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Medical Science Monitor: International Medical Journal of Experimental and Clinical Research logoLink to Medical Science Monitor: International Medical Journal of Experimental and Clinical Research
. 2021 Dec 22;27:e933450-1–e933450-17. doi: 10.12659/MSM.933450

Permanent Maxillary Odontometrics for Sex Estimation Based on a 3-Dimensional Digital Method

Jialin Liu 1,E,*, Yanshi Liu 2,A,*, Jian Wang 3,C, Shupeng Ge 1,D, Yangyang Zhang 4,F, Xiaohe Wang 4,B, Lijuan Du 5,B, Huiyu He 1,G,✉
PMCID: PMC8711212  PMID: 34934039

Abstract

Background

In the field of forensic medicine, sex estimation is a critical step in personal identification. Teeth are the hardest tissue and have high temperature resistance and corrosion resistance. In cases such as an airplane crash or the corpse of an unknown person, teeth often play a crucial role in identification. This study applied 3-dimensional technology to obtain odontometrics of permanent maxillary teeth and to examine the sexual dimorphism, finding suitable discriminant indicators to construct appropriate equations for sex estimation.

Material/Methods

A total of 204 participants (104 men and 100 women) from the Han population in Kashgar were included. Plaster models of their maxillary dentition were obtained to scan and measure through an accepted and commonly used 3-dimensional digital method. Descriptive statistics, t tests, and discriminant analyses were statistically analyzed using IBM SPSS 23.0 software.

Results

This study showed high intra- and interexaminer reliability (intraclass correlation coefficient >0.950). There were statistically significant sex-related differences (P<0.05), with male values generally being higher for buccolingual distance, mesiodistal distance, intercanine distance, crown area, crown module, crown index, and maxillary canine index. Compared with other measurements, mesiodistal distance and crown area indicator exhibited distinct sexual dimorphism. In addition, several appropriate equations were constructed through different discriminant analyses that could be used to estimate sex in our specific population.

Conclusions

Three-dimensional digital technology offers a promising method for odontometry. Combining mesiodistal distance and buccolingual distance of particular teeth or using maxillary canine index in discriminant functions are acceptable auxiliary tools for sex estimation in the forensic field.

Keywords: Discriminant Analysis; Imaging, Three-Dimensional; Odontometry; Sex Differentiation

Background

Sex estimation is a crucial procedure for individual identification. The differences due to sexual dimorphism can be detected in tissue structure, shape, and size in the same population and are genetically based [1]. Identifying an individual’s sex through gene detection is an effective method, but extracting nuclear DNA from degraded samples can be difficult, resulting in complicated procedures and high costs [2]. Compared with the organs of the human body, teeth are the hardest and long-lasting tissue, and they can resist damage due to bacterial degradation, high temperature, and erosion in terms of their physical and chemical properties. Even in massive disasters, teeth can withstand destruction and breakage [3]. Dental sexual dimorphism has been extensively studied in recent years, and a variety of crown diameters, related indices, equation algorithms, and measuring methods have been developed for sex estimation in different populations.

In 1957, Jensen et al [4] first reported measurement of teeth based on a US population. Other researchers subsequently discovered that odontometrics differ by region, race, and sex, and many studies estimated sex based on different dental measurements or combined indicators. Owsley and Webb [5] correctly distinguished the sex of Whites in the United States based on odontometrics, showing that the combinational algorithms for different teeth can be used for sex estimation. At present, the dental measurements and indicators used to estimate sex include mesiodistal distance (MD), buccolingual distance (BL), crown area (CA), crown module (CM), crown index (CI), and maxillary or mandibular canine index (MCI). Researchers from various countries have applied different odontometrics and established various sex prediction models to estimate sex [6–10], and most propose that the sex differences in tooth size and the sex prediction accuracy from indicators or equations differ by region and ethnic group, suggesting the need to establish reference data and prediction models for specific populations in specific regions.

The purpose of the current study was to obtain sex-related differences in odontometrics of the Han population in Kashgar using a 3-dimensional (3D) technique and to evaluate the probability of a male or female individual from odontometrics. This work applied an advanced dental measuring method to provide updated odontometric data for the population, verified the accuracy of existing indicators in sex detection, and used statistical discrimination analysis to calculate new functions that are applicable for estimating sex based on teeth in the Han population.

Material and Methods

Study Participants

Study participants were 18- to 25-year-old Han students from local universities and vocational technical colleges in Kashgar, China. The sample size was designed to detect a mean difference of 0.2 mm with a standard deviation of 0.50 mm at 80% power and α=0.05, with a 95% level of confidence. The calculated sample size was 196 participants (98 men and 98 women). To compensate for the expected 20% data loss, we recruited 236 participants.

Using a simple random sampling method (computer random number), 236 dental plaster models were selected from the dentition plaster model library of Han students in Kashgar established by our research group in a previous project. We selected 204 dental plaster models based on inclusion and exclusion criteria; 32 were eliminated. The selected participants were 21.60±1.59 years old on average and included 104 men and 100 women. This investigation was designed and conducted according to the guidelines of Strengthening the Reporting of Observational studies in Epidemiology (STROBE), and we applied the STROBE specification in this article [11].

Ethical Approval and Consent to Participate

This study was conducted with the approval of the Ethics Committee in the Xinjiang Medical University Affiliated First Hospital, in accordance with the standards of the Declaration of Helsinki and International Ethical Guidelines for Biomedical Research Involving Human Subjects. We obtained written informed consent from all participants.

