Osteocyte Mechanotransduction in Orthodontic Tooth Movement

PURPOSE OF REVIEW

Orthodontic tooth movement is characterized by periodontal tissue responses to mechanical loading, leading to clinically relevant functional adaptation of jaw bone. Since osteocytes are significant in mechanotransduction and orchestrate osteoclast and osteoblast activity, they likely play a central role in orthodontic tooth movement. In this review, we attempt to shed light on the impact and role of osteocyte mechanotransduction during orthodontic tooth movement.

RECENT FINDINGS

Mechanically loaded osteocytes produce signaling molecules, e.g., bone morphogenetic proteins, Wnts, prostaglandins, osteopontin, nitric oxide, sclerostin, and RANKL, which modulate the recruitment, differentiation, and activity of osteoblasts and osteoclasts. The major signaling pathways activated by mechanical loading in osteocytes are the wingless-related integration site (Wnt)/β-catenin and RANKL pathways, which are key regulators of bone metabolism. Moreover, osteocytes are capable of orchestrating bone adaptation during orthodontic tooth movement. A better understanding of the role of osteocyte mechanotransduction is crucial to advance orthodontic treatment. The optimal force level on the periodontal tissues for orthodontic tooth movement producing an adequate biological response, is debated. This review emphasizes that both mechanoresponses and inflammation are essential for achieving tooth movement clinically. To fully comprehend the role of osteocyte mechanotransduction in orthodontic tooth movement, more knowledge is needed of the biological pathways involved. This will contribute to optimization of orthodontic treatment and enhance patient outcomes.

Force-Induced Nitric Oxide Promotes Osteogenic Activity during Orthodontic Tooth Movement in Mice

Objectives

The aim of this study was to investigate the effect of nitric oxide (NO) on orthodontic tooth movement and the regulatory effect on bone formation.

Design

A mouse orthodontic tooth movement model was established to measure the level of releasing NO. Besides, orthodontic tooth movement distance and the bone formation in the tension side of the orthodontic tooth were also analyzed. In vitro, human periodontal ligament stem cells (hPDLSCs) were cultured under tensile force stimulation. The production of NO and the expression level of nitric oxide synthase (NOS) were detected after mechanical stimulation. Furthermore, the downstream cellular signaling pathway regulated by NO was also explored.

Results

The generation of NO steadily increased throughout the orthodontic tooth movement in mice. Orthodontic tooth movement was decreased in the NOS inhibitor group while it was accelerated in the NO precursor group. Force-induced NO promoted the osteogenic differentiation of human hPDLSCs under tensile force stimulation. And force-induced NO in hPDLSCs regulated the PI3K/Akt/β-catenin signal pathway.

Conclusion

NO is involved in the regulation of orthodontic tooth movement and promotes bone formation on the tension side of the orthodontic tooth. The PI3K/Akt/β-catenin pathway is one of the downstream cell signal transduction pathways of NO in the orthodontic process.

Does puberty affect oxidative stress levels and antioxidant activity of saliva in patients with fixed orthodontic appliances?

PURPOSE

To evaluate and compare patients' oxidative stress parameters and antioxidant status with fixed orthodontic appliances during the pubertal and postpubertal growth periods.

METHODS

Saliva samples of 20 pubertal (mean age: 12.94 ± 0.34 years) and 20 postpubertal (mean age: 16.34 ± 0.45 years) patients were collected just before the application of fixed orthodontic appliances (T0), 4-5 h (T1), and 7 days (T2) after the initial orthodontic activation. Myeloperoxidase (MPO), nitric oxide (NO), 8‑hydroxydeoxyguanosine (8-OHdG) levels, and superoxide dismutase (SOD) activity in the saliva were examined. Repeated measures analysis of variance (ANOVA), least significant difference (LSD) pairwise comparison, and independent sample t‑tests were used to analyze the differences between the time points and growth periods, respectively.

