The basic requirements of an upper-limb prosthesis can be described by three things: cosmesis, comfort, and control. The focus of this conference was on the control aspect of prosthetic use: myoelectric versus body-powered. However, prosthetic control influences both cosmesis and comfort. For most users of upper-limb prostheses, cosmetic function is of primary importance, and this is certainly true in the beginning of rehabilitation.1,2 Cosmetic function refers to the natural appearance of the prosthesis and, thus, to the perceived degree of unobtrusiveness of the individual with an amputation with his/her prosthesis.1 Van Lunteren et al.1 break down cosmetic function into three categories: passive cosmesis, cosmesis of wearing, and cosmesis of use. They found that most persons with amputation found passive cosmesis to be very important. Those highly concerned about the cosmesis of wearing may tend to hide their prosthesis and those concerned with cosmesis of use tend to avoid activities that require unnatural arm motions.1 The comfort of an upper-limb prosthesis is also extremely important as the discomfort associated with harnessing and socket interface can quickly negate advantages in functional control. Thus, any considerations of prosthetic control options must include the associated influences of cosmesis and comfort.
One of the primary variables regarding the type of prosthesis that will best meet the needs of any person with amputation is level of amputation. To simplify this discussion and make it relevant to the largest population of persons with upper-limb amputation, it is most appropriate to focus primarily on issues of the individuals with a transradial amputation. More proximal levels such as transhumeral and shoulder-level amputations have additional considerations because of the greater number of joints that need to be replaced and other factors, including limited control sites, additional weight, and limited functional envelop for use. Many of the points raised in the discussion below can be applicable to more proximal levels, but because of these other factors, prescription recommendations for individuals with transradial amputation may not be applicable to these higher levels.
Myoelectrically controlled prostheses have a long history of use, having been commercially available in the United States for almost half a century. For management of the individual with transradial amputation, myoelectric control is considered by many of the professionals specializing in the treatment of individuals with upper-limb amputation as the standard of care.2 This is not to say that body-powered prostheses do not have a place in contemporary management of persons with upper-limb amputation. Body-powered prostheses have a long history of use, and many persons with amputation successfully use and prefer them. The purpose of this article is to discuss the clinical considerations involved in choosing between externally powered and body-powered prosthetic systems by canvassing the benefits, indications, drawbacks, and contraindications for both device types. Barriers to obtaining the most appropriate upper-limb prosthesis will also be discussed.
INDICATIONS/BENEFITS/CONTRAINDICATIONS/DRAWBACKS FOR MYOELECTRIC CONTROL
Most persons with amputation want a hand prosthesis to replace the hand they lost. This is often an expectation of the persons with amputation as well as their family and social network. The cosmetic function of a prosthesis is usually the first thing a person with amputation will focus on. For many individuals with amputation, the hand's appearance is as important as the function it provides, and both appearance and function are desired. A prosthesis that does not restore a normal appearance is often rejected.2 Myoelectric prostheses are best suited to provide both a functional and cosmetic restoration that best meets the desire of the individual with amputation to “fit in” socially and not be the subject of unwanted attention.1
The transradial myoelectric prosthesis can be self-suspending and self-contained, therefore eliminating the need for a harness. The harness is often cited by wearers as being uncomfortable, and discomfort has been cited as one of the causes for prosthetic abandonment.1,3,4 At the transhumeral level with myoelectric control of the elbow and hand, a harness is often used for auxiliary suspension. However, in this case, owing to the reduced role of the harness, it can be worn less tightly and therefore more comfortably, and it is less restrictive than a harness for a fully body-powered transhumeral prosthesis. With discomfort being one of the most reported complaints by users, any prosthetic feature that can increase comfort, such as elimination of the harness, cannot be overemphasized. When the harness can be eliminated, the prosthesis is easier to don and doff, especially if the user is already fully dressed.
