A model to predict the effect of ankle joint misalignment on calf band movement in ankle-foot orthoses

Accurate alignment of anatomical and mechanical joint axes is one of the major biomechanical principles pertaining to articulated orthoses, yet knowledge of the potential effects of axis misalignment is limited. The purpose of this project was to model the effects of systematic linear (proximal-distal and anterior-posterior) misalignments of single axis mechanical ankle joints in an ankle-foot orthosis (AFO) in order to determine the degree and direction of calf band travel that would occur over a functional range of motion. Sagittal plane misalignments of the ankle joint centres of an AFO were simulated using a simple two-dimensional model for both a range of ankle angles and a typical able-bodied ankle kinematic curve for self-selected normal walking speed. The model assumed that no movement occurred between the foot and the foot-plate of the AFO. The model predicted that for anterior (positive horizontal) misalignments, dorsiflexion movements would cause the calf band to travel proximally (i.e., up the leg) and plantar flexion movements would cause the calf band to travel distally (i.e., down the leg). The opposite was predicted for posterior (negative horizontal) misalignments. Proximal (positive vertical) misalignments would cause only distal movements of the calf band while distal (negative vertical) misalignments would cause only proximal movements of the calf band. Anterior-posterior misalignments were found to have a much larger effect on the amount of calf band travel than proximal-distal misalignments.