Purpose: To estimate the effect of prolonged continuous force on the shear bond strength (SBS) of orthodontic brackets bonded with different adhesive systems.
Materials and Methods: Eighty premolars were randomly divided into four groups (I, II, III and IV) according to the bonding adhesive used. Orthodontic metal brackets were bonded with one of the succeeding adhesives; Rely-a-bond, Transbond XT primer and adhesive, Transbond Plus self-etch primer (SEP) and Transbond XT adhesive and Smart Bond. Half of the brackets were subjected to 150 g of force for 12 months. All the specimens were thermocycled 1000 times between 5° C and 55° C. A Universal Testing Machine was used to measure SBS.
Results: Transbond XT primer and adhesive exhibited the highest significant SBS value (12.8Mpa). Smart bond showed the lowest value (7.7Mpa). Rely-a-bond and Transbond Plus SEP adhesive systems had middle values (8.8 and 7.8Mpa). There was a significant decrease in the SBS of all studied adhesives (P<0.05) accompanied force application (P <0.05). Smart Bond showed the higher reduction in SBS while Transbond XT primer + Transbond XT adhesive revealed the lesser reduction.
Conclusion: Application of 150 gram of force for 12 months produced a pronounced negative effect on SBS especially with either SEP or Smart Bond.