Introduction

The action of forces on the facial bones, especially during the growth phase, may lead to quantitative and qualitative functional changes and may also modify their functional patterns, thereby leading to asymmetry, according to Santos and Morosolli [19]. These authors accepted that the origins of this asymmetry could be genetic or congenital, or could be related to disease, trauma or local factors. According to Carlini et al. [2], the asymmetry could be caused genetically through interruption of cell proliferation and development, as occurs in macrosomia, and could also be acquired through a variety of diseases, such as osteochondroma. Regarding local factors, Sousa et al. [20] reported that forces generated by defects of mastication, or any other mechanism that might unilaterally or bilaterally overload the mandible unequally, could generate asymmetry. These mechanisms could thus be unilateral mastication [18, 19], occlusal interference [19], bad habits [18], tooth hypersensitivity [19] and lack of masticatory function [18]. Thus, the presence of unfavorable pressure, interruption of muscle balance and damage to the ossification location could, at a critical stage of development, have a powerful influence on the final morphology and functional pattern of structures that form part of the craniofacial complex [13].

Mandibular stimuli are present from the development stage to adulthood and can change the growth of the mandible, along with its bone remodeling. According to Gosh et al. [8], the mandible is subject to remodeling and morphological changes in several of its regions and parts, such as the gonial, antegonial, condylar and ramus regions.

The presence or absence of teeth is thought to be a triggering factor for this remodeling. The general distribution of the remodeling fields in edentate mandibles differs markedly from that of adolescents with a growing mandible [16]. Enlow et al. [7] reported that, with tooth losses, the functional and structural relationships of the entire adult mandible change and specific remodeling responses occur. These would involve altered occlusal relationships, mandibular rotation, realignment and remodeling of the mandibular body, alteration of the muscle–bone alignment and changes to the location of and reduction in the areas for muscle insertion. Huumonen et al. [9] considered that, in adults, morphological changes to the mandible are influenced by the subject’s occlusal condition and age. Xie and Ainamo [21] accepted that long duration of edentulousness and the possibility of smaller masticatory forces could be related to changes to the mandibular angle. According to Gosh et al. [8] this would occur because the dimensions of the facial skeleton are related to the activity of the masticatory muscles.

The objective of the present study was to investigate the influence of edentulousness on remodeling of the gonial and incisure angles in dentate and edentate mandibles.

Materials and method

The sample was formed by 85 dry mandibles belonging to the anatomy laboratories of the Federal University of Sergipe (UFS) and Tiradentes University (UNIT). Four mandibles with identification, which presented fragmentation of structures of relevance for the measurements, were excluded. The mean age was 54.72 years, with a range from 14 to 81 years; 41 mandibles (50.62 %) were female and 40 (49.38 %) were male; and 33 (40.74 %) were edentate and 48 (59.26 %) were dentate.

The mandibles were photographed using a camera that was coupled to a static support and positioned at a standardized distance of 20 cm from the object to be photographed. The images were digitized and saved in the jpeg format (Joint Photographic Experts Group).

The following variables were evaluated: gonial angle of the mandible (ÂG) and mandibular incisure angle (ÂI). The ÂG was measured by tracing out one line tangential to the lower margin of the mandibular body and another line tangential to the posterior margin of the mandibular condyle and ramus (Fig. 1). The ÂI was measured as the intersection between two lines tangential to the internal borders of the coronoid and condylar processes, and these were found at the deepest point of the mandibular incisure (Fig. 2). The measurements on each of these angles were made by three dentistry students at different times, using the Image J 1.42q software, which is public-domain software developed by the National Institute of Health (NIH) in the United States.

