Fluoride releasing restorative materials: Effects of pH on mechanical properties and ion release

Abstract

Objectives

Secondary caries and restorative fracture are the two main reasons for restoration failures. Fluoride ion (F) release can help inhibit caries. Plaque pH after a sucrose rinse can decrease to a cariogenic pH of 4–4.5. The objective of this study was to investigate the effects of solution pH and immersion time on the mechanical properties and F release of restorative materials.

Methods

Three resin-modified glass ionomers (Viremer, Fuji II LC, Ketac Nano), one compomer (Dyract Flow), and one composite (Heliomolar), were tested. Flexural strength and elastic modulus were measured before and after 84 d of immersion in solutions of pH 4, 5.5, and 7. F release was measured as a function of pH and immersion time.

Results

Immersion and material type had significant effects on mechanical properties. Vitremer had a flexural strength (mean ± sd; n = 6) of 99 ± 25 MPa before immersion; it decreased to 32 ± 9 MPa after 84 d of immersion ( p < 0.05). In comparison, Heliomolar had a smaller strength loss, decreasing from 99 ± 9 MPa to 65 ± 7 MPa ( p < 0.05). Solution pH had little effect on mechanical properties. For example, Fuji II LC had a strength of 63 ± 15 MPa at pH 4, similar to 61 ± 30 MPa at pH 5.5, and 56 ± 22 MPa at pH 7 ( p > 0.1). In contrast, solution pH had a significant effect on F release. F release at 84 d for Fuji was 609 ± 25 μg/cm 2 at pH 4, much higher than 258 ± 36 μg/cm 2 at pH 5.5, and 188 ± 9 μg/cm 2 at pH 7.

Significance

The restoratives tested were able to greatly increase the F release at acidic, cariogenic pH, when these ions are most needed to inhibit caries. However, mechanical properties of these F-releasing restoratives degraded significantly in immersion. Efforts are needed to develop F-releasing restoratives with high levels of sustained F release, as well as improved durability of mechanical properties for large stress-bearing restorations.

Introduction

Resin composites are increasingly used in tooth cavity restorations due to their direct-filling capability, esthetics, and improved performance . Filler and matrix compositions have been optimized, mechanical properties have been enhanced, and polymerization shrinkage has been reduced . However, secondary caries can develop at the tooth-restoration interfaces . These caries, along with restoration fracture, are the most frequent causes for the failure of restorations . Therefore, fluoride ion (F) releasing restoratives as well as calcium and phosphate ion releasing materials have been developed as they may reduce secondary caries at the restoration margins . There are several mechanisms involved in the anticariogenic effects of fluoride, including the reduction of demineralization, the enhancement of remineralization, the interference with pellicle and plaque formation, and the inhibition of microbial growth and metabolism .

Commercial F-releasing restoratives fall into four categories: glass ionomer cements, resin-modified glass ionomer cements, polyacid-modified composites (compomers), and fluoride releasing composites. The mechanical properties and fluoride release abilities vary between different materials. Compomers, glass ionomers, and resin-modified glass ionomers are generally weaker than composite resins . Therefore, the clinical applications of F-releasing materials are usually limited to relatively small-sized restorations in moderate load-bearing areas .

The anticariogenic effect of F-releasing materials depends on the amount and sustainability of F release . The F release from a restorative material is determined by the matrix of the restorative material, the mechanism by which it sets, and the amount of F-containing fillers . The matrix of resin composites is much less hydrophilic, and F incorporated in the resin composite is only released in small amounts . The pattern of F release is typically characterized by an initial rapid release, followed by a significant reduction in the rate of release after only a few days of immersion .

The mechanical properties and the F release rate of a material may also depend on the pH of the solution to which the material is exposed. The oral plaque pH after a sucrose rinse can decrease to 4–4.5 . A plaque pH of above 6 is considered to be safe, a pH of 6–5.5 is potentially cariogenic, and a pH of 5.5–4 is cariogenic. However, the effects of solution pH and immersion time on the mechanical properties and F release of currently-available F-releasing restorative materials are yet to be investigated.

Accordingly, the objective of this study was to investigate the effects of solution pH and immersion on the mechanical properties and F release of restoratives. Five F-releasing materials were studied, including three resin-modified glass ionomers, one compomer, and one F-releasing composite. Three hypotheses were tested: (1) solution pH will have a significant effect on F release, with higher release at acidic and cariogenic pH when the F ions are most needed; (2) solution pH will have little effect on the mechanical properties of the immersed F-releasing specimens; (3) materials with higher F release will in general have lower mechanical properties.

