Decreased mandibular cortical bone quality after botulinum toxin injections in masticatory muscles in female adults

Evaluation of Regio Temporalis in 3 Steps in Terms of Interventional Procedures

Regio temporalis is a site where botulinum neurotoxin is applied for various medical reasons, such as migraine, bruxism, and myofascial pain syndrome. The region is also one of the target regions in flap surgery. This study aimed to define the region topographically. In addition, it was aimed to reveal the intramuscular nerve distribution of the temporalis.11 fixed cephalus (cadaver head) and 2 fresh cephalus were used. The lateral canthus of the eye was marked as point A, and the middle of the ear tragus as point B. The transverse and vertical distances of the branching point of the superficial temporal artery to the A and B points were measured. Transverse distances of the superficial temporal artery and superficial temporal vein to A and B points were measured. The muscle was examined in 5 equal parts (L0-L1-L2-L3-L4), and each part's vertical muscle and tendon lengths were examined. Intramuscular nerve density was demonstrated by applying the modified Sihler staining to fresh temporalis'. Superficial temporal artery had an average transverse distance of 8.56±1.9 mm in women and 12.56±1.94 mm in men from the middle of the ear tragus. The artery was 64.21±5.59 mm posterior in females and 63.48±6.53 mm in males from the lateral canthus of the eye. Our study determined that the branching point of the superficial temporal artery was below the upper level of the arcus zygomaticus in 10% of cases and above it in 90% of cases. In our study, the L2 point had the highest vertical muscle length at 45.67 mm, while the L3 point had the highest vertical tendon length at 41.25 mm. The point where the muscle length had the highest ratio with 1.49 compared to the tendon length, was the L2 point. The temporalis' for which the modified Sihler staining was applied was examined in 5 quadrants. It was determined that the nerve densities were in the second and third quadrants from anterior to posterior. The distance of superficial landmarks to neurovascular structures is extremely important in interventions to the regio temporalis. Considering the average distances given in our study is important in avoiding damage in surgical procedures and not injecting into vascular structures. The point where the muscle length had the highest ratio of 1.49 compared to the tendon length was the L2 point. The area in line with this point is the most suitable area for injection. The L2 point is also the most suitable area for injection as it has the highest muscle length. Since the nerve densities were observed in the modified Sihler staining applied temporalis' 2 and 3 quadrants from anterior to posterior, botulinum neurotoxin injections to these areas will give more effective results.

Transcriptional Profiling of Muscle in Females With Distal Radius Fracture and Functional Sarcopenia

Skeletal muscle and bone interact with each other in mechanical and biochemical ways. This study aimed to investigate the molecular mechanisms of interaction between muscle and bone by analyzing the transcriptional profiles of total RNA from the muscle tissue of females with distal radius fractures. A total of 30 female participants (mean age 71.1 ± 8.9 years) with distal radius fractures were recruited. Participants were categorized into 2 groups: the NORM group consisted of participants with T score of the areal bone mineral density (aBMD) of the femoral neck higher than -1.0, handgrip strength greater than 18 kg, and gait speed faster than 1.0 m/s (n = 10). Otherwise, participants with T score of the aBMD of the femoral neck equal to or less than -1.0, handgrip strength lower than 18 kg, and gait speed slower than 1.0 m/s (n = 20) were categorized into EXP group. Pronator quadratus muscle samples were obtained from all participants. Total RNA was extracted from frozen muscle samples and sequenced. The gene ontology analysis demonstrated that the potential interactions between attached muscle function and the density of the associated bone would be linked with collagen biosynthetic activity and maintenance of extracellular matrix structures. The analysis of the pathway, network, and protein class exhibited that integrin signaling, inflammatory reactions, matrix metalloproteinase (MMP) activity, and extracellular matrix protein structure had possible associations with the molecular background of muscle-bone interaction. Through integrin signaling, MMP activity, inflammatory reactions, and collagen biosynthesis, muscle and bone may mutually interact with one another.

Unilateral injection of botulinum toxin type A into the masseter muscle induces mandibular asymmetry in adolescent rats by suppressing the angular process growth

INTRODUCTION

Mandibular asymmetry has negative impacts on maxillofacial aesthetics and psychological well-being. This study investigated the effects of unilateral injection of botulinum toxin type A (BTX-A) into the masseter muscle on mandibular symmetry.

