Masseter function and skeletal malocclusion

An Anthropological Investigation of the Sociocultural and Economic Forces Shaping Dental Crowding Prevalence

OBJECTIVE

The aim of the current study is to explore country of origin and social race category differences in dental crowding prevalence through an anthropological approach.

DESIGN

Data were collected from individuals within five countries (Australia, China, Japan, South Africa, United States; n = 1008) and seven social race groups in two countries (American Black, American White, Latinx, and Indigenous in the United States, South African Black, South African White, and South African Coloured; n = 654). Statistical significance between groups was assessed with a Kruskal-Wallis test, while a Dunn's post-hoc test identified which groups significantly differed.

RESULTS

Results indicate South Africa is characterized by the lowest frequencies of dental crowding, with Coloured South Africans yielding the highest and Black South Africans displaying the lowest frequencies. Individuals in the United States exhibited relatively high levels of minor dental crowding. American Blacks had lower crowding levels, while the Indigenous group had high levels of severe crowding. Individuals within China and Japan exhibited higher relative prevalence of severe crowding.

CONCLUSIONS

Overall, significant differences exist in dental crowding prevalence across countries and social race categories using an anthropological grading system to assess crowding. These differences are likely impacted by sociocultural (aesthetic preferences) and economic (access to dental care) factors.

ACTN3 genotype influences masseter muscle characteristics and self-reported bruxism

OBJECTIVES

Main aim of the study was to explore the association between genetic polymorphisms in ACTN3 and bruxism. Secondary objectives included masseter muscle phenotypes assessment between bruxers and non-bruxers and according to genetic polymorphisms in ACTN3.

MATERIALS AND METHODS

Fifty-four patients undergoing orthognathic surgery for correction of their malocclusion were enrolled. Self-reported bruxism and temporomandibular disorders status were preoperatively recorded. Saliva samples were used for ACTN3 genotyping. Masseter muscle samples were collected bilaterally at the time of orthognathic surgery to explore the muscle fiber characteristics.

RESULTS

There were significant differences in genotypes for rs1815739 (R577X nonsense) (p = 0.001), rs1671064 (Q523R missense) (p = 0.005), and rs678397 (intronic variant) (p = 0.001) between bruxers and non-bruxers. Patients with self-reported bruxism presented a larger mean fiber area for types IIA (p = 0.035). The mean fiber areas in individuals with the wild-type CC genotype for rs1815739 (R577X) were significantly larger for type IIA fibers (1394.33 μm² [572.77 μm² ]) than in those with the TC and TT genotypes (832.61 μm² [602.43 μm² ] and 526.58 μm² [432.21 μm² ] [p = 0.014]). Similar results for Q523R missense and intronic variants.

CONCLUSIONS

ACTN3 genotypes influence self-reported bruxism in patients with dentofacial deformity through specific masseter muscle fiber characteristics.

Contribution of Masticatory Muscle Pattern in Craniofacial Morphology: A Systematic Review

Context:

The objective of this systematic review is to analyze the pattern of masticatory muscle and its effects on craniofacial structures.

Method:

Electronic search was conducted for relevant studies in last 20 years (from January 1, 2002 to December 31, 2021) on the following databases: PubMed, Google scholar, Web of Science, Cochrane Library, and Wiley Library database. Hand searching of selected orthodontic journals was also undertaken. The selected studies were assessed for the risk of bias in Cochrane collaboration risk of bias tool. The “traffic plot” and “weighted plot” risk of bias distribution are designed in the RoB 2 tool. Two authors extracted the data which was then analyzed by another reviewer to obtain the final data.

Results:

Six studies fulfilled inclusion criteria. The risk of bias was high for all the studies. The sample size of the studies, methodologies used, muscles under study, parameters of muscles, and statistical analysis performed were extracted. The methodologies and results of the studies were found to be heterogenous.

