Chin development as a result of differential jaw growth

What is a trait? Lessons from the human chin

The chin, a distinguishing feature of Homo sapiens, has sparked ongoing debates regarding its evolutionary origins and adaptive significance. We contend that these controversies stem from a fundamental disagreement about what constitutes a well-defined biological trait, a problem that has received insufficient attention despite its recognized importance in biology. In this paper, we leverage paleoanthropological research on the human chin to investigate the general issue of character or trait identification. First, we examine four accounts of the human chin from the existing literature: the mandibular differential growth byproduct, the bony prominence, the inverted T-relief, and the symphyseal angle. We then generalize from these accounts and propose a three-stage framework for the process of character identification: description, detection, and justification. We use this framework to reinterpret the four accounts, elucidating key points of contention surrounding the chin as well as other morphological characters. We show that debates over the chin carry broad and important biological implications that extend beyond this trait and that are not mere semantic issues of definition.

Treatment change comparisons between skeletal Class I and II white adolescents with 3 different vertical divergencies-Part 2: Holdaway difference

INTRODUCTION

The Holdaway difference represents the relationship of the NB line to the mandibular incisor (NB-L1) and the bony pogonion (NB-Pog). This study aimed to evaluate treatment changes of NB-L1, NB-Pog, and the Holdaway difference in patients with skeletal Class I and II relationships with 3 different skeletal divergencies.

METHODS

This retrospective study was the second part of treatment outcome assessments of 135 white adolescent patients (females, n = 69; males, n = 66; mean age, 12.8 ± 1.4 years pretreatment and 15.0 ± 1.4 years posttreatment). The NB-L1, NB-Pog, and Holdaway differences (NB-L1 - NB-Pog) were measured. The mixed-model analysis of variance was used to assess within- and between-subject effects responding to horizontal and vertical skeletal discrepancies.

RESULTS

For the group with favorable profile changes, the means of the Holdaway difference were maintained in the hypodivergent and normodivergent subgroups and reduced in the hyperdivergent subgroups for patients with skeletal Class I and II relationships. The means of NB-L1 and Holdaway difference were significantly larger in the skeletal Class II group and became greater as skeletal vertical divergencies increased. The NB-Pog means were significantly different only between the hypodivergent and hyperdivergent subgroups.

CONCLUSIONS

Based on the findings of this study, the Holdaway difference should be adjusted to individualize the incisor positions, considering not only the anteroposterior but also the vertical skeletal relationships of the patients.

Face Mask Therapy and Comprehensive Orthodontic Treatment for Skeletal Class III Malocclusion: A Case Report

Aim

This article aims to report a case of face mask therapy and comprehensive orthodontic treatment for skeletal class III malocclusion in a 16-year-old girl.

Background

Treating skeletal class III malocclusion in a growing patient is crucial, as it can help avoid the need for additional surgery. Early treatment also lessens the negative impacts of the patient's facial abnormality on their social life because surgery is only done later.

Case description

In this case report, a 14-year-old female patient presented with skeletal class III malocclusion with primary complaints of anterior crossbite. There was no relevant medical history. Face mask therapy and fixed appliance therapy were components of the treatment approach that successfully corrected the malocclusion. The total period of treatment was 20 months.

Conclusion

The treatment resulted in a harmonious face, a well-aligned smile arch, stable dental and skeletal relationships, and significant esthetic improvements, including improved facial symmetry and profile.

Significance

A growing teen who has a skeletal class III malocclusion and a maxillary deficit may be helped by a combination of face mask therapy and thorough orthodontic treatment. This case report outlines the use of the aforementioned technique to successfully treat a 14-year-old child with class III malocclusion and maxillary deficiencies.Early management of skeletal class III malocclusion in developing adolescents is vital as it can potentially eliminate the necessity for future surgical intervention, leading to improved treatment outcomes.Careful case selection, patient cooperation, and long-term stability enable a successful, stable, and esthetically pleasing treatment outcome.

How to cite this article

Le LN, Do TT, P Le KV. Face Mask Therapy and Comprehensive Orthodontic Treatment for Skeletal Class III Malocclusion: A Case Report. Int J Clin Pediatr Dent 2024;17(3):368-376.