RESULTS

MPO levels in the saliva of patients in the pubertal period showed a significantly higher increase within the first days of treatment (T2-T1) than in patients in the postpubertal period (p < 0.05). The SOD antioxidant enzyme activity decreased in the samples from T0 to T1 in the patients in the pubertal and postpubertal groups and returned to baseline values (T0) at T2 (p < 0.01). No significant differences in the other biochemical parameters between groups were observed.

CONCLUSIONS

Comparing the pubertal and postpubertal groups, orthodontic force application with fixed orthodontic appliances did not change the final levels (on day 7) of antioxidant status or oxidative stress markers, except for MPO in saliva.

Role of nitric oxide in orthodontic tooth movement (Review)

Nitric oxide (NO) is an ubiquitous signaling molecule that mediates numerous cellular processes associated with cardiovascular, nervous and immune systems. NO also plays an essential role in bone homeostasis regulation. The present review article summarized the effects of NO on bone metabolism during orthodontic tooth movement in order to provide insight into the regulatory role of NO in orthodontic tooth movement. Orthodontic tooth movement is a process in which the periodontal tissue and alveolar bone are reconstructed due to the effect of orthodontic forces. Accumulating evidence has indicated that NO and its downstream signaling molecule, cyclic guanosine monophosphate (cGMP), mediate the mechanical signals during orthodontic-related bone remodeling, and exert complex effects on osteogenesis and osteoclastogenesis. NO has a regulatory effect on the cellular activities and functional states of osteoclasts, osteocytes and periodontal ligament fibroblasts involved in orthodontic tooth movement. Variations of NO synthase (NOS) expression levels and NO production in periodontal tissues or gingival crevicular fluid (GCF) have been found on the tension and compression sides during tooth movement in both orthodontic animal models and patients. Furthermore, NO precursor and NOS inhibitor administration increased and reduced the tooth movement in animal models, respectively. Further research is required in order to further elucidate the underlying mechanisms and the clinical application prospect of NO in orthodontic tooth movement.

Physiopathology of nitric oxide in the oral environment and its biotechnological potential for new oral treatments: a literature review

OBJECTIVES

A narrative review on the NO properties and their relationship with the oral environment describing NO's molecular origin, role, and perspectives regarding oral pathological, physiological, and regenerative processes for future applications and possible use as prevention or treatment in dentistry.

MATERIALS AND METHODS

Pubmed was searched using the word "nitric oxide." Reviews, clinical studies, and experimental studies were eligible for the screening process. Similar search procedures were then performed with the additional search words "conservative dentistry," "orthodontics," "endodontics," "implants," "periodontics," "oral cancer," "pulp revascularization," and "oral surgery." Furthermore, references of included articles were examined to identify further relevant articles.

RESULTS

There is a relationship between NO production and oral diseases such as caries, periodontal diseases, pulp inflammation, apical periodontitis, oral cancer, with implants, and orthodontics. Studies on this relationship and uses of NO, in diagnosis, prevention, and treatment, are being developed. Also, some NO and oral cavity patents have already registered.

CONCLUSIONS

The understanding of how NO can interfere in oral health maintenance or disease processes can contribute to elucidate the disease development and optimize treatment approaches.

CLINICAL RELEVANCE

NO has considerable biotechnological potential and can contribute to improving diagnostics and treating the oral environment. As a biomarker, NO has an important role in the early diagnosis of diseases. Regarding treatments, NO can possibly be used as a regulator of inflammation, anti-biofilm action, replacing antibiotics, inducing apoptosis of cancerous cells, and contributing to the angiogenesis. All these studies are initial considerations regarding the relationship between NO and dentistry.

Compression and tension variably alter Osteoprotegerin expression via miR-3198 in periodontal ligament cells

Background

Osteoclasts play a critical role in bone resorption due to orthodontic tooth movement (OTM). In OTM, a force is exerted on the tooth, creating compression of the periodontal ligament (PDL) on one side of the tooth, and tension on the other side. In response to these mechanical stresses, the balance of receptor activator of nuclear-factor kappa-B ligand (RANKL) and osteoprotegerin (OPG) shifts to stimulate osteoclastogenesis. However, the mechanism of OPG expression in PDL cells under different mechanical stresses remains unclear. We hypothesized that compression and tension induce different microRNA (miRNA) expression profiles, which account for the difference in OPG expression in PDL cells.

