Development of a Custom 3D Printed Plantarflexion Stop for Foot Drop

Foot drop, characterized by the inability to dorsiflex the ankle during gait, can stem from neurologic, traumatic, or compressive etiologies. Management often focuses on mitigating fall risk and restoring functional mobility, especially in progressive cases like Charcot-Marie-Tooth disease where reversal is unlikely. Ankle-Foot Orthoses (AFOs) are commonly prescribed to support dorsiflexion and prevent plantarflexion-related gait deviations. The Hermes brace by Icarus Medical Innovations offers a unique solution through its adjustable dorsiflexion tension system, customizable via a user-operated dial. Despite its innovations, the Hermes lacks a plantarflexion stop, limiting its efficacy for patients who exhibit excessive or uncontrolled plantarflexion. This study aimed to address this limitation by designing a modular plantarflexion stop compatible with the Hermes brace. Design criteria were established based on anatomical torque data, user comfort, brace weight constraints, and spatial compatibility with the existing frame. An iterative prototyping process was employed, culminating in a fixed mechanical stop consisting of top and bottom components that collide to limit ankle rotation. Finite Element Analysis (FEA) and material testing demonstrated that a Nylon 6/6 prototype met the required strength-to-weight ratio, yielding a safety factor of 2.2 while adding only 0.11 oz to the device. The proposed design provides sufficient plantarflexion restriction without compromising the lightweight, low-profile nature of the Hermes brace. While effective, the prototype requires further refinement to minimize potential pinch points and improve user safety. Compared to existing AFOs, functional electrical stimulation, and surgical interventions, the Hermes, with its now expandable modular features, offers a mechanically simple, scalable, and non-invasive solution suitable for a wide range of foot drop etiologies. Future development will focus on making the plantarflexion stop adjustable for customizable gait control and validating performance through torque testing and Failure Mode and Effects Analysis (FMEA).