Hot Topic - An Evidence-Based Look at the Therapeutic Properties of Heat

by Anne Ahlman, MPT

Advance for Directors in Rehabilitation. August 2005;14(8):63-6.
Reprinted with the permission of Advance magazine.

   Therapeutic heat is a useful “means to an end” for many painful complications that you may encounter in the clinic. While they’re rarely used as a sole remedy, thermal modalities can enhance rehabilitation by easing pain during manual therapy and therapeutic exercise.

The degree of options can be confusing, so it’s important to stay informed about the ever-evolving methods of thermal modality applications, as well as the underlying rationale for each option.

However, this can be challenging in the face of available research, since the number of published, controlled clinical studies examining thermal modalities is relatively small. Rehab clinicians should be skilled in assessing patients as individuals, rather than merely an injured body part, which is important when considering an appropriate thermal modality.

In addition, you should be able to understand and explain the mechanism by which a given modality works. It’s not good enough to simply follow clinic protocol or ask an aide to prepare or administer the modality without a thorough knowledge of indications, treatment rationale, precautions and contraindications.

The primary goals of thermal treatment are to reduce pain, improve circulation, increase tissue extensibility and speed healing. You should use ice in the acute phase of therapy to combat inflammation, reduce edema and improve range of motion. But, heat can coax flexibility from stiff and painful soft tissues to enhance functional mobility during subacute and chronic phases.

Here are the most common thermal modality options.

• Superficial heat. Most patients prefer the comfort of heat over ice for nearly every diagnosis. So it’s important to instruct patients on the appropriateness of heat for specific conditions. Common conditions that respond well to thermal modalities are muscle spasms, chronic stiffness, arthritis and certain chronic pain conditions.

Hydrocollator hot packs provide superficial heat up to 1 cm to 2 cm in depth, and may reduce pain, provide relaxation and promote movement. Hot packs heat tissues to a maximal temperature within eight minutes of treatment, so monitor patients closely for signs of excessive hyperemia.¹

While research shows that moist and dry heat are equally effective in the rate of thermal transfer and soft tissue temperature response for temperomandibular joint pain, moist heat is preferred in a limited number of patients.² You can also select paraffin wax baths to apply heat to small joints in the hands and fingers for short-term relief of arthritic hand pain.³

When treating elderly patients with decreased skin thickness, people with broken skin, and those with diabetes or altered sensation, use superficial heat with caution to avoid burns.

Fluidotherapy is a superficial dry heat modality that involves a warm air stream that moves fine particles against the extremity being treated. Fluidotherapy delivers a greater thermal effect than paraffin wax or traditional hydrotherapy at depths up to 1.2 cm.⁴ This modality also allows you to grade the strength of warm fiber circulation from a gentle to a forceful setting, which can assist with desensitization programs.

• Therapeutic ultrasound. Ultrasound is a form of deep heat that can increase tissue extensibility in “continuous” mode, and can improve tissue circulation by changing cell membrane permeability and boost cellular activity in “pulsed” mode.

Indications include bursitis, tendonitis, myositis, trigger points, and joint capsule and scar tissue contractures. But don’t use ultrasound when a patient has cancer or acute infection, and never apply this modality over the eyes, spinal cord or reproductive tract.

High-frequency ultrasound waves are produced by the contraction and expansion of a piezoelectric crystal via an electrical alternating current. These waves can penetrate tissues to depths of 0.8 cm to 1.6 cm with a 3 MHz sound head, and up to 2.3 cm to 5 cm with the 1 MHz sound head.⁵

Different tissues absorb ultrasound energy waves at different rates—tissue with the highest protein or collagen content tends to absorb the most, such as bone, nerve, muscle, tendon, ligament and fascial tissue. Skin and fat are least absorbent. Because of this, exercise caution when administering ultrasound, since it’s possible to burn deep tissues without causing pain. This occurs by using incorrect parameters or by improperly using the sound head—a standing wave or “hot spot” may form if you don’t keep the sound head moving. Also, use adequate pressure to avoid an “air gap” between the sound head and skin, which inhibits proper transmission of sound waves.

