Session: 02-09-01: Design for Healthcare Products and Processes

Paper Number: 141399

141399 - Passive Ankle-Foot Orthoses: A Novel Approach for Passive Support of Plantar and Dorsiflexion Based on Innovative Mechanism and Closed-Loop Logic 

Introduction and motivation

The shifting demographics worldwide signal a looming challenge: an increasingly aging population. Alongside this demographic transformation comes a surge in age-related health concerns, among which neurological afflictions such as strokes loom prominently. Patients who suffering on such diseases often have a so-called plantar and dorsal weakness, i.e. a reduction in lifting and lowering the foot during gait. The resulting gait abnormalities not only compromise mobility but also significantly elevate the risk of falls and associated injuries. An established treatment method is the use of so-called active or passive ankle foot orthoses (AFO). While active AFOs offer dynamic support through external energy sources, passive AFOs are more reliable because of the use of their inherent stiffness or the energy generated by the wearer itself. Despite their potential benefits, current passive AFOs predominantly focus on supporting dorsiflexion. Research into the passively support of plantar flexion is still at the beginning and is now to be significantly expanded by the innovative mechanism presented in this article.

Methods

The basic premise for a "normal" gait pattern is that the calf and shin muscles (plantar and dorsiflexors) can be activated to such an extent that the desired angular moments can be provided. In order to ensure that an AFO can provide a supporting moment, an appropriate database must first be created for the design process. After that, a reliable energy saving orthosis mechanism can methodically be developed. As the ankle joint angle and moment follow an anticyclical curse, it must be possible to switch the mechanism on and off at certain times. To test the principal functionality, an integration into a multi body simulation software (MBS) is needed. In this contribution a weakened torque curve of one test subject with medium gait speed and 50% plantar weakening (50% residual activity) into the Ansys Motion simulation environment was set. The corresponding activation points for the angle and moment curse are transferred directly as a Heaviside function. In order to be able to optimally scale the storage medium for the energy, 15 different design points were used.

Results

In the approach described in this contribution, the mechanism stored the required energy for the support in torsion springs. Furthermore, a OR/AND logic was developed to control the mechanism, which was first designed separately from the mechanism, then simulatively tested with Matlab Simulink and finally experimentally validated on a scaled model. As a closed system, this logic is able to transmit corresponding signals to the mechanism with so-called control pads that are placed on the sole and ball of the foot at the defined support times. The greatest moment due to the plantar flexion from a test subject occurs at the heel off (HO) with 99 Nm. The maximum torque provided by the mechanism during the HO was 79 Nm. The design points of the torsion springs were 3 Nm/° for plantar flexion and 0.4 Nm/° for dorsiflexion. The torque that could be applied by the residual muscle activity (50 Nm) was therefore increased by 58%.

Conclusion

The developed mechanism for supporting the entire gait cycle, including plantar flexion, showed very promising results in an initial simulation. Simulations demonstrated that simple torsion springs can provide the necessary support for weakened patients. Furthermore, a simple logic was used to simulatively and experimentally prove that by using just two sensor points on the sole of the foot, a mechanism can be controlled which supports the entire gait cycle. However, since a calculation required approx. 11 minutes of computing time and many further simulations must be carried out for a general formulation of the mechanism, a model order reduction with static condensation or a trained model should be considered. In future work, the logic will also be integrated directly into the simulation model in order to directly investigate its effects and interactions with the mechanism and the gait behavior.

Presenting Author: Patrick Steck Lehrstuhl für Konstruktionstechnik

Presenting Author Biography: Patrick Steck is a research assistant at the Department of Engineering Design at the Friedrich-Alexander University Erlangen-Nuremberg. His area of responsibility in the lightweight design team is mainly concerned with structural optimization, the design of fiber composites and compliant mechanisms, considering additive manufacturing techniques. Besides that, his expertise lies in the design and FEA/MKS-coupled calculation of orthoses with model order reduction.

Authors:

Patrick Steck Lehrstuhl für Konstruktionstechnik
David Scherb Lehrstuhl für Konstruktionstechnik
Jörg Miehling Lehrstuhl für Konstruktionstechnik
Sandro Wartzack Lehrstuhl für Konstruktionstechnik