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A New Externally Powered, Myoelectrically Controlled Prosthesis for Persons with Partial-Hand Amputations at the Metacarpals

Richard F.ff. Weir, PhD
Edward C. Grahn, BSME
Stephen J. Duff, AASET

A new externally powered, myoelectrically controlled partial-hand prosthesis, suitable for fitting those persons with amputations at or more proximal to the level of the metacarpals, is described. Although a wide range of devices has been fabricated for partial-hand amputees, there are no partial-hand prostheses that provide prehension and are cosmetic for persons with transmetacarpal amputations. Further, the majority of mechanical hands that are available for persons with transradial and transhumeral amputations are not suitable for persons with wrist disarticulations or partial-hand amputations because the resulting prostheses are too long. We believe that preservation of wrist motion for positioning of the terminal device is of paramount importance to achieving maximum function and cosmesis. In partial-hand prostheses, the only space for any mechanisms is in the digits. The challenge is to be able to fit all the requisite mechanisms and electronics in this highly confined space and still have reasonable performance.

Current Fitting Practices

Much of the current surgical and prosthetics practice for partial-hand amputations is well described in the Atlas of Limb Prosthetics by Ouellette et al.¹ and Michael,² respectively. In those cases in which the thumb is missing, an opposition post, cosmetic or not, is usually prescribed. With an intact thumb, even if the metacarpals have been somewhat shortened with an oblique amputation, an orthotic device can be fitted that will provide a post for the uninjured thumb to oppose. In general, only those cases in which all digits (thumb and all four fingers) have been lost at a level equal or proximal to the metacarpophalangeal joint is a functional hand prosthesis recommended. The range of prosthetic devices presently available for persons with partial-hand amputations can be broadly divided into the following groups: cosmetic prostheses, passive functional prostheses, and active functional prostheses.

Cosmetic Prostheses

Cosmetic prostheses for persons with partial-hand amputations consist of a cosmetic glove in which, generally, the fingers are filled with urethane foam and have wire reinforcements running through them. The glove is cast to look hand-like. For persons whose loss is not so severe as to merit a full glove, cosmetic digits can be worn. A lack of adequate retention is sometimes an issue with these prostheses. The more common means of retention are adhesives and adhesive tape. Polyvinyl chloride (PVC) and silicone are the predominant materials used in the manufacture of cosmetic gloves. PVC is used because it is relatively inexpensive and durable. However, PVC degrades over time upon exposure to sunlight and picks up contaminants easily during contact with everyday items, such as newspaper print. Silicone is the material of choice for the "high-end" cosmetic coverings. Although silicone will fade upon extended exposure to ultraviolet light, cured silicones are highly stable and will retain their original mechanical properties over time, even when heated or exposed to contaminants. Unfortunately, the silicones generally used for cosmetic skins have poor tear strength.

Passive Functional

ProsthesesPassive functional prostheses are highly specialized prosthetic devices that are donned on those occasions when they are needed and doffed once the task for which it was designed has been completed. The majority of devices in this category are work prostheses that bear little or no resemblance to the natural hand. These work prostheses may be a simple post to provide opposition, or they may incorporate specialized features to aid in certain occupations. The person who has a mobile sensate thumb but no other phalanges has an acute need for opposition, which these devices may provide.

Active Functional Prostheses
Body-Powered Prostheses

Body-powered prostheses for persons with partial-hand amputations fall, for the most part, into one of two groups: those devices that are powered by biscapular abduction and/or glenohumeral flexion and those devices that are powered by flexion or extension of the wrist. Shoulder-driven devices are often the system of choice for persons with bilateral partial-hand amputations. The use of shoulder power requires that a figure-of-nine harness to be worn to actuate the terminal device (usually a hook). Although position and force feedback via the harness is retained, the harness is restrictive and requires abnormal movements of the shoulders to activate the terminal device. In addition, the two most commonly used cable-operated partial-hand solutions are no longer commercially available.

The Handy Hook consisted of a standard hook attached to the palmar surface of a partial hand prosthesis socket by way of a Handy Wrist (USMC-United States Manufacturing Company, Pasadena, CA). Unfortunately, the Handy Wrist is no longer commercially available, so it must be machined in house. The other system used in shoulder-powered partial-hand fittings was the Robin Aids hand (USMC). The Robin Aids hand attempted to provide both function and cosmesis for partial-hand fittings and was the device of choice in the fitting of partial-hand amputations for many rehabilitation centers until it was discontinued in 1990. The adjustable mechanical Hard Hand (USMC) is the current version of the Robin Aids hand and might be suitably modified for partial-hand use.

