Repairing and extending composite restorations are common and well-accepted procedures for the dental practitioner. Evaluation of the repair strength has great research focus by using tensile and shear bond strength measurements. The present collaborative study from NIOM has assessed the effect of surface treatments and bonding systems on the repair bond strength between composite materials after 1 and 12 months of storage, using a tensile test method.

Composite cylinders mimicking the old restoration were mechanically roughened using 320-grit silicon carbide sandpaper (similar roughness as medium coarse diamond), etched with 37% phosphoric acid gel and rinsed with water. The surface of one group was left untreated, one group was sandblasted, and one group was treated with silane. Each group was further divided into subgroups that received the following bonding systems: one-step self-etching adhesive, two-step self-etching adhesive, and three-step etch-and-rinse adhesive. Fresh composite was placed and cured on top of the prepared substrate cylinders. The test specimens were aged by thermo cycling (10 000 × 5 °C and 55 °C) and tested after 1 month and 12 months of water storage.

The mean tensile strength for the repaired groups ranged from 25 MPa to 50 MPa at one month and 20 MPa to 40 MPa at 12 months. The repair strength was lower than the strength of the composite material itself, that was 55 MPa after one month and 50 MPa at 12 months. The best repair bond strength was achieved by using freshly mixed silane solution on the substrate in addition to the two-step self-etching adhesive. This adhesive also rendered the thinnest bonding layer.

Clinical Relevance
Repairing and extending composite restorations is enhanced by using a medium coarse diamond bur to roughen the margin of the old composite followed by application of silane. The silane must be freshly made from a two-bottle system. In addition, the use of an adhesive system is mandatory; preferably a system that renders a thin bonding layer.

The full study is described in the following
Eliasson ST, Tibballs J, Dahl JE.
Effect of different surface treatments and adhesives on repair bond strength of resin composites after one and 12 months of storage using an improved microtensile test method.
Operative Dentistry 2014 May 7. [Epub ahead of print]


NIOM Newsletter October 2014, no 1