Low-frequency pulsed electromagnetic fields alleviate the condylar cartilage degeneration and synovitis at the early stage of temporomandibular joint osteoarthritis

BACKGROUND

Temporomandibular joint osteoarthritis (TMJOA) is characterized by articular cartilage degeneration and progressive synovitis. How to effectively inhibit TMJOA in the early stage has been a hot topic in the biomedical field. As a non-invasive physiotherapy, pulsed electromagnetic field (PEMF) treatment has shown great potential in the treatment of osteoarthritis (OA) in extremity joints.

OBJECTIVE

This study aims to investigate the biological effect of PEMF intervention on TMJ cartilage degeneration and synovium inflammation at the early stage of TMJOA.

METHODS

PEMF (2.0 mT, 15 Hz, 2 h/day) treatment was given to rats in which TMJOA was induced by applying the unilateral anterior crossbite (UAC). Histological and immunohistochemical staining, TUNEL assay, real-time PCR and western blotting assay were performed to detect the changes of the morphology and the expression of pro-inflammatory and degradative factors in condylar cartilage and synovium.

RESULTS

Obvious condylar cartilage degeneration, characterized by decreased cartilage thickness, degraded cartilage extracellular matrix, increased expression of pro-inflammatory and degradative factors (TNF-α, IL-1β, MMP-13, ADAMTS-5, IL-6, MMP-3, MMP-9 and COL-X) and increased chondrocytes death, was observed in UAC group, accompanied by synovium hyperplasia and up-regulation of pro-inflammatory and degradative factors in synovium. PEMF intervention reversed the decreased cartilage thickness at 3 weeks and degraded cartilage extracellular matrix at 6 weeks. Moreover, the up-regulation of pro-inflammatory, degradative and hypertrophyic factors and chondrocytes death in condylar cartilage induced by UAC were inhibited to some extent. In addition, the synovium hyperplasia and the up-regulation of pro-inflammatory and degradative factors in synovium were inhibited at 3 weeks and 6 weeks.

CONCLUSIONS

Appropriate PEMF stimulation can reverse the loss of cartilage extracellular matrix, the chondrocytes death, the increased expression of pro-inflammatory and degradative factors in cartilage, the decreased cartilage thickness and synovium inflammation induced by UAC at the early stage of TMJOA to some extent. PEMF stimulation may be a promising method in clinical TMJOA treatment.

The effects of anterior bite plane on temporomandibular joint and mandibular morphology

Objectives

An anterior bite plane (ABP) is an orthodontic appliance that prevents posterior teeth from making contact. This appliance's functional concept is to reduce muscle activity, overcome deep overbite, and temporomandibular joint (TMJ) disorders (TMD). However, ABP treatment for malocclusion frequently results in unfavorable reversible and irreversible long-term effects. This problem presents difficulties for dentists in developing an appropriate treatment modification plan in order to achieve the best results. As a result, the goal of this study is to observe the effects of different ABP types on the TMJ and mandible.

Materials and Methods

Thirty-six three-month-old male Wistar strain rats were divided into three groups: control, upper flat, and upper-lower inclined ABP. The overbite and body weight were measured. TMJ was examined histologically using hematoxylin and eosin (HE). To observe the entire mandibular bone in response to ABP, mandibular planes and angulations were measured.

Results

After 7 days, the upper-lower inclined ABP group has significantly lower body weight than the control group. On days 7 and 14, overbite was significantly reduced in both the upper flat and upper-lower inclined ABP groups. The superficial layer of the condyle was depleted in both ABP groups, according to HE analysis. Mandibular angle analysis revealed that the upper-lower inclined ABP group had a greater incisal and ramus angle. Furthermore, lower incisor (Li)-condyle (Co) mandibular points increased significantly more in the upper-lower inclined ABP group than in the control group.

Conclusion

According to this study, various forms of ABP may have an impact on the TMJ and mandibular morphology, specifically on the length, angulation, and superficial surface of the condyle.

