Bionic knee

A bionic knee is a prosthetic knee that is designed to function as closely as possible to a natural knee through use of electronics, mechanics and biological knowledge. Thus, it is an advanced artificial replacement to a natural knee.

A few important notes

1. Some people hold an opinion that these microprocessor controlled knees can’t actually be regarded as bionic. According to them bionic should be used when a device manages to closely replicate or surpass the function found in nature. As today’s prostheses are still pretty far off this goal they can’t be called bionic. As there aren’t a universal and widely accepted definition of what is and what isn’t bionic my opinion is a bit different. This is also the point of view of this article.

2. Customer related issues that should be evaluated when choosing a prosthetic knee are not included in this article. These could include price, insurance issues, maintenance availability and costs.

3. This article is not based on actual experience and it is not written from a user’s point of view. If you’d like to read more about these devices from a user’s perspective and find out which knee could be the best option in a particular situation I encourage you to visit this site.


Initially, the term “bionics” was used in a slightly different manner. Initially it was a field that sought solutions to engineering problems in nature. Now this field is more often regarded as “biomimetics”, while bionics has a somewhat different meaning.

If you look at definitions, some describe bionics as intended by the inventor of the term, some as “biology+electronics” and some as “life like” from the Greek language. As I see it, “biomimetics” is used when a solution to engineering problems is sought in nature and “bionics” is used when a function found in nature is replicated using electronics, mechanics, mechatronics and other engineering sciences.

What’s the difference between these explanations? In “biomimetics” we have a problem at the beginning, for example – we want to make a robot that can climb walls. Then the solution is sought in nature – let’s see how geckos do it and then imitate it!

In “bionics” we have a certain biological function at the beginning. For example – we need to make a joint that functions just like a natural knee, then we try to replicate this function as closely as possible using electronics, mechanics, et cetera, the result is a bionic knee. Biomechatronics is another closely related field and bionics, biomimetics and biomechatronics often overlap and are used interchangeably. Keep in mind; this is only how I see it!

History of bionic knees

As you understand, it’s not like we have a thousand or even hundred years long history in this field. Of course, there was plenty of movies and fictional stories where people had artificial leg prosthetics, Darth Vader from Star Wars is one example.

This means that people generally thought it could be possible in future when an according level of technology would be achieved. It turns out now that people were right back then. Advances in numerous fields have made bionic knees possible, at least to some degree.

In reality, the history of a bionic knee begins in 1990s. In 1990 people at Blatchford began development of a microprocessor controlled prosthetic knee. It was released in 1993 under the name of Intelligent Prosthesis. Next generation of this design – Intelligent Prosthesis Plus was introduced in 1995.

In 1997 Otto Bock, a German prosthetics company introduced their microprocessor controlled artificial knee – the C-leg. A year later, in 1998, Blatchford released another smart knee – Adaptive Prosthesis.

In the next decade the development efforts continued. In 2005 Ossur announced its bionic knee – the RHEO KNEE, a year later they introduced the world to the POWER KNEE – a powered prosthesis. As far as I’m aware, Ossur also was the first company that used the term “bionics” to describe their creations. And last but not least in 2008 Freedom Innovations launched their Plie MPC knee.

Current development

As you see, the history of such devices is quite short, so current development largely overlaps with history and many of the above mentioned models are still significant. Every of these models have their own pros and cons. Let me describe you – what is what!

Microprocessor controlled knee prostheses

Most of the above mentioned prostheses fall into this category. Moreover, all manufacturers but one doesn’t denote their smart knee prostheses as bionic, they call them “microprocessor controlled” instead. In my opinion, “bionic” is a good description and it can be used as this is how people tend to call them.

OK, enough with the philosophy, I’m sure you want to know what exactly is this microprocessor controlled knee, what it does and what it does not. Oh, bear in mind that I’m not a prosthetist, so I may be wrong somewhere. If you’re a competent person in this field and you see that I’m mistaken somewhere, please contact me and notify about this!

by JabberWork, released under Creative Commons license.

So, have you noticed what happens when a human walks? How exactly do we do it? If you haven’t, let me shortly describe it to you. A human leg is, basically, a double pendulum. When we walk we move forward the hip hence bending the leg at the knee and swinging the lower leg forward.

After that we “land” the leg on the heel and lock the knee to keep the leg straight. Then we roll the leg from heel to toe, simultaneously moving forward the other hip to swing the other leg. As you see, the knee is needed to swing the lower leg forward when walking.

There was and there still are mechanical knees that do exactly that but they rely solely on mechanics and hydraulics or pneumatics to determine when to lock the knee, when to initiate the swing phase, etc. Usually, these can be fine-tuned and pretty good results can be achieved.

