This 2 arm study will compare the effect of oral Bonviva (150mg, monthly) and placebo on parameters of bone micro-architecture, assessed by CT scan, bone turnover and bone mineral density. Patients will be randomized to receive either Bonviva 150mg po or placebo monthly, and measurements will be taken at baseline, and at intervals over 2 years. The anticipated time on study treatment is 2+ years, and the target sample size is 100-500 individuals.