Micro-tensile bond strength to surface and subsurface enamel

Testing of bond strength to enamel has been undertaken on both surface and subsurface enamel on the assumption that it is the same substrate. Over recent years, the micro-tensile method has become widely used for the evaluation of dental adhesives. However, there have been no micro-tensile bond strength studies comparing surface and subsurface enamel using orthodontic adhesives. It has been proposed that the bond strength to subsurface enamel is greater than that to surface enamel. Objective The micro-tensile bond strength (MTBS) of Transbond (light activated paste, 3 M Unitek, USA) to surface and subsurface enamel was investigated. Methods Ten sound freshly extracted lower human first molars were divided into two groups of five each. Surface enamel of the first group was removed with 500-grit silicone carbide paper, whilst in the second group it was left intact. Gelatin capsules filled with Transbond composite paste were bonded perpendicular to the enamel using light cure adhesive Primer (Transbond™ XT, 3 M Unitek, USA) in both groups, and stored in distilled water at 37 °C for 24 h. Crowns of all specimens were sectioned bucco-lingually into vertical slabs of approximately 0.6 mm thickness. Slabs in each group were prepared into an hourglass shape of 1 mm width and 0.6 mm thickness. Micro-tensile bond strength was determined using a mini tensile testing machine (COMPACT Gauge, DILLON). Results Mean values of MTBS were: 31.2 MPa (SD14.5) for surface enamel and 47.9 (SD15.0) for subsurface enamel. Significance There was a significant difference between the two groups P = 0.001. It was concluded that subsurface enamel has significantly higher bond strength than surface enamel. This has implications for bonding in restorative dentistry and orthodontics.