On the other hand, a natural knee is controlled as needed – the lower leg is swung further and faster when the gait is faster and vice verse. Also, we stiffen or legs when additional loads are carried and so on. Although some of these effects can be achieved solely by mechanical means, better results can be achieved if continuous calculations and control are used.

This is where microprocessor controlled prostheses come into the story and where we can start talking about bionic knees. As you can understand, these knees typically have numerous sensors that measure various values such as speed, load, acceleration and such.

Then, depending of these readings and according to the software desirable response is found and executed. Thus, these microprocessor controlled knees can do a better job when compared to mechanical knees in responding to changes such as walking speed, walking surface inclination, applied load and others.

In addition to that, microprocessor controlled knees are more universal as it is typically possible to create numerous behavior configurations for different activities. Today, there are numerous bionic knee manufacturers and models.

Blatchford (Endolite)

Blatchford released the first commercially available knee of this type – the Intelligent Prosthesis. Today their line of microprocessor controlled knees include three models – Smart Adaptive, Smart IP and IP+, all distributed under Endolite brand name.

As I understand, the IP+ is the simplest option of these three, while Smart Adaptive is the most sophisticated. Description “smart” in the names of two of these prostheses indicates that these knees can learn the most appropriate behavior and adjust to different gaits.

The Smart Adaptive knee has a hybrid control – hydraulic for stance and pneumatic for the swing phase.

Otto Bock

Otto Bock’s bionic knee is called C-leg. At the moment I’m writing this, there are two models – C-leg and C-leg Compact. While both knees are designed for active users that have to walk and negotiate obstacles daily, the first one is designed for more active users, while C-leg Compact is designed for users that require additional stability and safety.

C-legs gather information from their sensors 50 times per second and they use hydraulics to regulate stance and swing phases. Numerous programmable behavior configurations are possible to get the most out of the device. As far as I’m aware, this is one of the most popular bionic knees.


The RHEO KNEE by Ossur is another bionic knee that falls under this category. As other knees, this one also employs numerous sensors and calculates the desirable behavior, but it is quite unique from a different point of view.

The interesting thing about this knee is the way it manages swing and stiffness control. If other knees generally use hydraulics, the RHEO KNEE has a different actuator – it uses magnetorheological fluid – a fluid that changes viscosity when exposed to electromagnetic field.

The information from sensors is gathered around 1000 times per second and the software calculates the most appropriate response. This knee also adjusts itself to different gaits, essentially learning them, as some other bionic knees do.

Freedom Innovations

Freedom Innovations is a USA based company that distributes one bionic knee – Plie MPC Knee. As other knees, this one uses hydraulics to regulate swing and stance as well. But this knee is interesting because of some other innovations that can be quite handy.

While this knee as all of the above mentioned knees have a battery life of at least 24-48 hours, this knee has a removable battery pack. So it is possible to charge one battery while using another and then switch them.

Another important thing is water resistance. Most, if not all, above mentioned knees aren’t very water resistant. Plie MPC, on the other hand, can be even briefly submerged in water. It is not actually recommended to submerge it but it is OK to use it in rain and nothing bad will happen if a cup of coffee is accidentally spilled on it.

Powered bionic knees

All microprocessor controlled knees are a great leap forward if compared to what was available before them. Many people that use this kind of prostheses say that it is the closest thing to natural walking available today.

Obviously, this is great. However, while above mentioned prostheses do pretty well when walking is concerned, there are things they can’t do. This is where powered lower limb prostheses come in trying to enable people to do even more.

When a limb is lost part of available muscle activity is lost as well. For example – a human with a sound leg can straighten it in the knee using his muscles or ascend stairs step by step. It is not possible using the above mentioned prostheses.

So, powered or active prostheses are being researched in order to create even more natural gait and to make activities such as step by step stair ascending possible. Not only that, as you can understand much larger distances could be walked with such devices.

As far as I know, the first fully tested and developed powered knee is the POWER KNEE by Ossur. It’s very expensive though and virtually unavailable to most potential users. Apart from that, numerous research institutions are involved in researches in this field. If you’re interested, this study about active knee prosthesis may be interesting to you.


Obviously, the ultimate goal is to create a knee that would be as functional as its natural counterpart. This means it would have to be active, water-proof and controllable by will. As you see, there’s still a lot to do here.

No matter how unbelievable it may seem at first, the chances are that such devices will be designed sooner or later. The battery and actuator technologies are evolving; these would make advanced powered prostheses possible.

Interfacing prosthesis to a human’s neural network is another difficult task. While the field is pretty much in its infancy, the current progress promises that this will be achieved as well. Osseointegration is also researched in the framework of lost limb replacement; this would eliminate many socket-related problems.

Go back from “Bionic knee” to “Medical robots”.

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