An ex vivo assessment of gingivally offset lower premolar brackets

Abstract

Objectives: To compare the force to failure of standard premolar brackets to that of gingivally offset brackets and evaluate the site of bond failure between the two bracket types through the use of the Adhesive Remnant Index (ARI).

Design: An ex vivo study.

Setting: Dental Materials Science Laboratory, Dundee Dental School, Dundee.

Materials and methods: Forty extracted lower premolar teeth (caries free, extracted as part of orthodontic treatment, all donors living in a non-fluoridated area), divided into two equal size sample groups, as follows: Group 1: Victory Series^™ (3M Unitek, Monrovia CA, USA) lower premolar brackets bonded to buccal surfaces with Transbond XT (3M Unitek, Monrovia CA). Group 2: Victory Series^™ Gingivally Offset Bicuspid Brackets (3M Unitek, Monrovia CA) bonded to buccal surfaces with Transbond XT (3M Unitek, Monrovia CA). Force was applied in the occluso-gingival direction using an Instron Model 4469 Universal Testing Machine (Instron Ltd, High Wycombe, UK) operating at a cross-head speed of 0.5 mm/min and its value at failure determined. Following debond, the site of bond failure and ARI were recorded.

Outcome: Force to failure, site of bond failure and adhesive remnant index.

Results: The Weibull analysis gave higher values for the force to failure at 5% level (200 v. 159 N) and at all other levels of probability of failure for the gingivally offset bracket. The non-parametric survival analysis using Gehan–Wilcoxon tests with Breslow’s algorithm (p < 0.0001) showed significant difference in force to failure between bracket types. Chi-square tests showed no significant (p = 0.55) relationship between the site of bond failure and the bracket types.

Conclusion: Ex vivo testing suggests that there is a significant difference in the force to failure between gingivally offset and standard lower premolar brackets when force application is from an occluso-gingival direction. The site of failure (as given by the ARI) is insensitive to bracket types and force to failure.

Keywords:

• Dentistry
• orthodontics
• orthodontic bracket bonding
• gingival offset brackets

Acknowledgments

We would like to thank 3M Unitek for providing the materials used in this study, the Orthodontic laboratory at Dundee Dental Hospital for help in preparing the specimens and Mr Chic Gibson for production of the test jig.

Contributors

Bikram Singh Thind was responsible for study design, conducting the experiment, data interpretation, drafting of the article and final approval of the article. Colin J. Larmour assisted with protocol of the study and was responsible for preparation of the specimens and proofreading of the article before submission. David R. Stirrups was responsible for the data interpretation, statistical analysis and contributed to the writing of the article. Charles Lloyd was responsible for design of the apparatus and test procedures, supervising the production/commissioning of equipment and contributed to the writing of the article. Bikram Singh Thind and David R. Stirrups are the guarantors.