At the transradial level, myoelectric control of the terminal device is physiologically natural. That is, the muscles used for opening and closing the myoelectric hand are the same as those used for opening and closing the natural hand. At higher amputation levels, hand opening is controlled by the extensor muscles that are natively associated with releasing. Similarly, hand closing is controlled by the flexor pattern muscles that are natively associated with closure. With the advent and development of targeted muscle reinnervation (TMR), terminal device control, even at proximal amputation levels, is directly associated with physiological hand function, realizing similar physiologically natural control as the individual with transradial amputation.5
Grip strength of a myoelectric terminal device is several times greater than a voluntary opening body-powered device. This increased grip is achieved with almost no increased effort by the user since only very small muscle contractions, measured in microvolts, are required to attain maximum grip force. Also, since the myoelectrically controlled hand maintains any set grip force without sustained control input, the user is not required to concentrate on maintenance of grip. With the advent of multifunctional hands, the advantages of these myoelectrically controlled hands, which are capable of producing many normal hand grip patterns, are more pronounced compared to body-powered options.
There is some evidence that wearing a myoelectrically controlled prosthesis may reduce contralateral arm overuse problems. Jones and Davidson6 found that half of the individuals with amputation in their survey reported overuse problems with their remaining arm. In a survey of overuse problems, Burger and Vidmar7 found an association of carpal tunnel syndrome with the type of prosthesis worn. The syndrome was reported by none of the wearers of myoelectric prostheses, one-third of the wearers of body-powered prostheses, one-half of the wearers of aesthetic prostheses, and all of the nonusers.7
The ease of terminal device interchangeability is improved with myoelectric control. The myoelectrically controlled hand is easily exchanged with a myoelectrically controlled hook or other terminal device, and this can readily be done with one hand. Users of myoelectric terminal devices also benefit from the available myoelectronically controlled wrist rotation units. These wrist rotators can position the terminal device for best orientation and are not restricted by the control cable compared to fitting an electronic wrist rotator with a body-powered terminal device.
A prerequisite for myoelectric prosthetic use is the presence of adequate muscle activity producing electric signals for myoelectric control. Without muscle signals, myoelectric control is impossible. Almost all persons with amputation will possess adequate signals given the high sensitivity of state-of-the-art electrodes available and the ability to adjust the signals in the microprocessor and associated software. Even the presence of one useable signal can provide adequate control of a myoelectrically controlled prosthesis. When no useable signal is available, an electronic prosthesis can still be used effectively using another control input device such as a linear transducer.
Myoelectric control does not presently afford direct proprioceptive feedback other than through visual, auditory, and vibratory feedback. Since control is accomplished by surface electrodes, any interference with continuous contact of the electrodes with the skin may cause a lack of control or inadvertent signal generation. This may occur if the prosthetic socket moves on the residual limb or if the electrodes break contact with the skin.
Myoelectrically controlled prostheses may be damaged by certain environmental factors such as water, dirt, and electronic interference. For these environments, a myoelectric prosthesis may not be suitable. However, improvements have been made to protect myoelectronic devices from damage such as sealing the mechanism to prevent damage from water and dirt. Some myoelectrically controlled hooks are now water resistant and sealed against intrusion of dirt. Improvements have also been made in electronic filtering to reduce electronic interference.
Myoelectric devices require a battery for operation that needs to be charged daily. Myoelectric devices are more expensive to purchase and maintain than body-powered options and sometimes need to be serviced by the manufacturer.
Myoelectronic prostheses are generally heavier than body-powered prostheses. This is sometimes a factor in the preference of an individual with amputation for a particular type of prosthesis and is increasingly relevant at more proximal amputation levels.
At higher amputation levels such as transhumeral and shoulder disarticulation, control of multiple prosthetic joints can make operation more complicated. Historically, we have relied on creative ways to switch power and control from one component to another in order to allow the limited control sources to operate two or more components. With the introduction of TMR surgery and the recent introduction of implantable electrodes and pattern recognition control, this problem is being minimized.
