Introduction

Nonmetric dental traits are significant to clinical dentistry (e.g., orthodontics, and prosthodontics), dental anatomy/morphology, anthropology, oral pathology, and forensic dentistry [1]. Differences in the shape and size of the teeth are an important factor related to the etiology of malocclusion [2, 3].

The teeth are resistant to decomposition, destructive agents, fire, and time; they can convey information about ethnicity and gender; they also allow assessment of origins, the migratory chain, and habits/diets of populations [4,5,6,7]. Among the biological variations existing in populations, nonmetric dental traits are key factors for scientists who search for the link between populations’ biological history and their phenotypes. Dental anthropology has great significance in the study of populations’ variations because of the existence of numerous independent traits, excellent preservation, evolutionary conservatism free of selective pressure, genetic determination, interpopulational variation, and the simple assessment of live individuals such as in fossils [6, 8,9,10,11].

Nonmetric characteristics of dental crowns are phenotypic forms of dental enamel that result from the indirect process of secretion of mineral mediators by dental morphogenesis proteins and are expressed by the human genome of each individual. They can be positive (cusp) or negative structures (pit and groove) that can appear in a specific location and to varying degrees [12]. These features are described using different designations. They have a high classification value and can be used for biological prediction between different populations and comparative analysis of the history, culture, and biological progress of early humans and modern humans [12].

Since there was no study or just a few ones on a large set of so many dental anomalies, this study was conducted.

Materials and methods

This cross-sectional study was performed on 662 maxillary and mandibular dental casts of 331 patients (over 9000 teeth). The patients were selected randomly from Iranian patients attending the Orthodontic Department and two private orthodontic clinics in Ahvaz, Iran. For collecting the data, all the available patients’ records, as well as their archival casts and radiographs, were consecutively evaluated until reaching the desired sample size. A total of 809 patient records (along with their casts and radiographs) were evaluated. The inclusion criteria were being of Iranian descent, 12–35 years old, and having a complete permanent dentition (except the third molars) with no more than 2 extractions. The exclusion criteria were patients with any systemic diseases affecting the teeth or syndromes, cleft lips or palates, any earlier histories of orthodontic, prosthodontic, or surgical treatments. Also excluded were patients who did not have all the permanent teeth completely (except cases of hypodontia, cases of sporadic excluded teeth, cases of one or two extracted teeth, and also except the third molars), or patients with more than two extracted teeth, and a lack of complete eruption of more than two of the existing permanent teeth (including the second molars). The other exclusion criteria were single teeth with visible restorations, caries, crown fractures, or veneers (or a history of them), and teeth that had not been fully erupted. Cases with poor cast quality or a lack of panoramic radiographs or lateral cephalograms in the patient file would be excluded. Information regarding the patients’ age, sex, and type of skeletal malocclusion (Angle classes I, II, and III) was recorded from their files, radiographs, and casts. The data collection was performed from 2018 to 2020. No patient was exposed to X-rays for this research, and all the used radiographs were archival and taken merely for treatment purposes. Protocol ethics were approved by the Research Committee of the University (ethical code: U-98142) under the Helsinki Declaration [13,14,15].

The sample size was pre-determined as 267 patients using the following formula with conservative parameters: \(n=({\mathrm{Z}}^{2}* p*(1- p) )/({\mathrm{d}}^{2})\) where Z = 1.96, p (prevalence) = 0.5 (as the most conservative prevalence, i.e., the prevalence yielding the maximum sample size within this formula), and d (precision) = 0.06 (as a conservative precision). The sample size was augmented to 331 patients to ensure greater precision. After obtaining the data, the average prevalence of the traits/anomalies was 20.65%; the highest prevalence was 82.18%. For these average and maximum prevalence rates, sample sizes of 175 and 157 cases would suffice respectively, indicating that the current sample size of 331 cases was adequately large.

