A Cable Excursion-Recovery Unit: A Preliminary Report

Ian W. Cochrane


For many years the amputee's inability to develop the cable excursion required for effective performance has proved to be a discouraging factor for both the prosthetist and the amputee, and has often led to the rejection of the prosthesis. After analyzing the problem, we believe that the fault lies in the characteristics of the dual-control cable system, which require approximately 3 inches of excursion to raise the forearm to maximum height and then another 1-1/2 or 1-3/4 inches of excursion to fully open the terminal device. Since the average high-level arm amputee can provide little more than 3 inches of total excursion, he obviously has little excursion available for opening the terminal device after he has flexed the forearm fully.

The amputee with a long above-elbow stump and even the short above-elbow amputee can manage to operate reasonably well by using a combination of humeral abduction and flexion plus biscapular abduction. The patient with a shoulder disarticulation amputation must rely on biscapular abduction or chest expansion. The forequarter amputee has even more difficulty, since he has only scapular abduction on the sound side or chest expansion to achieve the desired function.

After we had fitted several shoulder disarticulation and forequarter amputees at this Centre, the need for an excursion-recovery device became increasingly evident. Such a device could dramatically change the situation for the high-level amputee and put him on the same performance level as a patient with a long above-elbow amputation.

Some time ago, a cable excursion multiplier1 was designed and tested with nearly 100 percent efficiency when mounted on the shoulder cap of an arm assembly. By means of one large and one small nylon pulley mounted on a single shaft, this device stepped up excursion by a ratio of 1.9:1. However, this excursion step-up meant that the force required to operate the dual-control system was increased by approximately the same ratio.

Another cable shortener was described by Northrop Aircraft, Inc. 2 in a 1951 publication. This device was incorporated into an elbow unit, which limited its applicability.

Design Criteria

After we had reexamined the problem, it was the consensus at this Centre that the design for a new cable shortener should meet the following requirements :

  1. Have low cable friction characteristics.

  2. Not be a step-up device.

  3. Not require separate control.

  4. Be easy to install.

  5. Be installable in existing prostheses.

  6. Be both internally and externally mountable.

The present design consists of two pulleys mounted in a Delrin body. One pulley remains stationary, while the other travels an arbitrary distance of 1 inch away from the first, assisted by a tension spring, and locks in this position by means of a spring-loaded pawl. The control cable loops around both pulleys on its way to the terminal device.

When the arm is in extension, the two pulleys are in a position of close proximity (Fig. 1 and 2 ) and remain this way during the raising of the forearm. When the forearm is flexed to the desired level, the elbow is locked and the amputee relaxes pressure on the harness. At this point the movable pulley takes up the slack and locks in position (Fig. 1 and 2 ), thus taking up the cable or recovering the excursion used in elbow flexion. Since this cable does not have to be taken up again, the body excursion resources available can be applied to the operation of the terminal device. The control cable which unlocks the elbow also unlocks the excursion device for extension of the forearm.

Method of Application

Although the unit may be applied either outside or inside the humeral section of the prosthesis, the internal method of installation will be used exclusively for the models currently under design.

Fig. 3 shows the line of the control cable being marked on the humeral section of a shoulder prosthesis. A second line is drawn through the longitudinal axis of the arm (Fig. 4 ). Where the two lines intersect, a third line is drawn at 90 degrees to the second. This point indicates the approximate center of the opening to be cut. It is expected that a paper template will be provided to simplify this procedure.

An opening is cut in the humeral shell with a cast saw (Fig. 5 ), and the recovery unit is then inserted (Fig. 6 ). Since the device is fitted with a mounting plate that will cover any gaps, adequate clearance should be provided for insertion. Four self-tapping screws are used to anchor the assembly (Fig. 7 ).

The same installation procedure is used for the short above-elbow prosthesis, providing there is sufficient room in the shell below the socket. An access door on the medial wall of closed arm sections will be necessary to facilitate the connection of the activator cable. The Hosmer anchor plate C-710c makes an ideal cover for this opening.

An elbow of the internal-locking type is best for this application, but the external cable type can also be used by inserting the cable through a hole drilled just above the knurled skirt of the turntable. The housing terminates about 3/4 inch inside the arm section. The cable is linked up to the activator lever by means of a light tension spring. The latter is fastened to the cable with an adjustable collar.

As the excursion required for the device is less than that required by the elbow lock, the spring eliminates the need for close synchronization.

Plans for Future

Ten test models of the excursion recovery unit are now being fabricated for field study, during which time various materials and designs will be tried. It is hoped that when these trials are completed, reasonable criteria for production and installation will have been developed. We hope to report on the results of this program at a later date.

While this device may not be the complete answer to all the problems of high-level amputees, it should certainly serve to make the prosthesis functionally more acceptable.

Ian W. Cochrane is associated with the Prosthetics/Orthotics Research and Development Unit Manitoba Rehabilitation Hospital Winnipeg, Manitoba, Canada

References:
1. Gwynne, Gerald, "Control Cable Excursion Multiplier," Memorandum Report No. 39, Department of Engineering, University of California, Los Angeles, April 1957. 
2. "Contractors Final Report on Artificial Arm and Leg Research and Development (January 2, 1945, to December 31, 1950)," Northrop Aircraft, Inc., Hawthorne, California, February 1951.