We calculated the mean change in tibia trabecular volumetric BMD Z-score between baseline and 12 months, as measured by peripheral quantitative computed tomography (pQCT). The Z-score, or Standard Deviation Score, is a measure of the number of standard deviations that an individual is above or below the median value in a healthy child or adolescent of the same age, sex and race. For example, a Z-score of 0 means that an individual's result is equivalent to the 50th percentile in a healthy population. A Z-score of -1.0 means that an individual's result is equovalent to the 16th percentile in a healthy population.