A New Development of Powered Orthotic for Deficient Push-Off Power

XueCheng Liu, Luis A. Rodriquez, Zhiyuan Yang, Qiu Wang, Robert Rizza

Rudi Kobetic(rkobetic@fescenter.org)

Rong Luo, XueCheng Liu, Luis A. Rodriquez, Zhiyuan Yang, Qiu Wang, Robert Rizza, A New Development of Powered Orthotic for Deficient Push-Off Power(2017)SDRP Journal of Biomedical Engineering 1(1)

Background

During the gait cycle, power generated during the push-off stage by individuals with Cerebral Palsy (CP) is deficient. Associated with this power is a deficient moment about the ankle. Current ankle and foot orthotics (AFO) can restrain abnormal joint motion, improve the kinematics, and stabilize gait and posture but cannot provide augmented moment and power during push-off phase. Thus gait of CP patients is not improved during push-off by traditional orthotics.

Methods

In this study, a new powered orthotic that will supply the deficient power and control foot drop was developed. Fundamental principles and analysis of fluid flow were applied in the design. A design using a Pneumatic Artificial Muscle (PAM) was developed.

A dynamic model was established which uses the patient’s measured clinical gait motion and is able to predict the amount of foot drop and deficient power. This model was coupled with a control system to provide proper sequencing in the activation and deactivation of the powered device.

The device is designed so that it may be “tuned” to each patient based on the patient’s weight, foot size and dynamics of gait. The device is designed for CP children 6-9 years of age.

Results

For a specific patient, the chosen PAM was tested in the laboratory and the kinetics and kinematics of the device were established. It was found that the displacement (stroke) changes nonlinearly with time and the displacement reaches its maximum value which is 1.55 in (0.039 m) at about 0.2 sec. On the other hand, the force output of the PAM varies linearly with displacement, and it takes about 0.3 sec to reach the maximum value of the force which is 32 lbf (142.3 N). For a specific patient, with a specific foot size, this  generates a moment about the ankle equal to 7.73 N·m (68.4 lbf·in) This moment at 4.786 rad/s produces augmented power about the ankle equal to the deficient power in the CP patient.

Simulations using the established dynamic model were found to accurately predict the deficient amount of power for the CP patient. Also, it was found that the model was able to predict the time during the gait cycle when the PAM should be activated to provide augmented power.

Conclusions

The new powered orthotic device provides supplemental power to augment for the patient’s deficient power and further improves the quality of the second ground reaction force peak (GRF2) at push-off. Compared to traditional orthotic designs, the new powered design has the potential to increase power at the third rocker despite the CP patient’s reduced muscle strength and increased spasticity.

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