Winter 2006

Myoelectric Prostheses Offer Advantages

Refinements in myoelectric technology have made it hard to justify using conventional prostheses for the upper body, says UCSF orthopaedist Matthew Garibaldi, C.P.O. Although myoelectric prostheses are much more expensive than conventional prostheses, they afford the patient major benefits.

Myoelectric technology uses sensitive electrodes to read activity in specific muscle groups and send signals to processing units that employ a specific function in electric motors in the prostheses. Myoelectric prostheses were made available to the public in 1970s but didn't become popular until the late 1980s. "But the technology was very expensive and a lot of patients didn't get the proper training in their use," Garibaldi says. Improvements in technology and education have made myoelectric prostheses much more attractive, he says.

Conventional prostheses use harnesses to transfer movements in the upper body into arm and hand motions. "One problem with these systems is that they can be tiring," Garibaldi says. "It can require a significant amount of energy to produce very simple motions, which impedes a patient's ability to utilize a prosthesis for long period." The movements can also be counterintuitive. "Scapular protraction, for instance, is completely unrelated to the contraction of the hand," Garibaldi says.

Electric prostheses, on the other hand, are wired to muscles that are normally involved in an action. For instance, muscles that are normally involved in grasping are wired to provide signals to clench the fist. "It's much more intuitive for the user," Garibaldi says. "Training is easier and shorter."

Myoelectric prostheses have the added benefit of proportional control, which allows the system to cater to the patient's maximum potential. The result is that even those who are old and frail can do things that may have been difficult, even with full limb usage. "A 90-year-old can have just as strong a grip as a 20-year-old," Garibaldi says. Newer technology incorporates strain gauges and other sensors in the myoelectric hand that provide proprioceptive feedback much like a patient's own hand. Such sensors can be used to automatically grasp more tightly as a cup is filled with water, for instance.

In some situations, myoelectric prostheses are virtually the only solution. "In cases of high-level amputation, where the patient is missing the entire shoulder and scapula, conventional prostheses have very little chance of success," Garibaldi says. The prosthetics department at UCSF Medical Center has been successful in recent years in using myoelectric technology on patients with very high-level amputations. Such patients would be unable to use an arm prosthesis if conventional technology were the only available treatment.

Some factors still hinder adoption of the technology. The complexity of the device — with its electromechanical and signal processing hardware — requires extensive clinical training. Given the small percentage of upper extremity amputees, most prosthetic facilities choose not to invest the time and money required to be truly proficient in this arena. UCSF Medical Center is one of the few facilities in the San Francisco Bay Area that provides myoelectric prostheses, Garibaldi says.

Cost is another factor. Whereas a standard upper body prosthesis might cost $30,000, the myoelectric equivalent might cost $100,000. Many insurers and patients don't realize the significant benefits of the electric prosthesis, Garibaldi says. "When you look at it in terms of function, the extra cost is very justifiable."

Matthew Garibaldi can be contacted at (415) 476-1788.

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