Three-Dimensional Radiological Methods for Age Estimation in Adults by Using the Pulp/Tooth Relation: an Integrative Review

Assessment of secondary dentin apposition is an observable phenomenon widely used to estimate age. An integrative review was performed by searching the keywords "computed tomography AND age estimation AND pulp tooth volume ratio" and "microfocus x-ray AND age estimation AND pulp tooth volume ratio" in the electronic platforms Embase, PubMed, Scopus and Web of Science, in July 2020. Studies included: complete articles with age estimation from human teeth by use of computed tomography or micro-computed tomography, written in English, without time restriction. Excluded studies: not written in English or not in form of an article, clinical cases, literature reviews, if did not realize age estimations or if age estimation is done in animals teeth. The search resulted in 32 different articles. With application of the above criteria only 26 were reviewed. Data collected included: reference, year, country, tooth type, number of subjects, number of teeth, age group, image type, measuring instrument/software, type of analysis, coefficient of determination, correlation coefficient with age and accuracy. These data provided a quick global comparison of various methodologies that use the pulp/tooth relation, being practical for researchers and forensic team to which method they can use accordingly to a specific case and its expected accuracy.


Introduction
Age estimation in forensic and anthropological research has been extensively studied in recent years and it contributes to the creation of the individual's biological profile, whether alive or dead. Various parts of the body can be used to estimate age, but it is remarkable the strong resistance of teeth against mechanical, chemical and physical effects of time, and their ability to preserve their morphology for a longer period of time 1,2 . In adults, the dental maturation process is already completed, and their corresponding age estimation techniques are based on degenerative processes, most of them complex and invasive, requiring the extraction or destruction of the tooth by sectioning, which is unfeasible in living individuals 3 .
The evaluation of deposition of secondary dentine is an observable phenomenon that it does not need the destruction of the tooth. This deposition begins after tooth eruption, resulting in the gradual decrease in the size of the pulp cavity with age 4 , being a significant morphological criterion for age estimation in adults 5 .
This phenomenon can be evaluated using the methods of pulp/tooth area ratio or pulp/tooth volume ratio, through imaging scans 6 . According to Zhang et al. 7 one or two-dimensional measuremants cannot reflect real three-dimensional morphological changes because secondary dentin deposition occurs on all surfaces of the pulp cavities. So, this study will perform an integrative review with the aim to provide to researchers and forensic team a quick and global comparison of studies that used three-dimensional radiological methods to analyze this deposition with the purpose of age estimation.

Sources of information
An integrative method of literature searching and selection was employed in the preparation of this review. The international electronic platforms Embase, PubMed, Scopus and Web of Science were used for searching, using the following keywords: "computed tomography AND age estimation AND pulp tooth volume ratio" and "micro focus x-ray AND age estimation AND pulp tooth volume ratio". Research on these platforms was performed on 07/09/2020. There was no restriction as to the date of publication of the studies.

Inclusion and exclusion criteria
The studies were included if they are complete research articles reporting age estimation research in human teeth, using computed tomography or computed micro tomography, were written in English, and had no time restriction. The studies would be excluded if they were not written in English, were not in the form of an article, reported clinical cases, were literature reviews, did not realize age estimations or the age estimation is done in animals' teeth.

Selection of studies
An initial selection was based on the title and the abstract, applying the inclusion and exclusion criteria. Afterward, the remaining articles had their full text printed and the eligibility criteria were once more applied.

Data collection process
Two authors performed the search on the platforms and applied inclusion and exclusion criteria. The data from each study were collected by one reviewer and then checked independently by the second reviewer.

Data collected
The data collected were organized in a Microsoft Excel spreadsheet and they included: reference, year, country, tooth type, number of subjects, number of teeth, age group (years), image type, measuring instrument/software, type of analysis, coefficient of determination, correlation coefficient with age and accuracy.

Results
Search in the platforms resulted in a total of 32 different articles. The amount of articles found per platform is observed in Table 1. By applying the inclusion and exclusion criteria, 6 articles were excluded, and 26 were reviewed. The reasons for exclusion were: two of them did not realize age estimation 8,9 , two were not in the form of an article 10,11 , one was not a research article 12 and another one used digital panoramic radiographs 13 . Age estimation through analysis of the pulp/tooth relation can be performed using any type of tooth and, in order to be applicable in a population level, it is interesting that the study uses a significant sample. Such information of each study is presented in Table 2. Various software can be applied when analyzing three-dimensional radiographic images, allowing linear as well as area and volume measurements.
These data are listed in Table 3.   Table 4 presents the coefficient of determination and the correlation coefficient achieved by each study, and Table 5 shows the accuracy found. In the tables, the tooth number will be listed according to the dental numbering system of the FDI (Federation Dentaire International).