Skeletal and soft-tissue changes in humans with untreated normal occlusion throughout lifetime: a systematic review

Age-related skeletal and soft-tissue changes are important in orthodontics, especially due to the increase of adult patients seeking treatment. The aim of this study is to assess the available evidence regarding age-related skeletal and soft-tissue changes in untreated Angle Class I. Articles studying skeletal and soft-tissue changes in orthodontically untreated subjects with Angle Class I and comparing them between age groups were included. Studies focusing on a single age group or in languages other than English were excluded. Risk of bias was assessed with both the MINORS and ROBINS-I tools. 50 studies were included, showing high methodological heterogeneity and a lack of information in subjects over 60 years old. In subjects with Angle Class I, the mandibular plane inclination was reported to reduce from 7 and 20 years old, while the anterior and posterior facial height continue to increase in late adult life. The anterior cranial base length increases until 20 years old, afterwards decreasing slowly until late adulthood. Nasal width increases and the nasolabial angle decreases during adolescence. Upper lip length and lower lip length increase from 6 to 18 years along with retrusion of the lips in late adulthood. Age-related skeletal and soft-tissue changes are documented in the literature from childhood until the fifth decade of life, but studies mostly focus on subjects until 20 years old. Changes after the second decade of life are studied only for the vertical and sagittal dimensions. No changes are reported in the transversal dimension beyond 15 years for neither skeletal nor soft tissues. Well-designed, long-term prospective cohort studies considering all three dimensions of skeletal and soft tissues are needed for confirmation of these findings (PROSPERO: CRD42020203206).

A Novel Cephalometric Approach Aiming to Quantify a Normal Range of Bony Chin Protrusion

Introduction

Cephalometric analysis is an essential tool in the diagnostics and planning of orthognathic surgery. No objective criterion exists to facilitate decision making regarding genioplasties. Differing opinions amongst clinicians therefore leads to wide variability amongst treatment options offered to potentially suitable patients. This study has three aims. The first was to quantify the distribution of chin morphology amongst the average population using cephalometric analysis. Secondly, we sought to determine whether cephalometric parameters could be used to predict overlying soft tissue changes. Lastly, we consider the use of a new cephalometric angle, BNPg, for pre- and post-operative assessment of genioplasty patients.

Methods

This study retrospectively analysed 231 lateral cephalograms. The angle between the landmarks: B point, Nasion and Pogonion was measured to generate 'BNPg' a novel unit to quantify bony chin protrusion.

Results

The mean BNPg from all 231 samples was 1.12 degrees with a standard deviation of ± 1.35. Comparison between sexes showed no significant differences between male and females (P = 0.108). Furthermore, bony chin protrusion was found to strongly positively correlate with soft tissue chin appearance (r = 0.731), however, BNPg was found not to correlate with skeletal malocclusion (ANB, r = 0.085).

Conclusion

The novel unit BNPg may serve as a useful tool in contributing to the determination of treatment thresholds in osseous genioplasty for desirable aesthetic outcomes and may be used post-operatively to assess outcomes also. As this is a pilot study, further clinical studies would be required to validate this parameter in genioplasty patients, both pre- and post-operatively.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12663-022-01784-5.

Symphysis morphology and mandibular alveolar bone thickness in patients with β-thalassemia major and different growth patterns

Objective:

The present study aimed to assess the morphology of symphysis and alveolar bone thickness (ABT) surrounding mandibular incisors in thalassemic patients, as compared to unaffected individuals.

Methods:

This case-control study was conducted on lateral cephalograms of 60 thalassemic and 60 unaffected patients with Class II malocclusion seeking orthodontic treatment at Dental School, Shiraz University of Medical Sciences. The sample was divided into three subgroups including hyperdivergent, normodivergent, and hypodivergent, according to the Jarabak index. Symphysis dimensions and alveolar bone thickness surrounding mandibular incisors were measured using AutoCad software. Finally, the correlation between alveolar bone thickness and symphysis morphology was assessed.

Results:

In general, chin dimensions and bone thickness at different levels of mandibular incisor roots (cervical, middle, apical) were smaller in thalassemic adolescents than controls. Concerning the total sample as well as the normodivergent subgroup, significantly lower values were observed in thalassemic patients for symphysis width, total ABT at the cervical, and lingual ABT at the apical root area compared to controls (p < 0.05). The hypodivergent growth pattern was not associated with any statistical differences between the groups (p> 0.05). In both thalassemic and control subjects, symphysis width showed a weak to moderate positive correlation with ABT of lower incisors (p< 0.05), whereas symphysis height showed a moderate positive correlation with cervical ABT in only ß‐thalassemia patients (p< 0.05).

Conclusions:

Compared to controls, ß-thalassemia patients showed thinner alveolar bone at different levels of lower incisor roots and smaller symphysis dimensions. There were significant correlations between symphysis dimensions and alveolar bone thickness of mandibular incisors in the sample.

