Understanding Bionic Legs & Feet

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Everything you need to know about bionic legs and feet, with descriptions of all the current devices, technologies, and the latest research. Continuously updated, this page is intended as your one-stop repository for all the latest information on lower-limb bionics.

The Need for Bionic Legs & Feet

According to the Amputee Coalition, there are 2.1 million amputees in the U.S. Of these, 65 % involve a lower limb.

Lower limb amputations can occur at several points:

Lower Limb Amputation Points for Bionic Legs

Note, the numbers shown in this diagram are estimates based on extrapolations from a recent German study. Actual, up-to-date breakdowns are not available for the U.S.

The vast majority of lower limb amputations are caused by vascular disease, usually related to diabetes.

There have been several advances in non-bionic (passive) leg prostheses in recent decades, often highlighted by the impressive feats of parathletes. This raises the questions of whether bionic (i.e. powered) legs & feet are needed. To answer this question, watch the first 56 seconds of this video:

Everyday tasks are not sporting events involving identical motions over perfect terrain in short bursts. They are varied, nuanced, and occur all day long.

Additionally, as you will see in the sections below, the natural human knee, ankle, and foot are incredibly sophisticated and efficient. Passive leg prostheses, by comparison, can sometimes be clumsy and tiring.

Finally, there’s a safety issue. Numerous studies have shown that amputees are less likely to fall when aided by a bionic device.

So, the simple answer is: yes, many of our amputees need bionic legs and feet, the sooner the better.

Understanding the Technical Challenges for Bionic Legs

The Basic Mechanics of Walking

When humans walk, each leg alternates between a Stance phase (the foot has contact with the ground) and a Swing phase (the foot is off the ground). These phases can be divided into sub-phases:

Human-gait-cycle for bionic legs

Source: https://www.researchgate.net/figure/Human-gait-cycle-28_fig1_324346331

During the course of this gait cycle, the body employs a number of biomechanical functions, including:

  • shock absorption by muscles, tendons, ligaments, and cartilage from the moment of Heel Strike until the full body weight is assumed by the Stance leg;
  • antagonistic muscle pairs to move the foot, ankle, knee, and hip into desired positions; for example, consider all the required foot positions in the above diagram;
  • the use of elastic energy as tendons and muscles stretch during the first half of the Stance phase, then recoil toward the end of that phase;
  • the generation of mechanical energy through muscle contraction;

Each of these functions has a mechanical equivalent, such as dampers, actuators, springs, motors, etc.

The challenge for bionic legs is not mimicking these individual biomechanical functions. It is controlling and coordinating them in a smooth, integrated manner.

Bionic Leg Control Systems

When humans approach stairs, they glance at them to gauge their height, steepness, tread width, etc. To climb the stairs with natural legs, they automatically adjust their stride, foot placement, foot/knee angle, force, etc. They have a keen awareness of the position of their legs/feet relative both to their bodies and the stairway, allowing them to automatically adjust as they ascend. And they often do all this while talking to someone or looking elsewhere.

Recent bionic legs use built-in sensors to detect things like walking speed, force of the foot striking the ground, joint angles, etc. Microprocessors in the ankle and/or knee use this data to automatically adjust the mechanical or electrical elements accordingly. Adjustments take place hundreds or even thousands of times a second, making the system(s) highly responsive.

These legs have achieved significant improvement in walking gait, stability, and efficiency, as shown in this video.

Sensor-driven adjustments still have shortcomings, however. For example, there is no direct connection between the limb and the mind – the ultimate processor.

This is being addressed through the use of myolectric sensors, targeted muscle reinnervation, and new surgical techniques like Ewing Amputation. For more information, see Mind-Controlled Bionic Limbs.

Current commercial systems also lack sensory feedback. This is important not only to help amputees adapt to changing terrain. It also restores a sense of self and helps eliminate phantom limb pain. To understand how, see Bionic Touch.

Finally, surgical procedures like Osseointegration improve both mind control and sense of touch.

Bionic Ankles/Feet

There are a number of bionic ankles/feet on the market including:

This promotional video from Blatchford provides an excellent technical description of the benefits of such devices, starting at 1:10:

For whatever reason, it is difficult to find independent, first-hand consumer reviews of bionic ankles/feet. Most material on the subject is still either promotional or scientific.

