The Effect of Thermocycling on Shear Bond Strength of Two Types of Self Etch Primers

Aims: To determine the shear bond strength of two types of self–etch primer (Transbond plus 3M Unitek, USA made and Clearfil, Japan made) when used to bond metal orthodontic brackets to enamel surface after thermocycling test and water storage for two months and to check the failure site after debonding using adhesive remnant index. Materials and methods: sixty extracted upper first premolars for orthodontic reason were used in this study; 30 teeth were bonded with light curing Transbond plus 3M Unitek and the other 30 teeth were bonded with light curing Clearfil self–etch primers. For both groups, Dentaurum stainless steel orthodontic brackets were bonded to enamel surface with Transbond XT light curing composite. Then 10 samples from each groups were tested for shear bond strength after 24 hours, the other 10 samples from each groups were tested after 500 manual thermocycles between 5°C &55°C, the third 10 samples of each groups were tested after 500 thermocycles and 2 months water storage at room temperature. The adhesive remnant index was tested under 10X magnification lens. Results: Both materials demonstrated a very good shear bond strength before thermocycling (14.5825 MPa for 3M Unitek & 14.3966 MPa for Clearfil groups). After 500 thermocycles, there were no significant changes in shear bond strength for both materials (15.0567 MPa for 3M Unitek & 13.997 MPa for Clearfil groups) and this is clinically acceptable. After 500 thermocycles and two months water storage the shear bond strength of the 3M/Unitek reduced progressively below the acceptable clinical value (3.469 MPa), whereas the shear bond strength of Clearfil group still above the acceptable level (10.607 MPa). The tendency of bond failure at the enamel–adhesive interface was increased after thermocycling and water storage. Conclusions: this study was done in vitro and further in vivo investigations are needed to evaluate these 2 materials.