Fatigue of resin bonded amalgam restorations

Standardized MOD cavities were prepared in 80 human premolars, which were treated with either adhesive resin or copal varnish and then restored with amalgam. Fracture resistance of these groups was compared after 24 hours of storage, 4 weeks of storage with thermocycling, and after 500 days of storage. The buccal cusps were loaded at an angle of 30 degrees to the tooth long axis until fracture occurred. Additionally, survival curves were compared for adhesive resin and copal varnish groups that had been repeatedly load cycled until fracture occurred. The 24-hour adhesive resin group was significantly stronger than the corresponding copal varnish group (P < 0.05). However, no significant differences between adhesive resin and copal varnish were found for the other thermocycling, extended 500-day storage, or load cycling tests. In conclusion, the strengthening effect of an adhesive resin on teeth restored with MOD amalgam restorations was transient.