The imbalance of masticatory muscle activity affects the asymmetric growth of condylar cartilage and subchondral bone in rats

Transition of endochondral bone formation at the normal and botulinum-treated mandibular condyle of growing juvenile rat

OBJECTIVE

The aim of this study was to understand the temporal and spatial distribution of canonical endochondral ossification (CEO) and non-canonical endochondral ossification (NCEO) of the normal growing rat condyle, and to evaluate their histomorphological changes following the simultaneous hypotrophy of the unilateral masticatory closing muscles with botulinum toxin (BTX).

DESIGN

46 rats at postnatal 4 weeks were used for the experiment and euthanized at postnatal 4, 8, and 16 weeks. The right masticatory muscles of rats in experimental group were injected with BTX, the left being injected with saline as a control. The samples were evaluated using 3D morphometric, histological, and immunohistochemical analysis with three-dimensional regional mapping of endochondral ossifications.

RESULTS

The results showed that condylar endochondral ossification changed from CEO to NCEO at the main articulating surface during the experimental period and that the BTX-treated condyle presented a retroclined smaller condyle with an anteriorly-shifted narrower articulating surface. This articulating region showed a thinner layer of the endochondral cells, and a compact distribution of flattened cells. These were related to the load concentration, decreased cellular proliferation with thin cellular layers, reduced extracellular matrix, increased cellular differentiation toward the osteoblastic bone formation, and accelerated transition of the ossification types from CEO to NCEO.

CONCLUSION

The results suggest that endochondral ossification under loading tended to show more NCEO, and that masticatory muscular hypofunction by BTX had deleterious effects on endochondral bone formation and changed the condylar growth vector, resulting in a retroclined, smaller, asymmetrical, and deformed condyle with thin cartilage.

Early changes in asporin levels in osteoarthritis of the temporomandibular joint

OBJECTIVES

The present study aimed to elucidate the pathogenesis of temporomandibular joint (TMJ) osteoarthritis (TMJ-OA) in a mouse model. We investigated morphological and histological changes in the head of mandible cartilage and early immunohistochemical (IHC) changes in transforming growth factor (TGF)-β, phosphorylated Smad-2/3 (p-Smad2/3), a TGF-β signaling molecule, and asporin.

METHODS

TMJ-OA was induced in a mouse model through unilateral partial discectomy. Micro-computed tomography (micro-CT) and safranin-O staining were performed to morphologically and histologically evaluate the degeneration of the head of mandible caused by TMJ-OA. IHC staining for TGF-β, p-Smad2/3, and asporin was performed to evaluate the changes in protein expression.

RESULTS

In the experimental group, three-dimensional (3D) morphometry revealed an enlarged head of mandible and safranin-O staining showed degeneration of cartilage tissue in the early stages of TMJ-OA compared to the control group. IHC staining revealed that TGF-β, p-Smad2/3, and asporin expression increased in the head of mandible cartilage before the degeneration of cartilage tissue, and subsequently decreased for a short period.

CONCLUSION

The findings suggested a negative feedback relationship between the expression of asporin and the TGF-β/Smad transduction pathway, which may be involved in the degeneration of the head of mandible in the early stages of TMJ-OA. Asporin is a potential biomarker of the early stages of TMJ-OA, which ultimately leads to the irreversible degeneration of TMJ tissues.

Effects on Facial Growth Following Masseter Muscle Resection in Growing Rats-A Systematic Review

An individual's facial appearance is heavily influenced by facial symmetry. In the asymmetric mandible, periosteal apposition and endochondral ossification in one of the condyles may stimulate asymmetric growth of the body. Our aim was to review the impact on the growth following masseter resection. Relevant studies up to October 2022 were retrieved from PubMed, Scopus, and Web of Science. The PICOS method was utilized to determine eligibility, and the SYRCLE risk of bias tool was utilized to provide an estimate of potential bias. A predetermined algorithm was used to search the databases. The results of our systematic review of seven studies indicate that the masseter muscle strongly impacts craniofacial growth and development. Resection of the masseter muscle significantly reduces the sagittal and vertical development of the jaw in rats. In addition, the masseter muscle excision influences the mandibular morphology, including the condylar area, angle, and development direction of the jaw.