Inclusion and Exclusion Criteria

Inclusion criteria required participants to be 18- to 25-year-old residents of Kashgar (≥3 years) and Han ethnicity (to at least their grandparents’ generation) without any orthodontic treatment. Participants needed to have healthy gingival tissue and periodontium, individual normal occlusal dentition with the exception of the third molars, normal molar and canine relationships, normal overjet and overbite relationships (2–3 mm), and caries-free maxillary teeth.

The exclusion criteria included visible defects in the canine cusps, proximal-distal adjacent points, and labial/buccal/palatine surfaces of the maxillary teeth; damage to the dentition models during transportation or storage; moderate crowding (>4 mm) or visible spacing in the dentition; identification number mismatch between the dentition model and the corresponding participant; and incorrect, unclear, or missing recorded information.

Definition of Measurement Indicators

The linear measurements taken were the mesiodistal and buccolingual distances of maxillary teeth and the maxillary intercanine distance. The mesiodistal distance of each tooth was obtained by measuring the maximum distance between the contact points on the mesial and distal surfaces of the crown [4]. Buccolingual distance was recorded as the greatest distance between the buccal and lingual surfaces of the crown [12]. Intercanine distance was measured between the tips of the maxillary canines [13].

The following indices were calculated from linear measurements taken in accordance with El Sheikhi and Bugaighis [14]. Crown index (CI) was calculated as (BL/MD)×100, and it gave an indication of the overall tooth shape. Crown area (CA) was calculated as MD×BL, and it indicated the overall tooth size. Crown module (CM) was equal to (BL+MD)/2, and it presented an overall picture of tooth size. Maxillary canine index (MCI) was equal to the canine MD divided by the maxillary intercanine distance. The percentage of sexual dimorphism was calculated as (M/F-1)×100, where M and F represented the mean dimensions of the male and female parameters, respectively. In general, 29 linear measurements were recorded, and from those measurements, 44 indices were computed in each dentition model.

Three-Dimensional Digitization Method

The 3D digital models were obtained by scanning the dental plaster models through the CAD/CAM system (Langcheng Company, China) with a dl-100 type scanner (accuracy up to ±10 μm). The documents were exported in STL format and imported into Geomagic 2015 reverse engineering software (Geomagic Company, USA) to recalibrate the models, create the coordinate systems, mark the points, and measure the indicators. Details can be found in a previous study [15]. Linear measurements are shown in Figures 1 and 2.

Figure 1.

Figure 1

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Measurements on mesiodistal distance (A) and buccolingual distance (B) of maxillary teeth on 3-dimensional digital dentition.

Figure 2.

Figure 2

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Maxillary canine cusp points and intercanine distance on 3-dimensional digital dentition.

Data Analysis

Intraclass correlation coefficients (ICCs) were calculated by SPSSAU project (QingSi Technology Ltd, China) 2019 version 20.0 online application software (retrieved from http://www.spssau.com). The data were analyzed using IBM SPSS Statistics version 23.0 (IBM Corp., Armonk, NY, USA) with 5% (P≤0.05) confidence level to test for significance. Results are reported as mean±standard deviation. Independent samples t test was used to compare sex-based groups. General and stepwise discriminant analysis were utilized to determine discriminant score of functions established for sex estimation. Twenty 3D dental models were randomly selected for intra- and interobserver error calculations. The time interval between the first and second marked-point measurements was approximately 2 weeks.

Results

Intra- and Interexaminer Reliability

High intra- and interexaminer reliability was found by ICC analysis (ICC >0.950, Table 1), which confirmed that the results were reliable.

Table 1.

The intra- and interexaminer reliability (n=20).

Teeth Variables Reliability ICC
11 BL Intra-examiner 0.969
Inter-examiner 0.957
MD Intra-examiner 0.988
Inter-examiner 0.984
12 BL Intra-examiner 0.992
Inter-examiner 0.989
MD Intra-examiner 0.989
Inter-examiner 0.985
13 BL Intra-examiner 0.994
Inter-examiner 0.991
MD Intra-examiner 0.983
Inter-examiner 0.980
14 BL Intra-examiner 0.991
Inter-examiner 0.990
MD Intra-examiner 0.985
Inter-examiner 0.983
15 BL Intra-examiner 0.989
Inter-examiner 0.985
MD Intra-examiner 0.984
Inter-examiner 0.981
16 BL Intra-examiner 0.994
Inter-examiner 0.991
MD Intra-examiner 0.990
Inter-examiner 0.986
17 BL Intra-examiner 0.995
Inter-examiner 0.991
MD Intra-examiner 0.990
Inter-examiner 0.987
21 BL Intra-examiner 0.966
Inter-examiner 0.958
MD Intra-examiner 0.987
Inter-examiner 0.982
22 BL Intra-examiner 0.993
Inter-examiner 0.989
MD Intra-examiner 0.990
Inter-examiner 0.986
23 BL Intra-examiner 0.995
Inter-examiner 0.991
MD Intra-examiner 0.984
Inter-examiner 0.981
24 BL Intra-examiner 0.992
Inter-examiner 0.990
MD Intra-examiner 0.984
Inter-examiner 0.981
25 BL Intra-examiner 0.989
Inter-examiner 0.986
MD Intra-examiner 0.983
Inter-examiner 0.980
26 BL Intra-examiner 0.993
Inter-examiner 0.990
MD Intra-examiner 0.990
Inter-examiner 0.987
27 BL Intra-examiner 0.995
Inter-examiner 0.992
MD Intra-examiner 0.989
Inter-examiner 0.985
Maxillary inter-canine distance Intra-examiner 0.999
Inter-examiner 0.998
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11 – the right central incisor; 12 – the right lateral incisor; 13 – the right canine; 14 – the right first premolar; 15 – the right second premolar; 16 – the right first molar; 17 – the right second molar; 21 – the left central incisor; 22 – the left lateral incisor; 23 – the left canine; 24 – the left first premolar; 25 – the left second premolar; 26 – the left first molar; 27 – the left second molar; BL – buccolingual distance; ICC – intraclass correlation coefficient; MD – mesiodistal distance.

