Influence of c-factor and layering technique on microtensile bond strength to dentin

Objective. Due to polymerisation shrinkage of resin-based composites, a high configuration factor in deep Class I cavities leads to a certain amount of stress when the material is bonded. The aim of this in vitro study was to evaluate the influence of c-factor and different layering approaches on bonding to dentin with three different adhesive systems. Materials and methods. Dentin bond strengths of Z250 bonded with OptiBond FL, Single Bond, and One Up Bond F were measured on flattened dentin surfaces without cavity walls and on the cavity floor of Class I cavities (10 layering concepts). The resin composite increments were applied horizontally, vertically and obliquely, both with and without a flowable liner. The tests were carried out in a microtensile apparatus at a crosshead speed of 1 mm/min after 24 h of storage at 37 °C in water. Mean bond strengths were analysed using the Wilcoxon test and multiple comparisons according to the Mann–Whitney U-test. Specimens having failed prior to the bond strength test were included as 0 MPa. Results. The groups bonded on flat surfaces exhibited significantly higher bond strengths than specimens cut from filled cavities. Within the cavity groups, OptiBond FL and Single Bond exhibited no significant differences, however, being above One Up Bond F. Within the groups of each adhesive, major differences between the layering concepts were detectable. Bulk technique led to low dentin adhesion at the cavity floor, above all for Single Bond and One Up Bond F. Horizontal layers resulted in significantly higher bond strengths than did vertical or oblique. Lining with a flowable composite did not promote bond strength for OptiBond FL. For the other adhesives, a lining improved adhesion when vertical or oblique layers were applied, for horizontal increments no effect was evident. Conclusions. The c-factor is an influencing factor for dentin adhesion. However, using an appropriate layering technique, high bond strengths to deep cavity floors can be achieved.