Apex 2

Dual energy X-ray absorptiometry (DXA) scans of the lumbar spine, proximal femur and total body were obtained using Hologic, Inc. (Bedford, MA) bone densitometers (QDR4500A, QDR4500W, Delphi A and Apex models). Scans were analyzed at the University of California, San Francisco’s DXA Core Laboratory using Hologic software (Discovery 12.3 at baseline and Apex 2.1 at post-baseline study visits using the “compare” feature). Spine, total hip and femoral neck aBMD, and total body less head (TBLH) BMC values were adjusted based on the cross-calibration of DXA devices and longitudinal calibration stability. TBLH BMC (rather than aBMD) was used because it is the preferred measure of bone status for the total body when adjusted for body size^{(28 (link))}. aBMD and BMC Z-scores were calculated using the BMDCS reference values to account for known increases and sex differences in aBMD/BMC during growth^{(23 (link))}. aBMD and BMC-Z were adjusted for height-for-age Z-scores to minimize potential confounding by skeletal size^{(29 (link))}.

Whole-body DXA scans were acquired on Hologic, Inc. (Bedford, MA, USA) bone densitometers (QDR4500A, QDR4500W, Delphi A, and Apex models). Central analysis of all scans was performed by the DXA Core Laboratory (University of California, San Francisco, San Francisco, CA, USA) using Hologic software version Discovery 12.3 (baseline scans) and Hologic Apex 2.1 software. Total fat mass and lean body mass (excluding bone) were determined and corrected for intermachine differences and longitudinal drift.