MARCH 1981 P H Y S I O T H E R A P Y MICROWAVE DIATHERM Y 7 L. HAYWARD, B.Sc. (Physio) (Wits) and A. STATHAM, B.Sc. (Physio) (Wits)* SUMMARY A n extensive literature survey is summarised. Experi­ m e n t a t i o n consisted o f irradiation o f paraffin wax and human subjects with microwave o f frequency 2 450 M H z Results, conclusions and recommendations are briefly stated. A fu ll reference list is given. INTRODUCTION Microwave has been used therapeutically in physio­ therapy departments since 1951 (Scott, 1968a), and is now used almost worldwide. Most large physiotherapy depart­ ments possess at least one microwave generator and ^ jk r e has been an even greater increase and acceptance use by private practitioners (Scott, 1968a). This increase in usage should have been accompanied by an increase in research done into the effects and usages of microwave. Instead, few articles have been published over the past years. Enquiry has revealed that there exists a lack of know­ ledge amongst physiotherapists about the working of the machine, the effects of microwave on body tissues, the dosages to be prescribed and the danger if not properly administered. Much of the literature appears to be in conflict and different theories have been formulated, most of which have never been proven conclusively. Our study was confined to the therapeutic range of microwave diathermy, mainly that of 2 450 MHz. The aims of this study were as follows: • To review fully and report all available literature. • To illustrate all points of confusion. • To attempt to clarify by experimentation certain points: the effective depth of penetration of microwave; the effect of varying some of the determining factors of dosage; the actual amount of heating occurring in the skin at therapeutic dosages. • To assess the value of paraffin wax as a suitable biological tissue phantom for experimentation. ^ R 'ER A TU R E s u r v e y From the available literature several points of interest were extracted, viz. the physical characteristics of micro­ wave, the production of microwave by diathermy machines, the types of apparatus commonly used, the effect of microwave on body tissues, the physiological results of these effects, the therapeutic uses and indica­ tions for the use of microwave diathermy, contra­ indications to the use of microwave, method of micro­ wave application, advantages and disadvantages of microwave diathermy, dosage estimation and measure­ ment and the effects of excessive dosages. Several points o f , interest worthy of note emerged from this part of the study: • It is evident thqt the actual percentage of micro­ wave radiation which is reflected at the skin surface (and at other interfaces in the tissue) has not been established as yet although several values have been suggested (McLees and Finch, 1973: Michael- son, 1971; Scowcroft et a l, 1977). OPSOMMING ’n Vitgebreide oorsig van literatuur word opgesom. Eksperimentasie het bestaan uit irradiasie van parafien- was en proefpersone m et m ikrogolf m et ’n frekwensie van 2 450 M Hz. Resultate, gevolgtrekkings en aanbeve- lings word kortliks genoem. ’n Volledige lys van ver- wysigings word * Final year student project, 1978. Received 21 April 1980. • The depth of penetration of microwave is also not known — the standard quoted depth o f 3 cm being theoretical and in fact now thought to be incorrect. The depth of penetration is affected by several variables, for example, the thickness o f the skin, the dielectric constant of the tissues, and the number of interfaces within the tissues (microwave being reflected at interfaces) (Scott, 1968a; Scott, 1968b; Hollander, 1968; Scott, 1975; Lehman et al., 1962; Krusen et al., 1971; de Lateur et al., 1970). • Any physiological effects of microwave are as a result of heating of the tissues, these effects being an increased metabolism of the irradiated tissues, an increase in blood flow to the area, muscle relaxation and increased activity of sweat glands (Scott, 1968a; McLees and Finch, 1973; Vernier et a l, 1978; Fench et al., 1949; Krusen, 1950; Hovind and Nielsen, 1974). • It appears unlikely from studies done that micro­ wave can have any effect on tissues other than those of the skin and subcutaneous tissues as it is doubtful that heating occurs at any depth greater than 3 cm from the body surface. • Although it is reported that microwave has been successful in treating a number of conditions there is very little conclusive evidence to support these claims and all of these conditions may