| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
RESEARCH REPORT |
1 Division of Pediatric Dentistry, Hokkaido University, Graduate School of Dental Medicine, Kita 13, Nishi 7, Kitaku, Sapporo 060–8586, Hokkaido, Japan; and
2 Department of Dental Materials Science, Academic Center for Dentistry Amsterdam (ACTA), The Netherlands
* corresponding author, masanori-h{at}mue.biglobe.ne.jp
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
|---|
KEY WORDS: polymerization • contraction stress • dentin adhesive • resin composite
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
|---|
The observed decline in stress with time in the above-mentioned experiments (Bolhuis et al., 2006) may become more pronounced, and could reveal more details, when the adhesive is placed between glass and dentin without a composite. In this situation, the polymerizing adhesive pulls at the dentin surface, but with composite included, it is the pre-cured adhesive that is pulled by the contraction stress of the polymerizing composite.
The aim of this study was to investigate the course of the polymerization contraction stress development of thin adhesive layers between glass and dentin for total-etch systems and systems with self-etching primers. The hypothesis tested was that a greater decline of stress after light-curing would be seen for total-etch systems than for systems with self-etching primers. To monitor the effect on stress development when the adhesives are pre-cured, we also conducted measurements on these adhesives combined with composite. We performed experiments with the adhesives between glass and steel, to see the effect on the course of stress development when a bonding substrate other than dentin was used.
|
MATERIALS & METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
|---|
). The composite used was Filtek Z250/A2 (3M/ESPE, St. Paul, MN, USA).
|
). The flat end of each core was finally wet-ground with SiC paper up to grit 600, creating a smear layer at the bonding site, and stored in water until testing. Similarly shaped steel specimens were machined, and the flat surfaces were sandblasted.
|
Contraction Stress Measurements
The polymerization contraction stress was determined in a universal testing machine (ACTA Intense, ACTA, Amsterdam, The Netherlands) as described previously (Dauvillier et al., 2003) in a modified set-up (Fig. 1
). The dentin or steel cores were fixed in a specially machined steel cylindrical specimen-holder, from which 0.5 mm of the 6-mm-diameter part of the cores could protrude (Fig. 1), and were connected to the crosshead with the load-cell of a universal testing machine.
The adhesives were applied to the pre-treated dentin or sandblasted steel surface (for Excite and One-Step Plus in two coats and gently air-dried for 3 sec to allow the solvent to evaporate), and then the crosshead was lowered toward the glass plate and adjusted to a position to form an adhesive layer of 15 µm. To avoid forcing adhesive into the opened dentin tubules, which could influence the stress measurements, we lowered the crosshead at a very low speed, to keep the force on the adhesive at 0 N. The adhesive layer was then light-cured from underneath the glass plate for 20 sec (600 mW/cm2). The contraction stress development was recorded continuously, from the start of light-curing up to 30 min. With dentin, the number of experiments was n = 12, and with steel, n = 3.
For the adhesive/composite combinations, the applied adhesives were first light-cured for 20 sec, then the composite was applied and the crosshead lowered to obtain a layer thickness of 1 mm of the composite. Excess of expressed composite was removed, and the layer was then light-cured for 40 sec. The number of experiments was n = 12.
Statistical Analysis
One-way ANOVA and Tukey’s post hoc tests were used to analyze differences (p < 0.05) between the maximum stress values and between the 30-minute values within each experimental group. Differences between the maximum and 30-minute values within each group were analyzed with paired t tests (p < 0.05).
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
|---|
), except for the adhesives cured between glass and steel (Fig. 2C).
|
) or adhesive/composite combination (Fig. 3B).
|
), and for the adhesive/composite combinations (Fig. 3B), the differences between adhesives were not significant. |
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
|---|
However, between glass and dentin, the stress curves of the adhesives first reached a maximum approximately 1 min after the start of light-curing, and then decreased steadily. For the two total-etch systems, the decrease from maximum to 30 min was greater than 70%, and for the two systems with the self-etching primers, the decrease was approximately 30%. So the hypothesis was accepted, that a greater decline of stress after light-curing is seen for total-etch systems than for systems with self-etching primers. Clearly, dentin plays an important role in the stress decline, but the extent depends on the adhesive system used.
