Comparative shear bond strength of some orthodontic bonding resins to enamel

Transcription

1 European Journal of thodontic} 18 (1996) O 1996 European thodontic Society Comparative shear bond strength of some orthodontic bonding resins to enamel L M. Trimpeneers*, R. M. H. Verbeeck**, L R. Dermaut*, and M. G. Moors*** Departments of 'thodontics, **Dental Materials, and ***Nonferrous Metallurgy, University of Gent, Belgium SUMMARY The aim of this study was to compare the in vitro bond strength, to bovine enamel measured in shear, of the orthodontic adhesives Lee Insta-bond (LiB), Rely-a-Bond (RaB), Right-on (Ro), Concise precoating method (Cc), Concise mixed method (Cab), Super-C (Sc), and thon (), and of the glass ionomer cement Ketac-Cem (KC). The fracture surfaces after debonding were also examined in order to determine the sites of failure. The results indicate that there is a significant difference between the shear bond strength obtained with the different adhesives so that the mean shear bond strength decreases in the order {Ro~Cab~Sc}>{LiB~RaB~Cc}>>KC. Moreover, for Cab and Sc it was found that the shear bond strength varies depending on the location on the bovine tooth. The failure site was essentially at the resin-bracket interface, except for Concise, where only 50 per cent of the cases failed at the resin-bracket interface. Introduction Bonding of brackets has become a routine orthodontic procedure in fixed appliance therapy. This technique has considerably reduced the need for orthodontic banding. Various bonding agents were developed after the introduction of the acid etch technique. The first and most popular bonding resins were twopaste, chemical curing bonding systems. As the polymerization starts immediately after mixing, the operator cannot control the setting time, so that bracketing accuracy as well as positioning on the tooth surface are affected (Wilson, 1988). Moreover, hand-mixing can include air bubbles resulting in a weaker bond strength (Wilson, 1988). Next, one-step or no-mix orthodontic bonding materials were developed. A thin coating of liquid activator is applied to the dried etched enamel surface and to the bracket base. The paste is put on the coated bracket base and the bracket is then firmly placed on the enamel surface. The contact between the activator and the bonding paste initiates the polymerization of the resin. Early work on light-cured resins has been published by Tavas and Watts (1979). Most visible light-activated direct bonding materials, however, have recently become commercially available. A rapid polymerization occurs when visible light (usually nm wavelength; Pollack and Blitzer, 1982) is applied, producing a 'command set' which is of great advantage. Such snap-set results in a nearly unlimited working time, thus allowing more accurate bracket placement. The potential risk of enamel surface decalcification can be reduced by using glass ionomer cements for the bonding of brackets. Glass ionomer cements were introduced in 1972 (Wilson and Kent, 1972), primarily as luting agents and as direct restorative material. These cements have the ability to bond chemically to both enamel and dentine (Kent et al, 1973) and the unique property of being able to release fluoride ions (Forsten, 1977; Swartz et al, 1984). Within the last few years several fluoride containing bonding materials have also appeared on the market. Underwood et al (1989) have shown that fluoride added to composite can effectively reduce the demineralization of enamel around orthodontic brackets in vivo. In fixed appliance therapy, the bonding resin

