A Method-Comparison Study Highlighting the Disparity between Osseous- and Skin-Based Measures of Foot Mobility

Purpose: This study examined the validity of standard clinical measures of arch height mobility, midfoot width mobility (MWM), and foot mobility magnitude (FMM) relative to skin-based and osseous measures derived from radiographs. Methods: Skin-based clinical indices of foot mobility were calculated from standard, caliper-based measures of foot length, midfoot width, and dorsal arch height of the left limb of 20 healthy participants (8-71 yr) during non-weight-bearing and weight-bearing. Skin-based radiographic and osseous indices were derived from concurrent anteroposterior and lateral radiographs. Agreement between skin-based clinical and skin-based radiographic measures of foot mobility with those of osseous measures was investigated using the Bland and Altman approach. Results: Foot mobility indices derived from clinical measures were significantly higher (20%-50%) than skin-based radiographic measures (P < 0.01), which were, in turn, significantly higher (200%-250%) than osseous measures (P < 0.01). Clinical measures demonstrated significant levels of proportional bias compared with radiographic measures of foot mobility (P < 0.01). The contribution of osseous movement to skin-based clinical measures of mobility was highly variable between individuals, ranging between 19% and 81% for arch height mobility, between 4% and 87% for MWM, and between 14% and 75% for FMM. The limits of tolerance for clinical measures of foot mobility ranged from +/- 3.2 mm for MWM to +/- 6.6 mm for measures of FMM. The limits of tolerance for skin-based clinical and skin-based radiographic measures were generally larger than osseous movement with weight-bearing. Conclusions: Skin-based measures of foot mobility, whether clinical or radiographic methods, are not interchangeable and are poor indicators of osseous mobility. Although further research regarding the utility of osseous measures is warranted, these findings strongly caution against the use of skin-based clinical measures of foot mobility in clinical and research settings.