Passive-Dynamic Ankle-Foot Orthoses Affect Walking Stability on Sloped Terrain

Abstract

Sloped terrain is frequently encountered during walking and can lead to instances of instability, particularly in individuals with compromised gait. Individuals with lower limb injuries are thought to be less stable than able-bodied individuals, particularly in the medial-lateral direction. During gait, an individual is mediolaterally stable when the position of the center of mass is held inside the mediolateral edge of the base of support. Step width and center of mass can both alter this calculated margin of stability (MOS). PURPOSE: To quantify walking stability over level and cross-sloped terrain. METHODS: Twelve patients with orthopaedic lower limb injuries wore their clinically prescribed, passive-dynamic, carbon fiber ankle-foot orthosis (AFO) during the testing procedures. Twelve able-bodied control subjects also participated. Full body kinematic data were collected for one minute as individuals walked on level and cross-sloped terrain at a speed standardized by leg length. MOS was calculated as the minimum distance between the whole body center of mass and lateral border of the base of support (5th metatarsal head marker) in the mediolateral direction. A 2-way repeated-measures ANOVA with un-paired post-hoc t-tests compared the AFO limb to controls across the three terrain conditions. Effect sizes were also calculated (large effect: d>0.8). RESULTS: Individuals with lower limb injuries who wore passive-dynamic AFOs had a 10% greater MOS than controls when their AFO limb was downslope and effect sizes were large (control: 0.097±0.012 m, patient: 0.107±0.009 m, p=0.0279, d=0.96). The patient group achieved MOS values equivalent to controls when the AFO limb was upslope (control: 0.099±0.013 m, patient: 0.103±0.014 m, p=0.457, d=0.31) and on level terrain (control: 0.100±0.012 m, patient: 0.107±0.013 m, p=0.203, d=0.54). There were no differences in MOS across the three terrain conditions (control: p=0.404, patient: p=0.172). CONCLUSION: The semi-rigid design features of a passive-dynamic AFO may limit stability when walking on cross slopes, particularly when the AFO limb is downslope. These results are consistent with data from other populations and demonstrate a compensatory increase in MOS under some destabilizing conditions.