Effect of Metal Primer Application on the Shear Bond Strength of Orthodontic Brackets with Different Base Designs: An In Vitro Study

Background/Objectives: Direct bonding of orthodontic brackets is standard in fixed-appliance therapy, but clinical success depends on achieving reliable bond strength without compromising enamel integrity during debonding. This in vitro study evaluated whether applying a metal primer to stainless-steel bracket bases affects shear bond strength (SBS) to enamel, and whether primer coverage and bracket-base design influence adhesive-remnant distribution and enamel integrity. Methods: A total of 132 extracted human maxillary premolars were randomly allocated to six groups (n = 22 each) defined by bracket base design (mesh or integral) and primer coverage (none, full, or partial central coverage). Brackets were bonded with Transbond XT, and Reliance Metal Primer was applied according to group allocation. After 2000 thermal cycles (5–55 °C), specimens were debonded in shear at 1 mm/min. SBS was calculated, adhesive remnants were quantified with ImageJ and scored using the adhesive remnant index (ARI), and enamel damage was assessed using an enamel damage index (EDI). Results: Mean SBS values were 15.68 MPa (MC), 18.38 MPa (MF), 20.89 MPa (MP), 13.04 MPa (IC), 17.11 MPa (IF), and 12.66 MPa (IP). Two-way ANOVA showed significant main effects of bracket base design (F(1, 126) = 16.06, p < 0.001, partial η2 = 0.113) and primer coverage (F(2, 126) = 3.98, p = 0.021, partial η2 = 0.059), together with a significant bracket-base-design × primer-coverage interaction (F(2, 126) = 4.46, p = 0.014, partial η2 = 0.066). ARI distributions differed significantly across the six experimental groups (χ2(15) = 75.24, p < 0.001, Cramér’s V = 0.436), whereas EDI scores did not differ significantly among groups (p = 0.756). Conclusions: The effect of the metal primer on SBS depended on bracket base design and primer coverage. Partial coverage produced the highest SBS among mesh-base brackets and was significantly higher than the mesh control. Among integral-base brackets, full coverage yielded the highest numerical SBS, but within-design pairwise differences were not significant. ARI failure patterns also varied significantly across the six experimental groups, whereas no significant difference in stereomicroscopically detectable enamel damage was observed.