be success­ fully treated using other modalities (Scott, 1968a; Scott, 1968b; Scott, 1975; Krusen, 1950; Lehman et al., 1939; Lehman et al., 1954; Instruction Manual). • There are several contra-indications and conditions where care must be taken concerning the use of microwave diathermy. These include metal in the field, treatment of the eye (which may cause the development of lenticular opacities), poor circula- lation both physiological (i.e. areas having poor circulation e.g. testes) and pathological (e.g. circu­ latory diseases), bony prominences (due to the reflection of the electromagnetic waves of micro­ wave), water or moisture in the field, defective sensation, cardiac pacemakers, pregnancy and menstruation, tuberculous or oedematous areas, malignancy or areas of haemorrhage, areas which have received deep X-ray therapy in the last six months (McLees and Finch, 1973; Michaelson, 1971; Scowcroft et al., 1977; Scott, 1968b; Scott, 1975; Krusen et a 1. 1071; Feuch et al., 1949; Krusen, 1950; Richardson, 1955; K antor and Witters, 1976). 9 Excessive dosage of microwave results in severe soft tissue burns and there is thought to be some detrimental effect on bone. It may also cause damage to the testes and eyes if excessive dosages are given (Michaelson, 1971; Scowcroft et al., 1977: Scott, 1968b: Krusen e t al., 1971; Krusen, 1950). R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) 8 • Microwave is thought to be a convenient modality of treatm ent as it is easy to apply and there are no tuning or field problems. The duration of treat­ ment required is said to be, on average, approxi­ mately half that of short-wave diathermy. How­ ever, from the literature it is evident that the effective depth of penetration limits its use so as to make it fairly disadvantageous (Scott, 1968b; Hollander, 1968). • Several guidelines for application were stated. These followed very closely the precautions which should be kept in mind when using short-wave diathermy. In addition the microwave beam should never be directed at the patient’s eyes or at anyone else nearby (Scowcroft et al., 1977; Scott, 1977; Instructional Manual). • The estimation of dosage is one of the most con­ troversial points in the literature and there is uncertainty over whether an accurate measure­ ment of dosage (using the wattage output meter) should be employed or whether, as with short­ wave diathermy, the physiotherapist should rely on the patient’s sensation in order to estimate the dosage. It is said that the minimum dosage to produce any heating is an exposure of 0,02 W /cm 2, Exposure to 0,1 W /cm 2 for periods of an hour or more can produce pathological changes in tissue. Treatm ent times usually vary between ten and thirty minutes. The distance of the applicator makes very little difference to the dosage. As with short-wave diathermy, mild therm al doses are given in acute conditions and therm al doses in chronic conditions. It is also felt th at the dosage should be greatly reduced for whole body expo­ sures (McLees and Finch, 1973; Scott, 1968b; Lehmann et al., 1962; Krusen el al., 1971; Instruc­ tional M anual; Heath, 1974; Brown and Johnson, 1975). EXPERIMENTATION The experimental part of this study was divided into two sections. • Microwave irradiation of paraffin wax. • Microwave irradiation of human subjects. For both of the above studies the M icrotron 200 Microwave Diathermy unit with a frequency of 2 450 M Hz was used. The oval field applicator, which gives an even field, was used. Paraffin Wax An experimental rig consisting of a perspex frame which held chromel-alumel thermocouple wires at predetermined distances apart was immersed in the paraffin wax which was then allowed to solidify. The wires were on the surface, 1 mm, 5 mm, 10 mm, 20 mm, 30 mm, 40 mm and 100 mm below the surface. The thermocouple wires were attached to the recording apparatus which comprised a digital volt meter, elec­ tronic ice reference junction and multiway switch. This rig enabled us to measure temperature changes at precise depths of wax. After calibration the paraffin wax was irradiated with the applicator at varying distances from the surface, at different intensities of irradiation, and recordings were taken at five minute intervals during the entire exposure. In this way the effects of intensity, distance of applicator and duration of exposure could be estimated. Human Subjects Similar chromel-alumel thermocouples were inserted into the extensor surfaces of the forearms of the human subjects