Ossur Rheo Knee

Ossur Rheo Knee Feature Image

Ossur’s Rheo Knee is the only bionic knee to use a magnetorheological fluid (MRF) to manage its level of resistance. This makes the Rheo Knee more reactive to movements of the residual limb than its hydraulic competitors. Most active users consider this an advantage, while those with less control over their residual limb generally prefer a hydraulic knee.

A Quick Look at the Rheo Knee

The following video is, unfortunately, a promotional video from Ossur (we prefer independent third-party footage). However, it does show the Rheo handling a variety of tasks and terrain:

Notice that the user seems to have a high degree of confidence in his knee. He also mentions that it saves him a significant amount of physical energy. What he means is that when he moves his residual limb, the Rheo Knee follows along more quickly and easily than other bionic knees. To understand why, we have to examine the Rheo’s magnetorheological technology in more detail.

The Rheo Knee’s Key Features

Magnetorheological Fluid (MRF) Technology

The Rheo has a cylinder filled with blades and the MRF. When there is no magnetic field, the MRF flows freely between the blades and acts as a lubricant, allowing the knee to move freely. But when the Rheo generates an electromagnetic field, the metal particles in the MRF align to form a bridge between the blades, causing resistance. The stronger the field, the more resistance.

In a hydraulic knee, the hydraulic fluid must be pushed from one chamber to another to increase or decrease resistance. This takes longer than using an electromagnetic field. That’s why the Rheo reacts more quickly to movements by the residual limb.

There is also a lower limit to hydraulic resistance so even if the user wants the knee to swing freely (e.g. when walking very fast), the hydraulics will still cause a bit of drag and a slight delay.

In contrast, an MRF knee can quickly reduce its electromagnetic field to nearly zero, essentially eliminating all resistance.

For the true techies among you, here is a diagram of the difference between the Rheo MRF technology versus the hydraulic system used in Ottobock’s C-Leg:

Rheo Knee MRF Versus Hydraulic Knee Design

Source: https://www.researchgate.net/figure/Top-left-Rheo-Knee-XC-3-top-right-schematic-representation-of-the-magnetorheological_fig1_329607106

Microprocessor Capabilities

To understand how bionic knees work in general, please see Understanding Bionic Legs & Feet.

The short story is that the microprocessors in 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;
  • 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;

The Rheo Knee addresses most of these issues but it does not actively manage the Swing Phase after release from the Stance Phase. Instead, it becomes a free-swinging knee until the foot touches down again and reactivates the electromagnetic field to generate resistance and provide support.

During the Swing Phase, it is up to the user to ensure proper foot clearance to avoid tripping.

Benefits and Drawbacks of the Rheo Knee

The primary benefits of the Rheo Knee are the same as for any bionic knee. The sensors can detect the current position/situation for the prosthetic leg and the microprocessor automatically compensate for things like:

  • different terrains;
  • changes in stride length or pace;
  • the user carrying additional weight;
  • the initial phase of a stumble, where a bionic knee will automatically increase resistance to prevent a fall;

The biggest benefit specific to the Rheo Knee is its free-swinging nature, which makes it more fluid and quicker to respond to the movement of the residual limb. This, in turn, reduces the user’s energy consumption, as described by this user:

The biggest drawback is that if users do not have good control over their residual limb (typically K2 or lower K3 users), there is a greater chance of failing to attain sufficient foot clearance during the Swing Phase, which increases the risk of stumbling/falling.

Compatibility with Prosthetic Feet

As is the case with most companies that make both bionic knees and prosthetic feet, the Rheo Knee is best paired with Ossur feet. This includes Ossur’s bionic Proprio Foot as well as the company’s pure carbon fiber K3 feet.

Battery Life

The Rheo Knee’s battery can last for between 48-72 hours of continuous use, depending on the level of activity.

If completely drained, the battery requires three hours to fully charge.

As with most bionic limbs, it is best to charge the Rheo’s battery each night.

Water Proofing and Dust Resistance

The Rheo Knee has an IP rating of 34, meaning it is protected from occasional water splashing from any angle but it cannot be submerged. It is not corrosion resistant and should not be exposed to salt or chlorinated water.

The knee should also not come into contact with sand or be used in a dusty environment because it lacks protection against particles smaller than 2.5 mm.

Device Weight & User Weight Limit

The Rheo Knee weighs 1.6 kilograms. The maximum allowable user weight for the knee is 136 kilograms.

K Rating

The Rheo Knee is technically rated for high K2 use to low K4 use, though it is best suited for mid-to-high K3 users. For a thorough understanding of K-levels, please see the Amputee Coalition’s web page on this topic.

User Software

The Rheo Knee is supported by software called the Ossur Logic App, which is available for IOS and Android devices.

For users, this application provides view-only capabilities including step count and battery charge level, as well as functional training exercises.

Ossur Rheo Knee Logic App

Operating in Expert Mode, trained clinicians can use the application to change the knee’s settings.


Ossur offers a full three-year warranty on the Rheo Knee including any repair costs due to defects.

The warranty can be extended to five years at the user’s option. Under this option, the knee must be subjected to a mandatory inspection between 36 and 40 months after purchase.

Ossur provides a free service unit during repair and service inspections.


According to our information, the Rheo Knee typically sells for between $40,000 and $50,000 US including the socket, prosthetic foot, and all prosthetist fees.

Considerations Before Buying a Rheo Knee

The biggest consideration before buying a Rheo Knee, price issues aside, is whether you are sufficiently active (mid-to-high K3, low K4 user) to benefit from the Rheo’s MRF technology without incurring the increased risk of tripping if you are not.

That core issue aside, bionic devices that use unique technologies such as MRF tend to have a different feel to them compared to devices that use mainstream technologies like hydraulics. This makes personal compatibility/preference even more of an issue than it is with two devices that use similar technology. So, our advice is to make sure that you test at least one hydraulic knee, such as the Ottobock’s C-Leg or Genium, before making a final decision to purchase a Rheo Knee (and vice-versa).

Related Information

For more information on lower-limb bionics, see Understanding Bionic Legs & Feet.

Click here for more information on Ossur.