Polymerization stresses in low-shrinkage dental resin composites measured by crack analysis

Abstract

Objective

The objective of this study was to compare several dental restoratives currently advertised as low-shrinkage composites (Clearfil Majesty Posterior, Kalore, Reflexions XLS Dentin and Venus Diamond) with a microfill composite (Heliomolar) in terms of polymerization stress, polymerization shrinkage and elastic modulus.

Methods

Cracks were made at several distances from the edge of a precision cavity in a soda-lime glass disk. The composites were placed into the cavity and lengths of the cracks were measured before and after light curing. Polymerization stresses generated in the glass at 2 and 10 min after the irradiation were calculated from the crack lengths and K c of the glass. Polymerization shrinkage and elastic modulus of the composites also were measured at 2 and 10 min after irradiation using a video-imaging device and a nanoindenter, respectively. The data were statistically analyzed by ANOVAs and Tukey’s test ( p < 0.05).

Results

The stress was significantly affected by composite brand, distance and time. The stress was directly proportional to time and inversely proportional to distance from the edge of the cavity. Clearfil Majesty Posterior demonstrated the highest stress and it resulted in the fracture of the glass at 2 min. Venus Diamond and Heliomolar exhibited the greatest shrinkage at both times. The elastic moduli of Clearfil Majesty Posterior and Reflexions XLS Dentin were greatest at 2 and 10 min, respectively.

Significance

Among the four low-shrinkage composites, two demonstrated significantly reduced polymerization stress compared to Heliomolar, which has previously been shown in in vitro tests to generate low curing stress.

Introduction

Despite its popularity as a direct restorative material, concerns continue to exist during the clinical placement of resin composite as the material demonstrates significant polymerization contraction and polymerization stresses during curing. The polymerization stresses are capable of inducing deformation of the composite and tooth , debonding of the composite–tooth interface , and cracking in the tooth . The polymerization stress may be one of the factors causing postoperative problems such as sensitivity and marginal degradation, and contributing to recurrent caries with resin composite restorations. Dental composites with reduced polymerization stress can be achieved through reductions in curing shrinkage, materials stiffness and reaction rate . In fact, it is likely that the relatively low polymerization stress of at least one commercial composite, Heliomolar, is a result of all three. However, it is often desirable to produce composites with rapid curing (i.e., very photoresponsive) and high elastic modulus. Thus, reducing the polymerization shrinkage is being suggested as a more direct path to decrease the polymerization stress. Increasing filler content reduces the polymerization shrinkage but simultaneously increases the material stiffness, therefore, the increase of filler is inappropriate to reduces the polymerization stress. Usage of high-molecular weight monomer also reduces the polymerization shrinkage because of a smaller number of C C bonds, which is directly related to the shrinkage, than low-molecular weight monomers. Several high-molecular weight dimethacrylates were developed and applied to some composites advertised as low-shrinking, e.g., TCD-DI-HEA (bis-(acryloyloxymethyl)tricyclo[5.2.1.02,6]decane) in Venus Diamond (Heraeus Kulzer, Hanau, Germany) and DX-511 in Kalore (GC, Tokyo, Japan) . The composites have in fact demonstrated lower polymerization stress or tooth deflection than conventional restorative composites .

Yamamoto et al. proposed a simple method for calculating localized polymerization stress based upon the crack analysis method described by Chantikul et al. . Indentation cracks are made near a cavity in a brittle material that simulates dental enamel. The cracks lengthen when subjected to tensile stresses, such as those resulting from the polymerization contraction of a composite cured in the cavity under the conditions of good interfacial adhesion. The stress is calculated from the change in crack lengths and the known fracture toughness of the brittle substrate. Kubota and Yamamoto applied this method to bovine incisors for evaluating the polymerization stresses generated by dental adhesive restorations, and noted that this method was capable of reproducing specific clinical phenomena such as interfacial debonding and cracking in enamel, and thus may better representative the clinical situation for a class I cavity preparation than certain other methods for measuring polymerization stress. In addition, the crack analysis was reportedly able to detect relatively small change in polymerization stress from a few minutes after light-irradiation .

The objectives of this study were to: (1) calculate the polymerization stresses at 2 and 10 min after light-irradiation for several composites advertised as low-shrinkage, and (2) compare the stresses in the “low-shrinkage” composites to that of a conventional microfill composite that has previously been shown to produce relatively low stress. The hypothesis tested was that the polymerization stress of low-shrinkage composites calculated by the crack analysis method would be similar or lower than that of a conventional composite.

Materials and methods

Polymerization stress

A soda-lime glass ( E = 70 GPa, K c = 0.61 MPa m 1/2 ) was used because it could provide sufficient bond strength to resin composite without the resultant composite–glass interfacial debonding during curing and also because of its similarity to enamel in terms of its elastic modulus .

Twenty-five glass disks having a central cylindrical hole (12.0-mm-outer diameter, 3.0-mm-inner diameter and 2.0-mm-thick) were processed (Yokohama Sekiei Co., Ltd., Yokohama, Japan). The roughness of the top flat surface of the disks was less than 0.08 μm (Ra). The disks were annealed at 510 °C for 24 h and slowly cooled to release any residual stresses .