approximately 70 mm from the distal ante- cubital crease to a depth of 2,5 mm below the skin surface. The human subjects were then irradiated with microwave at the therapeutic dosage of 100 W for 20 minutes at a distance of 15,0 cm. RESULTS Several interesting phenomena emerged from the experimental p art of the study: • The greatest increase in temperature occurred in the first five minutes of exposure in both the wax and human subject trials. This increase in tempe­ rature continued (at a slower rate) for the next fifteen minutes in the wax. The temperatures recorded during the human subject trials, however, showed a decrease in temperature after the initial increase and in two of the subjects the temperature recorded at the end of exposure were in fact lower than at the start. This is thought to be as a result of a compensatory increase in subcutaneous blood flow in response to the initial temperatuw^ increase, resulting in the transport of heat awdf from the area via the venous capillary network (Stoll, 1960). • From the above it is also evident that no transfer of heat by conduction to deeper layers can occur as a temperature gradient does not exist. It is therefore unlikely that muscle tissue can be heated in this manner. • It is thought, therefore, that any therapeutic results which have been reported may have been as a result of the increased blood flow to the tissues. • Temperature increase of the thermocouples in the wax was proportional to increases in intensity of irradiation thus confirming that the output of the machine is an important factor when considering dosage whereas alterations of the distance did not affect temperature changes. • It was evident that paraffin wax, although it exhibited several interesting pertinent phenomena, is not a suitable biological tissue phantom for further studies. There are several incompatible differences (apart from the obvious lack of blood supply) between paraffin wax and body tissue. Absorption of microwave is largely dependent on water content (McLees and Finch, 1973; Michael- son, 1971) and since wax has no water content, the microwave is transmitted and caused an almost equal rise in temperature in all th e rm f,' couples. Microwave is also reflected at interface?* and as wax contains no interfaces the results are altered. CONCLUSIONS Several conclusions were drawn from this research: • There is still much contradiction in the literature which requires clarification before microwave diathermy can be used with confidence. • The effective depth of penetration of microwave is as yet unknown. • Although microwave is said to be an effective and useful treatment, there is no conclusive evidence of this. ® Microwave diathermy appears to produce no appre­ ciable temperature increase in subcutaneous tissue at therapeutic dosage. • The wattage output of the machine directly affects the heating occurring whereas the distance of the applicator does not. • Paraffin wax is not a suitable biological tissue phantom on which to conduct further studies. MAART 1981F I S I O T E R A P I E R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) RECOMMENDATIONS It is recommended that further research be carried out into several aspects of microwave diathermy: • The effective depth of penetration. • The amount of heating occurring at all depths in human skin. • The actual amount of reflection which occurs at the surface of the body. • The actual dosage which should be applied and the installation of a uniform system of estimation, • An attempt to find a suitable biological tissue phantom on which to perform these investigations. References Brown, B. H. and Johnson, J. G. (1975). Microwave diathermy. Physiother. 61, 117. Feuch, B. L., Richardson, A. W. and Hines, H. M. (1949). The effects of implanted metals on tissue hyperthermia produced by microwave. Arch. Phys. Med. and Rehab 30, 164. Heath, J. (1974). The effects of short-wave diathermy B microwave diathermy and ultrasonics on demand * pacemakers and ventrical inhibited pacemakers A ust. J. Physiotherapy 20, 144. Hollander, C. (1968). Correspondence. Physiother. 