Distribution of adhesive layer in class II composite resin restorations before/after interproximal matrix application

Objectives: This study aimed to morphologically investigate the distribution of the adhesive layer when placed prior, or subsequent, to matrix positioning in direct-bonded Class II RBC restorations. Additional aim was to evaluate possible differences when using two-steps (CSE, Clearfil SE Bond2) or one-step adhesive system (CU, Clearfil Universal Bond Quick). Methods: Standardized mesio-occlusal and disto-occlusal cavities were prepared on 20 human molars. Teeth were randomly allocated to two protocols according to the positioning of contoured sectional metal matrices before (M->A, n = 10), or after adhesive application (A->M, n = 10). Both adhesive systems were additioned with crystal violet dye (CV, 10 vol%). Specimen sections were evaluated using optical and scanning electron microscopy (SEM). Dynamic viscosity, pH, microshear bond strength test (μ-SBS) on enamel and dentin, and three-point bend test (3PB) of polymerized adhesive rods, were performed on both pristine and CV-additioned adhesives. Results: M->A produced a layer of adhesive both on tooth-restoration interface and on external restoration surfaces in contact with the matrix. A->M produced a thin layer of adhesive on external tooth surfaces, well beyond cavity and RBC restoration margins. In all restorations, excess RBC material with uneven margins was observed protruding over the cervical margin. CV addition slightly increased pH and decreased viscosity. μ-SBS: CU + CV showed a 10-fold reduction in adhesion forces on dentine. 3PB: CSE yielded higher flexural strength values than CU. CV addition reduced flexural strength of CSE. Conclusions: Both M > A and A > M generated adhesive placement disadvantages with adhesive materials being expressed in difficult to reach locations that may jeopardize complete adhesive polymerization. Clinical Significance: All cervical margins of RBC restorations should be carefully finished to improve longevity, no matter the clinical protocol adopted. CV addition labelled the tested adhesives without compromising their performances considerably.