Current research results are inconclusive as to the effectiveness of therapeutic ultrasound, though several studies point to its potential benefits. In one study, continuous ultrasound treatment of the lumbar paraspinal muscles over the facet joints using the 1 MHz sound head was found to significantly heat periarticular tissues within six minutes of treatment.⁶ Benefits of pulsed ultrasound include enhanced ligament repair and improved organization and growth of healing tendon fibers.^7,8

Furthermore, low-intensity pulsed ultrasound applied to delayed-healing and nonunion fractures resulted in an 86 percent success rate of bone healing in a select group of patients.⁹ This method of ultrasound delivery may significantly shorten fracture healing time, as evidenced by X-rays.¹⁰

The safety and efficacy of ultrasound may be affected by treatment frequency, intensity, treatment time, type of tissue being treated, and the interval of servicing and calibration of the machine.

• Deep heat. You can administer heat to deep subcutaneous tissues and muscle groups through diathermy. Shortwave diathermy delivers a high-frequency electrical current through a system of capacitor plates or inductive coils, while microwave diathermy uses electromagnetic radiation to facilitate vasodilation and heat deep tissues and muscles with high water content.

Instruct patients to remain still during treatment, and keep the treatment field free of all metal, such as implants, jewelry, pacemakers and even chairs, which can superheat adjacent tissues.

Since fluids accept this form of heating the most readily, avoid using diathermy over areas of sweating or swelling, or on pregnant women. Pulsed shortwave diathermy can heat large deep muscles, such as the gastrocnemius, which can improve soft tissue extensibility and significantly improve flexibility.^11,12

• Cryotherapy. With acute injuries, swelling or following surgery, cryotherapy is often indicated. Although ice provides a cooling effect that promotes vasoconstriction and edema reduction, it’s still considered a thermal modality due to its ability to change local body temperatures.

Cryotherapy demonstrates a consistent benefit to reduce edema, provide analgesia, slow nerve conduction velocity, and reduce muscle spasm and spasticity. Intramuscular tissue temperatures can be reduced as much as 3 degrees to 7 degrees centigrade with ice packs. In one study of patients recovering from ACL reconstruction, cryotherapy significantly produced an intra-articular decrease in knee joint temperature by 2.7 degrees centigrade, which may help limit postoperative swelling.¹³

Cryotherapy combined with compression following hip or knee replacement surgery resulted in a 50 percent reduction in postoperative analgesic medication use, and a 20 percent increase in range of motion.¹⁴ In addition, using a cryotherapy cuff following knee arthroscopy resulted in significant pain and swelling reduction, which reduced inhibition of the quadriceps extensor mechanism.¹⁵

Ice massage increases quadriceps strength, improves range of motion, and decreases swelling of the affected knee in patients with knee osteoarthritis, probably due to its analgesic effects.¹⁶

However, frostbite is always a risk with ice, especially in patients who have impaired circulation or decreased sensation. Temporary peripheral nerve injury has been noted when ice is applied over superficial nerves or for prolonged time periods.¹⁷

Following cryotherapy treatment, motor function and reflex activity may be impaired for up to 30 minutes.¹⁸ Patients with fibromyalgia, Raynaud’s phenomenon and those with cold hypersensitivity may not be able to tolerate cryotherapy.

• Hydrotherapy. Forms of hydrotherapy can deliver thermal and mechanical effects to tissues, and increase circulation, relaxation and analgesia.

Traditional hydrotherapy uses a whirlpool or immersion tank to increase muscle extensibility and relaxation. Hydrotherapy has the added benefit of significantly reducing the healing time of pressure ulcers.¹⁹

Contraindications for hot tub and whirlpool use include early pregnancy, due to the associated risk of miscarriage during the first five months, and cardiac conditions.²⁰

Dry hydrotherapy is a form of thermal treatment in which the patient lies on a specialized waterproof mattress, yet remains dry and fully clothed. Hydro-jets in the mattress circulate water-filled cells to provide pressure against a patient’s body, which creates a form of heated deep tissue massage. You can access various treatment areas on a patient simply by changing position on the table.

Thermal modalities are a valuable adjunct to active physical therapy treatment. Regardless of approach, however, you should be aware of the theoretical base and appropriateness of your choice of modality. Applying thermal modalities correctly relies on informed selection of the best option, combined with an ongoing reassessment of your patient’s needs.

