Microtensile Bond Strength of Resin-Resin Interfaces After 24-Hour and 12-Month Soaking

Repair of fractured denture bases is an office emergency frequently encountered in dental practice. In-house repair may be the only solution available because of time constraints and patient demands. Maximizing bond strength at the resin-resin interface is vital to prevent repeat fractures. The recurrent rate of fracture has been reported to be as high as 19.5% to 21.3% in all denture cases,1^,2 which indicates that additional studies are needed in the area of denture base repair.

Published studies analyzing factors related to strength of denture base repair have examined materials used for repair,3^,4 joint surface contour,5,^–7 joint surface gap,7,^–9 and deleterious effects of oral fluids.4^,10 Wu and McKinney11 reported that the oral environment contributed to the in vivo degradation of composite materials. The proposed surface damage was caused by the chemicals in the oral environment, which softened and eliminated portions of the polymer matrix. It is assumed by these same conditions that denture base repairs and oral fluids found within the mouth will have a degrading effect.

To simulate the oral environment, repair samples were soaked in distilled water for 24 hours and 12 months. Intra-oral conditions are more complex than the laboratory model that simulates the oral environment with the use of distilled water; however, using distilled water as a soaking material will also cause a negative effect on the repaired interface and denture base itself. Swelling results from the process of diffusion, in which the storage media penetrates the matrix and expands the opening between polymer chains.10 The study by Lee et al12 found that diffusion of moisture through resin may also lead to initiation and propagation of microcracks at the interface and through the resin. It is assumed that this action could further widen the path for diffusion. This process could create a path and a reservoir for agents to further penetrate into the repaired material and result in lessened tensile strength.

Laboratory test methods frequently use shear, tensile, and 3-point flexural load testing as a means to analyze the bond strength at the resin-resin interface. The microtensile testing method is routinely used to measure bond strength of resin enamel and resin dentin because the small cross-sectional areas used in microtensile testing are believed to improve stress distribution during testing.13 Large cross-sectional areas are presumed to have a higher probability of containing flaws and air bubbles that may act as stress raisers during testing. Published data on the use of microtensile testing for testing the bond strengths of denture resin-resin is limited. Applying this principle to resin-resin interfaces, may be a viable method for testing denture resin-resin “interfacial strengths.”

The purpose of this study is to use microtensile bond strength (μTBS) testing to evaluate interfacial bonding of 6 commercially available denture repair kits to poly(methyl metacrylate) (PMMA) (Lucitone 199, Dentsply International) and Triad (Dentsply International) visible light polymerized (VLP) denture resin bases after soaking for 24 hours and 12 months. It was postulated that using the microtensile testing method in this study would allow for the development of more uniform stressing within the resin-resin bonds so that they would fail adhesively. Fracture samples were analyzed under light microscopy to determine mode of failure.

Methylmethacrylate-based, heat-cured acrylic resin blocks of PMMA (Lucitone 199) were fabricated using the conventional compression mold technique and processed according to manufacturer's directions. Strips of Triad VLP resin (6 × 12 × 32 mm) were cut from a sheet form, adapted to a mold, and light cured in a Triad oven for 15 minutes. The blocks of PMMA were fractured, and the repair area was widened to approximately ¼ inch, and repaired either under pressure (75° C held at 20 psi for 20 min) or bench-top cold cure for 1.5 hrs using 6 commercially available repair products. The blocks of Triad materials were fractured and Triad bonding agent placed before repair. Repairs using Permabond 910 (Permabond LLC, Somerset, NJ), a cyanoacrylate product, followed manufacturer's directions regarding denture surface preparation. Table 1 lists the materials and testing conditions used in the study.

Twenty bars (1 × 1 × 30 mm) per group were sectioned from each block parallel to the long axis and ∼90° to the resin-resin repair interface and stored in distilled water (24 hours and 12 months, wet, 38.5° C) before μTBS testing in a servo-hydraulic tensile-testing machine (Instron Model 5569; Instron, Norwood, Mass) cross head speed = 1 mm/min, Bencor Multi-T jig). Intact PMMA bars that had been soaked for 24 hours and 12 months were tested for reference. Intact Triad bars that had been soaked for 24 hours were tested for reference. The condition of the fracture surface was examined under light microscopy to determine the mode of fracture.

Differences among the group means (±SD, MPa), and repair strength as a percent of original material strength and soaked strength are reported in Table 2. Repair strengths for PMMA after 24-hour soaking ranged from 52% to 84% of original strength. The pressure-cured materials from Acraweld and Lucitone and the cold-cured materials from Hygenic and Bosworth tested strongest after 24 hours, ∼80–84% of the original material strength. Triad repair tested 35% of original strength after 24-hour soaking. Soaking for 12 months resulted in a 20% decrease in strength for the PMMA control. Repair strengths for PMMA after 12 months of soaking ranged from 43% (pressure-cured Acraweld) to 74% (cold-cured Acraweld) of the 12-month soaked material strength. Permabond 910 to PMMA tested 47% of original strength after 24 hours of soaking and 26% of the 12-month soaked material strength. Permabond 910 to Triad tested 30% of original strength after 24 hours of soaking. Permabond 910 showed a 100% adhesive mode of failure. All other materials tested exhibited either an adhesive mode of failure at the denture base-repair-material interface or a complex cohesive failure within the repair-material interface (between the denture base and repair material). Soaking seemed to cause an increase in adhesion failure. Conditions of fracture are reported in Table 3. A relatively small standard deviation was reported for all groups.

