The effect of repeated bonding on the shear bond strength of different orthodontic adhesives.

One problem clinicians face during treatment is bracket failure. In a busy orthodontic practice, a significant number of teeth will need to be rebonded. The purpose of this study was to evaluate the effect of repeated bonding with 2 different adhesives, a composite and a cyanoacrylate, on the shear bond strength of orthodontic brackets. Thirty-one freshly extracted human molars were collected. Brackets were bonded with 1 of the orthodontic adhesives according to the manufacturer's instructions. In group I, the teeth were etched with 37% phosphoric acid, a sealant was applied, and the brackets were bonded with Transbond XT (3M Unitek, Monrovia, Calif) and light cured for 20 seconds. In group II, the teeth were etched with 35% phosphoric acid, and the brackets were bonded with SmartBond (Gestenco International, Göthenburg, Sweden). In each group, the teeth were bonded and debonded 3 times with the same adhesive. At each sequence, the brackets were removed within 30 minutes after bonding to simulate the clinical condition at which a newly bonded bracket is tied to the archwire. Student t tests and the analysis of variance repeated measure were used to compare the shear bond strength between adhesives and within each adhesive at different debonding sequences. The results indicated that, at the first debonding sequence, the 2 adhesives did not have significantly different shear bond strengths. Between debonding sequence 1 and 2, there was a significant (P <or=.05) decrease in the shear bond strength of both adhesives. SmartBond decreased from x = 5.7 +/- 3.8 MPa to x = 2.2 +/- 2.6 MPa, and Transbond decreased from x = 6.1 +/- 3.4 to x = 4.1 +/- 2.3 MPa. When the 2 adhesives were compared at the second debonding sequence, the shear bond strength of Transbond was significantly greater than that of SmartBond (P =.046). Between debonding sequences 2 and 3, there were no significant differences either between the adhesives or in the change within each adhesive. The highest values for shear bond strength were obtained after the initial bonding. Rebonded teeth had significantly lower shear bond strength. The composite adhesive had a higher shear bond strength than the cyanoacrylate adhesive at the second bonding/debonding sequence but not at the third. The changes in bond strength after repeated bonding may be related to changes in the morphologic characteristics of the tooth surface caused by adhesive remnants.