An insight into the regulatory mechanisms of cells involved in resorption of dental hard tissues

Pathogenesis of non-infection related inflammatory root resorption in permanent teeth: A narrative review

BACKGROUND

The mechanism of action of root resorption in a permanent tooth can be classified as infection-related (e.g., microbial infection) or non-infection-related (e.g., sterile damage). Infection induced root resorption occurs due to bacterial invasion. Non-infection-related root resorption stimulates the immune system through a different mechanism.

OBJECTIVES

The aim of this narrative review is to describe the pathophysiologic process of non-infection-related inflammatory processes involved in root resorption of permanent teeth.

METHODS

A literature search on root resorption was conducted using Scopus (PubMed and Medline) and Google Scholar databases to highlight the pathophysiology of bone and root resorption in non-infection-related situations. The search included key words covering the relevant category. It included in vitro and in vivo studies, systematic reviews, case series, reviews, and textbooks in English. Conference proceedings, lectures and letters to the editor were excluded.

RESULTS

Three types of root resorption are related to the non-infection mechanism of action, which includes surface resorption due to either trauma or excessive orthodontic forces, external replacement resorption and external cervical resorption. The triggers are usually damage associated molecular patterns and hypoxia conditions. During this phase macrophages and clastic cells act to eliminate the damaged tissue and bone, eventually enabling root resorption and bone repair as part of wound healing.

DISCUSSION

The resorption of the root occurs during the inflammatory phase of wound healing. In this phase, damaged tissues are recognized by macrophages and neutrophiles that secrete interlaukines such as TNF-α, IL-1, IL-6, IL-8. Together with the hypoxia condition that accelarates the secretion of growth factors, the repair of the damaged perioduntiom, including damaged bone, is initiated. If the precementum and cementoblast are injured, root resorption can occur.

CONCLUSIONS

Wound healing exhibits different patterns of action that involves immune stimulation in a bio-physiological activity, that occurs in the proper sequence, with overlapping phases. Two pathologic conditions, DAMPs and hypoxia, can activate the immune cells including clastic cells, eliminating damaged tissue and bone. Under certain conditions, root resorption occurs as a side effect.

RANKL+ senescent cells under mechanical stress: a therapeutic target for orthodontic root resorption using senolytics

In dentistry, orthodontic root resorption is a long-lasting issue with no effective treatment strategy, and its mechanisms, especially those related to senescent cells, remain largely unknown. Here, we used an orthodontic intrusion tooth movement model with an L-loop in rats to demonstrate that mechanical stress-induced senescent cells aggravate apical root resorption, which was prevented by administering senolytics (a dasatinib and quercetin cocktail). Our results indicated that cementoblasts and periodontal ligament cells underwent cellular senescence (p21⁺ or p16⁺) and strongly expressed receptor activator of nuclear factor-kappa B (RANKL) from day three, subsequently inducing tartrate-resistant acid phosphatase (TRAP)-positive odontoclasts and provoking apical root resorption. More p21⁺ senescent cells expressed RANKL than p16⁺ senescent cells. We observed only minor changes in the number of RANKL⁺ non-senescent cells, whereas RANKL⁺ senescent cells markedly increased from day seven. Intriguingly, we also found cathepsin K⁺p21⁺p16⁺ cells in the root resorption fossa, suggesting senescent odontoclasts. Oral administration of dasatinib and quercetin markedly reduced these senescent cells and TRAP⁺ cells, eventually alleviating root resorption. Altogether, these results unveil those aberrant stimuli in orthodontic intrusive tooth movement induced RANKL⁺ early senescent cells, which have a pivotal role in odontoclastogenesis and subsequent root resorption. These findings offer a new therapeutic target to prevent root resorption during orthodontic tooth movement.

Orthodontically induced external apical root resorption considerations of root-filled teeth vs vital pulp teeth: a systematic review and meta-analysis

INTRODUCTION

The purpose of this systematic review was to research the difference between root-filled teeth (RFT) and vital pulp teeth (VPT) in orthodontically induced external apical root resorption (EARR) and to offer suggestions for clinicians on therapeutic sequence and timing when considering combined treatment of endodontic and orthodontic.

