Orthodontic treatment is a medical treatment that aims to obtain good maxillofacial morphology and normal oral functions by applying orthodontic forces to the tooth roots, periodontal tissues, and maxillofacial surfaces. Tooth movement is achieved through bone remodeling by artificially applying loads to the teeth and alveolar bone, resulting in bone resorption by osteoclasts on the compression side and bone formation by osteoblasts on the traction side. Severe root resorption can occur when the load applied to the tooth root greatly exceeds the repair capacity of the cementoblasts (Abass et al., 2008). Among the medicalogenic disorders that may occur during the orthodontic treatment process (such as pain, root resorption, caries, and temporomandibular joint disorder), root resorption is irreversible and difficult to predict, prevent, or repair. It is an asymptomatic disorder and can only be radiographically and histologically diagnosed. Approximately 48–66 % of orthodontically treated teeth show root resorption < 2 mm, and histological root resorption occurs in > 90 % of cases (Wishney, 2017). Anterior teeth are particularly susceptible to root resorption, with 1–5 % of the teeth reported to have resorption ≥ 4 mm (Weltman et al., 2010). Nevertheless, the clinical and biological factors that cause root resorption remain unclear (Sameshima & Iglesias-Linares, 2021).

Root resorption is caused by multinucleated giant cells that appear in the vitreous-like degenerative tissue formed in the periodontal ligament on the pressure side during tooth movement. Odontoclasts are multinucleated giant cells that are structurally and functionally similar to osteoclasts; they reside on the surface of the resorbed root (Sahara et al., 1996) and are involved in hard tissue resorption. Osteoclasts are part of the normal bone structure, whereas odontoclasts are rarely found on normal cementum or tooth root surfaces (Abass et al., 2008).

Receptor activator of nuclear factor κB ligand (RANKL), a cytokine belonging to the tumor necrosis factor (TNF) family, is essential for the induction of osteoclastogenesis. RANKL is produced by osteoblasts and binds to the receptor activator of nuclear factor κB (RANK) receptors localized on the surface of osteoclast progenitor cells to promote differentiation into osteoclasts via signal transduction (Suda et al., 1999). Osteoprotegerin (OPG), a member of the TNF receptor family produced by osteoblasts, acts as a decoy receptor for RANKL and inhibits progenitor cell differentiation into osteoclasts by binding to RANKL, with a higher affinity than that of RANK, the true receptor of RANKL (Udagawa et al., 2021). Thus, the RANKL/RANK/OPG system in periodontal tissues is an important determinant that regulates orthodontic tooth movement, bone remodeling, and root resorption (Matsuda et al., 2017).

TNF-α and interleukin (IL)− 1β are proinflammatory cytokines involved in bone remodeling and are highly expressed at inflammatory sites. TNF-α induces the production of IL-1β (Dinarello et al., 1987, Kaushansky et al., 1988). Increased expressions of TNF-α and IL-1β have been detected in the gingival sulci of patients undergoing orthodontic treatment (Uematsu et al., 1996). The expression of IL-17, produced by helper T cells, is induced by excessive corrective force (Hayashi et al., 2012) and subsequently induces the production of IL-6 (Shimizu et al., 2013). These inflammatory cytokines are important mediators involved in root resorption that upregulate the production of RANKL, an osteoclast activator, and activate osteoclast progenitor cells through synergistic action with the RANKL/RANK/OPG system (Sato et al., 2006).

Stem cells from human exfoliated deciduous teeth (SHED) were first isolated by Miura et al. (2003) and are noninvasively available. SHED can multiply into osteoblasts, adipocytes, chondrocytes, and neurons; hence, they have recently attracted attention in the field of regenerative medicine. Nakajima et al. (2018) transplanted SHED, human permanent dental pulp-derived stem cells, and human bone marrow-derived stem cells into induced skull defects in immunocompromised mice and observed good bone regeneration at 12 weeks after transplantation, suggesting that SHED may be an excellent cell source for bone regeneration.

The paracrine action of cytokines secreted by mesenchymal stem cells (MSCs) has hitherto attracted attention; growth factors and cytokines secreted by MSCs may accumulate in culture supernatant (CM) by paracrine action, and many studies have reported tissue regeneration-inducing effects using MSCs-CM (Ando et al., 2014, Chen et al., 2018, Chen et al., 2020, Katagiri et al., 2021, Katagiri et al., 2013, Katagiri et al., 2017; Kawai et al., 2015, Kim et al., 2020, Ogasawara et al., 2020, Ogata et al., 2015, Tsai et al., 2018, Yamaguchi et al., 2015). Hiraki et al. (2020) reported that treatment of bone defects in the parietal bones of immunocompromised mice with SHED-CM and SHED grafts significantly increased regenerated bone mass in both groups than that of the control group. Furthermore, quantitative analysis revealed that SHED-CM contains a large amount of angiogenic (e.g., vascular endothelial growth factor [VEGF]), bone metabolic (e.g., OPG and bone morphogenetic protein [BMP]−2,4), and neurotrophic (e.g., brain-derived neurotrophic) factors. Furthermore, SHED-CM reportedly suppresses the expression of inflammatory cytokines (Shimojima et al., 2016). Ogasawara et al. (2020) reported that SHED-CM administration via the tail vein to mice with temporomandibular joint osteoarthritis markedly suppressed inflammation and triggered temporomandibular repair and regeneration.

Based on these findings, the paracrine action of proteins contained in SHED-CM is expected to enhance the ability of periodontal tissues to induce metabolic activity, thereby affecting tooth movement-induced root resorption during orthodontic treatment. However, to our knowledge, no study has reported on the effects of SHED-CM on root resorption during tooth movement. Therefore, this study aimed to investigate the effect of SHED-CM administration on root resorption and periodontal tissue during experimental tooth movement.