Repairing a resin denture base should produce a denture with the physical and mechanical properties as close to the original material as possible. In general, the transverse strength of a heat-cured repair is about 80% of the original material, and the transverse strength of a chemically cured repair is approximately 60% of the original material.14 Transverse strength is measured using 3-point bending flexural test.

In this study differences in the strength of the final repairs were noted, and these repair values fall within the acceptable range of repair values for PMMA and Triad. Factors affecting the repair of a denture base include the cure condition and choice of materials. Factors affecting strength may include size and number of voids within the bond and percent conversion of monomer during polymerization. Microtensile testing was used to measure the repair bond strength with the hypothesis that the small cross-sectional areas used would improve stress distribution during testing. In this study both pressure cure and cold materials were used; however, the reported denture distortion4 and complicated working process makes the heat-curing techniques unpopular.

Repair material should not deteriorate in the aqueous oral environment, and crazes should not form as a result of attack by solvents present in food, liquids, or medications. Water attacks the material and hydrolyses the bonds, resulting in reduced strength. As a function of storage time, a significant decrease in tensile strength was noticed (P < .05). The best product after 12 months of soaking was Acraweld. The strength reached 74% of the 12-month soaked strength.

This in vitro study evaluated the μTBS of several commercially available denture resin repair materials. However, it did not simulate the clinical condition ideally and did not accurately address the variables of accuracy of fit after repair or the changes in horizontal or vertical dimensions that may lead to ill-fitting, uncomfortable dentures that make them more prone to refracture. Further investigation is necessary to evaluate the bonding under more closely simulated clinical conditions.

The Triad VLP resin system was tested after a 24-hour soak. The superior strength, ease of fabrication, and ease of manipulation of Triad, in addition to its short polymerization time, have led to useful applications.9^,15,^–17 But Triad VLP denture bases have a brittle nature and a very low impact to resistance.18 In addition, they rely on adhesion to hold their broken parts together. In this study Triad repair strengths after a 24-hour soaking were 35% of the original material strength and demonstrated an adhesive mode of failure. Because adhesive materials are subject to hydrolysis by saliva byproducts and water,19 a further decrease in repair strength would be expected for the Triad material after prolonged soaking; however, these tests were not completed. A similar manner of failure through hydrolysis of the cyanoacrylate (Permabond 910) material is suspected.

From a clinical perspective the overall performance of the repair joint, as measured by the fracture strength, is important. The mode of fracture—adhesive, cohesive, or combination—prove valuable in the research and development of newer bonding agents and repair techniques. In this study, the repair joint preparation was standardized for the PMMA materials, therefore, any effects from the preparation method should have the same order of magnitude influence on all the PMMA test results. Triad and Permabond followed manufacturer's directions regarding surface preparation. Permabond and Triad showed a 100% adhesive mode of failure. All other materials tested exhibited either an adhesive mode of failure at the denture base-repair material interface or a complex cohesive failure within the repair-material interface (between the denture base and repair material).

Ideally, the bond strength of the repair should be as close to the strength of the original base material as possible. The strongest bond available would therefore be the most desirable. Some materials tested lower than expected, which may be explained by the fact that microtensile testing uses 1 mm × 1 mm samples fabricated from one or two larger sample blocks. Errors in fabrication of the large sample, such as poor resin penetration and reduced cross-linking between resins, would be manifest in all of the smaller test samples from that particular sample. When using microtensile testing it may be prudent to fabricate several large sample blocks and use a representative number of 1 mm × 1 mm test samples from each block for testing. Possible errors during fabrication can then be detected and traced to a specific sample if a large variance is noted during the microtensile testing.

The μTBS method can be used to analyze the resin-resin interface of repaired acrylics. Repair strengths for PMMA after a 24-hour soaking ranged from 52% to 84% of original strength, which falls within the reported range of repair strengths for denture resins. Triad repair tested 35% of original strength after a 24-hour soaking. A 12-month soaking resulted in a 20% decrease in strength for the PMMA control. Repair strengths after a 12-month soaking ranged from 44% to 76% of the 12-month soaked material strength. Permabond to PMMA tested 47% after a 24-hour soaking and 26% after 12 months. Permabond to Triad tested 30% of original strength after a 24-hour soaking. Permabond and Triad exhibited a 100% adhesive failure. All other materials tested exhibited either an adhesive mode of failure at the denture base-repair material interface or a complex cohesive failure within the repair-material interface (between the denture base and repair material). The relatively small standard deviation makes the μTBS approach attractive.

Supported by a grant from the Myerson Tooth Corp. Chicago Il.