Superficial Heating Agents

Superficial heating agents include hydrocollator packs, air-activated heat wraps, paraffin, heating pads, and Fluidotherapy (Chattanooga Medical, Chattanooga, Tennessee). These agents can increase tissue temperature as much as 2 cm in depth; therefore, they work well in the hand and wrist. The depth of penetration will depend on (1) the physical agent used, (2) the composition of the target tissue, (3) the duration of treatment, (4) the initial temperature difference, and (5) how long the agent stays hot.

Tissue temperatures must be elevated between 40°C and 45°C (104°F–113°F) to achieve therapeutic benefit. Above this range, there is the potential for tissue injury and below this range the therapeutic benefits may not be achieved because the heating is considered to be mild. The proposed benefits of therapeutic heating include vasodilation or increased blood flow, decreased pain, increased tissue extensibility, and decreased muscle tension.

The increase in blood flow is limited primarily to the skin located beneath the heat source. Vasodilation occurs in response to the heat to regulate tissue temperature to prevent a burn. There is a secondary vasodilation response in coordination with a spinal cord reflex that is stimulated by the heating of cutaneous afferents. No change in skeletal muscle blood flow is expected. Along with the increased blood flow, metabolic rate (cellular activity) and oxygen saturation within the local tissue increase. Although these effects are only observed while the tissue temperature is elevated, these hemodynamic and metabolic effects may facilitate tissue healing.

Elevation of tissue temperature creates some temporary neuromuscular effects that may reduce pain and muscle tension. These changes only occur when the tissue temperature is elevated and maintained. As tissue temperature decreases, the neuromuscular effects will reverse. Sensory nerve conduction velocity will increase with elevated temperature. Because the increase in temperature is temporary, the therapeutic benefit is unclear, but it may contribute to a reduction in pain. Elevation of muscle temperature decreases the firing rate of the muscle spindle afferents (type II afferents) and increases the firing rate of the type Ib afferents from the Golgi tendon organs. These changes in firing rates will lead to a decrease in alpha motor neuron activity and thus a reduction in muscle (extrafusal fiber) activity. Superficial heat may not sufficiently increase muscle temperature to achieve these neuromuscular effects, but the heating of skin does decrease gamma efferent firing, which will elicit a similar response. Basically, skeletal muscles heated to at least 42°C (107.6°F) will relax and be more flexible. The therapeutic benefits may be decreased pain and increased range of motion, especially if muscle guarding or spasm is present.

There are several proposed mechanisms for pain modulation with heat modalities. The counterirritant theory suggests that thermal receptors in the target tissue are activated and conduct impulses about temperature increases in the tissue environment more quickly than impulses from pain fibers. This physiologic response is related to the gating mechanism of pain. Increases in tissue temperature also increase the activation threshold of pain fibers (nociceptors), which may reduce the number of pain signals sent to the spinal cord. The elevated threshold of nociceptors may also be related to a decrease in chemical mediators within the target tissue that either activate or sensitize nociceptors. The increased blood flow associated with temperature elevation may remove these chemical mediators and decrease nociceptor activity in the local tissue.

Tissue extensibility also increases with temperature increases. The viscosity of ground substance in collagen connective tissues such as joint capsule, ligaments, and tendons decreases. Due to the neuromuscular effects, there is increased muscle flexibility. Although these effects are seen only while the tissue temperature is elevated, the decreased perception of joint stiffness allows the patient to participate in range of motion exercises.

