Design of an automated device to measure sagittal plane stiffness of an articulated ankle-foot orthosis

The purpose of this study was to design a new automated stiffness measurement device which could perform a simultaneous measurement of both dorsi- and plantarflexion angles and the corresponding resistive torque around the rotational centre of an articulated ankle-foot orthosis (AAFO). This was achieved by controlling angular velocities and range of motion in the sagittal plane. The device consisted of a hydraulic servo fatigue testing machine, a torque meter, a potentiometer, a rotary plate and an upright supporter to enable an AAFO to be attached to the device via a surrogate shank. The accuracy of the device in reproducing the range of motion and angular velocity was within 4% and 1% respectively in the range of motion of 308 (158 plantarflexion to 158 dorsiflexion) at the angular velocity of 108/s, while that in the measurement of AAFO torque was within 8% at the 08 position. The device should prove useful to assist an orthotist or a manufacturer to quantify the stiffness of an AAFO and inform its clinical use

Helical axis calculation based on Burmester theory: experimental comparison with traditional techniques for human tibiotalar joint motion

In prosthetics and orthotics design, it is sometimes necessary to approximate the multiaxial motion of several human joints to a simple rotation about a single fixed axis. A new technique for the calculation of this axis is proposed, originally based on Burmester’s theory. This was compared with traditional approaches based on the mean and finite helical axes. The three techniques were assessed by relevant optimal axis estimation in in vitro measurements of tibiotalar joint motion. A standard jig and radiostereometry were used in two anatomical specimens to obtain accurate measurements of joint flexion. The performance of each technique was determined by comparing the motion based on the resulting axis with the experimental data. Random noise with magnitude typically similar to that of the skin motion was also added to the measured motion. All three techniques performed well in identifying a single rotation axis for tibiotalar joint motion. Burmester’s theory provides an additional method for human joint motion analysis, which is particularly robust when experimental data are considerably error affected