Direct bonding of orthodontic brackets has been advocated since the late 1960s.1, 2, 3, 4, 5 With the introduction of photosensitive (light-cured) restorative materials in dentistry, various methods have been suggested to enhance the polymerization of the materials used, including layering and more powerful light-curing devices. In addition, other factors can potentially contribute to the bond strength between the enamel and the orthodontic bracket, including type of enamel conditioner, acid concentration, length of etching time, composition of the adhesive, bracket base design, bracket material, oral environment, and clinician's skill.6, 7, 8, 9, 10, 11, 12, 13, 14
Two critical factors that affect bond strength of adhesives are the times and the conditions at which testing is performed after bonding. Ferracane et al15 evaluated the long-term effect of aging in water at 37°C on the physical properties of composites for 1 day and 6, 12, and 24 months. They found a 20%-30% reduction in fracture toughness of the composites after 6 months with very little change thereafter.
Ruse et al16 investigated the bond strength after cyclic shear loading test of cylinders of light-cured hybrid composite resin (Scotch Bond Multi Purpose) bonded to flattened enamel surfaces of human teeth at 1 hour and 1, 7, and 30 days. They found a significant increase in the shear bond strength between 1 hour and 24 hours. By the seventh day, there was a significant decrease that was maintained by day 30.
Because orthodontic adhesives are routinely subjected to thermal changes in the oral cavity, it is important to determine whether these temperature variations introduce stresses in the adhesive that might influence its bond strength. Therefore, any new adhesive should be tested both at 24 hours and after thermal cycling. According to Gale and Darvell,17 it is difficult to measure the routine limits of temperature changes introduced by eating and drinking. This is because these activities are essentially erratic habits, and significant variations occur between subjects and within the same person depending on the location in the same mouth. They suggested that air temperature, humidity, and air velocity when breathing can also radically alter resting mouth temperature.17 Gale and Darvell17 pointed to the lack of agreement and standardization between the various thermocycling studies they reviewed.
Because of these difficulties, the International Organization for Standardization has suggested specific criteria for conducting such tests to enable investigators and industry to interpret and compare results.18
More recently, a new adhesive was introduced that does not require the use of a primer or a curing light during bonding. In a recent study, Örtendahl and Örtengren19 compared the bond strength of a cyanoacrylate adhesive with 8 other adhesives. They found that after 24 hours the new adhesive performed as well as or better than the composite resins used for bonding both metal and plastic brackets. The cyanocrylate adhesive has not been tested under conditions partially simulating the oral environment.
The purpose of this study was to compare the effects of thermocycling on the shear bond strength of a cyanoacrylate adhesive, specifically (1) after 24 hours of conventional storage in water when it has achieved most of its bond strength, and (2) after thermocyling between 5°C and 55°C to simulate some of the temperature changes in the oral cavity.
Material and methods
Forty freshly extracted human molars were collected and stored in a solution of 0.1% (weight/volume) thymol. The criteria for tooth selection included intact buccal enamel, not subjected to any pretreatment chemical agents (eg, hydrogen peroxide), no cracks from the pressure of the extraction forceps, and no caries.
The teeth were cleaned and then polished with nonfluoridated pumice and rubber prophylactic cups for 10 seconds.
Maxillary central incisor brackets were used to bond all teeth (Victory Series, 3M Unitek, Monrovia, Calif). The average surface area of the bracket base was determined to be 11.7 mm2.
Smartbond adhesive (Gestenco International, Göthenburg, Sweden) contains ethyl-cyanoacrylate. The bonding procedure followed the manufacturer's instructions. A 35% phosphoric acid etch was applied to the enamel for 10 seconds, and the teeth were washed thoroughly for 20 seconds and air dried. A moist cotton roll was used to wet the enamel surface before the adhesive was applied.
The manufacturer recommends 2 methods of applying the adhesive to the bracket base—either directly from the syringe containing the adhesive or with a microbrush. In this study, the brush method was used because it allowed for the controlled application of a uniform thickness of the adhesive on the bracket base.
Each bracket was subjected to a compressive force of 300 g with a force gauge (Correx Co, Bern, Switzerland) for 10 seconds; then excess bonding resin was removed with a sharp scaler. The teeth were placed in distilled water.
