Mesenchymal stem cell-derived exosomes: a potential cell-free therapy for orthodontic tooth stability management

Abstract

Orthodontic relapse (OR) occurs at a rate of over 70%. Retention is the current attempt at prevention, but it requires a considerable amount of time and cannot fully block OR. It’s imperative to find a safe and effective method for managing post-orthodontic tooth stability. Periodontal bone remodeling is one crucial biological foundation of OR. Mesenchymal stem cell-derived exosomes (MSC-Exo) show promise in relapse management by regulating periodontal bone remodeling. MSC-Exo can prevent relapse by regulating periodontal ligament function, osteoclast activity, osteoblast differentiation, macrophage polarization, and periodontal microcirculation. In recent years, exosome-loaded hydrogels, which achieve controlled exosome release, have demonstrated efficacy in promoting bone regeneration and remodeling, offering promising prospects for OR management. This review aims to highlight the use of MSC-Exo-based therapy for preventing OR, offering new insights for future research focused on improving tooth stability and enhancing orthodontic anchorage.

Background

The goal of orthodontic treatment is to obtain and maintain an optimal occlusion, characterized by well-coordinated functional dentition and facial morphology. However, the ensuing disturbances of the local mechanical environment due to the removal of orthodontic appliances might cause the teeth to drift away from their corrected position. This inevitable and unfavorable post-treatment tooth movement is defined as orthodontic relapse (OR), which remains one of the biggest challenges in orthodontics. Despite correct diagnosis and treatment planning followed by effective retention, OR still occurs at a high percentage. Studies have shown that more than 70% of patients experience OR after removal of retainers [1]. The relapse rate exceeds 19% after 3 years of orthodontic treatment [2]. Lifelong retention is the only strategy recommended to prevent OR [3].

The periodontal ligament (PDL) is a highly mechanoresponsive tissue. Mechanical forces play a vital role in maintaining periodontal homeostasis by triggering a well-orchestrated response of all PDL components. Orthodontic tooth movement (OTM) is facilitated by continuous alveolar bone resorption by osteoclasts adjacent to the compression side of the tooth root along with the stimulation of new bone formation by osteoblasts on the tension side — a process known as periodontal remodeling [4]. This involves complex processes such as osteoimmunology, cell signaling, mechanotransduction, and extracellular matrix (ECM) alteration. During OTM, different mechanical stresses (shear, compression, and tension) on teeth are transferred to the alveolar bone through PDL and activate a chain of genetic and protein modifications. Periodontal ligament cells (PDLC) perceive mechanical stimuli and convert them into cellular signals, leading to release of extracellular factors that modulate the local microenvironment. OR undergoes the same biological process as OTM but in the opposite direction.

Given the similarity in physiological processes between OTM and OR, chemical agents that reduce the rate of OTM by targeting periodontal bone remodeling pathways may also be effective in modulating OR. After decades of investigations, evidence shows that various medications and biologic factors can potentially modulate the process. The local administration of medicines, such as tetracycline [5], atorvastatin [6], simvastatin [7], raloxifene [8], osteoprotegerin [9], etc., showed an inhibitory effect on OR in a rat OTM model. Although specific medicines do have a potential effect on OR, the associated deleterious effects on other tissues must be carefully considered. None of these approaches have reached the point where they can be routinely used in the clinic. It’s urgent to find a safe and effective approach for post-orthodontic tooth stability management.

OR activity and the effect of retention on OR

OR activity

According to previous animal experiment observations, OR curves are characterized by exponential decay curves, consisting of two phases, namely a rapid relapse phase and a long-term relapse phase (Fig. 1) [10]. This pattern occurs regardless of the force applied during active OTM or the total OTM distance. OR might get initiated immediately once the teeth are no longer restrained by orthodontic appliances [11]. During the rapid relapse phase, approximately 70–80% of total relapse occurs [10, 12]. This phase only lasts for a few days before transitioning into the long-term relapse phase, where the relapse lasts for several weeks at a drastically decreased speed [10, 13]. However, in animal studies of OR, OTM distances were primarily measured using silicone dentition models or solely photographs, which introduces considerable manual measurement error. To enhance accuracy, micro-CT is suggested to be applied in future studies.

