Mitigating the Slope Effect

Life has its ups and downs, but for those with lower-limb loss, this phrase takes on new meaning. Navigating uneven terrain presents daily challenges that many of us take for granted. The natural ankle adapts seamlessly to slopes through approximately 30 degrees of sagittal plane motion at the tibiotalar joint alone, supported by dozens of additional joints throughout the foot and ankle complex. For people with limb loss, a prosthetic foot—and ideally a prosthetic ankle—must replicate this adaptability.

The human ankle realigns itself instantly to stabilize the knee and hip with minimal conscious effort. A prosthetic foot and ankle, however, are aligned around a fixed neutral point determined during static alignment and refined during dynamic adjustment. Here lies the fundamental challenge: The prosthesis is optimized for level ground. On any slope, the user must manage destabilizing torque at the knee and hip through the residual limb at the socket interface—a force that static-alignment prostheses were never designed to accommodate.¹ Over many years, prosthetists and manufacturers addressed this through gel liners, flexible sockets, and foam or rubber bumpers in single and multiaxial feet, reducing but never eliminating these torques.

A Brief History of Hydraulics

Hydraulic ankle technology emerged earlier than most clinicians realize, though initially only as part of integrated knee-ankle systems. Commercial hydraulic control began in the 1940s with the Hydra-Cadence and gained momentum in the 1960s with the Mauch SNS knee and Mauch’s prototype ankle designs. It was not until the early 2000s, when more durable hydraulic components became available, that prosthetists could effectively level the ground beneath users and unlock new functional possibilities.

Before compact hydraulic controllers, prosthetic joints operated at a static rate of motion—such as with friction knees or compressible bumpers. Hydraulic control in prosthetic knees revolutionized this by providing variable joint resistance and, consequently, variable speed. A patient slowing down would feel the knee slow with them; accelerating would produce a corresponding speed increase. Additional internal mechanics enabled stability and locking during walking. This same functional advantage was soon pursued for the ankle, with some innovations developing in parallel to, rather than downstream from, knee-hydraulics research.

Early designs faced material limitations that couldn’t withstand the internal pressures required, particularly when hydraulic locking was the goal. Mauch developed a rotary hydraulic ankle with a gravity-referenced ball riding in a track: As the shank passed vertical, the ball settled at the low point and closed a port, effectively locking the ankle to match the current slope. Clinically successful, the design ultimately failed due to hydraulic seal limitations and the maintenance burden of fluid leakage.^2,3 The Hydra-Cadence succeeded by using hydraulics to produce stance phase shock absorption and swing-phase dorsiflexion, becoming the first commercial design to harness hydraulic technology for both safety and functional benefit.⁴ This design established the functional foundation on which later reimbursement coding decisions for hydraulic technology were built.

The Rise and Limits of ESAR Feet

Early mechanical ankles were attached to rigid or semi-rigid feet, offering limited flexibility or energy return. Without reliable hydraulics, ankle designs centered on range of motion through single and multiaxial units with various static bumpers for comfort and shank biomechanics. The advent of energy storage and return (ESAR) feet in the 1980s and ’90s transformed foot design. Modern composite materials created the sensation of an ankle through inherent material flexibility, which not only improved gait patterns through cleaner tibial motion but also stored energy through deformation.

The explosion of possible shapes and materials ushered in a new era of foot design that continues today. Yet these feet retain significant limitations. Foot alignment is tied to a static position (level ground), set for a single heel height (even if manually adjustable), and optimized for a single cadence.¹ The wearer must actively manage torques through proximal joints as the foot creates forces that must be controlled through the socket interface. At every position except midstance, the body continuously manages stability, modulating speed and storing energy to accommodate a foot optimized for one cadence. Beyond stance-phase challenges, these feet require additional ipsilateral knee and hip flexion for toe clearance in swing, and they contribute to difficult sit-to-stand transitions and comfort issues when standing or sitting still.

Modern Hydraulic Ankles

Advances in sealing and composite materials renewed interest in hydraulic ankle control. Fillauer (Hosmer/Motion Control), Freedom Innovations, and Blatchford (Endolite) pursued various designs, incorporating both purely mechanical and electronic functionalities. On the mechanical side, the linear hydraulic Echelon arrived first, with Blatchford producing extensive literature demonstrating hydraulic benefits—their Clinical Studies Compendium remains an excellent resource on this technology.⁵ Patients who transitioned to hydraulic ankle-feet showed improved and more symmetrical gait, reduced socket pressures, enhanced stability, and, anecdotally, reported fewer stumbles and greater confidence on uneven or sloped ground.

Motion Control’s rotary hydraulic MotionFoot followed, offering full natural ankle motion range and locking capability for activities like driving. Freedom Innovations (now part of Proteor) introduced the Kinterra foot shortly after. These designs all leveraged hydraulic control to level the foot from heel strike to loading response and to hydraulically resist dorsiflexion from loading response to midstance. From midstance to terminal stance, they employed ESAR composite benefits to store and release energy, and in swing phase, hydraulics actively resisted plantarflexion for clearance.

