caracterizacion mecanica de cementos para postes de fibra de v

7/29/2019 Caracterizacion Mecanica de Cementos Para Postes de Fibra de V

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Mechanical characterization of resin cements used for luting

fiber posts by nanoindentation

Laura Ceballos a, Miguel Angel Garridob, Victoria Fuentes a, Jes ´ us Rodr´ ıguez b,∗

a Departamento de Ciencias de la Salud III, Universidad Rey Juan Carlos, C/Tulip ´ an s/n, 28933 M ´ ostoles, Madrid, Spainb Departamento de Ciencia e Ingenier´ ıa de Materiales, Universidad Rey Juan Carlos, C/Tulip ´ an s/n, 28933 M ´ ostoles, Madrid, Spain

a r t i c l e i n f o

Article history:

Received 29 July 2005

Accepted 6 December 2005

Keywords:

Resin cements

Fiber posts

Bonding

Mechanical properties

Nanoindentation

a b s t r a c t

Objectives. To evaluate the mechanical behavior of resin cements used for luting fiber post.

The influence of the curing mode is analysed.

Methods. Nanoindentationtechniques have beenapplied to determine hardness and Young’s

modulus of disc-shaped specimens of three types of cements: chemical- , photo- and

dual-cured, provided by Ivoclar-Vivadent. Results obtained have been compared with mea-

surements performed inside the post–cement–dentin system. Mechanical properties are

evaluated together with scanning electron micrographs showing the post–cement and

dentin–cement interfaces.

Results and Significance. Differences have been detected between mechanical measurements

performed inside the post–cement–dentin system and those carried out in laboratory disc-

shaped specimens. The close presence of post and dentin boundaries has a lot of influence

on the cement behavior. The nanoindentation measurements indicate that the photo-cured

cement exhibits a high hardness and stiffness, but with a more marked tendency to brittle

failure. The chemically cured cement is the material with higher ability to bear deformation

without damage, although its hardness and elastic modulus are significantly lower. Dual-

cured cements present the best combination of properties.

© 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1. Introduction

The use of fiber posts to restore teeth with excessive loss

of coronal structure due to caries, trauma or overaggressive

endodontic procedures is gaining widespread acceptance with

dental clinicians [1–3]. Several characteristics of these postshave contributed to their popularity. They have comparable

elastic properties to dentin [4], inducing a stress field similar

to that of natural tooth [5]. This results in a reduction in the

incidence of root fractures, demonstrated in both in vitro [6]

and in clinical studies [2,7].

Moreover, the chemical nature of the posts allows them to

be bonded to canal walls with adhesive systems in combina-

tion with resin cements [8], avoiding friction between dentin

∗ Corresponding author. Tel.: +34 914887159.E-mail address: [email protected] (J. Rodrıguez).

walls and the post [4], and reinforcing the remaining tooth

structure [9].

However, adhesive technique performance in root canals

is a further challenge, due to the variable ability of bond-

ing systems to demineralize and infiltrate the dentin walls,

the poor control of moisture, the influence of differentdensity and orientation of dentin tubules along the root

canal, and the accessibility during handling of the materials

[8,11].

Regarding resin cements, an adequate polymerization is

utterly necessary to provide mechanical properties, such as

modulus of elasticity and hardness, good enough to reinforce

weakenedroots [12] and clinically ensure retentionof the post.

According to the curing mode, three possible alternatives may

0109-5641/$ – see front matter © 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

doi:10.1016/j.dental.2005.12.007



d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 100–105 101

be chosen: chemical-, photo- or dual-curing resin cements

[10].

The use of chemically cured resin cements guarantees

polymerization without the influence of post space depth, but

offers worse handling characteristics due to the absence of

control of the setting reaction. Photo-curing resin cements

allow sufficient time and control for proper seating of the post

into thecanal [12]. However,even whentranslucentfiber postsare supposed to transmit the light into the post space [8,13],

a reduction of resin cement hardness with depth has been

determined [12,13]. Dual-curing resin cements are expected to

combine favorable properties of both, auto- and photo-curing

systems.

The curing mode can also influence the amount of shrink-

age producedafter polymerization. Shrinkage stresses of resin

cements in root canals are especially relevant due to the unfa-

vorable factor of configuration that restricts the flow of resin

cement, which may affect the integrity of the adhesive inter-

face [10].

