Effect of high fat diet and excessive compressive mechanical force on pathologic changes of temporomandibular joint

Critical signaling molecules in the temporomandibular joint osteoarthritis under different magnitudes of mechanical stimulation

The mechanical stress environment in the temporomandibular joint (TMJ) is constantly changing due to daily mandibular movements. Therefore, TMJ tissues, such as condylar cartilage, the synovial membrane and discs, are influenced by different magnitudes of mechanical stimulation. Moderate mechanical stimulation is beneficial for maintaining homeostasis, whereas abnormal mechanical stimulation leads to degeneration and ultimately contributes to the development of temporomandibular joint osteoarthritis (TMJOA), which involves changes in critical signaling molecules. Under abnormal mechanical stimulation, compensatory molecules may prevent degenerative changes while decompensatory molecules aggravate. In this review, we summarize the critical signaling molecules that are stimulated by moderate or abnormal mechanical loading in TMJ tissues, mainly in condylar cartilage. Furthermore, we classify abnormal mechanical stimulation-induced molecules into compensatory or decompensatory molecules. Our aim is to understand the pathophysiological mechanism of TMJ dysfunction more deeply in the ever-changing mechanical environment, and then provide new ideas for discovering effective diagnostic and therapeutic targets in TMJOA.

Impact of obesity on the structures and functions of the stomatognathic system: A morphofunctional approach

OBJECTIVE

The prevalence of obesity is increasing significantly worldwide, raising great concern among health professionals. This observational study evaluated the electromyographic activity and thickness of the masseter and temporalis muscles, in addition to the maximum molar bite force, in obese and eutrophic subjects.

METHODS

Sixty subjects were divided into three groups: I (7-12 years), II (13-20 years), III (21-40 years) and sex: with 10 men and 10 women for each group. Electromyographic recordings of the masticatory muscles were obtained during mandibular tasks. The masticatory muscles thicknesses were obtained at rest and during dental clenching. The maximum molar bite forces were measured on the right and left sides. The difference in outcome measures between the groups and sex was analyzed using Mann-Whitney U test (p < 0.05) and analysis of covariance (ANCOVA).

RESULTS

Electromyographic activity in the masseter and temporal muscles consistently displayed lower levels in obese subjects of both sexes across all three age groups during mandibular tasks. Additionally, greater thickness of the masticatory muscles was observed in obese subjects of both sexes across all three age groups. Obese women in Group II displayed higher values of molar bite force, both on the right and left sides, compared to eutrophic women. On the other hand, women in Group III exhibited higher values of molar bite force on the right side in comparison to eutrophic women.

CONCLUSIONS

This study underscores the potential impact of obesity on the morphofunctional aspects of the stomatognathic system in subjects aged 7 to 40 years.

[Role of Piezo mechanosensitive ion channels in the osteoarticular system]

Objective

To summarize the role of Piezo mechanosensitive ion channels in the osteoarticular system, in order to provide reference for subsequent research.

Methods

Extensive literature review was conducted to summarize the structural characteristics, gating mechanisms, activators and blockers of Piezo ion channels, as well as their roles in the osteoarticular systems.

Results

The osteoarticular system is the main load-bearing and motor tissue of the body, and its ability to perceive and respond to mechanical stimuli is one of the guarantees for maintaining normal physiological functions of bones and joints. The occurrence and development of many osteoarticular diseases are closely related to abnormal mechanical loads. At present, research shows that Piezo mechanosensitive ion channels differentiate towards osteogenesis by responding to stretching stimuli and regulating cellular Ca 2+ influx signals; and it affects the proliferation and migration of osteoblasts, maintaining bone homeostasis through cellular communication between osteoblasts-osteoclasts. Meanwhile, Piezo1 protein can indirectly participate in regulating the formation and activity of osteoclasts through its host cells, thereby regulating the process of bone remodeling. During mechanical stimulation, the Piezo1 ion channel maintains bone homeostasis by regulating the expressions of Akt and Wnt1 signaling pathways. The sensitivity of Piezo1/2 ion channels to high strain mechanical signals, as well as the increased sensitivity of Piezo1 ion channels to mechanical transduction mediated by Ca 2+ influx and inflammatory signals in chondrocytes, is expected to become a new entry point for targeted prevention and treatment of osteoarthritis. But the specific way mechanical stimuli regulate the physiological/pathological processes of bones and joints still needs to be clarified.