Electronic options emerged as well: Hosmer created the Raize foot with extensive hydraulic motion and patient-controlled alignment based on early patents granted to Wayne Koniuk, CP; Blatchford developed the Elan; and Freedom introduced the Kinnex. Small startups and university groups followed, with many later acquired and refined by Ottobock. Össur entered with the Proprio, an electric foot that replicated hydraulic functions through entirely electronic controls. The Proprio was the first to truly realize the early Mauch vision of slope adaptation by locking alignment to match the slope; however, real-time electronic terrain accommodation remains challenging. While these electronic systems offered expanded patient functionality, their cost, maintenance requirements, and charging needs have limited availability and prescription frequency.⁶ This raises an important question: Can passive feet deliver these functional benefits?

From Rollover Shape to a New Foot

During this period, Fillauer acquired Motion Control, Hosmer, and Emotis, and became a close follower of rollover shape research conducted by Andrew Hansen, PhD, director, Minneapolis VA Rehabilitation & Engineering Center for Optimizing Veteran Engagement & Reintegration (RECOVER), and his team.^7,8 Their findings became a driving force in design philosophy for Fillauer Composites, resulting in a series of well-characterized, high-performing ESAR feet.^8-10

In late 2015, conversations between Hansen and the Fillauer engineering team set new design work in motion, inspired by laboratory innovations. A 2016 Small Business Innovation Research grant enabled collaboration between the VA and Fillauer to develop something new. Regarding the motivation, Hansen explained, “We had heard from and seen many veterans with amputations struggling with mobility on uneven terrain. We felt this type of innovation would help improve their stability on uneven terrain and empower participation in important life activities.”

The Problem That Remained: True Slope Adaptation

Despite advances, recognized problems persisted with commercial hydraulic options—issues documented by Hansen’s Minneapolis VA RECOVER lab in a controlled study.¹ Durability remained problematic even with modern materials. Range of motion was limited regarding tibial inclination at heel strike, especially on minimal slopes. The biomechanics of initial contact through loading response on slopes were sacrificed to optimize level-ground walking. Most significantly, energy loss, a critical concern for higher-activity patients, resulted from the inherent limitations of damping control. These last two issues stemmed from suboptimal alignment relative to the slope. The fundamental questions became: Could a foot’s neutral alignment shift automatically to always match the slope, and could such a device withstand long-term use?

Building a Passive Slope-Adaptive Foot

Development began with creation of linear and rotary hydraulic designs meeting new durability and performance requirements. The Fillauer team included engineers experienced with robotic hydraulic actuators in theme parks and military applications where millions of cycles are routine. They developed the hydraulics that would power the MotionFoot SLX, a more durable platform with greater range of motion than other linear hydraulic systems.

The team settled on a 17-degree range of motion, balancing genuine slope accommodation with practical size and durability constraints. This range provides stability and control on terrain typical for K3 patients, the same range Proteor incorporated in the updated Kinterra 3.0. The slope-adaptive version gained additional plantarflexion range for enhanced energy storage and release through dorsiflexion. Continuous improvements to the MotionFoot SLX have hardened the system for an active patient population, specifically veterans and highly functional K3 ambulators likely to operate at K4 levels. This group had traditionally rejected hydraulic feet due to durability concerns and energy-storage limitations. A foot that maintained energy efficiency, varied energy storage with speed, and offered proven durability could prove transformative. Development efforts also expanded functional adjustment range, allowing fine-tuning of the hydraulic valve system to individual patient needs, avoiding common problems like foot slap at heel strike or inadequate plantarflexion during normal walking.

Several significant challenges emerged early. The additional plantarflexion required new composite construction capable of handling loads and surviving K4-level activities. The system also needed active dorsiflexion, matching normal muscle activation rates for adequate toe clearance, a critical problem since more than half of people with lower-limb amputations experience a fall annually, many from toe catch during swing phase.^11,3 The final challenge involved perfecting the locking mechanism to be both easy to activate and predictably controlled.

Early Results

These challenges were overcome, and prototypes were provided to Hansen’s group for patient testing in a VA pilot study presented at the 2024 and 2025 American Academy of Orthotists and Prosthetists annual meetings.^12,13 Early trials demonstrated strong patient preference for true slope-adaptive functionality. Following initial testing, over 25 users ranging from K2 to K4 activity levels evaluated the new technology. The composite system was optimized for high-energy return, flexibility, and durability at extreme use levels.

This slope-adaptive foot actively modifies alignment without electronics, building on the same passive slope-adaptive concept Hansen’s lab first prototyped in 2014.³ It leverages true slope adaptation to accommodate variable tibial angles at heel contact and stores significantly more energy in composite springs than statically held composite plates. Because slope adaptation is inherent, the foot instantly accommodates various heel heights while preserving alignment. This hydraulic accommodation delivers real benefits: improved residual limb comfort and slope stability where needed.^14,15 Additionally, because tibial angle increases with speed, plantarflexion range increases with speed. Greater early-stance plantarflexion flexes composite plates more, increasing stored energy and subsequent energy release through late-stance plantarflexion.³ Combined hydraulic control and slope adaptation create a superior mechanism for accommodating variable cadence—from leisurely walking to vigorous hiking on rough terrain.

