Static magnets: what are they and what do they do?

Magnetos estáticos: o que são e para que servem?

REVIEW ARTICLE

Static magnets – what are they and what do they do?

Magnetos estáticos – o que são e para que servem?

Laakso L^I; Lutter F^II; Young C^I

^ISchool of Physiotherapy and Exercise Science, Griffith University, Gold Coast, Queensland, Australia

^IIPhysiotherapy Service, Gold Coast Hospital, Gold Coast, Queensland, Australia

^{Correspondence to}

ABSTRACT

INTRODUCTION: Therapeutic static magnets have gained wide community acceptance for neuromusculoskeletal pain relief in many countries yet, apart from strong anecdotal reports of benefit, there is a paucity of scientific evidence for their use.

OBJECTIVES: In this review we describe the physical characteristics of traditional and commonplace unipolar and bipolar static magnets as well as newer quadripolar magnetic arrays; discuss what is known of the physiological effects of static magnets and the strength of the literature; and make suggestions for targeted future research for static magnets in the management of neuromusculoskeletal pain conditions.

Key words: magnetotherapy; pain; static magnets; quadripolar magnetic arrays.

RESUMO

INTRODUÇÃO: A magnetoterapia estática conquistou ampla aceitação da comunidade para alívio da dor neuromusculoesquelética em diversos países. No entanto, com exceção de relatórios anedóticos de seus benefícios, há uma grande escassez de evidências científicas para seu uso.

OBJETIVOS: Nesta revisão, descrevemos as características físicas dos tradicionais magnetos estáticos unipolares e bipolares comuns, assim como os mais recentes conjuntos magnéticos quadripolares; discutimos o que se conhece sobre os efeitos fisiológicos da magnetoterapia estática e o suporte da literatura; e fazemos sugestões para futuras pesquisas direcionadas à magnetoterapia estática no controle de condições de dor neuromusculoesquelética.

Palavras-chave: magnetoterapia; dor; magnetos estáticos; magnetos quadripolares.

Introduction

Magnetic devices have been used for treating human ailments since the 16^th century¹. Magnetic fields of varying strengths are employed in such diverse applications as energy production, transportation, information storage and medical imaging. Most modern magnets are much more powerful than the Earth's magnetic field. A magnetic field occurs perpendicularly to an electric field; it is generated in two ways². Firstly, a magnetic field is created when electrically charged particles flow through a coiled or looped conductor producing one of two field types: static or time-varying². A static field forms with direct current, while a pulsating time-varying field is generated by alternating current³. Secondly, electrons within certain materials have their own intrinsic magnetic fields that, when summed vectorially, give a net magnetic field. Such permanent magnets do not require a motile electric current. Static fields from permanent magnets are the subject of this review.

The SI unit for magnetic field strength is the Tesla (newton per amperemeter) (where 1 Gauss=10^-4 Tesla). The authors will describe magnetic field strength in units of Tesla (T) or milliTesla (mT), and convert Gauss to Tesla when citing the work of others. To put field strength into perspective, the fields of Magnetic Resonance Imaging devices are in the order of 1.5 to 3 T, while the earth's field is less than 0.05 milliTesla (mT). Therapeutic magnetic devices used for pain relief typically generate magnetic fields of 11-500 mT⁴. It can be useful to remember that the field strength is inversely proportional to the cube of the distance from the surface of the magnet.

Magnetic fields can be represented diagrammatically so that the density of lines reflects the strength of the magnetic field². Field lines form closed loops, emerging from the negative (South) pole of the magnet and enter through the positive (North) pole (e.g., Figure 1A). Field strength is the amount of force exerted by the magnet on charged particles within the field. For example, iron filings will align with the field to reveal patterns in the lines of force (e.g., Figure 1B). Field patterns vary with different orientations of the poles in arrays of magnets e.g., bipolar magnets and quadripolar magnetic arrays. The distinction may be important since the unique field pattern is reported to be the basis of effect for devices such as quadripolar magnetic arrays.

Static magnet therapy is classified under CAM Methodology #3 - Energy, by the US National Institutes of Health Centre for Complementary and Alternative Medicine⁵. Commonly, weak static therapeutic magnetic devices are made of ferrite (typically <0.4 T) with a single positive and negative pole. While there is no such thing as a 'unipolar magnet' the term is used to describe the application of one pole to the area to be treated, e.g., Figure 1A. For a bipolar application both poles are in contact with the part to be treated, such as with a horseshoe magnet⁶.

