Niobium pentoxide as a new filler formethacrylate-based root canal sealers

V. C. B. Leitune1, A. Takimi2, F. M. Collares1, P. D. Santos1, C. Provenzi1, C. P. Bergmann2

& S. M. W. Samuel1

1Dental Materials Laboratory, School of Dentistry Porto Alegre, RS; and 2Ceramic Materials Laboratory, Federal University of

Rio Grande do Sul Porto Alegre, RS, Brazil

Abstract

Leitune VCB, Takimi A, Collares FM, Santos PD,

Provenzi C, Bergmann CP, Samuel SMW. Niobium

pentoxide as a new filler for methacrylate-based root canal

sealers. International Endodontic Journal, 46, 205210, 2013.

Aim To evaluate the influence of several niobium

pentoxide (Nb2O5) concentrations on the radio-opacity,

flow, film thickness, microhardness and degree of

conversion of an experimental root canal sealer.

Methodology An experimental dual-cured root

canal sealer was produced with a methacrylate-based

comonomer blend. Nb2O5 was added at four different

concentrations: 0, 80, 100 and 120 wt%. Radio-

opacity was evaluated according to ISO 6876 using a

digital system (n = 5). Flow and film thickness weredetermined in accordance with ISO 6876 (n = 3).Microhardness was evaluated with 50 g for 15 s

(n = 5). Degree of conversion was evaluated withFTIR immediately after photocuring and after 1, 7

and 14 days. The data were analysed using ANOVA

and Tukeys test. The degree of conversion over time

was evaluated using RM-ANOVA (a = 0.05).

Results The groups with 80 wt% and 100 wt% of

filler showed no significant difference in radio-opacity

from that of equivalent 2 mmAl (P > 0.05). The addi-tion of 120 wt% resulted in radio-opacity values

higher than 2 mmAl (P < 0.05). The flow was notsignificantly different amongst the different groups

(P > 0.05). All groups had a film thickness of

The materials used for a root filling come in direct

contact with dentine and periapical tissues. Therefore,

the materials should not be cytotoxic and would ideally

be biological stimulators. Root canal materials that are

capable of ionic delivery have been proposed to produce

increased sealing in the apical region (Alani et al.

2009). However, these materials have high solubility

and degrade over time. Some inorganic fillers with a

monoclinic phase (e.g. niobium pentoxide) appear to be

less prone to degradation (Khan et al. 1992) and could

possess bioactive properties (Karlinsey et al. 2006).

Niobium pentoxide (Nb2O5) is an oxide of a transition

metal (atomic number of 41) with suitable properties

for biomedical uses (Khan et al. 1992, Karlinsey et al.

2006). This oxide has never been used as filler for poly-

meric-based root canal sealers.

The purpose of this study was to evaluate the

effects of several Nb2O5 concentrations on the radio-

opacity, flow, film thickness, microhardness and

degree of conversion of an experimental dual-cured

methacrylate-based root canal sealer. The null

hypothesis was that the addition of niobium pentoxide

will not influence the properties of the experimental

methacrylate-based root canal sealer.

Materials and methods

The monomers used to produce an experimental root

canal sealer were urethane dimethacrylate (UDMA),

glycerol 1,3-dimethacrylate (GDMA), ethoxylated

bisphenol A glycol dimethacrylate (BISEMA6), cam-

phorquinone (CQ), N,N-dihydroxyethyl-para-toluidine

(DHEPT) and benzoyl peroxide (BP), which were sup-

plied by Esstech Inc (Essington, PA, USA). These mate-

rials were used without further processing. Nb2O5,

with a mean particle size of 38.16 lm (Leitune et al.2012), was obtained from Companhia Brasileira de

Mineracao e Metalurgia (CBMM, Araxa, MG, Brazil).

