There are several reasons for cracks in the root of a tooth: First, if the crack is a longitudinal crack extending from the crown of the tooth, it is mostly due to the fracture of the tooth caused by eating too hard food or by trauma, and it mostly occurs in teeth with dead pulp and after root canal treatment. Secondly, if the crack at the root is transverse crack with wedge-shaped tissue defect, it is mostly due to brushing the tooth with brute force laterally, or due to local occlusal stress concentration, which makes the wedge-shaped defect at the root. Thirdly, root surface caries, when periodontal disease or other reasons lead to root exposure, because the root surface is not as smooth as the enamel, it will easily lead to root surface caries. Once the root surface caries is formed, it will cause local melanin deposition and tooth defect, and many serious root surface caries will show ring-shaped encirclement around the root, leading to pulpitis and tooth fracture.