The Traverse Foot

Over the following two years, hydraulic foot designs evolved through multiple patient-testing rounds. The Traverse line of hydraulic feet, introduced based on this work, accommodates different functional needs through hydraulic technology. The Traverse K2 has a soft, stable, multiaxial foot and SLX hydraulic control technology, also featured in the Traverse SLX (formerly MotionFoot SLX). Real-time slope adaptation—the culmination of more than 60 years of development—comes without complex electronics, batteries, or software. Instead, it combines slope stability, cadence matching, heel-height accommodation, and active dorsiflexion with established hydraulic benefits to deliver functionality.

The journey continues with an ongoing large-scale VA study to look at the benefits of this technology relative to traditional hydraulic control. Over the next three years, the RECOVER lab will be working with a broad variety of patient presentations to investigate patient selection and further document the benefits of slope-adaptive technology in a way that will help clinicians identify the real-world opportunities to improve patient outcomes.

Sam Hale, MSPO, CPO/L, is the vice president of Research and Development and Education for Fillauer and has provided care for patients as a licensed prosthetist/orthotist for more than 14 years.

References

1. Koehler-McNicholas, S. R., E. A. Nickel, J. Medvec, K. Barrons, S. Mion, and A. H. Hansen. 2017. The influence of a hydraulic prosthetic ankle on residual limb loading during sloped walking. PLOS One 12(3):e0173423.
2. Sowell, T. T. 1981. A preliminary clinical evaluation of the Mauch hydraulic foot-ankle system. Prosthetics and Orthotics International 5(2):87-91.
3. Nickel, E., J. Sensinger, and A. Hansen. 2014. Passive prosthetic ankle-foot mechanism for automatic adaptation to sloped surfaces. Journal of Rehabilitation Research and Development 51(5):803-14.
4. Veterans Administration, Prosthetic and Sensory Aids Service. 1964. The V.A. publishes a study of the Hydra-Cadence above-knee prosthesis. Orthotics and Prosthetics 18(1):51-5.
5. Blatchford Ltd. 2020. Clinical Studies Compendium: Hydraulic Ankle-Foot, Edition 3.
6. Wurdeman, S. R., P. M. Stevens, and J. H. Campbell. 2019. Mobility analysis of AmpuTees (MAAT 5): Impact of five common prosthetic ankle-foot categories for individuals with diabetic/dysvascular amputation. Journal of Rehabilitation and Assistive Technology Engineering 6:2055668318820784.
7. Hansen, A. H., D. S. Childress, S. C. Miff, S. A. Gard, and K. P. Mesplay. 2004. The human ankle during walking: implications for design of biomimetic ankle prostheses. Journal of Biomechanics 37(10):1467-74.
8. Hansen, A. H., M. R. Meier, P. H. Sessoms, and D. S. Childress. 2006. The effects of prosthetic foot roll-over shape arc length on the gait of trans-tibial prosthesis users. Prosthetics and Orthotics International 30(3):286-99.
9. Koehler-McNicholas, S. R., E. A. Nickel, K. Barrons, and K. E. Blaharski, , et al. 2018. Mechanical and dynamic characterization of prosthetic feet for high activity users during weighted and unweighted walking. PLoS ONE 13(9):e0202884.
10. Miller, T. A., J. H. Campbell, D. L. England, P. M. Stevens, and S. R. Wurdeman. 2022. OASIS 2: Mobility differences with specific prosthetic feet across procedure codes. Journal of Rehabilitation and Assistive Technology Engineering 9:1-7.
11. Miller, W. C., M. Speechley, and B. Deathe. 2001. The prevalence and risk factors of falling and fear of falling among lower extremity amputees. Archives of Physical Medicine and Rehabilitation 82(8):1031-7.
12. Lloyd, A., M. Laine Dyreson, N. Walker, and P. McCracken, et al. 2024. User experience with a passive slope adaptive foot-ankle prosthesis (podium presentation). American Academy of Orthotists and Prosthetists Annual Meeting, Chicago, IL, March 6-9, 2024.
13. Laine Dyreson, M., A. Lloyd, N. Walker, S. Koehler-McNicholas, B. Hafner, and A. Hansen. 2025. Mixed methods pilot study of a slope adaptive foot (podium presentation). American Academy of Orthotists and Prosthetists Annual Meeting, Atlanta, GA, February 26-March 1, 2025.
14. Portnoy, S., A. Kristal, A. Gefen, and I. Siev-Ner. 2012. Outdoor dynamic subject-specific evaluation of internal stresses in the residual limb: hydraulic energy-stored prosthetic foot compared to conventional energy-stored prosthetic feet. Gait Posture 35(1):121-5.
15. Bai, X., D. Ewins, A. D. Crocombe, and W. Xu. 2018. A biomechanical assessment of hydraulic ankle-foot devices with and without micro-processor control during slope ambulation in trans-femoral amputees. PLOS One 13(10):e0205093.