Conduction, the transfer of heat between two media in direct contact, is of great impor¬tance to the amputee. As the socket becomes warmed to skin temperature, it acts as an insu¬lator against further dissemination of heat from the surface of the stump. It appears prob¬able also that in the vicinity of principal load¬ing, especially along the medial, anterior, and posterior segments of the socket rim, heat is generated by the friction resulting from shear¬ing action between the skin and the socket rim. The insulating effect of the socket would, of course, tend to maintain any such local eleva¬tion of temperature. We are initiating a clinical study of this question, employing thermistors for the direct reading of skin temperatures while the prosthesis is being worn under vari¬ous conditions of normal use.
Just how significant increased local heating of the skin may be in adversely affecting skin hygiene and metabolism over a long period of time we cannot say at present. It is known that an increase in environmental temperature elevates the oxygen and nutritional require¬ments of most tissues. At the same time, the blood supply to the skin of a lower-extremity stump, if changed at all by the active use of a prosthesis, is probably reduced. One might speculate here whether the predilection of these weight-bearing sites for the development of recurrent "pressure sores" may not be re¬lated to increased local heat plus diminished nutrition, as well as to mechanical damage and to maceration from sweat. Certainly this area of stump hygiene merits further investigation.
REFLEX SWEATING
If, in the normal person, the environmental temperature is raised above a critical level between 31° and 32°C (88° and 90°F), there is a sudden, visible outbreak of sweating over the whole body. A similar response, termed "reflex sweating," may be observed when only a portion of the body surface is heated. When¬ever there is excessive heating of the stump, the conditions favor reflex sweating, even though the environmental temperature of the rest of the body is below the critical level nec¬essary for visible sweating. Certainly a valu¬able contribution, both to the comfort of the amputee and to the improvement of his stump hygiene, would be the development of new socket materials and designs which would pro¬vide for more rapid heat transfer by conduction and radiation to the outside air.
Loss of heat by evaporation from the stump is negligible in the case of the suction socket. Where the conventional socket is worn with a wool stump sock, however, the wicking action of the sock may well provide an avenue for evaporation and consequent cooling. A light stump sock for use with the suction socket may prove feasible. If so, the cooling effect, as well as the added support and protection afforded the stump skin, would be of benefit in main¬taining a healthy stump.
According to Rothman (15), sweating which is elicited by exercise begins at a lower skin
temperature than does sweating produced by external heat. Bazett (2) suggested that there may be, deeply situated near vascular plexuses, thermal receptors which are warmed by the working muscles. These receptors may in turn activate the sweat glands of the skin. What¬ever the true explanation may be, the com¬bination of excessive sweating (Fig. 5) and increased energy requirements for locomotion is all too familiar to the lower-extremity amputee.
Visible sweat secretion and heat loss can also occur independent of thermoregulatory needs. For example, sweating can be elicited with ease at air temperature below 31°C (88°F) by the ingestion of hot drinks, probably through a viscerocutaneous reflex. A variety of other nervous impulses unrelated to heat control may produce sweating. One of the most important of these is "emotional sweat¬ing," which may at times affect most of us to some degree. In dermatologic practice, we sometimes see patients in whom this condition has become so severe as to be almost incapaci¬tating. Serious limitations affecting social con¬tacts and employability result. The same disturbance of sweat mechanism may be ex-perienced by amputees. Although the emotional factor may be important in some amputees who have a troublesome hyperidrosis, it is apparent from some of the known physiologic mechanisms for sweating that there may be other reasons for such an increase.
STUMP HYGIENE AND GERMS
It has been a matter of frequent observation that the normal skin is not a sterile skin. Such a condition simply does not exist. Normal skin teems with immense numbers of unseen organ¬isms, some harmless and some pathogenic, that is, capable, under the right combination of circumstances, of causing an infection of the skin. Normally, the harmful bacteria and fungi are held in check by a number of different forces. Most of the time we live in some meas¬ure of harmony with this enveloping horde. But when resistance to infection is lowered by local skin damage, the presence of some gener¬alized disease, a metabolic disturbance such as diabetes, or any one of numerous other causes, then this harmonious balance is de¬stroyed and the avenue of invasion is opened. Two different classes of bacteria exist on nor¬mal skin under average conditions—the resident bacteria, which remain fairly constant, and the transients, which may be almost any¬thing (Fig. 6). In addition, a variety of fungi come and go, chiefly members of the yeasts and molds, although other types, such as those which cause ringworm of the feet and body, may be present.
