The correct insertion depth was achieved by measuring the distance from the skin surface to these marks. The depth of insertion for each thermocouple was controlled using a mark made 5 cm from the tip of the thermocouple. The adipose layer thickness was determined through a previously used method 26 of halving the skinfold-thickness measurement. The insertion depth for the thermocouple was 1 cm beyond the adipose layer. The loaded hypodermic needle was inserted perpendicularly through the skin directly in the center of the cleansed area. A sterilized, implantable thermocouple was inserted into a 21-gauge hypodermic needle until the tip of the thermocouple was visible but not protruding into the bevel of the needle. A 2 × 2–cm area of the anterior thigh, midway between the patella and anterior superior iliac spine, was cleansed with an alcohol pad for 10 seconds.
26 During each testing session, subjects assumed a supine position on a standard treatment table. Tissue temperature measurements were made using a protocol previously described by Merrick et al.
An additional purpose was to map the field strength and uniformity for a commercially available magnet. The purpose of our study was to determine whether a temperature change occurs at the skin surface over the quadriceps muscle or at a depth of 1 cm below the adipose layer during a 60-minute application of magnet, sham, or control treatment. 23 If these claims are accurate, there should be a measurable temperature change within the tissues during magnet application.Īlthough therapeutic magnets have been suggested to increase blood flow by increasing tissue temperature, this phenomenon has not been documented experimentally. Additionally, manufacturers claim that their products' magnetic fields have the ability to penetrate tissue more deeply and more safely when compared with other deep heating modalities such as ultrasound or diathermy. If magnets do, in fact, increase temperature and therefore blood flow, they may be effective as a thermal modality. However, the usefulness of these magnets as a thermal modality able to produce clinically significant temperature changes has yet to be examined. Many allied health professionals, as well as many professional, collegiate, and recreational athletes, use these magnets.
Most are constructed of rubber impregnated with a magnetic material and induce low-level, static magnetic fields with a field strength of approximately 0.1 T (1000 G). Today, commercially available “therapeutic” magnets are essentially small sources of magnetic fields. To date, few investigators have examined whether magnetic fields alter blood flow in vivo. In an effort to support their claim that magnetic fields caused vasodilation, magnet vendors reference the work of Pratt and Mishra, 21 which was presented at a symposium but has not been subsequently published. Pratt and Mishra, 21 in work with saline solution and glass tubing, suggested that an ionic solution had greater flow in the presence of a magnetic field than in the absence of the field, independent of the diameter of the tube. In addition to potential vasodilation, blood flow may be altered by another means. This migration against resistance may cause the production of heat that, in turn, would result in blood vessel dilation. 22 The movement of these particles is resisted because the particles are forced to accumulate against their normal direction of flow. The magnetic field–induced voltage produced by this accumulation of charged particles against a concentration gradient is known as Hall voltage. The charged particles in blood may, in the presence of a magnetic field, accumulate toward like poles. The Hall effect is an electromotive force that causes charged particles to accumulate with like charges in the presence of a magnetic field. The theory behind the use of magnetic fields to increase blood flow stems from the physics principle known as the Hall effect. Touting information from unpublished studies, 5, 20 magnet manufacturers and distributors have claimed that magnetic fields are associated with an increase in blood flow. 1, 2 More recently, magnetic fields have been used in the treatment of musculoskeletal injuries, 3 – 5 nerve dysfunction, 6 – 9 pain, 3, 10 – 14 fractures, 15 – 17 and osteoarthritis. The use of magnetic fields to aid the body's healing response dates back to ancient Egypt and Greece.
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