Discussion
The majority of studies assessed the maxillary canine and maxillary central incisor.
The chosen of the former is due to being the tooth with the greatest longevity in the dental arch 3,21,28 , suffering the effects of diet less than the posterior teeth, suffering less attrition and abrasion than the anterior teeth and being the single-rooted tooth with the widest pulp area and, therefore, easier to analyze 3,28 . And the chosen of the maxillary central incisor can be explained for having a shorter length than the canines, larger pulp cavities than the lower incisors and a simpler root morphology than premolars and molars 23 , also allowing a simple morphometric analysis.  About the sample, when one proposes to determine a method in a population level, a considerable sample amount is expected, and the more expressive number were in the studies of Kazmi et al. 4 , Zhan et al. 32 and Gulsahi et al. 5 .
The choice of analyze studies which used three-dimensional (3D) radiographic images derive from the evidence that is not appropriate to measure the volume of a three-dimensional object, like the pulp cavity, with 2D radiographs 18,19,32 .
The Micro-CT although provides better resolutions than CBCT and MDCT, increasing accuracy in detection of the boundaries between pulp and hard tissue 14,17,27 , is unpractical in examining living individuals because the tooth extraction or jaw dissection is often required because of the limited scanning area 17,24,32 .
The pulp/tooth volume ratio can be considered more reliable than calculating the pulp/tooth area ratio, because secondary dentin formation may not be uniform throughout the surface of pulp 15 and, consequently, measures of projected areas can give incorrect impressions of the extent of this process 5,6,23 .
Some authors made analysis of the pulp/tooth relation excluding the enamel 3,17 because of some reasons as the risk that the volume of the pulp cavity has decreased due to attrition of the enamel or the crown wears out due to attrition.
Others authors recommended only the use of pulp volume as an indicator for age estimation, without measuring any dental hard tissue 4,27 . In the study of Akay et al. 1 , the authors found a better correlation between variability of pulp volume and age than to use the tooth volume. However, Zhang et al. 7 reported that the use of pulp volume as the independent variable could be problematic because it varies from person to person and even people of the same age do not always have the same pulp volume.
For three-dimensional exams where there is volumetric calculation, various existing software allow a three-dimensional reconstruction based on segmentation of the different structures, which is achieved through established thresholds of different image densities. A manual segmentation allows a more controlled process on images segmentation than the automatic method 15 . And when segmentation of small structure details like the apical portion of the tooth is needed, a manual intervention can be necessary 15,18,19 .
About the influence of sex on age estimation, most studies found no significant influence 2,3,[5][6][7]15,18,19,22,24,28,29 . However, Porto et al. 23 and Someda et al. 27 found the greater accuracy of results for women, while Akay et al. 1 and Asif et al. 18 found a higher correlation between pulp/tooth volume or area and age for men.
Even without having found statistically significant differences on the influence of sex, there were authors who developed separate formulas for men and for women 15,32 and others that used a factor specific to sex in their equation improving the predicative power of the model 4,24 . For more reliable results when sex is known, Porto et al. 23 and Someda et al. 27 encourage the selection of an age estimation equation for one sex in particular.
Dental development shows variation between individuals of the same population and between individuals of different regional population groups, so Asif et al. 18  Three-dimensional radiological exams in which the deposition of secondary dentine and the pulp/tooth relation can be evaluated have an auxiliary role in age estimation, achieving in some studies great accuracies. So, the use of methods considering the pulp/tooth relation can be applied in conjunction with other validated technique of age estimation using tooth or other body parts in order to reduce the fluctuation age range.

Conclusion
According to the literature, it can be concluded that assessment of the pulp/tooth relation, is a significant and often used method for age estimation in adults. Although this method can be applied to all kinds of tooth, through several types of imaging exams, maxillary canine and maxillary central incisor were the most widely used, as well as cone-beam computed tomography, which was the most observed exam. In addition, it was a consensus among the evaluated studies that specific formulas must be developed for the specific population of the sample. This integrative review is limited in regards to the quantity and quality of studies selected. However, a review of the subject is important for updating researchers on the existing, most used and most accurate methods. Likewise, this review is also addressed to the forensic team, guiding them as to which method may be used and what its expected accuracy, in accordance with the tooth and the resources available for each case.

Conflict of interest
No potential conflict of interest was reported by the authors.