Locking action makes prosthesis lighter, more reliable

Kinna, Sweden-When his daughter lost a leg to amputation at age six, Finn Gramnas quickly learned about the inefficiencies of conventional prosthetics. Most were bulky and awkward, he found. To walk with them, users usually adopted a special gait-hopping two steps with the good leg for each step with the prosthetic limb.

For most amputees, walking was a challenge, Gramnas noticed. Walking downhill and descending a staircase was even more difficult. More active endeavors-such as running-were almost out of the question.

Now-seven years later-Finn Gramnas believes he's found the solution to those problems. His new design for a prosthetic knee enables amputees to do many of the things they struggled with before. Called the Total Knee Geometric Locking System, it offers greater flexibility and stability than other designs, while weighing only about half as much as competing hydraulic devices.

Critical to the new design is a geometry that inherently offers greater stability. The system employs three main parts: an aluminum chassis; a hydraulic head; and a triangular balancing block. By reacting as the user shifts his or her center of gravity in front of, or behind, the knee, the balancing block controls the knee's action.

To achieve that control, the block tilts as the user walks. When it tilts backward, it locks a bottom link into place at the base of the chassis. With the bottom link locked, the user can load body weight onto the prosthetic limb without fear of the knee buckling. "When you place your heel on the floor and you have your leg in front of you, you are in a critical position where the knee can easily collapse," Gramnas notes. That's especially true when walking down a hill or a set of stairs, he says.

To deal with this situation, Gramnas selected a locking mechanism that relies on geometry, rather than friction or hydraulic force. "Because it is a geometric locking function, it will never wear out," he explains. The new design also reportedly offers a performance advantage over friction brakes, which can engage when the knee bends. In contrast, the geometric system locks only when the leg is straight.

With the addition of a urethane bumper at the base of the chassis, the Total Knee can counter impact stresses in the same manner as a human leg: By bending a little when its heel hits the ground. The bumper, which can employ a variety of hardnesses depending upon the patient's size, softens the blow of a hard stop. At such times, the bottom link presses against the bumper as it reaches the end of its range of motion. In this way, the bumper absorbs impact that would otherwise be transferred to the user's legs and hips.

The importance of the balancing block, however, goes beyond the locking function. When a user bends the knee, the balancing block tilts backward, enabling the knee to bend as much as 160 degrees. That's particularly important for active endeavors, such as running or bicycling, which require users to bend their knees beyond 90 degrees. It's also especially critical for users whose legs are imputed immediately above the knee. For those amputees, conventional prostheses often limit knee flexibility to approximately 90 degrees.

To further enhance the Total Knee's performance, Gramnas also endowed it with a three-phase encapsulated hydraulic-resistance motion-control system. This rotary rack-and-pinion system, located in the top of the knee, makes it possible to control the speed of the leg from slow walking to running. The Total Knee provides low resistance from 0 to 60 degrees and higher resistance from 60 degrees to fully bent, preventing the knee from swinging backwards too high and "kicking the user in the back." To establish this control, a tiny hydraulic piston transmits resistance to the rack-and-pinion gear, through an axle, and into an upper link attached to the balancing block.

The resulting system not only offers greater stability and flexibility than more conventional designs, it also significantly reduces overall unit weight. At 620 gms, the Total Knee is said to weigh approximately half as much as typical hydraulic systems. The system, Gramnas says, is small and light enough for his daughter to wear.

Using the Total Knee, Gramnas' daughter and other amputees now can bicycle, run, and easily walk down hills and stairs. "I wanted the device to be small, lightweight, and have a safe lock mechanism that could operate without failure," he says. "In perfecting this design over the years, I think we've accomplished that."

Additional details...Contact Linda Steers, Century XXII Innovations Inc., Box 868, Jackson, MI 49204, (800) 788-9878.

Comments (0)

Please log in or to post comments.
  • Oldest First
  • Newest First
Loading Comments...