MAGNETIC OIL – GENTLER CONTROL

By developing a braking system with magneto-rheological fluid, Össur has successfully created knee prostheses with a lightning response to the user’s movements and gentle control of the knee.

Text: Anders Lindgren

agneto-rheological fluid, or MR fluid, is used in a number of product segments to achieve a rapid damping effect. In recent decades, the technology has been refined to the extent that it is possible to control MR fluid with a high degree of accuracy and in small magnetic fields. Össur utilises this in developing its microprocessor-controlled prosthetic knees RHEO KNEE and RHEO KNEE XC.

WHEN DEVELOPING a prosthetic knee, you want it to provide as responsive and dynamic a range of movement as possible. And there are several benefits from using MR fluid instead of conventional solutions involving hydraulic cylinders to regulate bending resistance in the knee. Above all, the response is faster, as the fluid can go from zero to 100% resistance within a few milliseconds.

“MR fluid gives the user a knee with gentler control, for a more natural, responsive gait. As the oil reacts instantly to changes in the magnetic field, the user can often walk at a faster natural pace than with other microprocessor-controlled prosthetic knees and also climb stair-upon-stair,” according to Sven Johansson, Clinical Specialist, Academy Össur North Europe.

ÖSSUR’S KNEE has approximately 10 ml of magnetic fluid between around sixty slats. The bending resistance is governed by the strength of the magnetic field. When the MR fluid is activated, a shear resistance is created for the slats. The stronger the current, the greater the viscosity and resistance.

The magnetic field, which is generated by a battery with up to 72 hours of battery life, is applied only to the knee’s braking system of slats and MR fluid. The knee’s sensors constantly read the rate of angle of movement, the angular position, gyro position, etc. The computer controls the strength of the current and the knee itself finds the best setting in each given situation.

“The system uses several hundred different values to produce as even and energy-efficient a gait as possible. And, because the braking

system shares the same centre as the prosthetic knee, it can brake equally well, regardless of the bend of the knee. All microprocessor-controlled knees respond to a large number of parameters, but with conventional solutions, resistance is difficult to control. Even the ambient temperature can affect the viscosity of the oil. But the system even compensates for that,” Sven Johansson reports.

All the data is stored in a program that is being constantly developed and fine-tuned for the specific user and that user’s prosthetic solution.

“The programme is autodidactic, but we don’t allow it to make too-big changes automatically. The basic settings established during commissioning remain in place until the orthopaedic engineer makes any changes or reprograms the prosthesis,” he adds.

IN ANY OIL-BASED system, there is a risk of the oil wearing out. But, despite tests involving many millions of repetitions, there is no sign of the MR fluid in Rheo Knee wearing out. This is because it is a low-pressure system, Sven explains.

“The MR fluid in Rheo Knee does not get squeezed through a narrow valve, so it is not exposed to the heavy load and heat development that that would entail. Being a low-pressure system also minimises the risk of oil leakage.

By using MR fluid, Össur has successfully created knees that allow very good and fast adjustment of the braking effect for the user. It can take a few weeks for the prosthesis to coordinate fully with its user. But it never stops learning.