title: An ex vivo assessment of gingivally offset lower premolar brackets.
abstract: OBJECTIVE
  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).
  
  
  METHODS
  An ex vivo study.
  
  
  METHODS
  Dental Materials Science Laboratory, Dundee Dental School, Dundee.
  
  
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Unit of Dental and Oral Health, Orthodontics Section, University of Dundee Dental School, Dundee, UK. bikram_chaman@yahoo.com
authors.forename: B S
authors.initials: BS
authors.lastname: Thind
authors.forename: C J
authors.initials: CJ
authors.lastname: Larmour
authors.forename: D R
authors.initials: DR
authors.lastname: Stirrups
authors.forename: C H
authors.initials: CH
authors.lastname: Lloyd
mesh_terms: D001641:Bicuspid; D016009:Chi-Square Distribution; D001840:Dental Bonding; D004868:Equipment Failure; D005881:Gingiva; D006801:Humans; D016382:Orthodontic Appliance Design; D016910:Orthodontic Brackets; D019279:Resin Cements / Q000737:chemistry; D018709:Statistics, Nonparametric; D013314:Stress, Mechanical; D013499:Surface Properties; D016019:Survival Analysis
chemical_list: D019279:Resin Cements; C477790:Transbond XT