Orthodontic in vivo bond strength: comparison with in vitro results

The purpose of the present study was to test a new in vivo debonding device and compare in vivo bond strengths recorded by this device with in vitro bond strengths recorded by a universal testing machine such as the Instron. For the in vitro part of the study, 60 extracted premolar teeth were divided into 2 groups of 30 each. Both groups of 30 teeth had 3M Unitek Victory Twin brackets, precoated with Transbond XT composite resin, bonded to them. Shear bond strength tests were carried out in vitro using the universal testing machine on one group of 30 teeth while the debonding device was used on the other group of 30 teeth. The mean shear bond strength of the group debonded using the universal machine was 11.02 MPa and that of the group debonded with the debonding device was 12.82 MPa. For the in vivo part of the study, 8 patients randomly assigned to the research clinician from patients in The University of Alabama School of Dentistry, Department of Orthodontics, had a total of 60 premolar teeth bonded with 3M Unitek Victory Twin brackets. Following comprehensive orthodontic treatment (average time of 23 months), shear bond strength tests were carried out using the debonding device, which can measure debonding forces in vivo. The mean shear bond strength recorded in vivo was 5.47 MPa. Statistically significant differences were found between all 3 groups tested. The results appear to indicate that mean bond strengths recorded in vivo following comprehensive orthodontic treatment are significantly lower than bond strengths recorded in vitro.