Comparison of fluoride ion release and alkalizing potential of a new bulk-fill alkasite.

AIM: This study was conducted to evaluate and compare fluoride ion release by Cention-N (self-cure and light-cure) and conventional glass-ionomer cement (GIC) at different pH and time intervals.
METHODOLOGY: Cavities of similar dimensions were prepared in mandibular molars and restored with Cention-N (by self-cure and light-cure techniques) and GIC. Samples were stored in deionized water, and the cumulative fluoride ion release and change in pH were assessed utilizing spectrophotometer and pH meter, respectively, at the end of 7 days, 14 days, and 21 days. The data thus obtained were statistically analyzed.
RESULTS: All the tested materials released fluoride ions in both acidic and neutral pH at all time intervals, and the fluoride ion release was significantly higher (<0.05) in acidic pH as compared to neutral pH except in GIC. All the groups showed a statistically significant increase in pH in acidic medium, whereas no significant increase was observed in neutral medium.
CONCLUSIONS: Cention-N (self-cure) has the highest fluoride ion release and alkalizing potential in acidic pH as compared to Cention-N (light-cure) and GIC.

INTRODUCTION

Continuous progress in restorative technology has made possible the availability of numerous direct filling materials to the modern dental practice – ranging from amalgams to glass-ionomer cement (GIC) and composites. Of these, GIC stands out for their excellent potential to release fluoride, which helps in preventing enamel demineralization, promoting remineralization, reducing plaque growth, and consequently helping to prevent dental caries.[1234]

However, GIC lacks flexural strength and hence is not indicated for stress bearing.[2] In this regard, a new alternative metal-free esthetic alkasite restorative material (Cention N, Ivoclar Vivadent, Liechtenstein) has been introduced, which has been claimed not only to release substantial levels of fluoride ions comparable to traditional GICs but also hydroxyl and calcium ions and the increased ion release may be attributed to its patented alkaline filler.[5]

The release of hydroxide ions from a restorative material may also aid in neutralizing the excess acidity during acid attacks by cariogenic flora, thus preventing demineralization.[6] Both these factors may thus work in tandem to increase the anticariogenic potential of Cention-N.

There are various factors influencing the amount and pattern of fluoride ion release from a restorative material such as temperature, pH, the technique of mixing of material, powder-liquid ratio, and surrounding media.[7] Anticariogenic effect of fluoride-releasing materials depends on the amount and sustainability of fluoride ion release, especially at pH below the critical level (5.5).[7] Apart from the above-mentioned factors, another factor which governs the release of fluoride ions is the time for which the material has been in the oral conditions.

Quantification of fluoride and hydroxyl ion can be done directly by spectrophotometer; however, hydroxyl ion release can also be evaluated by an alternative method of recording pH change or the buffering capacity of the material.

Hence, this study was conducted to evaluate the fluoride ion release and alkalizing potential of Cention-N (light-cure and self-cure) at different pH and time interval and compare it with conventional GIC.

METHODOLOGY

Freshly extracted human permanent mandibular molar teeth, extracted for periodontal reasons, were collected. Teeth were cleaned of calculus and organic debris with the help of an ultrasonic scaler and periodontal curettes.

Forty-five specimens were selected on the basis of inclusion and exclusion criteria. Inclusion criteria were noncarious molars and molars extracted due to periodontal reasons, whereas the exclusion criteria were teeth presenting with caries, fracture or crack, and hypoplasia or hypomineralization.

They were then disinfected with 0.1% thymol solution and stored in normal saline till the time of use. Teeth were sectioned at the level of the cementoenamel junction, and the root portion was removed. Each sample was then sectioned in four equal sections mesiodistally and buccolingually to obtain 180 samples.

Further, a flat-end cylinder diamond bur was used at a speed of 300,000 rpm under continuous air water to prepare the cavities with a depth and width of 2 mm.

The samples (n = 180) were randomly divided into the following three equal groups (n = 60): GIC (G), Cention-N – Self-cure (CS), and Cention-N – Light cure (CL).

The cavities in all the groups were restored with respective restorative materials which were manipulated according to the manufacturer's instructions. All the samples were incubated in 95% relative humidity at 37°C for 24 h. Further, two layers of nail varnish were used to coat the samples, leaving a margin of 1 mm around the restoration.

The samples were subdivided into two equal subgroups (n = 30) on the basis of pH (acidic pH – 4, neutral Ph – 6.8) of the solution used for testing. The subgroups representing acidic pH were GA, CSA, and CLA, and subgroups representing neutral pH were GN, CSN, and CLN.