To introduce initial cracks, four Vickers indentations were made on the flat glass surface adjacent to the perimeter of the hole using a force of 9.8 N delivered for 15 s with a microhardness tester (MVK-E, Akashi, Tokyo, Japan). Each indentation was located in a quadrant (one per quadrant) and centered at a distance ( h ) of 200, 300, 400 or 500 μm from the edge of the hole . The radial cracks were aligned parallel to the tangents of the edge of the hole. After indenting, the glass was set aside for a day in a desiccator at room temperature to allow for the slow crack growth . Two lengths from the indentation center to the crack ends ( c 1 and c 2 ) were measured at 500× magnification using a measuring microscope (STM-UM, Olympus Optical Co., Tokyo, Japan), and an average of the lengths ( c ) was obtained for each specimen.

The indentations were covered with removable tape. One drop each of a silane coupling agent (Clearfil Porcelain Bond Activator, lot no. 00227B, Kuraray Medical, Tokyo, Japan) and an adhesive primer (Clearfil SE Bond Primer, lot no. 00851A, Kuraray Medical, Tokyo, Japan) were mixed and applied to the inner surface of the hole for 20 s and gently dispersed with compressed air for 5 s.

The glass disk was fixed on a glass plate using utility wax with a clear polyester matrix between the disk and plate. The hole was filled with a single increment of one of the five composites ( Table 1 ). The ratio of bonded surface area to unbonded surface area, termed the configuration factor , was approximately 1.3. The composite was covered with a clear matrix and irradiated for 45 s at 540 mW/cm 2 using a programmable quartz halogen light source (VIP, Bisco, Schaumburg, IL, USA). The tip of the light guide was placed in contact with the matrix. The total radiant exposure was 24.3 J/cm 2 . The removable tape was peeled from the surface immediately after irradiation. The indentations and the cracks were checked microscopically to ensure they were not covered by the adhesive or the composite. The two lengths <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='(c′1andc′2)’>(c1andc2)(c′1andc′2)
( c ′ 1 and c ′ 2 )
were measured again as described previously. Following the previous study , the length measurement was started at 2 and 10 min after irradiation and finished within a period of 2.5 min. An average length <SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='(c′2minandc′10min)’>(c2minandc10min)(c′2minandc′10min)
( c ′ 2 min and c ′ 10 min )
was calculated ( n = 5). The glass specimens were kept in the desiccator at room temperature except during indenting, the measurement of the crack length, and the composite filling procedure.

Table 1
Resin composites evaluated.
Composite Manufacturer Filler (vol.%) Filler (wt.%) Composition Lot no.
Clearfil Majesty Posterior Kuraray Medical, Tokyo, Japan 82 92 Matrix: Bis-GMA, TEGDMA, hydrophobic aromatic dimethacrylate 012CC
Filler: alumina, glass-ceramic, silica
Kalore GC, Tokyo, Japan 69 82 Matrix: UDMA, dimethacrylate co-monomers, DX-511 monomer 0905214
Filler: fluoroaluminosilicate glass, strontium glass, silica, pre-polymerized filler
Reflexions XLS Dentin Bisco, Shaumburg, IL, USA 76 88 Matrix: ethoxylated Bis-GMA 0900005247
Filler: glass filler, amorphous silica
Venus Diamond Heraeus Kulzer GmbH, Hanau, Germany 64 81 Matrix: TCD-DI-HEA, UDMA 010038
Filler: barium aluminum fluoride borosilicate glass, Silica
Heliomolar Ivoclar-Vivadent, Schaan, Liechtenstein 46 67 Matrix: Bis-GMA, UDMA, decandiol dimethacrylate L47752
Filler: highly dispersed silicon dioxide, ytterbium trifluoride prepolymer

Values of c and <SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='c′2min’>c2minc′2min
c ′ 2 min
or <SPAN role=presentation tabIndex=0 id=MathJax-Element-4-Frame class=MathJax style="POSITION: relative" data-mathml='c′10min’>c10minc′10min
c ′ 10 min
obtained from the length measurements were used to calculate the stress around the crack ( σ crack ) generated by polymerization of the composite using the following equation :

<SPAN role=presentation tabIndex=0 id=MathJax-Element-5-Frame class=MathJax style="POSITION: relative" data-mathml='σcrack=Kc−Φ/c′3/2Yc′1/2′>σcrack=KcΦ/c3/2Yc1/2σcrack=Kc−Φ/c′3/2Yc′1/2
σ c r a c k = K c − Φ / c ′ 3 / 2 Y c ′ 1 / 2

where σ crack is the stress in the ceramic, K c is the fracture toughness of the glass, Φ is K c c 3/2 , Y is a geometrical term equal to 1.12 π 1/2 . The data were analyzed using three-way ANOVA with material, distance and time as independent variables at α = 0.05.

Tensile stress at the composite–glass bonded interface ( σ interface ) was calculated from σ crack at each distance ( h ) using the following equation :

<SPAN role=presentation tabIndex=0 id=MathJax-Element-6-Frame class=MathJax style="POSITION: relative" data-mathml='σcrack=σinterfaceR02{B2−(R0+h)2}(R0+h)2(B2−R02)’>σcrack=σinterfaceR20{B2(R0+h)2}(R0+h)2(B2R20)σcrack=σinterfaceR02{B2−(R0+h)2}(R0+h)2(B2−R02)
σ c r a c k = σ i n t e r f a c e R 0 2 { B 2 − ( R 0 + h ) 2 } ( R 0 + h ) 2 ( B 2 − R 0 2 )
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Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Polymerization stresses in low-shrinkage dental resin composites measured by crack analysis
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