https://orcid.org/0000-0001-7304-044X
Summary
To investigate the association between the number of third molars and craniofacial shape.
The study sample comprised 470 individuals (194 males and 276 females), out of whom 310 (124 males, mean age: 14.6 years and 186 females, mean age: 14.1 years) had a full permanent dentition including third molars and 160 (70 males, mean age: 13.7 years and 90 females, mean age: 13.9 years) had at least one missing third molar. Pre-orthodontic treatment cephalometric images were digitized using 127 landmarks to describe the shape of the entire craniofacial configuration, the cranial base, the maxilla, and the mandible. The shapes of the various configurations were described by principal components (PCs) of shape. The effect of third molar agenesis on craniofacial shape was evaluated with multivariate regression models, considering shape PCs as the dependent variables, and age and sex as predictors.
There was a strong association between third molar agenesis and the shape of all craniofacial configurations in both sexes. Individuals with missing third molars presented a less convex craniofacial configuration, a shorter anterior facial height and a more retrusive maxilla and mandible. In cases with third molar agenesis only in one jaw, shape differences were also evident in the opposing jaw.
Interpretation of study outcomes should take into consideration the two-dimensional data and the analysis of only white-European subjects.
There is a strong association between third molar formation and craniofacial shape. The effect is rather generalized than local and is potentially linked to an ongoing evolutionary mechanism that leads to smaller and fewer teeth, as well as smaller craniofacial configurations, in modern humans.
Introduction
Tooth agenesis in the permanent dentition is defined as the developmental absence of at least one permanent tooth, with prevalence in the range of 6.4%−8.5%, without considering the third molars. This prevalence varies depending on geographic region, gender, and tooth type [1]. Non-syndromic tooth agenesis is more common than that related to a syndrome, and it involves a varying number of teeth. The most commonly affected teeth are the mandibular second premolars, followed by the maxillary lateral incisors and the maxillary second premolars. Mild tooth agenesis ([1, 2] missing teeth) occurs most commonly representing 81.6% of cases, followed by moderate ([3–5] missing teeth) and severe agenesis (more than 6 missing teeth) that occur in 14.3% and 3.1% of the affected individuals, respectively [1].
Recent meta-analytical data show that the worldwide agenesis of third molars ranges from 20.64% to 24.76% [2]. Females are affected 14% more than males and third molar agenesis occurs 36% more often in the maxilla than in the mandible [2]. Affected individuals are usually missing one or two and less commonly three or four third molar teeth [2]. The severity and patterning of third molar agenesis do not differ between sexes [3]. However, in the absence of other permanent teeth, third molars are missing more commonly compared to cases with no agenesis of other teeth [3].
Non-syndromic tooth agenesis is genetically controlled and has been linked to mutations of various genes such as AXIN2, IRF6, FGFR1, MSX1, and PAX9 [4–6]. A number of these genes play a critical role in the overall morphogenesis of craniofacial structures [7–9]. Significant differences in the size and the shape of the maxilla, the mandible and the overall craniofacial configuration between individuals with permanent tooth agenesis and individuals without agenesis have been reported [10–12]. These findings support the theory of an evolutionary mechanism in humans leading to smaller and fewer teeth as well as smaller facial structures [13, 14]. Nevertheless, it is not known if the congenital absence of third molars alone is also associated with the craniofacial structures, similar to the agenesis of other permanent teeth. This question arises because of the significantly higher prevalence of missing third molars in the population and, thus, the assumption that the development of third molars is not governed by the same biological mechanism as other permanent teeth. Due to the lack of relevant information, most related studies thus far have excluded third molars from their sampling and analyses in order to avoid potential confounders [2].
In a recent study, we investigated differences in craniofacial size between individuals with and without missing third molars. The results indicated a significant reduction in maxillary size, mandibular size, and the size of the entire craniofacial configuration [15] in cases of third molar agenesis. While this finding supports the assumption that third molar agenesis might be part of the same biological mechanisms found in tooth agenesis at large, it remains unknown if and how the craniofacial shape is affected. Therefore, the present study aimed to investigate craniofacial shape in a large sample of individuals with one or more missing third molars and compare it to a group of individuals with full dentition, including third molars.