The effect of retention on OR

The long-term use of retainers is often suggested to reduce OR. In most animal studies of OR, however, the efficacy of retention is scarcely evaluated. According to the present two studies that compared the effect of retention on mouse and dog OR models, the total amount of relapse was decreased by approximately 58 − 65% compared to the group without retention (Fig. 1) [14, 15]. Intriguingly, Eric [14] pointed out that the duration of the relapse period is independent of retention, and retention has no significant reducing effect on relapse when OTM distance is smaller than 2 mm, indicating that relapse is partly retention-responsive.

Fig. 1

A schematic diagram of relapse curves with or without retention

Hypotheses of OR

Although the question of OR has been raised for decades, its biological mechanism is still not fully elucidated. OR involves a complex interplay between various cell types, including PDLCs, osteoclasts, osteoblasts, and macrophages, as illustrated in the diagram (Fig. 2). Current hypotheses regarding OR can be categorized into two aspects:

Periodontal ligament (PDL) remains under tension

The role of PDL remodeling in OR remains controversial. Traditionally, it was believed that PDL remains stretched after removal of orthodontic appliance, creating stretch-induced tension that contributes to rapid relapse. This theory has been supported by evidence showing that surgical fiberotomy, proteolytic nanoparticles targeting collagen type-I fibers and antifibrotic medications like sulforaphane can block this rapid relapse phase [16,17,18]. However, this theory has been challenged by histological findings indicating that collagen turnover in the PDL is fast enough for the gingival fiber system to completely remodel during orthodontic treatment [19]. Additionally, the persistence of OR despite normally stretched transseptal fibers suggests that collagen turnover might not be the primary factor in the OR process [10].

Unfinished periodontal remodeling and osteoclast activity on the relapse direction

Periodontal bone remodeling has gained increased attention in recent studies on OR. Osteoclasts, as multinucleated cells responsible for bone resorption, play a crucial role in this process [20]. When orthodontic force is applied, woven bone begins to form on the tension side. However, the bone turnover is not complete at the end of orthodontic treatment [15]. The newly deposited bone remains a disorganized fiber structure with poor mineralization, making it susceptible to external force and subsequent osteoclastic resorption. Initial resorptive activity can create a temporary void adjacent to the PDL neighboring the tooth roots, which facilitates the rapid relapse [9]. At the end of active orthodontic treatment, osteoclasts begin to distribute along the alveolar bone on the relapse direction [21]. The number of osteoclasts is positively correlated with the relapse activity [10, 18]. As OR progresses, the number of osteoclasts decreases over time [10]. Inhibition of osteoclast activity through bisphosphonates or Nrf2 activation has been shown to reduce OR by over 70% [22, 23], which further confirms the role of osteoclasts in OR.

Fig. 2

A schematic diagram that summarizes the role of periodontal ligament cell (PDLC), osteoclast, osteoblast, and macrophage in orthodontic relapse

Exosomes and mesenchymal stem cells-derived exosomes (MSC-Exo)

Exosomes are tiny extracellular vesicles secreted by cells through exocytosis, with an average diameter of 100 nm. These vesicles contain various constituents of a mother cell that releases them, including DNA, RNA, lipids, and metabolites. They also carry transmembrane proteins, heat shock proteins, lipoproteins, and some transport-related proteins, which serve as markers for exosome identification and help target them to specific cells [24]. Exosomes are produced by almost all cell types, among which those sourced from mesenchymal stem cells (MSCs) are most widely studied. They also exist in various body fluids such as plasma, urine, and saliva. The biogenesis of exosomes is influenced by many external factors like cell type, microenvironment, growth factors, and cytokines, which results in the heterogeneity of exosomes. Encapsulated by lipid membranes derived from their parental cells, exosomes show inherent biocompatibility and specific homing effect, which means they can target and enter specific cells or tissues under certain conditions. This is mediated by surface molecules such as integrins and glycans. They can be taken up by recipient cells via receptor–ligand binding, endocytosis, or membrane fusion, allowing them to regulate the function and behavior of distant recipient cells. This positions them as crucial intercellular communicators [25].