Fig. 1
figure 1

Gonial angle of the mandible A a line tangent to the posterior border of the ramus of the mandible, B a line tangent to the inferior border of the mandible, ÂG gonial angle of the mandible (the angle formed between A and B), MC mandibular condyle, CP coronoid process

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Fig. 2
figure 2

Mandibular incisure angle, C a line tangent to the anterior border of the mandibular condyle, D a line tangent to the posterior border of the coronoid process, ÂI mandibular incisure angle (the angle formed between C and D), MC mandibular condyle, CP coronoid process

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To compare the mean values of ÂI and ÂG found in relation to sex and side, was employed multivariate analysis. To determine the significance of the distribution of values of ÂI and ÂG, in relation to sex and age, was performed bivariate analysis (t test).

Results

The ÂI and ÂG measurements made by the three observers were similar, and the means between the observers are presented in Table 1.

Table 1 Mean ÂI and ÂM measurements in dry mandibles
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The reliability analyses on the morphometric data between the observers of ÂI and ÂG presented values that were considered excellent. For ÂI, this value was 0.929 (Cronbach’s alpha) and for ÂG, the value was 0.902 (Cronbach’s alpha) [12].

The multivariate analysis on the mean values of ÂI and ÂG showed that there was a difference between male and female mandibles (p = 0.004) with a medium effect dimension and power of 0.868. The effect observed between the sexes was registered for ÂI, such that the angle was 97.3 ± 0.9° for males and 101.2 ± 0.9° for females, with a mean difference favoring the females of 3.9 ± 1.3° with 95 % CI of 1.3–6.5 (p = 0.003). On the other hand, no significant difference was demonstrated for ÂG. Age was seen to have an influence on ÂG, with p = 0.007 and a small effect dimension. There was no significant effect regarding the side (p = 0.84), or between dentate and edentate mandibles (p = 0.24). Likewise, there were no significant interactions between sex and side (p = 0.82), sex and teeth (p = 0.63), side and teeth (p = 0.82) or sex, side and teeth (p = 0.85) (Table 2).

Table 2 Multivariate analysis on ÂI and ÂG (MANCOVA)
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On the other hand, bivariate analysis (t test) demonstrated that the female dry mandibles presented significantly greater MVÂI and MVÂG (p < 0.05) than shown by the male mandibles (Table 3). Regarding MVÂI, the difference found between the female and male mandibles was 3.6° ± 1.3, with 95 % CI of 1.1–6.1. For MVÂG, the difference found was 2.2° ± 1.2, with 95 % CI of 0.1–4.3. There was no significant difference (p > 0.05) with regard to the ages of the male and female mandibles (Table 3). Regarding the MVÂI and MVÂG of the left and right sides, there were no significant differences (p > 0.05) (Table 4). The MVÂG was significantly greater (p < 0.05) in the edentate mandibles, with a difference of 2.7° ± 1.1 and 95 % CI of 0.5–4.8. However, there was no significant difference (p > 0.05) in MVÂI between dentate and edentate mandibles (Table 5). The edentate mandibles presented significantly greater age (p < 0.05) than the dentate mandibles, with a difference in age of 6.7 ± 2.7 years and 95 % CI of 1.4–12 (Table 5).

Table 3 Comparison of means values for ÂI and ÂG in relation to age and sex
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Table 4 Comparison of mean values of ÂI and ÂG in relation to age and sides
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Table 5 Comparison of the mean values of ÂI and ÂM in relation to age and edentate and dentate mandibles
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Discussion

Enlow et al. [7] described the remodeling that occurs in the mandible after tooth losses as a functional and structural alteration represented by a new occlusal relationship and changes to muscle strength and muscle insertions. In this manner, the entire mandible would undergo changes that would be very visible through the shape of the gonium and size of the mandible. Most authors who have studied mandibular remodeling have used panoramic radiographs for morphometric evaluations on parts of the bone [4, 5, 8, 9, 17, 21]. In the present study, the Image J 1.42q software was used to obtain these measurements. Carvalho [3] considered that this method was more advantageous, since unlike panoramic radiographs, it does not depend on image quality, distortions, blotting and artifacts, or the professional’s technical knowledge or subjective radiographic interpretations.