Materials and methods
Ethical approval
The protocol of this case-control study was examined and approved by the Ethics Commission of the Canton of Bern, Switzerland (Project-ID: 2018-01340), and the Research Committee of the School of Dentistry, National and Kapodistrian University of Athens, Greece (Project-ID: 281, 9 February 2016). The STROBE reporting guidelines were adopted, and the subjects of the study provided written informed consent before their data were used.
Sample
The sample population has been described previously by Gkantidis et al. [15]; however, certain features and methodological considerations are repeated here to facilitate comprehension of the present investigation.
The sample was selected between 2002 and 2020 from the archives of orthodontic patients records, from multiple orthodontic clinics: (a) University of Bern, Switzerland (Project-ID: 2018-01340); (b) National and Kapodistrian University of Athens, Greece (Project-ID: 281, 9 February 2016); (c) two private clinics in Athens and two in Thessaloniki, Greece; and (d) one private clinic in Biel, Switzerland.
The following inclusion criteria were applied:
- Age between 9 years and 50 years at the time of pretreatment records. For patients younger than 12.5 years old, panoramic radiographs at older ages, higher than 12.5 years old, were checked to avoid misdiagnosis of late forming third molars.
- European (White) ancestry.
- Lateral cephalometric radiographs in maximal intercuspation, of diagnostic quality, with a reference ruler at the mid-sagittal plane for magnification adjustment.
- Panoramic radiographs of diagnostic quality for the identification of tooth agenesis.
Subjects were excluded from the study in the presence of any of the following conditions:
- Systemic diseases, syndromes, or any other conditions that affect craniofacial development, as reported in the subjects’ medical record.
- Agenesis of additional teeth, other than third molars.
- Presence of severe dental anomaly affecting tooth number, size, or form in any tooth except for third molars.
- History of previous interventions known to influence craniofacial morphology, such as orthodontic treatment, prior to cephalometric image acquisition.
Patients presenting with common jaw discrepancies (class II and class III skeletal patterns, and hypo- and hyper-divergency) were retained in the sample, as expressing the full craniofacial variability of the population, even if some of these could be considered for orthodontic and/or surgical treatment. Thus, more than 8000 orthodontic patient files were reviewed according to a previously reported methodology [15]. The final sample size comprised of 470 individuals (194 males and 276 females) who met the aforementioned criteria (median age: 12.7 years, interquartile range: 3.7 years). Out of those, 310 individuals (124 males, mean age: 14.6 years and 186 females, mean age: 14.6 years) had all teeth present, including third molars, and 160 individuals (70 males, mean age: 13.7 years and 90 females, mean age: 13.9 years) had one or more missing third molars. A more detailed description of the sample is provided in Supplementary Table 1 (reprint from: Gkantidis et al. [15]) and Supplementary Table 2. All relevant information for the included subjects was then inserted in an Excel sheet (Microsoft Excel, Microsoft Corporation, Redmond, WA, USA) and all agenesis patterns of third molars were recorded using the TAC system [16].
Craniofacial shape definition
For all included individuals, the data related to craniofacial shape were captured through landmarks placed on lateral cephalometric radiographs. These landmarks are the same as those used in a series of previous publications [10, 11, 15] and were analyzed through geometric morphometric methods based on a previously published protocol for similar outcomes [11]. All landmarks were identified and digitally traced in Viewbox 4 software (dHAL Software, Kifissia, Greece) to capture the morphology of the entire craniofacial configuration (without including the posterior and superior part of the cranium), including the cranial base, the maxilla, and the mandible (Fig. 1).
Material from: Elias S. Oeschger et al. Number of teeth is associated with facial size in humans, Scientific Reports, published 2020, Springer Nature, licensed under CC BY 4.0. Craniofacial morphology was captured through the depicted landmarks. Digitization of the craniofacial complex with 15 curves, which included 116 semi-landmarks (red crosses) and 11 fixed landmarks (red squares). Orange color represents the structures of the cranial base, yellow the maxillary structures, blue the mandibular structures, and all lines together the entire configuration.
A total of 127 landmarks were used to describe the morphology of the craniofacial structures of interest. Those based on local anatomy, such as the anterior nasal spines and posterior nasal spines, were considered fixed landmarks (11 points). The remaining 116 landmarks were defined as semi-landmarks and were initially distributed on 15 curves [17] (Fig. 1). Semi-landmarks were allowed to slide from their initial position along the corresponding curve to minimize the bending energy in relation to an average, reference configuration. This was performed in an iterative process during which the average was calculated in each step and used as the reference for the next iteration. This iterative process was repeated three times until there was no detectable change in the average shape.
