Overview of Modalities

Modalities are methods used by therapists to exchange energy with tissues with the goal of creating a therapeutic response. Passive modalities may include the application of heat, cold, sound waves, electricity, and electromagnetic waves to effect changes in tissue structures such as muscle, fascia, ligament, tendon, capsule, and nerve. Modalities comprise adjunctive treatments included as part of a rehabilitation program; they are rarely used in isolation. Modalities are most useful in treating acute pain where the specific underlying musculoskeletal abnormalities can be matched with an appropriate intervention. For most patients, the long-term use of modalities is discouraged, since they may reinforce passive coping behavior, and strong evidence for their lasting effectiveness in chronic conditions is lacking.

Heat and Cold

Therapeutic heat transfer occurs by one or a combination of mechanisms: radiation, conduction, convection, conversion, and evaporation. Radiation is the transfer of heat through thermal radiation at the surface. Conduction is heat exchange through direct contact. Convection is characterized by transfer of thermal energy through movement in a fluid medium, although the therapeutic energy exchange still occurs through conduction. Conversion occurs when a different type of energy is converted to heat energy. Evaporation results in loss of heat when a liquid on a surface undergoes a phase transition into a gas. These mechanisms may be used alone or in combination to transfer heat to or from tissues, resulting in physiologic changes. Of these mechanisms, only conversion can be used to transfer heat to structures deeper than several centimeters beneath the surface. Remaining mechanisms are able to provide only superficial exchange of thermal energy.

The application of heat causes improved elasticity of soft tissue as well as increased blood flow, metabolic activity, enzymatic activity, oxygen demand, and capillary permeability. Nerve conduction velocity increases with the application of heat. The heated tissues become suppler, and there are increases in healing cells and nutrients and decreases in metabolic waste. However, heat can also increase edema and bleeding. There is some evidence from animal models that heat improves chronic inflammatory conditions but may aggravate acute inflammation. The target temperature for these modalities is generally accepted to be 40°C–45°C, and the thermal pain threshold is normally about 45°C. Using the patient’s pain response to a modality can prevent excessive or even damaging temperatures, as long as the sensorium is intact. Box 58.1 summarizes the indications for heat modalities used for musculoskeletal pain management. Box 58.2 lists general contraindications and precautions for the use of therapeutic heat.

Superficial heat causes the greatest increase in temperature at the surface of the skin, with less heat penetrating to the deep tissues—about 1°C at a depth of 2–3 cm. Heat is often applied using hydrocollator packs, a variety of fluid baths, and infrared lamps. Hydrocollator packs are heated to 74.5°C. Several layers of towels are used to prevent burning of the skin and minimize loss of heat to the air. Although heat therapy is often used as an adjunct to rehabilitation programs, the evidence for its effectiveness is mixed and appears to depend on the specific condition being treated, the chronicity of symptoms, and the outcome measure being evaluated. When evidence from three randomized controlled trials involving 179 patients with knee osteoarthritis was reviewed systematically, it was determined that hot packs had no effect on edema compared with placebo or cold application. In looking at the use of heat before isokinetic exercises among women with knee osteoarthritis, heat was found to lead to improved exercise performance, reduced pain, and improved function. When used for low back pain a Cochrane review determined that there is moderate evidence that continuous heat wrap therapy reduces pain and disability in the short term, in a mixed population with acute and subacute low back pain (up to 3 months), and that the addition of exercise to heat wrap therapy further reduces pain and improves function. Immersion of body parts in water of around 40°C is another way of applying superficial heat, which allows for therapy to take place during heating. Paraffin baths are typically used for peripheral limbs, especially the hands and arms. Temperatures around 53°C are used because paraffin transfers less heat than water does. Most commonly this modality is used for rheumatoid arthritis, and while the evidence is insufficient to draw definitive conclusions regarding its efficacy, a recent systematic review did find that it safely relieves pain and stiffness immediately after its application; when followed by exercise it can significantly improve hand function. Superficial heat leads to mild analgesia and a sense of relaxation, but the mechanisms remain unclear and systematic reviews suggest that further well-designed studies are needed to draw definitive conclusions about its effectiveness in the treatment of painful musculoskeletal conditions.