Wrist flexion and extension devices, the second type of body-powered partial-hand prostheses, are functional hands that operate in a manner similar to a tenodesis type of hand orthosis. Tenodesis action is a method by which prehension of the index finger, middle finger, and thumb is achieved through active wrist flexion. Essentially, these devices operate by way of a linkage mechanism, which translates wrist flexion into finger pinch and wrist extension into finger opening. In Europe, this method is used extensively in the fitting of persons with partial-hand amputation. Unfortunately, the use of wrist flexion-extension to activate the prosthesis results in restriction of the range of motion of the wrist. Preservation of wrist motion maintains the individual's ability to freely position the hand in space, greatly adding to overall hand function and dynamic cosmesis.

Dynamic cosmesis can be thought of as how a device "looks" while it is in motion. The eye is tuned to the unexpected. If a prosthesis is moved in a "normal" way, it will be perceived as "normal" by the casual observer. The wrist is essential to this "normal" motion. A partial-hand prosthesis that is hand-like and does not inhibit the wrist will have both dynamic and static cosmesis. By powering the fingers, a functional hand-like prosthesis can achieve a more normal appearance.

Externally powered prostheses

Externally powered devices have recently become available for persons with partial-hand amputations. Blair and Kramer³ noted that powered prehensors would be valuable because there is no need for a harness and control cable, which break the "line" of the device. Externally powered devices can be entirely self-contained, which is advantageous to individuals when they don and doff the device. For persons with severe scar tissue, or the inability to generate sufficient force, an externally powered device is a viable alternative. A disadvantage of external power is the lack of proprioceptive feedback that is intrinsic in cable-driven systems. However, for single-degree-of-freedom myoelectric systems, visual feedback has been found to be perfectly adequate. Otto Bock Health Care (Duderstadt, Germany) has recently released an externally powered partial-hand prosthesis suitable for transcarpal amputations. In addition, the conventionally powered hand from Centri AB (Järfälla, Sweden) can be shortened by a creative prosthetist to enable its use in some short transcarpal cases. Motion Control (Salt Lake City, UT) also claims that its new hand can be shortened for use in short transcarpal cases. We hope that our own hand, which is described in this article, will be added to this list in due course.

New Transmetacarpal Hand Prosthesis

The motivation for this project and earlier work^4,5 came from a temporary fitting our laboratory performed for a patient of the Rehabilitation Institute of Chicago (Figure 1 ). This patient had partial-hand and shoulder disarticulation and transfemoral amputations. A fitting was performed to give the patient some hand function for the duration of his stay at the institute. Because of the presence of hypersensitive scar tissue, a conventional harness or wrist flexion-extension device was precluded. The patient was fitted with a self-contained, self-suspended, powered prosthesis. It consisted of a Michigan Hook (Hosmer Dorrance Corporation, Campbell, CA), electronics for single-site myoelectric control, and a battery mounted on a socket, which permitted free movement of the wrist. The muscles of the thenar eminence provided the myoelectric control signal. Exceptionally functional movements were observed during this fitting, movements that were made possible by the unrestricted motion of the wrist. The freedom of the wrist relieved the patient of many of the compensatory movements that are necessary when the wrist is fixed. No attempt was made at cosmesis because the emphasis was on providing function. Most impressive was that his movements were not "amputee-like" (ie, the patient did not require unnatural-looking elbow motions to compensate for a fixed wrist or unnatural-looking shoulder movements to effect control cable excursion in body-powered devices), which was attributed to the conservation of nearly normal wrist function.

The availability of small (10 mm in diameter) DC motors (Figure 2 ) from MicroMo Electronics (Clearwater, FL) made this project feasible. These motors are small enough to be placed within the fingers and run at a nominal voltage of 6 volts. To boost performance, they are over-voltaged to run at 9 volts, so that reasonable grip force can be attained. Unfortunately, even with over-voltaging, these motors cannot meet both the speed and force requirements simultaneously; therefore, they were set up in synergistic pairs. Through the use of synergetic prehension,6 a hand with both sufficient speed and force can be developed.

Synergetic prehension is produced by the combined effect of two or more motors working together. The act of grasping requires little real work, ie, it is typically a low-power activity. When an object is grasped, a force is exerted with very little excursion, while excursion of the grasping fingers usually occurs in space and requires very little force. In both cases, the work involved is minimal. This kind of prehension can be readily implemented using multiple motors that operate in synergy. A simple synergetic prehensor (Figure 3 ) consists of two motors that open and close a split-hook. One motor gives one finger of the hook high speed and excursion but little force (fast side); the other motor gives the other finger of the hook high force but little speed and excursion (force side). In this way, the motors work in synergy to boost overall performance. The alternative to multiple motors is an automatic transmission, which Otto Bock uses in its System Electric adult hands. The disadvantage of an automatic transmission is that it can add delays to the response time of the hand.

