The 3D-pQCT equipment allows the evaluation of changes occurring within the bone at "microarchitecture" level, without the need for invasive bone biopsies. The primary objective is to evaluate the sensitivity of the technology to detect a difference between those treated with risedronate 35mg OAW (once a week) or placebo. Early phase postmenopausal women with osteopenia have been chosen because they have a more rapid and higher level of bone loss during the first few years of the menopause.