Conclusions:

It is found that the vertical craniofacial dimensions can be better appreciated by masseter than by medial pterygoid. The volume of masseter is superior predictor than its cross-sectional area. The volume of the medial pterygoid affects the ramus of the mandible and the length and width of the lateral pterygoid are significantly correlated with transverse skeletal dimensions.

Introducing surface-to-surface matching technique to evaluate mandibular symmetry: A retrospective study

Objectives

This study introduced a three-dimensional (3D) surface-to-surface matching technique to evaluate the mandibular symmetry of teenagers and adults with unilateral second molar scissor bite.

Methods

The targets came from 73 cone-beam computed tomography (CBCT) images with unilateral second molar scissor bite, including teenagers (n = 30) and adults (n = 43). 73 images without scissor bite and matched in sex and age were selected as controls. The scans were developed into 3D mandible models and seven mandibular functional unit models, including condylar process (Co), coronoid process (Cr), mandibular ramus (Ra), mandibular angle (Ma), alveolar process (Ap), mandibular body (Mb) and chin process (Ch). The surface-to-surface matching technique was introduced. 3D deviation analysis and matching percentages calculation were performed and compared to evaluate the symmetry of the mandible.

Results

Comparisons were made between the study samples and control samples. For teenagers, the matching percentages of the entire mandible (55.31 ± 7.24%), Mb (69.04 ± 9.22%) and Co (65.19 ± 10.67%) in the study group were lower than that of the entire mandible (60.87 ± 6.38%) (P <0.01), Mb (75.0 ± 8.71%) (P <0.05) and Co (70.25 ± 8.20%) (P <0.05) in the control group. While Ap, Ra, Ch, Cr and Ma showed no statistically significant differences (P >0.05). For adults, the matching percentages of the entire mandible (48.88 ± 9.77%), Ap (65.83 ± 11.21%), Mb (64.43 ± 12.03%), Ch (79.17 ± 10.29%), Ra (64.11 ± 9.84%) and Co (61.08 ± 11.64%) in the study group were lower than the entire mandible (59.28 ± 5.49%) (P <0.01), Ap (73.65 ± 9.10%) (P <0.01), Mb (71.66 ± 8.40%) (P <0.01), Ch (83.86 ± 5.59%) (P <0.05), Ra (68.54 ± 7.87%) (P <0.05) and Co (66.20 ± 10.62%) (P <0.05) of the control group. Only Cr and Ma showed no statistically significant differences (P >0.05).

Conclusion

Mandibular asymmetry was observed in both teenagers and adults with unilateral second molar scissor bite. Moreover, compared with teenagers, more mandibular units of adult patients were affected.

Clinical significance

Based on the surface-to-surface matching technique, the symmetric and morphological information of the mandible can be converted into visual color maps and quantitative descriptions. This method can bring convenience to the study of the growth of mandible, orthodontic treatment and orthognathic surgery design.

Influence of genotype and perioral musculature on maxillary and mandibular development

OBJECTIVES

To determine whether there is an association between skeletal jaw position and perioral musculature, and if genotypes can predict skeletal growth.

MATERIALS AND METHODS

A prospective study on 42 patients over 1 year was performed. The study included 22 females and 20 males with and average age of 28.5 years. Lip strength was compared to radiographic cephalometric measurements. Allelic and genotypic frequencies from polymorphisms rs678397 and rs1815739 in ACTN3 and rs10850110 in MYO1H were compared to each variable. Chi-square and Fisher exact tests were used to determine if differences were statistically significant (alpha = 0.05).

RESULTS

The data showed significant differences between rs678397 genotype and allele frequencies and SNA angle (P = .01; P = .003, respectively); between rs1815739 allele frequency and SNA angle (P = .01); between rs678397 allele frequency and ANB angle (P = .049); between rs678397 genotype and allele frequencies and lip strength in females (P = .045; P = .02); and between rs678397 allele frequency and overall lip strength (P = .049), after mean strength values used as cut off being customized by sex.