Traditionally, the mechanical properties of dental materials

on a small scale have been evaluated by means of micro-hardness tests. In recent years, the use of nanoindentation

has become very popular in several fields, mainly for its pos-

sibilities in the characterization of very small quantities of

material and, additionally, for providing simultaneous mea-

sures of hardness and elastic modulus [14].

In this work, the mechanical behavior of resin cements for

luting fiber posts has been evaluated by depth sensing inden-

tation. Three types of resin cement were studied (chemical-,

photo- and dual-curing) comparing the results obtained from

laboratory disc-shaped specimens with those derived from

actual restored teeth.

2. Materials and methods

The materials evaluated in the present study are shown in

Table 1.

Disc-shaped specimens were prepared for each resin

cement evaluated, Multilink, Variolink II Base and Variolink

II Base plus Catalyst (Ivoclar-Vivadent Schaan, Liechenstein)

using a metallic mold, 14mm in diameter and 1 mm thick.

The discs were placed over a glass plate and the materials

were inserted into thediscs. After insertion, themold wascov-

ered with a mylarsheetand a glass microscope slide. Multilink

resin cement was allowed to autocure, while Variolink II Base

Table 1 – Materials used in the study

FRC Postec (Ivoclar-Vivadent) Glass–fiber reinforced

composite posts

Multilink primer A and B

(Ivoclar-Vivadent)

Self-etching adhesive

Multilink resin cement

(Ivoclar-Vivadent)

Self-etching and self-curing

resin cement

Excite DSC (Ivoclar-Vivadent) Dual-curing adhesive

Variolink II Base

(Ivoclar-Vivadent)

Light-curing resin cement

Variolink II Base plus Catalyst

(Ivoclar-Vivadent)

Dual-curing resin cement

and Variolink II Base plus Catalyst mixture were photopoly-

merized for 30 s (Astralis 10, program ESC; Ivoclar-Vivadent).

After 20min, specimenswere removed from themoldsand

stored in darkness until evaluation.

Additionally, teeth were restoredwith fiberposts in order to

compare the mechanical behavior of the luting resin cements

in a situation similar to the clinical one. For this aim, single-

rooted teeth, free of caries and extracted for periodontal rea-sons were selected. The crowns of the teeth were eliminated

by cutting them 1mm coronally to the cementum–enamel

junction with a diamond bur mounted on a high-speed hand-

piece under water cooling.

Endodontic treatment was performed with stainless-steel

K-files (Dentsply-Maillefer, Paris, France) in combination with

2.5% sodium hypochlorite irrigation. The canals were filled

with laterally condensed guttapercha and TopSeal sealer

(Dentsply De-Trey GmbH, Konstanz, Germany).

The post space was prepared with the drills provided by

the manufacturer of the posts (FRC Postec, Size 1; Ivoclar-

Vivadent), leaving 4 mm of guttapercha in the apical portion.

The specimens were randomly allocated to three experi-mental groups according to the adhesive system and resin

cement applied. In all cases, the glass–fiber reinforced com-

posite posts FRC Postec (Size 1: length 15mm and a 0.8mm

diameter in the apical extreme; Ivoclar-Vivadent) were used.

They were all cleaned with alcohol after try-in and silanized

for 60 s (Monobond S, Ivoclar-Vivadent).

In group 1, the specimens were treated with Multilink

Primer A/B (Ivoclar-Vivadent) and the self-curing and self-

etching cement Multilink(Ivoclar-Vivadent) according to man-

ufacturer’s recommendations. Multilink Primer A and B were

mixed and applied into the root canal walls by means of a

microbrush. The excessive adhesive solution was removed

with a paper point and after 15 s, it was dried but not light-cured. Multilink cement was mixed and the post was coated

with it and inserted into the post space.

In group 2, the posts were luted with the dual-curing

adhesive Excite DSC (Ivoclar-Vivadent) and the light-curing

cement Variolink II Base (Ivoclar-Vivadent). The root canal

walls were etched with 35% phosphoric acid (Ultradent, South

Jordan) for 15 s, water rinsed and then gently dried with paper

points. This adhesive is offered in a single-dose vessel with

an applicator indicated for root canals. Excessive adhesive

was removed with a paper point. The adhesive was polymer-

ized for 10 s (Astralis 10 using ADH program; Ivoclar-Vivadent).

Then the posts were covered with Variolink II Base cement

and seated into place. The cement was light polymerizedthrough the post for 30 s (Astralis 10, program ESC; Ivoclar-

Vivadent).