A partially generalized Procrustes superimposition of all landmark configurations was used to obtain the final Procrustes coordinates, which identified each subject’s location in shape space [18]. A principal component analysis was applied on the Procrustes coordinates to recover the most significant shape patterns, a typical procedure in geometric morphometric analyses [19, 20].
Statistical analysis
Statistical analyses were performed using Viewbox 4 software (Version 4.1.0.1) and IBM SPSS statistic for Windows (Version 28.0, IBM Corp., Armonk, NY, USA), following the same baseline methodology described in previous work by the same group [10, 11, 15]. Two-sided tests were performed in all cases, accepting a type I error of 5%. When applicable, a Bonferroni correction was used to adjust for multiple comparisons.
Sexual dimorphism in craniofacial shape was evaluated with permutations tests (100,000 permutations without replacement) on the Procrustes distance between male and female group means.
The effect of missing third molars on craniofacial shape was tested with a series of multivariate regression models in males and females, separately. Shape was expressed as principal components (PCs) of shape that were considered the dependent variables. The number of PCs needed to provide adequate shape information was determined according to the ‘broken stick’ method [21]. The predictor variables were age and number of missing third molars. In addition to effect sizes, the association between craniofacial shape and third molar agenesis was also depicted graphically with shape morphings. These were created using the shape coordinates of the extreme shape configurations according to the PC values included in each regression model. In this way, shape comparisons between individuals without any missing third molar and with four missing third molars, for example, can be made graphical. Specific areas of difference were visualized through the Procrustes superimposition of the two extreme shape configurations.
As a secondary analysis, the influence of the agenesis site (maxilla or mandible) on the shape of its respective configuration (maxillary or mandibular) was evaluated. For this purpose, two subsamples were created. One subsample included individuals without third molar agenesis or with third molar agenesis exclusively in the maxilla, while the other included individuals without third molar agenesis or with third molar agenesis exclusively in the mandible. To examine the impact of sex, age, and the number of missing third molars on the maxillary and the mandibular morphologies of individuals who either had no third molar agenesis or had third molar agenesis exclusively in the maxilla, a multivariate regression model was also developed. For patients with either no third molar agenesis or mandibular-only third molar agenesis, a similar model was applied. This assessment revealed possible local over-generalized effects of third molar agenesis on the corresponding maxillary and mandibular structures by comparing the results of these two models. Here, both sexes were tested together in order to avoid the confounding effect of multiple subgrouping and subsequent limited sample size within each group.
An additional secondary analysis was performed to test the magnitude of the effect of zero versus at least one-third molar agenesis on male and female craniofacial shape, using the mean Procrustes distances as a metric. This magnitude was compared to the analogous one regarding individuals that had at least one tooth agenesis, without considering the third molars, versus those that did not have any tooth agenesis in teeth other than third molars. A previously studied sample was used to calculate the latter outcomes [11].
Error of the method
The error of the entire study methodology has been reported previously [11]. There was no systematic error (P > 0.05) and the random error of 4.9%, indicating the percentage of total variance in shape space due to repeated digitization, was considered acceptable [12, 20, 22, 23].
Results
Sexual dimorphism in craniofacial shape
There were significant shape differences between males and females in the entire craniofacial configuration, as well as in all individual structures (Table 1). However, the average Procrustes distances between sexes were of small magnitude (Table 1) and this explains the minor differences in the respective average structures depicted in Fig. 2 and Supplementary Figs. 1–3.
Craniofacial shape differences between male and female subjects.
| Shape | Mean Procrustes distance | P value |
|---|---|---|
| Cranial base | 0.01975 | <0.001* |
| Maxilla | 0.01137 | 0.019* |
| Mandible | 0.00857 | 0.047*a |
| Entire craniofacial shape | 0.01046 | 0.003* |
| Shape | Mean Procrustes distance | P value |
|---|---|---|
| Cranial base | 0.01975 | <0.001* |
| Maxilla | 0.01137 | 0.019* |
| Mandible | 0.00857 | 0.047*a |
| Entire craniofacial shape | 0.01046 | 0.003* |
*P < 0.05.
aMarginally significant result.
