Background <p>Understanding dimensions of both sinus tarsi (ST) and tarsal canal (TC) in normal and flatfeet may influence the design, usage and possible control of postoperative complications of arthroereisis. This study was to assess 3D features of the tarsal sinus and canal complex (TSCC) in both normal control and flatfeet in children under non-weightbearing and weightbearing conditions.</p> Methods <p>Three-dimensional (3D) computer-aided design modeling from WBCT scans of 22 children with flexible flatfeet (age 9–14) and 14 with normal control feet (age 9–15) were used to evaluate volume and dimensions of TSCC. Correlations between Meary’s angle, hindfoot alignment, and volume of TSCC were calculated on non-weightbearing and weightbearing CT scans.</p> Results <p>The volume of TSCC was smaller in the flatfoot group compared with the control group both under weightbearing and non-weightbearing conditions. From non-weightbearing to weightbearing the volume of TSCC decreased by 19.1% in the flatfoot group and 13.1% in the control group while the long axis of TSCC moved superiorly and anteriorly with internal rotation in both groups. When bearing weight, TSCC volume was negatively linearly correlated with the hindfoot alignment and Meary’s angle.</p> Conclusions <p>This study demonstrates that weightbearing has different influences in 3D features of the tarsal sinus and canal complex in children with control and flatfeet. These findings may help understanding biomechanics of pediatric flatfoot and guiding treatment in particular when arthroereisis is used.</p> Level of evidence <p>Level III, Retrospective Comparative Study.</p>