Descriptive Statistics and Comparison Between Sexes

Among the study participants, 104 (50.98%) were men and 100 (49.02%) were women. The mean values of crown dimensions and intercanine distance were significantly higher in men than in women, with the exception of buccolingual distances of center and lateral incisors for which the differences were not significant (Table 2).

Table 2.

Comparison of crown dimensions and intercanine distance in men and women.

Teeth Variables Group Number (n) Mean (mm) SD (mm) 95% Confidence interval t-value p-value
Lower Upper
11 BL Male 104 5.942 0.690 5.808 6.076 0.508 0.612
Female 100 5.898 0.546 5.790 6.006
MD Male 104 8.558 0.467 8.467 8.648 3.341 0.001
Female 100 8.327 0.518 8.224 8.430
12 BL Male 104 5.446 0.655 5.318 5.573 0.643 0.521
Female 100 5.395 0.456 5.305 5.485
MD Male 104 6.995 0.530 6.891 7.098 2.429 0.016
Female 100 6.830 0.433 6.744 6.915
13 BL Male 104 7.151 0.656 7.023 7.278 2.710 0.007
Female 100 6.914 0.592 6.796 7.031
MD Male 104 7.846 0.414 7.766 7.926 5.580 <0.001
Female 100 7.531 0.393 7.453 7.609
14 BL Male 104 9.514 0.677 9.382 9.645 3.700 <0.001
Female 100 9.129 0.803 8.970 9.289
MD Male 104 7.002 0.454 6.914 7.090 4.410 <0.001
Female 100 6.760 0.321 6.696 6.824
15 BL Male 104 9.236 0.563 9.126 9.345 2.148 0.033
Female 100 9.055 0.635 8.929 9.181
MD Male 104 6.709 0.481 6.615 6.802 4.375 <0.001
Female 100 6.453 0.345 6.385 6.522
16 BL Male 104 10.284 0.642 10.159 10.409 2.793 0.006
Female 100 10.046 0.574 9.932 10.159
MD Male 104 10.516 0.519 10.416 10.617 5.961 <0.001
Female 100 10.122 0.418 10.039 10.205
17 BL Male 104 10.476 0.673 10.345 10.607 2.575 0.011
Female 100 10.257 0.540 10.149 10.364
MD Male 104 9.398 0.551 9.290 9.505 3.149 0.002
Female 100 9.159 0.528 9.055 9.264
21 BL Male 104 5.954 0.615 5.832 6.076 0.637 0.525
Female 100 5.897 0.651 5.771 6.024
MD Male 104 8.583 0.482 8.489 8.676 3.504 0.001
Female 100 8.340 0.506 8.240 8.441
22 BL Male 104 5.492 0.590 5.378 5.607 0.580 0.562
Female 100 5.449 0.468 5.356 5.542
MD Male 104 7.027 0.512 6.928 7.127 3.082 0.002
Female 100 6.818 0.456 6.727 6.908
23 BL Male 104 7.028 0.656 6.900 7.155 2.710 0.007
Female 100 6.791 0.592 6.673 6.908
MD Male 104 7.848 0.427 7.765 7.931 5.615 <0.001
Female 100 7.527 0.387 7.450 7.604
24 BL Male 104 9.465 0.633 9.342 9.588 2.557 0.011
Female 100 9.251 0.556 9.141 9.362
MD Male 104 7.067 0.465 6.977 7.158 4.654 <0.001
Female 100 6.785 0.397 6.707 6.864
25 BL Male 104 9.228 0.549 9.121 9.335 2.358 0.019
Female 100 9.042 0.576 8.928 9.157
MD Male 104 6.721 0.430 6.638 6.805 4.516 <0.001
Female 100 6.467 0.373 6.393 6.541
26 BL Male 104 10.424 0.697 10.289 10.560 2.736 0.007
Female 100 10.189 0.522 10.086 10.293
MD Male 104 10.528 0.503 10.430 10.626 5.856 <0.001
Female 100 10.124 0.480 10.029 10.219
27 BL Male 104 10.386 0.716 10.247 10.525 2.473 0.014
Female 100 10.152 0.629 10.027 10.277
MD Male 104 9.457 0.499 9.360 9.554 3.014 0.003
Female 100 9.242 0.517 9.140 9.345
Maxillary inter-canine distance Male 104 36.649 2.037 36.253 37.045 2.686 0.008
Female 100 35.888 2.006 35.490 36.286
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11 – the right central incisor; 12 – the right lateral incisor; 13 – the right canine; 14 – the right first premolar; 15 – the right second premolar; 16 – the right first molar; 17 – the right second molar; 21 – the left central incisor; 22 – the left lateral incisor; 23 – the left canine; 24 – the left first premolar; 25 – the left second premolar; 26 – the left first molar; 27 – the left second molar; BL – buccolingual distance; MD – mesiodistal distance; SD – standard deviation.

The indices calculated from the measurements are show in Table 3. The mean values were higher in men than in women for CA, CM, and MCI, whereas CI was the opposite. CA, CM, and MCI of canine; CA and CM of the first premolar and the second molar; CA, CM, and CI of the second premolar and the first molar were significantly different between men and women.

Table 3.

Comparison of crown indices between men and women.

