Ultimately, chemical gel etchants were developed to overcome limitations of the macroretentive and electrolytic etch technique. When a noble alloy is used, tin-plating is required because the method electrolytically deposits a layer of tin onto the noble alloy surface. Bonding strength to noble alloys is increased because tin creates organic oxides that readily react with resin cements. Resin Cements Intraoral forces are cyclic in nature and well below the highest reported in vitro bond strengths. When a bond is subjected to repeated stress cycles, it may fail well below its highest strength.
Bond strength depends on the type of alloy, surface treatment, film thickness, and resin cement.
Some specific combination of these factors will result in an optimum resin-alloy bonding system that is simple and produces consistently high bond strengths. This combination eliminates the handling of harmful chemicals, the need for acquiring an expensive armamentarium, and the production of the unpredictable etch patterns associated with electrolytic or gel etchants. The minimal anterior tooth preparation must include a rest to ensure a positive seat, definitive supragingival margins, distinct proximal grooves that provide resistance form and a single path of insertion, and lingual reduction to provide adequate alloy thickness and guide planes. An occlusal rest with definitive walls is prepared in the marginal ridge area to resist gingival and lateral displacement. The axial surfaces are prepared parallel with supragingival margins in enamel.