Photomedicine and Laser Surgery Volume 32, Number 9, 2014 ª Mary Ann Liebert, Inc. Pp. 512–516 DOI: 10.1089/pho.2014.3732
Effects of Different Surface Treatments on the Bond Strength of Acrylic Denture Teeth to Polymethylmethacrylate Denture Base Material Hakan Akin, DDS, PhD,1 Omer Kirmali, DDS, PhD,2 Faik Tugut, DDS, PhD,1 and Mehmet Emre Coskun, DDS, PhD1
Objective: The purpose of this study was to investigate the effects of various surface pretreatments in the ridge lap area of acrylic resin denture teeth on the shear bond strength to heat-polymerized polymethylmethacrylate (PMMA) denture base resin. Background data: Tooth debonding of the denture is a major problem for patients with removable prostheses. Methods: A total of 84 central incisor denture teeth were used in this study. Seven test groups with 12 specimens for each group were prepared as follows: untreated (control, group C), ground, with a tungsten carbide bur (group H), airborne-particle abrasion (group AA), primed with methyl methacrylate (group M), treated with izobutyl methacrylate (group iBMA), Eclipse Bonding Agent applied (group E), and Er:YAG laser irradiated (group L). Test specimens were produced according to the manufacturers’ instructions and mounted to a universal testing machine for shear testing with a crosshead speed of 1 mm/min. Data were evaluated by one way variance analysis (ANOVA) and Tukey’s test (a = 0.05). Results: Similar bond strength values were found between groups L and M, and these were the highest shear bond strengths among the groups. The lowest one was observed in group E. All surface treatments, except group E, exhibited significant difference when compared with group C ( p < 0.05). Conclusions: Lasing of the ridge lap area to enhance the bond strength of acrylic resin denture teeth to PMMA denture base resin might be an alternative to wetting with MMA monomer. To overcome tooth debonding, surface treatment of the ridge lap area should be performed as part of denture fabrication.
olymethylmethacrylate (PMMA) denture base resin, with its superior properties, has been the most popular choice of clinicians since 1937.1,2 Although acrylic resin teeth have the ability to chemically bond to PMMA denture base resins, tooth debonding comprises 22–30% of denture repairs, especially in the anterior region of the denture.2,3 The causes are related to several factors including remaining wax on the ridge lap area of the teeth because of incomplete elimination or contamination with tin-foil substitutes,4 careless application of the separating medium or insufficient available monomer during processing, the polymerization method used in the processing of denture base resins,3 or force overload during the time in service.5 According to Cunningham and Benington,6 prior to the application of a suitable denture base resin, complete wax elimination of the bonding surface is the most important step for high denture tooth bond strength. Furthermore, the presence of im1 2
purity was found to lower the stress threshold for cracks to propagate, thus increasing the risk of tooth debonding.2 A reliable bond between a denture tooth and a denture base material could be constructed with the contribution of an interpenetrating network (IPN) or a covalent bond, macromechanical retentions, and micromechanical retentions.5 Several attempts have been made by researchers to enhance bond strength between acrylic resin teeth and denture base materials. Mechanical preparation of the tooth surface7–11 etching it with chemicals,12–15 or wetting it with organic solvents16,17 were performed to obtain reliable bond strength. In addition, an IPN or a covalent bond were established by grinding and then wetting the surface of the denture tooth with methylmethacrylate (MMA) monomer.5 Moreover, the surface of the PMMA can be altered by airborne-particle abrasion18–24 and laser irradiation.18,22,25,26 Nevertheless, conflicting results with the use of chemical solvents and mechanical preparation were seen in the literature. Contradicting the results of Papazoglou and Vasilas,27
Department of Prosthodontics, Faculty of Dentistry, Cumhuriyet University, Sivas, Turkey. Department of Prosthodontics, Faculty of Dentistry, Akdeniz University, Antalya, Turkey.
