Fluoride releasing effect of a modified resin denture containing S-PRG fillers on salivary fluoride retention: A randomized clinical study

Abstract

This study aimed to evaluate the clinical effect of resin denture base containing 20wt% S-PRG (surface pre-reacted glass-ionomer) fillers on saliva fluoride concentration as surrogate evaluation of caries control. The study was a double blind randomized clinical trial with a total of 110 participants aged 35–60 years who wore a removable partial resin denture for 1.5 years. Subjects were randomly assigned to receive a partial resin denture fabricated either with or without 20wt% S-PRG fillers (n=55 per group). After 14 days of wearing the dentures, 5000 ppm fluoride gel was applied nightly on both sides of the denture before storing it in a denture box overnight every night for 1.5 years. Salivary fluoride concentration was measured at baseline, at days 1, 14, 15, and at 3 months and 1.5 years of wearing the denture. A significant increase in salivary fluoride concentration in the S-PRG containing denture group was observed from baseline to Day 1, followed by a significant decrease to an approximate baseline level. After recharge with 5,000 ppm fluoride gel, salivary fluoride increased markedly at Day 15 and remained elevated at month 3 through 1.5 years. The resin denture base containing 20wt% S-PRG fillers demonstrated initial fluoride release that increased saliva fluoride concentration, but the fluoride release from within the material was short-lived. The long term effect of sustained fluoride release from the denture when combined with a regular fluoride recharge regimen may be beneficial for caries prevention. [ClinicalTrials.gov, registration number NCT 02460562]

Introduction:

Resin dentures are a typical option for people who exhibit partial tooth loss, especially in older populations due to economic factors. This type of denture can replace multiple teeth and is characterized by ease of fabrication and low cost. However, resin dentures require wide coverage over surrounding abutment teeth which subsequently are more likely than are any other teeth to be affected by caries. As a means of preventing caries, the denture wearer must attend to oral care and fluoride delivery using a variety of recommended methods [Han et al., 2002]. One such method is by the use of fluoride-containing dentures given their ability to maintain a therapeutic concentration of fluoride over extended periods of time. Recently, Surface pre-reacted glass-ionomer (S-PRG) filler has been introduced. It is a new type of biological material which is being used as filling material in the formulation of dental products, partly because of its ability to release and recharge fluoride. The fluoride in turn alters the pH of the surrounding environment when it comes into contact with water or acidic solutions and this process exerts a significant effect on caries prevention [Nakatuka et al., 2003; Takada et al., 2007]. An optimal content of 20wt% S-PRG fillers in traditional resin denture base was reported to provide sufficient fluoride release and recharge capacity in vitro and efficient inhibition of demineralization of bovine dentine while still maintaining acceptable strength in compliance with ISO 1567 [Kamijo et al., 2009; Mukai et al., 2009]. Despite the potential benefit that incorporation of S-PRG fillers in resin denture base can provide to fitness and longevity of abutment teeth, very limited clinical data exist on fluoride release by denture base materials that incorporate S-PRG fillers. This clinical study aimed to examine the fluoride release and recharge capability of a daily-used partial resin denture containing 20wt% S-PRG fillers and revealed that use of such a denture increased saliva fluoride concentration in adults who wore it for 1.5 years. The null hypothesis was that no significant differences exist in the amount of fluoride released and the recharge of salivary fluoride between resin dentures with and without S-PRG fillers.

Methods:

This study was a 1.5-year double blind, intervention-controlled clinical study and was reviewed and approved by the Institutional Review Board of the University of Washington, U.S.A, and Thammasat University, Thailand prior to implementation. Power calculation considered that salivary fluoride concentration after the use of 1000 ppm dentifrice remained at 0.02 ppm at baseline [ten Cate, 2013], whereas fluoride release from experimental resin denture containing 20wt% S-PRG filler on the first day after storage in distilled water was 0.006 mg/ml [Kamijo et al.,2009]. A difference in mean fluoride concentration in saliva between the control and the intervention group after wearing the denture on day 1 was expected to be approximately 0.04 ppm. A sample size of 45 subjects per group would be required to generate 80% power to detect this difference in mean saliva fluoride concentration using an independent t-test with two-sided 5% statistical significance. Allowing for a possible drop-out rate of 20%, an initial sample size of 110 subjects (55 per group) was recruited.