Teeth Variables Group Number (n) Mean (mm) SD (mm) 95% Confidence interval t-value p-value
Lower Upper
11 CA Male 104 51.022 7.863 49.493 52.552 1.682 0.094
Female 100 49.273 6.942 47.895 50.650
CI Male 104 69.410 6.808 68.086 70.734 −1.703 0.090
Female 100 70.876 5.433 69.798 71.954
CM Male 104 7.250 0.510 7.151 7.349 1.995 0.047
Female 100 7.113 0.471 7.019 7.206
12 CA Male 104 38.258 6.731 36.949 39.567 1.657 0.099
Female 100 36.924 4.602 36.011 37.838
CI Male 104 77.959 8.075 76.389 79.530 −1.148 0.252
Female 100 79.144 6.623 77.830 80.459
CM Male 104 6.220 0.512 6.120 6.320 1.724 0.086
Female 100 6.112 0.373 6.038 6.186
13 CA Male 104 56.232 7.047 54.862 57.603 4.365 <0.001
Female 100 52.154 6.258 50.912 53.395
CI Male 104 91.184 7.446 89.736 92.632 −0.686 0.493
Female 100 91.898 7.415 90.427 93.370
CM Male 104 7.498 0.463 7.408 7.588 4.487 <0.001
Female 100 7.222 0.414 7.140 7.304
MCI Male 104 0.215 0.013 0.212 0.217 2.331 0.021
Female 100 0.210 0.013 0.208 0.213
14 CA Male 104 66.855 8.643 65.174 68.535 4.533 <0.001
Female 100 61.815 7.195 60.388 63.243
CI Male 104 135.129 6.058 132.948 137.121 −0.658 0.511
Female 100 135.943 10.995 134.765 137.311
CM Male 104 8.258 0.536 8.153 8.362 4.355 <0.001
Female 100 7.945 0.488 7.848 8.042
15 CA Male 104 62.142 7.569 60.670 63.614 3.728 <0.001
Female 100 58.529 6.233 57.292 59.766
CI Male 104 137.948 7.368 136.515 139.381 −2.096 0.037
Female 100 140.516 9.980 138.535 142.496
CM Male 104 7.972 0.478 7.879 8.065 3.454 0.001
Female 100 7.754 0.421 7.671 7.838
16 CA Male 104 108.376 11.090 106.219 110.533 4.620 <0.001
Female 100 101.834 9.069 100.035 103.634
CI Male 104 97.826 4.594 96.932 98.719 −2.300 0.022
Female 100 99.262 4.315 98.406 100.118
CM Male 104 10.400 0.533 10.297 10.504 4.570 <0.001
Female 100 10.084 0.451 9.994 10.173
17 CA Male 104 98.524 9.531 96.671 100.378 3.304 0.001
Female 100 94.135 9.435 92.263 96.007
CI Male 104 111.785 8.965 110.041 113.528 −0.321 0.748
Female 100 112.108 4.879 111.140 113.076
CM Male 104 9.937 0.476 9.844 10.029 3.388 0.001
Female 100 9.708 0.489 9.611 9.805
21 CA Male 104 50.785 7.739 49.280 52.290 0.876 0.382
Female 100 49.846 7.571 48.344 51.348
CI Male 104 68.705 6.277 67.485 69.926 −3.168 0.002
Female 100 71.384 5.776 70.238 72.530
CM Male 104 7.240 0.500 7.143 7.337 1.323 0.187
Female 100 7.147 0.504 7.047 7.247
22 CA Male 104 38.734 6.070 37.553 39.914 1.918 0.057
Female 100 37.251 4.886 36.281 38.220
CI Male 104 78.288 7.603 76.809 79.766 −1.801 0.073
Female 100 80.053 6.312 78.801 81.306
CM Male 104 6.260 0.472 6.168 6.351 2.066 0.040
Female 100 6.133 0.397 6.055 6.212
23 CA Male 104 55.304 7.277 53.889 56.720 3.143 0.002
Female 100 52.269 6.509 50.977 53.560
CI Male 104 89.570 7.113 88.187 90.954 −2.555 0.011
Female 100 92.114 7.107 90.704 93.524
CM Male 104 7.438 0.479 7.345 7.531 4.404 <0.001
Female 100 7.159 0.425 7.074 7.243
MCI Male 104 0.215 0.013 0.212 0.217 2.453 0.015
Female 100 0.210 0.013 0.208 0.213
24 CA Male 104 67.104 8.281 65.493 68.714 3.949 <0.001
Female 100 62.918 6.747 61.579 64.256
CI Male 104 134.084 6.604 132.799 135.368 −2.584 0.010
Female 100 136.491 6.702 135.161 137.820
CM Male 104 8.266 0.510 8.167 8.365 3.730 <0.001
Female 100 8.018 0.435 7.932 8.105
25 CA Male 104 62.178 7.068 60.804 63.553 3.820 <0.001
Female 100 58.585 6.328 57.330 59.841
CI Male 104 137.516 7.139 136.127 138.904 −2.287 0.023
Female 100 140.033 8.542 138.338 141.728
CM Male 104 7.975 0.446 7.888 8.061 3.637 <0.001
Female 100 7.755 0.417 7.672 7.837
26 CA Male 104 109.999 11.681 107.728 112.271 4.626 <0.001
Female 100 103.291 8.892 101.527 105.055
CI Male 104 99.015 4.548 98.131 99.900 −2.649 0.009
Female 100 100.734 4.719 99.797 101.670
CM Male 104 10.476 0.559 10.368 10.585 4.549 <0.001
Female 100 10.157 0.440 10.069 10.244
27 CA Male 104 98.378 10.253 96.384 100.372 3.084 0.002
Female 100 94.017 9.930 92.047 95.987
CI Male 104 109.944 7.140 108.555 111.332 −0.019 0.985
Female 100 109.961 5.860 108.799 111.124
CM Male 104 9.921 0.522 9.820 10.023 3.096 0.002
Female 100 9.697 0.511 9.596 9.799
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11 – the right central incisor; 12 – the right lateral incisor; 13 – the right canine; 14 – the right first premolar; 15 – the right second premolar; 16 – the right first molar; 17 – the right second molar; 21 – the left central incisor; 22 – the left lateral incisor; 23 – the left canine; 24 – the left first premolar; 25 – the left second premolar; 26 – the left first molar; 27 – the left second molar; CA – crown area; CI – crown index; CM – crown module; MCI – maxillary canine index; SD – standard deviation.

