Introduction

Long-term and excess fluoride consumption induces disturbed homeostasis of the bone and a series of chronic systemic diseases (Erdal and Buchanan, 2005). The World Health Organization has stated that the maximum safe limit of fluoride in drinking water is 1.5 ppm; however, more than 50 countries have high fluoride levels in drinking water (Rango et al., 2012; Perumal et al., 2013; Yu et al., 2018; Zhou et al., 2019). Fluorosis is endemic in at least 25 countries around the world, of which India and China are the most affected. East Africa and North Africa, Mexico, and Latin America are also endemic areas for fluorosis (Pramanik and Saha, 2017). Men are usually more likely to have severe fluorosis than women, and studies have shown that childhood fluorosis can affect bone development (Jha et al., 2011; Saeed et al., 2020). Skeletal fluorosis (SF) is the major clinical manifestation caused by the excessive accumulation of fluoride (Srivastava and Flora, 2020). The main pathological features of SF include joint pain, muscle weakness, and skeletal deformities (Wei et al., 2019). Clinical treatment is challenging when fluorosis patients have co-morbidities with osteoarthritis, fracture, or other severe bone trauma (Everett, 2011). However, no approaches for decreasing the concentration of fluoride in the body have been developed; these therapies may have inhibitory effects but cannot cure SF (Johnston and Strobel, 2020). Therefore, it is important to further understand the effect of fluoride on bone fracture healing for clinical treatment.

Fluoride directly affects the bones via two main mechanisms (Ciosek et al., 2021). In mineralized tissues, fluoride is incorporated into apatite crystals in the process of ion exchange, which leads to the formation of fluorapatite (Johnston and Strobel, 2020; Ciosek et al., 2021). Such conversion results in changes in crystallinity and a reduction in mechanical properties (Ciosek et al., 2021). In bioactive tissue, fluoride also stimulates osteoblasts and osteocytes in a concentration-dependent mechanism (Jiang et al., 2020a; Chu et al., 2020). These studies suggest that fluoride is important in bone metabolism, but there is no direct evidence that has reported the effects of excess fluoride on fracture healing.

Normally, 10–15% of these patients have unsatisfactory fracture healing (Hak et al., 2014; Loi et al., 2016; Ho-Shui-Ling et al., 2018). Unfortunately, when combined with bone metabolic diseases such as SF, the rate of delayed or non-union fractures was increased (Nampei and Hashimoto, 2009; Ellegaard et al., 2010; Marin et al., 2018). The events during the fracture healing cascade include the initial inflammatory phase, hematoma formation, progenitor cell recruitment, formation of an intermediate callus, maturation of the callus, and the final remodeling of the bony callus to the original bone’s structure and shape (Alexander et al., 2011; Einhorn and Gerstenfeld, 2015). Commonly, failure in the bone healing process is due to insufficient numbers of progenitor cells and disrupted vascularization (Pajarinen et al., 2019). Macrophages play an important role in recruiting progenitor cells and angiogenesis (Schlundt et al., 2018; Pajarinen et al., 2019). Macrophages may acquire distinct phenotypes with proinflammatory (M1) or anti-inflammatory (M2) functions at the fracture site, a phenomenon known as macrophage polarization (Schlundt et al., 2018). Up to now, the effect of fluoride on the polarization of macrophages and the M1/M2 ratio has been unknown.

In the present study, we aimed to explore whether excessive fluoride ingestion has effects on bone fracture healing. We first found that excessive fluoride exposure delayed bone fracture healing, especially at the stage of maturation of the callus. Subsequently, we presented in vitro evidence to support that high fluoride concentration can effectively inhibit M2 differentiation. The findings in this study suggest that the decrease of M2 may be related to delayed fracture healing and provide a new research direction for the treatment of skeletal fluorosis…

Conclusion

Our results revealed that the high fluoride intake impaired bone fracture healing. The high fluoride concentration affects the polarization of macrophages, resulting in a decrease in M2 differentiation. Although this study does not directly address the cause-and-effect relationship between fluoride-induced macrophage polarization and the impairment of fracture repair, it still provides an important reference for the clinical treatment of bone fracture patients with a history of high fluoride intake or SF. Meanwhile, it also provides a reference for the pathological study and treatment of SF.

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*Original abstract and full-free text online at https://www.frontiersin.org/articles/10.3389/fbioe.2022.791433/full