Recruitment:

Enrollment of subjects occurred from June 2015 through September 2015 and follow-up took place from June 2015 through March 2017. Recruitment proceeded through local and hospital media at three government hospitals (Thammasat University Hospital, Kratumban Hospital and Health Promotion Center Region 1 Hospital) in the vicinity of Bangkok, Thailand where water fluoride content in the neighboring villages was less than 0.01 parts per million (ppm). Approximately 35–40 subjects per hospital were randomly allocated into control and intervention treatment groups to receive an upper partial resin denture either without or with S-PRG filler. A block randomization was generated and coded as a numeric ID for each subject. The allocation list was provided to the dental assistant who assigned participants in sequential order to alternate treatment groups. The nature of group assignment was concealed from participants and clinical staff until the conclusion of the study.

Healthy Thai patients aged 35–60 years who exhibited tooth loss and desire to have a resin denture fitted in their mouth were approached to participate in this study. At baseline, subjects completed a questionnaire of general demographic questions regarding medical health and oral care background, then received an initial oral screening and underwent a simple test of salivary flow rate by the participating dentist. Subjects who exhibited a salivary flow rate in the range of normal values (unstimulated whole saliva flow rate > 0.15 ml/min), who retained at least 6 natural upper teeth including at least 2 posterior teeth in good condition were included in the study. Patients were excluded if they exhibited signs of xerostomia, had extensive dental caries or untreated periodontal disease, or were allergic to resin or metal that could interfere with the study. Information on study procedures was provided to subjects by participating dental assistants before written informed consent was obtained

Intervention:

Upper partial denture treatment was conducted by a participating dentist of the individual hospitals following standard protocols at baseline (Figure 1, 2). After models were poured and set, appropriate denture designs were drawn on the models and transcribed through generic lab script for communication with the laboratory that constructed the resin dentures according to their routine procedure. A notation on each script indicated to the laboratory whether the corresponding denture was to be made with polymethyl methacrylate resin (PMMA, URBAN RESIN®, Shofu Inc. Kyoto, Japan) or with PMMA resin containing 20wt% S-PRG fillers (URBAN RESIN® containing white S-PRG filler, particle size 4.1 µm, Lot. 140122, Shofu Inc. Kyoto, Japan). At the next appointment, dentures were fitted for their respective subjects and adjusted for comfortable wearing. To maintain blindness in the study, standard at-home instructions for denture wearers were provided to all participants following denture delivery focusing on all days of wearing the denture, brushing the remaining teeth with a standard fluoride toothpaste (Colgate Total®, Colgate Palmolive, Thailand, 1000 ppm) and cleaning the denture with water before storage in a denture storage box containing tap water while sleeping overnight for the first 14 days. After 14 days, 5000 ppm fluoride gel (PreviDent® Gel, Colgate Palmolive, Thailand) was supplied for all participants in both group to storage of the dentures in lieu of tap water from night 14 to 1.5 years. Participants were suggested to remove the denture to clean with water and once apply 1 gram of 5000 ppm fluoride gel (approximately 1 cm of length) to the denture on both sides before storage in the denture box overnight every night for 1.5 years. Participants were required to rinse the denture with water to remove the fluoride gel before daily reinsertion. A diagram of the full procedure for participants wearing the denture is given in Figure 3.

Figure 1.

Upper partial resin denture containing without S-PRG fillers.

Figure 2.

Upper partial resin denture containing 20wt% S-PRG fillers.

Figure 3.

Diagram of partial resin denture wearer

Saliva sample collection:

2 ml of unstimulated saliva samples were collected from subjects at baseline before denture delivery and while wearing the denture on days 1, 14, 15, and at 3 months and 1.5 years of wearing the denture, under condition that the participants wore the denture for at least one hour before and refrained from tooth brushing for at least two hours before saliva sampling. All samples were kept frozen at - 80°C until fluoride analysis.

Fluoride analysis:

Samples were transferred to the stomatology laboratory, Prince of Songkhla University, Thailand for measurement of salivary fluoride content in duplicate, using the direct method of fluoride Ion-Selective Electrode. Standard fluoride solutions with concentrations of 0.02 and 0.2 were used to calibrate the measuring device. Prior to measurement, 0.2 ml of total ionic strength adjustment buffer III (TISAB III, Cole-Palmer, USA) was added to each test solution, then a fluoride ion electrode (item number 27502–19, Cole-Palmer, USA) was connected to a 710A plus fluoride ion meter (item number 067952, Thermo Orion, USA). Fluoride concentration (ppm) was measured while the solution was stirred at room temperature. Two readings for each sample were recorded and the mean value was calculated. Samples were discarded after completed analysis.

