Effect of Pre-Cure Movement on Orthodontic Bracket Bond Strength

In clinical practice, it is common for auxiliaries to perform initial bracket placement before the orthodontist achieves final positioning of the appliances and the adhesive is light-cured. Objectives: The purpose of this study was to determine whether movement of the bracket, after initial placement and before light-curing, affects the shear bond strength achieved. Methods: 100 extracted premolars were bonded using a conventional acid-etch procedure. Brackets were placed on the teeth with a measured 1.5N force. Within 30 seconds and before light curing, each group (N=20) was subjected to the following movements: 1) No movement (Control), 2) 2mm linear movement, 3) 4mm linear movement, 4) 45° rotation, and 5) 180° rotation. Shear bond strengths, Weibull survival, and ARI scores were compared among groups. Results: Mean bond strengths ranged from 12.3±6.0 MPa for the 180° rotation group to 15.7±6.5 MPa for the 2mm movement group (Control: 14.0±8.4 MPa) but were not significantly different from each other (P=0.57). Weibull 95% survival thresholds were between 3.4 and 4.3 MPa. ARI scores differed significantly among groups (P<0.005) but failures were mostly cohesive in all groups. The control group had the highest proportion (30%) of adhesive failures at the bracket-composite interface. Conclusions: Movement of orthodontic brackets after initial placement and before light curing does not appear to adversely affect the bond strength attained. The light force used during initial placement of the brackets in this study may have been a factor. (This study was supported in part by the MCV Orthodontic Education and Research Foundation)