INDICATIONS/BENEFITS/CONTRAINDICATIONS/DRAWBACKS FOR BODY POWER
When considering the benefits of body-powered control, it is necessary to make a distinction between body-powered hooks and hands. Body-powered hooks are very functional for a variety of activities. They are lightweight, relatively low cost, and simple in design. Hooks have proven to be very functional and are especially well suited for use for heavy-duty activities. They are less prone to damage in excessively dirty, wet, and corrosive environments than externally powered devices are. Body-powered hands, however, do not have these same attributes, as will be discussed below. Body-powered control offers good proprioceptive feedback through the cable and harness system. This type of feedback has been described as extended physiological proprioception where the control system provides feedback regarding force, position, and velocity of the prosthetic component.8,9
Body-powered hooks do not restore a normal appearance and therefore are rejected by persons who desire a cosmetic prosthesis. Body-powered hands are mechanically inefficient; they are not lightweight and require a great amount of force to operate.3 Smit et al.3 found that most body-powered voluntary opening hands do not provide enough prehensile force to accomplish activities of daily living. This finding raises a critical point when discussing the functional capabilities of body-powered prostheses. That is, body-powered hooks may be very functional but are not cosmetic, and body-powered hands, although more cosmetic, are not functional and are often little more than passive devices. Knowing that cosmesis is often of high importance to the end user, hand prostheses are appropriate and indicated for most persons with upper-limb amputation. Yet, a body-powered hand prosthesis is generally contraindicated functionally. By contrast, because body-powered hook prostheses tend to be cosmetically unappealing yet highly functional, they may be contraindicated cosmetically in many cases but indicated functionally in others.
Discomfort caused by the control harness required for use of a body-powered prosthesis is also a major drawback. Prosthetic comfort is affected by control in several ways. Body-powered control requires a harness that is cited by many users to cause discomfort.1,3 Successful use of a body-powered prosthesis requires that the user has sufficient force and excursion to operate the system. The control harness is the means of transmitting force and excursion to the prosthesis. These forces can be quite large3 and are often uncomfortable.3,10,11 Because of the high forces that are transmitted to the prosthesis at the prosthetic socket, local discomfort or pain may be experienced where the residual limb contacts the socket. Stabilization of the harness, often referred to as the anchor point, is usually accomplished at the sound side axilla, also causing discomfort and sometimes impingement of the neurovascular structures in this region. The harness is also more difficult to don and doff than a prosthesis that does not require a harness. For persons who place high value on the cosmesis of use, body-powered control may be rejected due to the unnatural body motions associated with activation of the prosthesis.
Although so-called quick-disconnect wrists are common in body-powered systems, they are often difficult for the user to manage with their one intact hand owing to the need to disconnect and reconnect the control cable. This problem can be further complicated when the glove of the prosthetic hand covers the control cable at the disconnection point.
INDICATIONS/BENEFITS/CONTRAINDICATIONS/DRAWBACKS FOR HYBRID DESIGNS
There are few examples of hybrid designs (body-power and myoelectric) at the transradial level, so it is not generally indicated at that level. Hybrid could also mean combining various externally powered inputs, but the term generally refers to combining body power with myoelectric control. A body-powered elbow with a myoelectrically controlled terminal device is the most common example. Without TMR surgery with or without pattern recognition control, limited control sites are available at higher levels, yet more components need to be controlled. Therefore, use of multiple types of control inputs can improve the use of the prosthesis for these users.
Cable control of the elbow is faster and more accurate in positioning than electronic elbow options because the user has proprioceptive feedback through the cable and harness regarding prosthetic velocity, force, and position. This type of hybrid control offers the potential for simultaneous or seamless sequential control of the prosthetic elbow and hand. For some users, a hybrid transhumeral prosthesis with body-powered control of the elbow and myoelectric control of the terminal device is a less complicated control strategy compared with mode selection routines commonly used to allow two control sites to control two or more components. For a hybrid control strategy where the terminal device is myoelectrically controlled and the elbow is cable actuated, the harness provides a similar function as that of a completely body-powered system; however, the excursion requirement is cut in half. This hybrid control strategy also eliminates the need to lock the elbow to operate the terminal device. A prosthesis that uses a body-powered elbow can be provided at a lower cost than a fully electric option such as an electronic elbow and hand versus cable elbow and myoelectric hand.
