The role of masticatory muscles in the continuous loading of the mandible

Effects of orthodontic treatment on masticatory muscles activity: a meta-analysis

Context: A comprehensive meta-analysis was carried out to investigate the impact of orthodontics on masticatory muscles.Methods: A thorough search of various databases, including CNKI, Wan Fang, VIP, CBM, MEDLINE, PubMed, Cochrane Library, EMBASE, Web of Science, and Google Scholar, was performed to identify relevant studies on patients undergoing orthodontics or functional corrections. Six case-control studies were finally included in this analysis, which specifically examined the effect of orthodontic treatment on masticatory muscle function.Results: The results revealed that the mean masticatory muscle voltage in patients treated with orthodontics was found to be higher after treatment compared to before treatment [odds ratio (OR)=1.57, 95% confidence interval (CI) (0.57, 2.57), p = 0.002], which could potentially have an impact on masticatory muscle function, particularly in individuals with Class II Division 1 malocclusion.Conclusion: These findings contribute to our understanding of the effects of orthodontic interventions on masticatory muscles, further highlighting the importance of orthodontics in optimising masticatory function.

Functional magnetic resonance imaging evaluation of masticatory muscle dysfunction in unilateral exodontia rabbits

Objective:

Occlusal alteration due to tooth loss may cause overload of masticatory muscle and promote muscle dysfunction. This study explored the feasibility of using functional magnetic resonance imaging (fMRI) to evaluate muscle dysfunction in an established unilateral exodontia animal model.

Methods:

six rabbits were extracted right maxillary molars. T2 mapping, T2* mapping and Iterative Decomposition of water and fat with Echo Asymmetry and Least Square Estimation (IDEAL-IQ) were performed one day before extraction and every 2 weeks (2th~12th week) after extraction. The T2 and T2* values and fat fraction (FF) of bilateral temporal muscle (TM), masseter muscle (MM) and medial pterygoid muscle (MPM) were measured and compared between the extraction side-and the contralateral side. Parameters of three monitoring time points (0th, sixth, 12th week) were also analyzed.

Results:

T2 values of MM on extraction side-were significantly higher than those of contralateral side-from fourth week to 12th week after extraction (p < 0.05). T2 values of MM and MPM on extraction side-and TM on contralateral side-were significantly higher in 12th week than those in 0th week (p < 0.05). And FF of bilateral MM was significantly higher in 12th week than those in 0th week (p < 0.05). T2* value showed no significant difference between extraction side-and contralateral side-and also at above three time points.

Conclusion:

T2 and T2* value and FF can be used as indicators of masticatory muscle dysfunction. fMRI is expected to be a non-invasive method for in vivo and real-time evaluation of masticatory muscle functional abnormality.

Three-dimensional radiographic and histological tracking of rat mandibular defect repair after inferior alveolar nerve axotomy

OBJECTIVE

To sequentially track mandibular defect repair by using radiographic and histological techniques, so as to compare repair patterns of sensory denervated versus innervated mandibles.

DESIGN

Forty Sprague-Dawley rats were subjected to unilateral inferior alveolar nerve (IAN) axotomy and bilateral 3 mm full-thickness circular osteotomy of their mandibles. Micro-CT and histological staining were applied to track the repair process of the mandibular defects at 1, 2, 4, and 8 weeks after surgery.

RESULTS

The bone volume of both sides increased by 2 weeks post-operation, and then gradually decreased. The new bone volumes of the axotomy side were significantly less than that of the sham side at 1, 2, and 4 weeks post-surgery, whereas no significant differences were detected at 8 weeks post-surgery. Meanwhile, there were no significant differences in bone mineral density between the two sides during repair. Noteworthy, the repaired bone remained more vertically than horizontally aligned throughout the repair process.

CONCLUSION

IAN axotomy decreases the quantity of bone calluses during the early stage of mandibular defect repair, but with no effect on the degree of mineralization. The shape of the defect area appeared to be aligned with the direction of local mechanical force produced by masticatory muscles.

Effects of unloading by tail suspension on biological apatite crystallite alignment in mouse femur

The aim of this study was clarify the effects of reducing various functional pressures essential for the maintenance of bone homeostasis. Femoral bone mineral density (BMD) and biological apatite (BAp) crystallite alignment were measured in conventionally reared and hindlimb-unloaded mice. The femur was divided into 10 equal segments perpendicular to the longitudinal axis of the bone and measurements were performed on the cortical bone in the five segments closest to the midpoint of the femur. Significantly lower BMD and BAp alignment in the longitudinal (Z-axis) direction were observed in the hindlimb-unloaded group. The present findings suggest that unloading by tail suspension significantly decreases not only mouse femoral bone mass but also BAp crystallite alignment, although minimal uniaxial preferential alignment is retained.