2 90 L. M. TRIMPENEERS ET AL. should not only provide a sufficient adhesion between bracket and enamel, but upon debonding should also result in minimum clean-up and damage to the enamel. The present study compares the in vitro shear bond strength of metallic orthodontic brackets bonded to enamel using various bonding agents and the mode of failure as a function of the bonding system used, i.e. no-mix, two-paste, powder/liquid, and light-activated system. Materials and methods Two-hundred-and-forty metal flat-based Begg brackets on laminated minimesh (TP thodontics Inc., part no La Porte, Ind.) and 80 bovine teeth, divided at random into eight groups, were used for this study. The freshly extracted bovine incisors were stored individually at 5 C in small containers containing a 5 per cent thymol solution prior to use. In order to ensure a planar surface parallel to the direction of the shear forces during testing, um were removed from the labial surface of the incisors by fixing the tooth in a polishing block and grinding the enamel off with a wet silicon carbide polishing paper, grit value P3-P400 (WETORDRY, TRI-M-ITE PAPER-Waterproof). In order to have a standardized bonding area, the resulting planar surface was covered with a perforated tape so that three square areas with a mean (SD) area of 24.1 (1.2) mm 2 were available for the bracket placement. Six commercially available direct bonding resins and one glass ionomer cement were evaluated. The products, codes, type, and manufacturer are listed in Table 1. The enamel surfaces were etched between and 120 s according to the manufacturer's instructions (Table 1) with the etching solution supplied. The etched surfaces were washed in running water for 20 s and dried with oil-free compressed air for another 20 s. In all cases the frosty white appearance of etched enamel was noticed. After application of the bonding agents, the brackets were firmly pushed on the enamel surface withfingertippressure as equally as possible. Excessive resin was then removed. For the evaluation of each orthodontic resin, 10 teeth were used with three brackets placed per tooth. The product Concise was used in two ways. In the Concise precoating method (Cc) the Table 1 Products, codes, type, etchant, etching time, and manufacturer of the orthodontic resins tested. Product Lee Insta-bond Rely-a-Bond Right-on Concise tho Super-C thon Ketac-Cem Code LiB RaB Ro Cc Cab Sc KC Type No-mix system No-mix system No-mix system Two-paste mix system Two-paste mix system Powder/liquid Light activated paste Glass ionomer luting cement Etchant Paste Fluid Fluid Etching time (seconds) Manufacturer Lee Pharamceuticals, 1444 Santa Anita Avenue, SO. El Monte, CA 91733, USA Reliance thodontic Products, PO Box 678, Itasca, IL. 143, USA TP Laboratories Inc., La Porte, IN, USA Dental products/ 3M C, St Paul, MN, USA Dental Products, 3M C, St Paul, MN, USA Amco Manufacturing, Philadelphia, PA, USA thon Dental Inc., 4464, West Saanich Road, Victoria BC, Canada Espe GMBH & C KG, D-8031 Seefeld/Oberbay, Germany

3 SHEAR BOND STRENGTH 91 etched enamel surface was first primed with a mixture of sealant A + B, the mixed pastes A + B were then put on the bracket base and the bracket was placed on the sealed tooth surface. In the Concise mixed method (CaB) the sealants, as well as the pastes were mixed together, put on the bracket base and bonded, without first coating the etched surface. After placement of the brackets, the teeth were stored on a grid on top of a box filled with water at room temperature for 24 h in an attempt to imitate humidity conditions of the oral cavity. For the determination of the shear bond strength, the teeth were mounted in a sample holder of the type described by Surmont et al. (1992); Figures 1 and 2). The brackets were loaded under shear to failure by using an Instron Universal Testing Machine (Instron Ltd., Buckinghamshire, England) with a cross-head speed of 0.5 mm/min. The forces were automatically recorded and converted into the shear bond strength (MN/m 2 ). Figure 2 Prepared sample positioned in apparatus. Figure 1 Photograph of dismantled sample holder, displaying different parts.

4 92 L. M. TRIMPENEERS ET AL. The results were analysed by using Levene's test, a two-factor mixed ANOVA analysis and Newman-Keuls' Multiple-Range test. The fractured test specimens (brackets and teeth) were examined under a stereomicroscope at a magnification x 50 in order to determine the sites of failure. Results The mean value for the shear bond strength (SBS) for the products investigated is summarized in Table 2 as a function of the location on the tooth (incisal, 1; centre, 2; gingival, 3). According to Levene's test the variances on the measurements for the different groups in Table 2 are homogeneous (0.10<P<0.20), i.e. independent of the adhesive or the location. Hence, the pooled estimate for the variance based on all measurements can be calculated as Table 2 suggests that the SBS apparently depends on the adhesive used and on the location on the tooth. This is confirmed by an analysis of variance based on a two-factor mixed design as shown in Table 3. A Newman-Keuls' Multiple-Range test at a significance level P= 0.05 reveals that the shear bond strength does not depend on the location for LiB, RaB, Ro, Cc,, and KC. However, the difference in SBS between location 3 and 1 is significant for Cab and Sc. For these adhesives the bond strength at location 2 differs not significantly from that at location 1 or 3. On this basis a global mean shear bond strength can be calculated for each adhesive studied. The results are represented in Figure 3. A Newman-Keuls' Multiple-Range test at the significance level P= 0.05 shows that the global SBS decreases in the order {Ro~Cab~Sc}>{LiB~RaB~Cc}>>KC This ranking, as well as Figure 3, clearly demonstrates that the bonding strength of is drastically lower than that of the other adhesives studied, but is still greater than that found for the glass ionomer by a factor of 2. Moreover, there is a small but significant difference between Cc and Cab. The quantitative amounts of resin left on the tooth and the bracket were independently determined at least five times. A fracture of the Table 2 Mean shear bond strength (MN/m 2 ) of the adhesives as a function of the location on the tooth. Material Lee Insta-Bond Rely-a-Bond Right-on Concise C Concise ABAB Super-C thon Ketac-Cem Notation LiB RaB Ro Cc Cab Sc KC Location 1 ll.l± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±0.3 Table 3 Results of a two-factor mixed-design ANOVA of the data resulting in Table 2. Source Total Between samples materials error b Within samples location location x material error w ss df ms F P <