Anne Ahlman, MPT, is a manual physical therapist, medical writer, and editor based in the San Francisco Bay Area. She has served as Programs Chair for the Northern California Chapter of the American Medical Writers Association.

References

1.      Klein MJ. Superficial heat and cold. eMedicine.com. Last updated Dec 16, 2004. Accessed July 11, 2005. Available at: http://www.emedicine.com/pmr/topic201.htm.

2.      Poindexter RH, Wright EF, Murchison DF. Comparison of moist and dry heat penetration through orofacial tissues. Cranio. 2002;20(1):28–33.

3.      Robinson V, Brossuea L, Casimiro L, et al. Thermotherapy for treating rheumatoid arthritis. Cochrane Database Syst Rev. 2002;(2):CD002826.

4.      Borrell RM, Parker R, Henley EJ, et al. Comparison of in vivo temperatures produced by hydrotherapy, Paraffin wax treatment, and Fluidotherapy. Phys Ther. 1980;60(10):1273–6.

5.      Draper DO, Castel JC, Castel D. Rate of temperature increase in human muscle during 1 MHz and 3 MHz continuous ultrasound. J Orthop Sport Phys Ther. 1995;22:142–50.

6.      Morrisette DC, Brown D, Saladin ME. Temperature change in lumbar periarticular tissue with continuous ultrasound. J Orthop Sports Phys Ther. 2004;34(12):754–60.

7.      Takakura Y, Matsui N, Yoshiya S, et al. Low-intensity pulsed ultrasound enhances early healing of medial collateral ligament injuries in rats. J Ultrasound Med. 2002;21(3):283–8.

8.      Da Cunha A, Parizotto NA, Vidal B de C. The effect of therapeutic ultrasound on repair of the achilles tendon (tendo calcaneus) of the rat. Ultrasound Med Biol. 2001;27(12):1691–6.

9.      Mayr E, Mockl C, Lenich A, et al. [Is low intensity ultrasound effective in treatment of disorders of fracture healing?] Unfallchirurg. 2002;105(2):108–15.

10.  Busse JW, Bhandari M, Kulkarni AV, et al. The effect of low-intensity pulsed ultrasound therapy on time to fracture healing: a meta-analysis. CMAJ. 2002;166(4):437–41.

11.  Draper DO, Knight K, Fujiwara T, et al. Temperature change in human muscle during and after pulsed short-wave diathermy. J Orthop Sports Ther. 1999;29(1):13–18.

12.  Robertson VJ, Ward AR, Jung P. The effect of heat on tissue extensibility: a comparison of deep and superficial heating. Arch Phys Med Rehabil. 2005;86(4):819–25.

13.  Glenn RE Jr, Spindler KP, Warren TA, et al. Cryotherapy decreases intraarticular temperature after ACL reconstruction. Clin Orthop Relat Res. 2004;(421):268–72.

14.  Albrecht S, Le Blond R, Cordis R, et al. [Effects of continuous cryotherapy on the surgically traumatized musculoskeletal system. Perioperative Cryotherapy Study Group]. Unfallchirugie. 1996;22(4):168–75.

15.  Whitelaw GP, DeMuth KA, Demos HA, et al. The use of the Cryo/Cuff versus ice and elastic wrap in the postoperative care of knee arthroscopy patients. Am J Knee Surg. 1995;8(1):28–30.

16.  Brosseau L, Yonge KA, Robinson V, et al. Thermotherapy for treatment of osteoarthritis. Cochrane Database Syst Rev. 2003;(4):CD004522.

17.  Drez D, Faust DC, Evans JP. Cryotherapy and nerve palsy. Am J Sports Med. 1981;9(4):256–7.

18.  Mac Auley DC. Ice therapy: how good is the evidence? Int J Sports Med. 2001;22(5):379–84.

19.  Burke DT, Ho CH, Saucier A, et al. Effects of hydrotherapy on pressure ulcer healing. Am J Phys Med Rehab. 1998;77(5):394–8.

20. Li DK, Janevic T, Odouli R, et al. Hot tub use during pregnancy and risk of miscarriage. Am J Epidemiol. 2003;158:931–7.

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