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Repairability of Cross-linked BiopolymersDepartment of Prosthetic Dentistry, Justus-Liebig-University, Schlangenzahl 14, D-35392 Giessen, Germany Correspondence: markus.balkenhol{at}dentist.med.uni-giessen.de
Repair of biopolymers is a critical issue, especially with aged restorations. Obtaining a chemical bond to the repair surface might solve this problem. We hypothesized that certain repair liquids are suitable to establish a strong bond to an artificially aged dimethacrylate-based biopolymer for temporary restorations. Specimens made of a self-curing temporary crown-and-bridge material were prepared and thermocycled for 7 days (5000x, 5–55°C). Cylinders made of light-curing composites (n = 10) were bonded onto the specimen surface, either after grinding or after the application of 4 different experimental repair liquids (Bis-GMA:TEGDMA mixture = bonding, methylmethacrylate = MMA, bonding & acetone, bonding & MMA). A shear bond strength test was performed 24 hrs after repair. The highest bond strength was obtained with the bonding & acetone liquid (20.1 ± 2.2 MPa). The use of MMA significantly affected the bond strength (6.8 ± 1.9 MPa). MMA is inadequate as a repair liquid on aged composite-based biopolymers.
Key Words: composite resin • repair • temporary crown-and-bridge material • shear bond strength • SEM analysis
Apart from their application as biomaterials for fillings, one of the main indications for cross-linked methacrylate polymers in dentistry is the fabrication of temporary restorations (i.e., crowns and bridges). Temporary crowns and bridges are always required where prepared teeth are to be fitted with fixed restorations that have to be fabricated in the dental laboratory (Burke et al., 2005). The main criteria for a temporary restoration involve protecting the prepared tooth structure, stabilizing the tooth position, and ensuring correct mastication/phonetics (Christensen, 1996; Burns et al., 2003; Burke et al., 2005). The occlusal loading in the oral cavity places severe demands on the mechanical strength of biomaterials used for fabricating temporary crowns and bridges. Consequently, temporary restorations may be damaged during functional loading and may, in turn, require repair (Burke et al., 2005; Balkenhol et al., 2008). In addition, there are also clinical situations in which a highly filled, light-cured composite resin has to be added (Burke et al., 2005). A prerequisite for successful repair or addition of material is that the temporary crown-and-bridge material form a firm, long-lasting bond with the repair material (Gegauff and Holloway, 2006). Bonding to aged material is especially problematic (Mitsaki-Matsou et al., 1991; Balkenhol et al., 2008). This involves the question about a possible enhancement of the repairability of temporary crown-and-bridge materials by surface pre-treatment with specific repair liquids. The authors are aware of only two studies on this topic to date (Hagge et al., 2002; Rosentritt et al., 2004). However, since these studies involved commercially available repair liquids, any interpretation of the results is limited. The objective of the present study was therefore to test the following null hypothesis: The bond strength between a temporary crown-and-bridge material and the repair material applied is independent of the surface pre-treatment prior to repair. Since, additionally, the viscosity of the repair material might affect the repair strength (Causton, 1975; Bonstein et al., 2005), 2 composite filling materials of different viscosities were used for repair.
Materials The self-curing dimethacrylate Luxatemp AM Plus (DMG, Hamburg, Germany, shade A2, Lot 578669), which is based on urethane dimethacrylate, aromatic dimethacrylate, glycol methacrylates, and inorganic filler particles (44 wt%), was used as the base material. Four different experimental formulations were used as repair liquids (Table  ). The light-curing composite resins Venus Flow (A2, Lot 010115) and Charisma (A2, Lot 010224) were used as repair materials (Heraeus Kulzer, Hanau, Germany). All commercial materials were used according to their respective manufacturers’ instructions.