MATERIALS AND METHODS

An electronic search of published studies was conducted before November 2022 in PubMed, Web of Science and other databases. Eligibility criteria were based on the Population, Intervention, Comparison, Outcome, and Study design (PICOS) framework. RevMan 5.3 software was used for statistical analysis. Single-factor meta-regression analysis was used to explore the sources of literature heterogeneity, and a random effects model was used for analysis.

RESULTS

This meta-analysis comprised 8 studies with 10 sets of data. As there was significant heterogeneity among the studies, we employed a random effects model. The funnel plot of the random effects model exhibited a symmetrical distribution, indicating no publication bias among the included studies. The EARR rate of RFT was significantly lower than that of VPT.

CONCLUSIONS

In the context of concurrent endodontic and orthodontic treatment, priority should be given to endodontic therapy, as it serves as the foundation for subsequent orthodontic procedures. The optimal timing for orthodontic tooth movement post-root canal therapy is contingent upon factors such as the extent of periapical lesion resolution and the degree of dental trauma sustained. A comprehensive clinical assessment is essential in guiding the selection of the most suitable approach for achieving optimal treatment outcomes.

25-Hydroxycholesterol induces odontoclastic differentiation through RANK-RANKL upregulation and NF-κB activation in odontoblast-like MDPC-23 cells: An in vitro study

AIM

The physiological effects and cellular mechanism of 25-hydroxycholesterol (25-HC), which is an oxysterol synthesized from cholesterol by cholesterol-25-hydroxylase (CH25H) expressed under inflammatory conditions, are still largely unknown during odontoclastogenesis. This study aimed to evaluate 25-HC-induced odontoclastogenesis and its cellular mechanisms in odontoblast-like MDPC-23 cells.

METHODOLOGY

To investigate 25-HC-induced odontoclastogenesis of MDPC-23 cells and its cellular mechanism, haemotoxylin and eosin staining, tartrate-resistant acid phosphatase (TRAP) staining, dentine resorption assay, zymography, reactive oxygen species (ROS) detection, immunocytochemistry, and nuclear translocation were performed. The experimental values are presented as mean ± standard deviation and were compared using analysis of variance, followed by post hoc multiple comparisons (Tukey's test) using SPSS software version 22 (IBM Corp.). A p-value <.05 was considered statistically significant.

RESULTS

Lipopolysaccharide or receptor activator of nuclear factor-κB ligand (RANKL) induced the synthesis of 25-HC via the expression of CH25H in MDPC-23 cells (p < .01). Multinucleated giant cells with morphological characteristics and TRAP activity of the odontoclast were increased by 25-HC in MDPC-23 cells (p < .01). Moreover, 25-HC increased dentine resorption through the expression and activity of matrix metalloproteinases in MDPC-23 cells. It not only increased the expression of odontoclastogenic biomarkers but also translocated cytosolic nuclear factor-κB (NF-κB) to the nucleus in MDPC-23 cells. Additionally, 25-HC not only increased the production of ROS (p < .01), expression of inflammatory mediators (p < .01), pro-inflammatory cytokines, receptor activator of NF-κB (RANK), and RANKL but also suppressed the expression of osteoprotegerin (OPG) in MDPC-23 cells. In contrast, CDDO-Me, a chemical NF-κB inhibitor, decreased TRAP activity (p < .01) and downregulated the expression of the odontoclastogenic biomarkers, including RANK and RANKL, in MDPC-23 cells.

CONCLUSION

25-HC induced odontoclastogenesis by modulating the RANK-RANKL-OPG axis via NF-κB activation in MDPC-23 cells. Therefore, these findings provide that 25-HC derived from cholesterol metabolism may be involved in the pathophysiological etiological factors of internal tooth resorption.

PD-L1, a Potential Immunomodulator Linking Immunology and Orthodontically Induced Inflammatory Root Resorption (OIIRR): Friend or Foe?

