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The Use of Myoelectric Control in Upper Extremity Prostheses
By Robert Sobotka, CPO

Introduction

Upper-limb amputations typically fall into one of three major categories: (1) those resulting from traumatic accidents, as with farmers and industrial workers; (2) those intended to correct congenital anomalies or deficiencies; and (3) those performed in an effort to contain or stem the effects diseases such as cancer.. Cancers are the principal disease related cause of amputations. Upper-limb amputations include all amputations made from the fingers to the shoulder.

The extent of the upper-limb amputation plays a major role in determining the functional success of any prosthetic replacement. The restoration of the patient’s function will generally be more successful the more extensive the remaining portion of the limb. The longer the residual limb is, the easier it is to operate the prosthesis due to the remaining residual movements of the intact joints.

The nature and extent of the amputation, along with the patient’s goals and expectations, will determine the utility of any prosthetic system. The program that best meets the patient’s needs will depend on the doctor’s recommendations, the prosthetist’s selection of the prosthetic system and componentry, and the physical therapist’s training the patient to use his or her new prosthesis.

Three Categories of Upper-limb Prosthetics

There are three major categories of upper-limb prosthetics: (1) cosmetic, (2) body powered and (3) myoelectrically controlled self-powered prostheses. Each of these categories will be discussed below.

Fig. 1. Costmetic arm (above-elbow).

Cosmetic Prosthetics In some cases, the only appropriate or available prosthesis is cosmetic. A patient may decide that he would prefer to use his remaining limb for all of his functional needs, or may be unable or unwilling to undertake the training needed to operate either a body powered or myoelectric prosthesis. In very high-level amputations (i.e., those occurring closer to the patient’s body), the patient might not have enough body power to operate a working prosthesis. The cosmetic prosthesis is much lighter than the alternatives. This may be a good reason for its use with older or weaker patients. A cosmetic prosthesis restores only a limited portion of the functional aspects of the portion of the limb removed. It can be designed to passively grip or hold light objects. A cosmetic prosthesis might be used to replace a lost finger or partial hand or to replace an entire arm removed in a shoulder disarticulation.

Fig. 2. Below-elbow body-powered prosthesis.

Body-powered Prosthetics Other upper-limb prostheses, used to restore some of the functional aspects of the patient’s lost limb, belong to a category of prostheses known as body-powered cable-activated prostheses. A cable-activated prosthesis uses cables attached to a harness that secures the prosthesis to the patient. It depends on the motion of the residual limb relative to the patient’s body to control and power the functions of the prosthesis. The amputation level will determine the complexity of the prosthesis. For higher level amputations, double- or triple-cabled harnesses may be required. In the triple-cable system, the prosthetic terminal device, used to replace some of the grasping functions of the hand, utilizes one of the cables. The other two cables are used to restore elbow flexion and elbow locking to the above-elbow prosthesis.

Fig. 3. Myoelectric prosthesis.

Myoelectric Externally Powered Prosthesis The human hand and arm are by nature so complex that replacing their functions with an artificial prosthetic device poses monumental problems. The control of an upper-extremity prosthesis needs to occur almost subconsciously. One way to provide this level of control is to harness the power of the neuromusculature that remains after an amputation. During the last decade, technology has advanced to the point where externally powered components have been used with greater frequency and success in upper-limb fittings. A myoelectric prosthesis can be prescribed for a child as young as two years old. Generally a geriatric patient will not be a candidate for a myoelectric prosthesis because it is much heavier than a cosmetic or body-powered prosthesis.

How Myoelectric Prostheses Work

Myoelectric control of a prosthesis or other system utilizes the electrical action potential of the residual limb’s muscles that are emitted during muscular contractions. These emissions are measurable on the skin surface at a microvolt level. The emissions are picked up by electrodes and are amplified for use as control signals to the functional elements of the prosthesis. The myoelectric emissions are used only for control. Because the electrical signals are not powerful enough to operate the electric motors in the prosthesis, a rechargeable 6-volt battery, which is accommodated in a socket in the prosthesis, is used to operate the motors which, in turn, produce the movements of the prosthesis.

High-voltage electrical potential is a prerequisite to the successful use of the myoelectric prosthesis. Before a prescription for a myoelectric devise is considered, the patient’s electrical potential should be measured. The measurements are made with a very sensitive voltmeter, utilizing test electrodes to determine the locations where the highest electrical potentials, and thus the strongest signals, can be generated. The myoelectrically controlled prosthesis can function properly only if the control electrodes are properly and consistently placed on the body.

Patients who have become candidates for an upper-limb prosthesis as the result of an acquired amputation (as opposed to patients who require an upper-limb prosthesis due to congenitally missing limbs) are the most likely to benefit from myoelectric control. Prior to the initial myoelectric fitting a patient goes through a training process to make sure that he will be able to operate the prosthesis. The patient is asked to “open” and “close” his phantom hand. The remaining muscles are naturally activated when the patient attempts to move his “phantom hand.” It is these control signals, coming from the natural muscle movements, that, in turn, control the actions of the myoelectrical prosthesis.

In the case of the below-elbow myoelectric prosthesis, two sets of muscles provide the signals to operate the prosthesis: the wrist flexor and extensor muscles. The wrist flexor muscles that controlled the closure of the natural hand are used to produce the signals used to close the prosthetic hand; the wrist extensor muscles used to open the natural hand are in turn employed to provide the signals to open the prosthetic hand. The result is, in the case of the below-elbow prosthesis, that a natural relationship is maintained between the signals generated myoelectrically and the prosthetic action. This natural relationship means that the patient is likely to be able to easily and quickly learn control of his prosthesis. Once the patient has mastered the use of his myoelectric prosthesis he may even be able to lift and grasp an egg by using different intensities of muscular contraction.

