ARTÍCULOS ORIGINALES
Influence of polishing protocol on flexural properties of several dental composite resins
Robert Ramirez-Molina1, Andrea E. Kaplan2
1 Department of Operative Dentistry, University of Los Andes,
School of Dentistry, Mérida, Venezuela.
2 Department of Dental Materials, University of Buenos Aires,
School of Dentistry, Buenos Aires, Argentina.
CORRESPONDENCE Dra. Andrea E. Kaplan. Catedra de Materiales Dentales Facultad de Odontologia, Universidad de Buenos Aires Marcelo T. Alvear 2142 3B (C1122AAH) Ciudad Autonoma de Buenos Aires Argentina. e-mail: akaplan@odon.uba.ar
ABSTRACT
The aim of this study was to determine the influence of the finishing protocol (FP) on flexural properties of several composites (CR). Twenty composite samples (25x2x2 mm) were prepared: G1 HelimolarR; G2 Filtek™ Z350;G3TetricR N Ceram, G4 Point 4™, G5 Premisa™; G6 Esthet.XR HD, G7 ice, G8 Vit-L-escenceR, G9 GrandioR, G10 TPHR3, G11 AmelogenR Plus, G12 Brilliant Enamel; G13 Filtek™ Z100 and randomly divided into four groups according to the finishing system: C control, J JiffyR, SS Super SnapR, AA AstropolR /AstrobrushR. Each sample was polished for 10 seconds with each sequence instrument, and stored in distilled water for 24 hours, after which a three-point flexure test was applied to determine flexural strength (FS) and modulus (Flexural modulus). Data were analyzed using a two-way multivariate ANOVA and means were compared with Tukey's test. Results were: FS level CR p=0.000 with significant differences. FS level FPp= 0.093 with significant differences. In order: FM level CR p 0.00 with significant differences. FM level PS p=0.001; with significant differences. Under the study conditions, the polishing systems based on silicone rubber decreased the flexural properties of composite resins.
Key words: Composite resins; Elastic modulus; Dental polishing.
RESUMEN
Influencia del protocolo de pulido sobre las propiedades flexurales de varias resinas reforzadas
El objetivo de este trabajo fue determinar la influencia del protocolo de pulido sobre las propiedades flexurales de varios composites. Se prepararon veinte probetas de composite (CR) (25x2x2 mm) en cada grupo: G1 HelimolarR; G2 Filtek™ Z350;G3TetricR N Ceram, G4 Point 4™, G5 Premisa™; G6 Esthet.XR HD, G7 ice, G8 Vit-L-escenceR, G9 GrandioR, G10 TPHR3, G11 AmelogenR Plus, G12 Brilliant Enamel; G13 Filtek™ Z100 y se dividieron aleatoriamente en cuatro grupos segun el sistema de pulido (SP) utilizado: C control, J JiffyR, SS Super SnapR, AA AstropolR /AstrobrushR. Cada probeta se pulio durante 10 segundos con cada secuencia de instrumentos, se almaceno en agua destilada durante 24 hs y se aplico un ensayo de resistencia flexural de tres puntos a fin de determinar la resistencia (RF) y el modulo (MF) flexural. Los valores obtenidos fueron analizados con un ANOVA multivariado de dos vias y comparacion de medias de Tukey. Los resultados obtenidos fueron: nivel de RF para CR p=0,000con diferencias significativas. RF nivel SP p= 0,093 con diferencias signi - ficativas. Para MF a nivel de CR p=0,000con diferencias signifi cativas. MF a nivel SP p=0,000, con diferencias signifi - cativas. En las condiciones de este trabajo se puede concluir que el uso de sistemas a base de gomas siliconadas disminuyen las propiedades flexurales de las resinas reforzadas restaura - doras. : composite resins; elastic modulus; dental polishing.
Palabras clave: Resinas reforzadas; Modulo elastico; Pulido dental.
INTRODUCTION
Composite resin is universally accepted as an
anterior direct restorative material, even for large
restorations. In posterior restorations its application
is becoming more popular, probably because of its
high esthetic potential, acceptable longevity, tissue
preservation potential, tooth structure bonding, low
temperature conductivity and increased demand
from patients 1. Initially these materials had many
disadvantages, such as high polymerization
shrinkage; occasional patient-referred post-operative
sensitivity and accelerated wear in restorations,
mainly located in the molar area, leading to loss of
occlusal and proximal anatomy. A meticulous
restoration technique, the right case selection,
improvement in material formulations and better
knowledge of their performance have enabled safe
use of these materials.
When considering desirable properties for clinical
performance, mechanical properties are described
as very important, especially because they are
closely related to the long-term success of these
restorations. Improvement in mechanical properties
in recent formulations has led to increased
toughness and resistance to abrasion and attrition
wear2. Mechanical properties depend mainly on
composite microstructure and composition; therefore
filler amount, size, morphology and distribution3 are critical for composite selection. In addition,
variations in matrix chemistry should be taken into
account, since it has been reported that they could
significantly affect flexural strength and modulus.
