Retentive strength of dental composite to metal surfaces

Retentive strength of the metal-composite interface was studied with tensile tests for 10-mm and 4-mm diameter specimens. In both series nonperforated and perforated metal surfaces with various numbers of holes were used. The specimens were tested after 1 day of storage in air and after thermocycling in water at 7 degrees C and 60 degrees C, respectively. The nonperforated specimens had the highest retentive strength values for both small and large specimens. The retentive strength decreased with increasing number of perforations and for nonperforated specimens with large retentive area. Thermocycling reduced the tensile force required to break the specimens by 4% to 50%. The fracture surface was mostly located close to the metal surface, indicating that this is the zone of stress concentration. With increasing length of the bond edge a reduction of the retentive strength was observed.