Statistical analysis:

Data were analyzed with SPSS statistics software version 22.0 for Windows at α = 0.05. Statistical significance of observed differences in mean salivary fluoride concentrations (ppm) between the two-treatment groups were assessed at different time points using repeated measures analysis of variance. The saliva fluoride concentration from each groups were statistically compared with baseline concentration using a t-test.

Results:

In total, 110 adults (24 males, 86 females) with a mean age of 49.7±10.4 years fulfilled the study inclusion criteria and participated in this study. Participant drop-out rates of 5–20% (by reason of absence or illness) were observed at each follow-up time point (see participant flow in Figure 4). No harm or adverse reaction was observed during the study. At baseline, mean number of remaining natural teeth in the upper arch was 13 teeth for the control group and 12 teeth for the S-PRG filler group. Both groups had an average of 3 teeth replaced with artificial teeth on the upper partial resin denture. No significant differences in baseline demographics of the participants were observed between the two groups (Table 1). The mean concentration of saliva fluoride at baseline demonstrated no correlation with oral care characters of the participants including frequency of tooth brushing, use of fluoride toothpaste and mouthwash (t-test, p>0.05).

Figure 4.

Flow diagram of study

Table 1.

Baseline demographics of participants

	Control   denture	S-PRG     denture	p value
Number of participants	55	55
Average age, years	49.3±10.9	49.8±10.1	0.79
Gender			0.96
Male	20.8	21.1
Female	79.2	78.9
Teeth present	12.4±1.9	11.9 ±1.9	0.11
Teeth replaced with denture	2.4±1.5	2.7±1.6	0.26
Fluoride concentration (ppm)	0.05±0.04	0.05±0.05	0.38
Tooth brushing			0.83
Twice a day	58.5	75.4
3 or more times a day	24.5	19.3
Not identified	17.3	5.3
Fluoride toothpaste			0.15
Yes	56.6	63.2
No	9.4	17.5
Not identified	34.0	19.3
Fluoride Mouthwash			0.07
Yes	26.4	43.9
No	58.5	50.9
Not identified	15.1	5.3

Data are presented as mean ± SD or percentage (%) as appropriate.

P value compared the baseline demographic of participants between the two groups.

The mean salivary fluoride concentration (ppm) of both groups at measured time intervals are given in Table 2. Fluoride concentration increased significantly from baseline to day 1 of denture wearing in both groups, followed by a significant decrease to approximately baseline level by 14 days (p<0.05). Mean salivary fluoride concentration was significantly higher for the S-PRG containing group compared to the control at 1 day but no difference between the two groups was observed at baseline and at day 14. However, when the resin denture was recharged with 5000 ppm fluoride gel, the mean salivary fluoride concentration in the test group increased significantly on day 15, and at 3 months and 1.5 years, and was significantly greater than the control group, particularly after 3 months and 1.5 years. This result indicates that the differences in mean fluoride concentration of the intervention group were predominantly due to recharging capability with the 5,000 ppm fluoride gel daily from day 15 to 1.5 years. Although sustained fluoride release was observed in the control group after recharge with fluoride, measured salivary fluoride concentration in this group began to decrease after 3 months, and was not significantly different from baseline concentration after 1.5 years. Fluoride concentrations in the control group were significantly lower than those for the experimental group at all time-points assessed following nightly fluoride recharging.

Table 2.

Mean ± SD salivary fluoride concentration (ppm) over time

Group	Baseline	Day 1	Day 14	Day 15	3 Months	1.5 Years
Control	0.05 ± 0.04	a0.09 ± 0.07	0.06 ± 0.06	a0.11 ± 0.11	a0.12 ± 0.12	0.08 ± 0.12
S-PRG	0.05 ± 0.05	a0.22 ± 0.12	0.06 ± 0.05	a0.23 ± 0.13	a0.26 ± 0.15	a0.32 ±0.20
p-value	0.658	<0.001	0.733	<0.001	<0.001	<0.001

P-value indicates the level of significant difference in salivary fluoride concentration between the test and control groups at each time point.

