Bone Response of Loaded Periodontal Ligament

Potential Application of Non-Invasive Optical Imaging Methods in Orthodontic Diagnosis

During orthodontic treatment, the early diagnosis of microscopic changes in soft and hard tissues, including periodontal tissue, is very important to prevent iatrogenic side effects like root resorption and periodontal diseases. Cervical periodontal tissue is the most critical area that reacts first to mal-habits or orthodontic forces, and it is also the place where bacteria deposits in the early stage of periodontal diseases. The early diagnosis of hard tissue changes, such as demineralization, is also very important in maintaining a patient's health during orthodontic treatment. Many diagnostic devices, including radiographic equipment and intra-oral scanners, are helpful in diagnosing these problems, but have certain limitations in invasiveness and precision. The purpose of this study is to verify the possible utilities of non-invasive diagnostic devices in the orthodontic field that can compensate for these limitations. For this, non-invasive optical diagnostic devices, including optical coherence tomography and optical Doppler tomography, were used in vivo with animal and human examination for hard and soft tissues. These devices can provide real-time three-dimensional images at the histological scale. The results of this study verified these devices can be used in clinical practice during orthodontic treatment and introduced a new diagnostic paradigm differentiating microstructural changes in tissues in orthodontic diagnosis.

Compression Promotes the Osteogenic Differentiation of Human Periodontal Ligament Stem Cells by Regulating METTL14-mediated IGF1

BACKGROUND AND OBJECTIVES

Orthodontic treatment involves the application of mechanical force to induce periodontal tissue remodeling and ultimately promote tooth movement. It is essential to study the response mechanisms of human periodontal ligament stem cells (hPDLSCs) to improve orthodontic treatment.

METHODS

In this study, hPDLSCs treated with compressive force were used to simulate orthodontic treatment. Cell viability and cell death were assessed using the CCK-8 assay and TUNEL staining. Alkaline phosphatase (ALP) and alizarin red staining were performed to evaluate osteogenic differentiation. The binding relationship between IGF1 and METTL14 was assessed using RIP and dual-luciferase reporter assays.

RESULTS

The compressive force treatment promoted the viability and osteogenic differentiation of hPDLSCs. Additionally, m6A and METTL14 levels in hPDLSCs increased after compressive force treatment, whereas METTL14 knockdown decreased cell viability and inhibited the osteogenic differentiation of hPDLSCs treated with compressive force. Furthermore, the upregulation of METTL14 increased m6A levels, mRNA stability, and IGF1 expression. RIP and dual-luciferase reporter assays confirmed the interaction between METTL14 and IGF1. Furthermore, rescue experiments demonstrated that IGF1 overexpression reversed the effects of METTL14 knockdown in hPDLSCs treated with compressive force.

CONCLUSIONS

In conclusion, this study demonstrated that compressive force promotes cell viability and osteogenic differentiation of hPDLSCs by regulating IGF1 levels mediated by METTL14.

Functional Load Capacity of Teeth with Reduced Periodontal Support: A Finite Element Analysis

The purpose of this study was to investigate the functional load capacity of the periodontal ligament (PDL) in a full arch maxilla and mandible model using a numerical simulation. The goal was to determine the functional load pattern in multi- and single-rooted teeth with full and reduced periodontal support. CBCT data were used to create 3D models of a maxilla and mandible. The DICOM dataset was used to create a CAD model. For a precise description of the surfaces of each structure (enamel, dentin, cementum, pulp, PDL, gingiva, bone), each tooth was segmented separately, and the biomechanical characteristics were considered. Finite Element Analysis (FEA) software computed the biomechanical behavior of the stepwise increased force of 700 N in the cranial and 350 N in the ventral direction of the muscle approach of the masseter muscle. The periodontal attachment (cementum-PDL-bone contact) was subsequently reduced in 1 mm increments, and the simulation was repeated. Quantitative (pressure, tension, and deformation) and qualitative (color-coded images) data were recorded and descriptively analyzed. The teeth with the highest load capacities were the upper and lower molars (0.4-0.6 MPa), followed by the premolars (0.4-0.5 MPa) and canines (0.3-0.4 MPa) when vertically loaded. Qualitative data showed that the areas with the highest stress in the PDL were single-rooted teeth in the cervical and apical area and molars in the cervical and apical area in addition to the furcation roof. In both single- and multi-rooted teeth, the gradual reduction in bone levels caused an increase in the load on the remaining PDL. Cervical and apical areas, as well as the furcation roof, are the zones with the highest functional stress. The greater the bone loss, the higher the mechanical load on the residual periodontal supporting structures.

