1. Introduction
Forensic odontology is a branch of dentistry that complements forensic medicine by offering its knowledge of the dental landscape. It is not simply a matter of mere notions for the purpose of assessing, in the event of litigation, who is right between the two parties, but also a set of skills that allow oral anatomical structures to be analyzed in order to attribute them to a certain individual, ethnic group or race when normal methods prove to be ineffective [
1].
The two sexes have important anatomical differences (one need only think of the urogenital apparatus or the conformation of the pelvis [
2]), and over the years there has been a focus on the possibility of discriminating between them through an analysis of the oral cavity, with all the limitations that this entails.
Oral structures designed for this purpose must be stable over time and difficult to alter. For example, dental elements are subject to wear and tear, and the pathologies that can affect them lead to their subsequent reconstruction, with the alteration of certain anatomical details such as grooves, ridges or dimples, which, naturally, are not reproduced to identically match the original state of the tooth.
In contrast, structures such as the hard palate [
3] and the lips [
4] are stable in time and are rarely altered, except in cases of oral surgery, for example in connective tissue sampling in the case of the palate or biopsies in both cases.
Several studies have investigated the possibility of discriminating between the two sexes by evaluating palatine rugoscopy or cheiloscopy. However, none of these studies appear to be exhaustive on sex identification and most focus only on one type of analysis.
It is the authors’ opinion that the lack of comprehensiveness could be due to several factors, such as the efficiency of the classifications or the method used.
Palatine rugae are anatomical structures that are unique to each individual [
5], consisting of a large proportion of connective tissue covered by the keratinized tissue present in the hard palate. They are studied in forensic odontology by means of palatine rugoscopy by assessing their number, shape and size [
6].
Cheiloscopy, on the other hand, deals with the study of labial impressions, which, like palatine rugae, are stable over time and have a pattern unique to each individual [
6].
This study aims to discriminate the two sexes on the basis of the morphology of the palatine rugae and labial wrinkles using palatine rugoscopy and cheiloscopy, respectively.
4. Discussion
The aim of this study was to identify morphological differences between the sexes in palatine rugae and labial rugae. Through a comparison of our results with those found in literature, numerous differences can be observed.
With regard to palatine rugoscopy, substantial differences can probably be attributed to variations between populations, as shown in two studies based on different ethnicities [
9,
10]. Furthermore, not all studies use the Thomas and Kotze classification.
A study on the Iranian population found that the most common pattern of palatine rugae is straight, followed by wavy and curvy, without distinction between the sexes [
11]. In our study, the straight pattern was the one that was most present in both sexes, with no statistically significant differences. However, the diverging pattern was seen to be more common than the curvy one, which was still more frequent in women than men but without statistically significant differences.
A further study on the Tibetan and Indian population identified the diverging pattern to be the most common [
12]. Similarly, a study on Iranian children found no difference in the total number of rugae between the sexes, but indicated a greater presence of rugae on the right side in the female group and a predominance of the curvy pattern in men, in contrast with our results [
13].
The data collected in our sample confirm what has been reported with regard to a population in Kerala, namely that the circular pattern is rare [
14]. However, in our study, we observed no significant differences between men and women. Similar to a study on the Central Indian population, we found that men tend to have more wavy rugae and women more straight rugae, although these differences were not statistically significant in our study [
15].
In a study on the Maharashtrian population, the wavy pattern prevailed in males, but this was a very small population [
16]. In concordance with a study on the Dravidian population, we observed more curvy rugae in women than in men, with a statistically significant result [
17].
A study on children in Davangere showed differences between men and women in converging and diverging patterns. These differences were not statistically significant in our study [
18]. However, the curvy pattern was more frequent in men, a situation not observed in our data, where the total number of curvy rugae was greater in the female group.
In a study on 100 Sudanese, a higher frequency of converging rugae on the left side of the palate was observed in males, a finding also observed in our study [
19]. A study on the Mediterranean population found no statistically significant differences between the groups [
20], while a study on five different Indian populations found significant differences in the number of rugae between the right and left side in the female group [
21]. In our study, this difference was observed in the male group with statistical significance. In contrast, significant differences were found between the male group and the female group in circular and converging rugae in 100 subjects from Meerut [
22].
In general, with palatine rugoscopy, we note how difficult it is to obtain concrete data capable of discriminating between the two sexes, probably due to the limited size of the samples studied. Some studies have proposed differential functions, such as a population study in coastal Andhra [
23]. One study revealed similarities between relatives, suggesting a certain heritability of palatine rugae patterns [
24].