Hybrid control of a transhumeral prosthesis costs more than a fully body powered system does. Configuring and fitting a hybrid transhumeral prosthesis sometimes require a higher technical knowledge to combine components that the manufacturers may not have planned to be used in combination. Harnessing can be difficult especially at the short and very short transhumeral levels, where the user may not have enough force and/or excursion to fully and efficiently operate the elbow. Elbows with a very strong spring lift assist mechanism have been a considerable improvement for this user group whenever this hybrid approach is implemented.
WHAT OBSTACLES PREVENT THE APPLICATION OF THE RIGHT TECHNOLOGY?
There are many reasons that application of the “right” technology may not occur. Availability of funding for the most appropriate prosthesis is certainly a major obstacle. Payers sometimes have exclusions that dictate the type of prosthetic control without consideration of user needs. In my opinion, there needs to be a change in how payers and other concerned parties view upper-limb prosthetic rehabilitation. Commonly, only the gripping function of the prosthesis is considered. However, loss of an upper limb involves much more than loss of a gripping tool. The hand is a superb gripping and manipulation tool, but considering this function alone fails to consider the psychosocial, emotional, and cosmetic value of the human hand. As mentioned earlier, the cosmetic function of the prosthesis is of primary importance to a large number of persons with amputation. In my experience, I have witnessed that many persons with amputation either reject a hook prosthesis outright or sometimes agree to use it in therapy but then refuse to wear it in public or resort to hiding it under clothing. Desmond and MacLachlan12 state that “for upper limb loss, in particular, the combined reduction in function and cosmetic appearance, alongside the inability to easily hide the amputated hand or arm out of sight can result in a traumatic experience.” In discussing “coping with the amputation,” Van Lunteren et al.1 says, “socially the amputee may feel stigmatized, sometimes being confronted with undesired attention.” In light of these findings, a prosthesis that best addresses both the gripping and the cosmetic functional needs of the person with amputation should be considered.
Another obstacle is seen in the long delays between amputation and prosthetic fitting. These often are the result of delays in obtaining authorization from payers. Malone et al.13 demonstrated that the best rehabilitation results are obtained when prosthetic fitting is performed within the first 30 days after amputation. Van Lunteren et al.1 also reported that “a short time between the amputation and provision with a prosthesis has a positive influence on the use of the gripping function.”
Determining the “right technology” for any individual person with amputation can be difficult without his/her input regarding preferences. Persons with new amputations cannot make any truly informed decisions until they experience wearing a prosthesis. For this reason, many prosthetists who specialize in upper-limb prosthetic care prefer to fit a trial prosthesis that can be configured to allow the user to test various components and controls. Unfortunately, it is often difficult or impossible for the prosthetist to be reimbursed for this service, and without sufficient reimbursement, the procedure is not possible.
Because of the small number of upper-limb patients and the specialized knowledge required to optimally manage these cases, it is sometimes difficult for individuals with amputation to find a qualified prosthetist close to where they live. The same issue is common for finding other specialized professionals such as therapists and other medical services providers related to amputation rehabilitation.
Because no one prosthesis is able to replace all of the exquisite features of the human hand, it is sometimes necessary to provide more than one type of prosthesis.
Determination of the most appropriate upper-limb prosthesis must take into account control options as well as comfort and cosmesis in all its forms. Thus, the indications and contraindications associated with device type (i.e., myoelectric, body power, etc.) cannot be confined to their immediate impacts on functional control. To do so at the expense of comfort and cosmetic considerations may undermine users' acceptance and ultimate use of their prosthesis. With clear benefits and drawbacks associated with both design types, it is incumbent upon the rehabilitation team to identify those prosthetic design options that will be most beneficial to the individual user. Likewise, the health care system has a responsibility to ensure access to and reimbursement for the most appropriate prosthetic care from a qualified provider in a timely manner.