Muscle-Bone Crosstalk in the Masticatory System: From Biomechanical to Molecular Interactions

The masticatory system is a complex and highly organized group of structures, including craniofacial bones (maxillae and mandible), muscles, teeth, joints, and neurovascular elements. While the musculoskeletal structures of the head and neck are known to have a different embryonic origin, morphology, biomechanical demands, and biochemical characteristics than the trunk and limbs, their particular molecular basis and cell biology have been much less explored. In the last decade, the concept of muscle-bone crosstalk has emerged, comprising both the loads generated during muscle contraction and a biochemical component through soluble molecules. Bone cells embedded in the mineralized tissue respond to the biomechanical input by releasing molecular factors that impact the homeostasis of the attaching skeletal muscle. In the same way, muscle-derived factors act as soluble signals that modulate the remodeling process of the underlying bones. This concept of muscle-bone crosstalk at a molecular level is particularly interesting in the mandible, due to its tight anatomical relationship with one of the biggest and strongest masticatory muscles, the masseter. However, despite the close physical and physiological interaction of both tissues for proper functioning, this topic has been poorly addressed. Here we present one of the most detailed reviews of the literature to date regarding the biomechanical and biochemical interaction between muscles and bones of the masticatory system, both during development and in physiological or pathological remodeling processes. Evidence related to how masticatory function shapes the craniofacial bones is discussed, and a proposal presented that the masticatory muscles and craniofacial bones serve as secretory tissues. We furthermore discuss our current findings of myokines-release from masseter muscle in physiological conditions, during functional adaptation or pathology, and their putative role as bone-modulators in the craniofacial system. Finally, we address the physiological implications of the crosstalk between muscles and bones in the masticatory system, analyzing pathologies or clinical procedures in which the alteration of one of them affects the homeostasis of the other. Unveiling the mechanisms of muscle-bone crosstalk in the masticatory system opens broad possibilities for understanding and treating temporomandibular disorders, which severely impair the quality of life, with a high cost for diagnosis and management.

Mandibular Bone Loss after Masticatory Muscles Intervention with Botulinum Toxin: An Approach from Basic Research to Clinical Findings

The injection of botulinum toxin type A (BoNT/A) in the masticatory muscles, to cause its temporary paralysis, is a widely used intervention for clinical disorders such as oromandibular dystonia, sleep bruxism, and aesthetics (i.e., masseteric hypertrophy). Considering that muscle contraction is required for mechano-transduction to maintain bone homeostasis, it is relevant to address the bone adverse effects associated with muscle condition after this intervention. Our aim is to condense the current and relevant literature about mandibular bone loss in fully mature mammals after BoNT/A intervention in the masticatory muscles. Here, we compile evidence from animal models (mice, rats, and rabbits) to clinical studies, demonstrating that BoNT/A-induced masticatory muscle atrophy promotes mandibular bone loss. Mandibular bone-related adverse effects involve cellular and metabolic changes, microstructure degradation, and morphological alterations. While bone loss has been detected at the mandibular condyle or alveolar bone, cellular and molecular mechanisms involved in this process must still be elucidated. Further basic research could provide evidence for designing strategies to control the undesired effects on bone during the therapeutic use of BoNT/A. However, in the meantime, we consider it essential that patients treated with BoNT/A in the masticatory muscles be warned about a putative collateral mandibular bone damage.

Condylar Degradation from Decreased Occlusal Loading following Masticatory Muscle Atrophy

Objective

The masticatory muscles are the most important contributor to bite force, and the temporomandibular joint (TMJ) receives direct occlusal loading. The present study aimed to investigate condylar remodeling after masseter muscle atrophy in rats.

Methods

Sixty 5-week-old female Sprague-Dawley rats were divided into the following 3 groups: the control group, soft diet (SD) group, and botulinum toxin (BTX) group. The cross-sectional area (CSA) of the masseter muscles was investigated as well as atrogin-1/MuRF-1 expression. Changes in the condylar head were evaluated by H-E, toluidine blue staining, and contour measurements. The biomechanical sensitive factors PTHrP Ihh, Col2a1, and ColX of condylar cartilage were detected by immunohistochemical staining and western blotting. Furthermore, micro-CT and tartrate-resistant acid phosphatase (TRAP) staining were performed to determine the osteopenia in subchondral bone.

