## 1 Introduction  
Osteoporosis is a systemic skeletal disorder that is characterized by low bone mass, decreased bone strength, and the microarchitectural deterioration of bone tissue. Osteoporosis often developsasymptomatically, and it is generally found at a first occurrence of fragility fracture. Due to the serious consequences of osteoporosis, fractures often cause a heavy economic burden, severely impact patients' quality of life, and increase disability and mortality, especially when a fracture occurs at the hip [1].  
Clinical therapies for the treatment of primary osteoporosis (i.e., menopause- and aging-related osteoporosis) are mainly classified as antiresorptive and anabolic. The anti-osteoporosis drugs approved by the United States Food and Drug Administration (FDA) include bisphosphonates (BPs) (i.e., alendronate, risedronate, ibandronate, and zoledronic acid, etc.), hormone replacement therapy (HRT), raloxifene, denosumab, and calcitonin as antiresorptive, and teriparatide as anabolic [2].  
Decreasing the risk of fracture and prevention of future fragility fractures are the primary goals for the clinical treatment of osteoporosis. Therefore, the prediction of fracture risk is an important subject and worthy of in-depth research. The relationship between bone mineral density (BMD) measurement and fracture risk has been clearly established, and BMD remains the best test for assessing fracture risk. However, BMD (bone mineral density) cannot identify individuals who will develop a fracture in the future [3]. Due to a general overlap in the bone densities of patients who tend to develop a fracture and those who do not, a high ratio of bone turnover over time has been considered an additional factor that contributes to future fracture [4]. Bone turnover markers (BTMs) are a series of proteins and peptides that are released during the metabolic process of bone remodeling and include markers of bone formation and bone resorption [5]. They provide a noninvasive approach for studying bone turnover and are easily and precisely measured, especially by automated analyzers. In most studies, high levels of BTMs were associated with an increased fracture risk of spine and hip in women [3; 4; 6].  
Clinically, BTMs and BMD are independent predictors of fracture risk for clinicians. However, fracture risk increases with the combined consideration of alterations in both factors rather than each factor individually. Nevertheless, BTMs seem a better choice for physicians in the monitoring of early responses to anti-osteoporosis therapy because of the delayed response of BMD to clinical treatment. Since both predictors are independent and essential, many researchers studied the relationship between the changes in biochemical bone markers and BMD in treated and untreated osteoporoptic patients to evaluate the value of bone markers in predicting long-term BMD changes. However, the results of these studies are sometimes controversial and generally exhibit significant disparity across studies. The present study performed a systematic review and meta-analysis on the correlations between clinically used markers of bone turnover and BMD at various bone sites in patients treated for osteoporosis to examine the potential use of biochemical markers of bone turnover in predicting long-term treated BMD changes and to identify some promising biochemical markers.