High-accuracy methodology for the integrative restoration of archaeological teeth by using reverse engineering techniques and rapid prototyping

Highlights

• Methodology for the integrative restoration of sampled archaeological teeth;

• Using computer microtomography, reverse engineering, computer-aided design, and rapid prototyping techniques to fabricate missing parts;

• 3D printing for cultural heritage preservation, research and exhibition.

Abstract

The reconstruction of the original morphology of bones and teeth after sampling for physicochemical (e.g., radiocarbon and uranium series dating, stable isotope analysis, paleohistology, trace element analysis) and biomolecular analyses (e.g., ancient DNA, paleoproteomics) is appropriate in many contexts and compulsory when dealing with fossil human remains. The reconstruction protocols available to date are mostly based on manual re-integration of removed portions and can lead to an imprecise recovery of the original morphology.

In this work, to restore the original external morphology of sampled teeth we used computed microtomography (microCT), reverse engineering (RE), computer-aided design (CAD) and rapid prototyping (RP) techniques to fabricate customized missing parts. The protocol was tested by performing the reconstruction of two Upper Palaeolithic human teeth from the archaeological excavations of Roccia San Sebastiano (Mondragone, Caserta, southern Italy) and Riparo I of Grotte Verdi di Pradis (Clauzetto, Pordenone, north-eastern Italy) (RSS2 and Pradis 1, respectively), which were sampled for physicochemical and biomolecular analyses.

It involved a composite procedure consisting in: a) the microCT scanning of the original specimens; b) sampling; c) the microCT scanning of the specimens after sampling; d) the reconstruction of the digital 3D surfaces of the specimens before and after sampling; e) the creation of digital models of the missing/sampled portions by subtracting the 3D images of the preserved portions (after the sampling) from the images of the intact specimens (before the sampling) by using reverse engineering techniques; f) the prototyping of the missing/sampled portions to be integrated; g) the painting and application of the prototypes through the use of compatible and reversible adhesives.

By following the proposed protocol, in addition to the fabrication of a physical element which is faithful to the original, it was possible to obtain a remarkable correspondence between the contact surfaces of the two portions (the original and the reconstructed one) without having to resort to any manipulation/adaptation of either element.

Introduction

In paleoanthropological research, dental and osteological remains are an irreplaceable source of information about the life history of an individual and the community to which this individual/person belonged. In recent years, the application of physicochemical (e.g., radiocarbon and uranium, stable isotope analysis, paleohistology, trace element analysis) and biomolecular analyses (e.g., ancient DNA, paleoproteomics) has revolutionized the field of osteoarchaeology and paleoanthropology. Even though they involve, in most cases, destructive or micro-destructive analyses, their application has become fundamental in the bioarchaeological field, allowing the retrieval of information that is not accessible through the employment of other non-destructive methodologies (e.g. Bortolini et al., 2021, Lugli et al., 2019, Lugli et al., 2018, Nava et al., 2020, Slon et al., 2018, Sorrentino et al., 2018). Therefore, standard protocols are needed to plan integrative restoration before the samples are even collected and need to consider the state of preservation of the specimens (size and morphology, as well as physicochemical properties) and their possible use after restoration (e.g., further scientific research, exhibition, teaching).

Traditionally, the reconstruction requires a manual approach, which is strongly influenced by the experience and subjectivity of the operator, is highly invasive, and becomes more demanding the more severely damaged and morphologically complex is the region to be reconstructed. So far, the replacement of missing parts has involved the reproduction of the external integrity of the specimen either by applying dental wax or hot paste made of organic and inorganic components (modelling chalk, raw beeswax, resin, zinc white) or by using mold-based techniques for contact replication of the missing parts, as a means to facilitate future interpretations of the element (Cencetti, 2008, Colli et al., 2009, White et al., 2000, Zanolli et al., 2016).

In the past decades, high-resolution 2D and 3D imaging technologies have generated a considerable degree of interest for several applications. Examples of the fields of application are palaeoanthropology, archaeology, geology, civil engineering, archaeology, reverse engineering, medicine, and virtual reality (Higgins et al., 2020, Sansoni et al., 2009, Traversari et al., 2016, Vazzana et al., 2018). This has led to a remarkable development of virtual restoration methodologies with reverse engineering (RE) techniques (Cook et al., 2021, Haile-Selassie et al., 2019a, Senck et al., 2013), to the increasingly widespread use of rapid prototyping (RP) to create replicas and scale reproductions of movable and immovable objects (D’Urso et al., 2000, Pérès et al., 2004, Tucci and Bonora, 2012, Urcia et al., 2018) or, in rare cases, to the manufacture of missing parts that are useful for restoration (Fantini et al., 2008). A virtual anthropological approach (Benazzi et al., 2014a, Benazzi et al., 2011, Romandini et al., 2020, Senck et al., 2013, Weber, 2014, Weber and Bookstein, 2011, Zollikofer and Ponce de León, 2005) based on reverse engineering, computer-aided design (CAD) and rapid prototyping technologies can facilitate and improve these operations because they minimize the subjective choices of the operator and increase the reliability of the result.

At present, the virtual reconstruction and rapid prototyping of missing parts are mainly used in maxillofacial surgery, where the design of customized implants using CT-derived 3D models, combined with the development of new biocompatible materials and rapid prototyping technologies, has led to multiple advantages over traditional surgical techniques (Aimar et al., 2019, Chua et al., 2020, Giovacchini et al., 2021, Maglitto et al., 2021, Sandeep Kumar et al., 2018, Touri et al., 2019, Zhou et al., 2010). The ability to use and manipulate digital data from CT scans and form an exact replica of an osteo-archaeological object in different materials (resin, polylactic acid (PLA), acrilonitrile-butadiene-stirene (ABS), etc.) using RP technologies introduces a new dimension to modern osteology, restoration, and exhibition.

Here we provide clear guidelines for the reconstruction of dental elements (though also applicable to bones) from archaeological and palaeoanthropological contexts by combining traditional methods and tools developed in manufacturing industries, as well as in the field of medicine and other research fields. The present approach overcomes the limits of manual procedures by a) strongly reducing the handling of the specimen, ultimately reducing risks of damage; b) exploring alternative solutions for both digital and physical reconstructions; c) printing copies of the final product that can be used for, e.g., scientific purposes, exhibition, educational or promotional activities.