Orthodontically induced inflammatory root resorption (OIIRR) is considered an undesired and inevitable complication induced by orthodontic forces. This inflammatory mechanism is regulated by immune cells that precede orthodontic tooth movement (OTM) and can influence the severity of OIIRR. The process of OIIRR is based on an immune response. On some occasions, the immune system attacks the dentition by inflammatory processes during orthodontic treatment. Studies on the involvement of the PD-1/PD-L1 immune checkpoint have demonstrated its role in evading immune responses, aiming to identify possible novel therapeutic approaches for periodontitis. In the field of orthodontics, the important question arises of whether PD-L1 has a role in the development of OIIRR to amplify the amount of resorption. We hypothesize that blocking of the PD-L1 immune checkpoint could be a suitable procedure to reduce the process of OIIRR during orthodontic tooth movement. This review attempts to shed light on the regulation of immune mechanisms and inflammatory responses that could influence the pathogenesis of OIIRR and to acquire knowledge about the role of PD-L1 in the immunomodulation involved in OIIRR. Possible clinical outcomes will be discussed in relation to PD-L1 expression and immunologic changes throughout the resorption process.

Concentration of dentin sialoprotein at the initial stage of orthodontic treatment using self-ligating and conventional preadjusted brackets: A pilot study

OBJECTIVES:

This study evaluated differences in concentration of dentin sialoprotein (DSP) in gingival crevicular fluid (GCF) relating to orthodontically induced inflammatory root resorption (OIIRR) at the initial stage of orthodontic treatment using self-ligating and conventional preadjusted brackets.

MATERIALS AND METHODS:

Eighteen patients were assigned to three groups of equal size. Two experimental groups received non-extraction orthodontic treatment using passive self-ligating or conventional preadjusted bracket. The control group included patients without orthodontic treatment. GCF was collected from five proximal sites of maxillary anterior teeth at subsequent intervals: immediately prior to orthodontic treatment (T0), and at three and 12 weeks after initiation of treatment (T1 and T2). DSP concentration was evaluated by enzyme-linked immunoabsorbent assay and the differences in DSP levels were analyzed between and within groups.

RESULTS:

There were no significant differences in DSP levels within both experimental groups and the control group during T0-T1-T2 (P ≥ 0.05). A significant difference of DSP concentration was found between the conventional preadjusted bracket and the control group at T2 (P = 0.038). However, it was thought to be clinically insignificant.

CONCLUSION:

The study showed no significant difference in DSP concentration at the initial stage of orthodontic treatment with either self-ligating or conventional preadjusted bracket.

Radiographic features in 2D imaging as predictors for justified CBCT examinations of canine-induced root resorption

OBJECTIVES

This retrospective observational study aimed to evaluate the diagnostic accuracy of two-dimensional radiographs on canine-induced root resorption (CIRR) in lateral incisors and identify predictors of CIRR in patients with impacted maxillary canines (IMC).

METHODS

Ninety-nine patients aged 9-17 years, with 156 IMCs, were included in the study. All had CBCT-volumes and two-dimensional radiographs consisting of at least one panoramic radiograph. Two radiologists jointly viewed all cases twice. First, radiographic features related to the IMC and possible CIRR were recorded from two-dimensional radiographs. Then, CIRR was determined from CBCT and according to position and extension classified as mild, moderate and severe.

RESULTS

CIRRs was detected in 80% of lateral incisors (mild: 45%; moderate: 44%; severe: 11%). The sensitivity was generally low at mild and moderate cut-offs (29 and 29%), and somewhat higher for severe (50%). Corresponding specificities were 48%, 63% and 68%. Canine cusp-tip superimposing the lateral incisor's middle third and root/crown ratio >1 was positively associated with mild CIRR, with an odds ratio (OR) of 3.8 and 6.7, respectively. In addition, the root development stage was positively associated with moderate/severe CIRR when the canine root was nearly or fully developed (OR = 3.1).