With the total-etch systems, phosphoric acid demineralized the smear layer and the underlying intact dentin to expose collagen fibrils and the dentinal tubules. Since the adhesives were applied according to the wet-bonding technique on a moist surface, visibly wet, the monomers diffused, along with their solvents, among the collagen fibrils and penetrated the tubules. Upon light-curing of the thin adhesive layer between the dentin and glass, as was done in the present study, the developing contraction stress pulled at the underlying "open" tissue, which released residual water and/or solvent into the adhesive film and interfered with setting. This retarded not only stress development, but also ultimate stress. An interesting observation was that the ultimate stress (peak value) for One Step Plus (1.6 MPa) was significantly lower than that for Excite (4.4 MPa). This may be explained by the different solvents used in the two systems, acetone in One Step Plus and ethanol in Excite. Micromorphological studies have demonstrated that acetone-based adhesives are most effective in hybridization with the collagen fibrils and in tag formation (Gregoire et al., 2002; Mohan and Kandaswamy, 2005). The deep penetration of acetone may be the result of efficiently replacing water from the collagen network and dentinal tubules better than any other solvent carrier. But this could be a disadvantage in the evaporation step, just prior to light-curing, if the acetone evaporates completely. Residual acetone in the deeper dentinal structure, when "sucked up" by the contraction stress into the growing polymer network, may keep the polymer network dissolved and weak in the very first seconds of light-curing. With the ethanol-based Excite, this may not occur to the same extent, due to more complete evaporation (in the evaporation step) and a lesser solubility of the growing polymer in ethanol, which would allow for faster stiffening of the polymer network and thereby higher stress development. The steep decline of stress, starting approximately 1 min after the start of light-curing, for both One Step Plus and Excite, must be explained by further infiltration of water and/or solvents into the set polymerized resin, accelerated by the contraction stress. This strongly supports previous research on single-bottle adhesives, showing that the bonding resin layer behaves as a permeable membrane (Tay et al., 2004; Hashimoto et al., 2005). The transudation of fluids from the underlying dentinal structure into the resin layer (accelerated by the contraction stress) eventually reversed the contraction stress of One Step Plus into an expansion stress within the 30-minute measuring period, presumably by swelling of the poly-HEMA component in the adhesives. The stress curve for Excite did not cross the x-axis within 30 min, but from its slope, one could expect that this may occur later on, or that the stress will at least level to zero.
With the systems using self-etching primers, the smear layer and smear plugs were not removed. The milder etching by the primers resulted in hybrid structures being formed with the smear layer and smear plugs and with intact dentin just beyond the smear layer (Tay et al., 2000). After application of the adhesive and light-curing under restrained conditions, the contraction stress pulled at a "closed" dentinal structure, in contrast to the situation for the total-etch systems, where the stress pulled at an "open" structure. As a result, the stress curves of both adhesives, Liner Bond 2V and Protect Bond, showed only a moderate decline in stress after reaching a maximum value. The course of the stress development of the two self-etching systems was quite similar, which may be due to similar chemical formulations of their primers and adhesives. Since no solvents other than water were present in the two adhesive systems, the decline could be caused only by water, which was pulled into the adhesive layer from the hybridized dentinal regions and/or from intact dentin below it. The moderate decline in stress supports earlier findings that adhesive systems based on self-etching primers produce good dentin sealing (Hashimoto et al., 2004), but are also permeable (Tay and Pashley, 2003; Hashimoto et al., 2005).
