Safe Casting and Reliable Cusp Reconstruction Assisted by Micro‐Computed Tomographic Scans of Fossil Teeth

Abstract

Dental replicas are frequently utilized in paleoanthropological studies of perikymata and enamel hypoplasia. However, fossil teeth are often fragile and worn, causing two problems: (1) the risk of damage by removing enamel fragments when impression-making material is separated from the fossil tooth surface, and (2) the need to reconstruct worn portions of the crown to assess perikymata number, distribution, and hypoplasia timing. This study presents the advantages of μCT data of canines and lateral incisors for (1) detecting cracks along the enamel-dentine junction (EDJ) which could cause damage when casting, and (2) reliably and non-destructively reconstructing worn or broken cusps. Fragile teeth of Homo naledi, Miocene, and Pleistocene specimens were μCT-scanned: 2D virtual sections and 3D models allowed for inspecting crack pattern beyond the external surface and 2D virtual sections were used to digitally reconstruct cusp tips (only Homo naledi). Micro-CT scans allowed cracks running along the EDJ and communicating with radial cracks in the enamel to be identified prior to casting. Cusp reconstructions using μCT data were conducted as precisely as when using thin-sections or photographs, and with high intraand inter-observer agreement, while preserving the original specimen and affording numerous planes of virtual section. When available, μCT data should be inspected prior to tooth casting to exclude teeth that show a pattern of cracks that could lead to damage. Virtual sections allow for accessible, reliable, and non-destructive cusp reconstructions that may be used for developmental (e.g., perikymata and enamel hypoplasia) or enamel

Reconstructing tooth crown heights and enamel caps: A comparative test of three existing methods with recommendations for their use

“…Dental researchers, particularly those who study the teeth of nonhuman primates, fossil hominins, and recent humans, are tasked with extracting the most information possible from a small number of preserved teeth. Furthermore, in addition to sample availability, researchers are often hindered by the state of specimens, notably the presence and extent of wear (Guatelli-Steinberg, 2008;Guatelli-Steinberg, Ferrell, & Spence, 2012;Lockey, Alemseged, Hublin, & Skinner, 2020;Modesto-Mata et al, 2017O'Hara, Le Cabec, Xing, Skinner, & Guatelli-Steinberg, 2019;Saunders, Chan, Kahlon, Kluge, & FitzGerald, 2007;Skinner, Alemseged, Gaunitz, & Hublin, 2015;Smith, Kupczik, Machanda, Skinner, & Zermeno, 2012). To measure critical dental growth and enamel thickness variables, dental researchers require unworn teeth and/or accurate and reliable methods to reconstruct worn tooth crowns (e.g., Guatelli-Steinberg, 2008;Guatelli-Steinberg, Pampush, et al, 2018;Lockey et al, 2020;Martin, 1985;Modesto-Mata et al, 2017;Saunders et al, 2007;Schwartz, 2000;Skinner et al, 2015).…”

“…Three methods for reconstructing cusp tips have been established by different groups of researchers and on different taxa. In this study, the three methods are referred to as the "Pen Tool" method (Guatelli-Steinberg, Ferrell, & Spence, 2012;O'Hara et al, 2019;Saunders et al, 2007), the "Polynomial" method (Modesto-Mata et al, 2017), and the "Profile" method (Grine & Martin, 1988;Martin, 1983;Smith et al, 2011;Smith, Kupczik, et al, 2012). The present article offers the first comparative study of the three methods on a single sample, allowing for a direct comparison of their accuracy across four measurements: crown height (CH), cuspal enamel thickness (CET), regional enamel thickness (RegAET), and average enamel thickness (AET).…”

“…Typically, 2D sections are either generated digitally using microcomputed tomography scans (microCT) (Skinner et al, 2015) of teeth or through histological sectioning (a destructive process) (Martin, Olejniczak, & Maas, 2003). This article only employs microCT images, but its results also apply to reconstructing cusp tips on histological sections of teeth, as they represent the same dental structures and present the same challenges as virtually-rendered 2D planes of section (e.g., see O'Hara et al, 2019).…”

“…On 2D microCT and histological sections of unworn teeth, researchers can measure features such as CH (Molnar & Gantt, 1977;Saunders et al, 2007;Guatelli-Steinberg, Ferrell, & Spence, 2012;Modesto-Mata et al, 2017;O'Hara, 2017;Guatelli-Steinberg, Pampush, et al, 2018;O'Hara et al, 2019;Modesto-Mata et al, 2020;O'Hara & Guatelli-Steinberg, 2020), CET (Molnar & Gantt, 1977;Martin, 1985;Martin et al, 2003;Smith et al, 2007;Modesto-Mata et al, 2017), AET (Martin, 1985;Schwartz, 1997;Smith, Kupczik, et al, 2012;Smith, Olejniczak, et al, 2012;Skinner et al, 2015;O'Hara et al, 2019;Lockey et al, 2020), and regional distribution of enamel over the enamel cap (RegAET; Kono, 2004;Suwa & Kono, 2005;Skinner et al, 2015;Lockey et al, 2020). Measures of CH and CET are often used for developmental studies of teeth, including evaluating patterns of dental development, enamel growth differences between groups, and rates of juvenile dental development (Bromage & Dean, 1985;Dean et al, 2001;Guatelli-Steinberg, 2008;Guatelli-Steinberg, Floyd, Dean, & Reid, 2012;Guatelli-Steinberg, Reid, & Bishop, 2007;Modesto-Mata et al, 2017;Modesto-Mata et al, 2020;O'Hara, 2017;Reid & Ferrell, 2006;Rosas et al, 2017;Smith et al, 2007;…”

“…Generally, these studies have employed the Profile method to measure AET, though a few have also considered RegAET (Lockey et al, 2020;Skinner et al, 2015). Saunders et al (2007) established the Pen Tool method on histological sections of archaeological modern human canines and premolars; it has since been incorporated into studies utilizing casts of primate canines (Guatelli-Steinberg et al, 2009;Guatelli-Steinberg, Ferrell, & Spence, 2012;O'Hara, 2017;O'Hara & Guatelli-Steinberg, 2020), primate histological sections (Guatelli-Steinberg, Pampush, et al, 2018), hominin canine and molar microCT scans Martínez de Pinillos et al, 2020;O'Hara et al, 2019;Sorenti, Martin on-Torres, Martín-Francés, & Perea-Pérez, 2019). The Pen Tool method has most commonly been used to facilitate measures of CH for dental development studies (perikymata distribution or linear enamel hypoplasia studies), but a few recent articles (Martínez de Pinillos et al, 2020;O'Hara et al, 2019;Sorenti et al, 2019) have applied the method reliably to calculate molar enamel thickness.…”