Osteopenia, or low bone mass, refers to a reduction in bone mineral density (BMD) below normal reference values without fulfilling the diagnostic criteria for osteoporosis. BMD is measured using dual-energy x-ray absorptiometry (DXA). According to the World Health Organization (WHO), osteopenia is defined by a T-score between -1.0 and -2.5, while a T-score less than -2.5 indicates osteoporosis. Decreasing BMD values reflect a disruption in bone microarchitecture, leading to diminished bone strength. Both osteopenia and osteoporosis represent quantitative rather than qualitative disorders of bone mineralization.