Research has highlighted the pivotal roles of exosomes in cell-to-cell communication and immune modulation under various physiological and pathological conditions. Unlike the undesirable side effects that medications may bring about, stem cells and their exosomes are easier to target to treatment sites and have lower immunogenicity. This has made them the focus of regenerative medicine in recent years [26].

MSCs are self-renewing, multipotent, and regenerative progenitor cells. They can be differentiated into mesenchymal lineages, namely osteoblasts, chondrocytes, adipocytes, and endothelial cells [27]. MSCs can be easily isolated from different and accessible adult tissues like peripheral blood, skin, and dental pulp [28]. As prolific producers of exosomes, MSCs ensure sustainable and reproducible production of exosomes [29]. Compared with stem cells, exosomes have more advantages in promoting tissue regeneration and have lower risks of triggering immune response and tumorigenesis [30]. MSC-Exo have been shown to have therapeutic effects on various orthopedic diseases, including osteoporosis, osteoarthritis, and bone fractures, through their regulation of bone remodeling and angiogenesis [31,32,33,34].

MSC-Exo as a potential cell-free therapy for OR

Research has demonstrated the efficacy of various MSC-Exo in modulating periodontal bone remodeling, laying a foundation for its potential use in OR prevention. These include regulating PDLC function, inhibiting osteoclast activity, facilitating osteogenesis, modulating macrophage polarization, and promoting periodontal microcirculation (Table 1; Fig. 3). The following sections will elucidate these processes in detail.

MSC-Exo regulates PDLC function

PDL is a specialized connective tissue that surrounds the roots of teeth, functioning as a stress absorber and redistributor to the adjacent alveolar bone [35]. Orthodontic force can dramatically alter the pattern and density of PDL collagen [18]. The normal structure of the PDL is replaced by type III collagen fibers during OTM. These disorganized fibers are rapidly remodeled, restoring the normal PDL structure within 3 weeks of OR [11]. Additionally, PDL tends to thicken on the tension side and thin on the opposite compression side during OTM but recovers its original width shortly after the withdrawal of orthodontic force [36]. The force-induced degradation of PDL structure is closely linked to the instability of post-orthodontic teeth, and its recovery during early relapse corresponds with the rapid relapse ratio [18].

Among the heterogeneous cell populations within the PDL, PDLCs and periodontal ligament stem cells (PDLSCs) are the main components responsible for PDL remodeling and extracellular matrix (ECM) deposition. These cells can differentiate into periodontal cell types in vitro and generate PDL complex structures in vivo [37].

PDLCs play a significant role in modulating local bone metabolism [38]. The dynamic interaction of PDLCs, including osteoblasts, fibroblasts, cementoblasts, and ECM components, is crucial for alveolar bone formation and regeneration [39]. In the absence of mechanical forces, PDL cells attract osteoclast precursors through the presentation of intercellular adhesion molecule 1 (ICAM1) and support the expression of osteoclastogenesis-promoting molecules such as RANKL, macrophage colony-stimulating factor (M-CSF), HMGB1, and tumor necrosis factor-α (TNF-α) [40]. When mechanical forces are applied, PDLCs respond by upregulating the expression of osteogenic markers, including osterix (Osx), runt-related transcription factor 2 (Runx2), and osteoprotegerin (OPG), meanwhile downregulating the osteoclastogenic marker receptor activator of nuclear factor kappa-B ligand (RANKL) [41].

Multiple signaling pathways are involved in maintaining cellular homeostasis of PDLSCs and PDLCs. Mechanical stimulation modulates the proliferation and differentiation of PDLSCs, with Wnt signaling being critical for their activation [42, 43]. When mechanical loading is applied, it not only activates the Wnt pathway but also induces cytoskeletal rearrangements. This process involves the mechanosensory protein complex linking to a kinase cascade system, triggering the transcription of various regulatory genes in response to mechanical forces [44]. Force-induced processes like pyroptosis, apoptosis and autophagy in PDLSCs are also essential for maintaining alveolar bone homeostasis and regulating periodontal remodeling [45,46,47]. Compressive force can alter cell morphology and suppress collagen expression in CD90⁺ PDLSCs, which recovers to its original status after force withdrawal. Inhibition of the collagen expression in PDLSCs by blocking TGF-β can interrupt PDL collagen recovery and alleviate early relapse [18].

