Elsevier

Dental Materials

Volume 17, Issue 3, 1 May 2001, Pages 268-276
Dental Materials

The influence of water storage and C-factor on the dentin–resin composite microtensile bond strength and debond pathway utilizing a filled and unfilled adhesive resin

https://doi.org/10.1016/S0109-5641(00)00081-6Get rights and content

Abstract

Objective: To test the elastic wall concept utilizing adhesive resins of varying stiffness in a low- and high-C-factor cavity design after short- and long-term water storage.

Methods: A flat and box-shaped cavity was restored on occlusal dentin with a resin composite using a filled and unfilled adhesive resin from which microtensile specimens with a 0.5 mm2 cross-sectional area were formed. After storage for 30- and 150-days the microtensile bond strength (μTBS) was determined in a Zwick materials testing machine and the subsequent debond pathway was examined under scanning electron microscopy. Fisher's exact test was used to determine differences in joint and substrate failure modes and a Weibull regression model with gamma frailties was used to test for differences between failure distributions. Tests for three-way and two-way interactions were also completed for storage time, C-factor and adhesive. All tests were at 95% confidence levels.

Results: The characteristic strength (TBS°) for the Optibond FL adhesive applied on a flat cavity was 47.57 and 20.90 MPa and a box-shaped cavity was 49.26 and 17.49 MPa for short- and long-term storage, respectively, while the corresponding TBS° for the unfilled Optibond adhesive on the flat cavity design was 36.93 and 32.68 MPa and in a box-shaped cavity was 32.84 and 15.46 MPa. Combining all groups according to storage time revealed a three-fold increase in the debond pathway including the bottom of the hybrid layer.

Significance: Evidence suggests that the durability of the bonded joint is threatened by hydrolysis and the most susceptible region is the bottom half of the hybrid layer and in low C-factor cavity designs a more flexible adhesive resin liner was more durable.

Introduction

Resin composite restorative materials are presently placed in both anterior and posterior regions of the mouth primarily due to their esthetic qualities. Modern resin composites demonstrate improved strength and wear resistance with reduced polymerization shrinkage and water sorption as compared to earlier versions. Although the polymerization shrinkage may be reduced in some systems, the induced contraction stress on the dentin-adhesive resin–resin composite bonded joint is still believed to lead to restorative failures. Efforts are underway to eliminate or reduce this shrinkage stress or to make the adhesive bond resistant to its damaging effects. A popular theory proposed by several investigators is that a more flexible intermediate liner between the dentin and resin composite will preserve the dentin bond during the polymerization process [1], [2], [3], [4]. This is commonly referred to as elastic bonding or the elastic wall concept. However, the use of low modulus liners and adhesives may result in a weakened structure due to inadequate stress dissipation [5].

The primary bonding mechanism employed by dentin adhesive systems is the hybridization of a decalcified dentin surface with primer and adhesive resins, which subsequently co-polymerize with the applied resin composite. The thickness, density and quality of this hybrid layer have been theorized to effect the absorption of resin composite contraction stresses [6], [7]. However, if the depth of demineralization during the conditioning step is not met by the applied adhesive monomers and oligomers, then this non-impregnated region is subject to hydrolytic breakdown. This non-impregnated or poorly impregnated zone between the hybridized dentin and the unaltered dentin has been shown with transmission electron microscopy and microleakage evaluation [8], [9]. The tensile bond strength of PMMA bonded to human dentin with 4-META/MMA-TBB was evaluated with a “miniaturized dumbbell-shaped specimen” after conditioning the dentin with 10% citric acid: 3% ferric chloride for either 10, 30 or 60 s [10]. The groups conditioned for 30 and 60 s were significantly weaker and shown by scanning electron microscopy (SEM) to fail within a non-resin impregnated demineralized dentin. The strength of the 10-s group was attributed to the full impregnation of a narrower band of acid-exposed collagen. However, even if this decalcified zone is fully impregnated with resins the long-term stability of this interdiffusion zone or hybrid layer is unknown. Inadequate polymerization or defects within the zone could occur due to diffusion gradients created by dentinal tissues, moisture content, residual solvents or phase separation of monomers [11], [12], [13].

