Impact of weight-bearing on foot shape: a geometric and morphometric assessment using principal polynomial shape analysis

Weight-bearing computed tomography (WBCT) has set a new standard for assessing foot alignment under physiological loading conditions. However, few studies have systematically quantified morphological differences between weight-bearing (WB) and non-weight-bearing (NWB) conditions. Therefore, the purpose of this study was to evaluate WB-induced foot shape changes using Principal Polynomial Shape Analysis (PPSA), a recent non-linear technique in geometric morphometrics. In this retrospective matched-pair study, we analysed 78 feet from 39 subjects scanned under WB conditions (WBCT-based) and 78 feet from 39 matched subjects scanned under NWB conditions (MRI-based). 3D foot bone models were reconstructed, registered, and compared. Radiographic measurements were calculated and statistically analysed between groups. As a result, PPSA captured 65.4% and 69.6% of shape variance in the first three components for the WB and NWB groups, respectively. Distance mapping revealed morphological deviations of up to 13 mm between the two groups, with the most statistically significant differences localised at the talonavicular joint (P < 0.05). These findings were further supported by radiographic measurements (P < 0.05). This study demonstrates that PPSA effectively captured subtle yet systematic shape changes induced by WB, offering a more comprehensive representation of WB-NWB morphological differences than conventional 2D radiographic measurements.