Examinations

All archival dental casts had been poured with white dental stone for orthodontic application. All 4 quarters of each patient were examined carefully by two observers (an experienced orthodontist and a trained dentist), twice each. They tried to identify the 60 traits/anomalies mentioned in Table 1 and Figs. 1 to 13 (25 dental traits that might appear in 60 teeth) [6, 16] as well as supernumerary teeth (hyperdontia), microdontia (totaling 62 traits/anomalies), and crowding. Microdontia was defined as noticeably small but normally shaped teeth [16].

Table 1 Nonmetric dental traits examined in this study
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Fig. 1
figure 1

A shovel-shaped incisor. Figures 1 to 13 were created by taking screenshots of an educational Android program with direct permission from the developer and owner (3D Tooth Anatomy 1.0.3; Universal Hospital LP, Richmond, Virginia, USA; developed by Dr Rami Ammoun, assistant professor at Virginia Commonwealth University, Richmond, Virginia, USA) and editing some of the screenshots using 2D image editing software (Photoshop, Adobe, San Jose, California, USA)

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Fig. 2
figure 2

An incisor with a talon cusp

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Fig. 3
figure 3

The Carabelli cusp is visible on the palatal surface of the mesiopalatal cusp

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Fig. 4
figure 4

The protostylid trait

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Fig. 5
figure 5

The fifth cusp on the distal side of a maxillary first molar (arrow). The Carabelli cusp is as well visible on the mesiopalatal cusp

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Fig. 6
figure 6

The hypoconulid absence: The upper tooth is a mandibular first molar with all common cusps; the lower one is a mandibular first molar without the hypoconulid cusp

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Fig. 7
figure 7

The hypocone absence: The upper image shows a maxillary first molar with 4 cusps; the lower one shows a maxillary second molar without the hypocone cusp

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Fig. 8
figure 8

The deflecting wrinkle (arrow) on the mesiolingual occlusal surface of a mandibular molar

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Fig. 9
figure 9

The mesial ridge (yellow arrow) and distal accessory ridge (orange arrow) of the maxillary canine

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Fig. 10
figure 10

The distal trigonid crest connecting the mesiobuccal and mesiolingual ridges (orange arrow), the sixth cusp on the distal side (blue arrow), and the tuberculum intermedium on the lingual side (purple arrow)

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Fig. 11
figure 11

Accessory cusps on the mesial and distal marginal ridges

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Fig. 12
figure 12

The mesial and distal accessory ridges

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Fig. 13
figure 13

An accessory cusp on the lingual side of a 3-cusp premolar

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Of these 62 traits and anomalies, 44 were found (Table 2); the prevalence of the rest of them was 0%. For hyperdontia diagnosis, also panoramic radiographs were evaluated for possible impacted supernumerary teeth. The data spreadsheets were evaluated by the two observers as well as a third evaluator to find any inconsistencies (which were rather rare). Any cases of inconsistency were re-evaluated on dental casts by both observers. The third evaluator did not check the dental casts.

Table 2 The prevalence rates (and Wilson 95% CIs for prevalence rates) of 44 nonmetric traits/anomalies (and crowding) in the sample, men, and women
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The results were collected at two levels: (A) at the quarter level (hemi-mandible/hemi-maxilla), which showed each nonmetric trait/anomaly in each quarter; (B) at the patient level, which showed each trait/abnormality plus crowding and Angle classes in individual patients.

Statistical analyses

Thirty casts were reevaluated by one of the observers about one year after the original assessment, and the interrater and intrarater agreements were calculated to be high or excellent for all the found traits and anomalies (Kappa > 0.6, P < 0.05). Descriptive statistics and Wilson 95% confidence intervals (CIs) were calculated for prevalence rates. The ages of men and women were compared using an unpaired t-test. Data were also summarized for quarters. Associations between the presence of nonmetric traits besides hyperdontia/microdontia with genders, left/right sides, and skeletal malocclusions (Angle classes I, II, and III) were assessed using a chi-square test. Correlations across dental traits were assessed using a Spearman correlation coefficient. The level of significance was set at 0.05.

