typeof subjects' dentitions. Another important factor isthat this study's subjects live in an area where thedrinking water is fiuoridated, and their baseline fiu-oride level (0,08 ± 0,03) was four times higherthan the mean baseline value (0,02 ± 0,01 ppm|reported by Heintze and Petersson, -

The denture itseif seems to function as a fiuoridegel tray, but the space between the denture baseand the oral tissues was not equai in ail the subjects,as this study popuiation consisted of subjects whohad worn their dentures from 1 to 20 years. The fitof dentures may deteriorate with time;'"^ therefore,the space between the dentures and the supportingtissues may have differed in size among this popuia-tion, and the authors believe that this may have hadan effect on fiuoride clearance rates. To evaiuatethis hypothesis, the reiationship between the age of

10, Number 6, 1997 559 al of Prosthodontii

Sillivary Fluoride Level; After Fluoride Gel Application

the dentures and the fluoride clearance rate was ex-amined, but a significant correlation was not found.Some of the denture wearers were more skillful inwearing their dentures than were others, and theymight have cüntrolled their dentures during thesaliva collection so well that the fluoride was notreadily diluted into the saliva. This may be one rea-son for the large variation in the fluoride concentra-tions between subjects found in this study.

Some of the fluoride will be swallowed and ab-sorbed into the systemic circulation"'; however,the daily use of two drops of 0,5% fluoride gel inthe denture is still clearly under the recommentJedmaximum amount of exogenous fluoride,''^

Tooth surfaces are constantly undergoing de-mineralization and remineralization processes. If asufficient amount of fluoride is present during theremineralization process, the rate of caries progres-sion may decrease so that subclinical lesions willnot be detected clinically,

[n their longitudinal study, Fttinger andjakobsen''^ reported that frequency of brushing, useof home fluorides, and disease and drug effectswere the most significant predictive factors ofcaries susceptibility among overdenture wearers.They also noted that initially softened root surfaceareas could be remineralized within 2 weeks if thepatients were willing to use a fluoride gel daily.

Different patient populations need differentkinds of preventive fluoride treatment. However, togain good compliance it is essential that thesetreatments are easy to perform. This study showedthat salivary fluoride levels remained high for along period of time after fluoride appiication, espe-cially in those whose salivary flow rate was low.Based on previous reports of the clinical effect oftopical fluoride gels,' "*-* it appears that daily appli-cation of high-concentration fluoride gel is a sim-ple and effective method to prevent caries on over-denture abutments.

Conclusions

Salivary flow rates and fluoride levels were evalu-ated in 24 overdenture wearers after topical appli-cation of fluoride gel (Karigel-N). Evaluation wasperformed separately for subjects with normal un-stimulated whole saliva flow rates (> 0.1 g perminute; n = 16) and for those with low flow rates(< 0.1 g per minute; n = 8). The number of sub-jects, especially in low-flow-rate group, was lim-ited, but the following conclusions may be made:

1. Salivary fluoride concentrations decreased dur-ing the 1-hour study period in both groups ofsubjects, but remained at a higher level in sub-jects with a low flow rate,

2. In both groups of subjects salivary flow rate cor-related negatively with the remaining fluorideconcentration at tbe abutment-denture interface,

3. Fluoride levels remained high for at least 30minutes after use of the daily home fluoride gel.

Acknowledgments

Dr Närhi's Visifing Assistanf Professorship was parfly supportedby Oskar Öflunds' Foundaiion (Heisinki, Finland),

This siudy was supported by a grant from the NIDR/NIH |PÎOD£10126-05}.

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10, Number 6, t997 561 Journal ol Prosthodontics