In the clinical situation, adhesive layers are never cured under the constrained conditions as used in this study, but polymerization contraction stresses are also produced in free resin layers applied to the cavity walls. The stresses are generally shearing in nature, but at the outer borders of a layer, the stresses are normal to the surface (Feilzer et al., 1990), and probably also at regions where the adhesive layer changes in thickness. Whether these stresses are strong enough to pull fluids out of the underlying dentin surface cannot be concluded at this point, but in this study, the stresses produced when composite was applied to the adhesive and cured under constrained conditions were strong enough. All curves in the adhesive/composite combinations showed a maximum, followed by a decline. Constrained conditions can also occur in the clinical situation, and one may expect that fluids can be pulled from the underlying dentin surface into the adhesive layer, which is supported by previous research that adhesives are permeable (Hashimoto et al., 2004, 2005; Tay et al., 2004).
Limitation of stress development and decline of stress following light-curing would have obvious clinical benefits. However, the withdrawal of fluids from the hybridized structures, leaving channels (which will fill with water), and the uptake of these fluids by the adhesive layer may have a negative effect on the durability of the bonded joint (Sano et al., 1999; Shono et al., 1999; Hashimoto et al., 2000, 2002; Armstrong et al., 2001; Li et al., 2001; Tay et al., 2003; Reis et al., 2005).
It should be noted that the observed course of stress development, where a maximum is reached followed by a decline, may be caused partly by the viscoelastic properties of demineralized dentin (Pashley et al., 2003). However, its contribution will be small as it involves demineralized dentin of only a few micrometers. Another aspect that could have influenced the results was that there was no control of the amount of wetness and evaporation of solvents in the bonding procedures. These factors, which influence dentin shrinkage (Eddleston et al., 2003; Ito et al., 2005), may have contributed to the relatively large standard deviations of the data.
|
ACKNOWLEDGMENTS |
|---|
Received August 18, 2005; Last revision March 28, 2006; Accepted April 21, 2006
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Ausiello P, Apicella A, Davidson CL (2002). Effect of adhesive layer properties on stress distribution in composite restorations—a 3D finite element analysis. Dent Mater 18:295–303.
Bolhuis PB, de Gee AJ, Kleverlaan CJ, El Zohairy AA, Feilzer AJ (2006). Contraction stress and bond strength to dentin for compatible and incompatible combinations of bonding systems and chemical and light-cured core build-up resin composites. Dent Mater 22:223–233.
Braga RR, Hilton TJ, Ferracane JL (2003). Contraction stress of flowable composite materials and their efficacy as stress-relieving layers. J Am Dent Assoc 134:721–728.
Causton BE, Miller B, Sefton J (1985). The deformation of cusps by bonded posterior composite restorations: an in vitro study. Br Dent J 159:397–400.
Choi KK, Condon JR, Ferracane JL (2000). The effects of adhesive thickness on polymerization contraction stress of composites. J Dent Res 79:812–817.
Dauvillier BS, Aarnts MP, Feilzer AJ (2003). Modeling of the viscoelastic behavior of dental light-activated resin composites during curing. Dent Mater 19:277–285.
Eddleston CL, Hindle AR, Agee KA, Carvalho RM, Tay FR, Rueggeberg FA, et al. (2003). Dimensional changes in acid-demineralized dentin matrices following the use of HEMA-water versus HEMA-alcohol primers. J Biomed Mater Res 67:900–907.
Feilzer AJ, de Gee AJ, Davidson CL (1987). Setting stress in composite resin in relation to configuration of the restoration. J Dent Res 66:1636–1639.
Feilzer AJ, de Gee AJ, Davidson CL (1990). Relaxation of polymerization contraction shear stress by hygroscopic expansion. J Dent Res 69:36–39.
Ferracane JL (2005). Developing a more complete understanding of stresses produced in dental composites during polymerization. Dent Mater 21:36–42.
Ferracane JL, Mitchem JC (2003). Relationship between composite contraction stress and leakage in Class V cavities. Am J Dent 16:239–243.
