Dual-stiffness passive prosthetic ankle for direction-dependent stability

Background

Lower-limb amputees often struggle to maintain balance and stability during everyday activities, particularly when turning or navigating uneven surfaces, because the loss of natural ankle musculature and proprioception limits their ability to adapt to changing loads and directions. Most prosthetic ankles provide a single fixed stiffness, which does not match the variable mechanical demands of movements like turning, where the inside and outside of the foot require different stiffness levels to maintain control.

While powered or microprocessor-controlled devices attempt to address this, they remain expensive, heavy, and maintenance-intensive, leaving the majority of amputees dependent on simpler passive devices that do not adequately support dynamic movement. This gap underscores the need for prosthetic solutions that can deliver natural, adaptive stiffness without the complexity, cost, or power requirements of advanced electronic systems.

Technology overview

This technology introduces a passive dual-stiffness prosthetic ankle designed to automatically provide direction-dependent stiffness during gait, particularly during turning maneuvers. The device uses a mechanical system with offset internal grooves and two sets of springs housed within a split cylindrical structure, ensuring that only one spring set engages at a time depending on inversion or eversion. This allows the ankle to deliver higher or lower stiffness based on the direction of coronal-plane rotation, closely mimicking natural ankle behavior.

The ankle includes an adjustable preloading mechanism for baseline stiffness customization, is lightweight and compact, and can be integrated into standard prosthetic setups. Springs can be swapped or stacked to tailor mechanical response for individual users, and the system can be scaled or adapted for additional planes of motion. By providing passive, automatic adaptability without electronics or external power, the device delivers reliable, low-cost performance that improves stability during complex movements.

Benefits

• Provides automatic direction-dependent stiffness for improved balance
• Passive mechanical design eliminates need for power or electronics
• Adjustable preloading and customizable spring configurations
• Lightweight, compact, and compatible with existing prosthetic systems
• Enhances stability during turning and uneven-surface walking

Applications

• Lower-limb prosthetics
• Rehabilitation engineering
• Mobility assistance devices
• Pediatric and adult prosthetic systems
• Adaptive passive prosthetic components

Opportunity

• Offers a low-cost, scalable alternative to complex powered ankles
• Addresses a critical unmet need for dynamic stability during turning and off-axis motions
• Easily integrated into existing prosthetic product lines
• Available for exclusive licensing

Intellectual property

U.S. Provisional application serial no. 63/589,677 filed on 10/12/2023

PCT application serial no. PCT/US2024/050902 filed on 10/11/2024, published as WO2025/080929

https://patents.google.com/patent/WO2025080929A1/en?oq=WO+2025%2f080929