The filler was silanized using gamma-methacryloxypro-

pyltrimethoxysilane (c-MPTS; Aldrich Chemical Co.,Milwaukee, WI, USA). The filler was added to an ace-

tone (Labsynth Ltda., Diadema, SP, Brazil) solution

containing c-MPTS (5% of the filler wt) to form aslurry. The mixture was stored for 24 h at 37 C toensure complete solvent removal (Sideridou & Karabela

2009).

The experimental root canal sealers were made by

mixing 70 wt% UDMA, 15 wt% BISEMA6 and 15 wt%

GDMA. CQ, DHEPT and BP were added to produce

a dual-cure root canal sealer (Collares et al. 2012).

The filler particles were used in four different

concentrations (0, 80, 100 and 120 wt%). Consider-

ing the weight of the co-monomer blend, a relative

amount of filler was added (e.g. to 1 g of resin 1.2 g

of Nb2O5 was added in the group with 120 wt%).

The density of niobium pentoxide is 4.6 g cm3, ofUDMA is 1.12 g cm3, of GDMA is 1.12 g cm3 andof BisEMA6 is 1.10 g cm3. Consequently, the volu-metric concentrations of the experimental groups

were 0, 19.44, 24.25 and 29.16 vol%. The mixture

(resin/particles) was hand-mixed for 120 s, sonicated

for 480 s and hand-mixed for another 120 s. To per-

form sealer photo-activation, a light-emitting diode

activation unit (Radii Cal; SDI, Bayswater, Vic.,

Australia) was used. An irradiation value of

1200 mW cm2 was confirmed with a digital powermeter (Ophir Optronics, Danvers, MA, USA). Radio-

opacity, flow and film thickness were tested according

to ISO 6876 (Standardization 2001).

Radio-opacity

The radio-opacity of the experimental sealers was eval-

uated using five specimens per group. The specimens

were 6.0 mm (0.5 mm) in diameter and 1.0 mm(0.2 mm) in thickness. X-ray images were obtainedby a digital system with phosphorous plates (VistaScan;

Durr Dental GmbH & CO. KG, Bietigheim-Bissingen,

Germany) with an exposure time of 0.4 s and a focus-

film distance of 400 mm. The X-ray source (DabiAtlante

model Spectro 70X) was operated with a tungsten

anode at 70 kV and 8 mA. For each film, one specimen

from each group with the same concentration was

used, resulting in a total of five films per concentration.

For all images, an aluminium step-wedge was exposed

simultaneously with the specimens. The thickness of

the aluminium step-wedge ranged from 0.5 to 5.0 mm

in increments of 0.5 mm. The aluminium alloy used

contained Al, 99.12; Fe, 0.47; Mg, 0.41; and < 0.1 ofCu (mass%), and met ISO 6876 standards (Standardi-

zation 2001). The images were saved in TIFF format

and analysed using Photoshop software (Adobe Sys-

tems Incorporated, San Jose, CA, USA). The means and

standard deviations of the grey levels (pixel density) of

the aluminium step-wedge and the specimens were

measured in a standardized area of 1.5 mm2 (Collares

et al. 2010).

Flow test

The flow test was conducted according to ISO 6876

(Standardization 2001). A total of 0.5 mL of each

Nb2O5 for root canal sealers Leitune et al.

2012 International Endodontic JournalInternational Endodontic Journal, 46, 205210, 2013206

experimental sealer was placed on a glass plate

(40 9 40 9 5 mm) with a graduated 1.5-mL syr-

inge. At 180 5 s after mixing was started, anotherplate with a mass of 20 2 g and a load of 100 gwas placed on top of the material. Ten minutes after

mixing had been started, the load was removed, and

the major and minor diameters of the compressed

material were measured using a digital calliper. If

both measurements were within 1 mm of each other,

the results were recorded. If the major and minor

diameter discs were not uniformly circular or did not

fall within 1 mm of each other, the test was repeated.

The test was conducted three times for each experi-

mental group, and the mean value was recorded.