3D Electrospun Polycaprolactone Scaffolds to Assess Human Periodontal Ligament Cells Mechanobiological Behaviour

While periodontal ligament cells are sensitive to their 3D biomechanical environment, only a few 3D in vitro models have been used to investigate the periodontal cells mechanobiological behavior. The objective of the current study was to assess the capability of a 3D fibrous scaffold to transmit a mechanical loading to the periodontal ligament cells. Three-dimensional fibrous polycaprolactone (PCL) scaffolds were synthetized through electrospinning. Scaffolds seeded with human periodontal cells (10³ mL^-1) were subjected to static (n = 9) or to a sinusoidal axial compressive loading in an in-house bioreactor (n = 9). At the end of the culture, the dynamic loading seemed to have an influence on the cells' morphology, with a lower number of visible cells on the scaffolds surface and a lower expression of actin filament. Furthermore, the dynamic loading presented a tendency to decrease the Alkaline Phosphatase activity and the production of Interleukin-6 while these two biomolecular markers were increased after 21 days of static culture. Together, these results showed that load transmission is occurring in the 3D electrospun PCL fibrous scaffolds, suggesting that it can be used to better understand the periodontal ligament cells mechanobiology. The current study shows a relevant way to investigate periodontal mechanobiology using 3D fibrous scaffolds.

Effect of tension and compression on dynamic alveolar histomorphometry

Here, we tested the hypothesis that tensile and compressive stresses generated in the alveolar bone proper regulate site-specific cellular and functional changes in osteoclasts and osteoblasts. Thirty-two 13-week-old male mice were randomly divided into four groups: two experimental groups with vertical loading obliquely from the palatal side to the buccal side of the maxillary molar (0.9 N) 30 min per day for 8 or 15 days and unloaded controls (n = 8). Calcein and alizarin were administered 8 and 2 days before euthanization, respectively, to detect the time of bone formation. Undecalcified sections parallel to the occlusal plane were prepared on the palatal root and the surrounding alveolar bone in the middle of the root length. The alveolar perimeter was divided into 12 equal regions for site analysis, and the bone histomorphometric parameters were obtained for each region. Data from in vivo microfocus computed tomography were used to construct animal-specific finite element models. 2D stress distribution images were overlain on histology images obtained from the same location. Significant differences in the total perimeter between groups and between loading and unloading in each region were statistically analyzed (α = 0.05). Osteoclast counts and the alizarin label ratio were significantly higher in the loaded group than in the unloaded group in regions where the maximum von Mises and principal tensile stresses were the highest along the perimeter. The label ratio of calcein was significantly lower in the 8-day loaded group than in the unloaded group, indicating that the calcein-labeled surface was resorbed by osteoclasts that appeared during the loading period. The effect of loading was mitigated by an increase in the maximum principal compressive stress. We conclude that bone resorption and bone formation are functions of site-specific tension and compression in the alveolar bone proper, confirming our hypothesis. This finding is critical for the advancement of diagnosis and treatment planning in clinical dentistry.

Advances in the Study of the Mechanisms of Physiological Root Resorption in Deciduous Teeth

Physiological root resorption of deciduous teeth is a complex physiological process that is essential for the normal replacement of deciduous teeth and permanent teeth in clinical practice, but its importance is often overlooked due to the presence of permanent teeth. This physiological process includes not only the resorption of hard tissues of deciduous teeth, such as dentin and cementum, but also the elimination of soft tissues, such as pulp and periodontal ligament (PDL). However, the mechanisms of physiological root resorption are not yet clear. In this article, the advances of research on the mechanisms related to physiological root resorption will be reviewed in two main aspects: hard tissues and soft tissues of deciduous teeth, specifically in relation to the effects of inflammatory microenvironment and mechanical stress on the resorption of hard tissues, the repair of hard tissues, and the elimination and the histological events of soft tissues.

