Prostheses Powered By Carbon Dioxide
Camille Corriveau, C.P.&O.
Until quite recently the prostheses available for upper-extremity amputees were of a relatively simple design which permitted the wearer few active functions-pinch or grasp, plus elbow flexion for higher-level amputations. Today we have an arsenal of upper-extremity prostheses from which we can select the device that the patient can control and use to best advantage. Before making the selection, the physician and the prosthetist must give careful consideration to the patient's disabilities, his general physical fitness, and the functions expected from the prosthesis.
As a general rule, the possibility of effective control of an artificial arm varies directly with the length of the stump-long stump, good control; shorter stump, progressively less control. Normally a below-elbow amputee can be fitted with a prosthesis whose terminal device is operated by a simple cable control. However, for the above-elbow amputee controlling the prosthesis by direct cable presents increasing difficulties, finally becoming almost impossible as the level of amputation approaches the shoulder. In these cases and particularly when the amputation is bilateral, externally powered prostheses may be the only answer.
Externally Powered Prostheses
As of today, an externally powered prosthesis should be given only to an upper-extremity amputee who cannot operate and control a prosthesis by conventional methods. The most obvious patients to derive benefit from external power are those with high above-elbow amputations, especially bilaterals; those with phocomelic malformations; and those with limitations of function (Fig. 1 ).
In planning the fitting of an externally powered prosthesis, the selection of the type of socket material to be used is particularly important. For instance, in the case of a child amputee the shoulder cap should be made soft and pliable, thus avoiding the necessity of making constant adjustments to keep pace with the child's growth. In the case of phocomelia, the hands should not be imprisoned in the socket, as it is essential that the child retain and possibly develop and extend the range of movement and function of the phocomelic hands. If CO2 is the source of external power, the prosthesis will be larger and heavier than the conventional prosthesis because of the valves, pistons, CO2 tanks, belts, etc., that are required.
The harnessing should be called "suspension," since it is used only for this purpose and not, as is the case with a conventional prosthesis, for both suspension and operation. Almost inevitably the system of suspension is somewhat complex, more so than the harnessing of a conventional prosthesis, in spite of the dual function of the latter, and quite often may create problems when the wearer is dressing or undressing.
There are no fixed standard methods for fitting and adjusting externally powered prostheses, and we believe that each case presents its own difficulties and that the rehabilitation team must often proceed by trial and error. A certain standard pattern may be found in phocomelic children (Fig. 2 ), but it is unwise to assume that a procedure which has worked well with one child is the method to follow with the next.
CO2 valves should be set in the form of an arc and within easy reach of the phocomelic hand. Quite often it is necessary to use the acromion to activate the valves. In cases of severe malformations, valves of the pull type are preferable (Fig. 3 ).
The training of a patient fitted with a CO2 externally powered prosthesis is somewhat different from that of a patient fitted with a conventional prosthesis. A child should not be fitted with external power too early in life, and by early, we mean up to three years of age, depending to a certain extent on his maturity.
Many questions arise when it comes to the management of young patients with prostheses having external sources of power, such as CO2. Among these questions are: Which should be activated first-the terminal device, wrist joint, or elbow joint? For the patient with bilateral deficits, is it advisable to concentrate on only one prosthesis initially, or on more than one? Is it better to start with one side and one joint only and then add other functions progressively as the child learns how to use the prosthesis? Should an attempt be made first to use conventional methods for elbow flexion and limit the use of external power to the wrist or the terminal device?
There are no definitive answers to these questions. As the child grows up, it is quite possible that some functions originally activated by CO2 may be abandoned in favor of conventional methods. No rigid course of action is applicable to all cases because of the individuality of patients. This leads us to say that even though an externally powered prosthesis has been decided upon, and especially with children, the well-established conventional prosthesis should not be abandoned altogether.
For obvious reasons, externally powered prostheses are not generally on the market. Commercial firms, with rare exceptions, look upon these sophisticated prostheses as being still in the experimental stage. Also, the cost is high, and at the present time it is difficult to make provision for growth-related adjustments.
The CO2 prostheses are reliable and require very little maintenance. Naturally, the children should not play in sand; they must avoid the possibility of dust particles finding their way into the mechanism. At one time, gas leaks were one of our biggest problems, but we have found that if the child is properly trained and uses the prosthesis not as a toy, but as a delicate instrument that requires common sense, attention, and care, mechanical problems become minimal.
Before a juvenile amputee is discharged, the parents must be made aware of the problems related to the care and maintenance of the prosthesis. They are brought into the picture while the child is still in the centre, and they participate in his training. They are given instructions in the operation, care, and maintenance of the prosthesis, and are taught how to refill the small CO2 tanks at home. They are also reassured that traveling presents no problem, and particularly that no danger arises when they travel by air.
In our view, it is advisable at this stage of our knowledge to use well-tried and accepted conventional methods whenever possible, and resort to external power primarily when conventional methods have failed or are impossible to apply.
Camille Corriveau, C.P.&O. is Head of the Prosthetics Workshop Rehabilitation Institute of Montreal Montreal, Quebec, Canada