METHODS

Forty Wistar rats (4-week-old) were divided into 4 groups (n = 10): control, group 1 (1U BTX-A), group 2 (3U BTX-A), and group 3 (1U BTX-A for 3 times). BTX-A was injected into the right masseter of treatment groups. Cone-beam computerized tomography scans were taken before the injection and then at 2 weeks, 4 weeks, and 6 weeks after injection. Histologic and immunohistochemical staining were done for the condylar cartilage. RNA sequencing and quantitative reverse transcription polymerase chain reaction were used to detect gene expression in the angular process.

RESULTS

In Groups 2 and 3, the right angular process length and the ramus height were reduced 4 weeks after injection, resulting in the mandibular midline deviating to the right side; the right condylar cartilage had reduced thickness and decreased expression of RUNX2, SOX9, and COL II (P <0.05). Two hundred sixty-one genes were differentially expressed (256 downregulated) in the angular process at 3 days post-BTX-A injection, and the calcium signaling pathway was unveiled through the Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis. Furthermore, TRPC1, Wnt5a, CaMKII, Ctnnb1, and RUNX2 expression were significantly downregulated at 1 and 3 days postinjection.

CONCLUSIONS

Unilateral injection of BTX-A into the masseter muscle in adolescent rats induces mandibular asymmetry by suppressing the angular process growth on the injected side.

Adverse effect of botulinum toxin-A injections on mandibular bone: A systematic review and meta-analysis

INTRODUCTION

Botulinum toxin-A (BTX) is a potent neurotoxin that is emerging in the scope of dental practice for its ability to temporarily paralyse musculature and reduce hyperfunction. This may be desirable in diseases/disorders associated with hyperactive muscles such as the muscles of mastication, most implicated in painful temporomandibular disorders (TMDs). The use of BTX extends beyond its indications with off-label use in TMD's and other conditions, while potential adverse effects remain understudied. BTX is well-established hindlimb paralysis model in animals leading to significant bone loss with underlying mechanisms remaining unclear. The objective of this study is to systematically review the literature for articles investigating changes in mandibular bone following BTX injections and meta-analyse available data on reported bone outcomes.

METHODS

Comprehensive search of Medline, Embase and Web of Science retrieved 934 articles. Following the screening process, 36 articles in animals and humans were included for quantitative synthesis. Articles in human individuals (6) and three different animal species (14) presented mandibular bone outcomes that were included in the meta-analysis.

RESULTS

The masseter and temporalis muscles were frequently injected across all species. In humans, we observe a decrease of about 6% in cortical thickness of mandibular regions following BTX injection with no evident changes in either volume or density of bone structures. In animals, bone loss in the condylar region is significantly high in both cortical and trabecular compartments.

DISCUSSION

Our analysis supports the concept of BTX-induced bone-loss model in animal mandibles. Further, bone loss might be confined to the cortical compartments in humans. Most studies did not address the reality of repeated injections and excessive dosing, which occur due to the reversible action of BTX. More rigorous trials are needed to draw a full picture of potential long-term adverse effects on bone.

Clinical aspects of mastication myalgia-an overview

Mastication myalgia is the most common cause of non-odontogenic pain in the orofacial region and is often associated with a reduced quality of life. The purpose of this review is to provide an overview of the clinical aspects of myalgia based on available research. The review includes epidemiological, diagnostic, and etiological aspects. In addition, the potential risk factors related to the transition from acute to chronic myalgia are explored and treatment strategies are presented for its management. As a result, this review may increase clinical knowledge about mastication myalgia and clarify strategies regarding prevention, diagnostics, and management to improve prognosis and reduce patient suffering.

The effect of masseter muscle mass on the rate of experimental tooth movement in rats

BACKGROUND

Previous clinical observational studies have suggested that orthodontic tooth movement (OTM) is related, at least partly, to the mass and/or capabilities of the masticatory muscles.

OBJECTIVES

Our study aimed to examine the influence of masticatory muscle mass on the OTM in an animal experimental model in which the masseter muscle was modulated by botulinum neurotoxin type A (BTX) injection.

METHODS

Eighteen Wistar rats were equally divided into two groups: BTX injection and control. BTX was injected bilaterally into the masseter muscles. Three days after the injection, the maxillary left first molars were orthodontically moved for 14 days. At the end of the experiment, micro-computed tomography was performed to evaluate the rate of OTM and bone morphometry. The masseter muscles were weighed and prepared for histological analyses.