CONCLUSIONS

Polymorphisms in ACTN3 are associated with weak lips and larger SNA and ANB angles.

Great potential of ultrasound elastography for the assessment of the masseter muscle in patients with temporomandibular disorders. A systematic review

Objective:

To summarize the available evidence on the use of elastography in the assessment of the masseter muscle in healthy individuals and patients with masseter muscle disorders.

Methods:

Systematic literature review has been performed according to the Preferred Reporting Items for Systematic Reviews and Meta-analyses guidelines.

Results:

16 of 142 studies identified were analyzed. Elastography was used in seven studies. Heterogeneity was observed in terms of study protocols, devices, patients, units of measure, and results. Elasticity values showed a correlation between the left and right masseter muscle side in healthy people, but not in patients with temporomandibular disorders (TMDs). Elasticity values increased in TMD and were correlated with the severity of TMD symptoms. Phantom studies proved the high reliability of elastography.

Conclusion:

Elastography is a promising tool for the assessment of the masseter muscle elasticity, but the evidence is insufficient. Studies on larger groups are needed to determine the accuracy of elastography to characterize masticatory muscle disorders.

The ancient sarcomeric myosins found in specialized muscles

Striated muscles express an array of sarcomeric myosin motors that are tuned to accomplish specific tasks. Each myosin isoform found in muscle fibers confers unique contractile properties to the fiber in order to meet the demands of the muscle. The sarcomeric myosin heavy chain (MYH) genes expressed in the major cardiac and skeletal muscles have been studied for decades. However, three ancient myosins, MYH7b, MYH15, and MYH16, remained uncharacterized due to their unique expression patterns in common mammalian model organisms and due to their relatively recent discovery in these genomes. This article reviews the literature surrounding these three ancient sarcomeric myosins and the specialized muscles in which they are expressed. Further study of these ancient myosins and how they contribute to the functions of the specialized muscles may provide novel insight into the history of striated muscle evolution.

Functional and molecular outcomes of the human masticatory muscles

The masticatory muscles achieve a broad range of different activities such as chewing, sucking, swallowing, and speech. In order to accomplish these duties, masticatory muscles have a unique and heterogeneous structure and fiber composition, enabling them to produce their strength and contraction speed largely dependent on their motor units and myosin proteins that can change in response to genetic and environmental factors. Human masticatory muscles express unique myosin isoforms, including a combination of thick fibers, expressing myosin light chains (MyLC) and myosin class I and II heavy chains (MyHC) -IIA, -IIX, α-cardiac, embryonic and neonatal and thin fibers, respectively. In this review, we discuss the current knowledge regarding the importance of fiber-type diversity in masticatory muscles versus supra- and infrahyoid muscles, and versus limb and trunk muscles. We also highlight new information regarding the adaptive response and specific genetic variations of muscle fibers on the functional significance of the masticatory muscles, which influences craniofacial characteristics, malocclusions, or asymmetry. These findings may offer future possibilities for the prevention of craniofacial growth disturbances.

Genetic variants in ACTN3 and MYO1H are associated with sagittal and vertical craniofacial skeletal patterns

OBJECTIVE

This study aimed to evaluate the association of genetic variants inACTN3 and MYO1H with craniofacial skeletal patterns in Brazilians.

DESIGN

This cross-sectional study enrolled orthodontic and orthognathic patients selected from 4 regions of Brazil. Lateral cephalograms were used and digital cephalometric tracings and analyzes were performed for craniofacial phenotype determination. Participants were classified according to the skeletal malocclusion in Class I, II or III; and according to the facial type in Mesofacial, Dolichofacial or Brachyfacial. Genomic DNA was extracted from saliva samples containing exfoliated buccal epithelial cells and analyzed for genetic variants inACTN3 (rs678397 and rs1815739) and MYO1H (rs10850110) by real-time PCR. Chi-square or Fisher's exact tests were used for statistical analysis (α = 5%).