Near-Infrared Light-Activated Mesoporous Polydopamine for Temporomandibular Joint Osteoarthritis Combined Photothermal-Chemo Therapy

The treatments generally employed for temporomandibular joint osteoarthritis (TMJOA) involve physical therapy and chemotherapy, etc., whose therapeutic efficacies are impaired by the side effects and suboptimal stimulus responsiveness. Although the intra-articular drug delivery system (DDS) has shown effectiveness in addressing osteoarthritis, there is currently little reported research regarding the use of stimuli-responsive DDS in managing TMJOA. Herein, we prepared a novel near-infrared (NIR) light-sensitive DDS (DS-TD/MPDA) by using mesoporous polydopamine nanospheres (MPDA) as NIR responders and drug carriers; diclofenac sodium (DS) as the anti-inflammatory medication; and 1-tetradecanol (TD) with a phase-inversion temperature of 39 °C as the drug administrator. Upon exposure to 808 nm NIR laser, DS-TD/MPDA could raise the temperature up to the melting point of TD through photothermal conversion, and intelligently trigger DS release. The resultant nanospheres exhibited an excellent photothermal effect and effectively controlled the release of DS through laser irradiation to accommodate the multifunctional therapeutic effect. More importantly, the biological evaluation of DS-TD/MPDA for TMJOA treatment was also performed for the first time. The experiments' results demonstrated that DS-TD/MPDA displayed a good biocompatibility in vitro and in vivo during metabolism. After injection into the TMJ of rats afflicted with TMJOA induced by unilateral anterior crossbite for 14 days, DS-TD/MPDA could alleviate the deterioration of TMJ cartilage, thus ameliorating osteoarthritis. Therefore, DS-TD/MPDA could be a promising candidate for photothermal-chemotherapy for TMJOA.

miR-132-3p participates in the pathological mechanism of temporomandibular joint osteoarthritis by targeting PTEN

OBJECTIVE

This study aimed to investigate the role of miR-132-3p in the progression of temporomandibular joint osteoarthritis (TMJOA) and its potential pathological mechanism.

DESIGN

A TMJOA model was established using six rats via the unilateral anterior crossbite method. The differential expression of miR-132-3p in the TMJOA (n = 6) and control groups (n = 6) was detected via miRNA sequencing and verified via PCR. The chondrocytes in the condylar cartilage of the temporomandibular joint were cultured and stimulated with IL-1β to simulate TMJOA in vitro. The changes in the proliferation, apoptosis, inflammation and extracellular matrix of these chondrocytes were detected after the upregulation of miR-132-3p expression. The targeted relationship of miR-132-3p and PTEN in TMJOA was verified, and rescue experiments were conducted via co-upregulation of the expression of both miR-132-3p and PTEN.

RESULTS

Compared with that in the control group, miR-132-3p expression was lower in the cartilage tissues of TMJOA rats and IL-1β-induced TMJ chondrocytes. After upregulating the expression of miR-132-3p, the cell proliferation activity and expression levels of aggrecan and type II collagen of IL-1β-induced TMJ chondrocytes were increased, and the apoptosis rate and levels of inflammatory factors were decreased. miR-132-3p can regulate PTEN expression in a targeted manner, and upregulating PTEN expression could reverse the influences of the upregulation of miR-132-3p expression on TMJOA cells.

CONCLUSION

miR-132-3p is less expressed in TMJOA, and it regulates the proliferation, extracellular matrix, and inflammatory response of TMJOA chondrocytes and participates in TMJOA progression by targeting PTEN.

Unilateral loss of maxillary molars in young mice leads to bilateral condylar adaptation and degenerative disease

The adaptive response of the mandible and temporomandibular joint (TMJ) to altered occlusion in juvenile patients is presently unclear. To address this question, we established a mouse model in which all molars were extracted from the maxillary right quadrant in prepubertal, 3‐week‐old mice and analyzed morphological, tissue, cellular, and molecular changes in the mandible and condyle 3 weeks later. Unilateral loss of maxillary molars led to significant, robust, bilateral changes, primarily in condylar morphology, including anteroposterior narrowing of the condylar head and neck and increased convexity at the condylar surface, as determined by geometric morphometric analysis. Furthermore, both condyles in experimental mice exhibited a degenerative phenotype, which included decreased bone volume and increased mineral density near the condylar head surface compared to control mice. Changes in condylar morphology and mineralized tissue composition were associated with alterations in the cellular architecture of the mandibular condylar cartilage, including increased expression of markers for mature (Col2a1) and hypertrophic (Col10a1) chondrocytes, suggesting a shift toward differentiating chondrocytes. Our results show significant bilateral condylar morphological changes, alterations in tissue composition, cellular organization, and molecular expression, as well as degenerative disease, in response to the unilateral loss of teeth. Our study provides a relatively simple, tractable mouse tooth extraction system that will be of utility in uncovering the cellular and molecular mechanisms of condylar and mandibular adaptation in response to altered occlusion. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Animal Models of Temporomandibular Disorder