In the present study, no significant difference in the gonial angle of the mandible in relation to the presence or absence of teeth was found. This finding was in agreement with Dutra et al. [5], who did not find any increase in the mandibular angle on panoramic radiographs on edentate women. Ceylan et al. [4] had similar findings from mandibles of edentate and partially dentate individuals. However, Gosh et al. [8] and Huumonen et al. [9] found that the gonial angle of the mandible was greater in patients with edentate mandibles. These authors explained this finding as due to a decrease in masticatory function that occurred with the tooth losses, which would result in lower electromyographic activity and diminished muscle density in the patients with tooth losses. Ohm and Silness [14] found a close relationship between the size of the gonial angle of the mandible and the number of teeth present. Those authors studied three groups of mandibles: edentate, partially dentate and dentate. The gonial angle of the mandible was greatest in the group of edentate mandibles and smallest in the dentate mandibles, while the partially dentate group had an angle size between the other two groups. Those authors’ findings were different from what was found in the present study. It is possible that this lack of agreement was due to the difference in the methodologies used: those authors used panoramic radiographs or cephalograms, while we used the Image J 1.42q software.

The presence of strong activity of the masseter and temporal muscles would give rise to a broad posterior facial height and a small gonial angle of the mandible [10]. Huumonen et al. [9] found that the gonial angle of the mandible was greater among women and accepted that this was due to the greater masticatory force presented by men than by women. Similar results were also reported by Xie and Ainamo [21], Gosh et al. [8] and Karoshah et al. [11]. In our study, no significant difference was found between the gonial angles of the mandibles of men and women. This finding is concordant with those of Ohm and Silness [14], Raustia and Salonen [17] and Ayoubet al. [1]. Ceylan et al. [4] reported that gender alone could not be responsible for the change in the gonial angle of the mandible.

The mandibular ÂI did not present any significant difference between the dentate and edentate mandibles. Studies on the mandibular ÂI are still scarce, although there are studies that have correlated the presence or absence of teeth with the components that form the mandibular incisure, condylar process and coronoid process. Enlow et al. [7] reported that bone deposition would occur in the area of the mandibular ramus, immediately inferior to the mandibular incisure, during remodeling of edentate mandibles. This would result in elevation of the mandibular incisure in edentate individuals. Edwards [6] reported that the coronoid process of edentate mandibles would be shorter than the condylar process, in contrast with what would occur in dentate mandibles, in which the coronoid process would be larger than the condylar process. Huumonen et al. [9] found that the heights of the mandibular ramus and condyle would be greater in patients with dentate mandibles and that bone remodeling in these coronoid and condylar processes due to edentulousness could give rise to morphological changes to the mandibular incisure. According to these authors, remodeling of the mandibular incisure could be important from a clinical point of view because of its close relationship with the temporomandibular joint and signs of pain.

The angle of the mandibular incisure was greater in females than in males. The parameter (ÂI), a finding that was not described in the literature, even in works such as Osato et al. [15], when he studied five parameters mandibular. The significance of the change in the value of this angle, which was higher in females, may be related to the mechanical interaction between the mandibular condyle and coronoid process. Thus, we admit that more studies from this finding will be conducted to clarify its meaning, especially in edentate mandibles. It is worth highlighting that studies on edentulousness and the influence of sex in relation to the angle of the mandibular incisure are few in number or nonexistent in the literature.

In the present study, the gonial angle and the angle of the mandibular incisure were measured on both sides of the bone. Like Huumonen et al. [9], we did not find any statistically significant differences in the gonial angles of the mandible between the left and right sides. Thus, it was evident that no asymmetry occurred in the mandibles studied. We believe that a major limitation of this study took place, as in most studies of this type, is that we cannot know how much time the edentate people were edentate.

Conclusions

Although the incisure and gonial angles are larger in females and in edentate mandibles, no statistically significant differences were found.

We hope that studies in greater detail might be undertaken, on dentate, partially dentate and completely edentate mandibles, to achieve greater understanding of the effect of edentulousness on the mandible. We also believe that further studies on the incisure of the mandible may provide better explanation of the real effect of the teeth on mandibular remodeling.