The transmetacarpal prosthesis (Figure 4 ) uses three motors operating in synergy to achieve reasonable force and speed. Motors in the index and middle fingers provide force, and a third motor in the line of the knuckles provides speed for opening of the fingers. The finger drive mechanisms consist of MicroMo MM1016 motors with 256:1 gearheads and lead screws. The finger drive mechanisms are mounted in two housings made of aluminum to reduce the weight. The knuckle motor drive mechanism consists of a MicroMo MM1516 motor with a 41:1 gearhead in conjunction with a VASI back-lock mechanism (Variety Ability Systems, Inc., Ontario, Canada). The thumb is kinematically linked to move with the fast side, or knuckle motor, thus achieving a large width of opening (approximately 10 cm, or approximately 4 inches) (Figure 4 , right). The battery and electronics are housed inside the ring and little fingers or behind the backplate of the mechanism (Figure 5 ). To meet the force requirement, the two motors in the index and middle fingers operate independently. Each finger can generate at least 26 N (6 poundsf). The total force generated by the hand is the vector sum of the force from each finger (approximately 53 N, or approximately 12 poundsf). The speed-of-closing for this hand is approximately 2 radians/second (approximately 105°/second).

We are interested in preserving as much residual motion of the wrist as possible; therefore we are also interested in developing "soft" prosthetic interface techniques. At present, the prosthetic interface consists of a self-suspending silicone sleeve-socket to which the hand mechanism is attached (Figures 5 , 6 , and 7 ). The fingers are mounted on carbon fiber plate that is bonded to a build-up of high durometer laminating silicone. This platform was incorporated into a silicone sleeve made with nylon impregnated with laminating silicone. The nylon provides resistance to tear and enables the electrodes to be pushed through the silicone sleeve and to remain in place without tearing the silicone. The electrodes can then be screwed into their electronics, which are located on the outside surface of the sleeve. The associated electrode electronics, wires, myocontroller, and synergetic controller are fitted onto the dorsal surface of the hand on the silicone build-up for protection. An outer cosmetic glove is rolled over this inner silicone sleeve socket. The silicone sleeve socket and cosmetic glove are then rolled on and off together in a manner similar to that used to don and doff transtibial silicone suspension sleeves.

Proportional myoelectric control using electromyographic (EMG) sites on the residual partial hand is the preferred mode of control. Ideally there should be two such sites using the intrinsic muscles of the hand (Figure 6 ). For example, opening could be controlled by an electrode over the lateral aspect of the hypothenar eminence, and closing could be controlled by an electrode over the thenar eminence. Thinking about flexing/adducting the thumb would cause the hand to close, and abducting the little finger would cause it to open. This kind of control might not always be possible, but even if only one EMG site is available, three-state control could be used. The power source is a rechargeable 9-volt transistor battery. We prefer to use rechargeable batteries so that the hand can be recharged overnight. A rechargeable battery in the standard 9-volt transistor battery shape was chosen so that "off-the-shelf" batteries could be used should the rechargeable battery lose its charge during use.

Conclusion

We have completed development of an initial prototype transmetacarpal hand mechanism and its associated prosthetic interface (Figure 7 ). We are in the process of refining the mechanism's control and the prosthetic interface's fabrication process as a prelude to an initial trial fitting. This initial fitting will allow us to ascertain the robustness of the current prototype and help us identify areas of the design that need improvement. The weight of the current mechanism alone is 145 g (0.32 pounds) and its length, in the closed position, from back plate to finger tip is 85 mm (approximately 3.5 inches). The weight of the mechanism with battery and electronics is 210 g (0.46 pounds). The total weight of the mechanism and current silicone interface is 425 g (0.937 pounds, ie, less than 0.5 kg).

References:

1. Ouellette EA, McAuliffe JA, Carneiro R. Partial-hand amputations: Surgical principles. In: Bowker JH, Michael JW, eds. Atlas of Limb Prosthetics, Surgical, Prosthetic, and Rehabilitation Principles, 2nd ed. St. Louis: Mosby-Year Book; 1992:199-216.
2. Michael JW. Partial-hand amputations: Prosthetic and orthotic management. In: Bowker JH, Michael JW, eds. Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles, 2nd ed. St. Louis: Mosby-Year Book; 1992:217-226.
3. Blair SJ, Kramer S. Partial-hand amputation. In: American Academy of Orthopaedic Surgeons. Atlas of Limb Prosthetics: Surgical and Prosthetic Principles. St. Louis: CV Mosby; 1981:159-173.
4. Weir RFff. An externally-powered myo-electrically controlled synergetic prosthetic hand for the partial hand amputee [thesis]. Chicago: Department of biomedical engineering, Northwestern University, 1989.
5. Weir RFff. The design and development of a synergetic partial hand prosthesis with powered fingers. Paper presented at the RESNA 12th Annual Conference, New Orleans, LA, June 25-30, 1989.
6. Childress DS. An approach to powered grasp. In: Gavrilovic MM, Wilson AB, Jr., eds. Advances in External Control of Human Extremities, Proceedings of the Fourth International Symposium on External Control of Human Extremities, Dubrovnik, Yugoslavia, August 28-September 2, 1972. Belgrade, Yugoslavia: Yugoslav Committee for Electronics and Automation; 1973:159-167.