Evans et al. (11) have studied the resident bacterial flora in 146 sample scrapings from the skin of 17 adults over an eight-month period. They found that the anaerobic bacteria (those which grow in the absence of free oxy¬gen) outnumbered the aerobic bacteria (those which require free oxygen) by a ratio that ranged between 10:1 and 100:1. In most of the cases, one species of anaerobic bacteria pre¬dominated, the so-called "acne bacillus," Propionibacterium acnes . Of the aerobic bacteria, two species were observed regularly: Micrococcus epidermidis and Staphylococcus albus {Micrococcus pyog-enes), the latter a skin pathogen The ob¬servation was made that, at least in cultures, some types of bacteria inhibited the growth of others. This finding might constitute one explanation for the overgrowth of certain bac¬teria, especially the acne bacillus, at the expense of the others. It was also found that the sebaceous glands were the major site of growth of bacteria on the skin and that exer¬cise with sweating caused a transient minor increase in skin flora.
What effect might the wearing of an occlu-sive prosthesis be expected to have on common skin pathogens trapped under the socket? How might the normal defenses of the skin be affected by the conditions attendant upon the use of a prosthesis? To answer these questions, let us consider four common groups of organ¬isms which are likely to cause skin infections in the region of the amputee's stump—the gram-negative organisms like Escherichia coli, the staphylococci, the beta hemolytic strep¬tococci, and Proteus, some strains of which are secondary wound invaders.
We know that the normal skin surface has two important natural defenses against bac¬terial invasion—first, the ordinary drying action on the surface, facilitated, where the skin is uncovered, by the movement of air currents; second, the presence of unsaturated fatty acids (particularly oleic acid), which are components of the sebum, or oily secretion from skin oil glands.
Gram-negative organisms, that is, those organisms which do not retain the selective blue dye used in the Gram staining technique, are particularly sensitive to drying. This alone is effective in killing or inhibiting their growth. Unfortunately, the dry state never exists for any length of time over the stump skin during the use of a prosthesis.
Both the drying and the action of the fatty acids are slightly to moderately inhibitory against the staphylococcal organisms. In other words, neither factor offers sure protection against invasion by this group of germs, but both have deterrent value in the normal skin. Again, the moist state which usually exists under the socket tends to encourage the growth of staphylococci.
Although the beta hemolytic streptococcus is unaffected by drying, it is destroyed by oleic acid. But streptococci will grow in serous exudate, such as may be seen in a weeping eczematoid dermatitis of the stump, because the albumin in the exuded serum neutralizes the oleic acid, the chief natural antagonist of the streptococci. This relation of exudative lesions of the skin to secondary infection under¬lines the importance of adequate hygienic care in routine management of minor abrasions and irritations of the stump area. Further¬more, it should be apparent that there are times when the continued use of a prosthesis on a stump which is the site of a dermatitis, especially where a serous discharge is present, will prevent healing and is almost certain to invite a secondary infection.
The Proteus strains—the fourth group of organisms mentioned—multiply rapidly in a moist environment. Any occlusive dressing or cover, such as the socket, which tends to increase local moisture on the skin will favor a heavy overgrowth of Proteus.
Thus we see that, in all four of the examples cited, the use of a prosthesis may be expected in some measure to interfere with the defensive mechanisms of normal skin in its resistance to disease. This interference is augmented by prolonged or strenuous use of the prosthesis and by the presence of any pre-existing lesions, however minor they may seem to the amputee.
ELECTRICITY AND THE SKIN
The electrical behavior of the skin plays an important part in the preservation of good health. Normally, there is a negative electrical charge in the superficial layers of the skin. When an alkaline condition prevails, this electrical negativity is increased owing to adsorption of negatively charged hydroxyl ions. An acid condition of the skin, however, causes a discharge of this normal negativity, which is complete between pH 3 and pH 4. As the relative acidity of the skin increases, there is eventually a reversal of the charge, the skin surface becoming electrically positive. Fur-thermore, investigators have reported that scarring of the epidermis (14) and prolonged soaking in water or concentrated salt solutions (13) tend to cause a discharge of the normally negative charge of the skin. Both of these ab¬normal conditions may develop over the stump as the result of use of a prosthesis.