The role of inhibition of osteocyte apoptosis in mediating orthodontic tooth movement and periodontal remodeling: a pilot study

BACKGROUND

Orthodontic tooth movement (OTM) has been shown to induce osteocyte apoptosis in alveolar bone shortly after force application. However, how osteocyte apoptosis affects orthodontic tooth movement is unknown. The goal of this study was to assess the effect of inhibition of osteocyte apoptosis on osteoclastogenesis, changes in the alveolar bone density, and the magnitude of OTM using a bisphosphonate analog (IG9402), a drug that affects osteocyte and osteoblast apoptosis but does not affect osteoclasts.

MATERIAL AND METHODS

Two sets of experiments were performed. Experiment 1 was used to specifically evaluate the effect of IG9402 on osteocyte apoptosis in the alveolar bone during 24 h of OTM. For this experiment, twelve mice were divided into two groups: group 1, saline administration + OTM_24-h (n=6), and group 2, IG9402 administration + OTM_24-h (n=6). The contralateral unloaded sides served as the control. The goal of experiment 2 was to evaluate the role of osteocyte apoptosis on OTM magnitude and osteoclastogenesis 10 days after OTM. Twenty mice were divided into 4 groups: group 1, saline administration without OTM (n=5); group 2, IG9402 administration without OTM (n=5); group 3, saline + OTM_10-day (n=6); and group 4, IG9402 + OTM_10-day (n=4). For both experiments, tooth movement was achieved using Ultra Light (25g) Sentalloy Closed Coil Springs attached between the first maxillary molar and the central incisor. Linear measurements of tooth movement and alveolar bone density (BVF) were assessed by MicroCT analysis. Cell death (or apoptosis) was assessed by terminal dUTP nick-end labeling (TUNEL) assay, while osteoclast and macrophage formation were assessed by tartrate-resistant acid phosphatase (TRAP) staining and F4/80+ immunostaining.

RESULTS

We found that IG9402 significantly blocked osteocyte apoptosis in alveolar bone (AB) at 24 h of OTM. At 10 days, IG9402 prevented OTM-induced loss of alveolar bone density and changed the morphology and quality of osteoclasts and macrophages, but did not significantly affect the amount of tooth movement.

CONCLUSION

Our study demonstrates that osteocyte apoptosis may play a significant role in osteoclast and macrophage formation during OTM, but does not seem to play a role in the magnitude of orthodontic tooth movement.

Shear Stress Modulates Osteoblast Cell and Nucleus Morphology and Volume

Mechanical loading preserves bone mass and function—yet, little is known about the cell biological basis behind this preservation. For example, cell and nucleus morphology are critically important for cell function, but how these morphological characteristics are affected by the physiological mechanical loading of bone cells is under-investigated. This study aims to determine the effects of fluid shear stress on cell and nucleus morphology and volume of osteoblasts, and how these effects relate to changes in actin cytoskeleton and focal adhesion formation. Mouse calvaria 3T3-E1 (MC3T3-E1) osteoblasts were treated with or without 1 h pulsating fluid flow (PFF). Live-cell imaging was performed every 10 min during PFF and immediately after PFF. Cytoskeletal organization and focal adhesions were visualized, and gene and protein expression quantified. Two-dimensional (2D) and three-dimensional (3D) morphometric analyses were made using MeasureStack and medical imaging interaction toolkit (MITK) software. 2D-images revealed that 1 h PFF changed cell morphology from polygonal to triangular, and nucleus morphology from round to ellipsoid. PFF also reduced cell surface area (0.3-fold), cell volume (0.3-fold), and nucleus volume (0.2-fold). During PFF, the live-cell volume gradually decreased from 6000 to 3000 µm³. After PFF, α-tubulin orientation was more disorganized, but F-actin fluorescence intensity was enhanced, particularly around the nucleus. 3D-images obtained from Z-stacks indicated that PFF increased F-actin fluorescence signal distribution around the nucleus in the XZ and YZ direction (2.3-fold). PFF increased protein expression of phospho-paxillin (2.0-fold) and integrin-α5 (2.8-fold), but did not increase mRNA expression of paxillin-a (PXNA), paxillin-b (PXNB), integrin-α5 (ITGA51), or α-tubulin protein expression. In conclusion, PFF induced substantial changes in osteoblast cytoskeleton, as well as cell and nucleus morphology and volume, which was accompanied by elevated gene and protein expression of adhesion and structural proteins. More insights into the mechanisms whereby mechanical cues drive morphological changes in bone cells, and thereby, possibly in bone cell behavior, will aid the guidance of clinical treatment, particularly in the field of orthodontics, (oral) implantology, and orthopedics.