The emerging biological functions of MSC-Exo in regulating PDLSCs have garnered significant attention. Gingival MSC-derived exosomes (GMSC-Exo) can promote the osteogenic differentiation of PDLSCs via the cross-regulation of NF-κB and Wnt/β-catenin signaling pathways in a periodontal inflammatory environment [48]. Dental pulp stem cell-derived exosomes (DPSC-Exo), the optimal choice among oral MSC-derived exosomes, can enhance the proliferation, migration, and osteogenesis of PDLSCs and regulate inflammation by inhibiting the IL-6/JAK2/STAT3 signaling pathway [49]. Additionally, DPSC-Exo can alter the cell cycle and apoptosis of PDLCs by modulating MAPK and Wnt signaling pathways [50, 51]. These properties highlight the potential of MSC-Exo, especially oral MSC-derived exosomes, to enhance PDLSCs and PDLCs function and inhibit orthodontic relapse.

MSC-Exo inhibits osteoclast activity

Osteoclasts originate from the monocyte/macrophage hematopoietic lineage cells, which circulate in the bloodstream and adhere to the bone tissue [52]. During OTM, osteoclasts break down bone tissue, creating space to facilitate tooth movement and new bone formation. It’s been suggested that OR activity is positively correlated with the number of osteoclasts in the relapse direction [21]. After the withdrawal of the orthodontic appliance, newly formed bone tissue is absorbed and degraded by enhanced osteoclast activity, creating spaces for OR [9].

Osteoclast differentiation, survival, and activation are primarily regulated by two essential factors: M-CSF and RANKL [53]. In the presence of M-CSF, RANKL binds to its receptor RANK on osteoclast precursors. This process recruits the adapter protein TNF receptor-associated factor 6 (TRAF6), which triggers multitudinous intracellular signaling cascades, including mitogen-activated protein kinases (MAPKs), nuclear factor-κB (NF-κB) and nuclear factor of activated T-cells 1/2 (NFATc1/2) [54]. NFATc1, a master regulator of osteoclast differentiation, governs the expression of numerous genes involved in bone resorption, such as tartrate-resistant acid phosphatase (TRAP), cathepsin K (CTSK), and Atp6v0d2. Once activated, osteoclast precursors fuse into multinucleated osteoclasts, attach to the bone surface, and dissolve mineralized bone matrix by secreting acid and lysosomal enzymes [55].

Exosome-based regulation of osteoclast differentiation is an emerging area of research. Exosomes can influence osteoclast differentiation by delivering signaling molecules, modulating the microenvironment, and interacting with other cells involved in bone remodeling. Studies have demonstrated that MSC-Exo has the capacity to downregulate osteoclast activity. Local injection of bone marrow mesenchymal stem cell-derived exosomes (BMMSC-Exo) can inhibit the osteoclastogenesis of human monocytes by directly inhibiting osteoclast formation targeting 3’-untranslated regions of osteoclast stimulatory transmembrane protein (OCSTAMP) and C-X-C motif chemokine ligand 12 (CXCL12) [56]. GMSC-Exo can also reduce periodontal osteoclast differentiation by targeting the Wnt/β-catenin-mediated RANKL pathway, providing a therapeutic approach for periodontitis [57]. Additionally, orofacial mesenchymal stem cell-derived exosomes, which have a high expression level of miR-206-3p, inhibit osteoclastogenesis by suppressing NFATc1 [58]. These findings highlight the potential of MSC-Exo in regulating osteoclast activity and preventing OR. Ongoing research continues to uncover more details about the specific mechanisms and molecules involved in the regulation of osteoclast differentiation by MSC-Exo, paving the way for innovative treatments in orthodontics and bone health.

