In contrast with bisphosphonates,discontinuation of denosumab results in gradual loss of bone mineral density gains. The investigators aim to evaluate whether in patients treated with denosumab, a single zoledronic acid infusion would prevent the anticipated bone loss.

Discontinuation of denosumab results in a rebound rise of bone turnover markers and gradual loss of the achieved bone mineral density gains. In contrast, bisphosphonates, such as zoledronic acid, remain within the skeleton acting for several months or even years after discontinuation while maintaining bone mineral density despite the cessation of treatment. In this study, the investigators aim to evaluate changes in bone mineral density of the lumbar spine and the femoral neck, as well as in bone turnover markers one year after treatment discontinuation both in denosumab-treated women and in denosumab-treated women who switched to zoledronic acid infusion one year before treatment discontinuation.

Inclusion Criteria:

• postmenopausal women
• osteopenic (T-score > -2.5 but < -1.0) after treatment with denosumab

Exclusion Criteria:

• secondary osteoporosis;
• diseases that could affect bone metabolism;
• medications that could affect bone metabolism;
• history of any antiosteoporotic treatment other than denosumab prior to randomization
• severe liver or kidney disease (creatinine clearance < 60ml/min/1.73m2)