Hydraulic/Pneumatic Knee Control Units A Prosthetisf s Point of View

Hydraulic/Pneumatic Knee Control Units A Prosthetisf s Point of View

Charles H. Pritham, CPO *

As Mr. Wilson has demonstrated, the use of hydraulic and pneumatic control units had its genesis in the post World War II R & D effort. The objective, of course, was to fit the returning veteran AK amputee with the best prosthesis technology could provide. Such amputees were young and physically fit, prime candidates to benefit from the advantages of advanced control units. The prime advantage, usually cited, is cadence responsiveness. As the patient walks at different rates, the control unit automatically adjusts to control heelrise and terminal swing impact. Constant friction knees can not duplicate this feature. All hydraulic and pneumatic units provide this feature and one, the Mauch S-N-S, provides stance phase control as well. This means that the unit provides enhanced knee stability in the early portion of stance phase to increase the patient's safety.

In this mode, the S-N-S unit can be said to function in a fashion analogous to that of a conventional safety knee. In another mode, the function of the S-N-S can be likened to that of a simple manually locking knee. Two other knee control units, variants of Kingsley's Hydranumatic and USMC's Dynaflex, function in a similar fashion.

The Hydracadence, in addition to swing phase control, also provides heel height adjustability and toe pick-up. Otto Bock has recently introduced a modular knee that includes a hydraulic swing phase control.

As can be seen then, these are just a few of the variations available to the prosthetist and his patient. The principle advantages claimed for such control units are enhanced cosmesis and performance, and lower energy expenditure. Against these advantages the disadvantages must be weighed. Bulk, size, and weight of some of the units preclude their use by many patients. The considerable expense of most, if not all, hydraulic and pneumatic control units rules out others. Moreover, the control units have shown to be unreliable. Some patients derive satisfactory service from their units while other patients using the same brand unit are constantly having them replaced and repaired. As most of the units need to be factory serviced, the delay and expense of maintaining a unit under such circumstances can engender considerable frustration.

Given these circumstances, the pool of available amputees for whom such advanced control units are suitable is a small proportion of the total AK population, and most closely resembles the patients for whom they were originally developed: young traumatic males; i.e. veterans. It must be borne in mind that this pool today represents a less important proportion of the amputee population than it did some 25 years ago. Statistics demonstrate that the majority of civilian amputees in the Western World are geriatrics who lose a leg due to arteriosclerosis and are as often as not female. Indeed, the very amputees who were originally provided hydraulic units by the VA are not getting any younger. The day will come for each of them when they, and the clinic teams who attempt to address their needs, must make a reappraisal of their prescription. So, the use of hydraulic/ pneumatic control units for a considerable portion of the amputee population can be ruled out. Not only that, but it is possible to be very skeptical in considering the suitability of such units for patients for whom it is theoretically ideally suited.

Young, active traumatic amputees are probably, children aside, the hardest on their prostheses. Given the expense of purchasing and maintaining such a unit, does it make sense to fit an amputee with one if he is going to have more than average maintenance problems? Can he afford the time lost from work, interruptions in his daily life, and expense of repairs? Given the disproportionately rising cost of health care today, can society? Gait studies demonstrate that AK amputees walk slower than normal subjects and BK amputees because of increased energy expenditure. If this is so, is the prime advantage cited for hydraulic/pneumatic units, cadence response, relevant and worth the additional expense and problems? In another vein, given the aging nature of the population should further effort and money be devoted to developing newer and more sophisticated knee control units?

In any event, it can be said that a prosthetist in attempting to formulate a solution to his patient's problems is confronted with a number of questions and a wide variety of devices all intended to perform the same function. It is also true that the prosthetist has little more than personal experience, hearsay, and the competing claims of the manufacturers to aid him in making his decision. The natural tendency on the prosthetist's part is to provide his patient with the most sophisticated unit possible, for all of us gain considerable satisfaction from doing so and from working with such units. The patient also wants the best prosthesis possible. The fact remains, however, that such tendencies must be resisted and both prosthetist and patient must make a realistic appraisal of the situation and logically weigh the pros and cons.