CREB activation affects mesenchymal stem cell migration and differentiation in periodontal tissues due to orthodontic force

During the orthodontic tooth movement, cells in periodontal ligament could differentiate into osteoblasts to synthesize alveolar bone as well as affect the proliferation, migration and differentiation of mesenchymal stem cells, which also contribute to bone remodeling. However, the mechanism is still largely elusive. Here, we evaluated the expression of CREB at the tension site of mouse periodontal ligament under orthodontic mechanical strain and in the cyclic tension strain treated human periodontal ligament cells. Then, through gain and loss of function analysis, we revealed that CREB in PDLCs promotes SDF-1 and FGF2 secretion, which enhance the migration and osteoblastic differentiation of BMSCs. We further discovered that CREB transcriptionally activates FGF2 and SDF-1 expressions by binding to the promoter regions.In conclusion, this study confirms that CREB is an upregulated gene in periodontal ligament under orthodontic tension strain stimulation and plays an important role in regulating BMSCs' physiological activity in orthodontic tension strain-induced bone formation.

Immunorthodontics: in vivo gene expression of orthodontic tooth movement

Orthodontic tooth movement (OTM) is a “sterile” inflammatory process. The present study aimed to reveal the underlying biological mechanisms, by studying the force associated-gene expression changes, in a time-dependent manner. Ni-Ti springs were set to move the upper 1^st-molar in C57BL/6 mice. OTM was measured by μCT. Total-RNA was extracted from tissue blocks at 1,3,7 and 14-days post force application, and from two control groups: naïve and inactivated spring. Gene-expression profiles were generated by next-generation-RNA-sequencing. Gene Set Enrichment Analysis, K-means algorithm and Ingenuity pathway analysis were used for data interpretation. Genes of interest were validated with qRT-PCR. A total of 3075 differentially expressed genes (DEGs) were identified, with the greatest number at day 3. Two distinct clusters patterns were recognized: those in which DEGs peaked in the first days and declined thereafter (tissue degradation, phagocytosis, leukocyte extravasation, innate and adaptive immune system responses), and those in which DEGs were initially down-regulated and increased at day 14 (cell proliferation and migration, cytoskeletal rearrangement, tissue homeostasis, angiogenesis). The uncovering of novel innate and adaptive immune processes in OTM led us to propose a new term “Immunorthodontics”. This genomic data can serve as a platform for OTM modulation future approaches.

Role of autophagy in the periodontal ligament reconstruction during orthodontic tooth movement in rats

Background/purpose

Autophagy, a lysosome-based degradation pathway that is reportedly activated by mechanical stress and nutrient deprivation, plays an important role in various physiological and pathological events. The present study investigated the level of autophagy and tumor necrosis factor-α(TNF-α) expression in the periodontal ligaments (PDLs) of Sprague-Dawley (SD) rats to analyze the involvement of autophagy and inflammatory cytokines in orthodontic tooth movement (OTM) and maintaining periodontal tissue homeostasis.

