Comparison of retentive systems for composites used as alternatives to porcelain in fixed partial dentures

The bond strength between framework metals and composites, used as alternatives to porcelain, requires further study because adhesive failures can promote microleakage, discoloration, and stains in composites.

The aim of this in vitro study was to analyze and compare the tensile strength between the base metal Ni-Cr alloys: (Wirocer and Wiron 99) and 2 composites (Artglass and Solidex) with 2 types of mechanical retention (0.4-mm and 0.6-mm beads); 2 types of chemical treatment (Siloc system and Metal Photo Primer system) and the combination of both treatments on a metal framework; to verify whether these composite systems were strong enough to be used as alternatives to porcelain.

A total of 144 specimens, cylindrical metal rods (3 mm diameter x 60 mm length), were cast in Ni-Cr alloy (Wiron 99 or Wirocer), divided into 6 equal groups corresponding to 6 types of retention: airborne particle abrasion/control (A), chemical (C), mechanical with 0.4-mm (M4) and 0.6-mm beads (M6), and chemical/mechanical with 0.4-mm (CM4) and 0.6-mm beads (CM6). Composite rings formed around the rods (6.0 mm diameter X 2.0 mm thickness) of Artglass or Solidex were light polymerized in the UniXS unit (wavelength range of 320-520 nm for 540s) according to the manufacturer's specifications. Specimens were invested, and their tensile strength was measured at fracture with an EMIC-MEM 2000 universal testing machine at a crosshead speed of 2.0 mm/min and 500 kgf load cell. Statistical analysis was carried out with analysis of variance and Tukey's test at significance level P=.01.

The highest mean retention values were recorded for groups CM6 (63.57 MPa) and M6 (63.05 MPa). The lowest mean retention values were recorded for groups A (30.63 MPa) and C (25.07 MPa). No significant difference was found regarding the alloys used in this study (P<.01). There was a significant difference in shear strength (P<.01) between Solidex (52.33 MPa) and Artglass (43.18 MPa).

Within the limitations of this study, mechanical retention with 0.6-mm beads and chemical/mechanical retention with 0.6-mm beads increased the metal-composite tensile bond strength. No significant difference was found between these 2 retention systems. Chemical and airborne particle abrasion/control retentions were statistically weaker than the other types of retention tested. The retention means corresponding to the commercial-grade Ni-Cr alloys used were statistically different. The composite Solidex showed a tensile strength mean statistically higher than Artglass. All values observed were higher than that recommended for metal-ceramic dental restorative systems (25 MPa-ISO 9693).