Switching From Oral Bisphosphonates to Bazedoxifene to Evaluate Effects on Bone Mineral Density in Postmenopausal Women

The primary hypothesis is: "Comparing to control group (Calcium+VitD), Bazedoxifene group (20mg daily+ calcium+VitD) is efficient in reducing BMD in spine at 12 months of treatment. in postmenopausal women switching from daily, weekly or monthly bisphosphonates therapy. It is a non-inferiority study of Bazedoxifene compared to biphosphonates, being each patient its own control from the baseline. It is a superiority study of Bazedoxifene compared to Calcium+VitD. Taking into account the difference in the percentage change observed in the total BMD of the spine during 12 months in those patients treated with Bazedoxifene+Ca+VitD at least 110 patients should be included, 55 in each arm of treatment. This simple size is going to allow us to detect the differences in the percentage change observed in the total BMD of the spine during 12 months between the two treatment arms, being the same or over 0.25%, with a significance level of 95%, an 80% power. It is estimated a standard deviation of 0.5%, and a 10% lost follow-up rate.