Determination of Shear Bond Strength The mixed temporary crown-and-bridge material was dispensed into an 8-mm-wide hole of an acrylic-glass rod (25 mm in diameter and height), which was used as the specimen holder. All specimens were subsequently stored in an incubator for 10 min at 37°C before being thermocycled for 7 days (5000 cycles, 5–55°C, dwell time 45 sec). Prior to repair, the surfaces of the specimens were ground on SiC paper (grit 320) and rinsed with water for 5 sec before being dried for 5 sec with an air syringe. Subsequently, a split mold was used to cure a shearing cylinder (diameter, 3 mm; height, 2 mm) made of the repair material onto the surface. The materials were cured for 40 sec by means of a conventional halogen light-curing unit (Elipar Highlight, 3M ESPE, Seefeld, Germany, standard mode, output 800 mW/cm2). After polymerization, the specimens were removed from the mold and stored in water at 37°C for 24 hrs prior to being tested. In 4 further test groups, the ground surfaces were pre-treated with one of the repair liquids prior to placement of the repair material. The liquid was applied with a mini-brush (10 sec) and allowed to react for 30 sec before being light-cured (40 sec). To differentiate between the shear bond strength and the cohesive strength of the temporary crown-and-bridge material, we cast specimens in one piece (control group).
After storage, a shear bond strength test was conducted (n = 10) at a cross-head speed of 1 mm/min, and the maximum stress (MPa) prior to fracture was calculated. A visual fracture analysis was carried out, distinguishing among adhesive, cohesive, and mixed fractures (Fig. 1
SEM Analysis A morphological characterization of the roughened surfaces of the specimens and bonding areas (cross-section), respectively, was carried out with a Philips XL20 scanning electron microscope (Philips, Eindhoven, the Netherlands) at an accelerating voltage of 20 kV and a magnification of 1000x to 20,000x. The specimens were prepared in the same manner as described previously.
Statistical Analysis Since the repair material had no significant influence on the shear bond strength (ANOVA p > 0.05), the data for the 2 repair materials were pooled before being subjected to a multiple comparison with the Games-Howell test (p = 0.05). The t test (p = 0.05) for independent samples was used for pair-wise comparisons. All statistical analyses were carried out with SPSS for Windows (release 12.01, SPSS Inc., Chicago, IL, USA).
Shear Bond Strength The cohesive strength (control group) of the temporary crown-and-bridge material was 26.9 MPa (± 2.2 MPa). For all test groups, shear bond strength was significantly lower compared with that of the control group (p < 0.05).
A significantly higher bond strength (p < 0.05) and predominantly cohesive failures were observed for the bonding & acetone liquid compared with all other test groups (Fig. 2
SEM Analysis The SEM of the ground specimen surface prior to repair revealed porosities inside the polymer structure (Fig. 3a ). Disintegration between filler particles and polymer matrix were clearly visible (arrows).
The following SEMs (Figs. 3b–3f  ) show cross-sections through the shear bond strength specimens. A fine line was discernible on the specimens ground only at a magnification of 1000x. However, no evidence of disintegration between the 2 layers was apparent (Fig. 3b). The cross-section of the shear bond strength specimen showed partial gaps between the temporary crown-and-bridge material and the repair liquid, when bonding was used for pre-treatment (Fig. 3c). Where the bonding & acetone liquid was used, the SEM showed a uniform transition zone and no indication of disintegration. Direct intimate contact was observed down to the finest structures between the repair liquid and the temporary crown-and-bridge material (Fig. 3d). The contact area between the bonding & MMA liquid primarily exhibited a continuous transition zone; however, small vacuole-like structures within the bonding zone and at the contact area were visible (Fig. 3e). A hairline defect was seen on the SEM where the surface had been conditioned with MMA. At a magnification of 4000x, partial disintegration and fine gaps were noted between the temporary crown-and-bridge material and the composite added (Fig. 3f).
