title: Comparison of hydroxyapatite and dental enamel for testing shear bond strengths.
abstract: OBJECTIVE
  To investigate the feasibility of using artificial hydroxyapatite as a future biomimetic laboratory substitute for human enamel in orthodontic bond strength testing by comparing the shear bond strengths and nature of failure of brackets bonded to samples of hydroxyapatite and enamel.
  
  
  METHODS
  One hundred and fifty hydroxyapatite discs were prepared by compression at 20 tons and fired in a furnace at 1300 degrees C. One hundred and five enamel samples were prepared from the buccal and palatal/lingual surfaces of healthy premolars extracted for orthodontic purposes. Orthodontic brackets were bonded to each sample and these were subjected to shear bond strength testing using a custom-made jig mounted in an Instron Universal Testing Machine. The force value at bond failure was obtained, together with the nature of failure which was assessed using the Adhesive Remnant Index.
  
  
  RESULTS
  The mean shear bond strength for the enamel samples was 16.62 MPa (95 per cent CI: 15.26, 17.98) and for the hydroxyapatite samples 20.83 MPa (95 per cent CI: 19.68, 21.98). The difference between the two samples was statistically significant (p < 0.001). When the nature of failure was assessed with the ARI Index, 83 per cent of the enamel samples scored 2 or 3, while 49 per cent of the hydroxyapatite samples scored 0 or 1.
  
  
  CONCLUSIONS
  Hydroxyapatite was an effective biomimetic substrate for bond strength testing with a mean shear bond strength value (20.83 MPa) at the upper end of the normal range attributed to enamel (15-20 MPa). Although the difference between the shear bond strengths for hydroxyapatite and enamel was statistically significant, hydroxyapatite could be used as an alternative to enamel for comparative laboratory studies until a closer alternative is found. This would eliminate the need for extracted teeth to be collected. However, it should be used with caution for quantitative studies where true bond strengths are to be investigated.
authors.affiliation: Units of Orthodontics, UCL Eastman Dental Institute for Oral Health Care Sciences, London, United Kingdom.
authors.forename: Nishat
authors.initials: N
authors.lastname: Imthiaz
authors.forename: George
authors.initials: G
authors.lastname: Georgiou
authors.forename: David R
authors.initials: DR
authors.lastname: Moles
authors.forename: Steven P
authors.initials: SP
authors.lastname: Jones
mesh_terms: D001840:Dental Bonding; D003743:Dental Enamel; D003799:Dental Stress Analysis / Q000379:methods*; D017886:Durapatite; D006801:Humans; D008422:Materials Testing; D016910:Orthodontic Brackets; D019279:Resin Cements; D033081:Shear Strength
chemical_list: D019279:Resin Cements; C477790:Transbond XT; D017886:Durapatite