Gregoire GL, Akon BA, Millas A (2002). Interfacial micromorphological differences in hybrid layer formation between water- and solvent-based dentin bonding systems. J Prosthet Dent 87:633–641.
Hashimoto M, Ohno H, Kaga M, Endo K, Sano H, Oguchi H (2000). In vivo degradation of resin-dentin bonds in humans over 1 to 3 years. J Dent Res 79:1385–1391.
Hashimoto M, Ohno H, Sano H, Tay FR, Kaga M, Kudou Y, et al. (2002). Micromorphological changes in resin-dentin bonds after 1 year of water storage. J Biomed Mater Res 63:306–311.
Hashimoto M, Ito S, Tay FR, Svizero NR, Sano H, Kaga M, et al. (2004). Fluid movement across the resin-dentin interface during and after bonding. J Dent Res 83:843–848.
Hashimoto M, Tay FR, Ito S, Sano H, Kaga M, Pashley DH (2005). Permeability of adhesive resin films. J Biomed Mater Res 74:699–705.
Ito S, Saito T, Tay FR, Carvalho RM, Yoshiyama M, Pashley DH (2005). Water content and apparent stiffness of non-caries versus caries-affected human dentin. J Biomed Mater Res 72:109–116.
Kemp-Scholte CM, Davidson CL (1990). Complete marginal seal of Class V resin composite restorations effected by increased flexibility. J Dent Res 69:1240–1243.
Li HP, Burrow MF, Tyas MJ (2001). The effect of long-term storage on nanoleakage. Oper Dent 26:609–616.
Mohan B, Kandaswamy D (2005). A confocal microscopic evaluation of resin-dentin interface using adhesive systems with three different solvents bonded to dry and moist dentin—an in vitro study. Quintessence Int 36:511–521.
Pashley DH, Agee KA, Wataha JC, Rueggeberg F, Ceballos L, Itou K, et al. (2003). Viscoelastic properties of demineralized dentin matrix. Dent Mater 19:700–706.
Reis A, Grandi V, Carlotto L, Bortoli G, Patzlaff R, Rodrigues Accorinte Mde L, et al. (2005). Effect of smear layer thickness and acidity of self-etching solutions on early and long-term bond strength to dentin.J Dent 33:549–559.
Sano H, Yoshikawa T, Pereira PN, Kanemura N, Morigami M, Tagami J, et al. (1999). Long-term durability of dentin bonds made with a self-etching primer, in vivo. J Dent Res 78:906–911.
Shono Y, Terashita M, Shimada J, Kozono Y, Carvalho RM, Russell CM, et al. (1999). Durability of resin-dentin bonds. J Adhes Dent 1:211–218.
Tay FR, Pashley DH (2003). Have dentin adhesives become too hydrophilic? J Can Dent Assoc 69:726–731.
Tay FR, Sano H, Carvalho R, Pashley EL, Pashley DH (2000). An ultrastructural study of the influence of acidity of self-etching primers and smear layer thickness on bonding to intact dentin. J Adhes Dent 2:83–98.
Tay FR, Hashimoto M, Pashley DH, Peters MC, Lai SC, Yiu CK, et al. (2003). Aging affects two modes of nanoleakage expression in bonded dentin. J Dent Res 82:537–541.
Tay FR, Frankenberger R, Krejci I, Bouillaguet S, Pashley DH, Carvalho RM, et al. (2004). Single-bottle adhesives behave as permeable membranes after polymerization. I. In vivo evidence. J Dent 32:611–621.
Watts DC, Marouf AS, Al-Hindi AM (2003). Photo-polymerization shrinkage-stress kinetics in resin-composites: methods development. Dent Mater 19:1–11.
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| IADR Journals | Advances in Dental Research ® |
| Journal of Dental Research ® | Critical Reviews (1990-2004) |