Temporomandibular disorders (TMD) are a group of diseases in the oral and maxillofacial region that can manifest as acute or chronic persistent pain, affecting millions of people worldwide. Although hundreds of studies have explored mechanisms and treatments underlying TMD, multiple pathogenic factors and diverse clinical manifestations make it still poorly managed. Appropriate animal models are helpful to study the pathogenesis of TMD and explore effective treatment measures. At present, due to the high cost of obtaining large animals, rodents and rabbits are often used to prepare TMD animal models. Over the past decade, various animal models have been intensively developed to understand neurobiological and molecular mechanisms of TMD, and seek effective treatments. Although these models cannot carry out all clinical features, they are valuable in revealing the mechanisms of TMD and creating curative access. Currently, there are multitudinous animal models of TMD research. They can be constructed in different means and summarized into four ways according to the various causes and symptoms, including chemical induction (intra-articular injection of ovalbumin, collagenase, formalin, vascular endothelial growth factor, intramuscular injection of complete Freund’s adjuvant, etc.), mechanical stress stimulation (passive mouth opening, change of chewing load), surgical operation (partial disc resection, joint disc perforation) and psychological stress induction. Here, we summarize and discuss different approaches of animal models for determining neurophysiological and mechanical mechanisms of TMD and assess their advantages and limitations, respectively.

Effect of dental malocclusion on cerebellar neuron activation via the dorsomedial part of the principal sensory trigeminal nucleus

Occlusion has been proposed to play a role for body posture and balance, both of which are mediated mainly by the cerebellum. The dorsomedial part of the principal sensory trigeminal nucleus (Vpdm) has direct projection to the cerebellum. The experimental unilateral anterior crossbite (UAC) has an impact on the motor nuclei in the brain stem via trigeminal mesencephalic nucleus (Vme). The current aim was to explore whether UAC has an impact on Vpdm‐cerebellum circuit. The inferior alveolar nerve was injected into cholera toxin B subunit (CTb), the cerebellum was injected into fluoro‐gold (FG), and the Vpdm was injected into biotinylated dextran amine (BDA) to identify the activation of Vpdm‐cerebellum circuit by UAC. Data indicated that there were more neuronal nuclei (NeuN)/CTb/FG triple‐labelled neurons and NeuN/CTb/vesicular glutamate transporter 1(VGLUT1) triple‐labelled neurons in the Vpdm, and more NeuN/BDA/ VGLUT1 triple‐labelled neurons in the cerebellum of rats with UAC than in control rats. The VGLUT1 expression in the Vpdm and cerebellum in the UAC group was higher than that in control rats. These findings indicate an excitatory impact of UAC on the Vpdm‐cerebellum pathway and support the role of occlusion for body posture and balance.

Role of the SDF-1/CXCR4 signaling pathway in cartilage and subchondral bone in temporomandibular joint osteoarthritis induced by overloaded functional orthopedics in rats

OBJECTIVES

To (i) use a mandibular advancement appliance in rats to investigate the role of the stromal cell-derived factor/CXC receptor 4 (SDF-1/CXCR4) signaling pathway in temporomandibular joint osteoarthritis (TMJ OA) induced by overloaded functional orthopedics (OFO) and (ii) provide a cellular and molecular basis for efficacious treatment of skeletal class-II malocclusion and avoidance of TMJ OA.

METHOD

Male Sprague-Dawley rats (6 weeks) were divided randomly into control + normal saline (NS), EXP + ADM3100 (SDF-1 antagonist), EXP + NS, and control + ADM3100 groups. Changes in articular cartilage and subchondral bone after TMJ OA in these four groups were observed by hematoxylin and eosin (H&E), immunofluorescence double staining (IDS), Safranin-O staining, immunohistochemical (IHC) staining, real-time polymerase chain reaction, and micro-computed tomography at 2, 4, and 8 weeks.

