Cupping (concave wear) on the occlusal surfaces of natural teeth opposing porcelain crowns is caused by which process?

Cupping on occlusal surfaces is caused by attrition - mechanical wear from tooth-to-tooth contact. Porcelain (ceramic) restorations are significantly harder than natural enamel (Vickers hardness: porcelain ~600-800 VHN vs. enamel ~340 VHN) and cause accelerated wear of the opposing natural teeth. The cupped or concave appearance results from preferential wear of the softer surrounding enamel while central fissures may appear relatively elevated, or central wear creates a cup-shaped depression. This is a significant concern with full-coverage porcelain crowns opposing natural teeth. Tooth-colored composite restorations are recommended as antagonists to minimize this wear. Erosion creates smooth, shallow concavities by chemical dissolution (acid) and is not specific to occlusal contact areas.