Craniofacial shape differences between male and female subjects.
| Shape | Mean Procrustes distance | P value |
|---|---|---|
| Cranial base | 0.01975 | <0.001* |
| Maxilla | 0.01137 | 0.019* |
| Mandible | 0.00857 | 0.047*a |
| Entire craniofacial shape | 0.01046 | 0.003* |
| Shape | Mean Procrustes distance | P value |
|---|---|---|
| Cranial base | 0.01975 | <0.001* |
| Maxilla | 0.01137 | 0.019* |
| Mandible | 0.00857 | 0.047*a |
| Entire craniofacial shape | 0.01046 | 0.003* |
*P < 0.05.
aMarginally significant result.
Left: Average difference between males (blue) and females (light red) in entire cranial configuration shape as explained by PC1 (21.9%)–PC2 (15.4%) (top) and PC3 (7.4%)–PC4 (6.7%) (bottom). The numbers in the parentheses represent the percentage of variation explained by each PC. Right: Best fit superimposition of average male (blue) and average female (light red) craniofacial configurations.
Number of third molars and craniofacial shape
The regression analyses revealed a significant association between the number of missing third molars and the shapes of the entire craniofacial configuration, the cranial base, the maxilla, and the mandible, in both males and females. The age factor showed a significant effect on shape differences between groups, except from the shape of the cranial base in males (Table 2).
Multivariate regression of shape on age and number of missing third molars.
| Shape configurations | Sex group | Test factor | η2 | P value |
|---|---|---|---|---|
| Cranial Base (PC1–PC9) | Females (N = 276) | Age | 0.132 | <0.001 |
| Number of missing third molars | 0.062 | 0.044 | ||
| Males (N = 194) | Age | 0.069 | 0.152 | |
| Number of missing third molars | 0.128 | 0.002 | ||
| Maxilla (PC1–PC10) | Females (N = 276) | Age | 0.183 | <0.001 |
| Number of missing third molars | 0.099 | 0.002 | ||
| Males (N = 194) | Age | 0.204 | <0.001 | |
| Number of missing third molars | 0.156 | <0.001 | ||
| Mandible (PC1–PC7) | Females (N = 276) | Age | 0.148 | <0.001 |
| Number of missing third molars | 0.055 | 0.034 | ||
| Males (N = 194) | Age | 0.191 | <0.001 | |
| Number of missing third molars | 0.091 | 0.012 | ||
| Entire craniofacial shape (PC1–PC15) | Females (N = 276) | Age | 0.303 | <0.001 |
| Number of missing third molars | 0.199 | <0.001 | ||
| Males (N = 194) | Age | 0.423 | <0.001 | |
| Number of missing third molars | 0.211 | <0.001 |
| Shape configurations | Sex group | Test factor | η2 | P value |
|---|---|---|---|---|
| Cranial Base (PC1–PC9) | Females (N = 276) | Age | 0.132 | <0.001 |
| Number of missing third molars | 0.062 | 0.044 | ||
| Males (N = 194) | Age | 0.069 | 0.152 | |
| Number of missing third molars | 0.128 | 0.002 | ||
| Maxilla (PC1–PC10) | Females (N = 276) | Age | 0.183 | <0.001 |
| Number of missing third molars | 0.099 | 0.002 | ||
| Males (N = 194) | Age | 0.204 | <0.001 | |
| Number of missing third molars | 0.156 | <0.001 | ||
| Mandible (PC1–PC7) | Females (N = 276) | Age | 0.148 | <0.001 |
| Number of missing third molars | 0.055 | 0.034 | ||
| Males (N = 194) | Age | 0.191 | <0.001 | |
| Number of missing third molars | 0.091 | 0.012 | ||
| Entire craniofacial shape (PC1–PC15) | Females (N = 276) | Age | 0.303 | <0.001 |
| Number of missing third molars | 0.199 | <0.001 | ||
| Males (N = 194) | Age | 0.423 | <0.001 | |
| Number of missing third molars | 0.211 | <0.001 |
Each shape configuration was described with the number of PCs explaining more than 80% of shape variation, as assessed with the broken-stick method.