RESULTS

A total of 646 patients were included in the present study. There was statistically significant association of the genotypes and/or alleles distributions with the skeletal malocclusion (sagittal skeletal pattern) and facial type (vertical pattern) for the variants assessed inACTN3 (P < 0.05). For the genetic variant evaluated in MYO1H, there was statistically significant difference between the genotypes frequencies for skeletal Class I and Class II (P < 0.05). The reported associations were different depending on the region evaluated.

CONCLUSION

ACTN3 and MYO1H are associated with sagittal and vertical craniofacial skeletal patterns in Brazilian populations.

Heredity, Genetics and Orthodontics - How Much Has This Research Really Helped?

Uncovering the genetic factors that correlate with a clinical deviation of previously unknown etiology helps to diminish the unknown variation influencing the phenotype. Clinical studies, particularly those that consider the effects of an appliance or treatment regimen on growth, need to be a part of these types of genetic investigations in the future. While the day-to-day utilization of "testing" for genetic factors is not ready for practice yet, genetic testing for monogenic traits such as Primary Failure of Eruption (PFE) and Class III malocclusion is showing more promise as knowledge and technology advances. Although the heterogeneous complexity of such things as facial and dental development, the physiology of tooth movement, and the occurrence of External Apical Root Resorption (EARR) make their precise prediction untenable, investigations into the genetic factors that influence different phenotypes, and how these factors may relate to or impact environmental factors (including orthodontic treatment) are becoming better understood. The most important "genetic test" the practitioner can do today is to gather the patient's individual and family history. This would greatly benefit the patient, and augment the usefulness of these families in future clinical research in which clinical findings, environmental, and genetic factors can be studied.

Management of Brodie bite: Note on surgical treatment

Described and illustrated by Allan G. Brodie in 1946, Brodie's syndrome corresponds to an abnormal transverse occlusal relationship in which the palatal cusps of the upper molars lie outside the vestibular cusps of their mandibular antagonists. This article covers the diagnosis, clinical presentation, etiology (and, particularly, etiopathogenesis) and evolution of this syndrome, and also its management, which is often difficult. The main limitations and difficulties of orthodontics and surgery are presented, along with the surgical techniques used. These points are illustrated by some clinical cases highlighting the importance of three-dimensional analyses and 3D planning.

ENPP1 and ESR1 genotypes associated with subclassifications of craniofacial asymmetry and severity of temporomandibular disorders

INTRODUCTION

We investigated whether ACTN3, ENPP1, ESR1, PITX1, and PITX2 genes which contribute to sagittal and vertical malocclusions also contribute to facial asymmetries and temporomandibular disorders (TMD) before and after orthodontic and orthognathic surgery treatment.

METHODS

One hundred seventy-four patients with a dentofacial deformity were diagnosed as symmetric or subdivided into 4 asymmetric groups according to posteroanterior cephalometric measurements. TMD examination diagnosis and jaw pain and function (JPF) questionnaires assessed the presence and severity of TMD.

RESULTS

Fifty-two percent of the patients were symmetric, and 48% were asymmetric. The asymmetry classification demonstrated significant cephalometric differences between the symmetric and asymmetric groups, and across the 4 asymmetric subtypes: group 1, mandibular body asymmetry; group 2, ramus asymmetry; group 3, atypical asymmetry; and group 4, C-shaped asymmetry. ENPP1 SNP-rs6569759 was associated with group 1 (P = 0.004), and rs858339 was associated with group 3 (P = 0.002). ESR1 SNP-rs164321 was associated with group 4 (P = 0.019). These results were confirmed by principal component analysis that showed 3 principal components explaining almost 80% of the variations in the studied groups. Principal components 1 and 2 were associated with ESR1 SNP-rs3020318 (P <0.05). Diagnoses of disc displacement with reduction, masticatory muscle myalgia, and arthralgia were highly prevalent in the asymmetry groups, and all had strong statistical associations with ENPP1 rs858339. The average JPF scores for asymmetric subjects before surgery (JPF, 7) were significantly higher than for symmetric subjects (JPF, 2). Patients in group 3 had the highest preoperative JPF scores, and groups 2 and 3 were most likely to be cured of TMD 1 year after treatment.