MSC-Exo modulates osteoblast differentiation

Osteoblasts, derived from pluripotent MSCs in the bone marrow, are crucial for bone matrix synthesis, secretion, and mineralization [59]. Under the influence of Runx2 and other factors, MSCs differentiate into osteoprogenitor cells, which further turn into immature osteoblasts. These immature osteoblasts secrete ECM components to form the osteoid and mineralize it by releasing vesicles containing calcium and phosphate ions. As the mineralization progresses, immature osteoblasts gradually transform into fully functional osteoblasts. This differentiation process is primarily driven by key signaling pathways, including Wnt, TGF-β/BMP, and Notch.

In animal studies of OR, the newly formed alveolar bone exhibits numerous bone cell lacunae, indicating its immature state [7, 60]. After the active OTM, osteoblasts are seen lining along the newly deposited alveolar bone in the direction of relapse, suggesting incomplete periodontal bone remodeling [10]. In this context, promoting alveolar bone remodeling is an effective way to prevent OR. For instance, raloxifene has been shown to reduce OR by inducing bone formation through elevated expression of osteoblastogenesis and mineralization-related proteins [8, 61]. Similarly, local delivery of simvastatin can inhibit OR by enhancing the expression of bone morphogenetic protein-2 (BMP-2), collagen type I and osteocalcin [60].

Emerging evidence shows that exosomes mediate the cell-to-cell communication in the process of osteoblastic differentiation. Interestingly, MSC-Exo has been proven to regulate the activity of osteoblasts and affect their proliferation and differentiation. For example, BMSC-Exo can improve osteoporosis by promoting differentiation, enhancing cell proliferation and alkaline phosphatase (ALP) activity of hFOB1.19 [62, 63]. MSC-Exo can also promote osteoblast activity by upregulating key osteogenic genes, such as Runx2, partly due to the presence of pro-osteogenic miRNAs (miR-27a, miR-196a and miR-206) that activate classical osteogenic pathways [64, 65]. These effects of MSC-Exo on osteoblast activity highlight their potential as therapeutic tools for various orthopedic conditions, such as osteoporosis, bone fractures, and bone tissue engineering. Considering that insufficient remodeling of alveolar bone is a primary cause of OR, MSC-Exo could be a promising strategy to suppress unwanted post-orthodontic tooth movement. However, the specific molecular mechanisms underlying these effects warrant further investigation.

MSC-Exo modulates macrophage polarization

Macrophages are immune cells that engulf and digest cellular debris, pathogens, and cancer cells through phagocytosis [66]. They are typically classified into two types: M1 and M2 macrophages. M1 macrophages release inflammatory factors such as IL-1, IL-6, and TNF-α, initiating osteoclastogenesis. In contrast, M2 macrophages secrete TGF-β and IL-10, inhibiting osteoclast formation and supporting bone deposition.

During OTM, macrophages play a multifaceted role as progenitors of osteoclasts, modulators of inflammation, and effectors of mechanical force [67]. M1 macrophages are the predominant subtype that functions in OTM [68]. In the early stage of OTM, orthodontic force upregulates the expression of monocyte chemotactic protein 1 (MCP-1) in periodontal tissues, contributing to monocyte recruitment to regional sites [69]. Once recruited, macrophages polarize toward the M1 phenotype due to local compressive force and various inflammatory factors [70]. M1 macrophages then promote alveolar bone resorption and inflammation-mediated bone remodeling due to their production of pro-inflammatory cytokines, including IL-1β, IL-6, TNF-α, and M-CSF [68, 71]. Additionally, macrophages secrete matrix metalloproteinases (MMPs) to degrade ECM components and cellular debris during periodontal tissue remodeling in OTM [72].

M2 macrophages, although crucial for the cessation of bone resorption and promoting bone formation, are less understood in the context of OTM [73, 74]. Studies suggest that the M1/M2 ratio reduces during the retention phase, which might be important for tissue healing and protecting against orthodontically associated root resorption [68, 75]. Therefore, promoting macrophage polarization towards M2 phenotype could be an effective strategy for OR management.