Finally, in group3, theposts were also luted with ExciteDSC

adhesive (Ivoclar-Vivadent) and the dual-curing cement Var-

iolink II (Ivoclar-Vivadent). The adhesive system was applied

as previously described, but it was not light-cured. Variolink II,

Baseplus Catalyst, were mixedand applied covering the posts.

Fiber posts were seated and the cement was polymerized also

for 30 s (Astralis 10, program ESC; Ivoclar-Vivadent).

Samples were included in resin blocks and after 24 h sam-

pleswere sectioned perpendicularto theirlong axis(Accutom-

5, Struers, Copenhagen, Denmark), in order to obtain sections

from thecoronal andapical root areas. Surfaces were polished



102 d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 100–105

with alumina suspension slurry of 3 m and OP-A (Struers,

Copenhagen, Denmark).

Indentations were made on the disc-shaped specimens

and in different regions of the resin cement used to lute the

fiber post. Young’s modulus and hardnessof the cements were

determined from the load–displacement curves following the

Oliver–Pharr methodology [15].

2.1. Preparation of the samples for scanning electron

microscopy observation

Selected specimens from each group with their corresponding

posts were prepared for scanning electron microscope (SEM)

observation.

In order to observe hybrid layer formation, samples were

longitudinally sectioned, demineralized with 35% phosphoric

acid for 15 s, deproteinized with 2% sodium hypochlorite for

120 s and dehydrated with increasing concentrations of alco-

hol [8].

Other samples were immersed in a solution of 30% HCl for

24h and in2% sodiumhypochlorite for 10minin order to com-

pletely eliminate the dental tissues for better observation of

resin tags formation [8].

Finally, samples were mounted on aluminum stubs with

carbon cement and sputter coated with gold (Sputtercoater,

Baltec, SCD005) and observed under a Philips XL 30 environ-

mental scanning electron microscope (ESEM).

3. Results and discussion

3.1. Nanoindentation tests

Fig. 1 shows an example of the load–displacement curvesobtained from the nanoindenter for the disc-shaped speci-

mens. As can be appreciated, the nanoindentation technique

is quite sensitive to material behavior. Elaborate results are

includedin Table 2, where apart fromhardness, H, and Young’s

modulus, E, the ratios H /E and H2 /E are also included. The

magnitude H /E can be considered as a measure of the mate-

rial’s ability to bear elastic deformation. In other words, H /E

Table 2 – Nanoindentation data from disc-shapedspecimens

Cements E (GPa) H (GPa) H /E H2 /E(GPa)

Multilink (self-curing

cement)

4.9 (0.2) 0.21 (0.01) 0.042 0.009

Variolink II Base(light-curing cement) 8.8 (0.5) 0.31 (0.03) 0.035 0.011

Variolink II Base plus

Catalyst (dual-curing

cement)

7.4 (0.2) 0.31 (0.01) 0.042 0.013

Average values and standard deviations are in parentheses.

is related to the deformation limit before permanent dam-

age is caused. On the other hand, H2 /E can be understood

as a measure of the material’s ability to store elastic energy.

Both magnitudes are significant in the mechanical perfor-

mance of the resin cement within the tooth restored with a

fiber post. According to the results obtained, the chemically

cured cement, Multilink (Ivoclar-Vivadent), presents the low-est values of stiffness and hardness, although its ability to

being deformed without damage is superior to other cements.

Photo-cured cement, Variolink II Base(Ivoclar-Vivadent), is the

stiffest and hardest, but its low value of H /E is a warning of

brittle failure. Dual-cured cement, Variolink II Base plus Cata-

lyst(Ivoclar-Vivadent) seems to posses thebest combination of

properties. It is almostas stiffand hardas thephoto-cured ver-

sion, but with a much higher ability to be reversibly deformed.

One of the most valuable advantage of nanoindentation

is the possibility of testing the cements in situ, i.e., inside

the post–resin cement–dentin system. Obviously, the situa-

tion is clearly different from that in the disc-shaped speci-

mens, mainly due to the influence of viscosity, uncontrollablevoid formation, thickness, factor of configuration, etc. A very

thin layer of cement does not have to behave as the bulk

cement characterized from disc-shaped specimens. Table 3

summarized theresultsobtainedin each type of resincement,

measured in different regions. These results provide appeal-

ing conclusions. Multilink (Ivoclar-Vivadent), the chemically

cured cement, exhibited remarkable changes in the appar-

Fig. 1 – Nanoindentation load–displacement curves from disc-shaped specimens.




ent hardness and Young’s modulus. The values of H and E

measured in situ increase by almost 100%. Interestingly, the

ratio H /E is not so severely altered, remaining constant at the

coronal region and exhibiting increases of 40% at the apical

zone.