Modern therapeutic magnets are constructed of synthetic alloys with inherently strong, permanent, static fields. Magnetic alloys are categorized by the material content. Compounds of aluminum, nickel and cobalt (alnico) are sometimes mixed with iron, copper or titanium to create field strengths of up to 0.15 T. Rare earth or super (lanthanoid) magnets when blended with neodymium and sometimes iron and boron are typically of 0.2 to 1.2+ T, or, with samarium cobalt can be even stronger, up to 3.4 T.

Magnets have become popular with the lay public for the management of acute (including post-operative) and chronic pain in humans, racehorses and domestic pets. During the last two decades coinciding with the development of the quadripolar magnetic array, community expectations of magnetic therapy have increased due to anecdotal claims of 'miraculous' healing reported in the media. Such reports have created a multibillion-dollar, consumer-driven industry worldwide, while the evidence for use of these devices remains anecdotal and insufficient for acceptance by conventional healthcare practitioners⁷.

Controversy surrounding the therapeutic efficacy of static magnets was highlighted in a vigorous discussion in the on-line reader response section to an editorial appearing in the British Medical Journal⁸. The matter of evidence for (and against) applications of static magnets evokes strong opinions which are sometimes driven by commercial interests and at other times lacking in scientific rigor. However, the quality of research in this field is steadily improving. Herein, we have restricted our discussion to static magnetic fields, and attempt to understand the literature related primarily to clinical populations with symptoms of pain of musculoskeletal origin. We compare the physical characteristics of bipolar and quadripolar magnetic arrays, investigate their purported physiological effects (which necessarily requires an incomplete review of laboratory models), and discuss the results of clinical studies using static magnets.

What are bipolar static magnets and quadripolar magnetic arrays?

Permanent static magnets come in a wide range of shapes and sizes (e.g., disc and barshaped magnets), field strengths and patterns. Many traditional therapeutic magnets are disc or coin shaped (Figure 1), often embedded in personal jewelry, mattress and pillow covers, orthopedic external supports such as neck collars and back braces, which are available to the public 'over the counter' and with few instructions for application.

A 'quadripolar' magnetic array is usually composed of four magnetic discs, arranged with alternating polarity within a hypoallergenic plastic casing (Figure 2A). Pairs of positive and negative poles repel each other across the midline of an "X" while being attracted to the neighboring opposite pole. Manufacturers of quadripolar devices suggest that the alternating attraction and repulsion force creates a 'magnetic void' in the centre of the array. The result is 'steep field gradients' purported to produce effects beyond those of simple bipolar static magnets⁹. Figure 2 illustrates the magnet arrangement within a quadripolar magnetic array and the field map produced by scanning 3mm above the device with a gaussmeter; and the resultant field pattern in iron filings. Comparison between Figures 1 and 2 demonstrates that the bipolar and quadripolar magnetic arrays appear to be substantially different.

The magnets of a quadripolar array are typically constructed from magnetic alloys, measuring less than 3 cm in diameter, weighing approximately15 grams, and generating a field of approximately 200 mT. The manufacturers recommend that quadripolar magnetic arrays are applied directly to the skin in specific locations around a painful area and left in situ as required¹⁰.

Physiological effects of static magnets

Low strength static magnetic devices are marketed not only to provide pain relief but also to address a wide range of signs and symptoms including reduction in swelling, induction of more restful sleep, stress relief and for anti-infective properties. Charged particles in body fluids flowing through a magnetic field will drift further apart (the Hall effect) and paramagnetic elements such as oxygen or aluminum will reorientate to magnetic lines of force¹¹. However these effects are transient, minute, and may not be clinically important. Hypotheses proposed for therapeutic effects of static magnets include altering radical dependent biochemical processes, or lipid membranes, and exerting forces on cell intermediates or charged particles such as electrolytes¹². These mechanisms may alter the firing rate of neurons, change the rate of enzyme-mediated reactions, affect calcium channels, or increase local blood circulation^12,13. However, the supporting evidence for any of these effects is not strong⁴ and the issue of effect mechanism remains vexatious. Information regarding possible mechanisms of effect would assist in defining the specific conditions for which static magnetic field therapy may have benefit, optimize its application and thus promote improved research.