RESULTS

The masseter muscle mass in the BTX group was less than that in the control group, and histological findings showed atrophy of muscle fibers. The rate of OTM was significantly higher in the BTX group than in the control group. Furthermore, a negative correlation was detected between masseter muscle mass and OTM in the BTX group. Bone morphometry showed no difference between the control and BTX groups.

CONCLUSION

Decreased masseter muscle mass was found to be closely related to an increase in the rate of OTM in rats using BTX injection to modify the masseter muscle mass. Masseter muscle mass could be a predictive factor for OTM in rats injected with BTX.

Use of botulinum neurotoxin Type A in the management of primary bruxism in adults: An updated systematic review

STATEMENT OF PROBLEM

The psycho-emotional effects caused by the coronavirus pandemic have increased the intensity and number of cases of bruxism, for which no treatment has been considered fully effective. Botulinum neurotoxin Type A (BoNT-A) has shown positive results as an adjunct treatment of primary bruxism; however, this off-label use does not have an established protocol, and further studies are required.

PURPOSE

The purpose of this updated systematic review was to analyze the clinical outcomes of BoNT-A in the management of primary bruxism in adults.

MATERIAL AND METHODS

The review was registered under the International Prospective Register of Systematic Reviews (PROSPERO) registration number CRD42021287653, without funding. PubMed, Web of Science, Scopus, LILACS, Cochrane Library, and Open Grey Literature databases were searched by using the Mendeley Desktop software program without language restrictions up to June 6, 2021. The risk of bias of the selected randomized clinical trials was assessed by using RoB2, and the level of evidence was measured by 2 independent researchers using the Grading of Recommendations, Assessment, Development and Evaluations (GRADE) tool.

RESULTS

A total of 741 references were obtained from the 6 databases in this systematic review. The 11 randomized clinical trials selected achieved a reduction in bruxism symptoms by injecting BoNT-A into 211 participants with different protocols. The heterogeneity of the included studies did not permit a meta-analysis.

CONCLUSIONS

All the evaluated studies supported the effectiveness of BoNT-A injections in reducing bruxism symptoms. The reduction of symptoms can be obtained with doses lower than 25U applied exclusively in the masseter muscles.

Guidelines for botulinum neurotoxin injection for facial contouring

SUMMARY

The hypertrophied temporalis and masseter muscles give a muscular shaped and bulky contour to the face. Botulinum neurotoxin injection methods are commonly used for facial contouring; however, adverse effects have been reported owing to a lack of delicate anatomical information. The anatomical considerations when injecting botulinum neurotoxin into the temporalis and masseter muscles have been reviewed in the present study. Current knowledge on the localization of the botulinum neurotoxin injection point with more recent anatomical dissection and modified Sihler's staining procedures was assessed. We found that for the muscles, the injection point can be more precisely demarcated. Optimal injection sites are presented for the temporalis and masseter muscles, and the injection technique has been suggested. We propose the optimal injection sites in relation to external anatomical landmarks for the frequently injected muscles of the face to facilitate the efficiency of botulinum neurotoxin injections. In addition, these guidelines would aid in more precise practice without the adverse effects of botulinum neurotoxin.

Repeated botulinum treatment of rabbit masseter causes cumulative tissue damage

OBJECTIVE

Botulinum neurotoxins (BoNT) are used in masticatory muscles for pain relief, unloading of the mandible, and cosmetic facial contouring. Treatment is often repeated every few months as function returns. This study assessed masticatory function and musculoskeletal structure after multiple BoNT treatment of the rabbit masseter.

DESIGN

Female rabbits received 3 injections of BoNT (n = 13) or saline (n = 5) into one masseter muscle at intervals of 12 weeks. The contralateral side served as control. Periodic measurements of masticatory electromyography (EMG) and stimulated anterior bite force were made. After the final 12-week recovery interval, neuromuscular connection was investigated by stimulating the masseteric nerve to elicit an evoked EMG response. Mandibular specimens were collected for microCT analysis, and masseters were collected for histomorphometry and counts of replicating cells.

RESULTS

Control and saline-injected muscles maintained consistent masticatory EMG and anterior bite force throughout the study. BoNT-injected masseters showed strong declines after each injection; during the 12-week recovery period, masticatory EMG and anterior bite force improved, although only electrical activity reached normal levels. Multiple injection resulted in persistently atrophied muscle fibers with fibrosis, and notable loss of bone from the mandibular body and condyle. The uninjected masseters of the BoNT group also showed evidence of mild toxin-related changes.