Until the cyanoacrylate adhesive is placed on the wet enamel surface, it will not readily set. Once it comes into contact with the wet enamel surface, the clinician has 3 to 5 seconds to adjust the placement of the bracket before the adhesive starts to set. According to the manufacturer, the adhesive will be sufficiently set in 3 to 5 minutes, when the initial archwires can be ligated. According to the manufacturer, the adhesive attains 70% of its ultimate bond strength within 10 minutes, 80% within 1 hour, and full strength within 12 hours.
After bonding, the teeth were randomly divided into 2 groups of 20. The teeth in group I were debonded after 24 hours immersion in deionized water at 37° C. The teeth in group II were debonded after thermocycling.
According to the International Organization for Standardization's recommendation,18 test specimens were prepared at 23°C ± 2°C and stored in water at 37°C ± 2°C before testing at room temperature. The teeth were stored in water for 24 hours to discriminate between materials that can and cannot withstand a wet environment.18
The thermocycling test comprising 500 cycles in water, between 5°C and 55°C, was started after 24 hours storage in water at 37°C. The exposure to each bath was 20 seconds, and the transfer time between baths was between 5 and 10 seconds.
Before bonding, the teeth were embedded in acrylic placed in phenolic rings (Buchler Ltd, Lake Bluff, Ill). A mounting jig was used to align the facial surface of the tooth to be perpendicular with the bottom of the mold; ie, each tooth was oriented so that its labial surface would be parallel to the force during the shear strength test. A steel rod with a flattened end was attached to the crosshead of a test machine (Zwick GmbH, Ulm, Germany). An occlusogingival load was applied to the bracket, producing a shear force at the bracket-tooth interface. A computer electronically connected to the test machine recorded the results of each test. Shear bond strengths were measured at a crosshead speed of 5 mm/minute.
Descriptive statistics including the mean, standard deviation, and minimum and maximum values were calculated for each of the 2 groups tested. The Student t test was used to determine whether significant differences existed between the 2 groups. The significance for all statistical tests was predetermined at P ≤ .05.
Results
The descriptive statistics for the shear bond strength are presented in the Table.
The results of the
t test comparing the 2 experimental groups (
t = 6.84) indicated significant differences between them (
P = .0001). The mean shear bond strengths of the cyanoacrylate adhesive were 7.1 ± 3.3 MPa at 24 hours after bonding and 1.5 ± 1.4 MPa after thermocycling.
Discussion
Clinicians should be aware of the properties of the adhesive systems they use so that they can optimize their ability to handle them properly and efficiently. The present findings indicated that the shear bond strength of the cyanoacrylate adhesive was significantly stronger at 24 hours than after thermocycling.
Reynolds20 suggested that a minimum tensile bond strength of 5.9 to 7.8 MPa is adequate for most clinical orthodontic needs. Newman1 believes that orthodontic forces seldom if ever exceed 10 lbs (4.5 kg) or 200 psi (1.4 MPa) shear force. Nevertheless, masticatory forces alone or combined with forces from orthopedic-type appliances significantly exceed these values.21 According to Kiliaridis et al,22 maximum bite forces can have means as high as 651 ± 196 N in men and 556 ± 128 N in women. These means are equivalent to 60 to 70 kg of force. Furthermore, chewing bite forces vary among tooth types, from a mean of 113 N for incisors to 128 N for canines and 140 N for premolars in men and almost half these values in women.22 Therefore, these mean values range between 7 and 16 kg force with very large standard deviations. This significant variation in chewing bite force between people and between different sites in the same person's mouth provides 1 explanation for bracket failure. As a result, the adhesive bond strength must be able to withstand orthodontic and chewing forces that will be applied to the appliance in the oral environment.
Smartbond provided adequate shear bond strengths at 24 hours after bonding but not after thermocycling, as a result of an 80% drop in its mean shear bond force. The decision to use this adhesive is influenced by a number of factors, including (1) no need to use a curing light, (2) a relatively short working time of 5 seconds to properly position the bracket before it starts to set, and (3) a significant decrease in bond strength after thermocycling.
Conclusions
The present findings indicated that (1) Smartbond cyanoacrylate adhesive has adequate bond strength 24 hours after initial bonding, but its strength decreases by 80% after thermocycling between 5°C and 55°C; and (2) the clinician must consider the properties of the various adhesive systems available, including bond strength and working time.
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