Results

The histological and protein analysis demonstrated muscle hypofunction in the SD and BTX groups. Condylar cartilage contour was diminished due to different treatments; the immunohistochemistry and protein examination showed that the expressions of PTHrP, Ihh, Col2a1, and ColX were suppressed in condylar cartilage. A steady osteoporosis in subchondral bone was found only in the BTX group.

Conclusion

The current results suggested that a steady relationship between muscular dysfunction and condylar remodeling exists.

A distal 594 bp ECR specifies Hmx1 expression in pinna and lateral facial morphogenesis and is regulated by the Hox-Pbx-Meis complex

Hmx1 encodes a homeodomain transcription factor expressed in the developing lateral craniofacial mesenchyme, retina and sensory ganglia. Mutation or mis-regulation of Hmx1 underlies malformations of the eye and external ear in multiple species. Deletion or insertional duplication of an evolutionarily conserved region (ECR) downstream of Hmx1 has recently been described in rat and cow, respectively. Here, we demonstrate that the impact of Hmx1 loss is greater than previously appreciated, with a variety of lateral cranioskeletal defects, auriculofacial nerve deficits, and duplication of the caudal region of the external ear. Using a transgenic approach, we demonstrate that a 594 bp sequence encompassing the ECR recapitulates specific aspects of the endogenous Hmx1 lateral facial expression pattern. Moreover, we show that Hoxa2, Meis and Pbx proteins act cooperatively on the ECR, via a core 32 bp sequence, to regulate Hmx1 expression. These studies highlight the conserved role for Hmx1 in BA2-derived tissues and provide an entry point for improved understanding of the causes of the frequent lateral facial birth defects in humans.

Masticatory biomechanics in the rabbit: a multi-body dynamics analysis

Multi-body dynamics is a powerful engineering tool which is becoming increasingly popular for the simulation and analysis of skull biomechanics. This paper presents the first application of multi-body dynamics to analyse the biomechanics of the rabbit skull. A model has been constructed through the combination of manual dissection and three-dimensional imaging techniques (magnetic resonance imaging and micro-computed tomography). Individual muscles are represented with multiple layers, thus more accurately modelling muscle fibres with complex lines of action. Model validity was sought through comparing experimentally measured maximum incisor bite forces with those predicted by the model. Simulations of molar biting highlighted the ability of the masticatory system to alter recruitment of two muscle groups, in order to generate shearing or crushing movements. Molar shearing is capable of processing a food bolus in all three orthogonal directions, whereas molar crushing and incisor biting are predominately directed vertically. Simulations also show that the masticatory system is adapted to process foods through several cycles with low muscle activations, presumably in order to prevent rapidly fatiguing fast fibres during repeated chewing cycles. Our study demonstrates the usefulness of a validated multi-body dynamics model for investigating feeding biomechanics in the rabbit, and shows the potential for complementing and eventually reducing in vivo experiments.

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.

Variation of the mineral density in cortical bone may serve to keep strain amplitudes within a physiological range

Within-bone variation in mineral density could be functional. A heterogeneous mineral-density distribution might serve to maintain habitual amplitudes of bone strain within a non-harmful, i.e., physiological range. Regions of a bone that would be strained the most on the basis of architecture alone might have a higher mineral density to make them more stiff and resistant to strain. We hypothesised that the cortical bone of the rabbit mandible contains such a functional distribution of mineral density. We thereby expected similar mineral-density patterns in the mandibles of different individuals due to the shared masticatory function. Secondly, we hypothesised that the highest mineral densities occur in mandibular regions predicted to be exposed to the largest amplitudes of strain-when taking into account bone architecture only. Mineral-density maps of the cortical bone of rabbit mandibles were obtained using micro-computed tomography (μCT). The μCT scans of two rabbits were converted into finite-element models (FEMs). To predict mandibular deformation during biting, these models were loaded by muscle forces and reaction forces. The forces acted on the condyles and on either the incisal or molar bite point. The FEMs were assigned a homogeneous material stiffness to calculate the strain amplitudes that would occur when only the architecture of the mandibular bone would be of influence. We found the cortical bone-mineral density patterns to be similar in all six mandibles. The mineral density of the corpus was higher than that of the ramus. A second consistent feature of the mandibular mineral-density distribution was that the medial ridge of the temporal-muscle insertion groove contained more mineral than its surrounding regions. The strain amplitudes calculated with the FEMs were variable and did not feature clear corpo-ramal differences. However, specific mandibular bone sites calculated to be exposed to the largest amplitudes of strain, including the medial ridge of the temporal-muscle insertion groove, did correspond with high-mineral-density regions. We conclude that, in the rabbit mandible, the heterogeneous mineral-density distribution might serve to suppress bone-strain amplitudes in regions architecturally susceptible to the largest deformations during loading.