BOND STRENGTH OF DENTURE TEETH
Saavedra et al.,28 and Geerts and Jooste,29 Morrow et al.,30 and Spratley31 advocated that grinding and monomer application on the bonding surface did not significantly improved the bond strength. Furthermore, there is no study that evaluates effects of different types of surface pretreatments on the bond strength of acrylic resin denture teeth to PMMA denture base materials. Therefore, it was the aim of this study to investigate the effects of various pretreatment protocols in the ridge lap area of acrylic resin denture teeth on the shear bond strength to conventional heat-polymerized PMMA denture base resin. It was hypothesized that the bond strength of a denture tooth to the base resin is independent of the surface pretreatment of the tooth’s ridge lap area.
FIG. 1. Complete specimen used in this study for shear bond strength testing.
Methods Preparation of denture tooth specimens
The commercially available 84 central incisor denture teeth (Samet; Pharma Zahnfabrik) were used for all test groups in this study. Around the teeth, the ridge lap surfaces that remained intact were ground with a tungsten carbide bur (H129FSQ; Brasseler USA) at *40,000 rpm with air coolant, to arrange bonding surfaces in a 6 mm diameter by using a digital caliper (Altas 905; Gedore-Altas) with a – 0.03 mm tolerance. Subsequently, the bonding surfaces of the teeth were thoroughly cleaned with air/water spray and ethanol (70%). Seven test groups with 12 specimens for each group were prepared as follows: Group C: No treatment. Served as control. Group H: The ridge lap surfaces of teeth were modified by grinding with a tungsten carbide bur (Edenta AG, Hauptstrasse 7, Switzerland) at a speed of 15,000 rpm (K9, KaVo EWL, Leutkirch, Germany) for 10 sec. All grinding procedures were performed by the same operator. Group AA: Airborne-particle abrasion (Ney; Blastmate II) was performed on the ridge lap surfaces of teeth with 120 lm aluminum oxide (Al2O3) particles at 2 bar for 10 sec. Application distance was set at 10 mm with a special holder. After airborne-particle abrasion, for removing remnants, the specimens were placed under running tap water and then dried with oil-free compressed air; Group M: The bonding surfaces of teeth were primed with a MMA monomer by using microbrushes for 30 sec. Group iBMA: Izobutyl methacrylate (iBMA) was applied on the bonding surfaces of teeth by using microbrushes in a single layer for 30 sec. Group E: A thin layer of Eclipse Bonding Agent (Eclipse; Dentsply) was performed on the bonding surfaces of teeth by using microbrushes for 30 sec. Group L: Bonding surfaces of teeth were irradiated with an Er:YAG laser (Smart 2940D Plus; Deka Laser) in pulse mode with a wavelength of 2.94 lm, under water spray at a rate of 5 mL/min. A 4 mm diameter titanium articulated arm transmission system was aligned perpendicular to the bonding surfaces of teeth at 10 mm distance, and scanned the whole bonding area for 20 sec with a repetition rate of 10 Hz/300 mJ, output power of 3 W, and pulse duration of 700 ls.
Preparation of shear bond strength test specimens
Rectangular wax specimens (36 · 12 · 5 mm3) were prepared, and teeth were fixed to them. According to the manufacturer’s instructions, the heat-polymerized PMMA (Meliodent; Bayer Dental) specimens were processed in the molds in denture flasks. After polymerization, the specimens were stored in distilled water at 370C for 48 h before the test. Eventually, 84 specimens were obtained for shear testing (Fig. 1). Shear bond strength test
The specimens were subjected to the custom jig of a universal testing machine (Lloyd LF Plus; Ametek Inc, Lloyd Instruments) with a constant crosshead speed of 1 mm/min. A schematic diagram of a test specimen can be seen in Fig. 2. The maximum force value (in MPa) to produce fracture was noted for each specimen. Furthermore, to evaluate fracture pattern, failure types were analyzed by a stereomicroscope (SMZ 800; Nikon) at 40 · magnification for every specimen, and classified as either adhesive (total separation at the interface between the denture tooth and base material), cohesive (full break in the base material or tooth), or both (mixed) (Fig. 3). All fracture observations were conducted by one person. One way ANOVA (SPSS 13; SPSS Inc.) and Tukey post-hoc tests (a = 0.05) were performed on data to analyze the shear testing results. Results
The results of the one way ANOVA (df = 6, MS = 91.623, F = 31.577, p < 0.001) was presented in Table 1. One way ANOVA showed that group L had the highest bond strength value, followed by group M. No significant difference was found between groups L and M ( p = 0.285), whereas these two groups exhibited significant difference when compared with all other groups ( p < 0.05). Moreover, the lowest shear bond strength value was found in group E. For the comparisons performed with Tukey’s honestly significant difference (HSD) test, there was a significant difference in shear bond strength between group C and groups H, AA, M, iBMA, and L ( p = 0.017, p = 0.025, p < 0.001, p = 0.005, and
AKIN ET AL.