How did modern morphology evolve in the human mandible? The relationship between static adult allometry and mandibular variability in Homo sapiens

Key to understanding human origins are early Homo sapiens fossils from Jebel Irhoud, as well as from the early Late Pleistocene sites Tabun, Border Cave, Klasies River Mouth, Skhul, and Qafzeh. While their upper facial shape falls within the recent human range of variation, their mandibles display a mosaic morphology. Here we quantify how mandibular shape covaries with mandible size and how static allometry differs between Neanderthals, early H. sapiens, and modern humans from the Upper Paleolithic/Later Stone Age and Holocene (= later H. sapiens). We use 3D (semi)landmark geometric morphometric methods to visualize allometric trends and to explore how gracilization affects the expression of diagnostic shape features. Early H. sapiens were highly variable in mandible size, exhibiting a unique allometric trajectory that explains aspects of their 'archaic' appearance. At the same time, early H. sapiens share a suite of diagnostic features with later H. sapiens that are not related to mandibular sizes, such as an incipient chin and an anteroposteriorly decreasing corpus height. The mandibular morphology, often referred to as 'modern', can partly be explained by gracilization owing to size reduction. Despite distinct static allometric shape changes in each group studied, bicondylar and bigonial breadth represent important structural constraints for the expression of shape features in most Middle to Late Pleistocene hominin mandibles.

Predicted Archaic 3D Genome Organization Reveals Genes Related to Head and Spinal Cord Separating Modern from Archaic Humans

High coverage sequences of archaic humans enabled the reconstruction of their DNA methylation patterns. This allowed comparing gene regulation between human groups, and linking such regulatory changes to phenotypic differences. In a previous work, a detailed comparison of DNA methylation in modern humans, archaic humans, and chimpanzees revealed 873 modern human-derived differentially methylated regions (DMRs). To understand the regulatory implications of these DMRs, we defined differentially methylated genes (DMGs) as genes that harbor DMRs in their promoter or gene body. While most of the modern human-derived DMRs could be linked to DMGs, many others remained unassigned. Here, we used information on 3D genome organization to link ~70 out of the remaining 288 unassigned DMRs to genes. Combined with the previously identified DMGs, we reinforce the enrichment of these genes with vocal and facial anatomy, and additionally find significant enrichment with the spinal column, chin, hair, and scalp. These results reveal the importance of 3D genomic organization in understanding gene regulation by DNA methylation.

Early treatment of Class III malocclusion with facemask therapy

The facemask is a widely used device in the treatment of Class III malocclusion and is intended to anteriorly displace the superior maxilla or stimulate its growth in that direction. The main goal of this study was to evaluate the effects of treatment using orthopedic maxillary expansion with facemask therapy in patients with Class III malocclusion. Sixty-four patients, with a mean age of 8.14 ± 1.18 years at the start of treatment and a mean age of 9.78 ± 1.19 years at the end, were treated using orthopedic maxillary expansion and associated facemask therapy. The patients were evaluated using lateral head teleradiography before and after treatment, and the differences were analyzed. In addition, binary logistic regression was used as a model for predicting successful treatment. When comparing the changes achieved by treatment, statistically significant favorable changes were found at the skeletal level. Furthermore, an improvement in the airways at all levels was detected. Orthopedic maxillary expansion associated with facemask therapy has proven effective in treating early skeletal Class III malocclusion.

[Functional genioplasty in growing patients]

Objective - To evaluate the role of age as a moderator of bone regeneration patterns and symphysis remodeling after genioplasty. Method - Fifty-four patients who underwent genioplasty at the end of their orthodontic treatment were divided into three age groups: younger than 15 years at the time of surgery (group 1), 15 to 19 years (group 2), and 20 years or older (group 3). Twenty-three patients who did not accept genioplasty and had a follow-up radiograph two years after the end of their orthodontic treatment were used as a control group. Patients were evaluated at three time points: immediate preoperative (T1), immediate postoperative (T2) and two years postsurgery (T3). Results - The mean genial advancement at surgery was similar for the three age groups, but the extent of remodeling around the repositioned chin was greater in group 1, less in group 2, and still less in group 3. Symphysis thickness increased significantly during the two-years postsurgery interval for the three groups, and this increase was significantly greater in group 1 than in group 3. Remodeling above and behind the repositioned chin also was greater in the younger patients. This was related to greater vertical growth of the dentoalveolar process in the younger patients. There was no evidence of a deleterious effect on mandibular growth. Conclusion - The outcomes of forward-upward genioplasty include increased symphysis thickness, bone apposition above B point, and remodeling at the inferior border. When indications for this type of genioplasty are recognized, early surgical correction (before age 15) produces a better outcome in terms of bone remodeling.

