Inclusion Criteria:  Postmenopausal women or men over 50 years old Denosumab treatment for at least two years and less than three years (up to five doses).  Exclusion Criteria:  Estimated glomerular filtration rate <35 ml/min. Malignancy Continuous steroid treatment, hormone therapy or other medical treatment affecting bone metabolism Secondary osteoporosis Metabolic bone diseases Contraindications to ZOL Patients older than 80 years old Hypocalcemia