Multivariate regression of shape on age and number of missing third molars.
| Shape configurations | Sex group | Test factor | η2 | P value |
|---|---|---|---|---|
| Cranial Base (PC1–PC9) | Females (N = 276) | Age | 0.132 | <0.001 |
| Number of missing third molars | 0.062 | 0.044 | ||
| Males (N = 194) | Age | 0.069 | 0.152 | |
| Number of missing third molars | 0.128 | 0.002 | ||
| Maxilla (PC1–PC10) | Females (N = 276) | Age | 0.183 | <0.001 |
| Number of missing third molars | 0.099 | 0.002 | ||
| Males (N = 194) | Age | 0.204 | <0.001 | |
| Number of missing third molars | 0.156 | <0.001 | ||
| Mandible (PC1–PC7) | Females (N = 276) | Age | 0.148 | <0.001 |
| Number of missing third molars | 0.055 | 0.034 | ||
| Males (N = 194) | Age | 0.191 | <0.001 | |
| Number of missing third molars | 0.091 | 0.012 | ||
| Entire craniofacial shape (PC1–PC15) | Females (N = 276) | Age | 0.303 | <0.001 |
| Number of missing third molars | 0.199 | <0.001 | ||
| Males (N = 194) | Age | 0.423 | <0.001 | |
| Number of missing third molars | 0.211 | <0.001 |
| Shape configurations | Sex group | Test factor | η2 | P value |
|---|---|---|---|---|
| Cranial Base (PC1–PC9) | Females (N = 276) | Age | 0.132 | <0.001 |
| Number of missing third molars | 0.062 | 0.044 | ||
| Males (N = 194) | Age | 0.069 | 0.152 | |
| Number of missing third molars | 0.128 | 0.002 | ||
| Maxilla (PC1–PC10) | Females (N = 276) | Age | 0.183 | <0.001 |
| Number of missing third molars | 0.099 | 0.002 | ||
| Males (N = 194) | Age | 0.204 | <0.001 | |
| Number of missing third molars | 0.156 | <0.001 | ||
| Mandible (PC1–PC7) | Females (N = 276) | Age | 0.148 | <0.001 |
| Number of missing third molars | 0.055 | 0.034 | ||
| Males (N = 194) | Age | 0.191 | <0.001 | |
| Number of missing third molars | 0.091 | 0.012 | ||
| Entire craniofacial shape (PC1–PC15) | Females (N = 276) | Age | 0.303 | <0.001 |
| Number of missing third molars | 0.199 | <0.001 | ||
| Males (N = 194) | Age | 0.423 | <0.001 | |
| Number of missing third molars | 0.211 | <0.001 |
Each shape configuration was described with the number of PCs explaining more than 80% of shape variation, as assessed with the broken-stick method.
As depicted in Fig. 3, in females, an increased number of missing third molars was associated with a less convex craniofacial configuration at the sagittal level, attributed primarily to a retruded maxilla, and a shorter anterior facial height. In males, individuals with a higher number of missing third molars showed a slightly less convex skeletal profile and slightly reduced anterior facial height. In both sexes, third molar agenesis was associated with a more retrusive position of the maxilla and the mandible as related to the head. Furthermore, in males, there was an increase in the steepening of the cranial base angle, primarily attributed to the more anterior positioning of the posterior part of the base.
Regression of shape on number of missing third molars, in females (top row) and males (bottom row). The blue lines represent shape configurations in cases with no or minimum third molars (-4SD from average shape) and the red lines represent shape configurations in cases with a maximum number of missing third molars (+4SD from average shape). The following shape configurations are displayed: (a) Entire craniofacial shape, (b) Maxilla, (c) Cranial base and (d) Mandible.
Local versus generalized effects of third molar agenesis
This subsample analysis (“Statistical analysis” section) revealed a significant generalized effect of third molar agenesis, which was not only present at the jaw where the agenesis was located but also on the opposite one (Table 3). This means that when a third molar agenesis was present in the maxilla, there were distinct differences in the shape of the maxilla, as well as the mandible, of the affected individuals. In cases of isolated mandibular third molar agenesis, the effects were smaller, but still significant and detectable on both jaws. Age was a significant, controlled confounder in this subgroup analysis as well, whereas sex was not found to have a significant effect.
