A New Prosthesis For An Anomalous Forearm

Werner Steinvorth, M.D.


R.K.D., a 10-year-old boy (Fig. 1 ), was seen at the Shriners Hospital for Crippled Children with anomalies of both upper extremities. The left hand presented a cleft separating the thumb and index fingers from the little finger-adactylia of middle and ring fingers (Fig. 2 and 3 ). The two radial rays were normal; the little finger had a flexion contracture of 90 degrees at the proximal interphalangeal joint. The patient used this hand quite well and was able to dress himself, eat, write, and perform most everyday activities. The function of this hand was considered to be excellent, and no prosthetic device has been prescribed for it.

Our concern was the right upper extremity. Here the upper arm had an abnormal elbow; a bone, radiologically identified as the radius, was opposed to the humerus-complete ulnar paraxial hemimelia and acheiria (Fig. 4 and 5 ). This radius was at a resting angle of 160 degrees to the humerus and could be extended 20 degrees. In this position, a space of 2 inches opened

between radius and humerus. The patient had poor extension power, but he could flex his forearm with a force of 12 pounds (Fig. 6 and 7 ).

Prosthetic History

Previously the patient had been fitted with a conventional above-elbow prosthesis with two cables and an outside elbow lock. The radius was enclosed in the socket, thus precluding its functional use. The patient stated that his handicap was greater with the prosthesis than without it. Using his radius, he could grip, hold, and suspend objects more accurately and with more power than with the prosthesis. His radius also provided him with a sense of touch. The patient's

main problem was to hold objects at a distance with his humerus and radius. At school he had difficulty holding the paper on the desk for writing. His prosthesis did not help in these functions either. Positioning his above-elbow prosthesis was cumbersome, but his parents required him to wear it most of the time.

Our objective was to provide the patient with a prosthesis that used the range of movement and power available in the rudimentary forearm. The patient had excellent flexion of the radius, but poor extension. The 2-inch range of motion at the distal end of the radius could be used to perform one of several possible prosthetic functions: opening the hook, pronating and supinating the terminal device, flexing and extending the elbow, or locking the elbow.

Since the rudimentary radius constituted the patient's forearm, it seemed logical to use it to flex the prosthetic forearm. It was necessary to determine whether or not the available range of motion and power were sufficient for this purpose. If not, the next most logical function for the radius would be to operate an elbow lock. The third possibility would be to use the radius for pronation and supination. This is an important function, but the patient could compensate for its absence by positioning the hook and by humeral elevation. A mechanism powered by the radius would have had to be designed to produce pronation-supination-a complicated problem!

Use of the radius to open the hook would not have reduced the number of control cables of the prosthesis, and this possibility was therefore discarded.

The Prosthesis

The prosthesis fitted consisted of a double wall socket which enclosed the entire shoulder, arm, and forearm. A bulge in the anterior socket wall allowed the rudimentary elbow to operate through its full range of motion. The distal end of the radius protruded through an anterior-proximal opening. Here a loop of 1-inch Dacron tape was attached to a cable to provide flexion of the prosthetic forearm. Single-pivot hinges on the outside wall of the socket and forearm constituted the elbow. The forearm had a WE-200-N Hosmer wrist joint and a Dorrance 10X hook operated by a single cable. A Northwestern ring harness was used to operate the hook and suspend the prosthesis (Fig. 8 ).

Considerations

Tension exerted by flexion of the rudimentary radius against the Dacron loop was transmitted to a cable which flexed the forearm. The range of forearm flexion increased the closer the cable attachment was to the elbow hinges, but the power requirement also increased proportionally. The most efficient balance between flexion range and power requirements was obtained with the cables attached 1 inch distal to the elbow centers. This arrangement provided 90 degrees of forearm flexion (Fig. 9 and 10 ).

Flexion of the radius gave a posteriorly directed force. The counterforce was provided by the prosthesis, which was prevented from displacing backward by its fit on the anterior shoulder and elbow. These two areas needed careful fitting.

The loop which surrounded the end of the radius had to be snug enough to transmit the full excursion available in the stump. In the position of complete extension it had to be loose in order to prevent the end of the radius from being constricted between the loop and the anterior wall of the socket.

Technicalities

The construction of the forearm and the alignment of the elbow or wrist will not be described, since these fitting procedures were done in the conventional way. The problem was to provide space in the upper arm socket so as to allow complete range of motion of the radius. First a model was made from a plaster wrapping, which included the radius in the position of flexion. From this cast a wax check socket was made. The fit over shoulder and elbow was adjusted and reinforced with another plaster wrapping. Then the anterior wall was cut out to expose the radius. The radius was held in maximal extension by a thin cord and a new wrapping was applied over the entire check socket. The final socket was laminated over this new model, which was a replica of the stump with the radius in extension.

The single cable loop used to flex the prosthetic forearm was attached at both ends to a Dacron tape, which fitted over the radius (Fig. 11 ). The two ends of the cable were soldered to cross bars enclosed in the Dacron tape. The cable passed through two short lengths of cable housing, which served three purposes:

  1. When the prosthesis was held in extension, they rested on the cross bars at the ends of the cable and released the tension on the Dacron tape.

  2. They prevented the cable from crossing any sharp angles.

  3. They kept the cable in place proximally at the end of the radius and distally toward the forearm.

The cable reached the forearm 1 inch distal to the elbow joint, passed through holes in the distal straps of the hinges, and was attached to the center of the forearm shell by a screw. This screw permitted small adjustments in the initial amount of forearm flexion provided. Forearm extension was accomplished by gravity when tension on the cable was released.

The patient learned to use his prosthesis in a very short time. After two weeks of occupational therapy he was adequately trained. Positioning of the forearm was performed naturally and accurately (Fig. 12 ). The boy's only complaint was that his arm muscles tired rapidly. However, his endurance and muscle power are expected to increase with time and use.

Werner Steinvorth, M.D. is associated with the Shriners Hospital Winnipeg, in Canada