An artificial yet active prosthetic leg gives above-the-knee amputees a more natural gait thanks to a microprocessor that adjusts the knee to changing conditions. The controller on the C-Leg from Otto Bock HealthCare, Minneapolis (ottobock.com) takes data 50 times/sec from a joint-angle sensor, and a force-sensing strain-gauge pylon (shin) to determine the phase-of-gait (position of the leg for a particular moment in a stride) and the user's walking speed. A sensor in the lower leg measures pressure at the heel. Software then establishes the user's phase-of-gait based on thousands of gait cycles. The processor, which understands normal human gait, then sends instructions to a damping mechanism that makes the knee stiffer or more compliant.

Nature uses tendons and muscles for damping and to adjust limb movements with each step. Hence, people use their weight and surface conditions when walking, running, cycling or standing. And they rapidly make many adjustments unconsciously, often as pure reflexes.

Most leg replacements, no matter how high tech, are passive devices. They do not change during movement. More advanced prosthetics, such as the C-Leg, rely on microprocessor controls. Like natural limbs, the prosthesis adapts in milliseconds to changing situations made possible by hydraulic damping. The controller calculates damping values and valves in the hydraulic circuit generate them.

“Inside the leg, reaction to human activity moves a piston and rod up and down through hydraulic or damping fluid,” says Byron Backus, a certified prosthetist and educator with Otto Bock. “Two 10-mm motors turn a planetary gear set that adjusts two valves controlling hydraulic damping. One valve controls damping for leg extension and the other for flexion. The valves are continuously adjusted for best stability. With each step, the motors adjust damping from its maximum level to almost zero and up again,” adds Backus. The motors use precious-metal brushes to deliver the needed torque and speed. The manufacturer says the prostheses work without recharging batteries for up to two days.

Lightweight micromotors from MicroMo Electronics Inc., Clearwater, Fla., (micromo.com) combined with intelligent control on the C-Leg make it possible to walk in a way that feels more natural and with the benefits of safety and comfort.

The C-Leg lets users walk slowly or fast, run, walk down slopes or stairs in a step-over-step fashion, and ride a bicycle, and users say without paying too much attention to what the limb is doing. In fact, the knee joint reacts immediately to changes in speed or surface conditions. Even if the wearer stumbles, the real-time controller prevents the prosthesis from buckling. Over the long term, the prothesis' ability to react intelligently avoids undue wear and tear on the other joints and problems arising from poor posture, so the healthy leg is not overstrained.

When developing the C-Leg, Otto Bock's Vienna-based research-and-development team considered that an advanced prosthesis should be suitable for use worldwide and work well under all possible conditions. Bock's concept was an active prosthesis with smooth and reliable interaction between mechanics, electronics, and user. Hence, each prosthesis is fine tuned by a trained local prosthetist to suit each user.

The C-Leg can handle temperatures from -15 to 65C, dry or humid, and in salty air or desert sand. Important components are sealed with some getting an extra casing. Components are replaceable. And if the battery dies, the prosthesis goes to maximum damping, in effect functioning as a stiff leg so wearers can continue walking, although with less comfort.

“Damping levels are initially set by a certified prosthetist using C-Soft software and Blue Tooth technology,” says Karen Lundquist, a spokesperson for Otto Bock. “Users choose between two settings, for example one for walking and one for cycling. Active-stance-phase damping makes it easier to stand and walk, and it can be switched off for easy cycling, for example.” Recent designs let users switch between damping modes with a wireless remote control. A standing mode has been added to stabilize the C-Leg at any flexion angle between 7 and 70°. This takes weight off the sound limb, letting the user relax while standing.