Materials and methods

Fluoride releasing materials

Five materials were used ( Table 1 ) to investigate the effect of pH on the mechanical properties and F release: three resin-modified glass ionomers (Vitremer, Fuji II LC, and Ketac Nano), one flowable compomer (Dyract Flow), and one composite (Heliomolar). According to the manufacturers, Vitremer is indicated for Class III and Class V restorations, root caries lesion restorations, Class I and Class II restorations in primary teeth, and core buildup. Heliomolar is indicated for Class I and Class II restorations in the posterior region, Class III and Class IV anterior restorations, Class V restorations, and pit and fissure sealing in molar and premolar teeth. Dyract Flow is recommended for restoration of minimally invasive cavity preparations, pits and fissures, restoration of shallow Class V preparations, and as a base/liner under Class I and Class II restorations. Fuji II LC is indicated for Class III and Class V restorations, restoration of primary teeth, and core buildup. Ketac Nano is recommended for primary teeth restorations, small Class I restorations, and Class III and Class V restorations.

Table 1
Compositions of fluoride releasing restorative materials used in this study.
Material Formulation Filler level Manufacturer
Vitremer; shade B2; glass ionomer core buildup/restorative system; powder: lot 20081028, liquid: lot 20081111 Powder: radiopaque, fluoroaluminosilicate glass with microencapsulated potassium persulfate and ascorbic acid; liquid: aqueous solution of a polyalkenoic acid modified with methacrylate groups 71.4 wt% 3M ESPE (St. Paul, MN)
GC Fuji II LC (improved); shade A2; radiopaque light curing reinforced glass ionomer restorative; lot 0810081 Polyacrylic acid, 2-hydroxyethylmethacrylate, urethanedimethacrylate, camphorquinone 76.2 wt% GC Corporation (Alsip, IL)
Ketac Nano; shade A2; light curing nano-ionomer restorative; lot 20081023 Polycarboxylic acid modified with methacrylate groups, fluoroaluminosilicate glass 69 wt% 3M ESPE (St. Paul, MN)
Dyract Flow; shade A2; compomer restorative material with flow characteristics; lot 090310 Strontium-alumino-fluoro-silicate glass, highly dispersed silicon dioxide, ammonium salt of PENTA and N,N-dimethyl aminoethyl methacrylate, carboxylic acid modified macromonomers, diethylene glycol dimethacrylate, camphorquinone, ethyl-4-dimethylaminobenzoate, 2-hydroxymethylbenzophenone, butylated hydroxyl toluene, colorants, and titanium dioxide 65 wt% Dentsply DeTrey GmbH (Konstanz, Germany)
Heliomolar; shade A2; light curing resin-based microfilled composite; lot L43034 Resin matrix: dimethacrylates; fillers: highly dispersed silicon dioxide, ytterbium trifluoride and copolymers (particles sized 40–200 nm); additional contents: catalysts, stabilizers, and pigments 76.5 wt% Ivoclar (Ontario, Canada)

Vitremer and Fuji II LC are two part, powder/liquid systems. Specimens of both were fabricated using the manufacturer’s suggested powder/liquid ratio, which was 2.5/1.0 for Vitremer and 3.2/1.0 for Fuji II LC. Dyract Flow and Heliomolar are one component systems. Ketac Nano is a two part, paste/paste system and was dispensed using the Clicker™ Dispensing System. All materials were placed into 2 mm × 2 mm × 25 mm molds, photo-cured (Triad 2000, Dentsply, York, PA) for 1 min/side, and incubated for 24 h at 37 °C before immersion.

Mechanical testing

Specimens were tested in a three-point flexure using a crosshead speed of 1 mm/min with a 20 mm span on a Universal Testing Machine (5500R, MTS, Cary, NC). Flexural strength was calculated: S = 3 P max L /(2 bh 2 ), where P max is the maximum load, L is the span, b is the specimen width, and h is the thickness . Elastic modulus was calculated: E = ( P / d )( L 3 /[4 bh 3 ]), where load P divided by displacement d is the slope of the load–displacement curve .

Fluoride release

Following a previous study , a sodium chloride (NaCl) solution (133 mmol/L) was buffered to three different pHs: pH 4 with 50 mmol/L lactic acid, pH 5.5 with 50 mmol/L acetic acid, and pH 7 with 50 mmol/L HEPES. As in previous studies , three specimens of 2 mm × 2 mm × 12 mm were immersed in 50 mL of solution, yielding a specimen volume/solution of 2.9 mm 3 /mL, similar to the 3.0 mm 3 /mL in a previous study . F concentration was measured at immersion times of 1 day (d), 2 d, 3 d, 7 d, 14 d, 21 d, 28 d, 35 d, 42 d, 49 d, 56 d, 63 d, 70 d, 77 d, and 84 d. At each time point, aliquots of 1 mL were removed and replaced by the same volume of fresh solution. F concentration was measured with a F ion selective electrode (Orion, Cambridge, MA) as described previously .

One-way and two-way ANOVA were performed to detect significant effects of the variables. Tukey’s multiple comparison test was used at p of 0.05.

