How is the crown-to-root ratio defined on a radiograph?

• A. Length of radiographic root divided by crown length
• B. Crown length divided by alveolar crest height
• C. Length of enamel relative to root length
• D. Length of radiographic crown divided by length of radiographic root

Answer: (D)

Crown-to-root ratio on a radiograph is defined as the length of the radiographic crown divided by the length of the radiographic root. Crown length is measured from the cusp tip or incisal edge down to the cemento-enamel junction, while root length is from the CEJ to the apex. This ratio compares how much tooth structure is above the bone to how much is anchored in bone and helps gauge periodontal support and prognosis for restorations. A ratio around 1:2 (crown about half the root) is generally favorable; a higher ratio indicates relatively less root support. Remember, this is a radiographic estimate and bone levels can influence interpretation.