Three-Dimensional Distance Mapping Method to Evaluate Mandibular Symmetry and Morphology of Adults with Unilateral Premolar Scissors Bite

(1) Objective: This study aimed to evaluate the association between unilateral premolar scissors bite and mandibular symmetry of adults via the 3D distance mapping method. (2) Methods: A total of 53 cone-beam computed tomography (CBCT) images of adults with unilateral premolar scissors bite were set as study samples. A total of 53 age- and sex-matched samples without scissors bite were in the control group. Three-dimensional mandibular models and seven mandibular functional units, including condylar process (Co), coronoid process (Cr), mandibular ramus (Ra), mandibular angle (Ma), alveolar process (Ap), mandibular body (Mb), and chin process (Ch) were constructed and mirrored. After superimposition of the original and the mirrored models, 3D distance maps and deviation analysis were performed to evaluate the mandibular symmetry and morphology. (3) Results: In the study group, the matching percentages of the entire mandible (50.79 ± 10.38%), Ap (67.00 ± 12.68%), Mb (66.62 ± 9.44%), Ra (62.52 ± 11.00%), Ch (80.75 ± 9.86%), and Co (62.78 ± 13.56) were lower than that of the entire mandible (58.60 ± 5.52) (p < 0.01), Ap (73.83 ± 8.88%) (p < 0.01), Mb (72.37 ± 8.69%) (p < 0.01), Ra (68.60 ± 7.56%) (p < 0.01), Ch (85.23 ± 6.80%) (p < 0.01), and Co (67.58 ± 10.32%) (p < 0.05) in the control group. However, Cr and Ma showed no significant difference (p > 0.05). (4) Conclusions: The 3D distance mapping method provided a qualitative and quantitative mandibular symmetry and morphology assessment. Mandibular asymmetry was found in adults with unilateral premolar scissors bites. Mandibular functional units, including the alveolar process, mandibular body, mandibular ramus, chin process, and condylar process, showed significant differences, while no significant difference was observed in the coronoid process and mandibular angle.

Micro-CT-Based Bone Microarchitecture Analysis of the Murine Skull

X-ray micro-computed tomography (micro-CT) imaging has important applications in microarchitecture analysis of cortical and trabecular bone structure. While standardized protocols exist for micro-CT-based microarchitecture assessment of long bones, specific protocols need to be developed for different types of skull bones taking into account differences in embryogenesis, organization, development, and growth compared to the rest of the body. This chapter describes the general principles of bone microarchitecture analysis of murine craniofacial skeleton to accommodate for morphological variations in different regions of interest.

Changes of the condylar cartilage and subchondral bone in the temporomandibular joints of rats under unilateral mastication and expression of Insulin-like Growth Factor-1

OBJECTIVES

This study aimed to define changes in the rat condylar cartilage and subchondral bone using the unilateral mastication model.

MATERIALS AND METHODS

In this study, forty 4-week-old Wistar rats were randomly divided into experimental (n = 20) and control group (n = 20). In the experimental group, unilateral dental splints were placed on the occlusal surface of left maxillary molars. The rats were sacrificed at 1, 2, 3, and 4 weeks after placement of the splint. Micro-CT scanning and histological staining were performed to observe the changes in the mandibular condylar cartilage and subchondral bone. Levels of insulin-like growth factor-1 (IGF-1) were determined via immunohistochemistry to analyse the occurrence of osteogenic changes.

RESULTS

Micro-CT scanning findings demonstrated the occurrence of asymmetric growth of condyle in the experimental group. The condylar cartilage and subchondral bone exhibited degradation on the chewing side of the experimental group and showed decreased bone mineral density, thinner cartilage thickness, and increased degree of degeneration and osteoclast activity. Compared with the control group, the expression of IGF-1 was remarkably higher on the non-chewing side.

CONCLUSION

Long-term unilateral mastication can lead to the occurrence of degenerative changes in the condylar cartilage and subchondral bone during growth and development. IGF-1 may play a role in promoting the process of osteogenesis.

nkx3.2 mutant zebrafish accommodate jaw joint loss through a phenocopy of the head shapes of Paleozoic jawless fish

