Comparison of Sagittal Plane Stiffness of Nonarticulated Pediatric Ankle-Foot Orthoses Designed to be Rigid

Introduction: When studying the effect of ankle-foot orthoses (AFOs) on gait, it is important to know their sagittal plane stiffness. However, there are no established thresholds for stiffness of nonarticulated AFOs designed to be rigid. If wanting to implement published algorithms for ankle-foot orthosis-footwear combinations (AFO-FCs), the AFOs must be equally as stiff as those of the developer of the published AFO-FC algorithms. Hence, the aim of this work was to compare the sagittal plane stiffness of AFOs designed to be rigid, made for a clinical trial in the United States, and following algorithms for AFO-FC designs, to those made and used clinically in the United Kingdom by the developer of the AFO-FC algorithms. Materials and Methods: Stiffness of nine pediatric polypropylene AFOs was tested (United Kingdom, six; United States, three). A computer-controlled motorized device was used in which all AFOs were clamped with the calf shell in a fixed vertical component and the foot section in a rotating plate. Each AFO was tested for three trials, loading the foot plate 30 Nm toward dorsiflexion and 20 Nm toward plantarflexion. Torque-angle graphs were plotted, and deflection and stiffness were compared descriptively across AFOs. Results: Average deflection of AFOs was 3.42 degrees +/- 0.83 degrees for United Kingdom and 4.81 degrees +/- 1.05 degrees for United States. Average stiffness of AFOs was 14.34 +/- 3.34 Nm/degrees for United Kingdom and 10.30 +/- 1.92 Nm/degrees for United States. Conclusions: All tested AFOs deflected only a few degrees in either direction (range, 2.59 degrees-6.02 degrees), providing the first information reported for the stiffness of rigid pediatric nonarticulated AFOs. Overall, the UK AFOs were stiffer and deflected less than the US study AFOs. AFO design features should be carefully considered as they likely influence sagittal plane stiffness and deflection under load.