Conclusion

Piezo mechanosensitive ion channels give the osteoarticular system with important abilities to perceive and respond to mechanical stress, playing a crucial mechanical sensing role in its cellular fate, bone development, and maintenance of bone and cartilage homeostasis.

Causal effects of life course adiposity on temporomandibular disorders: A Mendelian randomization study

BACKGROUND

Previous studies investigated the associations between obesity and temporomandibular disorders (TMDs), but the evidence for the causal inferences was unclear.

OBJECTIVE

We aimed to investigate the causal link between life course adiposity and TMDs.

METHODS

Mendelian randomization (MR) studies were performed using genetic instruments for birth weight (BW) (N = 261 932), childhood body mass index (BMI) (N = 39 620), childhood body size (N = 454 718), adult BMI (N = 99 998), body fat percentage (N = 454 633) and TMDs (N = 211 023). We assessed the overall effect of each life course adiposity factor via inverse-variance weighted (IVW), weighted median, and MR-Egger methods and performed extensive sensitivity analyses. Additionally, multivariable MR was conducted to evaluate the direct and indirect effects of childhood BMI on TMDs while accounting for BW and adult BMI, and vice versa.

RESULTS

Univariable MR analyses revealed a causal effect of low childhood adiposity on an increased risk of TMDs (childhood BMI: IVW OR: 0.65, 95% CI: 0.54-0.78, p < .001; childhood body size: IVW OR: 0.56, 95% CI: 0.43-0.73, p < .001). No causal association existed between genetically predicted BW, adult BMI, or body fat percentage and TMDs. In the multivariable MR analyses, the effects of childhood BMI on TMDs occurrence remained significant and direct, even after adjusting for BW and adult BMI (multivariable IVW OR: 0.78, 95% CI: 0.61-0.99, p = .048). No pleiotropy and heterogeneity were detected (p > .05).

CONCLUSION

Low childhood BMI might causally increase the risk of TMDs through a direct pathway.

Risk of metabolic abnormalities in osteoarthritis: a new perspective to understand its pathological mechanisms

Although aging has traditionally been viewed as the most important risk factor for osteoarthritis (OA), an increasing amount of epidemiological evidence has highlighted the association between metabolic abnormalities and OA, particularly in younger individuals. Metabolic abnormalities, such as obesity and type II diabetes, are strongly linked to OA, and they affect both weight-bearing and non-weight-bearing joints, thus suggesting that the pathogenesis of OA is more complicated than the mechanical stress induced by overweight. This review aims to explore the recent advances in research on the relationship between metabolic abnormalities and OA risk, including the impact of abnormal glucose and lipid metabolism, the potential pathogenesis and targeted therapeutic strategies.

Age-Related Gene and Protein Expression in Mouse Mandibular Condyle Analyzed by Cap Analysis of Gene Expression and Immunohistochemistry

INTRODUCTION

Aging, an inevitable physiological process, leads to morphological and histological degenerative changes in the mandibular condylar cartilage (MCC); however, the molecular mechanism has not yet been elucidated, and little information is available on age-related factors. Therefore, this study was designed to identify age-related factors by investigating the age-related differentially expressed genes (DEGs) and localization of their translated protein expression in the mandibular condyle.

METHODS

Mandibular condyles were collected from 10- and 50-week-old mice. Total RNA was extracted from the samples and then analyzed using cap analysis of gene expression (CAGE) to identify age-related DEGs. Gene ontology (GO) enrichment analysis was performed to determine which biological processes were most affected by aging in terms of gene expression using Metascape. The mandibular condyle samples were processed for histology to investigate morphological changes caused by aging and for immunohistochemistry to localize the protein expression encoded by age-related genes identified with CAGE. Semi-quantitative immunohistochemistry was performed to assess age-related extracellular matrix (ECM) protein levels in the MCC. The histological sections were also used for Alcian blue histochemistry to detect glycosaminoglycans (GAGs).