Certain clinical situations demand the repair of temporary crowns and bridges with light-curing composite resins (Burke et al., 2005; Gegauff and Holloway, 2006). However, the influence of surface pre-treatment with specific repair liquids after artificial aging of the specimens remains to be clarified, as does a possible influence of the repair material added, i.e., its viscosity. Since repair of composite-based temporary crown-and-bridge materials is especially crucial after simulated artificial aging (Balkenhol et al., 2008), the specimens were thermocycled (Söderholm et al., 1984; Montes-G. and Draughn, 1986; Gale and Darvell, 1999). Prior to repair, all specimens were roughened on SiC paper (Crumpler et al., 1989), which created a surface roughness comparable with that produced by medium- to fine-grained diamond burs (Anusavice and Antonson, 2003). Since temporary crown-and-bridge materials have a reduced filler fraction compared with filling composites, it is assumed that the ground surface consists predominantly of organic polymers. Sandblasting was omitted on purpose, to comply with daily clinical procedures. Since we wanted to focus on the bond strength to the matrix by the repair liquids, no further silane treatment procedures for bonding to filler particles were applied. The shear bond strength test has proved to be an adequate means of measurement for evaluation of the repairability of composite-based materials (Hagge et al., 2002; Bonstein et al., 2005; Teixeira et al., 2005). In contrast, analysis of the fracture surface can be considered only supporting information, since it alone does not permit accurate differentiation between the repair techniques. The 2 repair materials differed in viscosity, although the chemical composition was similar according to the manufacturer’s information. However, in contrast to other investigations (Bonstein et al., 2005), an influence on the bond strength could not be observed. Conditioning a fractured surface of a temporary crown-and-bridge material with special repair liquids or enamel/dentin adhesives reportedly enhances the bond strength (Hagge et al., 2002). The most important function of a repair liquid is considered to be that it solubilizes and/or causes the base material to swell, penetrates it, and finally leads to chemical bonding with the repair material (Teixeira et al., 2005; Seo et al., 2007), reported as "liquid etching" (Mijovic and Koutsky, 1977)—for example, PMMA can be caused to swell by the application of acetone, chloroform, or monomer (Crumpler et al., 1989; Rached et al., 2004; Seo et al., 2007). The latter method is routinely recommended for the repair of monomethacrylate-based temporary crown-and-bridge materials or denture base materials (Vallittu et al., 1994). The degree to which liquid etching also functions with highly cross-linked bio-polymers is virtually unknown. It has merely been reported that the ability for liquid etching decreases with increasing cross-linking of the surface to be repaired (Shen et al., 1984; Kinloch, 1987). Different mechanisms may contribute to the bond strength between a composite-based temporary crown-and-bridge material and the composite added for repair: (1) a direct chemical bond between the monomers of the repair material applied and the matrix components of the exposed surface; (2) a chemical bond between the repair material and the surfaces of the exposed fillers; (3) micro-mechanical locking due to monomers of the repair material penetrating the finest structures/gaps of the roughened surface (Söderholm and Roberts, 1991; Brosh et al., 1997; Bonstein et al., 2005; Teixeira et al., 2005) and micro-cracks of the fractured surface, respectively (Kinloch, 1987; Seo et al., 2007); and (4) formation of an interpenetrating network between polymer chains of the base material and the repair material, without direct chemical linkage (Rawls, 2003; Hernandez et al., 2004; Powers and Sakaguchi, 2006). Most repair liquids are based on Bis-GMA/TEGDMA mixtures (Brosh et al., 1997). Since an optimal conversion rate can be expected at a Bis-GMA concentration of 50–75 wt%, a Bis-GMA:TEGDMA mixture of 60:40 wt% was selected for the bonding liquid (Lovell et al., 1999). The dilution of this base mixture with MMA and acetone, respectively, aimed to enhance the wetting properties of the liquid by reducing its viscosity (acetone, resp. MMA) and allowing low-molecular-weight, polymerizable components (MMA) to penetrate the roughened surface (Matsumura et al., 1995). None of the test groups attained the cohesive strength, which is in accordance with reports in the literature (Bonstein et al., 2005), although, in numerous cases, cohesive fractures occurred after repairs. It is hypothesized that the structure created by different materials resulted in unfavorable stress distribution at the interface, thus leading to premature cohesive failure within the temporary crown-and-bridge material (Hagge et al., 2002). Where the experimental bonding liquid was used, shear bond strength values were