Spatial determinants of the mandibular curve of Spee in modern and archaic Homo

OBJECTIVES

The curve of Spee (COS) is a mesio-distally curved alignment of the canine through distal molar cusp tips in certain mammals including modern humans and some fossil hominins. In humans, the alignment varies from concave to flat, and previous studies have suggested that this difference reflects craniofacial morphology, including the degree of alveolar prognathism. However, the relationship between prognathism and concavity of the COS has not been tested in craniofacially variant populations. We tested the hypothesis that greater alveolar prognathism covaries with a flatter COS in African-American and European-American populations. We further examined this relationship in fossil Homo including Homo neanderthalensis and early anatomically modern Homo sapiens, which are expected to extend the amount of variation in the COS from the extant sample.

METHODS AND MATERIALS

These hypotheses were tested using three-dimensional geometric morphometrics. Landmarks were recorded from the skulls of 166 African-Americans, 123 European-Americans, and 10 fossil hominin mandible casts. Landmarks were subjected to generalized Procrustes analysis, principal components analysis, and two-block partial least squares analysis.

RESULTS

We documented covariation between the COS and alveolar prognathism such that relatively prognathic individuals have a flatter COS. Mandibular data from the fossil hominin taxa generally confirm and extend this correlation across a greater range of facial size and morphology in Homo.

DISCUSSION

Our results suggest that the magnitude of the COS is related to a suite of features associated with alveolar prognathism in modern humans and across anthropoids. We also discuss the implications for spatial interactions between the dental arches.

Alveolar and Skeletal Chin Dimensions Associated with Lower Facial Height Among Different Divergent Patterns

INTRODUCTION

Alveolar and skeletal chin dimensions with facial height have an impact on diagnosis and treatment planning in orthodontic patients and serves as a reference landmark for facial aesthetics.

AIM

To evaluate the alveolar and skeleton chin dimensions among different divergent patterns, to evaluate the lower face height in different divergent growth patterns, to give an estimate to the oral surgeon for any adjunctive procedure.

MATERIALS AND METHODS

The study includes a total sample of 45 non-growing individuals (26 males and 19 females) from the archival records of the Orthodontic Department with age ranging between 18-25 years, classified into different divergent patterns based on the mandibular plane angle. The sample was divided into three groups, of which 15 patients in each group were evaluated: Group-1: Normodivergent, Group-2: Hypodivergent and Group-3: Hyperdivergent patterns respectively. Standardized digital lateral cephalograms and antero-posterior cephalograms of these patients were obtained in Natural Head Position (NHP).

RESULTS

The results of the retrospective cephalometeric study were statistically analyzed by using ANOVA and Tukey's multiple post hoc test which showed that hard tissue chin measurement were greater in hypodivergent group (13.7mm) and the increased lower facial height showed statistically significant difference in high mandibular plane angle group (59.6mm). In antero-posterior cephalograms the transverse width showed statistically significant difference in normodivergent group (32.60mm).

CONCLUSION

The anterio-posterior width of the symphysis was higher in hypodivergent growth pattern, whereas vertical height of the symphysis was greater in hyperdivergent group. The lower facial height among three groups showed statistically significant difference among hyperdivergent group. In comparison of all the three groups in Anterio-Posterior (AP) cephalograms, statistically significant difference was noted in normodivergent group when compared with the other two groups.

Symphyseal surface strain during in vitro human mandibular wishboning

OBJECTIVES

This research theoretically models and empirically records symphyseal surface strain during in vitro human mandibular wishboning (lateral transverse bending) in order to test one aspect of the hypothesis that the chin is an adaptive response to masticatory stresses. From a perspective of optimality, three questions were tested: 1) Do human mandibles function as curved beams during wishboning? 2) Is the presence of a chin associated with lower than predicted curved beam effects? 3) Are there relatively low strain gradients on the lingual and labial symphyseal surfaces respectively?

MATERIALS AND METHODS

Based on morphometric criteria, theoretical wishboning strains were calculated for five dentate adult human mandibles. The same mandibles were fitted with strain gauges and subjected to simulated wishboning loads. From the empirically-recorded strains, relative strains were calculated by dividing all strains by the absolute lowest strain in a given specimen. The theoretical and empirical results were compared in order to address the three related questions guiding this research.

RESULTS

Human mandibles behave as curved beams during wishboning (question 1). Empirical strain measures showed greater disparity both between and within the labial and lingual symphyseal surfaces than the theoretical models predictions (questions 2 and 3).