Recent research has extensively evaluated the impact of MSC-Exo on macrophage polarization. DPSC-Exo can reduce macrophage M1 polarization through the ROS/MAPK/NF-κB signaling pathway in treating spinal cord injury [76]. Similarly, exosomes from adipose stem cell-derived exosomes (ADSC-Exo) regulate M1/M2 macrophage polarization to promote bone healing via miR-451a/MIF [77]. BMSC-Exo increases the ratio of M2 to M1 polarization by inhibiting Akt phosphorylation [78]. Given the dual role of macrophages in OTM — promoting inflammation and bone resorption, as well as tissue remodeling and repair — modulating their polarization by MSC-Exo presents as a novel approach to ensuring proper tooth movement and maintaining periodontal health.

MSC-Exo promotes periodontal microcirculation

Periodontal microcirculation plays a crucial role in the regulation of OTM and OR. Vascular endothelial cells within this complex system interact with bone cells, such as osteoblasts, osteoclasts, and mesenchymal stromal cells, leading to the formation of microcapillary networks around newly formed bone tissue [79]. Two primary subtypes of vascular endothelial cells, H-type and L-type, are involved in osteogenesis, with H-type vessels playing a dominant role [80]. Notch signaling in H-type vascular endothelial cells is significant for osteogenic-angiogenic coupling [81].

Local microcirculation facilitates the recruitment of immune cells, such as macrophages, to the site of tooth movement, contributing to the inflammatory response and tissue remodeling. This process underscores the importance of local microvessel formation and nutrient supply through microvessels for tissue regeneration and repair during OTM [82]. Changes in periodontal microcirculation significantly influence OTM, with local blood vessel formation affecting interstitial tissue fluid pressures and alveolar remodeling. The number and density of periodontal blood vessels correlate with the processes of OTM and OR, highlighting the critical role of vascular dynamics in these phenomena [83].

Recent studies have shown the successful application of MSC-Exo to promote angiogenesis in bone remodeling process, especially in maxillofacial bones [84, 85]. For example, PDLSC-derived exosomes (PDLSC-Exo) can promote the bone regeneration of calvaria defects by promoting the vascularization process through increasing the vascular endothelial growth factor (VEGF)/ vascular endothelial growth factor receptor 2 (VEGFR2) expression [86]. Human exfoliated deciduous teeth-derived exosomes (SHED-Exo) contribute to periodontal bone regeneration by promoting neovascularization, attributed to the upregulated expression of angiogenesis-related genes VEGFR2, CXCL12, and fibroblast growth factor-2 (FGF-2) [84]. In this context, modulations of periodontal microcirculation to enhance periodontal remodeling present a promising therapeutic approach for preventing OR.

Fig. 3

A schematic diagram illustrating the mechanisms of MSC-Exo in modulating periodontal bone remodeling and preventing orthodontic relapse

Table 1 The efficacy of various MSC-Exo in the regulation of PDLSC/ PDLC function, osteoclast activity, osteoblast differentiation, macrophage polarization and periodontal microcirculation

MSC-Exo loaded biomaterials for OR management

 Minimizing invasive procedures is crucial for post-orthodontic tooth stability management, making injectable biomaterials like hydrogels a preferred choice for exosome delivery. Hydrogels, soft and wet materials that resemble the natural ECM in structure and composition, have garnered increasing attention in the biomedical engineering field [87]. Due to their plasticity, porosity, biocompatibility, and injectability, hydrogels are considered ideal candidates for tissue engineering, controlled drug delivery, and various other biomedical applications [88].

Exosomes offer substantial potential in tissue engineering, but their rapid degradation in vivo poses a challenge for clinical applications. For instance, subcutaneously injected exosomes are typically cleared within a day [89]. To address this issue, researchers are exploring the use of scaffolds to facilitate the loading and controlled release of exosomes, offering a stable and effective method for their application in vivo. Various techniques, including direct attachment, chemical cross-linking, specific binding, freeze-drying, and 3D printing, have been developed to combine exosomes with scaffolds [90]. Encapsulating exosomes in hydrogels can prolong their half-life and ensure a slower and more controlled release of exosomes in vivo for several weeks [89, 91,92,93].