CONCLUSIONS

Although muscle function is mostly regained after each injection, masseters receiving multiple doses of BoNT show extensive damage. In addition, mandibular bone density is decreased on the injected side.

Can Botulinum Toxin-A Contribute to Reconstructing the Physiological Homeostasis of the Masticatory Complex in Short-Faced Patients during Occlusal Therapy? A Prospective Pilot Study

The physiological homeostasis of the masticatory complex in short-faced patients is too robust to be disintegrated and reconstructed due to the powerful masseter muscle. This study innovatively introduced the botulinum toxin-A (BTX-A) into the field of dental occlusal treatment, providing a novel and minimally invasive therapy perspective for the two major clinical problems in these patients (low treatment efficiency and high rates of complications). In total, 10 adult patients with skeletal low angle seeking occlusal treatment (age: 27.0 ± 6.1 years; 4 males and 6 females) were administered 30−50 U of BTX-A in each masseter muscle and evaluated before and 3 months after injection based on cone-beam computed tomography (CBCT). We found a significant reduction in the thickness of the masseter muscle (MMT) (p < 0.0001). With regards to occlusion, we found a significant increase in the height of the maxillary second molar (U7-PP) (p < 0.05) with significantly flattened occlusal curves (the curve of Spee [COS] (p < 0.01), and the curve of Wilson [COW] (p < 0.05)). Furthermore, the variations in the temporomandibular joint exhibited a significant reduction in the anterior joint space (AJS) (p < 0.05) and superior joint space (SJS) (p < 0.05). In addition, the correlation analysis of the masticatory complex provided the basis for the following multiple regression equation: MMT = 10.08 − 0.11 COW + 2.73 AJS. The findings from our pilot study indicate that BTX-A, as a new adjuvant treatment attempt of occlusal therapy for short-faced patients, can provide a more favorable muscular environment for subsequent occlusal therapy through the adjustment of the biting force and may contribute to the reconstruction of healthier homeostasis of the masticatory complex. However, further research is required to establish the reliability and validity of these findings.

Growth differentiation factor-15 as a modulator of bone and muscle metabolism

This study aims to clarify the potential role of growth differentiation factor-15 (GDF-15) as a myokine in bone metabolism and muscle function in females with osteoporosis. In total, 45 female participants (71.0 ± 8.5 years) with distal radius fractures were recruited. Participants were classified as healthy/osteopenic (n = 28) (CON) or osteoporotic (n = 17) (OP) according to their T-score from the areal bone mineral density (aBMD) of the femoral neck. Body mass index, upper arm and calf circumferences, and handgrip strength were assessed. Total hip, femoral neck, and lumbar spine aBMD was measured via dual-energy x-ray absorptiometry. The focal bone quality of the distal radius was evaluated via 3D reconstructed computed tomographic images. Serum levels of GDF-15, insulin-like growth factor-1, and inflammatory markers such as tumor necrosis factor-α (TNF-α), interleukin-6, and interleukin-1β (IL-1β), as well as the corresponding mRNA levels in the pronator quadratus muscle were determined. Participants in the OP group had higher serum GDF-15 levels than those in the CON group. The mRNA levels of GDF-15, IL-1β, and TNF-α in the pronator quadratus muscle were significantly higher in the OP group than in the CON one. Levels of both serum GDF-15 and GDF-15 mRNA in muscle were positively correlated with age and negatively associated with the aBMD of the total hip and focal bone quality of the distal radius. Handgrip power was not correlated with circulating GDF-15 levels but was correlated with circumferences of the upper arm and calf, and levels of GDF-15 mRNA in muscle specimens. The mRNA levels of GDF-15 were correlated with those of inflammatory cytokines such as TNF-α and IL-1β. The mRNA levels of TNF-α were associated with circumferences of the upper arm and calf and with the aBMD of the total hip. The mRNA levels of GDF-15 in muscle were correlated with serum levels of GDF-15 and TNF-α. GDF-15 may have associations with bone metabolism in humans via paracrinological and endocrinological mechanisms. Maintenance of muscle mass and function would be influenced more by GDF-15 in muscle than by circulating GDF-15. The role of GDF-15 in bone metabolism and muscle homeostasis could be related to inflammatory responses.

Comprehensive evaluation of botulinum toxin treatment outcomes of a patient with persistent myofascial orofacial pain

The outcome evaluation method presented in this case study, including Axes I and II findings combined with the results of quantitative bite force and EMG measurements, provides a good tool for proper evaluation of the effect of BoNT-A on patients with myofascial orofacial pain and changes in jaw muscle function.

