RHEO KNEE® technology in depth

RHEO KNEE® technology in depth
Perhaps a product designer's greatest challenge is to replace all physiologic function of the anatomical limb. Emulating the human anatomical, physiological and neurological system is a holistic task that is both complex and humbling. Anatomically, the designer must recreate structures of the body by using materials that enable everything from fine motor skills to ambulation. Neurologically, the designer must create a feedback mechanism that senses changes in the external stimuli and that uses the character of these changes to alter physiological responses in real-time.

The difficulty of meeting these challenges explains why commonly available prosthetic devices have such limited functionality. It also explains why Ossur turned to the Massachusetts Institute of Technology (MIT) to identify the state-of-the-art software, hardware and biomechanical technologies needed to make bionic prosthetics a reality. The results as realized in the RHEO KNEE® are striking: a synergy of artificial intelligence, advanced sensor and magnetorheological actuator technologies that give the user the unprecedented natural movement essential for embracing life without limitations. The specific technologies that achieve this synergy are the:

Dynamic Learning Matrix Algorithm™ (DLMA).
The RHEO KNEE® utilizes a software based artificial intelligence to learn the individual's walking style and provide a mechanism for continual monitoring and optimization of swing control. From the moment the first step is taken, the Rheo Knee compares the maximum degree of knee flexion achieved in each step during swing phase to targeted values set by the prosthetist with the Rheo Logic software. Swing flexion resistance values are then updated to limit excessive heel rise or promote heel rise depending on the walking speed. Swing flexion resistance values will influence extension rate and extension dampening making the swing phase adaptation process complete. Sampling and optimization of values is continual within the RHEO KNEE® even after the initial program has been set for the user by the prosthetist. Through constant monitoring and optimization, the Rheo Knee provides a virtual prosthetic adjustment to ensure that the individual?s walking style is targeted most appropriately.


This advanced technology uses magnetic fields, MR fluid, and rotary blades to vary the RHEO KNEE?s resistance in real-time. Microprocessor control initiates rapid kinetic response to sensory inputs through inductance command as frequently as 1000 times/second. With application of the magnetic field, carbyliron spheres are drawn together in electromagnetic chains. As the knee rotates into flexion or extension, fine rotary blades shear the particle chains to create resistance. The result is minimized fluid drag within the knee restoring more natural pelvic position during pre-swing and reducing fatigue levels.

Microprocessor Controlled Stance.
Through advanced sensing and processing, the RHEO KNEE® provides multiple safeguards against inadvertent stance release. The knee must be fully extended, momentarily still, and achieve 20% of the average maximum extension moment during each step to release. The microprocessor samples force measurements at a rate of 1000 times per second and is always aware of how the user is loading the prosthesis. Disturbances in the user's path are automatically recognized by the force sensors. Stance support is instantaneously activated to protect the user from a potential stumble and fall. By detecting these patterns in their earliest stages, the RHEO KNEE® can take the preventive actions to reduce the likelihood of a fall.

Rheo Logic Software.
This intuitive icon-based programming software runs on an HP iPAQ palm-based computer and makes operation simple. With the use of specialized SETUP modes, initial programming is automatic. You simply power the unit and have the user walk. SETUP modes calculate baseline starting values for swing and stance control. When manual adjustments are necessary to further customize the RHEO KNEE®, the software makes this task a fast and easy one. Multiple users and settings per user may be stored on the PDA and transferred to a PC making this tool very versatile.


No magnetic field applied to the MR fluid. Ironparticles are diffusing randombly allowing the disks to rotate freely in the opposite direction. Low magnetic field applied to the MR fluid. The iron particles start forming chains and resistance between the blades increases. The strong magnetic field has now caused the fluid to form continous chains and the knee can now been felt as rigid through the joint.


Clinical test results
In 2003, a study described in Industrial Robot (International Journal, Vol. 30, ~1 2003, P 42-55) assessed the clinical effects of the RHEO KNEE® by examining kinematic data from four subjects who were unilateral, transfemoral amputees. The investigators concluded that a user-adaptive control scheme and local mechanical sensing were minimally sufficient to enable amputees to ambulate in a biologically realistic manner (as compared against the schemes and sensing available in mechanically passive prostheses).