To study miRNA expression profiles resulting from OTM, compression force (2 g/cm²) or tension force (15% elongation) was applied to immortalized human PDL (HPL) cells for 24 h, and miRNA extracted. The miRNA expression in each sample was analyzed using a human miRNA microarray, and the changes of miRNA expression were confirmed by real-time RT-PCR. In addition, miR-3198 mimic and inhibitor were transfected into HPL cells, and OPG expression and production assessed.

Results

We found that certain miRNAs were expressed differentially under compression and tension. For instance, we observed that miR-572, − 663, − 575, − 3679-5p, UL70-3p, and − 3198 were upregulated only by compression. Real-time RT-PCR confirmed that compression induced miR-3198 expression, but tension reduced it, in HPL cells. Consistent with previous reports, OPG expression was reduced by compression and induced by tension, though RANKL was induced by both compression and tension. OPG expression was upregulated by miR-3198 inhibitor, and was reduced by miR-3198 mimic, in HPL cells. We observed that miR-3198 inhibitor rescued the compression-mediated downregulation of OPG. On the other hand, miR-3198 mimic reduced OPG expression under tension. However, RANKL expression was not affected by miR-3198 inhibitor or mimic.

Conclusions

We conclude that miR-3198 is upregulated by compression and is downregulated by tension, suggesting that miR-3198 downregulates OPG expression in response to mechanical stress.

Expression of biological mediators during orthodontic tooth movement: A systematic review

OBJECTIVES

The aim of the present systematic review was to offer a timeline of the events taking place during orthodontic tooth movement(OTM).

MATERIALS AND METHODS

Electronic databases PubMed, Web of Science and EMBASE were searched up to November 2017. All studies describing the expression of signaling proteins in the periodontal ligament(PDL) of teeth subjected to OTM or describing the expression of signaling proteins in human cells of the periodontal structures subjected to static mechanical loading were considered eligible for inclusion for respectively the in-vivo or the in-vitro part. Risk of bias assessment was conducted according to the validated SYRCLE's RoB tool for animal studies and guideline for assessing quality of in-vitro studies for in-vitro studies.

RESULTS

We retrieved 7583 articles in the initial electronic search, from which 79 and 51 were finally analyzed. From the 139 protein investigated, only the inflammatory proteins interleukin(IL)-1β, cyclooxygenase(COX)-2 and prostaglandin(PG)-E2, osteoblast markers osteocalcin and runt-related transcription factor(RUNX)2, receptor activator of nuclear factor kappa-B ligand(RANKL) and osteoprotegerin(OPG) and extracellular signal-regulated kinases(ERK)1/2 are investigated in 10 or more studies.

CONCLUSION

The investigated proteins were presented in a theoretical model of OTM. We can conclude that the cell activation and differentiation and recruitment of osteoclasts is mediated by osteocytes, osteoblasts and PDL cells, but that the osteogenic differentiation is only seen in stem cell present in the PDL. In addition, the recently discovered Ephrin/Ephs seem to play an role parallel with the thoroughly investigated RANKL/OPG system in mediating bone resorption during OTM.

Inherent physical characteristics and gene expression differences between alveolar and basal bones

OBJECTIVES

The objective of this study was to evaluate the baseline differences between alveolar and basal areas of the rat mandible.

STUDY DESIGN

Rat mandibular alveolar and basal bones were evaluated using histology and micro-computed tomography to compare osteocyte number as well as bone density and architecture and polymerase chain reaction to measure gene expression levels.