Results

There were 74 men and 257 women in the study. The mean (SD) age of patients was 19.21 ± 4.87 years (range 12–35). Mean ages in men and women were 18.29 ± 20.49 and 18.55 ± 19.76 years, respectively. The sexes were balanced in terms of age (t-test, P = 0.716). Of the patients, 182 (55.7%), 127 (38.8%), and 18 (5.5%) were skeletal Classes I, II, and III, respectively (the Angle classes of four patients were missing). Crowding was observed in 89 out of 331 cases (26.9%).

Prevalence rates of nonmetric traits/anomalies and their 95% CIs are presented in Table 2. Sexual dimorphism was observed in a few traits: microdontia and accessory cusps on the marginal ridge of the second mandibular molars were significantly more common in women (Table 2). Canine talon cusp and distal trigonid crest of the second mandibular molars were more prevalent in men (Table 2). The prevalence of crowding was 26.9% (CI 22.4–31.9%). It was observed in 22 men and 67 women, without any sex dimorphism (chi-square, P = 0.532).

Associations with skeletal malocclusions

There were significant associations between the skeletal Angle classes with these traits: shoveling of the central and lateral and canine (all the 3 shovelings were rather more frequent in class II), talon cusp on the canine (rather more frequent in class II), canine distal accessory ridge (rather more frequent in class II), accessory cusp in the mesial marginal ridge of the second premolar (mandibular and maxillary combined) (rather more frequent in class I and less frequent in class II), accessory cusp in the first premolar (maxillary and mandibular combined, rather more frequent in class II), and mesial accessory ridge in the first premolar (both maxillary and mandibular, less frequent in class I, Table 3). Crowding was observed in 45, 41, and 2 cases of Classes I, II, and III, respectively with no significant difference across the classes (chi-square, P = 0.101).

Table 3 Contingency tables show the net frequencies (and %) of 44 nonmetric traits/anomalies and crowding in different skeletal Angle classes, as well as associations between the presence of dental traits and dental occlusion classes
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Quarter level analyses

The summary of all cases at the quarter level, in different hemimandibles and hemimaxillae, is presented in Table 4. All dental traits/anomalies were evenly distributed on the right and left sides (all chi-square P values > 0.1, Table 4).

Table 4 The net (and %) prevalence rates of 44 nonmetric traits/anomalies in different sexes, jaws, and sides
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Correlations among the dental traits/anomalies

There were numerous significant correlations among the traits/anomalies (Additional file 1). The significant correlations that also had moderate or strong positive effect sizes (Rho ≥ 0.4) comprised: correlations among shoveling of different anterior teeth (Rho ≥ 0.4, P < 0.0005) as well as correlations between the canine’s distal accessory ridge and shoveling of the anterior teeth (Rho ≥ 0.4, P < 0.0005), between talon cusp on the central incisor and talon cusp on the lateral incisor (Rho ≥ 0.4, P < 0.0005), correlation between the tuberculum sextum on the lower first molar and fifth cusp on the upper first molar (Rho ≥ 0.4, P < 0.0005), correlations among accessory cusp in first premolars with shoveling of canine and to a lower degree shoveling of the other anterior teeth (Rho ≥ 0.4, P < 0.0005), correlation between mesial accessory ridge on the second premolar with distal accessory ridge on the first and second premolars (Rho ≥ 0.4, P < 0.0005), and the correlation between distal accessory ridge of the first premolar and distal accessory ridge of the second premolar (Rho ≥ 0.4, P < 0.0005, Additional file 1).

Discussion

Studies on associations between the Angle classes with nonmetric dental traits and anomalies are scarce and limited to very few anomalies. A recent study [17] assessed the association between skeletal classes and five dental anomalies; they exhibited that microdontia was associated with class III malocclusion. We could not find such an association, however, even though in both studies skeletal malocclusions had been examined. Perhaps, factors such as the ethnic background as well as other methodological factors including sample characteristics may be responsible for the differing results. Still, both studies failed to find a link between hyperdontia and Angle classes. More studies are needed in this regard before explaining potential reasons for the disputes or agreements.