Fig. 2 – ESEM micrograhs showing: (a) gap at the interface

obtained with Multilink Primer and resin cement ( ×1000);

(b) interface obtained with Excite DSC and Variolink II Base

( ×800); (c) interface detected for Excite DSC and Variolink II

Base plus Catalyst ( ×800).

Fig. 3 – ESEM micrographs showing: (a) resin tags at

coronal and mid-zones for Multilink Primer and resin

cement ( ×1200); (b) resin tags for Variolink II Base ( ×80); (c)

resin tags at coronal and mid-zones for Variolink II Base

plus Catalyst ( ×500).



104 d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 100–105

Table 3 – Nanoindentation data from in situ specimens

Cement Region E (GPa) H (GPa) H /E H2 /E (GPa)

Multilink (self-curing cement) Coronal 8.1 (0.4) 0.38 (0.01) 0.047 0.018

Apical 7.1 (0.3) 0.43 (0.02) 0.062 0.027

Variolink II Base (light-curing cement) Coronal 9.4 (0.6) 0.29 (0.04) 0.030 0.009

Apical – – – –

Variolink II Base plus Catalyst (dual-curing cement) Coronal 7.0 (0.3) 0.38 (0.02) 0.054 0.021

Apical 6.9 (0.3) 0.31 (0.02) 0.045 0.014

Average values and standard deviations are in parentheses.

Photo-cured Variolink II Base cement (Ivoclar-Vivadent) is

virtually unaltered in all the properties measured. It is worthy

of note to explain here, that an improper curingprocessmakes

it difficult to obtain consistent results in the apical region. The

difficulties of attainingcomplete photo-curinghave previously

been reported and it is one of the drawbacks of this type of

cement, confirmed in this work.

Finally, as is expected, the dualcement, Variolink II (Ivoclar-Vivadent) brings together the best properties of the other

cements. Itshardness andstiffness values areas high as those

of the photo-cured cement, but with a deformation capability

similar to that of thechemically curedcement. No appreciable

differences are observed between coronal and apical regions

in this case.

It is clear that the boundary conditions are decisive when

the thickness of the cement layer is quite low. The close prox-

imity of thepost andthe dentin surfacesplays a displacement

constraint role, leading to higher values of hardness and elas-

tic modulus. It would be very significant to analyse if cracks

and discontinuities between resin cement and post or dentin

are observed. The presence of this type of defect may partiallycancel the shrinkage stresses caused by polymerization.

3.2. Scanning electron microscopy observations

The interface obtained when Multilink adhesive and resin

cement were applied showeda discontinuousgap between the

resin cement and the infiltrated dentin, while this situation is

not observed in the other cements (Fig. 2).

Moreover, at this level, there were zones with no resin tags

and others where they only plugged the tubules, probably due

to the reduced density of tubules [16]. Also, many small and

few bigger voids in the resin cement were visible. In the coro-

nal and mid-zones, few areas were detected with long andnumerous resin tags. Generally, resin tags were short and no

adhesive lateral branches were observed (Fig. 3a).

Unlike self-etching adhesive Multilink, the density of the

resintagswas very high in coronal andmid-zones when Excite

DSC was applied after acid etching, regardless of the resin

cement used. Resin tags were also very long and numerous

lateral branches were observed (Fig. 3b). Probably phosphoric

acid is more effective in penetrating thick smear layers and

demineralizing the underlying dentin [10,11].

When light-curing Variolink II was used, areas were the

cement was detached from the post surface could be detected

in its apical extreme, being visible only in small areas. More-

over, short resin tags were observed in these areas. It may be

thatthe higher viscosityof thiscementrestricted itsflow along

the post space or that it was not properly polymerized due to

the attenuation of light with depth (Fig. 3c).

Posts bonded with Variolink II in dual-curing mode,

were completely covered with the resin cement. When the

dentin–cement interface was examined, the adhesive could

not be clearly detected, probably because it wasnot photopoly-

merized before post luting.The thickness of the cement layer changed according

to the different root canal morphologies. Some voids were

observed in the three resin cements evaluated, probably due

to their viscosity that restricts their placement inside the

post space and the mixing procedure of the base and catalyst

pastes.

Acknowledgements

Authors are indebted to the company Ivoclar-Vivadent for

providing the materials used in this study. Thanks are also

extended to the University Rey Juan Carlosfor partiallyfinanc-ing this work.

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