^aindicates a significant difference in salivary fluoride concentration compared with baseline concentration within the same group (p<0.05).

Over 1.5 years of study, 7 subjects (4.6%) experienced broken dentures, 2 of which were from the control group and 5 of which were from the S-PRG group (p=0.44, Fisher’s exact test). These dentures were repaired or remade with materials according to the group assignment.

Discussion:

Among denture wearing patients, abutment teeth and teeth in contact with a removable denture are indicated as high caries risk [Featherstone et al., 2011]. The effect on caries risk of wearing a removable denture is controversial, but it may relate to the difficulty in access for cleaning or with increased plaque accumulation and risk of caries seen with the use of the dentures [Bergman et al., 1995]. To prevent dental caries of abutment teeth associated with removable dentures, fluoride delivery into the oral cavity is recommended. One possible mode of fluoride delivery is through the use of a fluoridated denture. In the present study, 20wt% S-PRG fillers were incorporated into a resin denture base and acted as a fluoride source. The resin dentures containing S-PRG were initially capable of fluoride release in a burst effect for 1 day of wearing the denture, which is a known and unique property of fluoride-containing materials [Dhondt et al., 2001]. The dentures exhibited a fluoride releasing phenomenon consistent with that observed previously in vitro [Kamijo et al., 2009; Wiegand et al., 2007]. However, the amount of initial fluoride release from within the dentures was reduced to baseline levels after 14 days. This observation indicated that S-PRG containing dentures lead to elevation of fluoride in the oral cavity, but the initial effect was short-lived, consistent with the expectation due to the kinetics of saliva fluoride clearance [ten Cate, 2013].

While the benefit of the initial burst lies in fluoride’s proven effect in caries prevention and remineralization of dental tissue, a sustained release of fluoride is associated with long-term anti-cariogenic action [Itota et al., 2004a]. A constant supply of low levels of intraoral fluoride from 0.03 to 0.1–0.3 ppm (e.g. a 5 to 10-fold increase) for prolonged periods is of most benefit in caries control [Sjogren and Birkhed, 1993; Duckworth and Morgan, 1991]. Importantly, S-PRG fillers have been reported to exhibit fluoride releasing capacity after recharge [Dionysopoulosa, 2014; Zafar, 2013]. The S-PRG containing denture in this study exhibited fluoride recharging capacity that appeared to consistently maintain fluoride release following overnight recharge with 5,000 ppm fluoride gel daily from day 15 through 1.5 years. The potential for a S-PRG containing resin denture to maintain a constant release of fluoride, thereby increasing fluoride level in the oral environment whenever the denture is frequently recharged with fluoride, coupled with the observed salivary fluoride concentration of 0.2–0.3 ppm for this type of denture has the potential to control caries on abutment teeth in contact with the denture.

The degree of fluoride-recharging ability from fluoridated resin depends on many factors, including level of fluoride content, kind and frequency of fluoride exposure and concentration of the fluoridating agent [Toumba and Curzon, 2005; Han et al., 2002]. In this study, recharge of the resin denture base containing S-PRG fillers provided an optimal salivary fluoride level after daily recharge with 5,000 ppm fluoride gel. This finding is consistent with a previous investigation that reported the ability of fluoridated glass filler within a denture base resin to facilitate fluoride release following recharge with 5,000 ppm NaF gel [Al-Bakri et al., 2014]. Thus, the use of 5,000 ppm fluoride concentration for recharge of fluoride may be realistic for clinical use. However, 5,000 pm fluoride gel is available by prescription only and given that it is more expensive than regular fluoridated dentifrices (1,100 ppm), some patients may reject the idea of applying it. A regular fluoridated dentifrice may be an alternative in such cases.

The mechanism of fluoride release from S-PRG containing denture base observed in this study is related to the presence of glass-ionomer phase around the glass core of the filler [Fujimoto et al., 2010]. As the S-PRG particle is a pre-reacted glass–ionomer filler in a resin matrix, its fluoro-alumino-silicate glass has been prereacted with polyacid to form a glass–ionomer matrix structure which is then blended with PMMA resin. An almost complete acid–base reaction has already taken place before incorporation into the resin, therefore this glass–ionomer matrix contains much complex fluoride and is easily penetrated by water which results in significant fluoride release from this material at the initial stage [Najma and Meena, 2015]. Although, the precise effect of fluoride release following the recharge of the denture is not fully understood, it is acknowledged that recharge ability is dependent on the extent of the hydrogel matrix within the S-PRG particles which serves as a reservoir for storage of fluoride and a vehicle for continuous release [Itota et al., 2004a]. Furthermore, PMMA denture base material, which is hydrophilic in nature, may absorb solution and play a role in dissolution of fluoridated glass fillers resulting in increase of fluoride uptake and release.

