Building a better leg: The technology of 21st-century prosthetics

"We are at the threshold of a new age when orthotic and prosthetic appendages will no longer be separate, lifeless mechanisms, but will instead be intimate extensions of the human body, structurally, neurologically, and dynamically," writes Hugh Herr, an assistant professor at the MIT-Harvard Division of Health Sciences and Technology and at Harvard Medical School's Department of Physical Medicine and Rehabilitation, in a recent review of cutting-edge "cyborg technology."

Given that the era of wooden legs is within living memory, Herr's sweeping claim seems, if anything, like an understatement. Indeed, startling advances in human-machine neural interfaces, the creation of muscle-like actuators (some synthetic, others made from animal tissue), and electronic knees with microprocessors able to recalibrate 1000 times per second—coupled with digital technology—are leading to a range of prosthetic devices that were the stuff of B-grade science fiction only a generation ago.

Here, then, are a couple of the brave-new-world technologies that have already begun to erase the gap in performance between human and artificial limbs and that could in the not-so-distant future yield prostheses stronger, faster, and more durable than the real thing.

Osseointegration

In the 1960s, Professor Per-Ingvar Branemark, a doctor at the Sahlgrenska Teaching University Hospital in Sweden, began testing titanium implants to anchor false teeth. As it became apparent that the risk of immune system rejection was almost nil, the technique was extended to artificial limbs, especially above-the-knee prostheses.

To date, Sahlgrenska—which remains the leading center for osseointegration in the world—has fitted nearly 100 patients with titanium extensions extruding directly from the marrow space of the leg bone through the surface of the residual limbs. Prostheses can be locked in or removed almost as easily as changing lenses on a camera.

A clinical review in 2001, when the procedure was still considered experimental, noted "enthusiastic feedback from participating amputees" and the likelihood that the technique would become widespread. Little wonder: being able to attach an artificial limb directly to an extension of the bone eliminates the chafing, swelling, and poor fit of more conventional prostheses. For a Paralympics runner, it would also eliminate the energy dissipation caused by undesired movement in the interface between the residual limb and the socket covering it. Because it is a passive mechanism, there is also no obvious reason why osseointegration would be forbidden in competition.

Intelligent prostheses

Both Germany's Otto Bock and Iceland's Ossur, the number one and two prosthetics makers in the world, manufacture microprocessor-controlled electronic knees built into a prosthetic leg for above-the-knee, or trans-femoral, amputees. Sensors in the ankle and shin of the prosthesis collect data that allows the microprocessors to calculate the correct level of resistance to apply, automatically adjusting, for example, to uphill or downhill terrain by modifying the viscosity of the fluid in a hydraulic or pneumatic damper.

Otto Bock's C-Leg, which has been on the market for nearly a decade, makes these adjustments 50 times per second and operates whether the amputee is moving or stationary.

(Play this video for a look.) Ossur's Rheo Knee—short for "magneto-rheological actuator"—runs calculations 1000 times per second and has only been available since the fall of 2004. These "smart legs" are a vast improvement over purely mechanical legs and have, according to clinical surveys and insurance company studies, been enthusiastically received by amputees who have tried them.

Both of these legs adapt to even minor changes in the inclination of a walking surface. In addition, they accumulate data that, over time, is used to perfect the gait patterns that are initially configured on the basis of a biomechanical analysis. More recently, researchers and prosthetists have developed a way to use Internet-based conferencing systems—with live video, audio, and remote-control interfaces—to reconfigure device settings from a distance.

And this fall, Ossur, in partnership with Quebec-based Victhom, has taken things a step further by introducing the first artificial leg equipped with a motor to replace muscle mass.