Percentage of Sexual Dimorphism

With regard to sexual dimorphism in incisors, premolars, and the first molars, the results were MD >BL and CA >CM >CI. For canines, the results were MD >BL and CA >CM >MCI >CI. In the second molars, we found MD≈Bl and CA >CM >CI. Generally, the sexual dimorphism was evident for CA in maxillary teeth. In the comparison of crown dimensions, MD was higher than BL. Among crown indices, CA was the highest followed by CM, MCI, and CI. In a comparison of the sexual dimorphism of maxillary teeth parameters, canines, premolars, and the first molars were prominent (Table 4).

Table 4.

Percentage sexual dimorphism of maxillary teeth parameters.

Teeth Parameter sexual dimorphism (%)
BL MD CA CI CM MCI
11 0.749 2.769 3.551 −2.068 1.931
12 0.938 2.416 3.612 −1.498 1.764
13 3.433 4.187 7.821 −0.777 3.826 2.031
14 3.580 4.210 8.152 −0.598 3.942
15 1.991 3.960 6.173 −1.828 2.810
16 2.374 3.896 6.424 −1.447 3.138
17 2.141 2.599 4.662 −0.288 2.357
21 0.960 2.907 3.753 −1.884 1.300
22 0.796 3.071 3.981 −2.205 2.060
23 3.495 4.264 5.808 −2.762 3.899 2.084
24 2.311 4.155 6.653 −1.764 3.091
25 2.055 3.934 6.132 −1.798 2.839
26 2.309 3.987 6.495 −1.706 3.145
27 2.302 2.320 4.638 −0.016 2.311
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11 – the right central incisor; 12 – the right lateral incisor; 13 – the right canine; 14 – the right first premolar; 15 – the right second premolar; 16 – the right first molar; 17 – the right second molar; 21 – the left central incisor; 22 – the left lateral incisor; 23 – the left canine; 24 – the left first premolar; 25 – the left second premolar; 26 – the left first molar; 27 – the left second molar; BL – buccolingual distance; CA – crown area; CI – crown index; CM – crown module; MCI – maxillary canine index; MD – mesiodistal distance.

Discriminant Analysis

Discriminant function analysis was performed to predict sex. Mesiodistal distance, buccolingual distance, MCI, CA, CM, and CI were selected as predictor variables. D=k+a1x1+a2x2+…+anxn was the discriminant function form, where D was the discriminant score, k was the y-intercept, a was the coefficient, x was the discriminant variable, and n was the number of discriminant variables.

The general discriminant analysis was carried out in crown dimensions (MD and BL) for sex estimation (Table 5), and the accuracy rate can be seen in Figure 3. The highest overall accuracy was 67.6%, corresponding to the left first premolar and the first molar. The highest male accuracy was 67.3%, based on the left and the right first molars. The left first premolar provided the highest accuracy (72.0%) in distinguishing women.

Table 5.

General discriminant analysis in crown dimensions for sex estimation.

Teeth Function Discriminant function Wilks’ Lambda p-value Accuracy (%) Cutting point*
Male Female Overall
11 Crown dimensions −15.435-0.812BL+2.397MD 0.937 0.001 61.5 60.0 60.8 −0.005
12 Crown dimensions −13.183-0.427BL+2.242MD 0.970 0.048 53.8 52.0 52.9 −0.003
13 Crown dimensions −19.218+0.1BL+2.407MD 0.866 <0.001 65.4 64.0 64.7 −0.008
14 Crown dimensions −17.470+0.496BL+1.866MD 0.906 <0.001 55.8 68.0 61.8 −0.006
15 Crown dimensions −15.124-0.133BL+2.482MD 0.914 <0.001 57.7 68.0 62.7 −0.006
16 Crown dimensions −20.407-0.557BL+2.525MD 0.842 <0.001 67.3 64.0 65.7 −0.009
17 Crown dimensions −20.185+0.734BL+1.355MD 0.944 0.003 59.6 64.0 61.8 −0.005
21 Crown dimensions −12.993-1.310BL+2.452MD 0.899 <0.001 61.5 64.0 62.7 −0.007
22 Crown dimensions −12.388-0.642BL+2.296MD 0.951 0.006 55.8 64.0 59.8 −0.005
23 Crown dimensions −18.764-0.082BL+2.514MD 0.865 <0.001 59.6 56.0 57.8 −0.008
24 Crown dimensions −14.488-0.447BL+2.695MD 0.900 <0.001 63.5 72.0 67.6 −0.007
25 Crown dimensions −15.641-0.177BL+2.617MD 0.908 <0.001 61.5 68.0 64.7 −0.007
26 Crown dimensions −19.480-0.486BL+2.371MD 0.848 <0.001 67.3 68.0 67.6 −0.009
27 Crown dimensions −19.577+0.579BL+1.458MD 0.952 0.007 61.5 64.0 62.7 −0.004
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11 – the right central incisor; 12 – the right lateral incisor; 13 – the right canine; 14 – the right first premolar; 15 – the right second premolar; 16 – the right first molar; 17 – the right second molar; 21 – the left central incisor; 22 – the left lateral incisor; 23 – the left canine; 24 – the left first premolar; 25 – the left second premolar; 26 – the left first molar; 27 – the left second molar; BL – buccolingual distance; MD – mesiodistal distance.