RESULTS

Micro-computed tomography data indicated that basal bone is denser and less porous than alveolar bone. Histologic analysis showed that alveolar bone has more osteocytes per unit area compared with basal bone. Real-time polymerase chain reaction results showed higher levels of expression of the following genes in basal bone than in alveolar bone: SOST, E-11, DMP-1, and MEPE.

CONCLUSIONS

Three of these gene products are associated with mature osteocytes, and this suggests that basal bone has more mature osteocyte phenotypes compared with alveolar bone. These findings are suggestive of fewer bone mineralization units and therefore a slower remodeling rate.

Production and physiological role of NO in the oral cavity

Nitric oxide (NO) is a free radical which is produced from a wide variety of cells and tissues in the human body. NO is involved in the regulation of many physiological processes, such as vascular relaxation, neurotransmission, immune regulation, and cell death. NO is generated by nitric oxide synthase (NOS), which has three identified isoforms: neuronal type NOS (nNOS), endothelial type NOS (eNOS), and inducible type NOS (iNOS). Different isoforms are expressed depending on the organs, tissues, and cells, and investigation of the types and functions of enzymes expressed in various tissues is underway. The oral cavity is a space in which marked changes have been detected in NO levels, and each tissue is constantly influenced by NO. NO is a component of saliva and is produced by oral bacteria in the oral cavity and released by NOS expressed in oral mucosa. NOS isoforms expressed under normal conditions differ among the oral organs. In addition, the overexpression of NOS was involved in carcinogenesis and tumor growth progression. This review summarized the expression of NOS and functions of NO in oral cavity organs, and their roles in diseases and the influences of treatments.

Prevention of root resorption in hypofunctional teeth by occlusal function recovery

OBJECTIVE

To clarify whether occlusal hypofunction is one of the key determinants for root resorption during tooth movement and root resorption is prevented by its recovery.

MATERIALS AND METHODS

The rats were randomly divided into one control and two experimental groups: hypofunctional and recovery groups. In the hypofunctional group, an anterior metal cap and bite plate were attached to the maxillary and mandibular incisors to simulate occlusal hypofunction. In the recovery group, the appliances were removed 7 weeks after their use, and the rats were allowed to bite for 4 weeks after removal. At the age of 16 weeks, the upper first molars were moved and after 0, 7, 14, and 21 days, the maxillae were resected. The resorption area was quantified morphohistologically and tartrate-resistant acid phosphatase (TRAP)-positive cells on the root surface were counted. We also examined the expressions of receptor activator of nuclear factor-κB ligand (RANKL), macrophage-colony stimulating factor (M-CSF), and interleukin (IL)-8 immunohistochemically.

RESULTS

The amount of root resorption and the number of TRAP-positive cells were significantly greater in the hypofunctional group than in the control and recovery groups. Moreover, immunoreactivity for RANKL, M-CSF, and IL-8 was detected in the periodontal ligament and on the root surface in the hypofunctional group.

CONCLUSION

Occlusal hypofunction is one of the critical factors for root resorption; however, root resorption may be prevented by recovery of occlusal function.

Nitric oxide in human gingival crevicular fluid after orthodontic force application

Nitric oxide (NO) is involved in bone remodelling and has been shown to play a role in regulating the rate of orthodontic tooth movement (OTM) in rat models. In humans, however, the role of NO in OTM remains less clear. In this study, NO concentration in gingival crevicular fluid (GCF) was measured in patients undergoing orthodontic treatment. Thirteen male participants (ages 11-18 years) planned for non-extraction fixed orthodontic therapy were recruited. Samples of GCF were collected from each maxillary central incisor and first and second molar immediately before (T0), 1h after (T1), and 3-4 days after (T2) application of light orthodontic forces. The maxillary second molars were not included in the appliance and served as controls. Measureable NO levels were consistently obtained from all sampled sites. Total NO levels showed significantly higher NO levels (p<0.05) at T1 at the buccal surfaces of the central incisors when compared to the first and second molars. The results indicate a possible role for NO in OTM at the pressure sites of incisors at early time points. Further studies are required to determine whether NO levels in the periodontal ligament tissues of human teeth during OTM are affected by a force gradient and the magnitude of the applied force.

