Bonding of amalgam restorations: existing knowledge and future prospects

A number of laboratory and clinical studies over the last 15 years have explored the potential advantages of bonding amalgam to tooth surfaces. Bond strengths have been reported to range from 2 to 20 MPa, with higher bond strengths reported for filled adhesives. Most studies agree that the use of bonding results in a considerable reduction of microleakage, when compared with copal varnish or no lining. The use of bonding provides retention in vitro that is equivalent to, or better than, the use of mechanical undercuts. Most studies on strength of restored teeth report an improvement in resistance to fracture or cuspal flexing as measured by strain gauges. Penetration of secondary caries along the interface has been found to be inhibited by bonding. The mode of failure of bonded amalgams has generally been reported to be mixed, but predominantly between the resin and amalgam. In vitro studies have reported one potential problem in the incorporation of resin into amalgam, which may cause a decrease in strength of the restoration. The clinical studies of bonded amalgams that have been published to date are of short duration, indicating that when traditional preparations are used, no problems are seen with bonding, but also no advantages, as measured by clinical assessment criteria. However, some studies show that bonded amalgam may be useful for procedures where non-bonded amalgams would be expected to be lost, namely in preparations with little, if any, mechanical retention. It was concluded that, while there are various in vitro studies demonstrating that bonded amalgams have advantages of improved retention and tooth reinforcement and decreased marginal microleakage and secondary caries, the operative technique is more complicated and there are few advantages yet evident from clinical studies in conventional preparations having mechanical retentive features. However, there is evidence accruing from clinical studies that bonding of amalgam can be favorably used to extend the range of usage of amalgam to non-retentive conservative preparations, and toward the other extreme, as an adjunct to other forms of retention in large compound restorations.