Instantaneous centers of rotation in dorsi/plantar flexion movements of posterior-type plastic ankle-foot orthoses.

Hingeless plastic ankle-foot orthoses (PAFOs) achieve ankle motion by flexing about the ankle joint. Instantaneous centers of rotation (ICRs) in dorsi- and plantarflexion movements, used as a measure of PAFO axes of movement, were measured to evaluate their fit to ankle motion. Thirty different PAFOs were fabricated and their stiffness modified in three stages. They were dorsi- and plantarflexed 16 degrees at 2 degrees-intervals using an original device. Displacement of two marks on the lateral calf-cuff were traced photographically, and ICRs were determined by plotting intersections of vertical bisectors for each displacement. The ICRs converged on the junction between the calf shell and the shoe insert. They deviated posteriorly from the anatomical ankle axis and caused the calf-cuff to move up-down during dorsi- and plantarflexion movements. However, this poor fit of the PAFO to ankle motion can be sufficiently compensated for by fastening straps more loosely.