Table 1. Mean Shear Bond Strength Values with Standard Deviations Groups
Group Group Group Group Group Group Group
12 12 12 12 12 12 12
8.79a 11.16b 11.03b 14.47c 11.42b 8.51a 16.03c
1.11 1.12 1.97 2.15 2.14 .84 1.98
C H AA M iBMA E L
Means with the same superscript are not significantly different ( p > 0.05).
in groups AA, M, and L mixed failure was the predominant failure mode (83%, 66%, and 66%). Moreover, cohesive failures were observed in groups M and L (16% and 33%). Discussion
Schematic diagram of shear testing.
p < 0.001). Furthermore, no significant difference was observed in bond strength between groups C and E ( p = 1). In addition, results of the failure types were demonstrated in Table 2. Adhesive failures were predominantly detected for groups C, iBMA, and E (66%, 75%, and 83%). Nevertheless,
This study hypothesized that the bond strength of a denture tooth to the base resin is independent of the surface pretreatment of tooth’s ridge lap area. Summarizing the results requires rejection of the null hypothesis, because not only grinding and MMA priming, but also air-borne particle abrading, iBMA wetting, and lasing of the denture teeth significantly influenced the bond strength values. Conflicting results with modification of the ridge lap area were seen in the literature. Consistent with the results of the current study, Can and Kansu8 found that the grooves on the tooth’s ridge lap area produced a stronger bond. Similarly, Vallittu9 obtained the highest bond strength by grinding grooves on an acrylic resin tooth’s ridge lap area. In the present study, grooves were not made on the bonding surface; however, it was ground with tungsten carbide bur to obtain macroretentive areas. However, Huggett et al.10 and
FIG. 3. Failure types of fractured specimens. (A) Adhesive failure. (B) Mixed failure. (C) Cohesive failure.
BOND STRENGTH OF DENTURE TEETH
Table 2. Failure Type Distributions of Groups for Each Specimen Groups
Group Group Group Group Group Group Group
12 12 12 12 12 12 12
8 6 2 2 9 10 —
— — — 2 — — 4
4 6 10 8 3 2 8
C H AA M iBMA E L
Cardash et al.11 advocated that modification of the ridge lap area by grinding or cutting retention grooves did not alter the bond strength of denture tooth to the base resin. On the other hand, Chung et al.24 reported that airborneparticle abrasion resulted in severe irregularities and undercuts on the bond surface in scanning electron micrographs, and in accordance with the present study, it was effective for strengthening the bond between denture tooth and denture base resin. Nevertheless, they also found that roughening the ridge lap area with grinding did not provide bond strength compared with untreated surface. This result is contradicted by the present study. In addition, 120 lm particles were used in the present study because they were exhibited as being the best size for roughening PMMA denture base resins compared with 50 or 250 lm.19 MMA monomer priming is another attempt that has been made by researchers to enhance bond strength between acrylic resin teeth and denture base materials. It was thought that monomer priming dissolved part of the PMMA of the tooth and enabled free double bonds that might copolymerize with the PMMA of the denture base resin.3 Nevertheless, it was also exhibited that monomer dissolved the surface of the tooth and provided a durable secondary semi-IPN structure.3 Therefore, in agreement with the present study, most of the researchers demonstrated that monomer priming to the ridge lap area of tooth surface before packing the resin significantly improved bond strength.3,4,6,13,14,27–29 However, this contradicted the studies of Morrow et al.30 and Spratley.31 Only a few studies have been conducted on the treatment of acrylic resin denture base materials with chemical materials including iBMA. Leles et al.15 reported that application of iBMA on bonding surfaces of denture base material did not significantly improved bond strength of hard chairside reline resin compared with an untreated group, whereas in the present study, iBMA priming was found to be an effective method for locking teeth to