Ethnic Differences of Root-Crown Ratios in the Permanent Dentition

Objectives: Root resorption due to orthodontic tooth movement may adversely affect the root-crown (R/C) ratios of permanent teeth, especially in patients with Short Root anomaly (SRA), a poorly understood disorder affecting tooth root development. Evaluation of SRA R/C ratios to normal dentition will facilitate diagnosis and orthodontic treatment planning. However, reference values are not available for all ethnic groups. Objective: Using panoramic radiographs to determine R/C ratios of fully developed permanent teeth and their relationship to gender and ethnicity.
Methods: A total of 404 patient radiographs (139 Caucasians, 119 African Americans and 147 Hispanics; 47% males and 53% females) were analyzed. Root lengths and crown heights of 6,371 teeth were measured with modified Lind’s method. R/C ratios were calculated for each tooth by dividing root length to crown height. A two-way ANOVA was used to evaluate the effects of gender, ethnicity and their interaction on R/C ratios. Gender and ethnicity stratified analyses were performed using one-way ANOVA. A Tukey’s method was used for all post-hoc comparisons at α=0.05.
Results: The mean R/C ratios varied from 1.77-2.16 for the maxillary teeth and 1.87-2.47 for the mandibular teeth. In both arches the lowest values were recorded from the central incisors and the highest for second premolars. Gender differences in R/C ratios were found to be significant only in the upper canines. African Americans demonstrated significant differences in the measurements of the maxillary lateral incisors and mandibular central incisors and first premolars when compared to Caucasians. However, gender differences showed statistical significant in the R/C ratios of most teeth between Hispanics and the other two ethnicities examined.
Conclusions: Our data shows for the first time that ethnicity is an important factor in R/C ratios of permanent teeth. Therefore, in diagnosing developmental conditions such as SRA, ethnic specific reference values should be used as guidelines.