Evaluation of oxidative stress biomarkers in patients with fixed orthodontic appliances

AIM

The aim of this study was to examine the changes in the levels of interleukine-1 beta (IL-1 β ), tumor necrosis factor alpha (TNF- α ), malondialdehyde (MDA), nitric oxide (NO), and 8-hydroxydeoxyguanosine (8-OHdG) in saliva and IL-1 β , TNF- α , and NO in gingival crevicular fluid (GCF) samples of patients with fixed orthodontic appliances.

MATERIAL AND METHOD

The subject population consisted of 50 volunteers who were in need of orthodontic treatment with fixed orthodontic appliances. GCF and saliva samples were obtained from all individuals before treatment, at 1st month of treatment and at 6th month of treatment. Periodontal clinical parameters were measured. Samples were investigated to detect IL-1 β , TNF- α , and 8-OHdG levels using ELISA method and NO and MDA levels using spectrophotometric method.

RESULTS

Since IL-1 β level detected in GCF at the 6th month of orthodontic treatment is statistically significant according to baseline (P < 0.05), all other biochemical parameters detected both in saliva and in GCF did not show any significant change at any measurement periods.

CONCLUSION

Orthodontic tooth movement and orthodontic materials used in orthodontic treatment do not lead to a change above the physiological limits that is suggestive of oxidative damage in both GCF and saliva.

Tooth eruption results from bone remodelling driven by bite forces sensed by soft tissue dental follicles: a finite element analysis

Intermittent tongue, lip and cheek forces influence precise tooth position, so we here examine the possibility that tissue remodelling driven by functional bite-force-induced jaw-strain accounts for tooth eruption. Notably, although a separate true 'eruptive force' is widely assumed, there is little direct evidence for such a force. We constructed a three dimensional finite element model from axial computerized tomography of an 8 year old child mandible containing 12 erupted and 8 unerupted teeth. Tissues modelled included: cortical bone, cancellous bone, soft tissue dental follicle, periodontal ligament, enamel, dentine, pulp and articular cartilage. Strain and hydrostatic stress during incisive and unilateral molar bite force were modelled, with force applied via medial and lateral pterygoid, temporalis, masseter and digastric muscles. Strain was maximal in the soft tissue follicle as opposed to surrounding bone, consistent with follicle as an effective mechanosensor. Initial numerical analysis of dental follicle soft tissue overlying crowns and beneath the roots of unerupted teeth was of volume and hydrostatic stress. To numerically evaluate biological significance of differing hydrostatic stress levels normalized for variable finite element volume, 'biological response units' in Nmm were defined and calculated by multiplication of hydrostatic stress and volume for each finite element. Graphical representations revealed similar overall responses for individual teeth regardless if incisive or right molar bite force was studied. There was general compression in the soft tissues over crowns of most unerupted teeth, and general tension in the soft tissues beneath roots. Not conforming to this pattern were the unerupted second molars, which do not erupt at this developmental stage. Data support a new hypothesis for tooth eruption, in which the follicular soft tissues detect bite-force-induced bone-strain, and direct bone remodelling at the inner surface of the surrounding bony crypt, with the effect of enabling tooth eruption into the mouth.

Occlusal hypofunction causes periodontal atrophy and VEGF/VEGFR inhibition in tooth movement

OBJECTIVE

To examine changes in microvasculature and the expression of vascular endothelial growth factor A (VEGF-A) and VEGF receptor 2 (VEGFR-2) in rat hypofunctional periodontal ligament (PDL) during experimental tooth movement.

MATERIALS AND METHODS

Twelve-week-old male Sprague-Dawley rats were divided into normal occlusion and occlusal hypofunction groups. After a 2-week bite-raising period, rat first molar was moved mesially using a 10-gf titanium-nickel alloy closed coil spring in both groups. On days 0, 1, 2, 3, and 7 after tooth movement, histologic changes were examined by micro-computed tomography and immunohistochemistry using CD31, VEGF-A, VEGFR-2, and the terminal deoxynucleotidyl transferase mediated dUTP nick end labeling (TUNEL) method.