The potentially damaging effect of polymerization contraction stress has been recognized in the research community almost since the introduction of resin composite materials [14]. This stress will either disrupt the adhesive bond to tooth or cause internal derangement in the resin composite or both [15]. Although the delayed effect of post-cure water sorption by the resin components may partially or totally relieve the residual stress, it is not clear as to degree or outcome of this physical process [16] and certainly occurs after disruption of the micromechanical dentin bond. Davidson et al. [17] studied the effect of polymerization contraction stress in two-dimensional and three-dimensional cavity models using both chemical- and light-cure resin composite restorative materials. Durable marginal sealing was demonstrated only on flat surfaces (two-dimensional) while all Class V cavity specimens (three-dimensional) demonstrated bond disruption regardless of the curing mode of the resin composite. The observed difference in bond durability was attributed to the relatively limited stress relieving flow possible in the three-dimensional cavity. They also hypothesized that a contraction stress of 20 MPa could be generated at the cavity margin in the Class V cavity design used in this study. This value was calculated by taking the proportion of the restorative material in contact with tooth structure (2/3) by the maximum contraction stress (32 MPa) reported for this particular chemical cure resin composite (Silar, 3M) [17]. It is noteworthy that Asmussen and Munksgaard [18] also reported this as the bond strength required for producing gap free margins.

Hansen and Asmussen [19] created cylindrical cavities in dentin 1.5 mm deep with diameters ranging from 1.8 to 6.4 mm with cavosurface margins from 90 to 160°. The cavities were restored with nine different adhesives using a light-cured resin composite (Silux, 3M Co.), subsequently; marginal gap formation was measured after either 10 min, 1 h or 1-day water storage. Combining the bond strength results [18] with this measured marginal gap in the butt joint margin design, an inverse linear relationship was established (−0.96). Extrapolation from this relationship yielded a required 20 MPa of bond strength to produce a gap free margin. In the clinical situation, the resin composite is applied almost immediately after curing of the adhesive bonding agent. The quality of bond between the adhesive resin and dentin at this point is unknown. The information above is often used in a simplistic argument for a desired minimum bond strength of 17–20 MPa for current dental adhesive systems. However, as stated, the immediate bond strength is unknown and the 24-h and beyond SBS of a dentin-adhesive resin–resin composite bonded joint may have little correlation with the resistance to these immediate contraction stresses. A method of assessing the integrity of the adhesive resin system to dentin after undergoing this stress is the evaluation of bond durability after long-term water storage. Damage or degradation of the adhesive bond may be elucidated after water storage due to hydrolytic attack of organic tissues or alterations in mechanical properties of resinous components.

The more highly constrained the resin composite material is upon setting the greater will be the contraction stress [17]. Feilzer et al. [20] attempted to measure this stress as a function of restoration shape. They simulated the ratio of bonded to unbonded surfaces (configuration factor or C-factor) for common clinical cavity designs (Class I, II, III, IV, V) using cylindrical resin composite forms bonded to flat metal discs. Although the geometries are dissimilar, a good approximation of bonded to unbonded surfaces can be examined. In this study it was determined that in most clinically relevant cavity designs, there was insufficient stress-relieving flow to prevent cohesive failure in the resin composite during polymerization. Only those specimens with a C-factor <1 stayed intact, a C-factor >1 and <2 showed some spontaneous failures and all those >2 failed spontaneously within the resin composite. Testing conditions could have contributed to the observed cohesive failures in the laboratory but the developing stress supports the hypothesis that the early maturing dentin-adhesive resin–resin composite bonded joint may fail.