A common claim is that therapeutic magnets result in physiological thermal effects that promote tissue healing. Sweeney et al.¹⁴ conducted a study to determine if skin or intramuscular temperatures were altered with the application of flexible therapeutic magnets to the quadriceps muscle for 60 minutes. The study was a repeated-measures, placebo-controlled design (n=13) and the results showed that neither skin nor intramuscular temperatures were significantly different across the three treatments at any time. The authors emphasized that the results of their study contradict one of the fundamental claims made by magnet distributors.

The primary physiological effect attributed to exposure by static magnetic fields is that of change in blood flow and circulation^eg,15. An effect on blood flow has been verified in studies of rats using 8 T whole body exposure¹⁶, and in rabbits using 0.25 T in ear chamber experiments¹⁷. The results have led to the magnetic field effects being described as biphasic, i.e., causing vasodilation when resting blood vessels are constricted prior to magnet application; and vasoconstriction when blood vessels are dilated in the area of the magnetic field¹⁸.

In humans, there are few studies that have specifically investigated the clinical or physiological effects of static magnetic fields. A randomized, double-blind, placebo-controlled crossover study examined the effects of static magnets on resting forearm blood flow and vascular resistance in young, healthy men¹⁵. The results of the study demonstrated that the average blood flow was not significantly different between the magnet and placebo conditions after 10, 20 and 30 minutes of treatment application (P>0.05).

Clarity regarding physiological effects has only become evident in recent times in a series of studies in which Mayrovitz^19-21 have investigated the effects of static magnetic fields on aspects of microcirculation and skin blood perfusion. After a number of attempts using different protocols, Mayrovitz and Groseclose²¹ were the first to use locally applied static magnets to demonstrate an effect on human skin blood perfusion noting an unexpected reduction in this outcome measure. The authors concluded that the reduction in skin blood perfusion was likely to be related to the biphasic responses noted earlier in rat studies¹⁶. This finding raises the possibility that investigating static magnetic energy in experimental models of pain is unlikely to be successful if there is no pathology, in particular no vascular component.

Magnet therapy and neuromusculoskeletal pain management

Ratterman et al.⁴ carried out a review of scientific peer-reviewed publications regarding magnetic therapy and found that while magnetic therapy was gaining popularity, the scientific evidence to support its efficacy in pain management was lacking. A more recent systematic review by Pittler, Brown and Ernst²² concluded that the available evidence does not support the use of static magnets for pain relief. We have further updated the search, and a summary of relevant literature of static magnets (of varying configurations) is presented in Table 1 (with an indication of study designs, range of pain conditions and experimental samples utilized, number of subjects, inclusion of placebo, polarity, application times, outcomes and study limitations).

Due to the fact that the devices are distinctly different in field characteristics, we have separated published studies of quadripolar magnetic arrays and presented these in Table 2 along with results from an in vitro study of this device.

We employed an inclusive approach to the literature search using as broad a range of search terms as possible to identify as many references to static magnetic therapy in case studies as well as clinical reports, and controlled trials. We searched Medline, PubMed, CINAHL, Web of Science and OVID as well as the grey literature through electronic sources (such as Google Scholar). Reference lists were cross-referenced in order to identify as many relevant sources as possible. The search had no start date limitation but was restricted to reports published by June 30, 2008. No language limits were set although the capacity to interpret non-English language reports was restricted by the translation resources available to the authors. Only full-text sources were considered. A priori search terms were not limited to any particular type of pain conditions although, to ensure that the search was comprehensive, a number of searches were cross-referenced with specific search terms limited to musculoskeletal pain. No restrictions were placed on study designs or methodologies. Subsequent to the completion of the literature search, post hoc limitations were set for reporting purposes to exclude non-neuromusculoskeletal conditions.

The following discusses, in more detail, the outcomes from some of the known research and then distils the information for consideration of further research.