*

A discriminant score less than the cutting point indicates a woman.

Figure 3.

Figure 3

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Comparison of the sex estimation accuracy through crown dimensions. 11– the right central incisor; 12 – the right lateral incisor; 13 – the right canine; 14 – the right first premolar; 15 – the right second premolar; 16 – the right first molar; 17 – the right second molar; 21 – the left central incisor; 22 – the left lateral incisor; 23 – the left canine; 24 – the left first premolar; 25 – the left second premolar; 26 – the left first molar; 27 – the left second molar.

The discriminant functions founded by indices for sex estimation are presented in Table 6, and the accuracy rate can be seen in Figure 4. Taking the MCI of the left canine into consideration, overall and female accuracy were up to 68.6% and 76.0%, respectively. While in males, accuracy was 69.2%, which corresponded to the CI of the right first molar.

Table 6.

Discriminant functions in indices for sex estimation.

Teeth Function Discriminant function Wilks’ Lambda p-value Accuracy (%) Cutting point*
Male Female Overall
11 Crown area −6.755+0.135CA 0.986 0.094 48.1 56.0 52.0 −0.002
Crown module −14.613+2.035CM 0.981 0.047 53.8 56.0 54.9 −0.002
Crown index −11.361+0.162CI 0.986 0.092 48.1 64.0 55.9 0.003
12 Crown area −6.499+0.173CA 0.987 0.101 48.1 60.0 53.9 −0.002
Crown module −13.724+2.225CM 0.986 0.088 50.0 60.0 54.9 −0.002
Crown index −10.615+0.135CI 0.994 0.254 53.8 52.0 52.9 0.002
13 Crown area −8.128+0.150CA 0.914 <0.001 61.5 68.0 64.7 −0.011
Crown module −16.746+2.274CM 0.909 <0.001 63.5 68.0 65.7 −0.006
Crown index −12.318+0.135CI 0.998 0.493 53.8 56.0 54.9 0.001
Maxillary canine index −16.240+76.447MCI 0.974 0.021 51.9 64.0 57.8 −0.003
14 Crown area −8.082+0.126CA 0.908 <0.001 53.8 68.0 60.8 −0.007
Crown module −15.785+1.948CM 0.914 <0.001 53.8 68.0 60.8 −0.006
Crown index −15.351+0.113CI 0.998 0.511 50.0 44.0 47.1 0.001
15 Crown area −8.691+0.144CA 0.936 <0.001 57.7 64.0 60.8 −0.005
Crown module −17.458+2.220CM 0.944 0.001 59.6 64.0 61.8 −0.005
Crown index −15.916+0.114CI 0.979 0.037 50.0 52.0 51.0 0.003
16 Crown area −10.362+0.099CA 0.905 <0.001 55.8 64.0 59.8 −0.007
Crown module −20.724+2.023CM 0.906 <0.001 57.7 64.0 60.8 −0.006
Crown index −22.096+0.224CI 0.974 0.022 69.2 60.0 64.7 0.003
17 Crown area −10.162+0.105CA 0.949 0.001 57.7 68.0 62.7 −0.004
Crown module −20.372+2.074CM 0.946 0.001 59.6 68.0 63.7 −0.004
Crown index −15.428+0.138CI 0.999 0.751 53.8 60.0 56.9 0.001
21 Crown area −6.572+0.131CA 0.996 0.382 48.1 56.0 52.0 −0.002
Crown module −14.341+1.993CM 0.991 0.187 46.2 56.0 51.0 −0.002
Crown index −11.598+0.166CI 0.953 0.002 51.9 60.0 55.9 0.004
22 Crown area −6.883+0.181CA 0.982 0.057 50.0 56.0 52.9 −0.003
Crown module −14.192+2.290CM 0.979 0.040 51.9 56.0 53.9 −0.003
Crown index −11.307+0.143CI 0.984 0.073 51.9 48.0 50.0 0.003
23 Crown area −7.786+0.145CA 0.954 0.002 53.8 72.0 62.7 −0.005
Crown module −16.095+2.204CM 0.913 <0.001 59.6 68.0 63.7 −0.006
Crown index −12.773+0.141CI 0.969 0.011 61.5 58.0 59.8 0.004
Maxillary canine index −16.677+78.527MCI 0.971 0.015 61.5 76.0 68.6 −0.003
24 Crown area −8.595+0.132CA 0.928 <0.001 55.8 60.0 57.8 −0.005
Crown module −17.164+2.107CM 0.936 <0.001 55.8 60.0 57.8 −0.005
Crown index −20.333+0.150CI 0.968 0.010 51.9 52.0 52.0 0.004
25 Crown area −8.997+0.149CA 0.933 <0.001 57.7 64.0 60.8 −0.006
Crown module −18.205+2.314CM 0.939 <0.001 55.8 64.0 59.8 −0.005
Crown index −17.656+0.127CI 0.975 0.023 53.8 36.0 45.1 0.003
26 Crown area −10.253+0.096CA 0.905 <0.001 59.6 68.0 63.7 −0.007
Crown module −20.483+1.985CM 0.908 <0.001 61.5 68.0 64.7 −0.006
Crown index −21.555+0.216CI 0.966 0.009 57.7 64.0 60.8 0.004
27 Crown area −9.532+0.099CA 0.955 0.002 55.8 64.0 59.8 −0.004
Crown module −18.990+1.936CM 0.955 0.002 57.7 64.0 60.8 −0.004
Crown index −16.802+0.153CI 1.000 0.985 55.8 48.0 52.0 0.000
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11 – the right central incisor; 12 – the right lateral incisor; 13 – the right canine; 14 – the right first premolar; 15 – the right second premolar; 16 – the right first molar; 17 – the right second molar; 21 – the left central incisor; 22 – the left lateral incisor; 23 – the left canine; 24 – the left first premolar; 25 – the left second premolar; 26 – the left first molar; 27 – the left second molar; CA – crown area; CI – crown index; CM – crown module; MCI – maxillary canine index.

