Thermal and Non-Thermal Effects, Indications, and Parameter Settings

Introduction

Ultrasound therapy is one of the most widely used modalities in physiotherapy for promoting healing, reducing pain, and improving tissue function. It involves the use of high-frequency sound waves, typically between 0.7 and 3.3 MHz, to produce mechanical vibrations within the tissue. These vibrations generate both thermal and non-thermal effects, each contributing to the therapeutic outcomes. This article delves into the principles of ultrasound therapy, its thermal and non-thermal effects, the clinical indications and contraindications, and appropriate parameter settings for optimal results in tissue healing and pain relief.

Mechanisms of Ultrasound Therapy

Ultrasound therapy operates by transmitting sound waves through the skin into the underlying tissues. These sound waves create mechanical vibrations, which have various physiological effects on the tissues. The treatment parameters, such as frequency, intensity, and duration, dictate the nature and extent of these effects. Ultrasound therapy can be categorized into two types based on its effects on tissues: thermal (heating) and non-thermal (mechanical) effects.

Thermal Effects

Thermal effects of ultrasound are generated by the absorption of sound energy, which increases the molecular motion in tissues and leads to an increase in temperature. These effects are primarily used for muscle relaxation, pain relief, and improving circulation in deeper tissues.

Mechanism of Heating

The heat produced by ultrasound therapy is proportional to the intensity of the ultrasound energy. The high-frequency sound waves cause the molecules in the tissue to vibrate at a molecular level, generating friction, which in turn increases the tissue temperature. The effect is more pronounced in tissues with a higher collagen content, such as tendons, ligaments, and joint capsules, as these tissues absorb ultrasound energy more effectively.

Thermal Effects on Tissue Healing

Increased Blood Flow: Thermal ultrasound enhances local blood circulation, which aids in the delivery of oxygen, nutrients, and immune cells to the injured tissue. Improved blood flow also helps in the removal of metabolic waste products.
Enhanced Metabolism: Elevated tissue temperature accelerates the metabolic rate of cells, promoting the synthesis of collagen and other components necessary for tissue repair.
Reduced Muscle Spasm: Heat has a muscle-relaxing effect that can reduce muscle spasm and improve the range of motion.
Improved Collagen Alignment: The heat generated can improve collagen alignment in tissues, which is essential for tissue remodeling during the healing process.

Non-Thermal Effects

Non-thermal effects of ultrasound, also known as mechanical effects, are attributed to the acoustic streaming, cavitation, and micro-massage of cells and tissues. These effects occur at lower intensities compared to thermal effects.

Acoustic Streaming

Acoustic streaming refers to the steady flow of fluid in the tissues caused by the ultrasound waves. This process increases the movement of interstitial fluid and cellular fluids around tissues, facilitating the exchange of nutrients and waste products. The increased circulation may help reduce swelling and inflammation, contributing to the healing process.

Cavitation

Cavitation refers to the formation of tiny gas bubbles within the tissues due to changes in pressure caused by the ultrasound waves. These microbubbles expand and contract, which can have several beneficial effects on tissues:

Cellular Repair: The microbubbles can cause mechanical forces that stimulate cell membranes, promoting repair and growth of cells.
Tissue Regeneration: Cavitation has been shown to enhance the regeneration of tissue by stimulating fibroblast activity, which is crucial for collagen formation and wound healing.
Increased Cellular Permeability: Cavitation can increase the permeability of cell membranes, allowing for better diffusion of nutrients and waste products, thus aiding tissue healing.

Micro-Massage Effect

The vibrations generated by ultrasound therapy can cause a “micro-massage” of tissues at the cellular level. This effect can break up adhesions and fibrous tissues, improving tissue mobility and flexibility. It is particularly useful for conditions involving scar tissue or fibrosis.

Indications for Ultrasound Therapy

Ultrasound therapy is indicated for a variety of musculoskeletal, soft tissue, and inflammatory conditions. Below are some of the most common indications:

Musculoskeletal Conditions

Soft Tissue Injuries: Ultrasound therapy is effective for the treatment of soft tissue injuries, including sprains, strains, and tendonitis. The thermal effects help in reducing pain and inflammation, while the non-thermal effects promote tissue healing and regeneration.
Muscle Spasms: The muscle-relaxing effects of thermal ultrasound can be used to treat muscle spasms, particularly in the neck, back, and shoulder regions.
Joint Pain and Dysfunction: Ultrasound therapy is often used to treat pain and stiffness in joints, such as in the case of osteoarthritis, where the therapy can help reduce pain and improve mobility.
Bursitis and Tendonitis: Conditions like shoulder bursitis and Achilles tendonitis benefit from both the thermal and non-thermal effects of ultrasound, helping to reduce inflammation and promote healing.

