HR-pQCT precision errors were between 1 and 8% (LSC 3-21%) for trabecular bone outcomes and 1 and 11% (LSC 2-30%) for cortical bone outcomes at the radius and tibia in children. Cortical bone outcomes obtained using the advanced cortical evaluation appeared to have lower precision errors than cortical outcomes derived using the standard evaluation. These findings, combined with better-defined cortical bone contours with advanced cortical evaluation, indicate that metrics from advanced cortical evaluation should be utilized when monitoring cortical bone properties in children.

High‐resolution peripheral quantitative computed tomography (HR‐pQCT) imaging, together with computational finite element analysis (FEA), offers an attractive, noninvasive tool to quantify bone strength development in pediatric studies. Evidence of annual changes and errors in repeated HR‐pQCT measures is limited, and time intervals required to reliably capture changes in children's bone strength or microarchitecture have not yet been defined. Our objectives were: (1) to quantify annual changes in bone strength and microarchitectural properties; (2) to define precision errors for pediatric bone strength outcomes; (3) to characterize annual changes in contrast to pediatric precision errors; and (4) to estimate monitoring time intervals (MTIs) required to reliably characterize bone development at the distal radius and tibia. We obtained distal radius (7% of ulnar length) and tibia (8%) bone properties using HR‐pQCT and FEA from 38 follow‐up study participants (21 girls) at baseline (mean age 10.6 years, SD 1.7 years) and after 1 year; and from 32 precision study participants (16 girls) at baseline (mean age 11.3 years, SD 1.6 years) and after 1 week. We characterized mean annual changes (paired t tests) contrasted to pediatric precision errors (CV%RMS) and estimated MTIs. Annual increases in bone strength, total area, cortical thickness, and density ranged between 3.0% and 25.3% and 2.4% and 15.6% at the distal radius and tibia, respectively. Precision errors for all bone strength outcomes were ≤6.8% and ≤5.1% at the distal radius and tibia, respectively, and appeared lower than annual gains in bone strength at both sites. Cortical porosity decreased 19.6% at the distal radius and 6.6% at the distal tibia; these changes exceeded respective precision errors, indicating cortical bone consolidation. MTIs ranged between 0.5 years and infinity at the distal radius and 0.5 and 5.9 years at the distal tibia. Estimated MTIs suggest that pediatric bone strength, cortical bone density, and porosity development can be reliably monitored with annual measurements. © 2019 American Society for Bone and Mineral Research.