The vertebrate jaw is a versatile feeding apparatus. To function, it requires a joint between the upper and lower jaws, so jaw joint defects are often highly disruptive and difficult to study. To describe the consequences of jaw joint dysfunction, we engineered two independent null alleles of a single jaw joint marker gene, nkx3.2, in zebrafish. These mutations caused zebrafish to become functionally jawless via fusion of the upper and lower jaw cartilages (ankylosis). Despite lacking jaw joints, nkx3.2 mutants survived to adulthood and accommodated this defect by: (a) having a remodeled skull with a fixed open gape, reduced snout and enlarged branchial region; and (b) performing ram feeding in the absence of jaw-generated suction. The late onset and broad extent of phenotypic changes in the mutants suggest that modifications to the skull are induced by functional agnathia, secondarily to nkx3.2 loss of function. Interestingly, nkx3.2 mutants superficially resemble ancient jawless vertebrates (anaspids and furcacaudiid thelodonts) in overall head shape. Because no homology exists in individual skull elements between these taxa, the adult nkx3.2 phenotype is not a reversal but rather a convergence due to similar functional requirements of feeding without moveable jaws. This remarkable analogy strongly suggests that jaw movements themselves dramatically influence the development of jawed vertebrate skulls. Thus, these mutants provide a unique model with which to: (a) investigate adaptive responses to perturbation in skeletal development; (b) re-evaluate evolutionarily inspired interpretations of phenocopies generated by gene knockdowns and knockouts; and (c) gain insight into feeding mechanics of the extinct agnathans.

Condyle modeling stability, craniofacial asymmetry and ACTN3 genotypes: Contribution to TMD prevalence in a cohort of dentofacial deformities

Craniofacial asymmetry, mandibular condylar modeling and temporomandibular joint disorders are common comorbidities of skeletally disproportionate malocclusions, but etiology of occurrence together is poorly understood. We compared asymmetry, condyle modeling stability and temporomandibular health in a cohort of 128 patients having orthodontics and orthognathic surgery to correct dentofacial deformity malocclusions. We also compared ACTN3 and ENPP1 genotypes for association to clinical conditions. Pre-surgical posterior-anterior cephalometric and panometric radiographic analyses; jaw pain and function questionnaire and clinical examination of TMD; and SNP-genotype analysis from saliva samples were compared to assess interrelationships. Almost half had asymmetries in need of surgical correction, which could be subdivided into four distinct morphological patterns. Asymmetric condyle modeling between sides was significantly greater in craniofacial asymmetry, but most commonly had an unanticipated pattern. Often, longer or larger condyles occurred on the shorter mandibular ramus side. Subjects with longer ramus but dimensionally smaller condyles were more likely to have self-reported TMD symptoms (p = 0.023) and significantly greater clinical diagnosis of TMD (p = 0 .000001), with masticatory myalgia most prominent. Genotyping found two significant genotype associations for ACTN3 rs1671064 (Q523R missense) p = 0.02; rs678397 (intronic SNP) p = 0.04 and one significant allele association rs1815739 (R577X nonsense) p = 0.00. Skeletal asymmetry, unusual condyle modeling and TMD are common and interrelated components of many dentofacial deformities. Imbalanced musculoskeletal functional adaptations and genetic or epigenetic influences contribute to the etiology, and require further investigation.

Temporomandibular Joint Hypofunction Secondary to Unilateral Partial Discectomy Attenuates Degeneration in Murine Mandibular Condylar Cartilage

Mechanical overloading of the temporomandibular joint (TMJ) promotes both the initiation and progression of TMJ osteoarthritis (OA). New preclinical animal models are needed for the evaluation of the molecular basis of cellular load transmission. This would allow a better understanding of the underlying mechanisms of TMJ-OA pain and disability, and help identify new therapeutics for its early diagnosis and management. The purpose of this study was to evaluate the role of mechanical loading in the progression of TMJ-OA in surgical instability arising from unilateral partial discectomy (UPD) in a murine model. In the theoretical modelling employed, lower joint reaction forces were observed on the chewing (working) side of the TMJ in the murine craniomandibular musculoskeletal system. Hypofunction was induced secondary to UPD through surgically manipulating the working side using an unopposed molar model. When the working side was restricted to the same side as that on which UPD was performed, late-stage degeneration of the cartilage showed a significant reduction (p<0.05), with diminished fibrillation and erosion of the articular cartilage, cell clustering, and hypocellularity. Condylar remodelling and proteolysis of proteoglycans were less affected. Thus, select and specific late-stage changes in TMJ-OA were contextually linked with the local mechanical environment of the joint. These data underscore the value of the UPD mouse model in studying mechanobiological pathways activated during TMJ-OA, and suggest that therapeutically targeting mechanobiological stimuli is an effective strategy in improving long-term biological, clinical, and patient-based outcomes.

