E16. Discuss the importance of properly finishing composite resin surfaces. Explain the reason behind two techniques of finishing composite resins upon removal matrix and after 24 hours.

Because of polymerization shrinkage, composites try to pull away from the margin when they cure. Since modern composites are bonded to surrounding hard tissues, the materials induce rather high tensile stresses around the margins. According to some researchers, these stresses can exceed 10 MPa (values exceeding 30 MPa have been reported). If an instrument (gold-knife or scalpel) is used to remove excess, the edge of the instrument should not be pushed toward the restoration, because such a movement will add more tensile stresses to the marginal regions, stresses that could result in a marginal debonding. Instead, all trimming should be done by moving the cutting edge from the bulk of the restoration toward the tooth margin.

If it is possible to trim the margins at a later occasion, one should try to delay marginal trimming for at least 24 hours. The reason is simply that during the first 24 hours, stress relaxation and water swelling occurs, two factors that will reduce the marginal stresses.