|New techniques for fitting shoulder amputees, combined with more comfortable materials (ProFlex with Silicone pictured, from Fillauer, Inc.) allow much more successful suspension to proximal level amputees. (Photo courtesy Advanced Arm Dynamics)|
A recent U.S. prosthetics practice survey revealed that upper-limb prosthetics represents only 5-percent of work performed by prosthetists nationwide. One might conclude that since research efforts and dollars tend to be spent in a similar proportion, the needs of arm amputees could be overlooked in favor of larger groups such as leg amputees.
Conversely, this is a very exciting period for arm amputees in search of a better prosthesis. New technologies such as batteries, microcomputers, small but sturdy circuit boards, and other electronic advances allow greater function than ever before. And new materials such as composite plastics, silicones, etc., are creating lighter-weight designs and more natural-looking cosmesis for both adult and child upper extremity amputees.
Media attention is drawn to high-tech devices such as myoelectric arms, but progress also continues in body-powered developments. The future will reveal even more advancements in body-powered devices, offering the benefits of lower cost and simplicity. Lets take a look at some of these new technologies for consumers.
New electronic components
The trend is very clear in new electric hands - the microprocessor is here to stay! The latest controllers for electric hands from Motion Control, Otto Bock, and Liberty Technology all utilize microcomputers, and they have more automatic functions than ever before. Smaller than ever, they now consume less power - a big plus for the electric arm wearer.
Proportional control is now accepted as the preferred method for myoelectric hands because of the improved precision it affords the electric hand wearer. Proportional means that the hand power is in proportion to the muscle contraction. Formerly requiring a sacrifice in component space, the new microcomputers simply implement proportional control in their software, all within a very small space.
Hands and Hooks Terminal devices (in the prosthetics vernacular) have also evolved greatly. A much wider variety in shapes and sizes is available. For adults, in addition to the Otto Bock hands, the Ultralite Centri Hand is available at 1/3 less weight than the Otto Bock version. Hand weight is especially important to reduce weight at the end of the prosthesis; however, the lighter hand provides far less pinch force. Work-type devices like the Otto Bock Greifer and Hosmer Synergetic Prehensor can be interchanged with the hand for rugged environments, or where the tool-like gripping shapes better suit the task.
Childrens electric hands have proliferated widely, and are used quite commonly in most clinics, although the body-powered hooks will always have a place for kids rugged activities. The Flexi-Wrist (by Liberty Technology) allows a hand to bend at the wrist, which is useful when grasping a tricycle handle bar or in other play activities.
In combination with the electric terminal devices, electric wrist rotation is now used much more commonly and effectively with the new controllers allowing easy control of both hand and wrist. The Utah ProControl introduced co-contraction switching, which allows the wearer to turn on the electric wrist with a quick snap of the two control muscles. Now utilized by other systems, co-contraction requires some training for the new wearer, but has proven to be the most convenient method for wrist control, and has led to much wider usage. The benefit to the wearer is the freedom to reposition the hand without the trouble of reaching over with the sound hand (for unilateral amputees) or awkward knee manipulation (by bilateral amputees). The result is a much more natural movement for the wearer.
Progress in these technologies, as well as the fitting techniques of prosthetists have changed the traditional attitudes toward electric hands. Only a decade ago, electric hands were considered a very specialized prosthesis, and rarely prescribed for patients. Now, in up-to-date clinics serving arm amputees in the U.S. and Europe, the majority of arm prostheses utilize electric hands.
In another relevant trend, a new breed of prosthetic specialists has emerged those who fit upper limb prostheses predominately. These new upper limb specialists usually travel to the patients location, allowing patients anywhere in the country to receive the highest level of prosthetic technology, fitted by an experienced practitioner. The large prosthetic chains have such specialists, but smaller independent prosthetic laboratories can contract with either a manufacturers specialist or a free-lance specialist, if their own experience is lacking.
Other new electric prostheses include the Utah Arm 2 (U2) by Motion Control, which reworks the sophisticated prosthesis with new sturdier circuit technology, similar to a cell phones circuits, stronger plastics for more rugged use, and new nickel-metal-hydride batteries for faster charging and longer wear. New technologies like the U2 offer consumers dependability and high performance, with a very low effort to operate compared to body-powered arms.
Body-Powered Developments and Cosmetic Covers
Though progress is not celebrated as much as the high-tech electric arms, advances in this technology have the potential to reach many more amputees worldwide. The lightweight Advantage Arm by Sarcos, Inc., features internal cables, that go directly from the harness into the top of the elbow unit. Flexible polymer cables are utilized that pull much more smoothly than steel cables. A mechanism inside the elbow allows cable recovery, that is, after locking the elbow (with a second cable) the wearer relaxes the pull on the elbow cable, then pulls again to open the hook or hand terminal device.
Otto Bock has also introduced another new mechanical elbow system, the Automatic Forearm Balance (AFB), which features an internal mechanism providing a spring assist to lifting the elbow. The wearer uses a ballistic motion to initiate lifting the elbow, i.e., from an extended position (hanging down) the wearer will swing the elbow forward, at which point the forearm balance kicks in and raises the elbow to a level position. It could be said that wearers control the elbow position by swinging their shoulders. One version of the AFB, called the Ergo Elbow, is designed to be used with the electric hand. The combination of electric hand with body-powered elbow is often called generically a hybrid arm system, offering the wearer lower weight and the independence of elbow and hand function. For example, both elbow and hand could theoretically be operated at the same time; however, good shoulder motion is required, and some training is usually needed to learn to separate the control cables (for the elbow and lock) from the control signals for the muscle signals (for the hand and wrist) separately.
Silicone Hand Covers
|Silicone hand covers are now much higher in quality than previously available and importantly, are now strong enough to be used with electric hands. Until just a few years ago, the natural-looking silicone covers were not durable enough to be used with electric hands, which could pinch through the glove with their high force. Now, several manufacturers make high-strength silicones for the more functional electric hands, providing the best combination of high-grip strength with near-natural appearance.|
Fitting improvements for high-level shoulder amputees
With the improvements in electric elbows, hands and wrists, it has become possible to provide more function to even the most challenged of arm amputees those amputated at the shoulder level. Prosthetists are also learning new ways to fit these difficult cases, using easily-formed yet strong plastic materials, which make innovative designs possible.
A new shoulder component, the MICA/Liberty locking shoulder joint allows the wearer to securely lock the shoulder preventing front-to-back motion, but can then be unlocked with a lever (pushed with other hand, or chin) to freely move the arm to other positions, or for dressing more easily. Shoulder amputees are finding more success now with electric components, since better prosthetic techniques hold the prosthesis more securely on the body.
Arm amputees, who struggle to balance the needs of function, comfort, appearance, and cost of their prostheses, represent a great challenge to the manufacturers and prosthetists seeking to satisfy those needs. The fact that arm amputees represent a small fraction of the total prosthetic market further discourages advances for several reasons. Besides the obvious economic problem, there are also the difficulties of educating the decision makers about benefits of new technologies, as well as prosthetists who need to learn and apply the new technology into a practical limb for the amputee.
Two trends are working to the advantage of arm amputees:
- New technologies developed for other industries allow improvements in the design and function of arm prostheses.
- The trend toward specialization of prosthetic practice results in an elite group of arm specialists, prosthetists whose motto is have tools . . . will travel. They provide the link needed to disseminate modern fitting techniques, and the knowledge of the full range of electric and body-powered arm components.
About the Author
Harold H. Sears, Ph.D., is General Manager of Motion Control, Inc., Salt Lake City, Utah