RESULTS

GO enrichment analysis revealed that the genes related to "extracellular matrix organization," including Acan, Col1a1, Col1a2, Col2a1, Mmp3, Mmp9, and Mmp13, were most differentially expressed in the aged mandibular condyle. Among these seven genes, Mmp3 was upregulated, and the others were downregulated with aging. Histological examination showed the age-related morphological and histological changes in the MCC. Immunohistochemical investigation showed the localization of matrix metalloproteinases (MMPs)-3, -9, and -13 and their substrate proteins, aggrecan, type I collagen, and type II collagen, in the mandibular condyle at 10 and 50 weeks, indicating different localizations between the young and the aged. In the aged MCC, semi-quantitative immunohistochemistry showed a significant decrease in the aggrecan protein level, and Alcian blue histochemistry showed a decrease in GAGs.

CONCLUSION

MMP-3, MMP-9, and MMP-13 contribute to the remodeling of the ECM of the MCC and subchondral bone during aging by degrading ECM proteins at specific times and sites under the regulation of their production and secretion.

Causal association between body mass index and temporomandibular disorders: a bidirectional two-sample Mendelian randomization analysis

BACKGROUND

Observational studies have shown that body mass index (BMI) is highly correlated with the occurrence of temporomandibular disorders (TMDs). However, these studies failed to present a causal relationship. Thus, we aimed to performed a Mendelian randomization (MR) study to investigate causality between BMI and TMDs.

METHODS

We performed a two-sample bidirectional MR analysis using large-scale genome-wide association studies (GWAS). Data were obtained from a large-scale BMI dataset (N = 322,154), TMDs dataset (N = 134,280). The causal effects were estimated with inverse-variance weighted (IVW) method, MR Egger, weighted median. Sensitivity analyses were implemented with Cochran's Q test, MR-Egger intercept test, MR-PRESSO, leave-one-out analysis and the funnel plot.

RESULTS

In the forward MR analysis, a genetic prediction of low BMI was causally associated with a higher risk of TMDs (IVW OR: 0.575, 95% CI: 0.415-0.798, p: 0.001). Similar results were obtained using other complementary methods (MR Egger OR: 0.270, 95% CI: 0.104-0.698, p: 0.009; weighted median OR: 0.496, 95% CI: 0.298-0.826, p: 0.007). In the reverse MR results, TMDs was shown to have no significant effect on BMI (all p > 0.05). No pleiotropy and heterogeneity were detected in the bidirectional analysis (p > 0.05).

CONCLUSION

A lower BMI might be causally associated with increased risk of TMDs, supporting the importance of weight control for the prevention of TMDs. Clinicians should pay more attention to the low-BMI patients among those seeking medical advice due to temporomandibular joint discomfort.

Effects of the advanced mandibular spring on mandibular retrognathia treatment: a three-dimensional finite element study

Background

The Advanced Mandibular Spring (AMS) was newly developed as a dentofacial orthopedic appliance in conjunctive use of clear aligners to treat Class II malocclusion with mandibular retrognathia in adolescents. This study aimed to launch a biomechanical assessment and evaluate whether the stress patterns generated by AMS promote mandibular growth.

Methods

A three-dimensional finite element model was constructed using images of CBCT and spiral CT. The model consisted of craniomaxillofacial bones, articular discs, retrodiscal elastic stratum, masticatory muscle, teeth, periodontal ligament, aligner and AMS. Mechanical effects were analyzed in three types of models: mandibular postural position, mandibular advancement with AMS, and mandibular advancement with only muscular force.