Flexural modulus means material stiffness, which
is important because it influences composite
selection in high stress situations, such us mediumlarge
sized restorations in proximal or occlusal
locations, or for incisal angle replacement or when
used to replace cusps 4.
In a three-year clinical study2, 102 class 4 restorations
made with 4 different composite formulations were
evaluated and a correlation between mechanical
properties and clinical performance was found,
which is an association between wear, modulus and
defect size.
Clinical procedures that may damage or prematurely
harm restorations should be accurately performed
because the primary cause of failure during the first
five years is related to technique and material
selection 5. Optimal finishing and polishing of
composites is one of the most important steps when
performing a restoration, since it not only results in
optimal esthetics, but also favors gingival health,
restoration of marginal integrity over time and
patient comfort 6,7, as well as increasing resistance
to pigmentation and wear 8,9 and possibly influencing
restoration longevity. Surface roughness creates a
favorable microenvironment for bacterial adhesion
and growth, which enables the development of
secondary caries, gingival inflammation and
restoration staining. Polishing aims to reduce
surface roughness and lines progressively until they
are smaller than visible light wavelength. A
traumatic technique might overheat and damage a
composite surface, 10-12 resulting in accelerated
wear. This research group has published previous
data evaluating how filler morphology, matrix
composition and finishing protocol correlate with
flexural properties and mass loss in several
composites13 . We concluded then that all variables
evaluated correlate with a stronger weight on
modulus.
There is no clear evidence of how different clinical
polishing protocols might affect the flexural
properties of composite resins reinforced with
different fillers; thus, the aim of this study was to
evaluate the effect of three polishing protocols on
flexural properties of thirteen restorative composites.
MATERIALS AND METHODS
Thirteen groups of twenty samples of light cured composite resins, shade A2, from eight manufacturers, were prepared: G1 Heliomolar (Ivoclar- Vivadent, Schaan, Liechtenstein), G2 Filtek Z350 (3M/ESPE. St. Paul, USA), G3 Tetric N Ceram (Ivoclar-Vivadent, Schaan, Liechtenstein), G4 Point 4 (KERRR- Sybron dental Specialties, Orange, USA), G5 Premisa (KERR- Sybron dental Specialties, Orange, USA), G6 Esthet.X HD (DENTSPLYCaulk, Milford, USA), G7 Ice (SDI Limited. Victoria, AU), G8 Vit-L-escence (Ultradent Products, INC. South Jordan, USA), G9 Grandio (VOCO America INC. Sunnyside, USA), G10 TPH3 (DENTSPLYCaulk, Milford, USA), G11 Amelogen Plus (Ultradent Products, INC. South Jordan, USA), G12 Brilliant Enamel (Coltene Whaledent, Altstatten, Switzerland), G13 Filtek Z100 (3M/ESPE. St. Paul, USA). They were randomly divided into four groups of five samples each according to the polishing protocol to be applied: C control, J Jiffy (Ultradent Products, INC. South Jordan, USA), SS Super Snap (Shofu Dental Corporation, San Marcos, USA), AA Astropol /Astrobrush (Ivoclar- Vivadent, Schaan, Liechtenstein-batch H32042). Samples were prepared according to ISO Standard 4049 for flexural strength and ANSI/ADA standard 27, using a standardized 25x2x2 mm aluminum mold and verified by means of a digital micrometer 500 (Mitutoyo Corporation, Japan) with an accuracy of 0.01 mm. Composite was placed in 2 mm increments, each of which was light cured for 40 seconds using an Astralis 3 (Ivoclar-Vivadent, Schaan, Liechtenstein) unit at 600 mW/cm 2. Samples were normalized using moist 400 grit sandpaper. One hour later each sample was submitted to the polishing protocol, applying 10 seconds per step and following the manufacturer's instructions with a handpiece at 10,000 rpm NSK (NAKANISHI, Kanuma, Japan), being very careful to maintain water refrigeration and not to press too hard. Samples were stored in distilled water at 37oC for 24 hours. Sample dimensions were determined, after which they were submitted to a three point flexural test (ISO4049/2000 - 27 ANSI/ADA specification) using a universal testing machine 1011 (INSTRONR, Norwood, USA) with a crosshead speed of 1 mm/min until fracture. Data were recorded and processed applying the equations in Table 1 in order to determine flexural strength and modulus for each sample. Statistical analysis was performed using two-way ANOVA, and DHS Tukey test was applied for multiple comparisons. (Statistical Package for the Social Sciences 15.0R).
Table 1: Flexural strength and modulus.
