What is crown-to-root ratio?

• A. Crown height divided by root width
• B. Length of radiographic crown divided by length of radiographic root
• C. Length of root divided by crown length
• D. Crown height divided by root length

Answer: (B)

Crown-to-root ratio is the comparison of how much of the tooth is visible above the supporting root versus how much root length is anchored in bone. In practice this is measured on a radiograph by taking the crown height from the cementoenamel junction to the cusp tip and the root length from the cementoenamel junction to the root apex, then dividing the radiographic crown length by the radiographic root length. This radiographic approach is used because it consistently reflects the tooth’s vertical support within the bone, which is important for assessing prognosis for restorations and overall tooth stability. The other forms aren’t correct because one uses width instead of length, another reverses the ratio, and the last omits the radiographic context that is standard for this measurement.