DISCUSSION

Human symphyseal form, with its distinctive chin, is unlikely to be adapted for countering wishboning loads. Chins are associated with larger than expected strain gradients within and between symphyseal surfaces, which runs counter to the optimality criterion typically invoked in assessing trait performance for signs of adaptation. The implications are twofold: 1) wishboning may not, in fact, be a regular feature of human mastication or 2) wishboning may not pose the same structural risks in human jaws as this load does in other anthropoid primates.

The ontogeny of the chin: an analysis of allometric and biomechanical scaling

The presence of a prominent chin in modern humans has been viewed by some researchers as an architectural adaptation to buttress the anterior corpus from bending stresses during mastication. In contrast, ontogenetic studies of mandibular symphyseal form suggest that a prominent chin results from the complex spatial interaction between the symphysis and surrounding soft tissue and skeletal anatomy during development. While variation in chin prominence is clearly influenced by differential growth and spatial constraints, it is unclear to what degree these developmental dynamics influence the mechanical properties of the symphysis. That is, do ontogenetic changes in symphyseal shape result in increased symphyseal bending resistance? We examined ontogenetic changes in the mechanical properties and shape of the symphysis using subjects from a longitudinal cephalometric growth study with ages ranging from 3 to 20+ years. We first examined whether ontogenetic changes in symphyseal shape were correlated with symphyseal vertical bending and wishboning resistance using multivariate regression. Secondly, we examined ontogenetic scaling of bending resistance relative to bending moment arm lengths. An ontogenetic increase in chin prominence was associated with decreased vertical bending resistance, while wishboning resistance was uncorrelated with ontogenetic development of the chin. Relative to bending moment arm lengths, vertical bending resistance scaled with significant negative allometry whereas wishboning resistance scaled isometrically. These results suggest a complex interaction between symphyseal ontogeny and bending resistance, and indicate that ontogenetic increases in chin projection do not provide greater bending resistance to the mandibular symphysis.

Functional genioplasty in growing patients

Objective: 

To evaluate the role of age as a moderator of bone regeneration patterns and symphysis remodeling after genioplasty.

Method: 

Fifty-four patients who underwent genioplasty at the end of their orthodontic treatment were divided into three age groups: younger than 15 years at the time of surgery (group 1), 15 to 19 years (group 2), and 20 years or older (group 3). Twenty-three patients who did not accept genioplasty and had a follow-up radiograph 2 years after the end of their orthodontic treatment were used as a control group. Patients were evaluated at three time points: immediate preoperative (T1), immediate postoperative (T2,) and 2 years postsurgery (T3).

Results: 

The mean genial advancement at surgery was similar for the three age groups, but the extent of remodeling around the repositioned chin was greater in group 1, less in group 2, and still less in group 3. Symphysis thickness increased significantly during the 2-year postsurgery interval for the three groups, and this increase was significantly greater in group 1 than in group 3. Remodeling above and behind the repositioned chin also was greater in the younger patients. This was related to greater vertical growth of the dentoalveolar process in the younger patients. There was no evidence of a deleterious effect on mandibular growth.

Conclusion: 

The outcomes of forward-upward genioplasty include increased symphysis thickness, bone apposition above B point, and remodeling at the inferior border. When indications for this type of genioplasty are recognized, early surgical correction (before age 15) produces a better outcome in terms of bone remodeling. (Angle Orthod. 2015;85:360–373.)

Functional and morphological correlates of mandibular symphyseal form in a living human sample

Variation in recent human mandibular form is often thought to reflect differences in masticatory behavior associated with variation in food preparation and subsistence strategies. Nevertheless, while mandibular variation in some human comparisons appear to reflect differences in functional loading, other comparisons indicate that this relationship is not universal. This suggests that morphological variation in the mandible is influenced by other factors that may obscure the effects of loading on mandibular form. It is likely that highly strained mandibular regions, including the corpus, are influenced by well-established patterns of lower facial skeletal integration. As such, it is unclear to what degree mandibular form reflects localized stresses incurred during mastication vs. a larger set of correlated features that may influence bone distribution patterns. In this study, we examine the relationship between mandibular symphyseal bone distribution (i.e., second moments of area, cortical bone area) and masticatory force production (i.e., in vivo maximal bite force magnitude and estimated symphyseal bending forces) along with lower facial shape variation in a sample of n = 20 living human male subjects. Our results indicate that while some aspects of symphyseal form (e.g., wishboning resistance) are significantly correlated with estimates of symphyseal bending force magnitude, others (i.e., vertical bending resistance) are more closely tied to variation in lower facial shape. This suggests that while the symphysis reflects variation in some variables related to functional loading, the complex and multifactorial influences on symphyseal form underscores the importance of exercising caution when inferring function from the mandible especially in narrow taxonomic comparisons.