The results obtained in our study on palatine rugoscopy, although based only on the shape of the rugae and not on their size, determine that there are differences in the number of curvy rugae between the two sexes and that there are differences between the total number of male rugae on the right and left side, but these differences were not found in the female group. The only statistically significant results obtained in rugoscopy do not allow discrimination between the two sexes as it can correctly discriminate sex only in a few individuals. However, if there were to be a database, it could be used in rare cases to identify an unrecognizable subject [
25].
Even in the field of cheiloscopy, data are not always in agreement. On the one hand, this may be attributable, in part, to variations between populations, but on the other hand, several discrepancies arise from the numerous methods used in literature [
26] and the difficult interpretation of the data collected, where operator-dependent errors may be frequent [
31].
Unlike in rugoscopy, where the methods used are analogue, with alginate impressions, and digital, with the aid of intraoral scanners, in cheiloscopy we can photograph the subject’s lips directly, use a non-porous surface (e.g., a mirror) to photograph latent impressions, apply lipstick or other transfer medium and press the lips onto paper or adhesive tape, press the lips onto a suitable surface and develop impressions with fingerprint powder or magnetic powder [
27,
28]. There is an incalculable margin of error that derives from the methodology chosen [
29], both with regard to the different materials that can be used [
30], and the difficulty of taking impressions that present all the lip lines intact, without smears and easily readable [
31]. Furthermore, the operator-dependent error derived from the analysis of the impressions themselves must also be taken into account [
32].
The literature reports extremely discordant results concerning the possible use of cheiloscopy to determine sex, and the articles that have found statistically significant differences do not always agree with each other.
In a study of 2112 individuals from the population of Calicut, Kenya, a predominance of Type 1 and 1′ was found in male subjects and Type 4 and 5 in female subjects [
33], a difference partially observed in our study and limited to the second sextant, where Type 1 was found to be predominant in men and Type 4 in women. This predominance of Type 4 in the female gender has also been observed in other studies [
34,
35].
A study of 100 students at the Shri Sathya Sai Medical College and Research Centre also divided the lip prints into sextants and analyzed the samples, finding statistically significant differences at sextants 1, 3, 4 and 6 [
36], the exact opposite of the results in our study, which show sex-related differences only in the second sextant, although these were not statistically significant.
Another study of 600 individuals from rural and urban locations in Aurangabad, Maharashtra, India, observed that in the upper lip, Type 4 was more common in the lateral segments of females than males, while Type I was more common in the lateral segments of males than females, resulting in statistically significant results between the sexes in the lateral segments [
37], while this difference was not significant in the medial segments, a situation again not observed in our study.
Because of the data obtained in our study, which show the absence of statistically significant results, and because of the great heterogeneity of the results found in literature, we believe that cheiloscopy is not a reliable approach for determining sex. Even two recent systematic reviews report discordant opinions. The first [
26] agrees with us regarding the problems of methodology and considers cheiloscopy invalid for determining the sex of a subject; the second, which analyses both rugoscopy and cheiloscopy, finds the method even more reliable than rugoscopy, with an accuracy of 80%, [
38].
Despite this, we consider cheiloscopy to be a valid tool for 1-1 matching, e.g., in the field of criminology if a latent print can be found, as lip prints are unique and temporally stable [
8,
39,
40,
41,
42,
43].
In the light of the results observed in the literature, the effectiveness of both palatine rugoscopy and cheiloscopy is poor. The data obtained on small populations certainly make an anthropological contribution but do not allow the two sexes to be discriminated effectively, even within the same populations.
We therefore suggest that although studies on small populations may have statistically valid results on anatomical traits of palatine rugae and labial rugae, they do not allow effective discrimination between sexes, even within the same populations.
On the basis of our results and on the comparison with the various studies in literature, we can state that, despite some differences in the number of curvy rugae between the sexes and between the sides of the palate in men, these are not sufficient for effective sex discrimination. The data obtained in our study indicate a poor potential of palatine rugoscopy and a total ineffectiveness of cheiloscopy in discriminating between sexes. The combined use of palatine rugoscopy and cheiloscopy, therefore, did not provide statistically significant results sufficient to discriminate the sexes efficiently..
Author Contributions
Conceptualization, G.A.S. and P.M.; methodology, A.T., E.D.V., F.M.S., S.V., L.R.; software, ; validation, A.A., D.A. and S.Z.; formal analysis, E.C.; investigation, A.T., E.D.V., F.M.S., S.V., L.R. ; data curation, X.X.; writing—original draft preparation, A.T., E.D.V., F.M.S., S.V., L.R.; writing—review and editing, E.C., G.B.; supervision, P.M., G.A.S.
All authors have read and agreed to the published version of the manuscript.