Results

The stress generated by AMS was distributed to all teeth and periodontal ligament, pushing mandibular teeth forward and maxillary teeth backward. In the temporomandibular joint area, the pressure in the superior and posterior aspects of the condyle was reduced, which conformed to the stress pattern promoting condylar and mandibular growth. Stress distribution became even in the anterior aspect of the condyle and the articular disc. Significant tensile stress was generated in the posterior aspect of the glenoid fossa, which conformed to the stress pattern stimulating the remodeling of the fossa.

Conclusions

AMS created a favorable biomechanical environment for treating mandibular retrognathia in adolescents.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12903-022-02308-w.

Temporomandibular disorders, bite force and osseous changes of the temporomandibular joints in patients with hypermobile Ehlers-Danlos syndrome compared to a healthy control group

Background

Ehlers‐Danlos syndrome (EDS) is a hereditary disorder that affects the connective tissue and collagen structures in the body characterised by joint hypermobility, skin hyperextensibility and tissue fragility.

Objective

The aim was to investigate temporomandibular disorders (TMD), bite force, teeth in occlusal contact and osseous changes of the temporomandibular joints (TMJs) in 26 patients with hypermobile EDS (hEDS), differentiated by a genetic test, compared to 39 healthy controls.

Methods

Clinical examination according to Diagnostic Criteria for Temporomandibular Disorders (DC/TMD), radiological examinations of the TMJs by cone‐beam‐computed tomographic (CBCT) scans, registration of bite force and teeth in occlusal contact was performed. Statistical analyses included Fisher's Exact Test, multiple logistic and linear regression models adjusted for age, gender and Body Mass Index (BMI).

Results

Single symptoms and signs of TMD occurred significantly more often in hEDS (p = .002; p = .001; p = .003; p = <.0001; p = .012) and maximum mouth opening was significantly smaller in hEDS compared to controls (p = <.0001). The DC/TMD diagnosis myalgia, myofascial pain with referral, arthralgia, headache attributed to TMD, disc displacement disorders and degenerative joint disease occurred significantly more often in hEDS compared to controls (p = .000; p = .008; p = .003; p = .000; p = <.0001; p = .010, respectively). No significant differences were found in bite force and in teeth in occlusal contact between the groups (p > .05). On CBCT of the TMJs, subcortical sclerosis occurred significantly more often in hEDS compared to controls (p = .005).

Conclusion

Symptoms and signs of TMD and osseous changes of the TMJs occurred significantly more often in hEDS. Bite force and teeth in occlusal contact were comparable to controls.

Animal Models of Temporomandibular Joint Osteoarthritis: Classification and Selection

Temporomandibular joint osteoarthritis (TMJOA) is a common degenerative joint disease that can cause severe pain and dysfunction. It has a serious impact on the quality of lives of patients. Since mechanism underlying the pathogenesis of TMJOA is not fully understood, the development of effective tools for early diagnosis and disease-modifying therapies has been hindered. Animal models play a key role in understanding the pathological process of diseases and evaluating new therapeutic interventions. Although some similarities in disease processes between animals and humans are known, no one animal model is sufficient for studying all characteristics of TMJOA, as each model has different translatability to human clinical conditions. For the past 4 decades, TMJOA animal models have been studied by numerous researchers and can be broadly divided into induced, naturally occurring, and genetically modified models. The induced models can be divided into invasive models (intra-articular injection and surgical induction) or non-invasive models (mechanical loading, high-fat diet, and sleep deprivation). Different types of animal models simulate different pathological expressions of TMJOA and have their unique characteristics. Currently, mice, rats, and rabbits are commonly used in the study of TMJOA. This review sought to provide a general description of current experimental models of TMJOA and assist researchers in selecting the most appropriate models for different kinds of research.

Molecular signaling in temporomandibular joint osteoarthritis

Objective

Temporomandibular joint (TMJ) osteoarthritis (OA) is a type of TMJ disorders with clinical symptoms of pain, movement limitation, cartilage degeneration and joint dysfunction. This review article is aiming to summarize recent findings on signaling pathways involved in TMJ OA development and progression.

Methods

Most recent findings in TMJ OA studies have been reviewed and cited.

