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A Myoelectrically Controlled Wrist- Hand Orthosis for Brachial Plexus Injury: A Case Study

Mendal Slack, CO(c), BS
David Berbrayer, MD, FRCP(c)

Abstract

A myoelectrically controlled powered wrist-hand orthosis was developed for an individual with a unilateral brachial plexus injury. The lightweight orthosis, which incorporated supracondylar suspension, provided up to 14 lbs. of pinch force and had a closure time of one second. The device also offered the individual added independence during selected two-handed activities.

Introduction

The brachial plexus is a complex of nerves that provides muscle function to the arm. The nerves typically originate from the lower four cervical (C5,6,7 & 8) and the first thoracic (T1) nerve roots. Injuries to this area are most often the result of motor vehicle accidents, specifically those involving motorcycles (1-3). Such injuries can leave the patient with total or partial paralysis of the upper extremity. While surgery is frequently beneficial, orthoses or prostheses are still needed in some cases (2,4).

Patients with brachial plexus injuries can often function quite well with their contralateral hand or with simple assistive devices. However, in some cases the individual challenges the team to provide devices that allow more complex two-handed function. This can present a major challenge.

Early designs used carbon dioxide and valve-controlled McKibben muscles (5,6). Later designs employed linear actuators that were controlled by sip and puff or by butterfly rocker switches mounted on wheelchair trays (7,8). Myoelectric signals now control many devices, but cumbersome or remotely placed control units and slow hand closure times still present problems (9-11).

In addition, several of the current devices for brachial plexus injury provide shoulder and elbow support, but they do not offer any type of prehension function other than terminal attachments, such as hooks (12-14). The few devices that do provide prehension of the hand have been designed with the quadriplegic in mind, and thus, they are limited in their methods of control (15).

Ambulatory patients require compact, portable systems which must be very lightweight so they do not interfere with ambulation or balance. Adequate suspension is also necessary since the limb is no longer supported by the chair or tray, and these people will perform rigorous tasks, increasing dynamic loads on the orthosis. Furthermore, the device should be very responsive and have a simple control system.

Patient Profile

A 20-year-old male suffered a right brachial plexus injury due to a motorcycle accident. He sustained severely contracted fingers and thumb as well as loss of distal sensation. Wrist extension was by gravity only and restricted to neutral; however, some active wrist flexion and reasonable elbow and shoulder function were retained.

The patient had previously used resting wrist-hand orthoses (WHO) for position and contracture control only; they did not provide any dynamic function. In addition, he did not have enough strength to adequately power a wrist-driven WHO with an extension return spring. The wrist flexors were tested for possible myoelectric control, and the sites were found to be adequate. A rate sensitive myoelectric control was also tested, but the patient was unable to master the system. It was abandoned in favor of a threshold-type control system that allowed more versatile control calibration.

The Device

A two-piece myoelectric WHO was fashioned from a thermosetting acrylic resin (see Figure 1 ). The proximal section, section 1, containing the three-state electrodes, was fit directly to the patient's forearm. The proximal trimlines extended superiority to encase the elbow's epicondyle, providing suspension and proper electrode position. The distal trimlines ended just proximal to the wrist. The main section of the orthosis, section II, was linked by a three-conductor shielded cable to section 1 and consisted of the electronics, an actuator, a hand shell and a proximal forearm shell. The electronics consisted of a VASI three-state myoelectric system linked to a modified VV 5-9 power bridge and grip force control circuit (16-17). All electronic components were enclosed within a moisture-resistant barrier beneath the battery housing in the forearm shell (section II). A rechargeable six-volt/220ma Otto Bock nickel-cadmium battery supplied power (18). The linear actuator was a VASI model that provided a one second closure time over a range of three inches and could deliver up to 14 lbs. (62N) of pinch force. In this instance, pinch force was limited to 4 lbs. (18N), which was found to be adequate for this person.

The molded hand shell and thumb enclosure that encased the insensitive fingers and thumb provided protection from external hazards (i.e., burns) and from pressure during prolonged opposition. This allowed the patient to hold onto objects without concern of ischemia of the digits distally. The semirigid hand shell and thumb enclosure were covered with non-slip grip material on the palmar side, which provided a firm hold and adaptation to items of various shapes and sizes.

Donning

To don the device, the forearm was slipped into section I. The hand and thumb were then placed in the hand shell and thumb enclosure of section II. The forearm and section I were then lowered into the overlapping proximal shell of section II, which clasped around section I. The two sections were then secured together with Velcro.

Results

The proximal supracondylar brim provided adequate suspension, and the patient was able to control the device exceptionally well throughout a full range of motion. The self-contained device weighed only 24 ounces (680 grams), battery included (see Figure 2 ). The final product was a cosmetically acceptable, functional device with which the patient could pursue selected two-handed activities with greater independence and ease.

Conclusion

While the device performed well, a great deal of effort was exerted in designing and fabricating the orthosis. Technically, it was a major undertaking, and several fittings were necessary before the device could be dispensed.

Before undertaking such complex, expensive projects, an extensive team assessment of the individual should be performed. All surgical options and simple assistive devices for single-handed use should be explored. A definitive device such as this should not be attempted until maximum expected recovery has been achieved.

In addition, the patient must be highly motivated and understand that the device will take considerable fitting and training time. Since many patients with such injuries develop many compensatory mechanisms, one needs to know what activities the individual hopes to pursue and to establish realistic expectations of what the device can do.

Acknowledgments

The authors wish to acknowledge the contributions of Ihsan Al-Temen, Sharon Arscott, Greg Bush, Rinchen Dakpa, Mike Lymburner and Martin Mifsud to this project.

MENDAL SLACK, CO(c), BS, is coordinator of orthotic research in the Orthotic Services Department at the Hugh MacMillan Rehabilitation Centre, 350 Rumsey Road, Toronto, Ontario, Canada M4G 1R8.

DAVID BERBRAYER, MD, FRCP(c), is clinic physiatrist at the Sunnybrook Health Sciences Centre and The Hugh MacMillan Rehabilitation Centre in Toronto, Ontario.

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16. The VASI three-state myoelectric system is available from Variety Ability Systems Inc., 3701 Danforth Ave., Scarborough, Ontario, Canada MiN 2G2.
17. The VV 5-9 power bridge is a Variety Village component from VASI.
18. The rechargeable nickel-cadmium battery is available from Otto Bock Canada, 251 Saulteaux Crescent, Winnipeg, Manitoba, Canada R3J 3C7.

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