Bonding Properties of Young and Mature Human Premolars

Following eruption of a tooth into the oral cavity, enamel continues to calcify. Recently, there has been a significant increase in the number of adult patients seeking orthodontic treatment. Enamel maturation levels may affect the bond strength of brackets. Purpose: The aim of this study was to compare the shear bond strength and mode of bond failure of brackets bonded to mature versus young teeth. Methods: Extracted human premolars were randomly assigned to young (9-14 yrs) or mature (>21 yrs) groups. Edgewise brackets were bonded using either the conventional or self-etching technique following manufacturer's instructions. The 4 groups tested were: mature/conventional (N=16), young/conventional (N=16), mature/self-etch (N=16) and young/self-etch (N=16). After 7 days, brackets were debonded in an Instron universal testing machine and the shear bond strength was calculated. The site of bond failure was scored according to the Adhesive Remnant Index (ARI). Etched enamel surfaces were examined using scanning electron microscopy (SEM) and representative photomicrographs were taken. Statistics included two-way ANOVA, Kruskal-Wallis and Weibull survival analyses. Results: The mean shear strengths were 13.69 ± 4.75, 12.16 ± 5.49, 13.42 ± 7.74 and 9.77 ± 5.00 MPa for the mature/conventional, mature/self-etch, young/conventional and young/self-etch groups, respectively. There were no significant differences in the mean shear bond strengths and ARI scores among the groups (P>0.20). Mature teeth tended to have more cohesive bond failures whereas young teeth had more adhesive failures at the enamel-composite interface which was somewhat surprising. SEM analyses showed microstructural characteristics of the etching technique used. Conclusions: Based on the findings of this study, self-etching primer and conventional etching techniques do not differ in the bond strength values when comparing them on mature and young enamel.