Etching gel

All teeth were cleaned with water and fluoride-free pumice for at least 30 s and then dried with an oil-free air syringe. The enamel was then etched for 30 s with 37% orthophosphoric acid (etching gel, 3M, Monrovia, CA, USA), and the Primer (Transbond XT, 3M, Monrovia, CA, USA) was applied with a small brush and spread with oil-free compressed air. The composite (Transbond XT, 3M, Monrovia, CA, USA) was applied on the bracket base, and the attachment was positioned on the tooth surface. Composite excess was removed with a probe before polymerization. The composite was polymerized with a LED lamp (Opticore L3; Marslev Byvej, Denmark) for 80 s per bracket (20 s for side: mesial, distal, occlusal and gingival).

The extracted teeth were inspected to ensure that the part that would be restored is non-carious. After inspection, teeth were stored in 0.01% (w/v) thymol solution (pH 7.0) until use. Teeth were mounted vertically using self-curing acrylic resin (Unifast III, GC Corporation, Tokyo, Japan), exposing the crowns until 2 mm below the cementoenamel junction. After mounting, a box preparation (Class II) was conducted on the proximal surfaces of the teeth with the following dimensions: 4 mm occlusogingival, 2 mm mesiodistal and 2 mm buccolingual. One standardized operator prepared all teeth, then the prepared teeth were randomly assigned to the study’s groups. Before the restorations were placed, etching was conducted using 35% phosphoric acid (Cica Etching Gel, Neumünster, Germany) for 15 s and then dried for 5 s, and bonding was applied using self-priming resin (3M ESPE Single Bond Universal Adhesive, Neuss, Germany) and was cured for 20 s. Following etching and bonding, the restorations were placed using a single operator, and the curing was conducted using a light-emitting source (Satelec Mini LED Curing Light 1250 mW/cm², A-dec Inc., Newberg, OR, USA) for 20 s.