CONCLUSIONS

The diagnostic accuracy of two-dimensional radiographs was inadequate for detecting CIRR amongst patients referred for CBCT examinations. Based on our results, none of the suggested two-dimensional radiographic features could predict moderate/severe CIRR except for root development stage. IMC in a later stage of root development seems to be associated with a higher risk of moderate/severe CIRR.

Pathophysiological mechanisms of root resorption after dental trauma: a systematic scoping review

Background

The objective of this scoping review was to systematically explore the current knowledge of cellular and molecular processes that drive and control trauma-associated root resorption, to identify research gaps and to provide a basis for improved prevention and therapy.

Methods

Four major bibliographic databases were searched according to the research question up to February 2021 and supplemented manually. Reports on physiologic, histologic, anatomic and clinical aspects of root resorption following dental trauma were included. Duplicates were removed, the collected material was screened by title/abstract and assessed for eligibility based on the full text. Relevant aspects were extracted, organized and summarized.

Results

846 papers were identified as relevant for a qualitative summary. Consideration of pathophysiological mechanisms concerning trauma-related root resorption in the literature is sparse. Whereas some forms of resorption have been explored thoroughly, the etiology of others, particularly invasive cervical resorption, is still under debate, resulting in inadequate diagnostics and heterogeneous clinical recommendations. Effective therapies for progressive replacement resorptions have not been established. Whereas the discovery of the RANKL/RANK/OPG system is essential to our understanding of resorptive processes, many questions regarding the functional regulation of osteo-/odontoclasts remain unanswered.

Conclusions

This scoping review provides an overview of existing evidence, but also identifies knowledge gaps that need to be addressed by continued laboratory and clinical research.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12903-021-01510-6.

Distinct tooth regeneration systems deploy a conserved battery of genes

Background

Vertebrate teeth exhibit a wide range of regenerative systems. Many species, including most mammals, reptiles, and amphibians, form replacement teeth at a histologically distinct location called the successional dental lamina, while other species do not employ such a system. Notably, a ‘lamina-less’ tooth replacement condition is found in a paraphyletic array of ray-finned fishes, such as stickleback, trout, cod, medaka, and bichir. Furthermore, the position, renewal potential, and latency times appear to vary drastically across different vertebrate tooth regeneration systems. The progenitor cells underlying tooth regeneration thus present highly divergent arrangements and potentials. Given the spectrum of regeneration systems present in vertebrates, it is unclear if morphologically divergent tooth regeneration systems deploy an overlapping battery of genes in their naïve dental tissues.

Results

In the present work, we aimed to determine whether or not tooth progenitor epithelia could be composed of a conserved cell type between vertebrate dentitions with divergent regeneration systems. To address this question, we compared the pharyngeal tooth regeneration processes in two ray-finned fishes: zebrafish (Danio rerio) and threespine stickleback (Gasterosteus aculeatus). These two teleost species diverged approximately 250 million years ago and demonstrate some stark differences in dental morphology and regeneration. Here, we find that the naïve successional dental lamina in zebrafish expresses a battery of nine genes (bmpr1aa, bmp6, cd34, gli1, igfbp5a, lgr4, lgr6, nfatc1, and pitx2), while active Wnt signaling and Lef1 expression occur during early morphogenesis stages of tooth development. We also find that, despite the absence of a histologically distinct successional dental lamina in stickleback tooth fields, the same battery of nine genes (Bmpr1a, Bmp6, CD34, Gli1, Igfbp5a, Lgr4, Lgr6, Nfatc1, and Pitx2) are expressed in the basalmost endodermal cell layer, which is the region most closely associated with replacement tooth germs. Like zebrafish, stickleback replacement tooth germs additionally express Lef1 and exhibit active Wnt signaling. Thus, two fish systems that either have an organized successional dental lamina (zebrafish) or lack a morphologically distinct successional dental lamina (sticklebacks) deploy similar genetic programs during tooth regeneration.