CONCLUSIONS

Posteroanterior cephalometrics can classify asymmetry into distinct groups and identify the probability of TMD and genotype associations. Orthodontic and orthognathic treatments of facial asymmetry are effective at eliminating TMD in most patients.

ACTN3 R577X genotypes associate with Class II and deepbite malocclusions

INTRODUCTION

α-Actinins are myofibril anchor proteins that influence the contractile properties of skeletal muscles. ACTN2 is expressed in slow type I and fast type II fibers, whereas ACTN3 is expressed only in fast fibers. ACTN3 homozygosity for the 577X stop codon (ie, changing 577RR to 577XX, the R577X polymorphism) results in the absence of α-actinin-3 in about 18% of Europeans, diminishes fast contractile ability, enhances endurance performance, and reduces bone mass or bone mineral density. We have examined ACTN3 expression and genetic variation in the masseter muscle of orthognathic surgery patients to determine the genotype associations with malocclusion.

METHODS

Clinical information, masseter muscle biopsies, and saliva samples were obtained from 60 subjects. Genotyping for ACTN3 single nucleotide polymorphisms, real-time polymerase chain reaction quantitation of muscle gene message, and muscle morphometric fiber type properties were compared to determine statistical differences between genotype and phenotype.

RESULTS

Muscle mRNA expression level was significantly different for ACTN3 single nucleotide polymorphism genotypes (P <0.01). The frequency of ACTN3 genotypes was significantly different for the sagittal and vertical classifications of malocclusion, with the clearest association being elevated 577XX genotype in skeletal Class II malocclusion (P = 0.003). This genotype also resulted in significantly smaller diameters of fast type II fibers in masseter muscles (P = 0.002).

CONCLUSION

ACTN3 577XX is overrepresented in subjects with skeletal Class II malocclusion, suggesting a biologic influence during bone growth. ACTN3 577XX is underrepresented in subjects with deepbite malocclusion, suggesting that muscle differences contribute to variations in vertical facial dimensions.

Epigenetic influence of KAT6B and HDAC4 in the development of skeletal malocclusion

INTRODUCTION

Genetic influences on the development of malocclusion include heritable effects on both masticatory muscles and jaw skeletal morphology. Beyond genetic variations, however, the characteristics of muscle and bone are also influenced by epigenetic mechanisms that produce differences in gene expression. We studied 2 enzymes known to change gene expressions through histone modifications, chromatin-modifying histone acetyltransferase KAT6B and deacetylase HDAC4, to determine their associations with musculoskeletal variations in jaw deformation malocclusions.

METHODS

Samples of masseter muscle were obtained from subjects undergoing orthognathic surgery from 6 malocclusion classes based on skeletal sagittal and vertical dysplasia. The muscles were characterized for fiber type properties by immunohistochemistry, and their total RNA was isolated for gene expression studies by microarray analysis and quantitative real-time polymerase chain reaction.

RESULTS

Gene expressions for fast isoforms of myosins and contractile regulatory proteins and for KAT6B and HDAC4 were severalfold greater in masseter muscles from a patient with a deepbite compared with one with an open bite, and genes related to exercise and activity did not differ substantially. In the total population, expressions of HDAC4 (P = 0.03) and KAT6B (P = 0.004) were significantly greater in subjects with sagittal Class III than in Class II malocclusion, whereas HDAC4 tended to correlate negatively with slow myosin type I and positively with fast myosin gene, especially type IIX.

CONCLUSIONS

These data support other published reports of epigenetic regulation in the determination of skeletal muscle fiber phenotypes and bone growth. Further investigations are needed to elucidate how this regulatory model might apply to musculoskeletal development and malocclusion.