4. Suboptimal response to bisphosphonate treatment, defined as either one of the following: (1) failure of lumbar spine, femoral neck or total hip BMD values to increase; (2) values of lumbar spine, femoral neck or total hip BMD remain osteoporotic, ie. T scores < -2.5 or Z scores < -2.0; or (3) development of new fragility vertebral or non-vertebral fractures despite at least 2 years' treatment with good compliance.