Ultrasound

Ultrasound is considered a deep heating agent. It is probably the most widely used physical agent in hand therapy practice. Treatments are likely to be ineffective if the (1) ultrasound treatment is not focused on the correct structure, (2) dosing parameters are incorrect, and (3) transducer is moved too quickly. Electrical energy is converted into mechanical energy (sound propagation) via the piezoelectric effect. A ceramic or quartz piezoelectric crystal located within the transducer expands and contracts from the applied electrical current ( Fig. 117-2 ). The crystal creates an electric voltage potential that replicates the sound wave pattern determined by the selected frequency. The sound waves are initially propagated longitudinally into the target tissue and then transversely at bone or metal interfaces. These sound waves leave the transducer as a collimated focused beam similar to a flashlight. The larger the sound head, the more collimated the beam. Molecules and cells within the target tissue expand and contract to produce vibration ( Fig. 117-3 ). The vibration increases kinetic energy of the molecules, which increases tissue temperature, provided the ultrasound was delivered in the continuous mode with sufficient intensity.

The shaded area is the piezoelectric crystal (transducer) mounted in the ultrasound applicator. The effective radiating area (ERA) of the transducer is noted.

(Diagram with permission of Henley International, Sugar Land, Texas.)

Diagram of a collimated ultrasound beam (B) coming from an ultrasound (US) applicator. The associated pressure wave is diagrammed above. Areas of increased molecular concentration (:::) are condensations. Areas of decreased molecular concentration are rarefactions (:.:).

(From Ziskin MC, McDiarmid T, Michlovitz SL. Therapeutic ultrasound. In: Michlovitz SL, ed. Thermal Agents in Rehabilitation , 2nd ed. Philadelphia: FA Davis, 1990.)

Frequency determines the depth of penetration within the target tissue. The collimated beam is more divergent with the 1-MHz transducer, which will transmit sound waves through the superficial tissue layers so that the ultrasound energy will be absorbed in deeper tissues at 2 to 5 cm. Energy is absorbed in the superficial tissue layers up to 2 cm with the 3-Hz sound head. Ultrasound is absorbed within tissues with a high protein content (e.g., collagen); thus, collagen-rich connective tissues such as tendon, ligament, and joint capsule absorb sound waves well.

Acoustic impedance occurs at tissue interfaces such as bone. Sound waves will be reflected away from the bone either in the opposite direction or transversely. Standing waves may occur if the longitudinal incident waves are superimposed on reflected sound waves. Standing waves are known as “hot spots” and have the potential to cause tissue damage. Excessive heating of the periosteum at the bone interface may also cause pain due to standing waves. If this occurs during treatment, the patient usually pulls away from the transducer and reports pain. This is a clinical sign of standing wave formation, and treatment should be modified or discontinued for that session. Standing waves can be avoided if the transducer is kept moving, if the 3-MHz frequency is selected, and if the intensity is not too high.

The effective radiating area (ERA) is the portion of the transducer that actually produces and emits sound waves. The beam nonuniform ratio (BNR) compares the maximum point intensity on the transducer (spatial peak intensity) with the average spatial intensity across the sound head (spatial average intensity). The lower the BNR is, the more evenly distributed the energy from the transducer. Optimally, the BNR should be 1 : 1, but manufacturing guidelines require the BNR to be 6 : 1 or lower. Most units on the market today have an ERA equal to the size of the transducer face plate with the perimeter emitting less energy than the center of the face plate and a BNR of 6 : 1. The manufacturer specifications or equipment manuals supply this information. A variety of sizes of applicators are available for use, ranging from 0.5 to 5.0 cm ² .

Ultrasound waves will markedly attenuate in air. Therefore, air between the applicator face and body surface must be eliminated or minimized. A coupling medium such as a commercially available water-soluble gel is spread in a layer between the applicator and skin surface. If a small irregular area is to be treated with ultrasound, a small applicator with a gel pad interface can be used. Coupling through a water bath or water-filled balloon is less efficient.

