Repair of fractured denture bases is an

REPORT MICROTENSILE BOND STRENGTH OF RESIN-RESIN INTERFACES AFTER 24-HOUR AND 12-MONTH SOAKING Curry Leavitt; Kenneth G. Boberick, DMD; Sheldon Winkler, DDS Evaluate the bond strengths of denture base-repair materials to minimize recurrent failure rate. Use microtensile bond strength (ltbs) testing to evaluate the interfacial bonding strength of 6 commercial denture repair materials after 24-hour and 12-month soaking. Blocks of poly(methyl metacrylate) (PMMA) and Triad were fabricated, fractured, and repaired. Twenty bars (1 3 1 3 30 mm) per group were sectioned from each block parallel to the long axis and ;908 to the resin-resin repair interface and stored before ltbs testing in a servo-hydraulic tensile-testing machine. Intact PMMA and Triad bars that had been soaked for 24 hours and 12 months were tested for reference. The 24- hour repair strengths for PMMA ranged from 52% to 84% of original strength. Soaking for 12 months resulted in a 20% decrease in strength for the PMMA control. The 12-month repair strengths for PMMA ranged from 43% to 74% of the 12-month soaked material strength. Triad repair tested 35% of original strength after soaking for 24 hours. Permabond (cyanoacrylate) to PMMA tested 47% of original strength after 24 hours of soaking and 26% of the 12-month soaked material strength. Permabond to Triad tested 30% of original strength after 24 hours of soaking. 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. The ltbs approach can be used to analyze the resin-resin interface of repaired acrylics. The relatively small standard deviations make the ltbs approach attractive. In this study, ltbs was used to evaluate the repair strength of 6 denture repair materials enabling clinicians to make clinical judgments regarding the strongest repair bond available. Key Words: microtensile bond strength; denture base repair; resin-resin interface strength INTRODUCTION Curry Leavitt, is a senior dental student at Temple University School of Dentistry. Kenneth G. Boberick, DMD, is an associate professor of restorative dentistry at Temple University School of Dentistry, 3223 North Broad Street, Philadelphia, PA 19140. Address correspondence to Dr. Boberick (e-mail address: KBoberick@ dental.temple.edu). Sheldon Winkler, DDS, is an adjunct professor of dentistry at the College of Dental Medicine, Midwestern University, Glendale, Arizona. He formerly served as professor and chairman of the Department of Prosthodontics at Temple University School of Dentistry. 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 resinresin 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 310 Vol. XXXIII/No. Five/2007

Curry Leavitt et al 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 McKinney 11 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 al 12 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 (ltbs) 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. MATERIALS AND METHODS 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 3 12 3 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 (758 C held at 20 psi for 20 min) or benchtop 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 3 1 3 30 mm) per group were sectioned from each block parallel to the long axis and ;908 to the resin-resin repair interface and stored in distilled water (24 hours and 12 months, wet, 38.58 C) before ltbs testing in a servo-hydraulic tensile-testing machine (Instron Model 5569; Instron, Norwood, Mass) cross head speed ¼ 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. RESULTS Differences among the group means (6SD, 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 Journal of Oral Implantology 311

MICROTENSILE BOND STRENGTH OF RESIN-RESIN INTERFACES TABLE 1 Product name, manufacturer, and curing method for materials tested* Name Company Curing method PMMA Triad Permabond 910 Permabond LLC (Somerset, NJ) Cc x x Triad bonding agent Dentsply International (York, Pa) Cc x Acraweld repair material Henry Schein / Zahn Dental (Melville, NY) Cc x Pc x Repair resin Hygenic Coltene Whaledent (Cuyahoga Falls, Ohio) Cc x Lucitone repair material Dentsply International (York, PA) Cc x Pc x Tru repair Bosworth (Skokie, Ill) Cc x *Cc indicates cold curing; Pc, pressure curing; PMMA, poly(methyl metacrylate); x, testing combinations. 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. DISCUSSION 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 crosssectional areas used would improve stress distribution during testing. In this study both pressure cure and cold materials were used; however, the reported denture distortion 4 and complicated working process makes the heat-curing techniques unpopular. Repair material should not deteriorate in the aqueous oral environment, and crazes should not TABLE 2 Fracture strength data of the repair products after 24 hours and 12 months of soaking given in MPa and as a percent of control strength* 24-Hour Soaking (PMMA and Triad) 12-Month Soaking (PMMA only) Repair Material PMMA Triad Repair Strength as a % of Original Material Strength PMMA Repair Strength as a % of 12-Month Soaked Material Strength Control 43 6 6(Pc) 516 10 (Cc) 35 6 5 (Pc) Permabond 910 20 6 6(Cc) 156 4 (Cc) PMMA 47% Triad 30% 9 6 3 (Cc) 26% Triad 18 6 7 (Cc) 35% Acraweld 27 6 8 (Cc) 63% 26 6 4 (Cc) 74% Acraweld 36 6 8 (Pc) 84% 15 6 4 (Pc) 43% Repair (Hygenic) 36 6 8 (Cc) 84% 21 6 2 (Cc) 60% Repair (Lucitone) 22 6 6 (Cc) 52% 18 6 2 (Cc) 51% Repair (Lucitone) 34 6 7 (Pc) 80% 20 6 2 (Pc) 57% Tru Repair (Bosworth) 36 6 11 (Cc) 84% 22 6 3 (Cc) 63% *Cc indicates cold cure; Pc, pressure cure; PMMA, poly(methyl metacrylate); ltbs, microtensile bond strength. 312 Vol. XXXIII/No. Five/2007

