Ossur’s Power Knee is the only bionic knee that attempts to replace lose muscle power with electric power. This reduces the user’s energy expenditure while also assisting in tasks like standing and ascending stairs. It also helps ensure greater ground clearance during the Swing Phase.
What’s On This Page?
- A Quick Look at the Power Knee
- Power Augmentation
- Sensor & Microprocessor Capabilities
- Benefits & Drawbacks of the Power Knee
- Compatibility with Prosthetic Feet
- Water & Dust Resistance
- Device Weight & User Weight Limit
- K-Level Rating
- User Software
- User Feedback Survey & Results
- Considerations Before Buying a Power Knee
- Related Information
A Quick Look at the Power Knee
We would have preferred to show you a more recent video of the Ossur Power Knee but this video from 2013 remains the best quick introduction:
If you watch this video closely, you will likely notice that Allison’s gait is not as smooth as the gait exhibited by the users in our videos of other bionic knees.
This has more to do with her level of amputation rather than the device. Power Knee users tend to have a higher level of amputation than the users of other bionic knees. They may even have had a hip disarticulation. Generally speaking, the higher the level of amputation, the more difficult it is to control a lower-limb prosthesis due to missing musculature and reduced leverage.
The Power Knee partly compensates for this with motorized power, which is what makes it so unique.
Ossur Power Knee Key Features
When walking, there is a point where the load-bearing foot pushes off from the Stance Phase to begin its Swing Phase. It is also the point where the user swings that foot/leg forward as part of the next step:
In a natural foot/leg, these actions are powered by muscles and tendons. The quadriceps, in particular, plays a key role in lifting the leg and swinging it forward, as well as generating most of the power to rise from a sitting to a standing position or to ascend stairs.
Most Power Knee candidates are missing part or even all of their quadriceps muscle. To compensate, the Power Knee replaces the missing power with an electric actuator. For example, when ascending stairs, it is the actuator that helps straighten the knee to lift the user. During the Swing Phase, the actuator causes the knee to bend, which helps pull the foot up to create more clearance between the toes and the ground.
Sensor & Microprocessor Capabilities
To understand how bionic knees work in general, please see A Complete Guide to Bionic Legs & Feet.
The short story is that the microprocessors in most bionic knees have three main tasks:
- automatically adjusting the resistance in the knee to ensure the proper level of support through each stage of the Stance Phase regardless of terrain;
- ensuring the optimal release point for the knee to begin the Swing Phase and also the proper foot clearance during this phase, especially when ascending stairs, ramps, etc.;
- assisting in stumble recovery.
As far as we can tell, the microprocessor in the Power Knee manages the first two tasks but does not have an automatic stumble recovery feature. Its emphasis is more on stumble avoidance. It also has an additional task: to determine when to apply power to the knee actuator to generate push and/or lift.
Benefits and Drawbacks of the Power Knee
The benefits of the Power Knee are considerable. As always, we prefer to show you rather than make you read text, so we dug up this training video. Again, it is a bit older than we’d like (2011) but it does a great job of demonstrating the Power Knee’s key functions starting at the 8:20 mark:
To summarize, the Power Knee uses motorized power to help:
- propel the user forward while walking on level ground;
- lift the prosthetic leg to ensure better ground clearance during the Swing Phase;
- provide added push while ascending stairs or slopes.
This is in addition to the typical bionic knee benefits of managing knee resistance (support) throughout the Stance Phase and triggering the Swing Phase.
The drawbacks of the Power Knee are:
- cost, which is $70,000 to $90,000 US (or even more) for a complete solution;
- weight; at 2.7 kilograms, it is too heavy for some users;
- noise; the Power Knee does make an unmistakable noise while moving;
- length; at 37.5 centimeters, it is too long for people with longer residual limbs.
For a thorough review of the Power Knee, we recommend this article from The O & P Edge.
Compatibility with Prosthetic Feet
As is the case with most companies that make both bionic knees and prosthetic feet, the Power Knee is best paired with Ossur feet — in particular their Pro-Flex line of feet.
The Power Knee’s battery can last for up to 12 hours of continuous use depending on the level of activity.
If completely drained, the battery requires 3.5 hours to fully charge.
As with most bionic limbs, it is best to charge the Power Knee’s battery each night.
Water & Dust Resistance
We have been unable to find an official IP rating for the Power Knee. The only official statement from Ossur is contained in the Power Knee’s user manual, which states that it “is suitable for use in any environment except where spillage or immersion in water or any other fluid is possible”. The manual also contains a warning to avoid intense dust.
This sounds similar to the water and dust ratings for Ossur’s Rheo Knee and Rheo Knee XC, both of which have an IP rating of 34.
Device Weight & User Weight Limit
The Power Knee weighs 2.7 kilograms. This is significantly heavier than other bionic knees. However, due to its power augmentation, the knee typically feels much lighter to the user.
The maximum allowable user weight for the knee is 165 kilograms.
The Ossur Power Knee is rated for K3 users. It is particularly useful for lower-to-mid-level K3 users who have a shorter residual limb (or hip disarticulation) and lack their full above-the-knee musculature.
For a thorough understanding of K-levels, please see the Amputee Coalition’s web page on this topic.
Ossur does not offer software to end-users of the Power Knee. They only offer a software package to clinicians and physical therapists called the PowerLogic Workbench.
Ossur offers a full three-year warranty on the Power Knee including any repair costs due to defects. Under this warranty, the knee must be sent to Ossur for mandatory maintenance within 20 months after purchase.
The warranty can be extended to five years at the user’s option. Under this option, the Power Knee must be sent to Ossur within 20 months and within 40 months after purchase.
Where possible, Ossur provides a free service unit during repair and service inspections.
According to our information, the Power Knee typically sells for between $70,000 and $90,000 US including the socket, prosthetic foot, and all prosthetist fees.
User Feedback Survey & Results
Are you or have you previously been a Power Knee customer? If so, please share your insights with others looking at the Power Knee as a possible prosthesis.
We do not yet have a sufficient number of survey participants to publish fair and accurate results for the Power Knee.
As soon as we do, we’ll update this section.
Considerations Before Buying a Power Knee
There are three main considerations before purchasing Ossur’s Power Knee:
- Activity Level: the Power Knee is rated for K3 users only and is best suited for lower-to-mid users in this group.
- Price: very few people can afford to purchase a Power Knee outright. While there is some private insurance coverage, most purchases of the Power Knee are made through military healthcare programs, worker’s compensation, or accident insurance coverage.
- Need: does the user already have sufficient upper-leg musculature to opt for a less expensive bionic knee?
For a list of competitor devices, see All Bionic Feet.
For a comprehensive description of all current lower-limb technologies, devices, and research, see our complete guide.
Click here for more information on Ossur.