Therapists have the option to select continuous or pulsed-wave ultrasound treatments. With pulsed-wave ultrasound, the intensity is periodically interrupted so that no ultrasound energy is being produced during the off time in the duty cycle. Continuous-wave ultrasound produces thermal and nonthermal effects, whereas pulsed-wave ultrasound emphasizes the nonthermal effects of ultrasound because heat does not really accumulate. This is a result of the dissipation of heat by conduction during the off time of the pulse period. Duty cycles may be calculated by dividing the duration of the pulse (on time) by the pulse period (on time + off time). Therefore, if the pulse is 2 seconds of a 10-second pulse period, the duty cycle would be 20%. Figure 117-4 shows a typical pulsed-wave ultrasound pattern. The physiologic effects of a particular duty cycle are unclear. Low-duty cycles of 10% or 20% have been studied in chronic wound healing, but no other work has been demonstrated at high-duty cycles.

A typical pulsing pattern. The total pulse period is 10 msec. The pulse duration is 2 msec. The duty cycle is 0.20 (20%).

(From Ziskin MC, McDiarmid T, Michlovitz SL. Therapeutic ultrasound. In: Michlovitz SL, ed. Thermal Agents in Rehabilitation , 2nd ed. Philadelphia: FA Davis, 1990.)

Spatial average intensity is the rate at which ultrasound energy (in watts) is delivered and averaged over the area of the transducer (in square centimeters). Intensity is the strength of the ultrasound treatment. All factors held constant, the greater the intensity is, the greater the tissue temperature elevation with continuous ultrasound. Most units display spatial average intensity. When using pulsed-wave ultrasound, the temporal average intensity can be calculated by multiplying the duty cycle by the spatial average intensity. For example, if the spatial average intensity selected was 2.0 W/cm ² and the duty cycle is 20%, the temporal average intensity is 0.4 W/cm ² . Although the temporal average intensity over the pulse period is low, it is still 2.0 W/cm ² during the pulse period, which may be too high for some target tissues in the upper extremity. There are no clear-cut guidelines on how to select a treatment intensity, but the World Health Organization does set an upper limit of 3.0 W/cm ² ultrasound units. The correct intensity will achieve the desired goal and do no harm.

Selecting continuous- or pulsed-wave mode determines whether the thermal or nonthermal effects will predominate during the ultrasound treatment. Thermal effects are related to temperature elevation and are the same as those for the superficial heating agents discussed previously. The two primary nonthermal effects are cavitation and acoustic streaming. Cavitation is the formation of gas bubbles within the cells in response to the vibration within the target tissue. Stable cavitation may enhance cellular diffusion, but unstable cavitation may results in cell implosion and tissue damage. Unstable cavitation can be avoided by selecting the pulsed-wave mode and using lower treatment intensities in the continuous-wave mode. Acoustic streaming is the movement of fluids and gas bubbles within cells in response to vibration. Acoustic streaming is theorized to facilitate cell membrane permeability and ion exchange, which have potential tissue healing benefits.

Ultrasound has all the same clinical indications as the superficial heating agents discussed previously, but it focuses on smaller treatment areas and may target structures greater than 2 cm in depth. Most structures in the wrist and hand can be sufficiently heated to elevate tissue temperature to a therapeutic level, but ultrasound as a deep heating agent is needed for similar indications at the forearm, elbow, and shoulder. Ultrasound may promote tissue healing first by increasing blood flow and oxygenation due to thermal effects and second as a result of the nonthermal effects. Studies of delayed-healing wounds have demonstrated that low-intensity pulsed-wave ultrasound, which emphasizes nonthermal effects, releases growth factors from macrophages, increases angiogenesis, and facilitates collagen production. These benefits promote the fibroplasia phase of tissue healing. During remodeling, tensile strength may increase when thermal ultrasound is combined with controlled stress exercises. Increased tissue extensibility will facilitate collagen reorganization and gains in tensile strength.

Cryotherapy

The physiologic effects of cold are for the most part the exact opposite of those associated with thermotherapy. Table 117-3 reviews these comparisons. Cold is primarily used after acute injury or surgery to control pain and edema. The normal patient response to cold in terms of sensory changes is the feeling of cold followed by burning, pain/discomfort, tingling, and numbness. Although the timing is not standardized among patients, the order of these sensory changes is consistent. It is this author’s experience that patients do not tolerate ice bags or ice baths applied directly to the hand, but do find ice applied directly to the shoulder or elbow tolerable. Commercial cold packs are more commonly used at the hand and wrist.