*

A discriminant score less than the cutting point for CA, CM, and MCI indicates a woman. A discriminant score less than the cutting point for CI indicates a man.

Figure 4.

Figure 4

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Comparison of the sex estimation accuracy through crown indices. 11 – the right central incisor; 12 – the right lateral incisor; 13 – the right canine; 21 – the left central incisor; 22 – the left lateral incisor; 23 – the left canine; 14 – the right first premolar; 15 – the right second premolar; 16 – the right first molar; 17 – the right second molar; 24 – the left first premolar; 25 – the left second premolar; 26 – the left first molar; 27 – the left second molar; CA – crown area; CI – crown index; CM – crown module; MCI – maxillary canine index.

The stepwise discriminant analysis is shown in Table 7. Crown dimensions, CA, and CM were selected as categorical variables, and all teeth of the corresponding variable were included in the homology equation. Crown dimension function was exploited to distinguish whether an individual was male or female with superior rates (69.2% in men, 72.0% in women, and 70.6% in overall), followed by the CA and CM functions (Figure 5).

Table 7.

Stepwise discriminant analysis for sex estimation from maxillary teeth.

Teeth Function Discriminant function Wilks’ Lambda p-value Accuracy (%) Cutting point*
Male Female Overall
All Crown dimensions −20.461+0.838 ×(BL13)-1.219×(BL21)+1.122 ×(MD13)+1.275×(MD16) 0.758 <0.001 69.2 72.0 70.6 −0.011
All Crown area −8.521+0.342×(CA13)-0.083 ×(CA21)-0.24×(CA23)+0.066×(CA26) 0.788 <0.001 63.5 68.0 65.7 −0.010
All Crown module −18.094+2.107×(CM13)+1.359×(CM16)-1.578×(CM21) 0.841 <0.001 63.5 64.0 63.7 −0.009
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BL13 – buccolingual distance of the right canine; BL21 – buccolingual distance of the left central incisor; CA13 – crown area of the right canine; CA21 – crown area of the left central incisor; CA23 – crown area of the left canine; CA26 – crown area of the left first molar; CM13 – crown module of the right canine; CM16 – crown module of the right first molar; CM21 – crown module of the left central incisor; MD13 – mesiodistal distance of the right canine; MD16 – mesiodistal distance of the right first molar.

*

A discriminant score less than sectioning point indicates a woman.

Figure 5.

Figure 5

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Comparison of the sex estimation accuracy by stepwise discriminant equations.

Discussion

Estimating the sex in damaged bodies or from bones is important in forensics and anthropology identification. Teeth, as the hardest and most durable tissue, play a critical role. However, the standards for identification based on teeth vary among different populations [16,17], and data concerning sexual dimorphism in specific populations are still needed. Thus, we investigated the Han population, making identification possible through readily available dental measurements.

The electronic handheld digital caliper is conventionally used for tooth measurements due to its accuracy, practicality, portability, and low cost. However, it has some inherent drawbacks. It is only accurate to 0.01 mm, which is lower than 3D technology. In addition, samples are not convenient for storage and retrieval and may be easily damaged through direct contact by tools, affecting reuse. Continuously evolving technology enables multiple measurements obtained from 3D dental models, achieving great reliability, reproducibility and validity [18]. At the same time, the use of 3D techniques also facilitates research [19].

The current study estimated the sex of individuals from a Han population through odontometric data obtained with 3D technology. The Han underwent similar evolution, but no odontometric data had been collected until our study. The maxilla is a fixed bone that is not easy to dissociate. In our investigation, we used maxillary odontometrics to simulate scenarios in which only a skull with the maxilla is available, establishing an equation algorithm for sex estimation. Our data present the crown dimensions, intercanine distance, and the calculated indices in maxillary teeth of 18- to 25-year-old individuals of the Han population in Kashgar. As tooth wear is minimal in this age group, odontometric information could be maximized. Furthermore, discriminant functions were established for the local population.

The Reliability of Examiners

In other fields of forensic science, complex comparisons between different evaluators are necessary [20]. The current study obtained high intra- and interexaminer reliability, which provides a credible guarantee for subsequent measurement results. On one hand, the high degree of consistency in the formulation of a complex judgment reflects that the forensic dental professionals have well-established and shared understanding and knowledge. On the other hand, the controllability and repeatability of 3D software should not be ignored.

Crown Dimensions and Sexual Dimorphism

The mesiodistal distance was significant in estimating an individual’s sex based on maxillary teeth, with measurements from men generally being greater than those from women, a finding that agreed with earlier studies [14,21–24]. The reason may be that the greater thickness of enamel in men due to long period of amelogenesis compared with women and the Y chromosome producing slower male maturation [25]. MD values in our sample were found to be smaller than those of Brazilians [26], Africans [27], and Malaysian Tamils [28]; equivalent to those of Indians [29] and Japanese [27]; and larger than those of Greeks [30] and Whites [27]. In our study, the percentage of sexual dimorphism for MD ranged from 2.32% to 4.26%. The results of the anterior teeth were closest to those from the Indian study [29]. Sexual dimorphism of the premolars was similar to that found for Africans [27]. The first molar sexual dimorphism matched that of Americans [31] and the second molar was equivalent to Brazilians [26].