Materials and methods

SD rats (n = 100) were randomly divided into a control group (n = 10) and an experimental group (n = 90). An orthodontic appliance was placed in each rat in the experimental group, and 10 rats were randomly euthanized 15 min, 30 min, 1 h, 2 h, 4 h, 12 h, 1 d, 3 d and 7 d after mechanical loading. The OTM distance was then measured. Hematoxylin and eosin (HE) staining was used to analyze the morphology of the PDL. Immunohistochemical (IHC) staining and tartrate-resistant acid phosphatase (TRAP) staining were also performed.

Results

After the application of orthodontic force and under the dual effects of mechanical force and starvation caused by compressed vessels, the level of autophagy and TNF-α expression in the PDL fluctuated and exhibited a similar trend.

Conclusion

Our data suggest a significant correlation between the initiation of autophagy and TNF-α expression, which both exerted positive effects on PDL remodeling during OTM in rats.

Local orthodontic force initiates widespread remodelling of the maxillary alveolar bone

Objectives

To clarify the effects of a local orthodontic force on alveolar bone by analysing bone remodelling in different regions of the maxilla during orthodontic tooth movement (OTM).

Methods

An OTM model was established in rats. Histological changes in the maxilla were analysed using TRAP staining, IHC staining for CTSK and haematoxylin and eosin (H and E) staining. The root bifurcation region of the alveolar bone of the first (M1), second (M2) and third (M3) molars were selected as the regions of interest (ROIs), which were further divided into a cervical and an apical level. Sequential fluorochrome labelling was performed to analyse bone deposition rates.

Results

The maxillary left first molars were moved mesially. TRAP staining and IHC staining for CTSK showed orthodontic force increased osteoclast numbers in all six ROIs at both the cervical and apical levels. H and E staining indicated elevated osteoblast numbers in the OTM group in all induced regions. Sequential fluorochrome labelling exhibited increased bone deposition rates around M1, M2 and M3 in the OTM group.

Conclusions

An orthodontic force applied to the first molar could initiate widespread remodelling of the maxillary alveolar bone, which was not restricted to the tension and pressure sites. This may revise the orthodontic biomechanical theory and provide new insights for clinical work.

[Expression of autophagy-related protein Beclin-1 and microtubule-associated protein 2 light chain 3 in periodontal ligament cells in orthodontic tooth pressure areas]

OBJECTIVE

To investigate the expression of autophagy-related protein Beclin-1 and microtubule-associated protein 2 light chain 3 (LC3Ⅱ) in periodontal ligament cells in orthodontic tooth pressure areas.

METHODS

Sixty male SD rats were randomly divided into a blank control group and nine experimental groups. In the experimental groups, 0.392 N orthodontic force was used to move the first right upper molars for 15 min, 30 min, 1 h, 2 h, 4 h, 12 h, 1 d, 3 d, or 7 d. The blank control group did not receive any treatment. The rats were euthanized. Changes in the morphology of the periodontal membrane in the pressure areas were observed through hematoxylin and eosin (HE) staining. The expression levels of Beclin-1 and LC3Ⅱ were detected by immunohistochemical staining, and tartrate-resistant acid phosphatase (TRAP) staining was performed for the counting of osteoclasts.

RESULTS

The HE stains showed that the hyalinization of the periodontal ligament appeared in the pressure areas after 1 day of exertion and was gradually aggravated. The immunohistochemical stains showed that the expression levels of Beclin-1 and LC3Ⅱ in the experimental groups gradually increased, peaked after 1 h, and then gradually decreased. The expression levels peaked again after 1 d, then decreased to baseline levels at 7 d of exertion. Beclin-1 and LC3Ⅱ were expressed in the osteoclasts. The TRAP stains indicated that the number of osteoclasts started to increase after 1 day.

CONCLUSIONS

Autophagy may participate in the process of periodontal ligament reconstruction in orthodontic tooth pressure areas by mediating the hyalinization of periodontal ligament and affecting the biological effects of osteoclasts.

Effect of high-frequency vibration on orthodontic tooth movement and bone density

OBJECTIVES:

Previous reports have shown that high-frequency vibration can increase bone remodeling and accelerate tooth movement. The aim of this study was to evaluate the effects of high-frequency vibration on treatment phase tooth movement, and post-treatment bone density at initiation of retention, with cone-beam computed tomography (CBCT).