Conclusions

We propose that the expression domains described here delineate a highly conserved “successional dental epithelium” (SDE). Furthermore, a set of orthologous genes is known to mark hair follicle epithelial stem cells in mice, suggesting that regenerative systems in other epithelial appendages may utilize a related epithelial progenitor cell type, despite the highly derived nature of the resulting functional organs.

Transcriptomic profiling of feline teeth highlights the role of matrix metalloproteinase 9 (MMP9) in tooth resorption

Tooth resorption (TR) in domestic cats is a common and painful disease characterised by the loss of mineralised tissues from the tooth. Due to its progressive nature and unclear aetiology the only treatment currently available is to extract affected teeth. To gain insight into TR pathogenesis, we characterised the transcriptomic changes involved in feline TR by sequencing RNA extracted from 14 teeth (7 with and 7 without signs of resorption) collected from 11 cats. A paired comparison of teeth from the same cat with and without signs of resorption identified 1,732 differentially expressed genes, many of which were characteristic of osteoclast activity and differentiation, in particular matrix metalloproteinase 9 (MMP9). MMP9 expression was confirmed by qPCR and immunocytochemistry of odontoclasts located in TR lesions. A hydroxamate-based MMP9 inhibitor reduced both osteoclast formation and resorption activity while siRNA targeting MMP9 also inhibited osteoclast differentiation although had little effect on resorption activity. Overall, these results suggest that increased MMP9 expression is involved in the progress of TR pathogenesis and that MMP9 may be a potential therapeutic target in feline TR.

In vivo effects of different orthodontic loading on root resorption and correlation with mechanobiological stimulus in periodontal ligament

Orthodontic root resorption is a common side effect of orthodontic therapy. It has been shown that high hydrostatic pressure in the periodontal ligament (PDL) generated by orthodontic forces will trigger recruitment of odontoclasts, leaving resorption craters on root surfaces. The patterns of resorption craters are the traces of odontoclast activity. This study aimed to investigate resorptive patterns by: (i) quantifying spatial root resorption under two different levels of in vivo orthodontic loadings using microCT imaging techniques and (ii) correlating the spatial distribution pattern of resorption craters with the induced mechanobiological stimulus field in PDL through nonlinear finite-element analysis (FEA) in silico. Results indicated that the heavy force led to a larger total resorption volume than the light force, mainly by presenting greater individual crater volumes ( p < 0.001) than increasing crater numbers, suggesting that increased mechano-stimulus predominantly boosted cellular resorption activity rather than recruiting more odontoclasts. Furthermore, buccal-cervical and lingual-apical regions in both groups were found to have significantly larger resorption volumes than other regions ( p < 0.005). These clinical observations are complemented by the FEA results, suggesting that root resorption was more likely to occur when the volume average compressive hydrostatic pressure exceeded the capillary blood pressure (4.7 kPa).

Root Resorption Classifications: A Narrative Review and a Clinical Aid Proposal for Routine Assessment

Root resorption (RR) refers to noninfectious damage related to the loss of hard and soft dental tissue that results from clastic cell activity. It is observed as a pathologic process that is predominantly asymptomatic in the permanent dentition and physiological during the shedding of primary teeth. Roots are protected by unmineralized organic cementoid and predentine. RR occurs because of the inability of the clastic cells to adhere to unmineralized surfaces. Since the first RR classification was described by Andreasen in the 1970s, several classifications have been proposed with diverse terminology. A narrative literature review was undertaken on the current classification, diagnosis, pathophysiology, and treatment of RR. A bibliographic search resulted in 434 titles and abstracts, and from those, 17 articles were obtained that alluded to 15 RR classifications. A total of 28 articles that met the inclusion criteria were included. Results showed that Andreasen’s classification is the most widely used for classifying RR. In terms of epidemiologic data, studies are scarce, although RR pathophysiology is well described in the literature. Overall clinical guidelines are summarized in a new RR classification diagram. The choice of RR treatment application should be in line with the RR type and the dentist’s experience. However, an earlier and correct diagnosis will significantly improve final outcomes and long-term prognosis, especially with the current evolution of advanced imaging techniques, such as cone-beam computed tomography and bioceramic-based endodontic sealers.