Until that situation changes, we’ll focus on showing the latest advances, such as this bionic ankle/foot for more active amputees:

And on why all of this is so important:

Bionic Ankles/Feet Affordability

Based mainly on user feedback, most bionic ankles/feet sell for between $18,000 to $25,000 US. This makes sense given that the Medicare/Medicaid reimbursement rates max out at around $18,000 US for this type of device.

The most expensive bionic ankle/foot is Ottobock’s Empower Ankle, which sells for between $40,000 to $50,000 US. That’s quite a leap, but if you watch our videos on the Empower, it’s also quite a leap in technology.

For more information, see our Bionic Foot Price List.

Bionic Knees

The role of the knee differs from that of the ankle. For example, the knee is not responsible for as much power generation as the ankle and its attached muscles/tendons when walking.

Instead, the knee acts as a hinge in the Swing phase of the gait cycle. In the Stance phase, its attached muscles, cartilage, etc., play a key role in shock absorption and load support.

The ability of a bionic knee to accurately and smoothly transition between these two modes is crucial to mobility. Again, sensors and microprocessors play a vital role in fulfilling these responsibilities.

Here is a video of a young man using Ottobock’s Genium X3 bionic knee for a number of high-level activities:

Paired with the correct prosthetic foot, there are obviously few limits on the use of this knee.

So with a knee like this, why would someone need a bionic ankle? First, using the knee alone is more tiring without the augmenting power of a bionic ankle. It also isn’t as smooth or proficient when walking up or down stairs or other inclines.

This of course leads to the question: what happens if you combine these two technologies?

Full Bionic Legs (Bionic Knees Plus Bionic Ankles)

There is only one bionic leg we know about that fully integrates a bionic knee and ankle. This is Blatchford’s Linx leg:

It is inspiring to see amputees being so physically active. It’s proof that, technically, we are getting close to eliminating lower limb disabilities.

The problem is one of availability. This is limited not only by price, but also by insurance policies.

For example, Ossur combined its Rheo bionic knee with its Proprio bionic ankle to create a Symbionic leg product. But the Symbionic leg had poor sales. Why? Because public and private medical insurance resisted paying for it.

Bionic Leg Affordability

The technologies presented in the previous sections are brilliant.

Their price tag is not. These solutions can cost tens of thousands of dollars, which is beyond the financial means of a vast majority of lower limb amputees.

We have to find a way to bring these prices down.

One way to do this may be through use of open source solutions.

Open Source Bionic Legs & Feet

For decades now, the open source approach has flourished in the software industry. It works by allowing the entire user community to contribute to software solutions. It speeds up innovation, reduces costs, and fosters standardization. Just on the issue of standardization alone, this makes software more modular and easier to reuse.

Now compare this to the world of bionic legs & feet. Despite the fact that the tasks faced by one bionic limb manufacturer are quite similar to those faced by another, each takes a proprietary approach. This has the unfortunate effect of slowing progress and making end products more expensive.

Fortunately, the University of Michigan is spearheading an Open-Source Leg initiative.

The objectives for this program are not yet as far-reaching as a typical open software initiative, as it is focused more on standardizing aspects of the R & D process. But the benefits of this approach will ultimately trickle down into the commercial market.

Latest News for Bionic Legs / Feet

The Revolution in Prosthetic Aesthetics

There is something transformative happening in the prosthetics industry — a long-overdue awakening among product designers, those with limb differences, and the general public that is helping us take a big step toward a more inclusive society.

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Bionic Foot Price List

This price list is intended to give potential users a general idea of the price for each bionic foot currently available on the market.

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Current Options for Bionic Feet

Want a quick list of the bionic feet currently on the market? For your convenience, this page shows you each of the devices that we’ve reviewed along with links to the full reviews.

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Fillauer Raize Foot

Fillauer’s Raize Foot is a lightweight, low-profile microprocessor foot/ankle. Its microprocessor actively manages flexion resistance for a smoother rollover during the Stance Phase of the gait cycle. It also allows users to adjust heel height to accommodate different shoes.

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