RESULTS

OFO led to increased expression of SDF-1, CXCR4, and matrix metalloproteinase (MMP) 13 and decreased expression of collagen II. The thickness of the hypertrophic cartilage layer was reduced at 4 weeks in the EXP + NS group, and damage to subchondral bone was observed at 2 weeks. Using ADM3100 to inhibit SDF-1 signaling could attenuate expression of MMP13, cartilage damage, and osteoblast differentiation. IDS showed that the areas of expression of SDF-1 and OSX in subchondral bone overlapped.

CONCLUSIONS

Overloaded functional orthopedics (OFO) induced TMJ OA. The destruction of subchondral bone in TMJ OA caused by OFO occurred before damage to cartilage. SDF-1/CXCR4 may induce the osteogenic differentiation and cause cartilage degradation in TMJ OA caused by OFO.

Calcium-/calmodulin-dependent protein kinase II in occlusion-induced degenerative cartilage of rat mandibular condyle

Activated calcium-/calmodulin-dependent protein kinaseII (CaMKII) is important to promote chondrocytes from proliferative to pre-hypertrophic state, which probably plays a role in osteoarthritis (OA), a widespread degeneration disease with enhanced aberrant chondrocyte differentiation. Our aim was to detect the role of CaMKII, and its relationship with the feedback loop of Indian hedgehog (Ihh) and Parathyroid-related peptide (PTHrP) in the temporomandibular joints (TMJs) OA. KN93, the competitive inhibitor of CaMKII, was added to the culture medium in vitro and was locally injected to rats TMJs (n = 54, female) every other day for 4 weeks from the beginning of the 5th and 9th week after installing of unilateral anterior crossbite (UAC), termed as 4 wk+4 wk and 8 wk+4 wk, accordingly. The RNA expression of CaMKII α (1.49 ± 0.09), CaMKII β (3.36 ± 0.20), Ihh (1.88 ± 0.06) and PTHrP (1.87 ± 0.12) was all enhanced, especially at 24 dyn/cm² in vitro (all P < .05), accompanied with downregulated expression of cartilage matrix, but upregulated markers of chondrocytes differentiation (all P < 0.05). Similarity was observed in the 4 wk+4 wk group in vivo. In the 8 wk+4 wk group, UAC upregulated the RNA expression of CaMKII α (1.81 ± 0.24), CaMKII β (1.36 ± 0.07) and Ihh (1.70 ± 0.21), however, down-regulated PTHrP (0.53 ± 0.04) (all P < .05), in consonance with the protein expression. All these changes were attenuated by KN93 (all P < .05). In conclusion, CaMKII took a role, via Ihh and PTHrP pathways, in promoting biomechanically induced TMJ chondrocytes differentiation, the initiation issue of UAC stimulated osteoarthritic changes in rodent TMJs. Inhibiting CaMKII is helpful to rescue the biomechanically stimulated cartilage degradation and prospective to be a target treatment of OA.

Osteochondral Interface Stiffening in Mandibular Condylar Osteoarthritis

Osteoarthritis (OA) of the temporomandibular joint (TMJ) is associated with dental biomechanics. A major change during OA progression is the ossification of the osteochondral interface. This study investigated the formation, radiological detectability, and mechanical property of the osteochondral interface at an early stage, the pathogenesis significance of which in OA progression is of clinical interest and remains elusive for the TMJ. Unilateral anterior crossbite (UAC) was performed on 6-wk-old rats as we previously reported. TMJs were harvested at 4, 12, and 20 wk. The progression of TMJ OA was evaluated using a modified Osteoarthritis Research Society International (OARSI) score system. Osteochondral interface was investigated by quantifying the thickness via von Kossa staining of histological slices and in vivo calcium deposition by calcein injection. Tissue ossification was imaged by micro-computed tomography (CT). Mechanical properties were measured at nanoscale using dynamic indentation. Time-dependent TMJ cartilage lesions were elicited by UAC treatment. Geometric change of the condyle head and increased value of the OARSI score were evident in UAC TMJs. At the osteochondral interface, there was not only enhanced deep-zone cartilage calcification but also calcium deposition at the osseous boundary. The thickness, density, and stiffness of the osteochondral interface were all significantly increased. The enhanced ossification of the osteochondral interface is a joint outcome of the aberrant deeper cartilage calcification at the superior region and promoted formation of subchondral cortical bone at the inferior region. The micro-CT detectable ossification from an early stage thus is of diagnostic significance. Although the environment of the cartilage and subchondral bone could be changed due to the stiffness of the interface, whether or not the stiffened interface would accelerate OA progress remains to be confirmed. With that evidence, the osteochondral interface could be a new diagnostic and therapeutic target of the mechanically initiated OA in the TMJ.

