Development of a Motorized Device for Quantitative Investigation of Articulated AFO Misalignment

The objective of the study is to develop a motorized device to quantitatively investigate the AFO alignment and mechanical properties. The motorized device consists of a servo motor and an inline gear box with a 25:1 gear ratio. A thermoplastic articulated AFO with TamarackTM dorsiflexion assist flexure joint was investigated in the study. The motor shaft was used to mimic the anatomical ankle joint. The joint of the AFO was aligned in such a way that the offset could be systematically manipulated (aligned exactly with the center of motor shaft, 10mm superior, inferior, anterior or posterior with respect to the center of motor shaft). The AFO was passively moved in the range from 30 degrees plantar flexion to 20 degrees dorsiflexion at a prescribed speed of 3 deg/s. 11 complete cycles were run lasting about 400 s. Index of hysteresis was calculated as the ratio of area within the loop to the area below the ascending limb and presented in percentage (%). In addition, passive resistance torque and quasi-static stiffness were quantified at prescribed positions. Based on the preliminary results the perfect alignment will result in the lowest resistance torque, quasi-static stiffness and moderate hysteresis index. However, the alignment of 10 mm inferior showed the largest hysteresis index indicating an excessive energy loss during unloading phase and higher resistance torques during the loading phase. It suggested that alignment excessively below the anatomical joint should be avoided in clinical practice. The alignment of 10 mm superior showed comparable quantities to the perfect alignment and could be recommended in practice if perfect alignment is hard or not possible to implement. In summary, we have demonstrated that the device could be a powerful quantitative tool for practitioners and researchers. It is expected that the information obtained will be valuable to clinicians and provide recommendation on decision-making. In addition, the quantitative results will be useful in simulation and modeling studies.