Association between clinical parameters and size of three-dimensionally reconstructed anatomical abnormalities in patients with lateral epicondylitis: a cross-sectional study

Background

The association of the severity of clinical symptoms and level of functional performance with the degree of magnetic resonance imaging abnormalities in patients with lateral epicondylitis has not been fully elucidated. This study aimed to investigate the association between the degree of anatomical abnormalities by evaluating three-dimensional magnetic resonance imaging models of the common extensor tendon and clinical parameters in patients with lateral epicondylitis.

Materials and methods

A total of 61 patients (24 men and 37 women) with lateral epicondylitis were included in this study. 3-Tesla magnetic resonance imaging was performed for all patients, and clinical parameters, including pain visual analog scale score, Quick Disabilities of Arm, Shoulder and Hand questionnaire score, elbow range of motion, and demographic factors, were evaluated. The proportion of lesion volume of common extensor tendon was adopted for three-dimensional model analysis. To determine the factors associated with clinical parameters, univariate, and multivariate linear regression analyses were performed.

Results

The proportion of lesion volume of common extensor tendon was not associated with clinical parameters. Gender and muscle edema were independently associated with pain visual analog scale scores. However, demographic factors and magnetic resonance imaging abnormalities were not associated with the Quick Disabilities of Arm, Shoulder, and Hand questionnaire score or elbow range of motion.

Conclusions

The three-dimensional volumetric lesion size of common extensor tendon was not associated with clinical symptoms and functional performance in patients with lateral epicondylitis. The clinical parameters of lateral epicondylitis may be influenced by several factors.

Bone mineral density, bone microstructure, and bone turnover markers in females with temporomandibular joint osteoarthritis

OBJECTIVE

The pathogenesis of the temporomandibular joint osteoarthritis (TMJ OA) has not been clearly revealed. This study aimed to investigate the pathogenesis of TMJ OA based on bone metabolism.

METHODS

Fifty-nine young (mean age 23.4 ± 3.4 years) and 41 post-menopausal females (mean age 57.2 ± 4.6 years) were enrolled. Areal bone mineral density (aBMD) was measured via dual-energy X-ray absorptiometry of the lumbar spine, femoral neck, total hip, and ultradistal radius. Levels of four bone resorption markers, serum ionized calcium and C-telopeptide of type I collagen (CTx) and urinary N-telopeptide of type I collagen and deoxypyridinoline, two bone formation markers, serum bone alkaline phosphatase and osteocalcin, and serum 25-dihydroxyvitamin D were analyzed at baseline and after 12 months. Condylar bone quality was assessed by 3D reconstructed CT images.

RESULTS

Significant differences in condylar bone quality and aBMDs of the lumbar spine in accordance with TMJ OA stages were observed in young and post-menopausal females. The level of CTx was significantly associated with the development and progression of TMJ OA only in young females, whereas 25-dihydroxyvitamine D demonstrated significant associations in young and post-menopausal females. Progression of TMJ OA was accompanied by reduced condylar bone quality and concomitant with lower lumbar spine aBMDs in young and post-menopausal females.

CONCLUSION

Bone metabolism and condylar quality might be involved in the development and progression of TMJ OA.

CLINICAL RELEVANCE

CTx could be considered as a potential diagnostic and monitoring marker in young females, and vitamin D showed a therapeutic potential for TMJ OA.

Preoperative and postoperative assessment of temporal and masseter muscle size with magnetic resonance imaging in patients undergoing unilateral temporomandibular joint surgery