Proprioceptive mechanisms in occlusion-stimulated masseter hypercontraction

Neurons in the trigeminal mesencephalic nucleus (Vme) have an axon that branches peripherally to innervate the orofacial region and projects centrally to the trigeminal motor nucleus (Vmo). They function as the primary neurons conveying proprioceptive messages. The present study aimed to demonstrate the presence of a periodontal-Vme-Vmo circuit and to provide evidence for its involvement in an experimental unilateral anterior crossbite (UAC) model, which can induce osteoarthritis in the temporomandibular joint. Cholera toxin B subunit (CTb) was injected into the inferior alveolar nerve of rats to help identify the central axon terminals of Vme neurons in the Vmo. The levels of vesicular glutamate transporter 1 (VGLUT1) expressed in the periodontal region, Vme, Vmo, and masseter, and the level of acetylcholinesterase (AChE) expressed in the masseter, were assessed in UAC rats and controls. In CTb-treated rats, many CTb-labeled cell bodies and endings were identified in the Vme and in the Vmo, respectively. In UAC rats, VGLUT1 was expressed at a statistically significantly higher level in the periodontal ligament, Vme, Vmo, and masseter than it was in control rats. The level of AChE protein was 1.97 times higher in UAC rat masseter compared with control rat masseter. These findings reveal a trigeminal mechanism underlying masseter hyperactivity induced by an altered occlusion.

Activation of α2A-adrenergic signal transduction in chondrocytes promotes degenerative remodelling of temporomandibular joint

This study tested whether activation of adrenoreceptors in chondrocytes has roles in degenerative remodelling of temporomandibular joint (TMJ) and to determine associated mechanisms. Unilateral anterior crossbite (UAC) was established to induce TMJ degeneration in rats. Saline vehicle, α2- and β-adrenoreceptor antagonists or agonists were injected locally into the TMJ area of UAC rats. Cartilage degeneration, subchondral bone microarchitecture and the expression of adrenoreceptors, aggrecans, matrix metalloproteinases (MMPs) and RANKL by chondrocytes were evaluated. Chondrocytes were stimulated by norepinephrine to investigate signal transduction of adrenoreceptors. Increased α2A-adrenoreceptor expression was observed in condylar cartilage of UAC rats, together with cartilage degeneration and subchondral bone loss. Norepinephrine depresses aggrecans expression but stimulates MMP-3, MMP-13 and RANKL production by chondrocytes through ERK1/2 and PKA pathway; these effects were abolished by an α2A-adrenoreceptor antagonist. Furthermore, inhibition of α2A-adrenoreceptor attenuated degenerative remodelling in the condylar cartilage and subchondral bone, as revealed by increased cartilage thickness, proteoglycans and aggrecan expression, and decreased MMP-3, MMP-13 and RANKL expressions in cartilage, increased BMD, BV/TV, and decreased Tb.Sp in subchondral bone. Conversely, activation of α2A-adrenoreceptor intensified aforementioned degenerative changes in UAC rats. It is concluded that activation of α2A-adrenergic signal in chondrocytes promotes TMJ degenerative remodelling by chondrocyte-mediated pro-catabolic activities.

TNF Accelerates Death of Mandibular Condyle Chondrocytes in Rats with Biomechanical Stimulation-Induced Temporomandibular Joint Disease

Objective

To determine if temporomandibular joint chondrocyte apoptosis is induced in rats with dental biomechanical stimulation and what a role TNF takes.