Flexural strength Flexural modulus
Two series of samples of each composite resin were prepared and metalized with gold and argon laser for SEM observation at the Advanced Microscopy Center, University of Buenos Aires, Argentina, with a Supra 40 Scanning Electron Microscope, Carl Zeiss, Germany, with a field emission gun. Micrographs were taken and composites were classified into five groups: spherical, sphericalconglomerates, irregular, irregular + pre polymers and pre-polymers. (Table 2).
Table 2: Classification of the composite according
to filler morphology.
RESULTS
Flexural strength: descriptive statistics (mean and standard deviation) are shown in Table 3. ANOVA for flexural strength (FS) showed statistical differences among groups p<0.001 (Table 4). HSD Tukey (Table 3) proved composites reinforced with spherical (G13 Filtek™ Z100) and irregular particles (G9, G7, G10, G6, G12, G8 and G11) to have higher flexural strength than composites with mixes of pre-polymerized/ irregular (G3, G4 and G5), spherical conglomerates (G2 Filtek™ Z350) and pre-polymerized particles (G1 HeliomolarR). When flexural strength was analyzed related to polishing system, ANOVA showed significant differences among groups p<0.05 (Table 4). Descriptive statistics are shown in Table 5 with mean and typical deviation expressed in MPa. HSD Tukey test showed that there was no significant difference among J, SS and AA and only AA differed significantly from C. Composite-polishing system interaction on flexural strength showed p=0.416, indicating that flexural properties are independent from it. Modulus: ANOVA performed for flexural modulus showed significant differences among materials p<0.001. Descriptive statistics (mean and standard deviation in GPa) (Table 5) were G13:15.03(1.09)= G9:14.50(1.50)> G11:9.79(0.52)= G10:9.76(1.27)= G2:9.72(2.20)≥ G12:8.89(0.4)= G7:8.56(0.73)≥ G4:7.98(1.01)= G6:7.88(0.46)= G8:7.57(0.50)= G3:7.19(0.66)≥ G5:6.53(0.71)> G1:4.65(0.38). When modulus was analyzed with relation to polishing system, ANOVA showed significant differences among groups, p<0.001 (Table 6). Descriptive statistics are shown in Table 7 with mean and standard deviation expressed in GPa. Groups were as follows: C= SS≥ AA: > J. HSD Tukey test proved that all experimental groups showed lower modulus values than the control, although SS had no significant difference. JiffyR had the lowest values and differed significantly. Composite-polishing system interaction on flexural modulus showed p<0.001 (Table 4), indicating that flexural modulus is dependent on it.
Table 3: Descriptive statistics and multiple comparisons
for flexural strength at composite level.
Table 4: Tests of Between-Subjects Effects (ANOVA).
Table 5: Descriptive statistics and multiple comparisons
for flexural modulus at composite level.
Table 6: Descriptive statistics and multiple comparisons
for flexural strength at polishing level.
Table 7: Descriptive statistics and multiple comparisons
for flexural modulus at polishing level.
Figures 1-5 show SEM photographs taken of each type of composite and their classification in terms of filler morphology.
Fig. 1: SEM image at 10,000x of Filtek Z100 composite resin
reinforced with spherical fillers.
Fig. 2: SEM image at 10,000x of Filtek Z350 composite resin
reinforced withspherical conglomerate fillers.
Fig. 3: SEM image at 10,000x of composite resin reinforced
with irregular fillers. From right to left upper and lower
images of Esthet.XR HD, Ice, Vit-L-escence, Grandio, TPH3
and Amelogen Plus.
Fig. 4: SEM image at 10,000x of composite resin reinforced
with irregular fillers + pre-polymer fillings. From right to left
upper and lower images of Point 4, Premisa, Tetric N Ceram
and Brillant Enamel.
Fig. 5: SEM image at 10,000x of Heliomolar composite resin
reinforced with pre-polymers.
DISCUSSION
Two out of the three polishing protocols evaluated in
this investigation are based on silicone impregnated
with varied grain size abrasives, mostly silicon
carbide (Jiffy and Astropol). The third (Super Snap)
is based on flexible paper discs impregnated with
silicon carbide and aluminum oxide.
In order to calculate flexural performance, we carried
out a three point stress test which applies one force
in one sense that generates three types of loads (shear,
tensile and compressive)14. Some authors consider
that these results cannot be transferred to clinical
situations, primarily because failure distribution is
different. However, the model we applied in order to
determine mechanical properties here is accepted
worldwide because it is easy to reproduce, serves in
terms of comparative purposes and represents
material behavior in biomechanical efforts and
determines critical performance during function 15,16.
