**2. BMD overview**

#### **2.1 Bone mineral density**

Bone mineral density is the most-used parameter for the prediction of fracture risk in adults. According to the World Health Organization (WHO), bone could be subdivided into four groups based on the BMD values' variation. BMD values are obtained based on DXA images. Based on the calculation of T-score, which represents the number of standard deviation (SD) between studied BMD value and the average value of normal bone of adults of the same sex, the quality of bone can be determined. As far as adults are considered, there are some categories that are more susceptible to get osteoporosis than others; hence, physicians highly recommend them to undergo frequent BMD screenings. By way of illustration, we mention men above 50 years of age who have a historical fracture [5–8], women over 65 years of age, and sometimes the younger women who have an elevated risk of fracture [9]. Based on the fact that osteoporosis is a silent disease and since fragile bones are not painful until the occurrence of fracture, an early screening is always beneficial to detect this disease emergence [10].

For adults, a normal BMD value can be easily determined relative to a specific population. Nevertheless, BMD's use remains difficult for children because of the variation of their bone density and structure during growth. Generally, in this period, bone size and mass grow to reach 90% of the peak bone mass at 18 years of age [11]. Therefore, researchers cannot find a particular normal BMD value to evaluate the bone mineral density levels. In addition to age for children, several variables can also influence the BMD normal value, such as gender, body size, pubertal stage, skeletal maturation, and ethnicity [12]. All these limitations make a diagnosis of osteoporosis more complex [13].

### **2.2 Dual energy X-ray absorptiometry**

DXA imaging technic consists of sending x-rays through the human body, which allows the creation of interior body images based upon the variations of material absorption. Among the many advantages of this technic, we can note its short scanning time, its low radiation dose, and its low cost compared to other imaging technics. DXA provides 2D images of the scanned bone, which can help to extract areal bone mineral density (aBMD) in addition to the hip fracture index (HFRI), and to create a 2D model of the zone of interest, which can be used during finite element modeling. DXA-based finite element models arrive to some extent to determine bone strength and bone behavior vis à vis the mechanical loads. According to [14], DXA-based FE models permit to provide up to 74–77% of experimental femoral strength results. However, despite its advantages, DXA has a poor resolution of images, reducing their

quality, which makes the distinction between cortical and trabecular bone impossible. Moreover, DXA 2D images avoid bone structure, which affects the accuracy of obtained a BMD values.