Methods

Thirty-two rats were divided into 4 groups (n = 8/group) and exposed to incisor mal-occlusion induced by unilateral anterior crossbite biomechanical stimulation. Two groups were sampled at 2 or 4 weeks. The other two groups were treated with local injections of a TNF inhibitor or PBS into the temporomandibular joints area at 2 weeks and then sampled at 4 weeks. Twenty-four rats either served as unilateral anterior crossbite mock operation controls (n = 8/group) with sampling at 2 or 4 weeks or received a local injection of the TNF inhibitor at 2 weeks with sampling at 4 weeks. Chondrocytes were isolated from the temporomandibular joints of 6 additional rats and treated with TNF in vitro. Joint samples were assessed using Hematoxylin&eosin, Safranin O, TUNEL and immunohistochemistry staining, real-time PCR, fluorogenic activity assays and Western blot analyses. The isolated chondrocytes were also analyzed by flow cytometry.

Results

Unilateral anterior crossbite stimulation led to temporomandibular joint cartilage degradation, associated with an increase in TUNEL-positive chondrocytes number, caspase-9 expression levels, and the release of cytochrome c from mitochondria at 2 weeks without changes in TNF and caspase-8 levels until after 4 weeks. TNF stimulated apoptosis of the isolated chondrocytes and up-regulated caspase-8 expression, but did not change caspase-9 expression levels. Local injection of TNF inhibitor down-regulated caspase-8 expression and reduced TUNEL-positive cell number, but did not reverse cartilage thickness reduction, caspase-9 up-regulation or cytochrome c release.

Conclusions

Unilateral anterior crossbite stimulation induces mitochondrion-mediated apoptosis of articular chondrocytes. TNF accelerated the unilateral anterior crossbite induced chondrocytes apoptosis via death-receptor pathway. However, anti-TNF therapy does not prevent cartilage loss in this model of temporomandibular joint.

β2-Adrenergic signal transduction plays a detrimental role in subchondral bone loss of temporomandibular joint in osteoarthritis

The present study tested whether activation of the sympathetic tone by aberrant joint loading elicits abnormal subchondral bone remodeling in temporomandibular joint (TMJ) osteoarthritis. Abnormal dental occlusion was created in experimental rats, which were then intraperitoneally injected by saline, propranolol or isoproterenol. The norepinephrine contents, distribution of sympathetic nerve fibers, expression of β-adrenergic receptors (β-ARs) and remodeling parameters in the condylar subchondral bone were investigated. Mesenchymal stem cells (MSCs) from condylar subchondral bones were harvested for comparison of their β-ARs, pro-osteoclastic gene expressions and pro-osteoclastic function. Increases in norepinephrine level, sympathetic nerve fiber distribution and β2-AR expression were observed in the condylar subchondral bone of experimental rats, together with subchondral bone loss and increased osteoclast activity. β-antagonist (propranolol) suppressed subchondral bone loss and osteoclast hyperfunction while β-agonist (isoproterenol) exacerbated those responses. MSCs from experimental condylar subchondral bone expressed higher levels of β2-AR and RANKL; norepinephrine stimulation further increased their RANKL expression and pro-osteoclastic function. These effects were blocked by inhibition of β2-AR or the PKA pathway. RANKL expression by MSCs decreased after propranolol administration and increased after isoproterenol administration. It is concluded that β2-AR signal-mediated subchondral bone loss in TMJ osteoarthritisis associated with increased RANKL secretion by MSCs.

Installing and thereafter removing an aberrant prosthesis elicited opposite remodelling responses in growing mouse temporomandibular joints

Temporomandibular joint (TMJ) displays a high remodelling capability. The current purpose was to investigate the differences between mandibular condylar remodelling responses of growing mice to installation and removal of unilateral anterior crossbite (UAC) prosthesis. Twenty-four mice were divided into one mock control group and two UAC groups. Unilateral anterior crossbite was created by installing a pair of prosthesis to left-side maxillary and mandibular incisors. Unilateral anterior crossbite was removed in removal group at 3 weeks but remained in UAC group. Temporomandibular joints were sampled at 7 weeks. Changes in condylar cartilage and subchondral bone were assessed by histology and in vivo micro-CT. Real-time PCR and immunohistochemistry were performed to evaluate expression changes in ADAMTS-5, MMP-3, MMP-9, MMP-13, IL-1, TNF-α, OPG and RANKL. Statistical analysis was performed at α = 0.05. Temporomandibular joint cartilage degradation was induced by UAC as previously reported but was reversed by removal of UAC. The dropped cartilage thickness, chondrocyte number and collagen II-positive area, the increased expression levels of Adamts-5, Mmp3, 9, 13, Tnf-α and Il-1β in cartilage, the decreased ratio of OPG/RANKL in both condylar cartilage and subchondral bone, the loss of TMJ subchondral bone and the increase in the TRAP-positive cells in subchondral bone were all reversed in the removal group (P < 0.05). The growing mouse TMJ condyle displays a high remodelling capability which can be degenerative and rehabilitative, respectively, in response to placement and thereafter removal of the aberrant prosthesis. Eliminating aberrant prosthesis is helpful to promote the degraded condyle to recover.

