A Wrist-Driven Partial-Hand Prosthesis
HANS RICHARD LEHNEIS, C.P.O.
In the past, little success has been achieved in filling prostheses for partial-hand amputations. The reason for this lack of success is that no prosthetic system has been specifically devised for this amputation level which, essentially, has been provided with the same control system as is used in a long-below-elbow prosthesis. Thus, the hardware and harnessing required are the same as for a higher level amputation. The frequent rejection of the prosthesis as a functional device may be related to this factor. This rejection is particularly evident with congenital amputees who rarely wear a functional prosthesis for any amputation distal to the wrist. Most likely the elimination of sensory feedback when the prosthesis is worn is a major but much underestimated factor. Furthermore, proportionality of grasp is lost in voluntary-opening hands such as the Robin-Aids hand, which is usually prescribed because of its adaptability to a long stump, while a voluntary-closing hand would make the limb too long. Moreover, without the prosthesis, the patient is able to use voluntary wrist motion to hold objects against the body, table or any other surface with a force proportional to the weight of the object and/or the activity to be performed. In partial-hand prostheses the socket often extends above the wrist joint, immobilizing it and taking away an important degree of freedom.
A wrist-driven partial-hand prosthesis was developed to overcome these problems. This prosthesis utilizes residual wrist motion to provide proportional finger prehension force and feedback by means of a Bowden cable between the forearm cuff and the prosthetic hand.
In July 1966, J.L.. a white. 12-year-old male, sustained an explosion injury to his left hand. All soft tissue and bones distal to the carpals were completely destroyed. Surgery was performed on the hand for debridement of tissue fragments distal to the carpals and for tendon transplants. In November 1966 the patient was filled with a partial Robin-Aids hand and glove, double-wall socket extending two-thirds of the way up the forearm, flexible hinges, a triceps pad. and a figure-eight ring harness ( Fig. 1 ). He was able to obtain full elbow flexion with this prosthesis However, pain was present along the incision line and in the distal aspect of the stump because of a neuroma. In April 1970 this boy was evaluated for a wrist-driven partial-hand prosthesis, which he has worn intermittently since that lime.
The prosthesis consists of a Robin-Aids hand attached to a plastic-laminated socket with a Silastic soft insert extending to the wrist joint, a mechanical wrist joint (Hosmer SP 100), and a forearm cuff ( Fig. 2 ). Initially, the hand was activated by reciprocal wrist extension/finger flexion, assisted by spring closure of the hand, while wrist flexion produced hand opening ( Fig. 3 ).
Dent 1 in 1970 described a mechanism functionally similar, utilizing commercially available components for a wrist-driven (tenodesis) orthosis. Similarly, a wrist-actuated hand prosthesis was previously available from Robin-Aids 2 but can no longer be obtained. Both of these configurations left much to be desired cosmetically since the protrusion of the actuating levers and rod did not permit the application of a PVC cosmetic glove. Furthermore, tenodesis action used in these devices was not applicable in one case because of an imbalance in range of motion and motor power of the wrist flexors and extensors. Although the initial design eliminated all of the harnessing previously required, it did not provide voluntary closing and proportional force control since hand closing was spring-assisted.
The hand mechanism was subsequently modified by reversing the spring to produce hand opening, and the Bowden cable was reversed to produce finger closing. A retainer for the cable housing was attached over the dorsal aspect of the socket to act as a reaction point and the cable terminated on the dorsal aspect of the forearm cuff. Thus, wrist flexion produced proportional force control for finger flexion ( Fig. 4 ) while relaxation of the flexors and active wrist extension in combination with the opening spring produced finger opening ( Fig. 4 ). The reason this mode of operation was chosen was that the wrist flexors were of good plus strength and wrist extensors were of good minus strength. In addition, wrist extension was limited to 180 deg., i.e., to the neutral position ( Fig. 5 ), while wrist-flexion range was approximately 75 deg. ( Fig. 5 ). Thus, this system provides not only proportional control of prehension force but position feedback as well since the hand opening is proportional to the wrist position.
This type of wrist-driven hand prosthesis for a partial-hand amputation offers certain definite advantages over conventional prostheses. These advantages are proportional force control and position feedback plus the provision of a greater degree of freedom at the shoulder and at the elbow through the elimination of conventional harness systems.
While the patient has been quite pleased with the performance of the prosthesis and optimistic that he will eventually use it full-time, certain problems still remain. Since this patient had not used his wrist for approximately four years, he fatigues readily when using the prosthesis. In order to maintain a rather normal-looking position of the hand he needed to constantly exert some degree of wrist flexion to keep the fingers approximated to the thumb. To alleviate this problem, a semiautomatic locking mechanism was installed ( Fig. 6 ). This lock consists of a strap extending from the volar aspect of the forearm cuff to the socket. A button at the distal end of the strap fits into a recess in the volar aspect of the socket. When the fingers are in normal approximation to the thumb the button is inserted into the recess and the spring tension tending to open the hand prevents the button from slipping out. Thus, this position can be maintained without muscular exertion. When the patient is ready to use the prosthesis, a slight increase in wrist flexion causes the button to drop out of the recess and the hand is once again ready for use. Other problems encountered are not as readily solved. For example, after the cosmetic glove was applied, the force required to operate the prosthesis increased 120 percent.
The implication of this fitting experience is that a hand prosthesis which provides reciprocal wrist-finger motion for proportional force control and feedback should be made available commercially so that modification is unnecessary. A glove which does not inhibit finger flexion or does so only minimally is also needed so that force can be transmitted efficiently from the wrist to the fingers.
The contributions of Ludwig Greilinger, CP., in the fitting and construction of the prosthesis and of Glenn Goldfinger, R.P.T., and Tamara Sowell, R.P.T., in the collection of patient data cited in this report, are gratefully acknowledged.
1. Dent, Leslie, Wrist-driven prostheses for a bilateral partial-hand amputee. Artif. Limbs, 14 :2:81-82, Autumn 1970.
2. Robinson, George, Robin-Aids Co., Vallejo, Calif Personal Communication, 1972