Multivariate regression of maxillary and mandibular shape on sex, age, and number of missing teeth performed on two subsamples that had no agenesis and third molar agenesis only in the maxilla or only in the mandible.
| Shape configurations | Test factor | η2 | P value |
|---|---|---|---|
| Individuals with third molar agenesis only in the maxilla (N = 37) or no agenesis (n = 310) | |||
| Maxilla (PC1–PC10) | Sex | 0.027 | 0.484 |
| Age | 0.105 | <0.001 | |
| Number of missing teeth | 0.090 | <0.001 | |
| Mandible (PC1–PC8) | Sex | 0.006 | 0.952 |
| Age | 0.177 | <0.001 | |
| Number of missing teeth | 0.066 | 0.002 | |
| Individuals with third molar agenesis only in the mandible (N = 49) or no agenesis (n = 310) | |||
| Maxilla (PC1–PC10) | Sex | 0.019 | 0.753 |
| Age | 0.112 | <0.001 | |
| Number of missing teeth | 0.060 | 0.016 | |
| Mandible (PC1–PC8) | Sex | 0.007 | 0.926 |
| Age | 0.193 | <0.001 | |
| Number of missing teeth | 0.043 | 0.031 | |
| Shape configurations | Test factor | η2 | P value |
|---|---|---|---|
| Individuals with third molar agenesis only in the maxilla (N = 37) or no agenesis (n = 310) | |||
| Maxilla (PC1–PC10) | Sex | 0.027 | 0.484 |
| Age | 0.105 | <0.001 | |
| Number of missing teeth | 0.090 | <0.001 | |
| Mandible (PC1–PC8) | Sex | 0.006 | 0.952 |
| Age | 0.177 | <0.001 | |
| Number of missing teeth | 0.066 | 0.002 | |
| Individuals with third molar agenesis only in the mandible (N = 49) or no agenesis (n = 310) | |||
| Maxilla (PC1–PC10) | Sex | 0.019 | 0.753 |
| Age | 0.112 | <0.001 | |
| Number of missing teeth | 0.060 | 0.016 | |
| Mandible (PC1–PC8) | Sex | 0.007 | 0.926 |
| Age | 0.193 | <0.001 | |
| Number of missing teeth | 0.043 | 0.031 | |
Each shape configuration was described with the number of PCs explaining more than 85% of shape variation, as assessed with the broken-stick method.
Multivariate regression of maxillary and mandibular shape on sex, age, and number of missing teeth performed on two subsamples that had no agenesis and third molar agenesis only in the maxilla or only in the mandible.
| Shape configurations | Test factor | η2 | P value |
|---|---|---|---|
| Individuals with third molar agenesis only in the maxilla (N = 37) or no agenesis (n = 310) | |||
| Maxilla (PC1–PC10) | Sex | 0.027 | 0.484 |
| Age | 0.105 | <0.001 | |
| Number of missing teeth | 0.090 | <0.001 | |
| Mandible (PC1–PC8) | Sex | 0.006 | 0.952 |
| Age | 0.177 | <0.001 | |
| Number of missing teeth | 0.066 | 0.002 | |
| Individuals with third molar agenesis only in the mandible (N = 49) or no agenesis (n = 310) | |||
| Maxilla (PC1–PC10) | Sex | 0.019 | 0.753 |
| Age | 0.112 | <0.001 | |
| Number of missing teeth | 0.060 | 0.016 | |
| Mandible (PC1–PC8) | Sex | 0.007 | 0.926 |
| Age | 0.193 | <0.001 | |
| Number of missing teeth | 0.043 | 0.031 | |
| Shape configurations | Test factor | η2 | P value |
|---|---|---|---|
| Individuals with third molar agenesis only in the maxilla (N = 37) or no agenesis (n = 310) | |||
| Maxilla (PC1–PC10) | Sex | 0.027 | 0.484 |
| Age | 0.105 | <0.001 | |
| Number of missing teeth | 0.090 | <0.001 | |
| Mandible (PC1–PC8) | Sex | 0.006 | 0.952 |
| Age | 0.177 | <0.001 | |
| Number of missing teeth | 0.066 | 0.002 | |
| Individuals with third molar agenesis only in the mandible (N = 49) or no agenesis (n = 310) | |||
| Maxilla (PC1–PC10) | Sex | 0.019 | 0.753 |
| Age | 0.112 | <0.001 | |
| Number of missing teeth | 0.060 | 0.016 | |
| Mandible (PC1–PC8) | Sex | 0.007 | 0.926 |
| Age | 0.193 | <0.001 | |
| Number of missing teeth | 0.043 | 0.031 | |
Each shape configuration was described with the number of PCs explaining more than 85% of shape variation, as assessed with the broken-stick method.