Researchers from various branches of anthropology have documented human variations in different populations and have concluded that phenotypic and genotypic characteristics of populations are related to the geographical distance between them [4, 18,19,20,21]. Regionalized anthropological research can replace the classical division of humankind (as Caucasians, Negroids, and Mongoloids) as an important path for historical, evolutionary, and forensic studies [22,23,24,25]. Skeletal assessments comprise metric and nonmetric methods that can allow for designing a biological profile and identification [6, 26].

The characteristics of nonmetric traits are primarily used to predict human identity, gender, and origin [14], although as confirmed in this study, few nonmetric traits might have sex dimorphism [27]. It seems that the nonmetric properties of tooth crowns rarely have gender differences. The statistical relationship between these features is small. A significant geographical variation is seen in the frequency of these features [12]. These characteristics also play a critical role in racial and legal identification. So far, over 135 dental features have been identified in the human dental system, but few have been studied in global research [14] on dental casts, direct clinical evaluation, radiography, and digital photography [14, 28]. The characteristics of nonmetric traits are easily observed and recorded, so they provide us with information about genetic and ethnical variations that occur, to organize populations according to the group-specific evolution process [12]. It should be noted, however, that certain dental traits can disappear due to tooth wear and caries [7, 14].

In the present study, three cases of sexual dimorphism had higher frequencies in women compared to men. Regarding the Carabelli’s cusp, it had a male incidence (27.1%) significantly higher than the female (12.3%) in Brazilians [6], but in the present study and another research [29], no significant sex dimorphism was observed for this trait. The absence of hypocone was higher in women in Brazilians [6] and samples from Southeast Asia, North America, India, and North Africa [29]; however, it was not present in our study. Traits like shoveling, fifth cusp, and absence of hypoconulid showed no significant sexual dimorphism in Brazilians [6], and in the study of Hanihara [29] and Aguirre et al. [12].

It should be noted that all the subjects in this study were orthodontic patients with malocclusion and not normal people. Therefore, their results might not be generalized to the normal population. Since there was no similar study on so many dental traits in normal populations (or even orthodontic patients) sampled from any countries, we could not compare our results extensively. Future studies are warranted to evaluate these traits in the normal populations of different countries. Another limitation was the inclusion of cases with one or two extracted teeth or some excluded teeth, since such teeth might have had some traits or abnormalities and their exclusion confound the findings. Due to the difficulty in collecting the cases, we were limited to tradeoff between a few extracted or excluded teeth in some patients versus discarding the whole patient and all the available precious information altogether. Hence, we preferred to keep all the other information obtainable from a patient at the cost of introducing some rather subtle noise to the data by the extracted or excluded teeth. Finally, the sample was not equally distributed in terms of sex or the Angle classes. Hence, it may influence the finding in regards of association between dental abnormalities with sexual dimorphism or skeletal malocclusion.

Conclusions

  1. 1.

    The prevalence rates (and 95% CIs) of 44 nonmetric shape/number/size dental traits/anomalies in the Iranian orthodontic patients were documented: they might range between 0.3% and 73.1%, with similar prevalence rates on the right and left sides.

  2. 2.

    Sex dimorphism was uncommon in nonmetric traits/anomalies. (A) It was shown that microdontia, hypocone absence, and accessory cusps on the marginal ridge of the mandibular second molars might be more prevalent in women. (B) Canine talon cusp and distal trigonid crest of the second mandibular molars might be more prevalent in men.

  3. 3.

    The skeletal malocclusions were associated with certain dental traits/abnormalities: (A) Shoveling of all the anterior teeth, talon cusp on the canine, canine distal accessory ridge, and accessory cusp in the first premolar might be more prevalent in skeletal Angle class II; whereas (B) accessory cusp in the mesial marginal ridge of the second premolar might be rather more prevalent in skeletal class I, and (C) mesial accessory ridge of the first premolar might be less frequent in skeletal class I.

  4. 4.

    The occurrences of a few dental traits/anomalies were positively correlated with each other to a moderate or strong extent.