Wnt5a/Ror2 mediates temporomandibular joint subchondral bone remodeling

Increased subchondral trabecular bone turnover due to imbalanced bone-resorbing and bone-forming activities is a hallmark of osteoarthritis (OA). Wnt5a/Ror2 signaling, which can derive from bone marrow stromal cells (BMSCs), takes a role in modulating osteoblast and osteoclast formation. We showed previously that experimentally unilateral anterior crossbites (UACs) elicited OA-like lesions in mice temporomandibular joints (TMJs), displaying as subchondral trabecular bone loss. Herein, we tested the role of BMSC-derived Wnt5a/Ror2 signaling in regulating osteoclast precursor migration and differentiation in this process. The data confirmed the decreased bone mass, increased tartrate-resistant acid phosphatase (TRAP)-positive cell number, and enhanced osteoclast activity in TMJ subchondral trabecular bone of UAC-treated rats. Interestingly, the osteoblast activity in the tissue of TMJ subchondral trabecular bone of these UAC-treated rats was also enhanced, displaying as upregulated expressions of osteoblast markers and increased proliferation, migration, and differentiation capabilities of the locally isolated BMSCs. These BMSCs showed an increased CXCL12 protein expression level and upregulated messenger RNA expressions of Rankl, Wnt5a, and Ror2. Ex vivo data showed that their capacities of inducing migration and differentiation of osteoclast precursors were enhanced, and these enhanced capabilities were restrained after blocking their Ror2 signaling using small interfering RNA (siRNA) assays. Reducing Ror2 expression in the BMSC cell line by siRNA or blocking the downstream signalings with specific inhibitors also demonstrated a suppression of the capacity of the BMSC cell line to promote Wnt5a-dependent migration (including SP600125 and cyclosporine A) and differentiation (cyclosporine A only) of osteoclast precursors. These findings support the idea that Wnt5a/Ror2 signaling in TMJ subchondral BMSCs enhanced by UAC promoted BMSCs to increase Cxcl12 and Rankl expression, in which JNK and/or Ca(2+)/NFAT pathways were involved and therefore were engaged in enhancing the migration and differentiation of osteoclast precursors, leading to increased osteoclast activity and an overall TMJ subchondral trabecular bone loss in the UAC-treated rats.

The influence of altered occlusion on pro-inflammatory cytokine levels in the TMJ synovial tissues of rats

OBJECTIVE

The aim of this study was to evaluate whether altered occlusion affects both the condylar cartilage thickness and the cytokine levels of the TMJs of rats.

DESIGN

Thirty adult-male rats (n=30) were randomly assigned to three experimental conditions: a control group that underwent sham operations with unaltered occlusion; an FPDM group that underwent functional posterior displacement of the mandible that was induced by an incisor guiding appliance; and an iOVD group in which the increased occlusal vertical dimension was induced in the molars. The rats were subjected to the FPDM or iOVD model for 14 days and then killed. Both the right and left TMJs were removed and randomly assigned to examination with staining or immunoassay techniques. Toluidine blue staining was used to measure the thicknesses of the four layers of the articular cartilage (i.e., the fibrous, proliferating, mature, and hypertrophic layers). ELISA assays were used to assess the concentrations of the pro-inflammatory cytokines IL-1α, IL-1β, IL-6, and tumour necrosis factor (TNF-α). The measurements of the articular cartilage layers and cytokine concentrations were analyzed with ANOVA and Tukey's tests and Kruskal-Wallis and Dunn tests, respectively (α=5%).