Discussion
This investigation tested the effect of third molar agenesis on craniofacial shape and found notable associations. In a previous study, the absence of third molar formation was found to be associated with a reduction in the size of the maxilla, the mandible, and the entire facial configuration [15]. This is consistent with the present findings detecting a significant association with the shape of these structures. An increased number of missing third molars is related to a less convex skeletal profile, a retruded maxilla, and a shorter anterior facial height in both males and females, with limited differences between sexes. The effect of third molar agenesis on the craniofacial shape is similar to the effect of agenesis of teeth other than third molars, which has also been reported previously [11]. The only qualitative difference in effects lies in the position of the mandible, which was found to be more protruded in case of missing teeth other than third molars, as compared to controls [11]. This was not confirmed here, neither in females nor in males.
In the present study, the shape of the anterior cranial base was found to be significantly affected by the number of third molars formed in the dentition, an association present more strongly in males. The differences in cranial base shape were more evident in the outline of sella, the anterior clinoid process and the extreme posterior region of the sphenoid bone (basion). This is in contrast to previous findings regarding the effect of generalized tooth agenesis on craniofacial size [10] and shape [11], and in contrast to previous reports regarding the effect of third molar agenesis on craniofacial size [15]. In the above cases, no effect was shown on the configuration of the cranial base. However, from the results of these previous studies, it became evident that the agenesis of third molars has a more profound effect (approximately 3-fold) on craniofacial size, as compared to the rest of the teeth in the human dentition [10, 15]. Regarding craniofacial shape, a similar pattern was also present, as extrapolated from the results of a previous study with similar shape outcomes related to the agenesis of teeth other than third molars [11], when compared to the present results regarding third molar agenesis. Supplementary Tables 3 and 4 provide evidence of this, showing significantly larger Procrustes distances between subjects missing one or more third molars and subjects without missing third molars, compared to the Procrustes distances between subjects missing one or more teeth other than third molars and subjects with full permanent dentition, without considering the third molars. These exploratory results confirm the significantly larger association of third molar agenesis with craniofacial shape compared to the agenesis of teeth other than third molars.
The latter comparisons are robust considering the age and sex factors (Supplementary Tables 2–4) and are also valid when accounting for the number of missing teeth per compared group. The group with missing third molars, but no other tooth in the dentition, had on average 2.5 missing third molars and was compared to a group of subjects with no missing tooth. The group with other missing teeth in the dentition, without considering third molars, had on average 2.7 missing teeth per subject and was compared to a group with no teeth other than third molars missing. In the latter groups, the third molars were not considered, but this could only strengthen our argument, since subjects with agenesis in teeth other than third molars present more often third molar agenesis compared to controls [3].
The third molars are characterized by variability in the time of formation, as well as an increased risk for developmental disturbances because they are the last teeth emerging in the dentition [2, 24, 25]. When considering the high developmental instability of third molars, a less robust association of the third molar agenesis to craniofacial size and shape would have been expected as compared to agenesis of other permanent teeth that present a more robust development. However, the opposite was shown to be true. It is well documented that the agenesis of third molars is associated with other dental anomalies [26]. Previous reports have also shown that third molars are more often missing bilaterally than unilaterally, and this effect is accentuated when teeth other than third molars are also absent [3]. In addition, no sexual dimorphism in the patterns or the severity of third molar agenesis has been found, despite the higher vulnerability compared to other teeth [27]. The aforementioned findings indicate that the third molars are more often and more globally affected by genetic or epigenetic factors involved in tooth agenesis, with no differences between sexes. This provides further support to the theory that the higher incidence of third molar agenesis in modern humans occurs within the context of the evolutionary mechanism that leads to a reduction in the number and size of teeth along with a reduction of the facial structures in humans [13, 14, 28], a mechanism which is apparently still active in modern times. Humans present a significantly higher incidence of tooth agenesis than other primates [29, 30], which might indicate a response to the changing functional needs. When considering that the distal molars are consistently the teeth that are reducing in number over the years, the present findings provide further support to the argument that this evolutionary mechanism is still active in modern humans. Another argument in favor of this theory derives from the fact that the number of third molars present in the dentition was found to affect the shape of the middle cranial base, which is a highly conserved structure among species and has reduced variation among humans [31, 32]. The importance of this finding becomes obvious when considering that the development of the cranial base does not overlap timely with the development of the third molars, in contrast to other permanent teeth that develop much earlier.