Ultrasound waves, shortwaves, and microwaves can safely penetrate deep into tissues before the energy they carry is converted to thermal energy. Ultrasound diathermy (as distinguished from diagnostic ultrasound) is the only method commonly used at this time. It can easily heat the bone-muscle interface up to 45°C, even in deep structures such as the hip. Ultrasound generators convert electrical energy into vibratory energy through the piezoelectric properties of a crystal transducer. When ultrasound vibrations are directed into tissue, they generate heat based on the water and protein content of the tissue and in areas of transition between tissue densities, such as at the interface between bone and muscle. Tissues that heat poorly due to high water concentration are fat and skin, and tissues that heat well due to high protein concentration are ligament, tendon, muscle, bone, and nerve, with bone and nerve heating the most. Ultrasound is safe for use near metal implants because the heat energy is rapidly conducted away, but caution must be used near prosthetic cements, which do not release heat as easily. Ultrasound may also cause gaseous cavitation and acoustic streaming effects that do not transmit thermal energy but may increase tissue pressures and cellular metabolism and disrupt cell membranes. Duration of treatment is 5–10 minutes and is based on the size of the treatment area. Although ultrasound diathermy has superior deep-heating capability, it does not produce the same degree of analgesia or relaxation as superficial heat modalities. Box 58.3 lists some common uses and Box 58.4 lists precautions for ultrasound.

Despite a long history of clinical use, the evidence for the effectiveness of ultrasound diathermy in the treatment of musculoskeletal disorders is largely lacking. A systematic review of five small trials totaling 341 patients with knee osteoarthritis evaluating a mixture of pulsed and continuous ultrasound found that treatment with this modality compared with control decreased pain by 1.2 cm on a 10-cm visual analog scale and also positively affects function without causing any adverse events. However, the poor quality and heterogeneity among the included studies suggest that further investigations are needed to draw more definitive conclusions. When evaluated in the treatment of soft tissue complaints such as lateral epicondylitis, shoulder pain, degenerative rheumatic disorders, ankle distortions, temporomandibular pain, or myofascial pain, most studies do not support clinical or statistical differences in favor of ultrasound therapy. When studied specifically in the treatment of myofascial trigger points, the evidence is mixed, with one study demonstrating that low-dose ultrasound evokes short-term segmental antinociceptive effects and others suggesting that ultrasound provides no reduction in pain in this population.

The application of cold through conduction, convection, or evaporation results in loss of heat from tissues, leading to vasoconstriction followed by vasodilation, decreased local metabolic activity, decreased enzymatic activity, and decreased oxygen demand. Tissues and muscles become stiffer, nerve conduction slows, and the activity of muscle spindles and Golgi tendon organs decreases. Muscle isometric strength increases and the rate of muscle fatigue decreases. Cold also results in analgesia and relaxation. Boxes 58.5 and 58.6 summarize general indications and contraindications for cryotherapy.

Cold is often used during the first 48 hours after an acute musculoskeletal injury to decrease inflammation, edema, and pain. The application of cold should not exceed 30 minutes and should not be placed directly over superficial nerves so as to prevent neurapraxia. Cold is normally applied in ice packs at −12°C with towels layered over the pack to protect the skin. As with the application of superficial heat, the surface of the skin is affected first and most, but after 20 minutes, tissues 2 cm deep are cooled by about 5°C. Although the local application of cold is known to decrease the temperature of the skin as well as superficial and deeper tissues, the evidence to support its use is not strong. A systematic review of cryotherapy in the treatment of acute soft tissue injuries found marginal evidence that ice in addition to exercise is most effective after ankle sprain and postoperatively, with few studies assessing the effectiveness of ice on closed soft tissue injuries. In patients with osteoarthritis, ice massage improved range of motion, function, and knee strength but did not significantly impact pain. Immersion in cold water (5°C–13°C) can be used but is generally poorly tolerated, although muscle temperatures can decrease by about 6°C after 30 minutes of immersion. Vapocoolant spray is used for cutaneous anesthesia and is used by some practitioners in conjunction with passive stretching to treat myofascial pain. Evaporation of the spray induces cutaneous cooling, with postulated cutaneosomatic reflex effects at the level of the muscle spindle.

Cold and heat can be used together in contrast baths, with alternating warm and cold immersion, to cause cyclic vasodilation and vasoconstriction, with beneficial effects hypothesized for pain from rheumatologic and neuropathic conditions.

Thermal modalities should be used in conjunction with exercises for motion and flexibility. The long-term effects of most of these treatments on functional outcomes and range of motion are minimal when used alone.