The aim of this study was to assess any change between the preoperative and postoperative sizes of temporal and masseter muscles with magnetic resonance imaging (MRI) in patients undergoing unilateral temporomandibular joint surgery. This study was designed and implemented retrospectively. For clinical evaluation, a visual analog scale (VAS) and maximum mouth opening (MMO) were used. In order to make a preoperative diagnosis and perform a 6-month control, muscle size was measured in millimeters (mm) on T1 axial sections in MRI. Statistical analyses were performed using the SPSS 23.0 software package. Numeric variables were compared between two dependent groups using the Wilcoxon signed rank test. Statistical significance was set at p < 0.05. Twelve patients who underwent unilateral discectomy plus dermis-fat grafting, with classical preauricular inverse L incision, were included in the study, and data for eleven female patients were evaluated. The difference in size between the operated and non-operated sides was found to be statistically insignificant at the preoperative stage for both masseter muscle (operated side m^M: 13.264 ± 1.822 mm; non-operated side m^M: 13.264 ± 2.315 mm; p^M = 0.929) and temporal muscle (operated side m^T: 20.345 ± 2.609 mm; non-operated side m^T: 20.582 ± 2.366 mm; p^T = 0.594). There was a significant size reduction in the masseter muscle on the operated side in the postoperative period (preop m^M: 13.264 ± 1.822 mm; postop m^M: 12.036 ± 1.728 mm; p^M = 0.018). Although there was also a size reduction in the operated side of the temporal muscle in the postoperative period, that difference did not reach statistical significance (preop m^T: 20.345 ± 2.609 mm; postop m^T: 19.445 ± 1.603 mm; p^T = 0.182). On the non-operated side, there were no significant postoperative changes in the sizes of either the masseter muscle (preop m^M: 13.264 ± 2.315 mm; postop m^M: 12.682 ± 2.059 mm; p^M = 0.248) or the temporal muscle (preop m^T: 20.582 ± 2.366; postop m^T: 19.891 ± 3.487 mm; p^T = 0.625). Considering the study findings as a whole, a size reduction was observed in the operated side of the masseter muscle after TMJ surgery. The etiology of this change may be surgical trauma to the temporal and masseter muscles, skeletal alteration resulting from condylar change secondary to discectomy, and patients restraining themselves from application of maximum bite force as a result of a self-protection mechanism due to postoperative pain.

Association between forearm cortical bone properties and handgrip strength in women with distal radius fractures: A cross-sectional study

OBJECTIVES

Mechanical and biochemical bone properties are influenced by muscles. However, the muscle-bone interaction has not been fully elucidated regarding the upper extremities. The objective of the present study was to evaluate the mechanical muscle-bone interaction at the forearm by evaluating the relationship between the properties of three-dimensional (3D) forearm cortical bone models derived from conventional computed tomography (CT) images and handgrip strength (HGS).

METHODS

A total of 108 women (mean age, 75.2 ± 9.4 years; range, 62-101 years) with a distal radius fracture who took conventional CT scans for the assessment of the fracture were included in this study. Distal radius 3D models were reconstructed and the average cortical bone density (Cd) and thickness (Ct) of the region of interest (ROI), which might be affected by the forearm flexor muscles, were calculated using a 3D modeling software. Clinical parameters including HGS, lumbar and hip bone mineral densities (BMDs), and other demographic factors were also obtained. A multivariate linear regression analysis was performed to identify relevant factors associated with HGS.

RESULTS

HGS was found to be independently associated with height and Cd, but no significant difference was found between HGS and Ct, age, weight, as well as lumber and hip BMDs.

CONCLUSIONS

Cortical bone density might be associated with HGS, which is generated by the forearm flexor muscles. Hence, the mechanical muscle-bone interaction in the upper extremities could be supported by the present study.

Effect of multiple injections of botulinum toxin into painful masticatory muscles on bone density in the temporomandibular complex

Background

Adverse effects of masticatory muscle injections of Botulinum Toxin (Btx) have been noted in animal and, less dramatically, human studies.

Objective

Among women treated in multiple community‐based private practices, to compare TMJ bone density and mandibular condylar volume between patients with myofascial TMJD receiving multiple masticatory muscle Btx treatments and similarly diagnosed women not receiving such treatment.

Methods

Cohorts consisted of women whose treatment charts indicated a diagnosis of myofascial TMJD: 35 received at least 2 Btx treatment cycles; 44 received none. Bone density at pre‐specified regions of interest (ROI) was defined by grey scale values from Cone Beam CT, adjusting for a fixed density phantom included in each scan. Mean bone density and mandibular condyle volume were compared between groups. Dose‐response effects were tested within the Btx‐exposed group.

Results

The mean density of primary and secondary ROIs was similar between exposure groups, as was condylar volume. Among Btx‐exposed women, increasing dose of Btx to the temporalis muscle was inversely proportional to the density of the trabecular area of the mandible body. Many Btx‐exposed women received smaller doses of Btx to the masseter muscles than in most TMJD Btx clinical trials.

Conclusion

Masticatory muscle injections of Btx failed to produce clinically significant TMJ bone‐related changes. Should Btx receive regulatory approval for treatment of myofascial TMJD, a phase IV study is recommended to evaluate potential adverse effects of Btx on bone and muscle when administered at higher doses and/or for more treatment cycles.

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.