RESULTS

The thickness of articular cartilage in the FPDM group (0.3±0.03mm) was significantly greater than those of the control (0.2±0.01mm) and iOVD (0.25±0.03mm) groups. No significant difference was observed between the control and iOVD groups. The four articular cartilage layers were thicker in the FPDM group than in the control and iOVD groups, and the latter two groups did not differ one from each other. Both the FPDM and iOVD groups exhibited higher cytokine levels than did the control (p<0.05) group. Compared to the FPDM group, the iOVD group exhibited significantly higher levels of IL-1β and TNF-α.

CONCLUSION

Both models induced inflammation in the TMJ and caused significant structural changes in the TMJ and surrounding tissues.

Connexin43 hemichannels mediate small molecule exchange between chondrocytes and matrix in biomechanically-stimulated temporomandibular joint cartilage

OBJECTIVE

Connexin (Cx) 43 hemichannels play a role in mechanotransduction. This study was undertaken in order to determine if Cx43 hemichannels were activated in rat temporomandibular joint (TMJ) chondrocytes under mechanical stimulation.

METHODS

Sprague-Dawley rats were stimulated dental-mechanically. Cx43 expression in rat TMJ cartilage was determined with immunohistochemistry and real-time PCR, and Cx43 hemichannel opening was evaluated by the extra- and intracellular levels of prostaglandin E2 (PGE2). Both primary rat chondrocytes and ATDC5 cells were treated with fluid flow shear stress (FFSS) to induce hemichannel opening. The Cx43 expression level was then determined by real-time PCR or Western blotting, and the extent of Cx43 hemichannel opening was evaluated by measuring both PGE2 release and cellular dye uptake.

RESULTS

Cx43 expression and intra- and extracellular PGE2 levels were increased in mechanically-stimulated rat TMJ cartilage compared to the unstimulated control. The FFSS treatment increased Cx43 expression and induced Cx43 hemichannel opening in primary rat chondrocytes and ATDC5 cells indicated by enhanced PGE2 release and dye uptake. Furthermore, the Cx43 hemichannel opening could be blocked by the addition of 18β-glycyrrhetinic acid, a Cx channel inhibitor, Cx43-targeting siRNA, or by withdrawal of FFSS stimulation. The migration of cytosolic Cx43 protein to the plasma membrane in ATDC5 cells was still significant after 8 h post 2-h FFSS treatment, and the Cx43 protein level was still high at 48 h, which returned to control levels at 72 h after treatment.

CONCLUSION

Cx43 hemichannels are activated and mediate small molecule exchange between TMJ chondrocytes and matrix under mechanical stimulation.

Reducing dietary loading decreases mouse temporomandibular joint degradation induced by anterior crossbite prosthesis

OBJECTIVE

Dietary loading has been reported to have an effect on temporomandibular joint (TMJ) remodeling via periodontal-muscular reflex. We therefore examined whether reducing dietary loading decreased TMJ degradation induced by the unilateral anterior crossbite prosthesis as we recently reported.

METHODS

Forty 6-week-old female C57BL/6J mice were randomly divided into two experimental and two control groups. One experimental and one control group received small-size diet and the other two groups received large-size diet. Unilateral anterior crossbite prosthesis was created in the two experimental groups. The TMJ samples were collected 3 weeks after experimental operation. Histological changes in condylar cartilage and subchondral bone were assessed by Hematoxylin & Eosin, toluidine blue, Safranin O and tartrate-resistant acid phosphatase staining. Real-time polymerase chain reaction (PCR) and/or immunohistochemistry were performed to evaluate the expression levels of Collagen II, Aggrecan, a disintegrin and metalloproteinase with thrombospondin motifs 5 (ADAMTS-5) and RANKL/RANK/OPG in TMJ condylar cartilage and/or subchondral bone.

RESULTS

Thinner and degraded cartilage, reduced cartilage cellular density, decreased expression levels of Collagen II and Aggrecan, loss of subchondral bone and enhanced osteoclast activity were observed in TMJs of both experimental groups. However, the cartilage degradation phenotype was less severe and cartilage ADAMTS-5 mRNA was lower while OPG/RANKL ratio in cartilage and subchondral bone was higher in the small-size than large-size diet experimental group. No differences of histomorphology and the tested molecules were found between the two control groups.

CONCLUSIONS

The current findings suggest that a lower level of functional loading by providing small-size diet could reduce TMJ degradation induced by the biomechanical stimulation from abnormal occlusion.