Utilizing photoelastic analysis, Kinomoto and Torii [21] described the polymerization contraction stresses generated by light-cured resin composite in box-shaped cavities in both bovine teeth and transparent resin moulds. They were unable to observe internal stresses in the restorations placed in bovine teeth, due to the formation of marginal gaps, evidenced by fuchsin dye leakage. No dye penetration was observed in the transparent resin moulds enabling the description of internal stress distributions. The highest principal stresses occurred near the internal line angle with the normal tensile stress ranging from 8–23 MPa along the lateral cavity wall and 11–23 MPa on the cavity floor. Numerous assumptions were made in this study, but it is clear that the stress distribution imposed upon a tooth-restorative joint by a polymerizing resin composite is highly complex and non-uniform. Suffice to say that high stresses, relative to known bond strength values, may be imposed upon the early maturing dentin adhesive resin bonding system and that this stress is detrimental to the long-term integrity of the adhesively bonded joint.

The most desirable method to eliminate the possibility of contraction stress induced marginal failure would be to eliminate shrinkage of the polymerizing restorative material. Reports of an expanding spiroorthocarbonate/epoxy copolymer for use in resin composites shows much promise [22], as does the relatively reduced shrinkage observed with oxybismethacrylate monomers and oligomers [23]. However, these representative systems are in the early development stages and are not expected to be available for clinical application in the near future. Other solutions to the polymerization shrinkage stress problem are outlined in review articles by Carvalho et al. [16] and Davidson and Feilzer [24]. In addition to the lower configuration cavity design and elastic wall concept mentioned above, they mention the use of chemical-cured resin systems, glass inserts, increased filler loading, modulation of curing initiation and incremental filling as means to reduce polymerization shrinkage stress. However, many of these suggestions remain controversial. For instance, shrinkage vectors of auto- and light-cured resin composite and the benefit of incrementally filling a restorative cavity, once thought to be universally understood, have recently been questioned [25], [26].

Choi et al. [27] have recently demonstrated that thicker layers of relatively low-modulus adhesive can significantly reduce the contraction stress of composite restorations and reduce the overall degree of microleakage in marginal areas. Fillers added to adhesive resins will reduce the amount of polymer matrix available for polymerization contraction but simultaneously increase the stiffness. According to Hooke's law, the higher the stiffness per given contraction strain the higher will be the contraction stress [27]. If adhesive resins possessing dissimilar stiffness are applied to cavity walls with equivalent film thickness the more flexible adhesive should be able to absorb more strain energy during the polymerization contraction of the overlying resin composite. Bond integrity may be preserved if the adhesive resin can adequately absorb the stress from resin composite polymerization.

The purpose of this study is to test the elastic wall concept utilizing adhesive resins of varying stiffness. The bond strength durability and debond pathway will be determined in dentin-adhesive resin–resin composite joints formed in two different cavity designs after short- and long-term water storage.

Section snippets

Materials and methods

Forty intact, non-carious, non-restored, human molars were randomly selected from a pool of extracted teeth that had been stored in 0.5% chloramine T at 4°C for less than three months. They were cleaned of soft tissue, pumiced, roots notched, and mounted in dental stone with a custom-fabricated tooth-mounting device and then placed in tap water. A grinder/polishing machine (Buehler Ecomet V, Buehler Ltd, Lake Bluff, IL, 60044) was used to flatten the occlusal surface perpendicular to the long

Results

The film thickness for Optibond FL and Optibond unfilled adhesive resins was not significantly different (p=0.83) with mean values of 38.0(30.3) and 41.0(27.5) μm, respectively. Bias in film thickness may exist for fractographic measurements involving only non-joint failures, but baring this condition it appears that the film thickness were equivalent. Relative to Optibond FL (378 N/mm), the unfilled version of Optibond (233 N/mm) was X1.6 more flexible (p-value <0.001). Due to the adhesives

Discussion

Due to statistical software limitations, the failure distributions in Fig. 1 are presented as if from an independent research design, however, this research design does not assume independence of tooth, which has been commonly done in previous bond strength studies [33], [34], [35], [36]. There is a possibility that different results will be obtained if the analytic method does not match the study design (future studies). The data from this investigation are treated as dependent upon the

Acknowledgements

This investigation was supported in part by Research Grant K16 DE00175-11 from the National Institute for Dental Research, Bethesda, Maryland, USA. The adhesive resin and resin composite was provided by Kerr. We would like to thank Lester Kirchner for statistical support.

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