5 SHEAR BOND STRENGTH as a particular type of fracture (Joseph and Rossouw, 1990). SBS (MN/m2) Cab Sc LIB RaB Cc KC Figure 3 Mean shear bond strength (SBS) of the orthodontic resins investigated. The horizontal lines group the products whose SBS is not statistically different. enamel or the bracket was not observed on debonding. Moreover, a splitting fracture within the resin material parallel to the enamel surface was never observed. On the basis of these results a classification of the failure mode was performed as follows: A. Adhesive resin/bracket failure (0-33 per cent of the adhesive left on the bracket). B. Adhesive resin/enamel failure ( per cent of the adhesive left on the bracket). C. Mixed failure at resin/bracket and resin/ enamel interface (33-66 per cent of the adhesive left on the bracket). The sites of fracture are shown in Figure 4. More than two-thirds of the fracture had to be evident in anyone of the sites to classify it % failure 0M R/E SR/B Ro Cab Se UB RaB Cc Kc Figure 4 Percentage of adhesive failure after debonding at resin/bracket interface (R/B), at resin/enamel interface (R/E) and at R/B and R/E interfaces (M). Ro Discussion The results of the present study show that with the exception of thon and the glass ionomer there is a small, but significant difference in the bond strengths of presently available materials. The highest shear bond strength is found for the orthodontic adhesives Right-on, Concise mixed method, and Super-C (Figure 3). Of all products involved, Ketac-Cem has the lowest SBS. This glass ionomer cement is a water-hardened formulation in which the polyacrylic acid is freeze-dried and mixed with the alumino-silicate powder. The setting occurs upon mixing the powder with water (McLean et al, 1984). The shear bond strength of glass ionomers to enamel has been previously reported to be per cent of that of composite resins (Coury et al, 1981; Murray and Yates, 1984; Klockowski et al, 1989; Cook and Youngson, 1989). Apart from the lower strength due to their brittleness, glass ionomer cements are sensitive initially to moisture contamination and later to dehydration (Knibbs, 1988; Swift, 1988). On the other hand, they have the advantage of preventing decalcification due to fluoride ion release, and of easier debonding and clean-up procedures (White, 1986). For this reason it has been suggested that Ketac-Cem may be suitable for clinical use in the upper labial segment (Fox et al, 1991). The visible light-cured resin system, thon, shows a bond strength which amounts to approximately one-half of the average shear bond strength found for the chemically cured resins. This reduced SBS may be due to an incomplete polymerization (Greenlaw et al, 1989). This is substantiated by the study of Swartz et al. (1983) which demonstrates an increased polymerization of the visible lightcured resins with time. As has been mentioned above, the authors of this study only applied the manufacturer's instructions. It was not the purpose of this study to evaluate this parameter. However, bracket placement and excess removal were found to be much easier with a lightactivated composite than with an autopolymerizing system (Sonis, 1988). Although the SBS of thon and Ketac-Cem