RESULTS

Hypofunctional molars inclined more than normal molars and did not move notably after day 1 of tooth movement. Blood vessels increased on the tension side of the PDL in normal teeth. Immunoreactivities for VEGF-A and VEGFR-2 in normal teeth were greater than those in hypofunctional teeth during tooth movement. Compressive force rapidly caused apoptosis of the PDL and vascular endothelial cells in hypofunctional teeth, but not in normal teeth.

CONCLUSIONS

Occlusal hypofunction induces vascular constriction through a decrease in the expression of VEGF-A and VEGFR-2, and apoptosis of the PDL and vascular cells occurs during tooth movement.

Sinusoidal electromagnetic field stimulates rat osteoblast differentiation and maturation via activation of NO-cGMP-PKG pathway

Nitric oxide (NO) is an important intracellular and intercellular messenger, critically affecting bone metabolism. The purpose of this research is to investigate whether the effect of sinusoidal electromagnetic field (SEMF) on the differentiation and maturation of osteoblasts is mediated by the NO-cGMP-PKG signal pathway. We examined the impact of SEMF on nitric oxide synthase (NOS) activity, and found that L-NAME, nitric oxide synthase's inhibitor, prevents SEMF-mediated increase in NOS activity and NO levels. We showed that an inhibitor of soluble guanylyl cyclase (ODQ) blocks the increase in cGMP levels triggered by exposure to SEMF. The inhibitor PDE5, which hydrolyzes 3',5'-cyclic-GMP to 5'-GMP, prevents the SEMF's stimulation of PKG activity. We also blocked the NO-cGMP-PKG pathway to determine whether the maturation and mineralization of osteoblasts, stimulated by SEMF, would be inhibited. This was evaluated by measuring alkaline phosphatase (ALP) activity, osterix gene expression and mineralized bone modulus. After treatment with SEMF, the NOS activity increases in comparison with the control group (P<0.01), reaching the highest level after 0.5h. Osterix gene expression, ALP activity and mineralized bone nodules in the SEMF experimental group also increase significantly. However, these effects are partially blocked in the L-NAME treated cultures. Surprisingly, all the osteogenic markers in the SEMF+L-NAME group were slightly higher than in the control culture, but lower than in the cells exposed to SEMF only. We conclude that the NO-cGMP-PKG signal pathway is activated by SEMF treatment, the stimulatory effect of SEMF on the differentiation and mineralization of osteoblasts is attenuated when the pathway is blocked.

Molecular markers of early orthodontic tooth movement

OBJECTIVE

To understand the molecular basis of early orthodontic tooth movement by looking at the expression of KI-67, runt-related transcription factor 2 (Runx2), and tumor necrosis factor ligand superfamily member 11 (RANKL) proteins.

MATERIALS AND METHODS

We employed a rat model of early orthodontic tooth movement using a split-mouth design (where contralateral side serves as a control) and performed immunohistochemical staining to map the spatial expression patterns of three proteins at 3 and 24 hours after appliance insertion.

RESULTS

We observed increased expression of KI-67, a proliferation marker, and RANKL, a molecule associated with osteoclastic differentiation, in the compression sites of the periodontal ligament subjected to 3 hours of force. In contrast, there was increased expression of KI-67 and Runx2, a marker of osteoblast precursors, in tension areas after 24 hours of force. Decreased KI-67 expression in the mesial and distal regions of the periodontal ligament was observed at the midpoint of the tooth root.

CONCLUSIONS

The early RANKL expression indicates that at this early stage cells are involved in osteoclast precursor signaling. Also, decreased KI-67 expression found near the midpoint of the tooth root is believed to represent the center of rotation, providing a molecular means of visualizing mechanical loading patterns.