Typically, exosome-scaffold combinations exhibit an initial rapid release followed by a slower and stabilized release over a period of more than a week, contrasting sharply with the rapid degradation seen with directly injected exosomes. For example, exosomes loaded onto ionically cross-linked alginate hydrogels release over 50% within the first three days, with the remainder releasing slowly over time [89]. In freeze-dried scaffolds, exosome release rates are 71.4% and 75.4% at 14 and 28 days, respectively [94]. Meanwhile, radial scaffolds created via 3D printing retain more than 56% of exosomes after 14 days in vitro [90].

There are mainly three ways of preparing exosomes-loaded hydrogels: The “breathing method” takes advantage of the hydrogel’s water retention and absorption properties. After removing excess water from the swollen hydrogel using a solvent, the exposed voids can be filled with exosomes. However, the effectiveness of this method depends on the appropriate pore size of the hydrogel to accommodate the exosomes; too small, and the loading process may fail; too large, and there is a risk of premature exosome leakage [95]. Another approach involves directly mixing exosomes with the hydrogel precursor solution using a crosslinking agent or through simple physical mixing. Additionally, polymers capable of self-assembly, such as hyaluronic acid-adamantane, can be added to the exosomes suspension, rapidly forming a hydrogel that encapsulates exosomes [96]. Another technique involves mixing exosomes with polymers and co-injecting them with a crosslinking agent into the target site using a double-lumen syringe. This method allows for gel formation directly at the application site, but it is essential to select non-toxic crosslinking agents to avoid adverse reactions in vivo [97].

Studies have reported the application of hydrogels in bone remodeling and regeneration, particularly in periodontal bone. For instance, a clinical trial utilizing basic fibroblast growth factor-gelatin hydrogels in patients with periodontal alveolar bone loss showed significant improvements in periodontal bone regeneration [98]. Additionally, controlled release of bisphosphonate risedronate with a topically administered gelatin hydrogel has shown its efficacy in decreasing OR by inhibiting osteoclast activity [99]. Intriguingly, MSC-Exo-loaded exosomes have shown efficacy in bone healing. BMSC-Exo loaded on bilayer-hydrogels can regulate macrophage polarization and scavenge reactive oxygen species to promote cartilage repair [100]. Similarly, the combination of PDLSC-Exo with collagen hydrogels has yielded promising results in a rat model for alveolar bone regeneration by enhancing osteogenic properties [101].

Conclusions, challenges, and future prospectives

Despite the effective use of orthodontic retainers, OR remains at a high rate. It is presumed that periodontal bone remodeling plays a major role in OR. Studies have shown that promoting periodontal bone remodeling by inhibiting osteoclastogenesis and enhancing osteogenesis is effective in preventing OR, suggesting a strong link between local bone metabolism and OR.

Exosomes, as natural carriers of biochemical signaling molecules, are secreted by almost all cell types. Their constituents can vary depending on the disease state, making exosomes possible for disease diagnosis and treatment. MSC-Exo, with or without biomaterials, has shown promising application prospects in preventing OR. Oral stem cells-derived exosomes are particularly advantageous due to their easy availability, excellent immune regulation and regeneration abilities.

This review summarizes novel strategies using MSC-Exo in orthodontic tooth stability management and discusses the possible mechanisms, outlining new directions for developing advanced therapeutic materials aimed at reducing the retention period and enhancing long-term post-orthodontic stability. While MSC-Exo-based therapies hold significant potential as a novel approach for managing orthodontic tooth stability, several challenges remain that must be addressed before clinical application. Variability in exosome content, based on cell sources, cell types, and culture conditions, may result in differences in biological functions. This highlights the need for standardized exosome isolation, purification, and characterization methods, as well as determining optimal dosing and treatment protocols for sustained efficacy. Furthermore, the long-term safety and potential immunogenicity of repeated exosome treatments require careful investigation. Future research should focus on elucidating the specific mechanisms underlying their efficacy in managing orthodontic tooth stability and conducting comprehensive clinical trials to establish their safety and effectiveness in managing OR.