Curry Leavitt et al TABLE 3 Fracture mode of the repaired denture base specimens* Repair Material 24-Hour Soaking (PMMA and Triad) 12-Month Soaking (PMMA only) PMMA Triad PMMA Permabond 910 A A A Triad A Acraweld (Cc) A þ D A þ D Acraweld (Pc) A þ D A þ D Hygenic Repair A þ D A þ D Lucitone Repair (Cc) A þ D A þ D Lucitone Repair (Pc) A þ D A þ D Bosworth Tru Repair (Cc) A þ D A þ D *Cc indicates cold cure; Pc, pressure cure; PMMA, poly(methyl metacrylate); A, adhesive failure at the denture base-repair-material interface; B, cohesive within denture base; C, cohesive within the repair material; D, complex cohesive failure within the repairmaterial interface (between the denture base and repair material). 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 ltbs 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 3 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 3 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. CONCLUSION The ltbs method can be used to analyze the resinresin 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 Journal of Oral Implantology 313

MICROTENSILE BOND STRENGTH OF RESIN-RESIN INTERFACES 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 ltbs approach attractive. NOTE Supported by a grant from the Myerson Tooth Corp. Chicago Il. REFERENCES 1. Smith DC. The acrylic denture mechanical evaluation of mid-line fracture. Br Dent J. 1961;110:257 267. 2. Hargreaves AS. The prevalence of fractured dentures. Br Dent J. 1969;126:451 455. 3. Anderson JN. Applied Dental Materials. 5th ed. Oxford: Blackwell Scientific; 1976:269 274. 4. Andreopoulos AG, Polyzois GL. Repair of denture base resins using visible light-cured materials. J Prosthet Dent. 1994;72:462 468. 5. Beyli MS, von Fraunhofer JA. Repair of fractured acrylic resin. J Prosthet Dent. 1980;44:497 503. 6. Ward JE, Moon PC, Levine RA, Behrendt CL. Effect of repair surface design, repair material, and processing method on the transverse strength of repaired acrylic denture resin. J Prosthet Dent. 1992;67:815 820. 7. Ishigamik K, Maeda R. Basic studies on visible light-cured resin as a denture base. Part 17: Transverse and tensile strengths of repaired denture base resin using a trial repair resin. J Nihon Univ Sch Dent. 1993;35:36 42. 8. Ishigamik K, Shirane M, Aoyama Y. Basic studies on visible light-curing resin as a denture base. Part 4: Its strength in the repair of fractured parts of heat-curing denture base resin. J Nihon Univ Sch Dent. 1986;28:287 293. 9. Fellman S. Visible light-cured denture base resin used in making denture with conventional teeth. J Prosthet Dent. 1989;62: 356 359. 10. Lin CT, Lee SY, Tsai TY, Dong DR, Shih YH. Degradation of repaired denture base materials in simulated oral fluid. J Oral Rehabil. 2000;27:190 198. 11. Wu W, McKinney JE. Influence of chemicals on wear of dental compositions. J Dent Res. 1982;61:1180 1183. 12. Lee SY, Greener EH, Mueller HJ, Chiu CH. Effect of food and oral simulating fluids on dentin bond and composite strength. J Dent. 1994;22:352 359. 13. Shono Y, Terashita M, Pashley EL, Brewer PD, Pashley DH. Effects of cross-sectional area on resin-enamel tensile bond strength. Dent Mater. 1997;13:290 296. 14. Craig RG, Powers JM. Restorative Dental Materials. 11th ed. St Louis, Mo: Mosby; 2002:665. 15. Ogle RE, Sorensen SE, Lewis EA. A new visible light cured resin system applied to removable prosthodontics. J Prosthet Dent. 1986;56:497 506. 16. Shifman A. Clinical applications of visible light cured resin in maxillofacial prosthetics. Part 1: denture base reline materials. J Prosthet Dent. 1990;64:578 582. 17. Stipho HD, Talic Y, Assery M. Transverse strength of various resin joints repaired with visible light cured reline material. Saudi Dent J. 1999;11:23 29. 18. Al-Mulla MA, Huggett R, Brooks SC, Murphy WM. Some physical and mechanical properties of a visible light activated material. J Dent Material. 1988;4:197 200. 19. Ping Chaing BK. Polymers in the service of prosthetic dentistry. J Dent. 1984;12:203 214. 314 Vol. XXXIII/No. Five/2007