Results

TMJ OA is caused by inflammation, abnormal mechanical loading and genetic abnormalities. The molecular mechanisms related to TMJ OA have been determined using different genetic mouse models. Recent studies demonstrated that several signaling pathways are involved in TMJ OA pathology, including Wnt/β-catenin, TGF-β and BMP, Indian Hedgehog, FGF, NF-κB, and Notch pathways, which are summarized in this review article. Alterations of these signaling pathways lead to the pathological changes in TMJ tissues, affecting cartilage matrix degradation, catabolic metabolism and chondrocyte apoptosis.

Conclusion

Multiple signaling pathways were involved in the pathological process of TMJ OA. New therapeutic strategies, such as stem cell application, gene editing and other techniques may be utilized for TMJ OA treatment.

The translational potential of this article

TMJ OA is a most important subtype of TMJ disorders and may lead to substantial joint pain, dysfunction, dental malocclusion, and reduced health-related quality of life. This review article summarized current findings of signaling pathways involved in TMJ OA, including Wnt/β-catenin, TGF-β and BMP, Indian Hedgehog, FGF, NF-κB, and Notch pathways, to better understand the pathological mechanisms of TMJ OA and define the molecular targets for TMJ OA treatment.

Involvement of an FTO gene polymorphism in the temporomandibular joint osteoarthritis

Objectives

The FTO gene has been reported as an obesity-associated gene and is also considered a risk gene for osteoarthritis (OA). However, its exact function is unclear, and there is conflicting evidence on the involvement of FTO polymorphisms in OA via obesity. The purpose of this study was to determine the effects of FTO polymorphism rs8044769 alleles on OA in the temporomandibular joint (TMJ), which is minimally affected by body weight.

Materials and methods

A total of 324 TMJs (113 with OA and 211 without OA, serving as controls) from 162 Japanese patients with temporomandibular disorders and undergoing MRI examination were analyzed. Genotyping was conducted, and multivariate analysis was performed after adjusting for the effects of age, sex, body mass index, and TMJ disc abnormalities.

Results

Mean age, BMI, and sex did not differ between the TMJs with OA and the TMJs without OA, but a significant difference was found for positional and dynamic disc abnormalities (P < 0.05). The allele frequency of FTO polymorphisms also differed significantly between the TMJs with OA and the TMJs without OA (P = 0.011). Moreover, logistic regression analysis showed no significant association between BMI (P = 0.581) and the occurrence of TMJOA but also indicated that the CC allele of rs8044769 is a risk factor for TMJOA (P = 0.040).

Conclusions

Our results show that rs8044769 in the FTO gene might be involved in TMJOA.

Clinical relevance

The present study provides a basis for a deeper understanding of the mechanism underlying degenerative skeletal diseases and the more effective selection and development of treatment strategies based on the patients’ genetic characteristics.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00784-021-04278-9.

Posterior Mandibular Displacement-A Systematic Review Based on Animal Studies

Treating extreme mandibular growth is challenging. The mandible is pushed backwards to address itsprotrusion. Nevertheless, conclusions after such displacement in animals have been contradictory. The aim of the present review is to present measurable alterations of the mandible and the condyle following retractionin healthy rats or rabbits. PubMed, Scopus and Web of Science were accessed for relevant studies up to October 2020. Eligibility was determined by the PICOS process, while the risk of bias was estimated with SYRCLE's risk of bias tool. Retraction resulted in a more distal molar occlusion and the condyle rested more posteriorly. Mandibular anteroposterior bilateral growth restriction was achieved, the condylar process measured smaller and its angulation increased. The condylar neck thickened, its posterior surface flattened, the coronoid process was measured longer, and enlarged retromolar density was registered. Differences in the ramus height and the intercondylar distance were insignificant. Changes persisted for the period of study and subsequently the mandible resumed its inherited growth pattern. The timing of mandibular shaping and TMJ outcomes might depend on the properties of the applied force. Stability is of concern and well-structured, long-term studies are expected to resolve the issue and further clarify the results of posterior mandibular displacement.