Finally, each of the subgroups was further divided into three groups on the basis of duration (7 days, 14 days, and 21 days) for which testing was done.

One-hundred and eighty plastic containers were prepared each containing 5 ml of deionized water/acidic medium. Ten samples from each of the subgroup were stored in each of these plastic containers. After 24 h, the containers were thoroughly shaken; samples were removed; and the storage medium was collected. The samples were then reimmersed in the plastic container-containing fresh 5 mL of deionized water. The same procedure was repeated for 7 days for subgroups – GN7, CSN7, CLN7, GA7, CSA7, and CLA7, for 14 days for subgroups – GN14, CSN14, CLN14GA14, CSA14, and CLA14, and for 21 days for subgroups – GN21, CSN21, CLN21GA21, CSA21, and CLA21.

All the samples were incubated in 95% relative humidity environment at 37°C until the period of testing. The cumulative fluoride ion release and change in pH were assessed at the end of 7 days, 14 days, and 21 days. The data so obtained were subjected to statistical analysis using ANOVA-F, Paired “t”, and Unpaired t-test.

RESULTS

All the tested materials released fluoride ions in both acidic and neutral pH at all time intervals, and the fluoride ion release was higher in acidic pH as compared to neutral pH [Table 1]. Subgroup CSA released significantly higher amounts of fluoride ion when compared to subgroup GA and CLA, whereas subgroup GN released significantly higher amounts of fluoride ion when compared to subgroup CSN and CLN at all time intervals except at 21 days where the fluoride ion release of subgroup GN and CSN was similar. The fluoride ion release of subgroup CL was lower than that of subgroup CS and G at all periods.

Table 1

Mean values of fluoride ion release (ppm) and pH change from different subgroups at 7, 14, and 21 days

Days	Parameter	GN	GA	CSN	CSA	CLN	CLA
7	ppm	4.56	5.11	2.88	7.94	2.55	6.45
	pH change	+0.03	+0.39	+0.03	+1.44	+0.01	+0.97
14	ppm	2.08	5.13	0.73	7.84	0.98	5.30
	pH change	+0.03	+0.81	+0.01	+0.98	+0.01	+0.21
21	ppm	1.49	5.64	1.49	7.14	0.48	4.77
	pH change	+0.04	+0.19	+0.04	+0.51	+0.05	+0.14

Fluoride ion release from all the tested materials decreased with increasing period except in subgroup GA, where the amount of fluoride ion release increased. All the groups showed a statistically significant increase in pH in acidic medium, whereas no significant increase was observed in neutral medium. Change in pH or alkalization in acidic medium was significantly higher in subgroup CS as compared to subgroup CL and G [Table 2].

Table 2

Probable values of paired t-test between subgroups of Groups G, CS, and CL for fluoride ion release and pH change

Time points (days)	Parameter	Probable values of paired t-test in subgroups for fluoride ion release and pH change
		GN	GA	CSN	CSA	CLN	CLA
7-14	ppm	0.0000*	0.9292	0.0000*	0.5784	0.0000*	0.0000*
	pH	0.8934	0.0000*	0.3243	0.0000*	0.0608	0.0000*
7-21	ppm	0.0000*	0.0190*	0.008*	0.0004*	0.0000*	0.0000*
	pH	0.7892	0.0098*	0.4653	0.0003*	0.0850	0.0000*
14-21	ppm	0.1677	0.0040*	0.0391*	0.0321*	0.0001*	0.0036*
	pH	0.7543	0.0013*	0.2534	0.0028*	0.1201	0.2143

*A statistically significant difference at 0.05 level of significance (P<0.05)

DISCUSSION

Cention-N is a recently introduced bulk-fill restorative material which is both self-cure and light-cure, thus making the curing depth theoretically unlimited. It exhibits a sustained release of fluoride and hydroxyl ion in carious conditions (below critical pH) as claimed by the manufacturers.[5] However, this aspect has not been researched yet. Hence, this study was conducted to evaluate its fluoride ion release capability at different pH and time intervals and compare it with GIC.