Mandibular lateral deviation induces alteration in vascular endothelial growth factor expression and oxidative stress/nitric oxide generation in rat condyle, synovial membrane and masseter muscle

OBJECTIVE

We aimed to investigate alteration in cellular signaling mediated by vascular endothelial growth factor (VEGF) and parameters of oxidative stress/nitric oxide generation, superoxide dismutase (SOD) and neuronal nitric oxide synthase (nNOS), underlying altered functional mechanical loading of TMJ (temporomandibular joint) during lateral mandibular deviation.

DESIGN

Thirty-eight 5-week-old male Wistar rats were divided into experimental group, which received acrylic resin appliance that shifted mandible to the left during closure, and control group. Computed tomography and histomorphometry were used for condyle analyses, while samples of condyle, synovial membrane and m. masseter were analyzed with enzyme-linked immunosorbent assay and spectrophotometry to determine VEGF and nNOS protein concentrations, and SOD activity.

RESULTS

Experimental group of rats developed smaller and asymmetrical mandibles. Less of new bone and cartilage formation and larger bone marrow cavities area were found in the experimental group. Higher VEGF expression in condyle and m. masseter as well as higher nNOS expression in m. masseter and synovial membrane were found in the experimental compared to the control group. Alteration of SOD activity was found in m. masseter and synovial membrane in the experimental group.

CONCLUSIONS

Lateral mandibular deviation induces mandibular and condylar morphological changes as well as significant cellular signaling alterations in condyle, synovial membrane and masticatory muscle. Cellular VEGF protein overexpression and oxidative stress/nitric oxide disbalance could be the mechanisms underlying unbalanced functional TMJ loading due to mandibular deviation.

Mandibular Vertical Growth Deficiency After Botulinum-Induced Hypotrophy of Masticatory Closing Muscles in Juvenile Nonhuman Primates

The purpose of this study was to investigate the relationship between masticatory muscular hypotrophy and mandibular growth in juvenile nonhuman primates (cynolmolgus monkeys, Macaca fasicularis). We hypothesized that botulinum toxin (BTX)-induced neuro-muscular junctional block and its resultant hypotrophy of masticatory muscles would produce mandibular growth disturbances in size and shape. Ten male cynomolgus monkeys were divided into three groups: group I (control; n = 3), group II (unilateral BTX; n = 4), and group III (bilateral BTX; n = 3). The unilateral or bilateral muscular hypotrophy of major masticatory closing muscles was induced by synchronous BTX application to masseter, medial pterygoid, and temporal muscle. Mandibular growth was tracked by linear, angular, area and volume measurements using three-dimensional (3D) computed tomography imaging before BTX treatment and after 3 and 6 months. After unilateral hypotrophy of masticatory muscles in group II, vertical growth deficiency was prominent on the BTX side, with compensatory overgrowth on the control side. The bilateral muscular hypotrophy in group III also showed smaller ramal height and width than that of control (group I) and control side (group II). Moreover, ramal sagittal angles (posterior tilt) increased on the BTX side of both groups II and III, but coronal angles (lateral tilt) did so on the BTX side of group II, resulting in asymmetry. The results confirmed our hypothesis that functional activity of masticatory closing muscles is closely related to mandibular growth in size and shape of juvenile nonhuman primates. In addition, the focused growth disturbances on the ramal height and posterior-lateral tilt suggested the possible role of masticatory closing muscles for ramal vertical and angular growth vector of the mandible.

Three-dimensional analysis of mandibular functional units in adult patients with unilateral posterior crossbite: A cone beam study with the use of mirroring and surface-to-surface matching techniques

OBJECTIVES

To use three-dimensional (3D) mirroring and surface-to-surface techniques to determine any differences in mandibular functional unit shape and morphology between the crossbite side and non-crossbite side in adult patients with posterior unilateral crossbite who had not received any corrective treatment for malocclusion.

MATERIALS AND METHODS

Cone-beam computed tomography (CBCT) records from 24 consecutive adult white patients (mean age, 27.5 years; range 22.6-39.7 years; 14 women and 10 men) seeking treatment for maxillary transverse deficiency were assessed in this study. The control group comprised CBCT scans from age- and sex-matched patients. Segmentation masks were generated to obtain 3D surface mesh models of the mandibles and analyze the six skeletal functional units, which were further analyzed with reverse engineering software.

RESULTS

Statistically significant differences in the mean surface distance when comparing the study sample and the control sample were found at the condylar process, mandibular ramus, angular process (P ≤ .0001), and alveolar process (P ≤ .01); no statistically significant differences were found for the coronoid process, the chin, and the mandibular body (P ≥ .5).