Neuromusculoskeletal pain management with bipolar static magnets

Vallbona, Hazelwood and Jurida²⁴ conducted a study to determine if the chronic pain experienced by post-polio patients could be relieved by the application of magnetic devices over an identified painful trigger point. The study is reviewed here in detail as it is commonly used as a basis to promote magnetic products. Vallbona, Hazelwood and Jurida²⁴ designed a double-blind, randomized clinical trial of 50 patients diagnosed with post-polio syndrome and self-reported muscular and arthritic pain. The McGill Pain Questionnaire was used to measure subjective pain levels experienced following firm application of a blunt object over an active trigger point. Placebo or active magnetic devices (30 - 50 mT) were applied to the affected area for 45 minutes to identify if the magnets had analgesic effects. Patients in the active device group experienced an average reduction in pain score of 4.4 +/-3.1/10 (p<0.0002) while those with the placebo device experienced a decrease of 1.1 +/-1.6/10 (p<0.005).

Vallbona, Hazelwood and Jurida²⁴ concluded that application of magnetic devices over painful trigger points in participants with post-polio pain results in significant and prompt relief of pain, however the results of the study should be viewed with caution due to a lack of adequate experimental controls. For example, the researchers did not measure nor standardize the pressure applied to trigger points before and after application of the magnetic device, hence the dependent variable may not have been reliably measured. Secondly, the mean age of participants in the experimental group was lower and there were twice as many women than in the control group. The possible effects of age and gender were not matched across groups. The results of the above study are yet to be reproduced.

More recent studies are indicative of the conflicting results noted for magnet research. Alfano et al.³¹ tested the effectiveness of therapeutic magnets in individuals with fibromyalgia. The randomized, placebo-controlled study investigated sleep pads with static magnetic fields compared to placebos and usual treatment, in decreasing patient pain perception (pain intensity ratings, tender point count and tender point pain intensity score) and improving functional status (Fibromyalgia Impact Questionnaire) after six months of treatment. All groups showed improvements in functional status, pain intensity level, tender point count and tender point intensity. With the exception of pain intensity level, the improvements observed in the real magnet groups did not differ significantly from the placebo group or usual care group (p=0.25). The results of the above study did not show strong evidence for the efficacy of therapeutic magnets.

A study by Hinman, Ford and Heyl³⁴ aimed to determine the effects of static magnets on the level of pain and functional limitation associated with chronic knee pain from degenerative joint disease. A double-blind, randomized controlled trial was conducted in which subjects with chronic knee pain wore pads containing magnets or placebos over the knee joint for two weeks. The results revealed a significantly greater improvement in ratings of pain and physical function in the group wearing magnets (P=0.002). In another randomized, controlled trial that investigated the effects of magnetic insoles on plantar heel pain, the investigators found that wearing magnetic insoles daily for 8 weeks did not provide significant reductions in daily foot pain and employment performance when compared to placebo³⁵. In contrast, Wolsko et al.¹² found in a randomized, placebo-controlled trial that magnets showed statistically significant reductions in osteoarthritic knee pain compared to placebo treatment (P<0.05) at four hours but not at 6 weeks.

The results of our search demonstrate that the literature relevant to static magnet therapy is increasing and that of the 20 clinical studies that have investigated the efficacy of magnets on neuromusculoskeletal pain, 11 studies have shown at least some benefit for a variety of outcome measures. Beyond this observation, it is difficult to be more definitive about the effects, and pooling of data is not possible due to the disparate nature of the studies and protocols utilized.

Neuromusculoskeletal pain management with quadripolar magnetic arrays

As outlined earlier, quadripolar magnetic arrays produce a magnetic field pattern substantially different to that produced by traditional therapeutic magnets. As such, research pertaining to quadripolar magnetic arrays and their effects on pain are considered separately herein (Table 2). The available research on quadripolar magnetic arrays is more limited than reported for traditional therapeutic magnets, reflecting the fact that the devices are comparatively new and have been subject to patent controls until recent times.

There are few clinical studies that have examined the hypoalgesic effects of quadripolar magnetic arrays^1,43,44. Statistically significant reductions in pain have been noted and are discussed below. Despite some positive findings, skepticism exists regarding the efficacy of using quadripolar magnetic arrays in the treatment of pain. In particular, a number of studies have been carried out by researchers with affiliations and financial interests with the manufacturer rather than by independent investigators. As well as the factors noted earlier regarding physical parameters of magnets, known studies of quadripolar magnets present various inadequacies such as the lack of placebo or control conditions, inadequate control of confounding variables and insufficient subject numbers.

