As is a chronic inflammatory disease, mainly involving sacroiliac joint and spine, but also involving peripheral joints. It is characterized by inflammation of tendon and ligament attachment points, and then, leads to bone destruction and heterotopic ossification. The prevalence rate of AS is about 0.25%, and the age of onset is 20-30years old. Epidemiological investigations found that the prevalence of osteoporosis and osteopenia of AS was more than 60%[2]. The risk of vertebral fracture in AS patients caused by osteoporosis and osteopenia was significantly increased. Malochet-Guinamand et al.[13] found that: about 6.2% of AS patients had at least one vertebral fracture. In the 2-year follow-up study, 6% of AS patients had new fractures during the follow-up period[14]. As a result of abnormal bone metabolism in the early stage of AS, it is particularly important to identify the changes of bone metabolism early to prevent the occurrence of osteoporosis and fracture. In this study, all objects enrolled were man with the age between 18 and 45 years, in order to eliminate interference factors.
Bone remodeling was an equilibrium process, which involved bone resorption by osteoclasts and bone formation by osteoblasts. Vasikaran et al.[15]found that BTMs could be used to rapidly indicate the condition of bone loss or formation. BTMs were generally subdivided into bone resorption markers, bone formation markers and osteoclast regulatory proteins. The representative marker of bone resorption was β-CTX. While bone formation markers were OC and P1NP. Studies were reported that BTMs were used to understand bone metabolism, predict fracture risk and bone healing[16]. However, BTMs were not tested routinely, and they were expensive to evaluate or treatment monitoring in osteoporosis or osteopenia in AS patients. Therefore, UA was a convenient and economical laboratory index. UA was reported to closely related to bone metabolism, and played an important role in the pathogenesis of osteoporosis[17]. Most reports found an increased serum UA level was associated with a lower risk of fracture[18]. In our study, we found that the levels of UA were significantly higher in healthy controls than in AS patients. It showed that UA played an important role in osteoporosis of AS patients. Yan et al.[19] reported that serum UA levels were significantly associated with osteoporosis, and serum UA levels were positively correlated with the BMD of the femoral neck, total hip, and L1-4 in females. Furthermore, UA was negatively correlated with OC and β-CTX in females. However, there was no relationship between UA and BTMs in males. We found that UA was significantly negative correlated with OC and β-CTX in healthy young males. Bone resorption and bone formation are the process of dynamic balance. It again confirmed that serum UA levels were closely associated with bone metabolism.
In low BMD group of AS patients, UA was significantly positive correlated with β-CTX which was the marker of bone resorption. The mechanism of bone metabolism is complex in AS. It is characterized by both abnormal bone formation and bone resorption[20]. Therefore, UA played an important role in bone resorption of AS patients with low BMD. UA could reflect bone resorption in AS patients with low BMD, and could be used as a marker for the treatment of osteoporosis (or osteopenia) and following-up. Whether reducing UA can improve osteoporosis in AS with low BMD remains to be further studied.
In conclusion, UA was associated with bone metabolism, and it could reflect bone resorption in AS patients with low BMD. UA could be used as a marker for the treatment of AS with low BMD and following-up.