denture base resin. Methacrylate monomer with high numbers of alkyl groups could interact with C-H groups and establish hydrogen bonds. On the other hand, Akin et al.32 demonstrated that surface treatment of Eclipse Bonding Agent resulted in a significant improvement in bond strength between urethane dimethacrylate (UDMA) denture base resin and PMMA denture teeth. Therefore, Eclipse Bonding agent was used in the present study. Eclipse Bonding Agent consists of methyl acetate, and the molecular weight of methyl acetate is lower than that of MMA. Therefore, theoretically, this manipulation should benefit the bond site and result in stronger bonds by penetration of methyl acetate into the denture network. However, application of Eclipse
Bonding Agent on the tooth’s adhesive surface had an adverse effect on the bond. This result can be explained. The network between methyl acetate and PMMA was not constructed in the same way as the network between methyl acetate and urethane-based oligomers. In addition, methyl acetate adhesive can be ineffective for dissolving and roughening the PMMA denture base resin surface. Laser irradiation of the adhesive surface was found to be ineffective in improving bond strength of acrylic resin denture teeth to UDMA denture base resin.32 Nevertheless, modification of ridge lap area can be achieved by lasing. Akin et al.26 demonstrated that increased tensile bond strength of the soft lining material to UDMA resin may be achieved by laser irradiation of the bonding surface. Furthermore, amending the adhesive surface of PMMA by Er:YAG laser was found to significantly improve the bond strengths in PMMA/soft liner specimens.19 In the present study, Er:YAG laser irradiation on the ridge lap area was found to be an effective surface treatment method to enhance the bond strength between denture teeth and base resin. This result contradicted that of Akin et al.32 However, this could arise from the difference in the denture base materials used in these studies. Moreover, Tugut et al.25 demonstrated that adhesive interface pretreatment of PMMA with Er:YAG laser at 10 Hz, 3 W, and 300 mJ with a long pulse duration (700 ls) was effective for providing mechanical interlocking. Therefore, the parameters of Er:YAG laser irradiation were based on Tugut et al.25 in the current study. When failure patterns were assessed, it was seen that there was a correlation between bond strength values and failure types. Cohesive failure types were seen only in groups M and L; 16.6% of adhesive failure was seen in group M, whereas group L specimens did not show any adhesive failure type. It was determined that the higher the bond strength value, the higher the percentage of mixed or cohesive failures. Nevertheless, contrary to this correlation, group AA specimens predominantly exhibited mixed failure. One of the limitations of this study was that in vivo conditions were not simulated. Short-term water storage was used in the present study. Cyclic loading as well as human chewing patterns and thermocycling effect were not evaluated in the present study. Therefore, future investigations should focus on overcoming the limitations of in vitro tests by performing an intraoral evaluation of bond strength between denture teeth and base resin. Clinicians or dental laboratory technicians should at least consider ridge lap modification by tungsten carbide bur grinding or MMA monomer priming to accomplish debonding of denture tooth, because these are easy processes. Conclusions
Within the limitations of this study, surface treatment on the ridge lap area of denture tooth before packing denture base resin should be performed as part of denture fabrication. MMA monomer priming or lasing of the ridge lap area of denture teeth were the best choices for enhancing the bond strength of denture teeth to denture base resin. Author Disclosure Statement
No competing financial interests exist.
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Address correspondence to: Hakan Akin Faculty of Dentistry Department of Prosthodontics Cumhuriyet University 58140 Sivas Turkey E-mail: [email protected]