Inhibitory effect of Thai propolis on human osteoclastogenesis

BACKGROUND/AIM

Avulsed teeth should be immediately replanted into the socket or otherwise kept in a physiologic storage medium to maintain periodontal ligament cell viability. A previous study has demonstrated that Thai propolis extract can maintain viability of human periodontal ligament cells. However, root resorption by osteoclasts often occurs when the avulsed teeth are replanted. The aim of this study was to determine the inhibitory effect of Thai propolis extract on human osteoclastogenesis in vitro.

MATERIALS AND METHODS

Human peripheral blood mononuclear cells were isolated for osteoclast precursors and cultured in the presence or absence of various non-toxic concentrations of propolis extract, as determined by the alamarBlue® assay, during in vitro induction of osteoclastogenesis. Osteoclast formation was examined by tartrate-resistant acid phosphatase staining, actin ring formation, and real-time polymerase chain reaction. The resorption pit assay was performed to determine osteoclast function.

RESULTS

Non-toxic concentrations of propolis extract suppressed osteoclast formation by significantly decreasing the percentages of tartrate-resistant acid phosphatase-positive multinuclear cells and the ratios of cells with F-actin ring formation (P < .01) in a dose-dependent fashion. Expression of several osteoclast-specific genes was significantly downregulated by propolis in a dose-dependent manner (P < .05). The percentages of resorption areas on dentin slices were significantly decreased by propolis (P < .05).

CONCLUSIONS

Thai propolis can inhibit human osteoclast formation and function, which may be beneficial for prevention of root resorption following replantation of avulsed teeth.

Clinical Characteristics of Localized Aggressive Periodontitis in Primary Dentition

OBJECTIVES

Due to the low prevalence of localized aggressive periodontitis (LAP), clinical characteristics of LAP in primary dentition are derived from a few case reports/series in the literature. The goal of this study was to determine common clinical characteristics such as bone and root resorption patterns, in a series of cases with LAP in primary dentition. We hypothesize these cases present aggressive periodontal bone destruction starting mostly around first primary molars and atypical root resorption patterns.

STUDY DESIGN

We have evaluated 33 LAP cases in primary dentition for pattern of bone destruction, root resorption and early exfoliation.

RESULTS

Cases evaluated were aged 5-12 (mean=8.7 years). Thirty cases presented more severe bone loss on first than second molars, with relatively fast progression to second molars, altered pattern of root resorption, mostly external (n=16) and early exfoliation of primary teeth due to periodontal bone loss, rather than physiologic root resorption (n=11).

CONCLUSIONS

This study showed common clinical characteristics found in LAP in primary molars, including possible initiation on first primary molars and abnormal root resorption patterns. These characteristics are important to be early identified and treated in order to prevent possible progression into the permanent dentition.

The cell biology and role of resorptive cells in diseases: A review

Resorptive cells are responsible for the resorption of mineralized matrix of hard tissues. Bone-resorbing cells are called osteoclasts; however, they can resorb mineralized dental tissues or calcified cartilage and then they are called odontoclasts and chondroclasts, respectively. Resorptive cells form when mononuclear precursors derived from a monocyte–macrophage cell lineage are attracted to certain mineralized surfaces and subsequently fuse and adhere onto them for exerting their resorbing activity. These cells are responsible for degradation of calcified extracellular matrix composed of organic molecules and hydroxyapatite. The activity of these cells can be observed in both physiological and pathological processes throughout life and their activity is mainly required in bone turnover and growth, spontaneous and induced (orthodontic) tooth movement, tooth eruption, and bone fracture healing, as well as in pathological conditions such as osteoporosis, osteoarthritis, and bone metastasis. In addition, they are responsible for daily control of calcium homeostasis. Clastic cells also resorb the primary teeth for shedding before the permanent teeth erupt into the oral cavity.