Film thickness

The film thickness was evaluated according to ISO

6876 (Standardization 2001). Two glass plates that

measured 5 mm in thickness and 10 mm in length

were placed together and their combined thickness

was measured. The centre of one of the plates was

covered with 0.5 mL of experimental sealer, and a

second plate was placed on top of the material. At

180 5 s after the start of mixing, a load of 150 Nwas applied vertically on top of the glass plate. Ten

minutes after the mixing had been started, the thick-

ness of the two glass plates and the interposed sealer

film was measured using a digital calliper. The differ-

ence in the thickness of the two glass plates, with and

without sealer, was recorded as the film thickness of

the experimental sealer material. The mean value of

three measurements for each sealer was recorded as

the film thickness of the material.

Knoop microhardness

The specimens used for the evaluation of radio-

opacity were also used to determine the Knoop

microhardness. For each experimental sealer, five

specimens were embedded in acrylic resin and

polished using a manual polisher (Model 3v; Arotec,

Cotia, SP, Brazil) with a felt disc saturated with in an

alumina suspension (Alumina 1.0 lm; Arotec). Thesamples were then dried at 37 C and stored for 24 hThe specimens were subjected to a microhardness test

in which five indentations (50 g 15 s1), which wereplaced 100 lm apart from each other, were assessedusing a digital microhardness tester (HMV 2; Shimadzu,

Tokyo, Japan). The calculation of the hardness

value was carried out using the following equation:

Knoop microhardness 14228 cd2

Where 14228 is a constant, c is the load in grams, d

is the length of the larger diagonal, in micro meter.

The Knoop microhardness number (KHN) was the

mean of the five indentations for each specimen.

Degree of conversion

The degree of conversion of the experimental root canal

sealers was evaluated using real-time Fourier transform

infrared spectroscopy (RT-FTIR) with a Vetrex 70

(Bruker Optics, Ettlingen, Germany) spectrometer

equipped with an attenuated total reflectance device

composed of a horizontal diamond crystal with a mir-

ror angle of 45 degrees. A support was coupled to the

spectrometer to fix the light-curing unit and standard-

ize the distance between the fibre tip and sample at

5 mm. Opus software (Bruker Optics) was used in the

monitoring scan mode, with Blackman-Harris 3-Term

apodization in a range of 4000400 cm1 and resolu-tion of 4 cm1. With this setup, one spectrum wasobtained prior to photocuring and one immediately

after photocuring. The same samples were analysed on

days 1, 7 and 14. The samples (3 lL) were directly dis-pensed onto the diamond crystal and light-activated for

40 s (n = 3). The degree of conversion was calculatedas described in a previous study (Collares et al. 2011),

considering the intensity of carbon-carbon double bond

stretching vibration (peak height) at 1635 cm1, andusing the carbonyl group at 1720 cm1 from the poly-merized and unpolymerized samples as an internal

standard (Stansbury & Dickens 2001).

Statistical analysis

The normality of the data was evaluated using the

KolmogorovSmirnov test. Statistical analysis was

performed using one-way ANOVA (Nb2O5 concentra-

tion) and Tukeys post hoc test. The degree of conver-

sion over time was evaluated using repeated

measures ANOVA at the 0.05 level of significance.

Results

The radio-opacity data, measured in pixels, are shown

in Table 1. The group with a ratio of 80 wt% and

100 wt% showed no significant difference from that of

equivalent 2 mm of Al (P > 0.05). The addition of120 wt% increased the radio-opacity, resulting in

Leitune et al. Nb2O5 for root canal sealers

2012 International Endodontic Journal International Endodontic Journal, 46, 205210, 2013 207

values >2 mmAl (P < 0.05), but 0.05).*Indicates that was not possible to perform the flow test for group with 0% of Nb2O5.

Nb2O5 for root canal sealers Leitune et al.

2012 International Endodontic JournalInternational Endodontic Journal, 46, 205210, 2013208

because the ability to create a thin film of sealer could

promote a better seal (de Deus et al. 2003). Although

the group containing 120 wt% of Nb2O5 had flow

values that were not in accordance with ISO 6876

(Standardization 2001), a commercially available and

widely used resin-based root canal sealer has lower

values than established by standardization (Almeida

et al. 2007). The flow property could determine the

effectiveness of filling accessory canals and voids

between core materials (McMichen et al. 2003).

