"The greater function and reliability provided by today's state-of-the-art electronics are further supported by new materials and interface designs which increase comfort and longevity," says Miguelez. "Thermoplastic materials permit easy interface adjustments for residual limb volumetric fluctuation without the need to replace the inner socket," he points out. "New flexible materials combined with advanced socket designs produce lighter weight prostheses which increase range of motion while enhancing comfort. Advanced socket designs perhaps are best demonstrated by the Micro-Frame design of shoulder and interscapular thoracic level prostheses that reduce socket to skin surface area by as much as 65% from traditional interface designs, dramatically diminishing weight and heat buildup.

"Cosmetic restoration materials also have improved to resist staining while closely duplicating the sound limb," he continues. "Several manufacturers provide custom shaped and painted silicone materials which can now be applied to myoelectric prostheses. For the first time, one prosthesis can meet an amputee's functional and cosmetic requirements."

Prostheses also can be designed for partial hand amputations, Miguelez notes. "If some aspect of the digits is left intact, a silicone restoration can be provided. If no aspect of the digits is remaining, a modified electric hand can provide the best functional results. Using an Otto Bock System 2000 pediatric electric hand attached to a microprocessor with touch pads can provide good grasping function without adding significant length." Miguelez explains.

"Using an adult Otto Bock hand and rotating the motor and transmission provides more grip strength but also bulk," he continues. "I fit a man in the Midwest several years ago with a partial hand electric prosthesis because he wanted the increased grip strength that the adult Otto Bock electric hand produced, so that he could ride his Harley­and he did."

The reimbursement climate also is improving, say Sears and Rendi. "Although the cost of myoelectric prostheses is still significantly greater than for body-powered arms, reimbursement by third-party payors is becoming more commonplace. The positive clinical experience since their introduction, technical improvements, and the greater familiarity of all players in the clinical environment contribute to an improved climate for reimbursement in many respects, even though the trend towards managed care presents greater and greater hurdles in the opposite direction."

Advantages and Disadvantages

What are the primary advantages and disadvantages of both the body-powered and the electric prostheses?

A mechanical prosthesis can operate easily in most physical environments and can achieve a high level of accuracy and speed during functional performance, says Carl D. Brenner, C.P.O., Michigan Institute for Electronic Limb Development, Livonia, Michigan, in the Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles (Second Edition, Mosby-Year Book, 1992.)

Its primary disadvantages are discomfort caused by the shoulder harness and the non-cosmetic appearance of the hook terminal device, he notes. The advantages and drawbacks of the electric prosthesis are just the reverse, he points out, with the electric prosthesis providing more comfort and a stronger grip force in the electronic hand, which is inherently more cosmetic. However, the electronic terminal device may be slower to operate, and the electronic prosthesis is not suitable for environments with frequent dirt, water, dust, grease, and solvent contact.

Myoelectric control from remnant muscle contraction feels more natural, point out Sears and Rendi, adding, "The latest generation of myoelectric devices also can be adjusted to operate at a very low level of muscle contraction, so that the wearer is less fatigued than with earlier systems." Most electric prostheses can now be supplied with fast battery chargers which charge in about two hours, with improved battery packs having two or three times the life of previous ones, making battery maintenance much more convenient, they note, adding, "The dependability of electric prostheses is improving in many other ways as well, with the availability of modern circuit board manufacturing techniques, improved electric motors, higher-strength plastics, and long-life motors."

Often providing the best of both worlds is the combination mechanical-electronic, or "hybrid" prosthesis, designed for the specific needs of an individual user. "When properly understood and applied, a hybridly designed prosthesis can offer the individual the greatest degree of function and reliability a prosthesis has to offer," says John Billock.

Billock has designed highly functional prostheses which incorporate myoelectric control, switch control, Bowden cable control, and passive mechanical control. One of his most notable successes was creating a prosthesis for famed photojournalist Mohamed "Mo" Amin, who alerted the world to the devastating famine in Ethiopia in the 1980s. Amin had lost his left arm above the elbow when he was struck by a missile in an ammunition depot explosion in Addis Ababa in 1991. Amin's first prosthesis incorporated a myoelectrically controlled hand, switch-controlled wrist, Bowden cable-controlled elbow, and passive mechanical friction humeral rotation, as well as a universal ball-and-socket passive mechanical friction wrist. (Mo Amin later died in the crash of a plane which had been commandeered by hijackers.)

Control Systems

Various control systems exist for electronic prostheses. Myoelectric control systems use the existing neuromuscular system for either digital or proportional control. Electromyographic (EMG) potentials are monitored with surface electrodes placed over muscles or muscle groups within the residual limb. A proportional control feature allows the user to control the output voltage to the electrically powered hand, hook, wrist, or elbow, causing, for example, the hand to move faster and grip harder.

