The Effect of Metal Primer on the Shear Bond Strength of Denture Base Resin to Base Metal Alloy after Electrolytic Etching and Laser Etching: An In Vitro Study

Background: The purpose of this study was to compare and evaluate the effects of metal primer on the shear bond strength of denture base resin to base metal alloy after electrolytic and laser etching. Materials and Methods: A total of 48 cobalt‐chromium (Co‐Cr) alloy specimens were fabricated. The specimens were divided into 6 groups of 8 specimens each. Group I: Sandblasting (control group). Group II: Sandblasting and metal primer application before the packing of acrylic resin. Group III: Electrolytic etching. Group IV: Electrolytic etching and metal primer application before the packing of acrylic resin. Group V: Laser etching. Group VI: Laser etching and metal primer application before the packing of acrylic resin. Each of these specimens was then subjected to shear load in Lloyd’s universal testing machine. Results: Laser etching with metal primer application showed the highest bond strength, followed by laser etching alone. Electrolytic etching without metal primer application showed the least bond strength. Conclusion: Within the limitations of this in vitro study, the metal primer application after the metal surface treatments significantly increased the shear bond strength of denture base resin to Co‐Cr alloy. Key Words: Electrolytic etching, laser etching metal primer, sandblasting, shear bond strength Introduction Metal‐resin bonding systems may be categorized as mechanical, chemical or a combination of the two.1 Framework design must incorporate certain elements that provide mechanical retention of the resin.2 The absence of a chemical bond directly affects the metal-resin interface. Microscopically, a space exists between the metal framework and the resin denture base. This contributes to increased microleakage of oral fluids, and microorganisms resulting in discoloration, foul odor, adverse soft tissue response, and deterioration of the denture base material.3 Recently, several chemical methods that enhance the bonding of resin to cast alloys have been developed, such as adhesive heat‐ cured opaque resin,4 Silicoater system,5 silane coupling agent and adhesive metal primer,6 self-curing 4-methacryloxyethyl trimellitate anhydride or methylmethacrylate‐tri‐n‐butylborane (MMA‐TBB) opaque resin7 and some commercially available adhesive bonding promoters. Kojima et al.8 synthesized 6‐(4‐vinylbenzyl‐n‐propyl) amino‐1,3,5‐triazine 2,4‐dithione (VBATDT) monomer and reported that an MMA‐TBB resin bonded strongly to the precious metal with the use of VBATDT primer. Barrack et al.9 refined the metal framework by electrolytically etching the non‐precious castings to improve the resin bond to the casting. Recently, laser surface treatment to enhance the micromechanical retention of bands and brackets has been introduced,10 but the effect of this treatment on the bond strength of resin to non‐precious alloy has not been investigated. Hence, the purpose of this study was to examine the effect of metal primer on the shear bond strength of denture base acrylic resin to cobalt‐chromium (Co‐Cr) alloy after electrolytic etching and laser etching. Materials and Methods The materials used in this study have been listed in Table 1. Preparation of alloy specimen (Figures 1a and b, and 2a and b) Disk shaped specimens (10 mm in diameter and 2 mm in thickness) of base metal alloy were casted from the wax patterns using a standardized mold, sandblasted and divided into 6 groups of 8 specimens each. Sandblasting was done using 150 μm alumina oxide particles, keeping the sandblasting point. 5 mm away from the surface to be etched for 60 s at a pressure of 0.15 MPa. The specimens were then cleaned in an ultrasonic cleaner for 10 min with distilled water. Each specimen was air dried and examined for bonding surface defects. Specimens with irregularities and