Buccolingual distance of maxillary teeth other than incisors showed sex differences. Men’s values were greater than women’s, and the observations were roughly consistent with previous reports [14,28,31–33], whereas some results were contrary to Babu et al [34] and Dash et al [35]. In comparison with measured values from Turks [36], Nepalese [37], Indians [38], and Brazilians [39], the BL distances of anterior teeth and molars in our sample were smaller, but premolars were larger. Our results for the sexual dimorphism percentage ranged from 0.75% to 3.58%, which was smaller than the other populations above. Previous studies have suggested that the reason could be evolution, which results in overlapping sex-based measurements, as well as environmental, cultural, and genetic factors [40].

In this research, MD had a higher sexual dimorphism percentage than BL distance, which accorded with previous reports that MD is better than BL for sex estimation. For a certain tooth, Garn et al [12], El Sheikhi and Bugaighis [14], and Eboh et al [41] reported that BL distance is more dimorphic than MD. Other researchers, such as Acharya [42], indicated that sex estimation had higher accuracy when MD and BL distances were used simultaneously. Thus, crown dimensions can serve as simple and reliable parameters for sex estimation from teeth. Furthermore, MD and BL distance can be used to obtain other indicators that are used by forensic experts as auxiliary means to estimate sex in catastrophes.

Crown Indices and Sexual Dimorphism

CA, CM, and CI were selected as crown indices and were calculated from linear odontometrics. In comparison with Brazilians [39], CA, CM, and CI values of the anterior teeth were smaller in both men and women in our sample. For posterior teeth, the results for CM of the first premolars and CI of the premolars were larger. In addition, the CA of premolars, CI of molars, and CM of the second premolars and molars were similar in these 2 populations.

Among these indices, CA presented the strongest sexual dimorphism. In comparison with Brazilians [39], except for the first premolars, the sexual dimorphism of the rest teeth was less, manifesting as a smaller tooth size overall. Regarding CI, center incisors, canines, and the first premolars showed apparent differences in tooth shape between these 2 populations. Moreover, the results on CM demonstrated significant differences between male and female, especially in premolars and the first molars.

Maxillary Canine Index and Sexual Dimorphism

Several studies have been conducted to establish canine dimorphism, especially the mandibular canines, which have the highest degree of dimorphism [43–45]. However, our research was limited to maxillary dentition. There were sexual differences in both canine mesiodistal distance and intercanine distance, in keeping with Gupta et al [46]. In our results, sexual dimorphism of the MCI was 2.03% and 2.08% for the right and the left, respectively, in contrast to Phulari et al [47], who found −28.13% on average. Consequently, the accuracy of using the same index to classify the sex of an individual was different. Differences in the results between our study and previous studies may be attributable to racial, ethnic, regional, and genetic elements that affect dental measurements, as well as the different methods employed.

Discriminant Function

We united MD and BL dimensions in our equation, with the prediction rates ranging from 52% to 72%. When the indices were applied to create functions separately, the accuracy ranged from as low as 36% up to 76%. With all the maxillary teeth included in the classified indicators, stepwise discriminant analysis for sex determination was performed to establish functions, improving the holistic accuracy with a limit of 63.5–72%. The discriminant function on crown dimensions displayed a more acceptable and stable accuracy of 69.2–72%, in men and women and overall.

Limitations and Expectations

There are some limitations in this study. First, this study only estimated the sex of individuals in a Han population by linear dental measurements and only as an auxiliary sex estimation method. Second, we used only maxillary odontometrics to simulate scenarios in which only a skull with the maxilla is available. In the future, we will conduct comprehensive studies on the mandibular odontometrics and the overall indexes. Third, we only used good dentitions and high-quality models, which limited further applications. Special circumstances such as prostheses, conservative treatments, malpositions, wear, and so forth could yield different results.

Conclusions

We are the first to present a study on odontometrics of all permanent maxillary teeth by using a 3D technique for sex estimation of the Han population in Kashgar. Crown dimensions, maxillary intercanine distance, and crown indices exhibited descriptive statistics and sexual dimorphism. For possible future studies estimating sex through permanent maxillary teeth measurements in our population, we suggest combining BL and MD distances with the left first premolar and the left first molar for relatively high accuracy that can be used to aid sex estimation. Compared with other indices, the function established by the left canine index had a higher accuracy and could serve as an auxiliary method for estimating sex. The stepwise discriminant function on crown dimensions including BL13, BL21, MD13, and MD16 can be used for sex estimation with more acceptable and stable accuracy.

In this study, 3D digital technology offered a promising method for odontometry and a starting point for the application in the forensic field. However, differences between populations, the variability based on the time when a sample is found, and the influence of the environment must be taken into consideration. Thus, the data derived from 3D measurements in this study are not generalizable.

Acknowledgments

We acknowledge the contributions of the Department of Prosthodontics in the First Affiliated Hospital of Xinjiang Medical University for providing facilities for the study. Our thanks go to all the participants of the study for their valuable co-operation.

Footnotes

Conflict of interest: None declared

Declaration of Figures Authenticity

All figures submitted have been created by the authors who confirm that the images are original with no duplication and have not been previously published in whole or in part.

Financial support: This work was supported by grants from Xinjiang Uygur Autonomous Region Science and Technology Support Project Foundation of China (No. 201591191)

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