In terms of FS, all evaluated composites comply
with ISO Standard 4049. However, there were some
differences among commercial brands. In our study,
composites with spherical filling proved to be
similar to composites with irregular particles. Small
variations might be given by filler quantity and
mean size because the manufacturing processes are
essentially the same. Therefore, it can be speculated
that recent formulations with optimized matrix
containing nanometric spherical filler do not seem
to improve FS. It could be considered that variations
(although not significant) in FS occur because
irregular particles concentrate stress at dead angles
in the polymerized matrix. On the other hand,
differences in monomer proportions might
significantly change mechanical properties 4, where
a slight TEGDMA increase might cause a strength
reduction and a UEDMA increase would turn into
an increase. Furthermore, BisGMA variations do
not seem to affect the final result. It can thus be
speculated that TEGMA content may be responsible
for preventing Grandio high filling content and
spherical particles in FilteK Z-100 from being
significantly superior in mechanical properties to
the other irregular particles composites. Finally, the
lower part of the table shows the results for prepolymerized
+ irregular reinforced composites
(Tetric N Ceram, Point 4, Premisa), spherical
conglomerate (Filtek Z-350) and pre-polymerized
(Heliomolar). It could be explained by several
factors such as their lower sized fillers, which
increase the surface to be covered by the matrix,
leading to less filler content. Also more silane could
be present and diminish the polymerization degree,
modifying their mechanical properties18. All
composites, with the exception of Tetric N Ceram,
have TEGMA in their composition and smaller
particles are less effective in preventing crack
propagation because they are easier to go around.
Regarding composites with pre-polymerized
particles, it could be speculated that the high
conversion degree might affect bond strength with
unpolymerized matrix, leading to lower mechanical
properties. In Filtek Z-350, it can be inferred that
although the filler keeps its spherical shape, sintered
nanometric particles are not bonded enough to alter
the course of the fracture, causing a conglomerate
separation. Our group has previously reported a part
of this research focused on filling morphology,
matrix and polishing protocol correlation with
flexural properties and surface loss 13. We found that
flexural strength, flexural modulus and surface loss
were significantly correlated to the composition and
morphology of these composite resins. The highest
influence was found for modulus at 85.7%, while it
was lower for resistance and surface loss, at 41%
and 36.3% respectively. Although that paper also
analyzed the influence of the matrix composition,
we reported, for example, that spherical conglomerates
negatively influence mechanical properties. On the
other hand, independent spherical particles might
turn into a more regular structure that could help
distribute tension, thus increasing properties. This
might suggest that properties are influenced not only
by filler morphology, but also by filler distribution.
Finishing refers to the restoration´s adaptation
to the tooth and aims to obtain good contour,
occlusion, healthy relationship with adjacent teeth
and a smooth surface 9, while polishing refers
to the elimination of irregularities in order to create
a surface which is as smooth as possible, to
minimize gingival irritation, biofilm accumulation,
pigmentation, and recurrent formation of caries18-21.
We carried out the whole process using water
cooling in order to avoid some factors that may
affect resins when the procedure is performed
without water cooling22. It was also finished a
few minutes after initial curing, since lack of
polymerization and surface roughness in newly
cured composite resins 23 have been rejected in
recent formulations 24,25.
There are more variables that might influence
restoration properties, such as operator, time,
humidity and matrixes were controlled, pressure
applied to polish the samples. That pressure might
increase tension in the sample, and even transmit
pneumatic handpiece vibrations, which might
induce some kind of initial defect in the material.
All these variables were controlled by limiting all
procedures to a single operator.
The efficacy of abrasive systems is related to the
flexibility of the supporting material. Abrasive
hardness in this case is very close in the different
systems (2500 Knoop H Kg/mm2 silicon carbide and
2100 Knoop H Kg/mm2 aluminum oxide), instrument
geometry and application . Differences between
flexible discs and silicone rubber reside in the fact
that discs wear filler particle and matrix in a similar
way 7, whereas rubbers wear matrix by plastic
deformation, leading to temperature increase and
either leaving particles on the surface or removing
them, both of which leave irregularities that translate
into a less bright surface. They also create defects
where the cracks initiate, leading the material to its
strength limit. Moreover, physical stress caused by
temperature increase might result in the formation of
micro-cracks, micro-pores or interface spaces
between matrix and filler, which might affect physical
properties 9. Our results differ from those reported by
Gordan et al. 2003 10, who found an increase in FS
when polishing maneuvers were carried out.
CONCLUSIONS
Under the conditions of this study, flexural properties of composite resins were affected by the polishing systems tested. FS is higher in composites reinforced with spherical and irregular particles. Flexural strength is diminished when the Astropol/ Astrobrush system is used. Regarding stiffness, Filtek Z100 and Grandio proved to be superior to the other resins tested. However, both resins showed lower stiffness when systems based on silicone rubber such as Astropol/Astrobrush and Jiffy were used.
ACKNOWLEDGMENTS
The authors thank Prof. Marc Heft for his assistance in manuscript preparation. This study was supported by UBA Grant 2002009010017 and ULA CDCHTA O-221-08-07-B.
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