similar to those in the test group which had only been ground, whereas a higher percentage of adhesive fractures was observed in the latter group. In both groups, this was possibly caused by the hydrophobicity of the monomers, which impeded penetration into the smallest cracks and gaps of the water-saturated surface. Wetting phenomena may have also played a role. This hypothesis is corroborated by the fact that fine gaps were observed in the SEM in the transition zone between the temporary crown-and-bridge material and the bonding liquid. Conditioning the surface with MMA led to significantly lower bond strength values compared with those in all other test groups. Therefore, conditioning the surface with MMA, as postulated for PMMA resin-based biomaterials, cannot be recommended for the temporary crown-and-bridge materials used. This raises the question of the cause-and-effect principle for this phenomenon: It has been reported that thermocycling leads to microcracks and degradation phenomena in composites, as reflected by the SEMs. In turn, this results in increased water uptake/solubility (Söderholm et al., 1984; Montes-G. and Draughn, 1986; Gale and Darvell, 1999). The MMA may have combined with residual water in the specimen surface and formed an azeotropic mixture (Lide, 2006), which does not evaporate completely and therefore inhibits bonding to the repair material (Jacobsen and Söderholm, 1995). Where the surface was conditioned with bonding & MMA, low shear bond strength values (< 15 MPa) were observed, although the SEM showed a relatively uniform transition zone. Again, this may be related to the polymerization-inhibiting effect of water entrapped in the surface and/or the formation of a lower load-bearing polymer network triggered by MMA. The vacuoles observed on the SEM may be an indication for this hypothesis. Additionally, it has been reported that combining dimethacrylates (UDMA) with MMA leads to incomplete curing (Matsumura et al., 1995). The highest shear bond strength values were observed with the bonding & acetone liquid, corroborated by the SEM, which showed intimate, gap-free transitions to the temporary crown-and-bridge material. Various mechanisms may have contributed to this phenomenon: (1) Acetone reduces the viscosity of the liquid and thus improves its wetting properties, resulting in intimate interdigitation with the roughened surface (Holmes et al., 2007). Since acetone is a good carrier for monomers (Holmes et al., 2007), it conveys them into the most minute cracks and gaps and, where relevant, even into spaces between polymer chains (Powers and Sakaguchi, 2006). (2) Acetone is a very good solvent and tends to solubilize or to swell PMMA-polymer surfaces (Rached and Del-Bel Cury, 2001; Rawls, 2003). Swelling may also be anticipated with highly cross-linked polymers (Rawls, 2003), as with the temporary crown-and-bridge material used, widening the spaces between the polymer chains. This creates room for monomers to penetrate, which is a prerequisite to the establishment of an interpenetrating network (Hernandez et al., 2004; Powers and Sakaguchi, 2006). (3) Acetone has a water-chasing effect, which is utilized in various dentin adhesives (Finger and Fritz, 1996; Holmes et al., 2007). Owing to its very high vapor pressure, acetone evaporates rapidly, leaving the polymerizable monomers on/in the surface applied (Powers and Sakaguchi, 2006; Holmes et al., 2007). Which of these mechanisms was actually relevant and to what degree could not be determined in the present study. This matter will have to be clarified in further investigations. In addition, it should be mentioned that the shear bond strength was tested after 24 hrs, and therefore a statement as to the long-term stability of the bond cannot be made. This question is currently under observation in ongoing experiments. Since the type of surface conditioning significantly influenced the bond strength, the null hypothesis must be rejected. In summary, repair liquids based on Bis-GMA/TEGDMA with acetone are best suited for conditioning aged surfaces of cross-linked temporary crown-and-bridge materials to enhance bond strength. The shear bond strength values were within the range considered clinically acceptable (Lucena-Martin et al., 2001), whereas the use of MMA as a repair liquid is inappropriate.
We thank Dr. Jürgen Riehl for his assistance with the statistical analysis. In addition, the authors appreciate the materials donated by the respective manufacturers. We thank Dr. Marcus Hoffmann and Mr. Astrit Kastrati (Heraeus Kulzer, Wehrheim, Germany) for supplying the raw materials to fabricate the experimental formulations. Finally, we thank Dr. Martin Hardt (Central Biotechnology Unit, Justus-Liebig-University Giessen, Germany) for his assistance with the SEM analysis. Received for publication July 3, 2008. Revision received September 27, 2008. Accepted for publication November 11, 2008.
Journal of Dental Research, Vol. 88, No. 2,
152-157 (2009)
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