Just what effect socket wear has on the nor¬mal electrical behavior of the stump skin, or how significant this may be in maintaining a healthy condition in the stump area, we do not know at the present time. This is, however, another problem that should receive further investigation. We do know that the negativity of normal skin can be a factor in the defense of the body against pathogenic organisms, which are also negatively charged and which tend to be repelled from, or bound to, the sur¬face of the skin according to variations in the electrical charge on the latter (Fig. 7). It is of interest, incidentally, to note here that in muscle the relationship of negative-positive electrical charges to normal and damaged tissue, as here described for the skin, is just reversed.
STUMP HYGIENE AND LOCAL pH or THE SKIN
Blank has confirmed earlier observations that the pH of healthy skin is always on the acid side, falling usually between 4.2 and 5.6. Furthermore, both eccrine sweat and apocrine sweat are normally acid. These facts have given rise to the concept of the so-called "acid mantle" of the skin, which is cited by some investigators as one of the body's natural defenses against disease. Schmid (17) found a significant shift toward the alkaline side in the surface pH of the skin in cases of eczema and in seborrheic dermatitis, an inflammatory disorder involving especially the hairy and more oily regions of the skin. In general, an even greater shift toward the alkaline side takes place in these inflammatory diseases if the intact skin is broken and neutral in charge or if alkaline extracellular fluid diffuses through, as in any acute, weeping dermatitis of the stump. With healing, the original acid pH returns.
BUFFERING ACTION OF NORMAL SKIN
Another important property of the skin is its buffering action. If the skin surface is exposed to dilute acids or alkalies, there is normally a corresponding shift of the pH locally; but this is temporary, and the former acid pH is rapidly restored. This behavior represents the neutralizing capacity of the skin. Probably the most important agents in this neutralizing property are the sweat constitu¬ents, especially the lactic acid-lactate system and the amphoteric amino acids. Any local damage to the sweat mechanism, such as might be caused by socket irritation, could conceivably impair this important function of the skin in the involved areas. Burckhardt {7,8) and others have established that there is a definite correlation between the acid and alkali neutralizing capacity of the skin and its tolerance for acids and alkalies.
Pursuing a discussion of acid-base balance brings to mind several unanswered questions with regard to the amputee's problem of stump hygiene. We would like to know, for example, what happens to the normally acid pH of stump skin during the daily wearing of an airtight socket. Does stump skin possess the same pH and buffering properties as the skin of an intact limb? What effect do different socket materials have on the pH of stump skin? Does an interior finish which gives an alkaline reaction necessarily cause more damage to the skin than does one with an acid reaction? These are questions which should receive further investigation in the light of their vital relationship to stump hygiene.
It might seem from the foregoing that the cutaneous surface which gives an acid reaction denotes a healthy skin, resistant to invasion and disease, while an alkaline-reacting skin surface denotes the presence of some disease state. Unfortunately it is not quite so simple. Some organisms grow readily on an acid medium. Pathogenic fungi, for example, flourish on certain media at pH 4.9. None¬theless, in general, it is desirable to maintain the surface of the skin at least slightly on the acid side.
Washing, even with plain water, causes moderate hydration of the horny layer, with a drop, according to Szakall (21), from pH 6.3 to pH 5.3 in 30 minutes. This information may also have some application to lower-extremity prosthetics, since the stump skin becomes soaked with sweat in most cases shortly after the prosthesis is put on. Further¬more, a single washing with soap removes about 50 percent of the surface lipid film, thereby facilitating the outward diffusion of carbon dioxide, the acid reaction of which helps to neutralize an alkaline state on the surface of the skin.
SURFACE pll AND DEGERMING OF THE SKIN
Control of surface pH is also important in degerming the skin. Blank, Coolidge, and others (4,5,6), in an extensive study of the surgical scrub, have investigated many differ¬ent germicidal agents and techniques of cleans¬ing. Among the agents studied were the quaternary ammonium compounds, like Ceep-ryn® and Zephiran,® which are widely used in surgical cleansing of the skin. While these compounds do exert a bacteriostatic or bac¬teriocidal effect, Blank et al. found that they also have the property of binding the bac¬teria to the skin. It was demonstrated that, at a pH a little higher than the isoelectric point of keratin, the quaternary ammonium com¬pounds change the normally negative charge on the surface of the skin to positive. Since the bacteria are negatively charged, they are attracted to the skin. If the pH is then in¬creased considerably, for example by rinsing with an alkaline soap, the charge on the skin will revert to negative and the bacteria will be released from the skin, as has been confirmed experimentally by analysis and culture of the rinse water.
