Ultrashort pulsed laser conditioning of human enamel: in vitro study of the influence of geometrical processing parameters on shear bond strength of orthodontic brackets

The surfaces of 63 extracted premolar teeth were processed with intense ultrashort laser pulses (λ = 795 nm; pulse duration, 120 fs; repetition rate, 1 kHz) to produce cross patterns with different pitches (s) in the micrometer range in order to evaluate the influence of such microstructures on the shear bond strengths of orthodontic brackets to enamel. The samples were classified in nine groups corresponding to the control group (raw samples) and eight different laser-processed groups (cross patterns with s increasing from 15 to 180 μm). Brackets were luted with TransbondTM XT adhesive resin to all the samples; after 72 h, they all were submitted to strength test in a universal testing machine. Additionally, a third of the samples underwent morphological analysis of the debonded surface by means of scanning electron microscope microscopy and an analysis of the failure mode based on the adhesive remnant index. The results showed that enamel microstructuring with ultrashort laser pulses remarkably increase the bond strength of brackets. Dense cross patterns (s  90 μm) give rise to smaller improvements of the bond strength. A strong correlation of this behavior with the predominant failure mode in both scenarios was found. So far, the best compromise between suitable adhesive efficiency, processing time minimization, and enamel surface preservation suggests the performance of cross patterns with pitches in the order of 90 μm.The authors acknowledge the financial support from the Ministerio de Economía y Competitividad (projects Consolider SAUUL CSD2007-00013 and FIS2009-09522) and Junta de Castilla y León (project SA086A12-2). We are grateful to the Centro de Láseres Pulsados (CLPU, Spain) for allowing free SEM observation of the specimens