A New Myoelectric Prehension Orthosis
Cl. Hamonet, M.D. A. DeMontgolfier
Because of the increasing number of quadriplegic patients with high medullary lesions coming to us for treatment, we have been forced to seek new solutions in providing upper-limb functions. Recently we have developed a new type of prehension orthosis (Fig. 1 ) as part of this program.
Principles of the Orthosis
A single muscle can control the orthosis thus simplifying its application and the training of the patients.
When the muscle contracts the hand closes, the speed of the movement being proportional to the intensity of the myoelectric signal. This functional characteristic seems essential to us since we wish to approach the physiological conditions of pinching as closely as possible. The pinch may be maintained by sustaining myoelectric activity. The pinching power is also proportional to the muscular contraction.
When the muscular contraction stops the hand opens. This "passive" or low-energy expenditure opening is similar to the normal action of the hand when it releases an object after it has been pinched. Active or more explosive "opening" can also be provided and is of particular use, for example, when an object is being thrown away. It is possible to stop the opening of the hand at any point and to maintain the fingers in a selected position if the patient desires to regrasp an object previously released.
Description of the Orthosis
The orthosis is made in Dural A.U.4.G (Aluminium plus small amounts of iron, copper, magnesium, silicon, and manganese) and in brass. It incorporates a wristband on which the elements that set the fingers in motion and the mechanical system of transmission are fixed. The finger pieces involve one part which maintains the thumb in opposition and another which includes the fixation rings for the second and third fingers.
The fingers are articulated at the metacarpophalangeal joints. A second articulation is provided at the proximal interphalangeal joint.
The mechanical transmission system and the motor are placed along the external side of the forearm in such a way that they can be hidden by the sleeve.
The mechanical system of transmission includes a cog wheel and a small driving-rod which transfer the motion of the motor to the fingers.
The orthosis weighs 180 grammes. The maximum opening of fingers is 8 centimeters. The pinch force is 400 grammes.
Powering the Motor
A small electric motor giving a large output torque and powered by a direct-current, 12-volt, nickel-cadmium battery is used. This battery can provide 8 hours of use.
Selection and Treatment of the Myoelectric Signal
The myoelectric signal is picked up by surface electrodes placed in the cutaneous projecting area of the extensor carpi radialis longus when this muscle can provide a signal of sufficient strength to command the function of the orthosis. Other muscles can also be used if they have little or no involvement in the motions of the hand when it is equipped with the orthosis (e.g., the platysma or the rhomboid).
The output of the myoelectric signal is sent to the pre-amplifier by shielded two-conductor cables connected to electrodes.
The shielded portion is connected to a metallic grounded electrode incorporated in the orthosis and in contact with the patient's forearm.
The whole system includes an operational amplifier and several filtering elements:
- a filter which attenuates the 50 Hz sources
- a filter which amplifies the signal from 100 to 1,000 Hz with a gain of 20.
- The signal is rectified and transformed into a continuous signal.
- The output amplifier gives a tension proportional to the level of the rectified continuous signal.
This orthosis has been adapted to eleven patients. Ten of them were quadriplegic patients due to medullary cervical-traumatic lesions. One of them had a partial lesion of the radial nerve and complete lesions of the median and ulnar nerves on the left arm. They could flex the elbow against resistance and abduct the shoulder. The extensor carpi radialis longus was insufficient for wrist extension against resistance, but the myoelectric signal from this muscle was adequate for orthosis control.
All the patients controlled the orthosis after 10 or 20 minutes training. Delicate manipulations were possible such as serving, or unscrewing the cap from a tube of toothpaste, squeezing the tube in order to get the necessary quantity of paste, and reapplying the cap.
The first results have been encouraging and fittings to a larger number of patients are in progress.
This research was supported by the Direction de la Recherche Scientifique et Technique, the Caisses Régionales d'Assurance Maladie and the Ministere des Anciens Combattants.
For their technical collaboration we thank R. Musson-Genon, R. Fournier, G. Carre, M. Bedoiseau, S. Pannier, D. Boulongne, and Ph. Lacert.
- Hamonet, Cl., A. de Montgolfier, M. Bedoiseau, Ph. Lacert, S. Pannier, J. C. Raoult, R. Fournier, G. Carre, D. Mezier, and A. Grossiord, Appareillage de la main par une or-thèse a commande myo-électrique chez le tétraplégique. Film 16 mm. Association Française pour l'Appareillage, Paris, Avril 1970. Annales de Médecine Physique, 14:1:161-169, 1971.
- Hamonet, Cl., and Th. Simard, Méthode d'étude de la commande myo-électrique par or-thèse du membre supérieur. Union médicale du Canada. 100:2371-2375, 1971.
- Hamonet, Cl., A. de Montgolfier, M. Bedoiseau, S. Pannier, and A. Grossiord, Orthèses de préhension à commande myo-électrique? Premières réalisations International Congress of Physical Medicine, Barcelona, July 1972.
- Hamonet, Cl., A. de Montgolfier, M. Bedoiseau, S. Pannier, Ph. Lacert, G. Carre, R. Fournier, G. Dothe, D. Boulongne, and A. Grossiord, Ortheses myo-électriques du membre supérieur. First International Congress on Prosthetic Techniques and Functional Rehabilitation, Vienna, March 1973.
Eng. Hôpital Henri Mondor Paris, France