Clinical studies

In one of the first pilot studies using quadripolar magnetic arrays, Holcomb, Parker and Harrison¹ investigated the ability of the devices to reduce pain in 54 patients with chronic low back and knee pain using a 2x2 randomized, double blind, cross-over design. Patients received one of two treatments consisting of either quadripolar magnetic arrays followed by placebo or vice versa. Base line and post-treatment measures of pain at one, three and 24 hours were obtained using the Visual Analogue Scale (VAS) and Verbal Rating Scale. Also, data was collected on analgesic and mood altering drug use during the treatment periods.

Prior to treatment the average pain rating was 52.9 +/-23.3 points (mean +/-standard deviation). With application of the devices, pain reduced by an average of 8.11 +/-3.38 points more than the placebo treatment (p=0.03). Treatment with quadripolar magnetic arrays reduced pain levels at all three time points, although only the one and 24 hour differences were statistically significant (p=0.032 and 0.03, respectively). There were no statistically significant differences in the amount of analgesics used during the treatment and placebo conditions (p=0.087). The results of this study suggest that quadripolar magnetic arrays might be effective in reducing low back and knee pain.

In a pilot study examining the efficacy of quadripolar magnetic arrays (190 mT) as an adjunct therapy for joint pain in patients with inflammatory (rheumatoid or psoriatic) arthritis and persistent knee pain, Segal et al.⁴³ measured a range of dependent variables (including patient's and physician's global assessments of disease activity (GADA), Westergren Sedimentation Rate (WSR), range of motion of the knee by goniometry, tenderness, swelling, patient's assessment of physical function, VAS and the modified Health Assessment Questionnaire (MHAQ) for difficulty in daily activities). The dependent variables were measured before and at consistent time intervals up to one week after placement of the magnets. Four quadripolar magnetic arrays were applied to the knee over the suprapatellar and infrapatellar bursae and over the medial and lateral collateral ligaments. The authors found that knee pain was reduced significantly on average by 67% compared to base line after one week of treatment with the devices (p<0.006). In addition, there was a statistically significant reduction in the rheumatologists' GADA rating (p<0.0005). Nearly all patients offered "extremely positive feedback" concerning the benefits obtained with the devices and elected to continue using the devices on completion of the study. The limitations of the study included the lack of a placebo or control condition and a high ratio of female to male participants (8:1).

Holcomb et al.⁴⁴ conducted a case study of two adolescents with debilitating, drug-resistant, chronic pain of the low back and abdomen with intermittent pain of the genitalia, diagnosed on MRI with intervertebral disc disease. Both patients had undergone multiple evaluations by several specialists and surgery without pain relief. In both patients, treatment with quadripolar magnetic arrays provided rapid relief of symptoms that was sustained for more than two years. The devices were taped to the skin over the pain associated spinal levels. One patient reported a rapid 90% reduction in pain while the other reported a "rapid and notable" (not quantified) reduction in pain. Adjusting the placement of the magnetic devices controlled recurrent pain. Holcomb et al.⁴⁴ reported that one of the patients gradually decreased his dependence on the devices and remained virtually pain free for the following 24-month follow-up period. Although the results seem remarkable and describe application of the devices in a clinical setting, the anecdotal nature of the results in single case reports means they cannot be used to extrapolate more broadly.

Holcomb et al.⁴⁴ claim that the success of quadripolar magnetic arrays is a common experience with more than 2000 people being treated with the magnetic devices, alone or in combination with medication, for low back pain over a period of ten years. The authors state that approximately 80% of patients received sufficient benefit to continue treatment. Many became pain free within minutes to hours, while others took weeks to months to achieve acceptable pain levels. Approximately 20% of patients with low back pain were reported by Holcomb et al.⁴⁴ to receive no benefit from treatment with quadripolar magnetic arrays. Such impressive claims in the absence of definitive research results require verification under controlled clinical trial conditions.

In addition to the problems identified later in this review, numerous other matters may have confounded the results of studies investigating quadripolar magnetic arrays. These factors include insufficient control over confounding variables, poor study design, insufficient subject numbers and gender inequality. Overall the research on quadripolar magnetic arrays is encouraging however the studies need to be replicated with large randomized controlled trials and by investigators without affiliations and financial interests in the manufacturer of the devices.