Another germicidal agent commonly used in disinfecting the skin is G-ll,® or hexa-chlorophene. Chemically it is 2,2'-methyl-enebis (3,4,6-trichlorophenol):
This compound has the double advantage of accumulating on the skin when used daily and of not being inactivated, as most germicides are, when combined with a detergent. If used only at infrequent intervals, G-ll is no more effective as a disinfectant than any nonmedi-cated soap. If used regularly, however, within five to seven days there will develop in the skin a concentration sufficient to cause a definite reduction in the bacterial flora. One contraindication to the use of this agent is the presence of a serous ooze, such as we see not infrequently on the stump in various types of eczematous skin conditions. Seastone (19) has reported that as little as 1.0 percent of sterile serum will reduce the bacteriostatic effect of this agent.
Hexachlorophene is available commercially in combination with various soaps and liquid detergents, in strengths varying from 0.75 to 3.0 percent. These include such brand names as Dial® soap, Gammaphen® soap, pHisoHex,® and Septisol.® Another useful preparation of G-ll is an alcoholic solution containing 0.1 percent of G-ll, with 0.5 percent of cetyl alcohol added as an emollient. This solution may be used as a two-minute rinse following soap-and-water cleansing of the stump.
A useful cleansing agent for stump skin has been found to be pHisoHex, especially where superficial infection is a problem. It consists of an emulsifying agent known as pHiso-derm,® to which 3 percent of G-ll has been added. Chemically, pHisoderm is sodium octylphenoxyethoxyethyl ether sulfonate, plus lanolin cholesterols, lactic acid, and petrola¬tum. Its pH is 5.5, approximately that of nor¬mal skin. It lowers the surface tension of water and is an active emulsifier.
There are many other agents for degerming the skin, many of which are too irritating for the type of regular use necessary to routine stump care. One of the more readily available of these is alcohol, which remains a useful bac¬teriocidal preparation. Isopropyl alcohol, for example, is germicidal up to 50-percent dilu¬tion. Too-frequent use of such solvents, how¬ever, will dry the skin excessively and may do more harm than good. Furthermore, any marked depression of bacterial flora over the stump skin cannot be maintained for long during use of the prosthesis.
SELECTIVE ABSORPTION AS A PROTECTIVE BARRIER
The healthy cutaneous envelope of the body is constantly active as a physicochemical bar¬rier against the outside world, retaining some substances and passing others through (Fig. 8). As early as 1904, Schwenkenbecher (18) showed that the intact skin is permeable to fat-soluble substances and to certain gases but is practically impermeable to water and most electrolytes. Most substances which are soluble in both water and lipids penetrate the skin and pass into the general circulation at rates comparable even to gastrointestinal or subcutaneous absorption. Phenolic compounds, lipid-soluble vitamins, and hormones pene¬trate rapidly. This property of the skin con¬ceivably could be of serious import in the indiscriminate use of socket materials or fin¬ishes capable of liberating absorbable toxic fractions which could be taken up by the stump skin.
In rare instances, individuals have demon-strated a peculiar sensitivity, known as an "idiosyncrasy," on first exposure to certain drugs and chemicals applied to the skin. Alex¬ander (1) described a case of iododerma, a form of iodine reaction, resulting in the death of a 37-year-old woman following routine pre-operative cleansing of the surface of the skin over the abdomen with iodine. This is not intended to suggest that any similar hazard exists in the use of present-day, conventional socket materials. It does, however, emphasize the fact that the skin may be, in certain rare cases, an open portal to the systemic circu¬lation.
Transfer of gases across the skin barrier may take place with ease in either direction. The biological significance of the movement of oxygen and carbon dioxide through the skin, which was once thought negligible, is given more importance now. Shaw and others (20) found that oxygen was given off through the skin when the oxygen content of the am¬bient air was reduced to about 2 percent and that it was absorbed more rapidly when the skin was surrounded by a gaseous mixture containing about 37 percent of oxygen than when surrounded by air. According to Cham¬bers and Goldschmidt (9), if the total skin surface is surrounded by nitrogen gas instead of air, there may be a compensatory, increased uptake of oxygen by the lungs.