Switch and servo mechanisms also can be used when needed. Switch control systems require much less force and excursion than a Bowden cable-controlled system to actuate and control a terminal device and can use different types of switches, such as pull, rocker, push-button, and toggle, notes Billock. "This type of control is typically indicated in situations when limited body motion and forces are available for Bowden cable control and/or when EMG potentials are inadequate or inappropriate for terminal device control," he explains. Some conditions making myoelectric control unfeasible include a lack of muscle strength after nerve damage, extensive or fragile scar tissue over otherwise acceptable muscle sites, and very high-level amputations.

What patients are good candidates for electronic prostheses? Harold Sears and Joanna Rendi provide a profile of regular and consistent wearers:

• They are able to tolerate a significant prosthesis weight. Slightly built patients are not disqualified, of course, but their weight tolerance should be carefully evaluated.
• The most active wearers often use two or more terminal devices, and they may use a body-powered prosthesis as well as an electric.
• They have a functional drive to use the prosthesis. Whether at work or play, some of the user's essential activities require the use of a prosthesis.
• A body image need exists. The user is usually not accustomed to a one-armed appearance nor performing normal two-handed activities with only one hand.

"Motion Control provides a video mini-course for therapists to address this problem­the lack of training and experience for the occupational therapist," says Joanna Rendi. "It is free of charge to therapists and prosthetists," she adds.

Carl Brenner's approach involves giving the patient a chance to try out different components through different preparatory prostheses until the optimum combination of components from both the patient's and the prosthetist's perspective has been found, which will then become the definitive prosthesis. Before fitting, an extensive evaluation and consultation is held with the patient, which includes discussing his ideas and feelings and the technology available to him.

The patient begins with a body-powered prosthesis with a mechanical terminal device. Most patients are fitted within 48-72 hours after surgery, although fittings can be done within 24 hours. "As long as a patient is fitted within 30 days, we feel that we have eliminated the tendency to become one-handed," Brenner explains. As the amputee explores what is best for him by using different componentry, he also is building muscle strength and learning how to perform functional tasks. When the change is made from a mechanical to an electronic prosthesis, the skills remain; the amputee simply learns a new way to control the prosthesis, says Brenner. "For example, sometimes a patient won't like myo control of the elbow as much as a switch control, but he wants myo control of the hand, because that gives him proportional control."

A limb bank is the key to this system. Limb banks involve the collection over a period of time of a variety of electronic components which can be loaned to the patient on a trial basis for a modest leasing charge, Brenner explains in the Atlas of Limb Prosthetics. For a fraction of the cost of new hardware, necessary componentry for a preparatory/training prosthesis can be provided. Brenner lists three types of limb banks: a private limb bank, maintained by an individual prosthetic laboratory, a commercial limb bank, sponsored by a manufacturer of electronic limb components, and an institutional limb bank, which is generally organized and supported by either a hospital or a charitable organization.

Electronic prostheses may not be right for every amputee, says John Miguelez, "but they need to know that they exist and what the options are." There are about 10,000 new upper-limb amputees a year; about half choose not to wear prostheses, he estimates. Practitioner experience is a vital factor in positive outcomes, he notes, but with the small number of upper-extremity patients each year, the average practitioner may see only one or two cases a year. "Furthermore, the limited availability of advanced practitioner training (often a prerequisite for purchasing upper-extremity components) requires practitioners to leave their busy practices plus additional expenditure."

To address these needs, John Miguelez provides a specialized upper extremity consulting service, Advanced Arm Dynamics, Inc., Rolling Hills Estates, California. Previously he was Vice President and Senior Clinical Director for the National Upper Extremity Prosthetic Program at NovaCare, Inc.

How can electronic upper-limb technology benefit amputees? "In a nutshell, it means better comfort and a more natural appearance," say Sears and Rendi. Sums up John Miguelez, "With detailed information and the flexibility to apply it creatively, an experienced practitioner can expedite his patients' rehabilitation and enhance their quality of life."

• Lighter, stronger components made of titanium;
• Faster speeds. Hand (one second); wrist (two seconds); elbow (three seconds);
• Higher hand/hook grip force­32-34 pounds;
• Wrist with high torque;
• Smaller, higher-capacity and lighter weight batteries­this technology is here but not yet readily available;
• Smaller electronic circuits;
• Multi-positional hands (fingers);
• Greater interchangeability between components;
• More aesthetic and durable prosthetic skin gloves;
• Better self-suspension techniques;
• More occupational therapists trained in upper-limb ADL.

• Electric wrist flexion and extension;
• Sensory feedback;
• Remote diagnostic abilities;
• Smaller controllers and batteries, especially for pediatric components.

• Better gloves­stain resistant and tear resistant­at a reasonable cost;
• More reliable batteries;
• Practical, commercially available touch-sensor systems to provide user feedback.