In vitro studies

The mechanism of hypoalgesic effects of quadripolar arrays reported in previous studies remains unsubstantiated. Results from in vitro studies suggest that the analgesic effects of quadripolar magnetic arrays are due to strong magnetic field gradients moving membrane components such as voltage-sensitive ion channel proteins, or changing the phosphorylation state of ion channels in sensory neurones, consequently reducing or blocking action potential (AP) firing^42,46.

The first published study of cellular effects using quadripolar magnetic arrays, found that exposure of adult mouse dorsal root ganglion cells in culture to a 10 mT quadripolar field reduced or blocked action potential (AP) firing⁴⁶. AP firing was stimulated by brief 1-3 msec pulses of depolarizing current. The reduced or blocked AP firing was reversible with slow recovery of firing occurring over several minutes. Arrays of four magnets with like polarity (i.e., all positive or all negative) (32-35 mT) reduced AP firing but resulted in fast recovery of firing following removal of the field. An alternating dipolar array (13.7 mT) or a single magnet had no effect. The neurons utilized resembled mechanoceptive and noci-ceptive neurons in humans suggesting the results observed could be applicable to the human nervous system. Complete blockage of APs was achieved in 83% of the 'nociceptive type' neurons and 92% of the 'mechanoceptive type' neurons within 3-7 minutes⁴⁶.

In another study, the same researchers examined the AP blocking effects of quadripolar magnetic arrays and found that 66% of stimuli failed to elicit an AP in neurons in cell culture when exposed to an 11 mT field compared to less than 5% during the control period (p<0.02)⁴². The number of firing failures was maximal after approximately 200-250 seconds of exposure to the field and returned gradually to baseline over 400-600 seconds following removal of the magnets. The authors proposed that a direct or indirect effect on the conformation of AP generating sodium channels could account for these results⁴² however there have been no molecular or cellular studies to confirm this claim.

McLean et al.⁴² determined several features of the biological effects caused by quadripolar arrays. These include the finding that maximal reduction of action potential firing in the quadripolar field required several minutes to evolve (indicating time dependency) and recovery of action potentials occurred over minutes after removal of the field. Other field patterns had different or no effects (p>0.05). A single magnet (88 mT) or two magnets of alternating polarity (28 mT) had no significant effect. To determine if the gradient of the field or the field strength was the principal determinant of the reduction of AP firing, a weaker quadripolar array was produced which had 1% of the field strength of the original array. It was noted that the weaker array reduced action potential firing as much as the stronger array. McLean et al.⁴² propose that the effectiveness of the quadripolar magnetic array is due to the steep gradient between the centre of the array and the magnetic poles and not the strength of the magnet. To date, it seems no in vivo nerve conduction studies have been performed to establish a link between in vitro effects and the analgesic responses observed in pain studies.

In the above studies, quadripolar magnetic arrays were placed at a distance of 0.5 to 1cm from neurons in cell culture. There is no literature to suggest that similar AP blocking effects would occur with the device placed further from neurons. Hence, the findings may not be applicable in vivo when distances between the skin surface, where the device is applied, and the sensory neurons carrying nociceptive or mechanoceptive signals may be greater than 0.5-1cm. This is of particular importance in clinical settings where patients will have varying amounts of soft tissue overlying nerves. Nerves pass close to the skin surface in various locations; however studies of the pain relieving effects of quadripolar magnetic arrays have found relief of pain when applying the device over more deeply located nerves. In Holcomb et al.⁴⁴ case study of two patients with severe pain from intervertebral disc disease discussed earlier, the devices were applied at the skin surface, a distance that may be two or three times greater than that used in laboratory studies. The factor of distance from magnet to target tissue raises the possibility that another mechanism may be responsible for the analgesic effects that have been observed in previous studies.

It is clear that much research remains to be done in order to identify effect mechanisms and clinical outcomes for both static magnets and magnetic arrays. A range of research design and methodology factors that may influence research outcomes is discussed below.

Methodological and design-related issues for static magnet research

The examples of research findings of static magnetic therapy cited herein and the effects on pain related measures demonstrate the disparate nature of magnet studies. Some of the issues influencing research outcomes have been raised in the discussion of quadripolar magnet arrays. Although authors generally have attempted to design randomized, placebo-controlled studies, the variety of approaches used has not resulted in clear outcomes. One of the main factors complicating interpretation of results is that there are many 'dosing' and application variables to consider when applying magnetic therapy, e.g., polarity, field strength and penetration, and perhaps configuration of field patterns.