Mandibular molars were selected for the sample preparation since these teeth have a larger surface area allowing easier sectioning at various levels.[8] For the better simulation of clinical conditions, instead of artificial molds (as used in various other studies),[9101112] the materials were restored in the prepared cavities on the tooth surface. Cavities were made of similar dimensions to maintain standardization. Flat-end cylinder diamond bur was used for cavity preparation to achieve the exact dimensions of the cavity with smooth axial walls and flat pulpal floor. The unprepared enamel surface of the samples was coated with two layers of nail varnish, leaving a 1-mm window around the cavity margins to prevent ion release from the tooth surface which may cause overestimation of results.[13]

Deionized water was preferred as the storage medium over artificial saliva, due to the high viscosity and the presence of ions in the latter one. These ions may affect the release of fluoride ions from the restorative materials, thus leading to an inaccurate estimation of fluoride ion.[14]

The storage medium was changed in every 24 h due to the possibility of saturation of released fluoride ions in the storage medium, which interferes with further release of fluoride ions.[910]

The results of this study showed higher fluoride ion release in acidic pH as compared to neutral pH in all the groups, indicating that when conditions become acidic due to cariogenic challenges, GIC and Cention-N would release relatively more fluoride ion. This finding is in corroboration with studies conducted by Gandolfi et al.,[15] Mungara et al.,[10] and Jingarwar et al.[11] which assessed fluoride ion release in GIC in similar conditions. This may be because a decrease in pH of solvent may lead to an increase in the surface dissolution of the materials, thus increasing fluoride ion release.[16]

In neutral pH, GIC released significantly higher amounts of fluoride ion when compared to Cention-N (self-cure and light-cure) at all time intervals. This may be because of relatively higher filler content in GIC (99.9%).[17] The fillers of Cention-N comprise barium aluminum-silicate glass filler, ytterbium trifluoride, an isofiller (Tetric N-Ceram technology), a calcium barium aluminum fluorosilicate glass filler, and a calcium fluorosilicate (alkaline) glass filler.[5] Out of this 78.4% filler content, only 24.6% of the final material is responsible for fluoride ion release.[5] In addition, fillers in Cention-N are surface modified, thus becoming resistant to degradation and may lead to the release of a lesser amount of fluoride ions.[5] GIC has the presence of a thick 300 nm silica gel layer on its surface which after water sorption increases in its thickness. While in Cention-N, due to the formation of calcium fluoride and calcium phosphate, a 0.5-μm thick surface layer has been observed, which is resistant toward rinsing with deionized water.[5]

In acidic pH, Cention-N (self-cure) released significantly higher amounts of fluoride ion when compared to GIC at all time intervals. The possible explanation is that Cention-N may have reacted more aggressively in the presence of acidic environment, i.e., probably the surface resistant layer may have deteriorated faster as compared to GIC, thus exposing the matrix for increased release of fluoride ions.[5]

The amount of fluoride ions release decreased with an increasing period. Similar results have been observed by Kiran and Hegde,[18] Neelakantan et al.,[19] and Cardoso et al.[12] who compared GIC with different restorative material in a different context. The initial superficial rinsing effect or surface wash-off effect and fluoride burst may have led to initial high values and the rapid fall during subsequent days was likely due to only slower and constant diffusion through cement pores, fractures, and mass diffusion.[1115] However, in GIC, the fluoride ion release increased in acidic medium due to relatively higher degradation of GIC at low pH.

Cention-N in self-cure mode released significantly higher fluoride ions as compared to that of light-cure mode. The decline in the capacity of light cure to release fluoride ions may be due to a tightly bound or a less hydrophilic matrix due to photopolymerization of the alkasite restorative material.[20] The same reason could be valid for a better alkalizing ability in self-cure as compared to the light-cure mode of Cention-N.

Cention-N demonstrated a significantly high alkalizing potential in acidic pH. This may be due to the hydroxyl and calcium ions release by alkaline glass fillers from Cention-N, which are able to have a direct effect on the pH levels in the oral cavity, thus creating conditions whereby excess acidity due to cariogenic bacterial activity could be neutralized.[5]

A slight increase in pH in GIC was also noticed in acidic medium. Although GIC does not release hydroxyl ion, studies conducted by Nicholson et al.[21] have indicated that GIC does have a buffering effect. It has been seen that the exposure to an acidic challenge may lead to aluminum ions being leached out, aiding in the neutralizing action of GIC.[22]

CONCLUSIONS

Based on the results and within the limitations of the study, the following conclusions were formulated – The highest fluoride ion release potential was exhibited by Cention-N (self-cure) in acidic medium and GIC in neutral medium. Fluoride ion release was higher in acidic pH as compared to neutral pH for both Cention-N and GIC. Fluoride ion release decreased over the period in both acidic and neutral pH in all the groups except GIC (in acidic medium), where the fluoride ion release gradually increased over the period. All the groups demonstrated alkalizing ability in acidic medium only with Cention-N (self-cure) having significantly higher alkalizing potential than Cention-N (light-cure) and the lowest being in GIC.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.