Hediger (12) reported that, from a water chamber containing the dissolved gas, carbon dioxide passed into the skin as long as the water contained more than 4 percent of carbon dioxide. When the concentration dropped below 4 percent, carbon dioxide diffused out¬ward through the skin, as it does constantly under physiological conditions. Measurements cited by Rothman and Schaaf (16) showed that over a 24-hour period 7 to 9 grams of carbon dioxide escaped from the total skin surface, less that of the head, of an adult male. The amount suddenly increased when the temperature was raised to the critical tem¬perature of visible sweat secretion.
Cleansing of the skin with organic solvents such as ether, benzene, and, to a lesser degree, alcohol, enhances percutaneous absorption, that is, absorption across the skin barrier. Since such solvents are used frequently in the cleansing of the stump, as well as of the inte¬rior of the prosthetic socket, this effect upon the skin's absorption should be borne in mind. Moisture, almost constantly present in the wearing of a prosthesis, also promotes trans-epidermal absorption by an unexplained mechanismю
SUMMARY
Through the use of improved prostheses, many amputees have been able to return to relatively normal physical activity and to take again their rightful place in business and social life. It must be remembered, however, that the use of a prosthesis places upon the leg amputee new and heavy demands, includ¬ing not only muscular and emotional readjust¬ments but also the infliction of unaccustomed wear and tear upon his stump skin. Daily, for the rest of the amputee's life, his stump will be subjected to an abnormal environment that combines heat, moisture, and darkness with chemical and mechanical irritation. It becomes imperative then, in restoring the amputee to full activity, to make certain that he under¬stands the importance of systematic skin care. An adequate appreciation of the necessary requirements for good stump hygiene must be based on a knowledge of the functions and limitations of normal skin.
The skin provides for the other tissues a highly effective, tough and elastic outer cover¬ing, which has a great capacity for strengthen¬ing itself at points of stress and for repairing itself after injury. But this capacity of the skin for mechanical protection, the limits of which are of special interest in prosthetics design, is only one of its many important functions. The skin possesses, in addition, a variety of ana¬tomical structures, including the eccrine, apocrine, and sebaceous glands, the normal function of which is necessary for the preserva-tion of good skin hygiene. The eccrine glands are indispensable in the heat control of the body. All of the glands produce secretions, some of which are exceptionally copious. This normal function poses an important sanitary problem for the amputee and makes routine cleansing of both the skin and the prosthesis essential.
The natural defenses of the skin against germs depend upon good hygiene. Conditions inside the socket tend to impair the resistance of the skin to infection, but through adequate cleansing, frequent airing, and intelligent care of early lesions, serious infection may be avoided.
Knowledge is increasing concerning the electrical and chemical buffering properties of the skin and their role in the maintenance of skin health. There is usually a negative charge in the superficial layers of normal skin. It is, however, discharged by injury or by prolonged soaking in water or salt solution. Similarly, normal skin is slightly acid, but in the presence of inflammation of the skin a shift to the alkaline side usually occurs. The sweat con-stituents contribute largely to the capacity of the skin to neutralize or buffer dilute acids and alkalies to which it is exposed. Whether or not these properties are retained intact by the stump skin of amputees and, if so, how they are affected by the conditions of use of a prosthesis are important areas for further research.
Although the skin serves as a protective barrier, it is readily penetrated by certain substances. For this reason the stump should be protected from contact with materials potentially toxic. Similarly, the stump skin may be subject to a variety of local injuries— mechanical, chemical, or allergic in origin. Again the importance of early and close attention to minor lesions and to good pre¬ventive hygiene must be emphasized.
There have been two chief aims in this discussion of basic principles. The first was to impart an awareness of the complex nature of the problem of stump hygiene and the second to emphasize that good stump hygiene, far from being an academic matter, is one of the utmost importance to the amputee. Like the proverbial dispatch rider whose horse was crippled for want of a horseshoe nail, the amputee may suffer discomfort and serious disability because of neglect of a seemingly insignificant lesion or failure to follow a simple cleansing routine.