MATERIALS AND METHODS:

Thirty patients with initial Class I skeletal relationships, initial minimum-moderate crowding (3–5 mm), treated to completion with clear aligners and adjunctive high-frequency vibration, (HFV group) or no vibration, (Control group) were evaluated. The patients were instructed to change aligners as soon as they become loose. Changes in bone density associated with orthodontic treatment were evaluated using i-CAT cone-beam computed tomography (CBCT) and InVivo Anatomage^® software to quantify density using Hounsfield units (HU) between treated teeth in 10 different regions. HU values were averaged and compared against baseline (T1) and between the groups at initiation of retention (T2).

RESULTS:

The average time for aligner change was 5.2 days in the HFV group, and 8.7 days in the control group (P = 0.0001). There was significant T1 to T2 increase of HU values in the upper arch (P = 0.0001) and the lower arch (P = 0.008) in the HFV group. There was no significant change in average HU values in the upper (P = 0.83) or lower arches (P = 0.33) in the control group. The intergroup comparison revealed a significant difference in the upper, (P = 0.0001) and lower arches (P = 0.007).

CONCLUSION:

High-frequency vibration adjunctive to clear aligners, allowed early aligner changes that led to shorter treatment time in minimum-moderate crowded cases. At initiation of retention, the HFV group demonstrated statistically significant increase as compared with pre-treatment bone density, whereas control subjects showed no significant change from pre-treatment bone density.

A porous fibrous hyperelastic damage model for human periodontal ligament: Application of a microcomputerized tomography finite element model

The periodontal ligament (PDL) is a soft biological tissue that connects the tooth with the trabecular bone of the mandible. It plays a key role in load transmission and is primarily responsible for bone resorption and most common periodontal diseases. Although several numerical studies have analysed the biomechanical response of the PDL, most did not consider its porous fibrous structure, and only a few analysed damage to the PDL. This study presents an innovative numerical formulation of a porous fibrous hyperelastic damage material model for the PDL. The model considers two separate softening phenomena: fibre alignment during loading and fibre rupture. The parameters for the material model characterization were fitted using experimental data from the literature. Furthermore, the experimental tests used for characterization were computationally modelled to verify the material parameters. A finite element model of a portion of a human mandible, obtained by microcomputerized tomography, was developed, and the proposed constitutive model was implemented for the PDL. Our results confirm that damage to the PDL may occur mainly because of overpressure of the interstitial fluid, while large forces must be applied to damage the PDL fibrous network. Moreover, this study clarifies some aspects of the relationship between PDL damage and the bone remodelling process.

Non-invasive evaluation of periodontal ligament stiffness during orthodontic tooth movement

OBJECTIVES

To evaluate the longitudinal changes in periodontal ligament (PDL) stiffness during orthodontic tooth movement using the Advanced System for Implant Stability Testing (ASIST).

MATERIALS AND METHODS

ASIST measurements of maxillary canines that were actively retracted into an extraction space were collected approximately once per month for 12 adolescent female patients. The ASIST Stability Coefficient (ASC) values, which are directly related to PDL stiffness, were determined for each visit to examine longitudinal changes for individual canines as they were exposed to different forces (approximately 80 and 150 g) during retraction.

RESULTS

The pattern of longitudinal changes in ASC was similar for both canines (regardless of the two force levels applied) in individual patients and across patients. All patients showed some decrease in ASC, with an average maximum reduction in stiffness of 73.4 ± 7.7%. Some recovery was observed for most patients; however, none of the patients had the PDL stiffness return to the pre-treatment value at the final measurement appointment which was some time close after space closure was completed. On average, the ASC value at the final measured visit was 48.1 ± 12.2% of the initial value. No measurements are available after removal of orthodontic appliances and during retention.

CONCLUSIONS

The ASIST was able to detect changes in PDL stiffness during orthodontic treatment, providing some insight into the mechanical changes that occur at the tooth root interface.