By comparison, a similar pattern of fluoride release was observed for the control dentures as for the dentures containing S-PRG fillers, but the amount of salivary fluoride measured was significantly lower than that for a denture containing S-PRG fillers, and was not maintained following recharge for the entire 1.5 years of study. The reduced effect following initial release of fluoride from the control resin implies that only the fluoride adsorbed to the surface topography of the control denture was released. This fluoride release from the control denture may result from the physical properties of PMMA resins that are subjected to water sorption when placed in an aqueous environment [Cunha et al., 2009]. These resins absorb fluoride from the oral environment to the surface of the material and sustain fluoride release in the manner of fluoridated materials. Additionally, chemical reactivity of PMMA resin may be implicated in the form of remaining unreacted monomer in the denture base. Residual monomer may be leached and affected by bulk sites of material that facilitate the diffusion of salivary fluoride toward the external surface. However, once the bulk sites are fully occupied, their permeability may be limited resulting in a decrease of fluoride concentration, as observed in this study [Bhola and Mishra, 2010]

In the present study, some patients experienced broken dentures due to accidents such as dropping the denture or biting when the denture was not correctly in position. Broken dentures comprised less than 5% of the total dentures placed and no significant difference in incidence of broken dentures was observed between groups. Resin denture base containing 20wt% S-PRG filler did not appear to diminish the mechanical properties of the denture over the study period of 1.5 years. This concentration may be an optimal percentage of filler that satisfies the need for fluoride release while maintaining acceptable strength compliant with the requirement of ISO1567 [Kamijo et al., 2009].

An advantage of this study was that almost all participants were approached from the waiting list for removable partial dentures of each hospital, therefore the oral status of participants had been prepared readily for denture fabrication previously. Although more than three times the number of female participants were recruited than male participants (86 females and 24 males), no differences in performance were found between female and male participants. Participants in this study were followed with a similar intervention as that for a routine denture wearers. The difference between these groups was in the materials used for fabrication of their dentures. However, the dentures fabricated with and without S-PRG fillers were so similar in appearance and in color that they were difficult to distinguish visually.

A limitation of this study was in relation to the ion-selective electrode method used to determine saliva fluoride concentration in the study. This method requires TISAB buffer solution to control pH and this ensures that total fluoride ion concentration (free and complex fluoride) can be determined. Although the ISE with TISAB III is simple, cost effective and convenient for the measurement of fluoride release, the method is limited by the uncertainty of measuring at low fluoride concentrations, particularly given the difficulties of achieving a stable meter reading with no drift. The lower limit of fluoride determination with the electrode is 0.1 ppm. Below this concentration, the output from the electrode becomes meaningless [McCabe et al., 2002]. To overcome the shortcomings with ISEs, ion chromatography is a useful alternative method for fluoride determination at very low concentrations because of its high sensitivity which provides meaningful and reproducible measurements [Itota et al., 2004b].

This study demonstrated that sustained release of fluoride from the resin denture containing S-PRG filler through the recharging route was significantly greater than that released from within the material, and is expected to have the long term effect of increasing enamel and dentin resistance to new carious lesions on abutment teeth of partial denture wearers. The capacity of fluoridated resin denture base to act as a fluoride reservoir may be an important consideration in the selection of materials for clinical use in the near future. However, further investigation should reveal the efficacy of this material for the prevention of new carious lesions and also should focus on facilitation of incorporation of a higher content of S-PRG filler, which would satisfy the financial need of patients for fluoride recharge using the lowest and therefore least expensive possible fluoride concentration recharge material.

Within the limitations of the study, we concluded that the resin denture base containing 20wt% S-PRG fillers demonstrated initial fluoride release to increase saliva fluoride concentration, but the fluoride release from within the material was of short duration. The long term effect of sustained fluoride release from the denture requires combination with a regular fluoride recharge regimen, which in turn is expected to act as a fluoride reservoir for controlling of dental caries on abutment teeth of partial denture wearers.