Thermal or heat modalities have been popular treatments for soft tissue dysfunction and pain management for centuries. Hot baths, infrared lamps, water bottles, moist warm towels, warm compresses, ultrasound, and hot moist packs have been conventionally used for the treatment of muscle spasms and chronic pain. This chapter discusses the effects of heat on soft tissue and reviews the various types of heat modalities.
Physiologic Effects of Local Application of Heat
Heat energy applied to a localized area of the body produces changes not only at the site of application, but also in surrounding tissues and in distant regions. The resultant changes counter the threat to thermal equilibrium and assist the local structures to avoid tissue damage. The local effects are caused by the elevated temperature response of cellular function by direct and reflex action. The rise in temperature is accompanied by an increase in blood flow. Dilatation of the capillaries on exposure to local heat leads to an elevation of hydrostatic pressure and an increase in capillary permeability. The increase in permeability leads to an increase in leukocytic and antibody infiltration in the area, which is an important factor in controlling inflammation and promoting early healing. However, an increase in hydrostatic pressure may cause edema of the desired region. This effect would contraindicate the use of heat in conditions where swelling or edema is to be avoided.
Superficial heat remodels connective tissue by three mechanisms. First, heat creates several physiologic changes through subsequent vascular changes, causing vasodilation. This dilation gives an analgesic effect, which increases the patient’s pain threshold and results in greater tolerance of connective tissue stretching. Second, physiologic effect of heat is an alteration of the viscous flow properties of collagen, which results in the relief of joint stiffness. Third, there is an increase in the extensibility of collagen tissue, a major component of connective tissue, through changes in its viscoelastic properties following the application of heat.
In general, the transfer of heat (whether the purpose is heating or cooling) often is classified into four general types of heat transfer: conduction (hot pack, paraffin bath), convection (fluidotherapy, hydrotherapy), conversion (radiant heat), and radiation. The most commonly used in clinical application is conduction, through the agency of water. The level of the tissue temperature (usually 40 to 45° C), duration of the tissue temperature increase, rate of increase in the tissue temperature, and the size of the area being treated determine the extent of the physiologic response to heat.
Commonly Used Heat Modalities in Present Clinical Setting
Hot Packs or Hydrocollator Packs
Hot packs or hydrocollator packs contain silica gel which is heated to a temperature of about 170° F. These packs are placed in a thermostatically controlled hot water tank. The silica gel absorbs a large quantity of water and has a high heat capacity. Hot packs are applied over 4 to 6 layers of towels between the pack and the patient. The recommended treatment time is for 20 to 30 minutes. The method of heating is through conduction. Heat transmitted to the patient increases during the first few minutes of treatment as the towels get heated.
To prevent burning, adequate layer(s) of towel should be added during the duration of treatment and the patient should never lie on the packs because the patient’s body weight could squeeze hot water out of the pack into the towel and potentially cause a burn. It is highly recommended to check the patient every 5 minutes. Indications for the application of hot packs may include painful muscle spasms, abdominal muscle cramping, menstrual cramps, and superficial thrombophlebitis .
Contraindications may include peripheral vascular diseases, skin eruptions at the site of application, desensitized skin, new skin, and obtunded sensorium of the patient.
When applying the hot pack, have the patient remove clothing from the area to be treated. The patient is set in a comfortable position. The area to be treated can be supported or elevated when necessary. Apply one or several layers of towels to the area.
Determine the wrapping procedure to be used. Apply the hot pack to the area to be treated. Cover it with a dry towel. Check the patient every 5 minutes ( Figs. 44-1 to 44-3 ).
Ultrasound is the deepest heating modality used in physical therapy. The term ultrasonic refers to sound waves or vibrations that have a frequency above the audibility of the human ear. Therapeutic ultrasonography uses a frequency range of 0.8 to 1.0 MHz.
Ultrasonographic energy is generated by the piezo-electric effect. Electrical energy is applied to a crystal causing the crystal to vibrate at a high frequency and thereby to produce ultrasound waves. Ultrasound is delivered by continuous or pulsed high-frequency sound waves and provides a high heating intensity. The sound waves vibrate tissues deep inside the injured area. This creates heat that draws more blood into the tissues, which institutes the reparative process.
Ultrasound is the optimal form of heat treatment for soft tissue injuries. It is used to treat joint and muscle sprains, bursitis, and tendonitis. Ultrasound can produce many effects other than just the potential heating effect. It has been shown to cause increases in tissue relaxation, local blood flow, and scar tissue breakdown. The effect of the increase in local blood flow can be used to help reduce local swelling and chronic inflammation, and, according to some studies, promote bone fracture healing. The intensity or power density of the ultrasound can be adjusted depending on the desired effect. Knight and associates studied the effects of superficial heat and deep heat including ultrasound on the extensibility of plantar flexors. The results indicated that among the modalities tested, the use of ultrasound for 7 minutes prior to stretching may be the most effective for increasing ankle dorsiflexion range of motion (ROM).
The equipment used for therapeutic application of ultrasonic energy consists of a generator of high-frequency current and an applicator, also referred to as an ultrasound head or transducer head . The generator produces electric oscillations of the desired frequency, which cause the head to vibrate and generate sound waves. The application needs a coupling medium between the transducer head and the skin such as ultrasonic gel or water. This allows the ultrasonic waves to pass through air and air-free contact must be established between the ultrasound head and the area being treated. The gel is applied to the area being treated and the applicator is gently moved over the skin with light pressure.
Stroking motion is applied to larger areas such as the back or neck and circular motion is applied for smaller areas such as the shoulder, hip, or knee. Because the ankle/foot and wrist/hand have irregular surfaces, an underwater technique is recommended. When this technique is used, the ultrasound head is kept an inch away from the area being treated and slowly moved in a circular fashion. This would allow a uniform distribution of energy on the area being treated.
Ultrasound can also be used in phonophoresis. This treatment involves the application of a topical antiinflammatory. The antiinflammatory medication can be mixed with the ultrasound gel and applied to the area using the ultrasound head. The ultrasonic sound waves force the medication to migrate into the tissues, thereby reducing inflammation.
A typical ultrasound treatment will take from 3 to 5 minutes depending on the size of the area being treated. For underwater treatment, time is progressive, beginning at 5 minutes and increasing to a maximum of 8 to 10 minutes per treatment. The dosage depends on the intensity and time, and the product of these factors is usually expressed in watt-minutes. The intensity of sound energy available from the ultrasound head is usually expressed as watts per square centimeter. Therapeutic intensities of 0.5 to 2.0 watts per square centimeter have been determined to be useful.
Ultrasound treatment series is considered to be 6 to 12 applications, applied daily according to some authorities. Others say, every other day. It is also important to remember that when using this modality in acute conditions, low intensity treatment is required. Chronic conditions will require a higher intensity. Ultrasound treatment, just like any other deep heating modality is used to (1) relieve pain and inflammation, (2) speed healing, (3) reduce muscle spasms, and (4) increase ROM. This treatment modality is commonly used for joint contractures, joint adhesions, calcific bursitis, hematoma resolution, and neuromas. Other conditions that may benefit from ultrasound treatment include fibrosis, phantom limb pain, myofascial pain, reflex vasodilatation, and ulcer débridement. However, the therapeutic benefit maybe limited in these later conditions. Warden and associates have shown that conventional therapeutic ultrasound as traditionally used by physical therapists may be used to accelerate fracture healing.
Ultrasound treatment is contraindicated in the pregnant uterus, laminectomy sites, and ischemic areas. Application of ultrasound over surgically implanted hardware remains controversial and when performed in these areas it should be done with caution ( Figs. 44-4 and 44-5 ).