**3.2 C-terminal telopeptide of type 1 collagen (CTX)**

In the process of bone resorption, the mature type I collagen is cleaved and the C-terminal peptide and N-terminal peptide are removed. The common C-terminal peptides are α-CTX and β-CTX, which are isomers, and their production rate is equal to the degradation of type I collagen. CTX and NTX are released into the blood with the degradation of type 1 collagen molecules and can be excreted in the urine. Therefore, the concentrations of CTX and NTX in the blood and urine can specifically reflect the activity of osteoclasts and the level of bone resorption [67]. Since β-CTX has been studied more as a marker of bone resorption, it is clinically used as a sensitive and specific marker of bone resorption [68]. At the same time, CTX-I showed a circadian rhythm, and its concentration peaks usually appeared at night and early morning, and reached the lowest point in the afternoon [69]. And for the measurement of CTX-I, food intake has a greater impact on the results, so it is necessary to measure CTX-I in a fasting state [70].

A number of studies on women have suggested that elevated β-CTX is associated with fracture risk. Vilaca (2017) found that for each standard deviation increase in serum β-CTX, the risk of vertebral fractures increased by 1.4–2.2 times, and the risk of non-vertebral fractures increased by 1.8–2.5 times, and the results were basically

unchanged after adjusting for BMD, indicating that CTX has an independent predictive effect on fracture risk [66]. Fracture risk is better predicted if CTX is combined with BMD. The Swedish EPIDOS study showed that the 10-year fracture risk of postmenopausal women from high to low was as follows: ① Elevated serum β-CTX + history of fragility fracture; ② Elevated β-CTX + T value of BMD lower than - 2.5; ③ BMD Women with a T value below −2.5 + a history of fragility fracture; ④ elevated β-CTX or a history of fragility fracture; ⑤ BMD T value below −2.5 [66]. CTX may have a good application prospect in predicting the occurrence of osteoporotic fractures. However, it is still difficult to popularize and apply in clinical practice, and the results are still uncertain due to the high heterogeneity among different studies. Therefore, further large-sample, homogeneous prospective studies are still needed for detailed clarification in the future.

#### **3.3 Periostin**

Periostin is a newly discovered macromolecular glycoprotein. As a unique extracellular matrix protein, it is mainly expressed in the periosteum, also known as bone-specific factor 2 which is obtained from the osteoblast cell line MC3T3-E1 cDNA library by Takeshita et al. (1993). A bone adhesion have a molecular weight of 90-kDa [71]. Periostin mainly triggers signaling pathways such as NF-KB/STAT3, P13K/Akt and focal adhesion kinase (FAK) by binding to cell surface integrin receptors αvβ3 and αvβ5, and regulates the expression of downstream genes. It plays an important role in adhesion, tissue repairing and maintaining the integrity of connective tissue structure and function [72].

Basic research suggests that Periostin can regulate bone formation, promote bone development/remodeling, and increase bone strength. It is a key regulator of bone microstructure and plays a very important role in bone metabolism [73, 74]. Regarding the clinical study of periostin, Li et al. (2021) showed through crosssectional observation in postmenopausal women that periostin has no significant correlation with the overall BMD [75], but is positively correlated with cortical bone density, negatively correlated with cortical bone porosity. Periostin is primarily responsible for periosteal metabolism, so it is more closely related to long bones covered by periosteum and can better reflect cortical bone loss [76]. Further studies suggested that periostin was not associated with baseline BMD and was significantly elevated in women with fractures [77–79]. Kim emphasized that it was primarily a risk factor for nonvertebral fractures [80]. Rousseau proposed that periostin is an independent risk factor for fractures in postmenopausal women, and microarray analysis suggested that periostin mRNA was up-regulated twice in the process of osteoporosis and fracture repairing [77].

In conclusion, periostin, as a new-generation biochemical marker of bone metabolism, is an independent risk factor for fractures among postmenopausal women. Combined with bone mineral density testing, it can better evaluate and predict the risk of osteoporosis and fracture in patients, and provide a theoretical basis for early intervention.
