Triple Flexion, Explained: How SYNSYS Is Redefining Full-Leg Prosthetic Movement

Reading Time: 6 minutes

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Summary:

• PROTEOR’s SYNSYS links a microprocessor knee and ankle into one full-leg system.

• PROTEOR says the link allows triple flexion of the hip, knee, and ankle.

• In a 12-man trial, SYNSYS increased flat-foot time and toe clearance over users’ own prostheses.

• PROTEOR lists SYNSYS for K2 and low-to-moderate K3 activity levels.

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Uneven surfaces such as slopes, stairs, and sidewalks demand greater ankle mobility than flat ground. For individuals with above-knee limb loss using an energy storage and return (ESAR) foot without ankle articulation, these conditions can be challenging because these feet are designed for level surfaces and do not adapt well to slopes or stairs.

SYNSYS, a microprocessor full-leg system from PROTEOR, redefines this by linking the knee and ankle, allowing controlled “triple flexion” of the hip, knee, and ankle.

This guide explains what triple flexion does and what PROTEOR’s own clinical trial found.

What the SYNSYS full-leg system is  

SYNSYS pairs a microprocessor-controlled knee with a microprocessor-controlled ankle and foot and links the two, so they work as one unit. This makes SYNSYS a full-leg system, distinct from PROTEOR’s knee-only QUATTRO 2 microprocessor knee. The manufacturer’s product page lists SYNSYS for K2 and low-to-moderate K3 activity levels.

PROTEOR says the design grew out of a survey of 330 amputees that it ran in the first quarter of 2021. The company reports that respondents wanted more range of motion and less difficulty on uneven ground and slopes.

Triple flexion: how kinematic coupling works  

PROTEOR explains SYNSYS’s triple flexion as a synergistic movement involving the hip, knee, and ankle. The system achieves this through kinematic coupling between the knee and ankle, designed so these joints move together rather than independently. This coupling acts as an extension of the hip, with triple flexion occurring in coordination with hip movement.

A 2024 study in the journal Prosthesis states that a single hydraulic unit controls both joints. Sensors detect each step’s phase and walking conditions. PROTEOR’s white paper reports a knee flexion of 125° and a total ankle range of 42°.

The coupling manifests at two points during a step: in stance, the ankle automatically plantarflexes (the movement where the toes are pointed downward) at the start, helping the foot to ground more quickly; during slope and stair descents, the ankle dorsiflexes (the upward movement of the foot) as the knee flexes, allowing the prosthesis user to position the foot on the step. In swing, active dorsiflexion lifts the toes as the knee flexes, which PROTEOR’s white paper relates to increased toe clearance.

Clinical trial findings  

A randomized crossover trial compared SYNSYS to participants’ existing prostheses, involving 12 men with above-knee amputations at a low-to-moderate K3 level. Each participant used both setups—SYNSYS and their usual microprocessor knee with an energy-storing foot and a fixed ankle—for approximately four weeks in random order at home. They then visited the lab for gait analysis on flat ground, inclined slopes, stairs, a six-minute walk, and questionnaires. Data was collected between late 2018 and spring 2023 at two French rehabilitation centers.

The primary focus was flat-foot time, the percentage of each step in which the foot remains flat on the ground, which the authors say correlates with increased stability and reduced slip risk. Flat-foot time was significantly longer with SYNSYS on level ground, downhill slopes, and stairs, but not uphill, with an overall improvement of 92%.

The second measure was toe clearance, the gap between the toe and ground during the swing phase when the leg moves forward. Increased clearance reduces trip risk by preventing the toe from catching on obstacles. Participants' usual prostheses showed about 2 cm clearance on level ground, which increased to around 5 cm with SYNSYS, and about 4 cm on slopes, likely due to active dorsiflexion during swing.

While toe clearance is a proxy for trip risk, the study did not record actual trips or falls. It references earlier research linking swing dorsiflexion to lower fall rates, including studies on below-knee amputees. PROTEOR’s white paper suggests that higher clearance may also make crossing varied terrain easier with less mental effort.

Quality-of-life outcomes were mixed. The physical component of the SF-36 health survey improved significantly with SYNSYS, but the mental component did not. Scores for appearance and well-being on the Prosthesis Evaluation Questionnaire also improved significantly. However, walking distance, perceived exertion during the six-minute walk, and two mobility questionnaires showed no significant change.

Limitations include the fact that all participants were young, active men, mainly with trauma-related amputations, which the authors note as a constraint. The trial lasted about four weeks per setup and compared SYNSYS to participants’ own prostheses, not to other full-leg systems.

What it changes in daily movement  

PROTEOR says the following everyday activities may improve with the SYNSYS:

Stairs. The ankle’s range of motion lets wearers set the whole foot on the step when going down. The white paper links this to more confidence, less need to rearrange daily routines for safety, and a lower risk of falls.

Slopes and uneven ground. Active dorsiflexion during swing lifts the toes, which PROTEOR says lowers the chance of catching them on sticks or other unexpected obstacles. The ankle also conforms to hills and slopes so the foot reaches flat sooner, adding knee stability.

Sitting and squatting. With 42° of ankle motion, the foot can be tucked under a seat. Furthermore, the knee’s maximum flexion allows for squatting and bending down.

Long stretches on your feet. PROTEOR also indicates an automatic standing support feature for people who stand for long periods.

SYNSYS specifications at a glance  

Each system includes a protective cover (12 colors, plus one that the user can customize), a power supply kit, and an on/off tool. A patient app is available for iOS and Android.

Related Reading:

Computer-Controlled Prosthetic Legs: An Intro

An Overview of Prosthetic Foot Technology

Microprocessor vs Mechanical Prosthetic Knees: Which Gives Better Mobility and Value?

How Microprocessor Feet Improve Mobility