The radio-opacity of root canal sealers is essential

for clinical use (Collares et al. 2010); the clinician has

to be able to distinguish the filling material from hard

and soft tissues that surround the root (Krejci et al.

1991). For this purpose, the ISO standardization

(Standardization 2001) established the reference

value of 3 mm Al. In this study, all groups had val-

ues that were significantly different from 3 mm Al.

The groups contained large weight-based amounts of

filler material; however, when considering the volu-

metric fraction of Nb2O5 relative to resin, there was a

relatively low volumetric concentration of Nb2O5(max 29.16 vol%). However, greater amounts of

Nb2O5 could not be used without compromising the

flow of the sealer (Table 1). A viable alternative could

be the addition of a second filler with the goal to

increase radio-opacity without losing the biological

properties provided by Nb2O5 (Karlinsey et al. 2006).

The addition of inorganic particles to the polymeric

matrix could change its properties, including the degree

of conversion and microhardness, because the refrac-

tive index of a given substance may decrease the avail-

ability of light energy within the polymer (Shortall

et al. 2008). In this study, the addition of filler particles

to the root canal sealer led to an increase in microhard-

ness in all groups. The degree of conversion in groups

containing Nb2O5 was lower than that in groups with-

out added filler. However, considering the methacry-

late-based sealers, the values for degree of conversion

are similar to those reported previously (Lee et al.

2011). The evaluation of degree of conversion of the

experimental root canal sealers within a specific time is

important because chemical initiators (e.g. benzoyl per-

oxide) were used in these formulations and could

increase the degree of conversion of the sealer. The

group containing 80 wt% filler was less influenced

than groups with 100 wt% and 120 wt% because the

initial value of degree of conversion showed no signifi-

cant difference compared with days 1, 7 and 14. The

presence of less filler (80 wt%) could lead a more avail-

ability of light and consequently promote a higher ini-

tial degree of conversion than groups with 100 wt%

and 120 wt%.

Niobium pentoxide was added to improve the prop-

erties of methacrylate-based sealers and introduces a

new filler with bioactive properties (Karlinsey et al.

2006). In the oral environment, methacrylate resins

degrade over time (Ferracane 2006). This degradation

is thought to be a limiting factor because it could lead

to gap formation, fluid infiltration (Kim et al. 2010)

and leaching of monomers, resulting in undesirable

tissue reactions (Yamanaka et al. 2011). Theoreti-

cally, Nb2O5 could be deposited at the root canal sur-

face during the degradation process, thereby

increasing the biocompatibility of the sealer. Further-

more, some substances are known to have the capac-

ity to induce crystal growth on material surfaces,

such as octacalcium phosphate (Barrere et al. 2001),

amorphous calcium phosphate (Johns et al. 2010),

bioglass (Hashimoto et al. 2010) and ionomer cement

(Hashimoto et al. 2010). In the present study, Nb2O5was shown to be a novel filler material, especially

with the knowledge that this oxide could promote

crystal growth and biomimetic mineralization of the

surrounding tissues because of its known bioactive

properties (Karlinsey et al. 2006).

Conclusion

The addition of Nb2O5 increased radio-opacity and

microhardness and may be a promising filler for the

production of new endodontic sealers.

Acknowledgements

The authors gratefully acknowledge CAPES (Corde-

nacao de Aperfeicoamento de Pessoal de Nvel Supe-

rior) for the scholarship, the CBMM (Companhia

Brasileira de Metalurgia e Mineracao) for providing the

niobium pentoxide and the Durr Dental Brazil for pro-

viding the VistaScan Digital Radiograph System. This

article represents a part of a PhD thesis (V.C.B.L.)

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Nb2O5 for root canal sealers Leitune et al.

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