Materials and methods

Fluoride releasing materials

Five materials were used ( Table 1 ) to investigate the effect of pH on the mechanical properties and F release: three resin-modified glass ionomers (Vitremer, Fuji II LC, and Ketac Nano), one flowable compomer (Dyract Flow), and one composite (Heliomolar). According to the manufacturers, Vitremer is indicated for Class III and Class V restorations, root caries lesion restorations, Class I and Class II restorations in primary teeth, and core buildup. Heliomolar is indicated for Class I and Class II restorations in the posterior region, Class III and Class IV anterior restorations, Class V restorations, and pit and fissure sealing in molar and premolar teeth. Dyract Flow is recommended for restoration of minimally invasive cavity preparations, pits and fissures, restoration of shallow Class V preparations, and as a base/liner under Class I and Class II restorations. Fuji II LC is indicated for Class III and Class V restorations, restoration of primary teeth, and core buildup. Ketac Nano is recommended for primary teeth restorations, small Class I restorations, and Class III and Class V restorations.

Table 1
Compositions of fluoride releasing restorative materials used in this study.
Material Formulation Filler level Manufacturer
Vitremer; shade B2; glass ionomer core buildup/restorative system; powder: lot 20081028, liquid: lot 20081111 Powder: radiopaque, fluoroaluminosilicate glass with microencapsulated potassium persulfate and ascorbic acid; liquid: aqueous solution of a polyalkenoic acid modified with methacrylate groups 71.4 wt% 3M ESPE (St. Paul, MN)
GC Fuji II LC (improved); shade A2; radiopaque light curing reinforced glass ionomer restorative; lot 0810081 Polyacrylic acid, 2-hydroxyethylmethacrylate, urethanedimethacrylate, camphorquinone 76.2 wt% GC Corporation (Alsip, IL)
Ketac Nano; shade A2; light curing nano-ionomer restorative; lot 20081023 Polycarboxylic acid modified with methacrylate groups, fluoroaluminosilicate glass 69 wt% 3M ESPE (St. Paul, MN)
Dyract Flow; shade A2; compomer restorative material with flow characteristics; lot 090310 Strontium-alumino-fluoro-silicate glass, highly dispersed silicon dioxide, ammonium salt of PENTA and N,N-dimethyl aminoethyl methacrylate, carboxylic acid modified macromonomers, diethylene glycol dimethacrylate, camphorquinone, ethyl-4-dimethylaminobenzoate, 2-hydroxymethylbenzophenone, butylated hydroxyl toluene, colorants, and titanium dioxide 65 wt% Dentsply DeTrey GmbH (Konstanz, Germany)
Heliomolar; shade A2; light curing resin-based microfilled composite; lot L43034 Resin matrix: dimethacrylates; fillers: highly dispersed silicon dioxide, ytterbium trifluoride and copolymers (particles sized 40–200 nm); additional contents: catalysts, stabilizers, and pigments 76.5 wt% Ivoclar (Ontario, Canada)

Vitremer and Fuji II LC are two part, powder/liquid systems. Specimens of both were fabricated using the manufacturer’s suggested powder/liquid ratio, which was 2.5/1.0 for Vitremer and 3.2/1.0 for Fuji II LC. Dyract Flow and Heliomolar are one component systems. Ketac Nano is a two part, paste/paste system and was dispensed using the Clicker™ Dispensing System. All materials were placed into 2 mm × 2 mm × 25 mm molds, photo-cured (Triad 2000, Dentsply, York, PA) for 1 min/side, and incubated for 24 h at 37 °C before immersion.

Mechanical testing

Specimens were tested in a three-point flexure using a crosshead speed of 1 mm/min with a 20 mm span on a Universal Testing Machine (5500R, MTS, Cary, NC). Flexural strength was calculated: S = 3 P max L /(2 bh 2 ), where P max is the maximum load, L is the span, b is the specimen width, and h is the thickness . Elastic modulus was calculated: E = ( P / d )( L 3 /[4 bh 3 ]), where load P divided by displacement d is the slope of the load–displacement curve .

Fluoride release

Following a previous study , a sodium chloride (NaCl) solution (133 mmol/L) was buffered to three different pHs: pH 4 with 50 mmol/L lactic acid, pH 5.5 with 50 mmol/L acetic acid, and pH 7 with 50 mmol/L HEPES. As in previous studies , three specimens of 2 mm × 2 mm × 12 mm were immersed in 50 mL of solution, yielding a specimen volume/solution of 2.9 mm 3 /mL, similar to the 3.0 mm 3 /mL in a previous study . F concentration was measured at immersion times of 1 day (d), 2 d, 3 d, 7 d, 14 d, 21 d, 28 d, 35 d, 42 d, 49 d, 56 d, 63 d, 70 d, 77 d, and 84 d. At each time point, aliquots of 1 mL were removed and replaced by the same volume of fresh solution. F concentration was measured with a F ion selective electrode (Orion, Cambridge, MA) as described previously .

One-way and two-way ANOVA were performed to detect significant effects of the variables. Tukey’s multiple comparison test was used at p of 0.05.

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Nov 30, 2017 | Posted by in Dental Materials | Comments Off on Fluoride releasing restorative materials: Effects of pH on mechanical properties and ion release
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