The application of cold results in decreased blood flow and vasoconstriction in response to decreases in tissue temperature. Vasoconstriction is an immediate response to attempt to regulate temperature. There is a decrease in blood viscosity, which further reduces blood flow. Prolonged cold exposure such as in an ice bath below 10°C (50°F) may result in a reflex vasodilation called the hunting response. This response is usually observed in the skin, but it may occur in deeper tissues as well.

There may also be a decrease in the inflammatory response if cold is applied immediately after injury. Due to the decrease in blood flow, the chemical mediators associated with inflammation such as histamine and prostaglandins may not be able to circulate and promote vasodilation. Decreased blood flow also reduces metabolic activity, including the synthesis of prostaglandins that mediate inflammation. There is also a decrease in oxygen uptake within cells due to reduced metabolic demand. With prolonged cold exposure, this may result in hypoxia and tissue damage, as is observed in frostbite and peripheral nerve palsy.

The mechanisms of pain modulation are similar to those with heat based on the gating mechanism and counterirritant theory. In addition, decreased blood flow may lessen the inflammatory response and further modulate pain. Reduced edema means that there will be less tissue distention and therefore less pain. A decrease in the circulating chemical mediators that promote inflammation and activate or sensitize nociceptors will also modulate pain.

The patient may report an increase in joint stiffness or at least the perception of increased stiffness after the application of cold. The viscosity of the ground substance will increase with decreases in tissue temperature. Although muscle tension will decrease in response to cold, which may reduce muscle spasm, guarding, or spasticity, the muscles will be less flexible to movement. With cold exposure, decreased nerve conduction velocity and muscle spindle and Golgi tendon organ activity may also contribute to decreases in pain.

There are several cold sensitivity symptoms and conditions that need to be addressed before applying cold to hand and upper extremity patients. It is normal for the skin underlying the cold source to become red during cryotherapy because of hyperemia or vasodilation of skin blood vessels. The entire area should be red. However, if wheals or hives develop as a result of a histamine reaction, this is an allergic reaction to cold. It is called local cold urticaria, and the wheals are raised with red borders and blanching in the center. They are usually warm to touch ( Fig. 117-5 ). This is the most common cold sensitivity condition observed, and the cold treatment should cease if a wheal develops.

Wheal formation, or local cold urticaria, after ice massage to the knee.

Systemic urticaria is a systemic allergic reaction that results in flushing of the face, increased heart rate, and a sharp decrease in blood pressure that can lead to syncope or lightheadness. Raynaud’s disease or phenomenon is a vasospastic disorder that occurs in response to cold. It usually affects the fingers, and in response to cold, the fingers will blanch (pallor), become blue/purple (cyanosis), turn bright red (rubor), and then return to normal color. This cyclic response is associated with autoimmune disorders such as lupus erythematosus and rheumatoid arthritis. It has also been associated with nerve compression syndromes and trauma. Patients likely know whether they have ever had any of these responses to cold, so be sure to ask about them when obtaining your patient’s history during the clinical examination.

Cryoglobulinemia is associated with autoimmune conditions, chronic liver disease, multiple melanoma, and infections. In response to cold exposure, an abnormal blood protein forms a gel, decreasing blood flow and resulting in ischemia and eventually gangrene. Paroxysmal cold hemoglobinuria is a condition in which hemoglobin is released and lysed red blood cells can be identified in urine, although the urine is not red. Anemia develops in the patient, which may be mild or severe. Although neither of these cold sensitivity problems are common, therapists should ask patients about these conditions when obtaining their history and monitor the patients’ response to cryotherapy carefully because the sequelae can be severe.