6 94 L. M. TRIMPENEERS ET AL. is significantly smaller than that obtained with the other bonding systems studied, the question remains which minimum bond strength is required for clinical success. With respect to Concise, the results indicate that the use of the sealant resin separately as a primer does not increase the attachment of the bonding cement; on the contrary, the Concise mixed method (Cab) is significantly stronger than the Concise precoating method (Cc). The results of the present study are in agreement with those previously reported (Mitchem and Turner, 1974; Jorgensen and Shimokobe, 1975; Low et al, 1976; Espinosa, 1978). Therefore, it is suggested that sealant may not be necessary in two-paste orthodontic bonding (Wang and Tarng, 1991). On the other hand, in the absence of a precoating procedure the SBS may depend on the location on the tooth as evidenced by the results with Cab and Sc (Table 2). In view of the small differences found in SBS and the fact that the other adhesives studied show no such dependency, these differences may not be clinically significant. The failure site was essentially at the resinbracket interface, except for Concise, where this only occurred in approximately 50 per cent of the cases. Wang and Tarng (1991) found no statistical difference in the distributions of debonding interface between orthodontic bondings with and without sealants. This was confirmed in our study, where Cc and Cab showed nearly the same adhesive failure at the resin-bracket interface (48.3 versus 51.7 per cent). However, twice as many mixed adhesive failures for Cab and in consequence 10 per cent less failures at the resin-enamel interface, were found. Conclusions The results indicate that there is a significant difference between the shear bond strength obtained with the different adhesives, so that the mean shear bond strength decreases in the order {Ro~Cab~Sc} > {LiB~RaB~Cc} >>KC. Moreover, for Cab and Sc it was found that the shear bond strength varies depending on the location on the bovine tooth. The failure site was essentially at the resin-bracket interface, except for Concise, where only 50 per cent of the cases failed at the resin-bracket interface. Address for correspondence L. R. Dermaut University of Gent Dental School Department of thodontics De Pintelaan 185 B-9000 Gent Belgium References Cook P A, Youngson C C 1988 An in vitro study of the bond strength of a glass ionomer cement in the direct bonding of orthodontic brackets. British Journal of thodontics 15: Coury T L, Miranda F J, Wilier R D, Probst R T 1981 Adhesiveness of glass-ionomer cement to enamel and dentine: a laboratory study. Operative Dentistry 7: 2-6 Espinosa H D 1978 In vitro study of resin-supported internally etched enamel. Journal of Prosthetic Dentistry 40: Forsten L 1977 Fluoride release from a glass ionomer cement. Scandinavian Journal of Dental Research 85: Fox N A, McCabe F J, Gordon P H 1991 Bond strengths of orthodontic bonding materials: an in vitro study. British Journal of thodontics 18: Greenlaw R, Way D C, Galil K A 1989 An in vitro evaluation of a visible light-cured resin as an alternative to conventional resin bonding systems. American Journal of thodontics and Dentofacial thopedics 96: Jorgensen K D, Shimokobe H 1975 Adaptation of resinous restorative materials to acid etched enamel surfaces. Scandinavian Journal of Dental Research 83: Joseph V P, Rossouw F 1990 The shear bond strengths of stainless steel and ceramic brackets used with chemically and light-activated composite resins. American Journal of thodontics and Dentofacial thopedics 97: Kent B E, Lewis B G, Wilson A D 1973 The properties of a glass ionomer cement. British Dental Journal 135: Klockowski R, Davis I, Joynt R, Wieczkoski G, McDonald A 1989 Bond strength and durability of glass ionomer cements used as bonding agents in the placement of orthodontic brackets. American Journal of thodontics and Dentofacial thopedics 96: -64 Knibbs P J 1988 Glass ionomer: 10 years of clinical use. Journal of al Rehabilitation 15: Low T, Lee K W, Von Fraunhofer J A 1976 The direct use of composite materials in adhesive dentistry. British Dental Journal 41: McLean J W, Wilson A D, Prosser H J 1984 Development and use of water-hardening glass ionomer luting cements. Journal of Prosthetic Dentistry 52: Mitchem J C, Turner L R 1974 The retentive strength of acid-etched retained resins. Journal of the American Dental Association 89: Murray G A, Yates J L 1984 A comparison of the bond strengths of composite resins and glass ionomer cements. Journal of Pedodontics 8:

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