CONCLUSIONS

The condylar, angular, and alveolar processes plus the mandibular ramus appear to play a more dominant role than did the body, the coronoid, and the chin units in the asymmetry of the mandible in patients with unilateral crossbite.

Effects of masticatory muscle function affected by BTX on maxillofacial bone growth through the sutural modification

OBJECTIVES

To evaluate the effect of botulinum toxin type A (BTX)-induced masticatory muscle hypofunction on the maxillofacial suture bone growth of growing rats.

SETTING AND SAMPLE POPULATION

Department of Orthodontics at Taipei Medical University. Forty-eight male 4-week-old Wistar rats were divided into four groups. The N group received injections of normal saline into each of the masseter and temporalis muscles. The M group received injections of normal saline into each of the temporalis muscle and injections of BTX into each of the masseter muscle. The T group received injections of normal saline into each of the masseter muscle and injections of BTX into each of the temporalis muscle. The MT group received injections of BTX into each of the masseter and temporalis muscles.

MATERIAL & METHODS

Rats were sacrificed after 42 days of growth. Changes in body and muscle weight were measured. Anthropometric measurements of the maxillary arch, sutural bone mineral density and sutural bone deposition distances were recorded. Statistical comparisons were performed using analysis of variance.

RESULTS

No significant change in body weight was found across groups. However, significant decreases were observed in muscle weight, anthropometric measurements, sutural bone mineral density and bone apposition distance in the BTX-injected group.

CONCLUSIONS

Reduced masticatory muscle function in growing rats can affect maxillofacial suture bone growth.

Masseter muscle atrophy impairs bone quality of the mandibular condyle but not the alveolar process early after induction

BACKGROUND

Masseter muscle function influences mandibular bone homeostasis. As previously reported, bone resorption markers increased in the mouse mandibular condyle two days after masseter paralysis induced with botulinum toxin type A (BoNTA), followed by local bone loss.

OBJECTIVE

This study aimed to evaluate the bone quality of both the mandibular condyle and alveolar process in the mandible of adult mice during the early stage of a BoNTA-induced masseter muscle atrophy, using a combined 3D histomorphometrics and shape analysis approach.

METHODS

Adult BALB/c mice were divided into an untreated control group and an experimental group; the latter received one single BoNTA injection in the right masseter (BoNTA-right) and saline in the left masseter (Saline-left). 3D bone microstructural changes in the mandibular condyle and alveolar process were determined with high-resolution microtomography. Additionally, landmark-based geometric morphometrics was implemented to assess external shape changes.

RESULTS

After 2 weeks, masseter mass was significantly reduced (P-value <0.001). When compared to Saline-left and untreated condyles, BoNTA-right condyles showed significant bone loss (P-value <0.001) and shape changes. No significant bone loss was observed in the alveolar processes of any of the groups (P-value >0.05).

CONCLUSION

Condyle bone quality deteriorates at an early stage of BoNTA-induced masseter muscle atrophy, and before the alveolar process is affected. Since the observed bone microstructural changes resemble those in human temporomandibular joint degenerative disorders, the clinical safety of BoNTA intervention in the masticatory apparatus remains to be clarified.

[Influence on Indian hedgehog-parathyroid hormone-like related protein pathway induced by altered masticatory loading in the condylar cartilage of growing rabbits]

OBJECTIVE

To determine the influence of altered masticatory loading on Indian hedgehog (Ihh)-parathyroid hormone-like related protein (PThrP) pathway in the condylar cartilage of growing rabbits.

METHODS

A total of 48 10-day-old rabbits were randomly divided into two groups and fed different kinds of food, such as solid diet and soft diet. The animals were sacrificed after 2, 4, 6, and 8 weeks. Difference of Ihh and PThrP expression levels induced by altered masticatory loading was tested by hematoxylin-eosin (HE), immunohistochemistry, Western blot, and real-time polymerase chain reaction (PCR).

RESULTS

The thickness of condylar cartilage and expression levels of Ihh and PThrP proteins and mRNA of the solid diet groups exceeded those of the soft diet groups. The decreasing tendencies of the expression levels of Ihh and PThrP proteins and mRNA were observed at 2, 4, 6, 8 weeks.

CONCLUSIONS

Low masticatory loading may delay or inhibit the development of condylar cartilage and its growing factors Ihh and PThrP. Therefore, masticatory loading plays an important role in the development of condylar cartilage.