In the case of above-elbow amputations and shoulder disarticulations, the muscle sets used for control signals are so removed from natural hand control that significant training and therapy are needed to teach the patient to operate his myoelectrically controlled prosthesis.

Advantages and Disadvantages of Myoelectrically Controlled Prosthetics

The below-elbow myoelectric system is well suited for amputees such as salespersons, students, business people and professionals who are engaged in light work. The myoelectric unit is not usually recommended for patients involved in heavy work such as farming or construction. These users, however, may find the myoelectric unit appropriate as a secondary prosthesis.

The myoelectric system is more cosmetic than the conventional body-powered prosthesis. The cosmetic benefits make the myoelectric prosthesis the overwhelming choice of our female patients. Without the harness control of the body-powered prosthesis, the myoelectric controlled hand provides a greater range of motion, particularly when overhead motions are involved.

The biggest single drawback to myoelectrically controlled prostheses is their high cost. The myoelectric prosthesis is often triple the cost of the ordinary, body-powered device and it will require more care and maintenance than the body-powered hook prosthesis. Despite the high price tag of the system, with proper documentation to justify the device, many insurance companies cover the expense.

What Can Be Controlled with Myoelectrics?

Myoelectrical controllers can be used to control the function of the hand, wrist and elbow. Each of these important anatomical functions can be simulated mechanically, even if the mechanical replacements never actually duplicate the functions of the natural parts.

Fig. 4. Myoelectric hand.

Hand The Otto Bock System Electric Hand is the most commonly used electric hand in North America. It is available in three adult sizes, determined by the circumference at the knuckles. The 7 ¼" is suggested for females and juvenile males. The 7 ¾" and 8" sizes are used for adult males. Otto Bock also makes a very light child’s hand. The complete hand consists of a three-finger mechanical hand operated by an electric motor; an inner hand, or plastic form, that provides support and natural shaping; and an outer cosmetic hand or glove that provides a natural appearance with natural skin texturing. Gender differences and cosmetic coloration are provided by the cosmetic glove pulled over the plastic form.

Patients concerned with appearance will usually prefer the myoelectric hand over the alternative hooks band or other prehensile devices used in body-powered systems.

Problems with the hand include the need to replace the cosmetic glove every 6-12 months, as it soils easily. Additionally, the palmar prehension or the three-finger grasping technique used by the hand makes it hard for the patient to lift certain objects.

The Otto Bock System includes an electric Griefer or gripper hand that was developed as an alternative to the electric hand. It is designed to be used in working situations that require greater grasping force or that might injure or discolor the cosmetic hand. The Griefer is available in one size as either a left- or right-handed unit. The Greifer has two fingers and is longer and slightly lighter than the cosmetic electric hand. The Greifer closes faster than the cosmetic hand and has 50-75% greater grasping or prehensile force.

Fig. 5. Electric Greifer (myoelectric wrist).

Wrist Otto Bock is the only manufacturer of an electric wrist mechanism that can be myoelectrically controlled. Other wrist mechanisms are mechanical in nature and will require the patient to use his remaining hand and arm (in the case of unilateral amputees) to position the wrist in order to undertake the task at hand. The Otto Bock wrist can be controlled by either a cable pull switch or by using the myoelectric system used to control the hand mechanism. The benefit of the electric wrist is that it allows the patient to position his hand to grasp objects. This can be done manually (with the remaining arm and hand) in the case of a unilateral upper-limb amputee but, for the bilateral (loss of both) upper-limb amputees the electric wrist may be the only way to provide positioning. There are problems inherent in the use of an electric wrist. The electric wrist adds weight to the prosthesis. It also increases the power consumption of the prosthesis, thereby shortening its battery life. Finally, it adds maintenance problems to the prosthesis, and the patient’s training in the use the prosthesis is slightly more complicated.

Fig. 6. Myoelectric elbow

Elbow There are three major elbow systems available for either myoelectric or harness switched control. The available units are the New York Hosmer Electric Elbow (Hosmer), the Liberty Mutual Boston Elbow (Boston), and the Utah Arm manufactured by Motion Control (Utah). These elbows differ from one another in mechanical configuration, drive mechanisms, and collateral options. All of the units share the primary function of positioning the prehensile device. Bilateral amputees will derive the greatest benefit from the use of the electric elbow for positioning their prehensile devices since they will not be able to rely on their remaining upper limb to position the prosthetic elbow.

The electric elbow has the same disadvantages as the electric wrist. Including the electric elbow in a prosthesis design will add weight and power consumption, and will complicate both maintenance and patient training Another area of concern is the electric elbow’s lifting capacity. The Hosmer can lift a maximum weight of 2.5 lbs, the Boston 4 lbs, and the Utah somewhere in between the other two. The weight of the forearm, the wrist mechanism, and the prehensile device must be subtracted from these capacity values to determine the maximum weight of an object that can be held and lifted. (In comparison, the lifting capacity of the natural arm of an adult male is between 29 and 55 lbs.)

Heavier loads can be lifted by a prosthesis with an electric elbow, but in a passive manner. This is done by locking the elbow in place, after prepositioning it, using body motion to orient the prehensor to grasp the object, then straightening the body without actively moving the elbow joint. The Boston and Utah can be used to passively move an object of up to 49 lbs and the Hosmer can passively lift 17-19 lbs.

Conclusion

The use of upper-limb prostheses is not as common as the use of lower-limb prostheses. But a patient’s need is just as acute for the one as for the other. In both cases, it is the prosthetist’s job to maximize the functionality of the prosthesis. While all of the functions of a natural limb cannot be fully restored, a well-designed, upper-limb prosthesis can restore a patient’s confidence and much of his or her job productivity.