Despite the general observation that there is some evidence to support the use of magnets for neuromusculoskeletal pain, it is not yet possible to define the application parameters contributing to reported beneficial outcomes. This is exemplified by the fact that in 50% of the studies reviewed, the polarity of the magnets investigated was not stated (or at least unclear) and a variety of magnet types was utilized (e.g., magnet insoles, magnetic discs etc). Moreover, the number of magnets used varies widely from one study to the next (ranging from a single magnet through to 270 individual magnets, and in some cases not specified), as does the method of application (e.g., mattress underlays, necklaces etc).

In relation to magnet characteristics, the issue of field strength is an obvious variable. Few authors have recognized the variable and attempted to describe the field strength at the surface of the magnet device or at a distance from the device in order to quantify how much energy is delivered to the target tissue. A close inspection of the available literature suggests that this factor remains a probable confounding variable in relation to outcomes and would appear to be a mandatory requirement in future design and reporting of magnetic therapy research. In the studies listed in this review, field strength varied from 4 mT to 1080 mT.

The magnet application period is another factor potentially contributing to disparate results. As noted in the available literature in Table 1, application times ranged from as little as 30 minutes up to many weeks of continual application. Clearly this aspect of dosing is critical to clarify, if magnet therapy is to be considered as a legitimate non-pharmacological method of pain relief. Time of magnet application may have a bearing on factors such as immediacy of effect, sensitivity of neural structures (e.g., accommodation) and carry over of responses (which may have a bearing on washout periods in cross-over study designs). All of these factors require further research as contributing elements in studies of efficacy. Additionally, the placement of magnets can be specific (i.e., precisely placed over tender points) or general (as in magnetic blankets).

Arguably, the most important variable in clinical studies is that of an adequate placebo device permitting effective blinding. This factor can be controlled where the study methodology incorporates a short supervised magnet application period such as in a clinical laboratory setting. However, such a methodology may result in an inadequate period of application. Where a methodology tests extended periods over many hours of application over days or weeks, it becomes more difficult to control either accidental or intentional loss of blinding by the research participants, and makes imperative the inclusion of an adequate placebo. To preserve blinding, some researchers have gone to elaborate lengths such as: constructing sham magnets¹², using metal shielded or capped magnets³⁸, demagnetizing active magnets⁴⁷, and deliberate deception of research subjects as to the status of the control group³². The variety of ways in which this aspect of magnet research has been dealt with, is indicative of the problematic nature of this issue – one which needs to be addressed satisfactorily if the therapy is to be adequately investigated.

The disparate nature of the results of magnet research raises the possibility that there is some form of dose-responsiveness related to magnet therapy (perhaps a threshold for dose exists) and that by combining factors such as application time, polarity and field strength improved outcomes may result. Such factors should arguably be studied in cheaper laboratory (e.g., animal or experimental) models of pain to substantiate efficacy and matters related to dosing parameters before continuing with expensive clinical research in patient populations with questionable outcomes.

Conclusion

In a review of the known literature presented herein, it is not yet clear if static magnets have a significant role to play in the effective management of neuromusculoskeletal pain although some of the research is encouraging. If the clinical studies presented in this review are combined (Tables 1 and 2) then 13 of 24 clinical studies investigating neuromusculo-skeletal pain have demonstrated at least some efficacy using static magnetic therapy. However, there are significant issues related to dosing parameters and physical characteristics, as well as effect mechanisms that remain to be clarified prior to conducting further expensive clinical studies which are unlikely to demonstrate an effect until the methodological issues are attended to.

Acknowledgements

The authors are grateful to the Editor (Prof Dr Tania de Fatima Salvini) for the invitation to contribute this review to the Brazilian Journal of Physical Therapy. Thanks and appreciation goes to Prof Dr Nivaldo Parizotto (UFSCar) for his comments and advice on the manuscript.

Received: 12/01/2009

Revised: 21/01/2009

Accepted: 26/01/2009

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Publication Dates

• Publication in this collection
  06 Mar 2009
• Date of issue
  Feb 2009

History

• Accepted
  26 Jan 2009
• Reviewed
  21 Jan 2009
• Received
  12 Jan 2009