54, 415. Hovind, J. and Nielsen, S. L. (1974). The effects of short-wave and microwave diathermy on blood flow in subcutaneous and muscle tissue in man. Proceedings 7th International Congress WCPT. Instruction M anual for the use of the M icroton 200 Microwave Diathermy Unit. Electro Medical Supplies (Greenham) Limited. Kantor, G. and Witters, D. W. Jr. (1976). A comparative performance study of space applicators in microwave diathermy. J. Microwave Power 11, 164. Krusen, F. H. (1950). Medical applications of micro­ wave diathermy — Laboratory and clinical studies. Proc. Royal Soc. Med. 43, 641. Krusen, F. H., Kottke, F. J. and Ellwood, P. M. (1971). Handbook of Physical Medicine and Rehabilitation 2nd Ed. Saunders. MARCH 1981 9 de Lateur, B. J., et al. (1970). Muscle heating in human subjects using 915 M Hz microwave contact applica­ tor. Arch. Phys. Med. and Rehab. 51, 147. Lehman, J. F., et al. (1939). Comparative study of the efficiency of short-wave, microwave and ultrasonic diathermy in heating the hip joint, Arch.j Phys. M ed. and Rehab. 40, 510. Lehmann. J. F., et al. (1954). A comparison of u ltra­ sonic and microwave diathermy in the treatm ent of periarthritis of the shoulder. Arch. Physs Med. and Rehab., 29, 67. Lehmann, J. F., et al. (1962). H eating patterns produced in specimens by microwave of the frequency of 2 456 M Hz when applied with the “ A”, “B” , and “ C” directors. Arch. Phys. M ed. and Rehab 43, 538. McLees, B. C. and Finch, E. D. (1973). An analysis of the reported physiological effects of microwave rad ia­ tion. Advances in Biological and Medical Physics 14, 163. Michaelson, S. M. (1971). Biomedical aspects of micro­ wave exposure. American Industrial Hygiene A sso­ ciation Journal 32, 338. Richardson, A. W. (1955). The effect of microwave diathermy as a hyperthermic agent upon vascularized and avascular tissue. Brit. J. Phys. Med. 18, 143. Scott, B. O. (1968a). The principles of microwave therapy. Physiother. 54, 150- 153. Scott, B. O. (1968b). Microwave diathermy. Physiother., 54, 143 - 145. Scott, P. M. (1975). Clayton’s Electrotherapy and Actinotherapy 7th Edition. Bailli&e Tindall, London. Scowcroft, A. T., Mason, A. H. L. and Hayne, C. R. (1977). Safety with microwave diathermy. Physiother„ 63, 359. Stoll, A. M. (1960). The role of the skin in heat transfer. Transactions o f the A S M E J. o f H eat Transfer 82 239 - 242. Vernier, M., Ashby, P. and Crawford, J. S. (1978). Effects of therm otherapy on the electrical and mecha­ nical properties of human skeletal tissue. Physiother. Canada 30, 117. p h y s i o t h e r a p y WCPT NEW SLETTER REGIONAL SEMINARS The first regional seminar organised by W CPT was meld in Geneva in October 1980 and dealt with aspects of EMG, Statistics and Writing for Scientific Publica­ tions. A video-film introducing EM G Kinesiology is available in 16mm (either U-matic or V.C.R. system) from Mr. Yves Blanc, RPT, Laboratoire de Myocine- sigraphie, Hospital Cantonal Universitaire, 1211 Geneva 4, Switzerland at cost of return postage. An expanded text, with introduction by Prof. J. V» Bas- majian available from W CPT Headquarters, 16/19 Eastcastle Street, London W IN 7PA on provision of a large self addressed envelope plus 2 international postal reply coupons. The second regional seminar will be held in Geneva in October 1981 and will deal with the physiology, evaluation, psychological aspects and treatm ent of pain by means of TENS amongst others. WCPT NEWS The Zambia Society of Physiotherapy has been granted provisional approval. The Executive Com­ mittee met in Bangkok in January. ASIAN CONFEDERATION FOR PHYSICAL THERAPY .. Thailand hosted the first Assembly of the above at m e same time as the meeting of the Executive Com­ mittee. Associations from Indonesia, Japan, Korea, Philippines, Republic of China and Thailand participa­ ted. AUSTRALIA The APA is marking its 75th anniversary with an Austral-Asian Congress in Singapore from 31 May - 6 June 1981. A week-long intensive course in acupunc- .ture will precede the Congress, which will deal with manual therapy, neurology and general sessions. Details from Mr. M. Corfee, APA Congress ’81 Secretariat, 1st Floor, 33 Park Street, South Melbourne, Victoria 3205, Australia. EGYPT The first national Congress of E.P.T.A. was held in May 1980 in Cairo, as well as the Assembly of the African Physiotherapy Organisation with participation from Egypt, Canada, Ethiopia, Ghana, Nigeria, U.S.A. and Zambia. FRANCE A scientific data base of computer based information on physical therapy, physical medicine and rehabilita­ tion from physiotherapy journals from Canada, N ether­ lands, South Africa, Sweden and U.S.A. as well as monographs, doctoral theses and other documents has been established. The price of a question is 150 French francs (US $35, £16, Sw.kr. 150) available from BLDOC, B.P.12 60260, Lamorlaye, France. R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. )