1.Deep caries causes pulpitis and tooth fracture and exposed pulp often need root canal treatment. These teeth usually have large defects, which are difficult to fill directly (fillings), or easily fall off after filling. 2. Root canal treatment requires opening a “hole” (pulp hole) at the top of the tooth, and the tooth will become weaker. 3. After root canal treatment, the tooth will lose the nutrients supplied by the pulp, and will gradually become dry and brittle, and will be prone to fracture. Wearing a crown can repair the larger defect, wrap around to protect the remaining tooth, and avoid fracture of the tooth.