A novel system for classifying tooth root phenotypes

Abstract

Human root and canal number and morphology are highly variable, and internal root canal form and count does not necessarily co-vary directly with external morphology. While several typologies and classifications have been developed to address individual components of teeth, there is a need for a comprehensive system, that captures internal and external root features across all teeth. Using CT scans, the external and internal root morphologies of a global sample of humans are analysed (n = 945). From this analysis a method of classification that captures external and internal root morphology in a way that is intuitive, reproducible, and defines the human phenotypic set is developed. Results provide a robust definition of modern human tooth root phenotypic diversity. The method is modular in nature, allowing for incorporation of past and future classification systems. Additionally, it provides a basis for analysing hominin root morphology in evolutionary, ecological, genetic, and developmental contexts.

Patterns of variation in canal and root number in human post-canine teeth

“…Following the method developed by Gellis and Foley (2021) we used cone-beam computed tomography (CT) to analyse 5,970 post-canine teeth (Table 2) from the right sides of the maxillary and mandibular dental arcades of individuals (n= 945) from a global sample of humans (Table 1). While information for all teeth from both sides of the maxillary and mandibular arcades was recorded, only the right sides were analysed to avoid issues with asymmetry and artificially inflated sample size.…”

Canal number and configuration are predictors of external root morphology

“…Data used in this study were originally collected from CT scans of 945 individuals (Table 2) stored in osteological collections at the Smithsonian National Museum of Natural History, Washington D.C., USA, American Museum of Natural History, New York, USA, and the Duckworth Laboratory at the University of Cambridge, England (Gellis & Foley, 2021; Gellis, 2021). Only adult individuals, based on the eruption, occlusion, and closed root apices of M 3 s/M 3 s (or M 2 s/M 2 s in the case of congenitally absent M 3 s/M 3 s), were used in this study.…”

“…Canal count, configuration, and external root morphology for 5,790 teeth (Table 3) collected from a global sample of 945 individuals were downloaded from the Tooth Root Phenotypic Dataset (Gellis & Foley, 2021; Gellis, 2021) and analyzed with the R Project for Statistical Computing, version 3.6.3 (R Core Team, 2017). Only roots of from the right sides of the maxillary and mandibular dental arcades were analyzed to avoid issues with asymmetry and artificially inflated sample size.…”

“…In the case when two canals bifurcate from a single canal, if the degree of bifurcation is >= 33% of the total length of the canal structure, the canal count is two. The full method and its application are discussed in Gellis and Foley (2021, 2022), but are visualized in Figure 5.…”

“…For example, root morphologies described as ‘plate-like’ and ‘dumb-bell’ shaped, have been described in great apes, cercopithecoids, and Plio-Pleistocene hominins (Kullmer et al, 2011; Kupczik et al, 2019; Robinson, 1956). However, until recently the full range of morphological variants has been under-explored and with contradictory or complimentary definitions (Gellis & Foley, 2021). The anthropological literature has generally emphasized root number and rare or infrequent morphologies such as Tomes’ root, Taurodont molars and/or C-shaped molars.…”