Cementochronology - The Still Underestimated Old "New" Method for Age-At-Death Assessment
Stephan Naji and Katrin Koel-Abt*
Pentagon, Defense POW/MIA Accounting Agency, USA
Submission: July 20, 2017; Published: July 28, 2017
*Corresponding author: Katrin Koel-Abt, Pentagon, Defense POW/MIA Accounting Agency, USA.
How to cite this article: Stephan N, K Koel-Abt. Cementochronology - The Still Underestimated Old "New" Method for Age-At-Death Assessment. J Forensic Sci & Criminal Inves. 2017; 4(1): 555630. DOI: 10.19080/JFSCI.2017.04.555630
Accurate age-at-death assessment is still a major issue in establishing a biological profile for forensic settings and in archaeological (paleodemography and paleoepidemiology) contexts. Even if results of aging methods are satisfactory for growing sub adults, accuracy and reliability for aging adults are weak at best, especially for individuals over 50 years old [1,2]. The purpose of all age-at-death estimation methods is to explore potential biological indicators closely related to the actual "chronological" age. For adult age categories, all dental or skeletal methods are based on senescence phenomena. But these biological modifications of the human body are strongly dependent on and an adaption to multiple external variables, such as pathologies, physical activities or environmental exposures. For example, when assessed with conventional ageing methods an older individual in good physical health can seem much younger, as can a rather young adult easily be misinterpreted as an old(er) individual when the bones are in poor physical shape due to pathologies or "wear and tear" (e.g. due to extreme physical activity, as for example oftentimes noticed in competitive athletes or infantry soldiers). Consequently, no single biological indicator used for traditional aging methods in anthropology today has an acceptable accuracy for estimating individual age .
Only one method in anthropology truly allows for a direct observation of a skeletal tissue that does not remodel throughout life: cementochronology, a chronobiologic method based on the counting of dental acellular cementum growth increments. Chronobiology is the "discipline that measures and explores mechanisms of biological temporal structures and their relations to rhythmic manifestation of living matter” . Chronobiology includes all fields addressing the study of growth markers observable within organisms, in other words, "all histomorphological markers created inside hard growing tissues, regardless of their structure or meaning . Skeletochronology (a word coined by Castanet & colleagues  is dedicated to growing markers inside skeletal structures of vertebrae such as bones, calcified cartilage and teeth .
Cementum is a growing dental tissue deposited regularly throughout life and generally referred to as "cementum annulations” or "incremental lines” in forensic studies [7-10]. The analysis of cementum lines or cementochronology is used to estimate individual age-at-death and seasonality-of-death, thus mainly in archaeological contexts, with a very promising accuracy of ± 1-3 years from the actual chronological age of an individual [11-13]. Cementochronology was first implemented in the 1950's on marine mammals [14,15] and then largely on ungulates [16,17]. The method was quickly adapted to zoo archaeological problems of seasonality and mobility [18,19]. Subsequently, the identification of annual layers of cementum (1 dark + 1 light band = 1 year) and their use for age estimation in forensic dentistry was formally tested on known age cases in 1982 by Stott and colleagues who concluded that "This technique may be extremely valuable in forensic medicine, forensic dentistry, and anthropology”. This pilot study was quickly followed by positive validation studies [20-23] as well as negative ones [24-26]. Finally, the method was adapted for human archaeological remains  and most notably by Wittwer-Backofen and Buba  who then established the bases for the current protocols .
Overall, cementochronology has been successfully implemented on over 72 species of mammals across 21 families and 9 orders [17,29-31]. Cementochronology is one of the very few intensively test edaging methods in humans. It has been tested by 31 different laboratories in 11 countries on4 continents on well over 2000 known-age human teeth (Table 1). In all cases, a circa-annual rhythm of a cellular cementum deposition has been observed as a pair of alternating translucent and opaque increments, interpreted as a two-phase annual growth process in transmitted bright light microscopy. No other estimator in anthropology has been independently validated at this scale. Therefore, we can logically ask why this method is still not a standard method more widely used in forensic or bioarchaeological contexts?
There are a certain number of basic obstacles, technical and theoretical, preventing a wider implementation of cementochronology.
a) First, cementochronology is a histologic technique, which is generally not very appealing to most "anthropology users” who are often not familiar with this type of section preparation. Besides the initial and somewhat cost-intensive laboratory set up, the learning curve of properly and routinely using the method is rather long. The user must also be well- trained in reading and understanding dental histological features and thus be familiar with cementum biology. However, forensic anthropology is done within a forensic sciences laboratory which always includes a histology lab that routinely does histological analyses of soft tissues for pathological purposes. It would be quite simple to set up the labto include standard dental histology procedures to implement cementum analysis.
b) Second, the counting procedure of cementum lines is a strong impediment because of its apparent subjectivity and potential inter-observer errors. Yet with proper training, removing intra-observer variation is rather easy and reducing inter-observer error is definitely possible.
c) Third, cementochronology is a destructive technique. However, only the root is involved and cheap micro CT scanning options are available to make a permanent virtual record of the root's morphology and structure.
d) Fourth, there are still no answers to the question of what the etiology of cementum is and why cementum increments are semiannually deposited. As of today, there are no satisfying answers to these questions, although extensive research has been conducted [32-34]. Nonetheless, empirical evidence on over 70 mammal species has demonstrated that a cellular cementum growth is seasonal. Not understanding the detailed mechanism of a biological process should not be a deterrent to using it. Many other biological processes cannot be completely explained yet either. What for example are the exact biomechanical rules behind ageing of the pubic symphysis, or behind dentine transparency? None of these mechanisms can be fully explained, yet they are routinely part of standard operating procedures in forensic contexts, despite the fact that only very few validation studies exist and that they have all demonstrated a very weak correlation with chronological age at best .
e) Last but not least, what is the nature of the lines we are counting? Recent advances in cementum studies seem to favor the differential mineralization hypothesis over the collagen fiber orientation one [36-38].
Regardless of the nature of these layers, we know how to systematically visualize a cellular cementum increments through proper thin-section preparation [37,39,40]. Not only does empirical evidence allow us to estimate age-at-death, we can also estimate the season-at-death from the last forming increment [9,27,41]. With the increasing number of validation studies on several mammals, the volume of empirical evidence confirming the periodicity of cementum growth is overwhelming and use of the technique should be encouraged as a highly efficient aging tool.
This work was supported in part by the "Agence National de la Recherche" [project CemeNTAA, ANR-14-CE31-0011], and in part by an appointment to the Research Participation Program for the Defense POW/MIA Accounting Agency, administered by the Oak Ridge Institute for Science and Education through an agreement between the U.S. Department of Energy and Defense POW/MIA Accounting Agency. The opinions or assertions contained herein are the private ones of the author(s) and are not to be construed as official or reflecting the view of the DoD.