The primary cause of secondary caries developing under crowns is the deterioration of the crown’s marginal fit over time, or the dissolution of the adhesive cement in oral fluids, resulting in a loss of its sealing properties. Bacteria seeping through these gaps slowly decay the vital or non-vital tooth tissue beneath the crown. In metal-based crowns, these caries may not be visible on X-rays in the early stages because X-rays cannot pass through the metal. Diagnosis without X-rays is based on clinical findings such as the patient complaining of a bad odor or taste coming from the crown, the insertion of an examination instrument called a probe at the gum line, dark-colored caries fluid seeping from the crown margin causing discoloration of the gums, or unexplained spontaneous sensitivity to hot and cold if the tooth is vital.