In the subsample analysis performed in this study, there was no effect of sex on the shapes of the maxilla and the mandible in individuals that had third molar agenesis only in the maxilla or only in the mandible, compared to individuals that had no third molar agenesis. In a previous study considering an analogous outcome for tooth agenesis affecting all permanent teeth [11], there were significant differences between sexes in certain cases. These were, however, of a small extent and they were not evident when the third molars were included in the model, together with the other permanent teeth. After adolescence sexual dimorphism diminishes and differentiates with age and mostly presents itself in structures other than the jaws [33–35]. The third molars are the latest forming teeth in the dentition, and thus, they share more developmental timing to this latter period, compared to the rest of the dentition. Additionally, the present sample is smaller compared to the original sample, where only minimal sexual dimorphism was evident in the maxilla and the mandible (Table 1). The above could explain the absence of the effect of sex in the shape of the maxilla and the mandible in the present study.
The effect of size on craniofacial shape (allometry) was not controlled in this study, because it was tested thoroughly in a previous study with a similar sample and methods, and was found that within sex groups, the presence of allometry could be attributed to age [11]. In the statistical analyses applied here, the age factor was incorporated in the models, and thus, any effects on craniofacial shape were controlled in this manner.
When interpreting the outcomes of this study, it is crucial to recognize that craniofacial shape is influenced by a multitude of factors throughout development. From a biological standpoint, and, to a lesser degree, from a clinical perspective, the influence of third molars on craniofacial shape is noteworthy. It also becomes more pronounced as the number of missing third molars increases. In fact, the influence of the ‘missing teeth’ factor, as quantified by Eta-squared (η2), was at times comparable to that of the ‘age’ factor—a primary determinant of craniofacial morphology, particularly in growing individuals. The subtle differences in average male and female craniofacial features, as illustrated in Fig. 2, further underscore the substantial impact of third molar absence on craniofacial characteristics, depicted in Fig. 3.
Conclusion
The findings of this investigation clearly indicate a stronger association between third molar formation and overall craniofacial development, as shown by the effects on craniofacial size and shape, compared to the effects of other permanent teeth. The effect of third molar agenesis on craniofacial shape is rather generalized than local, which is evident by the shape differences in individual jaw configurations that are not affected by a missing third molar. A possible explanation for this observation might be that third molars are a more integral piece in the evolutionary mechanism that leads to smaller and lesser teeth as well as smaller craniofacial configurations in modern humans.
Acknowledgements
We would like to express our gratitude to all the good colleagues who contributed to the selection of this sample.
Author contributions
Georgios Kanavakis (Conceptualization [Equal], Formal analysis [Equal], Investigation [Equal], Methodology [Equal], Resources [Equal], Visualization [Equal], Writing – original draft [Supporting], Writing – review & editing [Equal]), Ragda Alamoudi (Data curation [Equal], Investigation [Equal], Resources [Equal], Validation [Equal], Visualization [Equal], Writing – original draft [Equal], Writing – review & editing [Equal]), Elias S. Oeschger (Data curation [Equal], Investigation [Equal], Project administration [Supporting], Resources [Equal], Visualization [Supporting], Writing – review & editing [Supporting]), Manuel Tacchi (Data curation [Equal], Investigation [Equal], Resources [Supporting], Visualization [Equal], Writing – review & editing [Supporting]), Demetrios Halazonetis (Methodology [Supporting], Resources [Equal], Software [Equal], Visualization [Supporting], Writing – review & editing [Supporting]), and Nikolaos Gkantidis (Conceptualization [Lead], Data curation [Equal], Investigation [Equal], Methodology [Equal], Project administration [Lead], Resources [Equal], Supervision [Lead], Writing – original draft [Equal], Writing – review & editing [Equal])
Conflict of interest statement
The authors declare that there is no conflict of interest.
Funding
This research received no external funding.
Data availability
The data underlying this article will be shared on reasonable request to the corresponding author.