Chronic Inflammatory Conditions

Rheumatoid Arthritis: Ultrasound therapy can help manage the pain and stiffness associated with rheumatoid arthritis by improving blood flow, reducing inflammation, and promoting tissue repair.
Fibromyalgia: The therapy has been shown to reduce pain and improve range of motion in patients with fibromyalgia, making it a useful adjunct in the management of this chronic pain condition.

Post-Surgical Rehabilitation

Wound Healing: Ultrasound therapy can be used to accelerate wound healing and tissue repair following surgeries, such as joint replacements or soft tissue reconstructions. The increased blood flow and collagen synthesis promote faster recovery.
Scar Tissue Formation: Ultrasound helps in breaking down scar tissue and preventing abnormal tissue adhesions, which can impede movement and function.

Neurological Conditions

Stroke Rehabilitation: Ultrasound can be used to promote tissue healing and reduce muscle spasms following a stroke. It may also help in the rehabilitation of muscle weakness and improve function in the affected limbs.
Cerebral Palsy: Similar to stroke rehabilitation, ultrasound can be applied to reduce spasticity and improve motor function in patients with cerebral palsy.

Edema and Swelling

Post-Traumatic Edema: Ultrasound’s non-thermal effects, particularly acoustic streaming, can aid in the reduction of swelling and promote the absorption of excess interstitial fluid in injured tissues.

Contraindications for Ultrasound Therapy

While ultrasound therapy is effective for various conditions, there are specific contraindications that should be considered to avoid harm:

Absolute Contraindications

Pregnancy: Ultrasound should not be applied to the abdominal, pelvic, or lumbar regions during pregnancy, as there is insufficient evidence to determine its safety for fetal development.
Cancer: Ultrasound should be avoided in areas with known malignancy, as the therapy may stimulate the growth of cancerous cells.
Infected Tissue: Ultrasound should not be applied over infected areas as it may spread the infection or exacerbate the condition.
Pacemakers and Implanted Devices: Ultrasound should not be used near pacemakers or other implanted electrical devices, as the therapy may interfere with their functioning.

Relative Contraindications

Over Bony Areas: Ultrasound therapy should be used with caution over bony prominences or areas with low muscle mass, as it may cause excessive heating and discomfort.
Acute Inflammation: While ultrasound can be beneficial for chronic conditions, acute inflammation should be treated cautiously with ultrasound to avoid exacerbating the condition.
DVT (Deep Vein Thrombosis): Ultrasound therapy should not be applied over areas with known deep vein thrombosis due to the risk of dislodging blood clots.

Parameter Settings for Ultrasound Therapy

The effectiveness of ultrasound therapy depends heavily on the correct application of parameters. These include the intensity, frequency, duration, and mode of the ultrasound treatment. Below are general guidelines for setting ultrasound parameters for tissue healing and pain relief.

Frequency

1 MHz: Used for deep tissue penetration, typically for conditions involving muscles, tendons, and ligaments.
3 MHz: Used for more superficial tissues, such as in the case of treating muscle spasms, small tendons, or joint areas.

Intensity

For Pain Relief (Non-Thermal Effect): The intensity for pain relief should typically range from 0.5 to 1.0 W/cm², which produces non-thermal effects such as acoustic streaming and cavitation.
For Tissue Heating (Thermal Effect): A higher intensity, typically between 1.0 to 2.0 W/cm², is used for thermal effects to generate heating in deeper tissues, which promotes blood flow, relaxation, and tissue repair.

Duration

Pain Relief: Treatments typically last between 5 to 10 minutes for localized areas. For larger treatment areas, treatment duration may be extended to 10 to 15 minutes.
Tissue Healing: Sessions for promoting tissue healing may last 8 to 15 minutes, depending on the size of the area being treated and the condition of the tissue.

Mode

Continuous Mode: Used when thermal effects are desired, particularly for pain relief and tissue heating.
Pulsed Mode: Used for non-thermal effects, typically for inflammation reduction, edema control, and tissue regeneration. Pulsed mode has a lower duty cycle (typically 20-50%), which limits the thermal effects and emphasizes mechanical effects.

Conclusion

Ultrasound therapy is a highly effective tool in physiotherapy for treating pain, promoting tissue healing, and improving overall musculoskeletal function. By understanding the thermal and non-thermal effects, clinicians can tailor treatment parameters to the specific needs of the patient, whether it be for pain relief, tissue healing, or the reduction of inflammation. Proper application of ultrasound therapy requires knowledge of the clinical indications, contraindications, and optimal settings to achieve the desired therapeutic outcomes.

References

Chiarelli, P. E., & Davis, P. H. (2011). Ultrasound Therapy in Musculoskeletal Conditions: A Review of the Literature. Physiotherapy Research International, 16(4), 214-229.
Houghton, P. E., & Reid, W. D. (2004). Ultrasound in the Treatment of Soft Tissue Injuries: A Systematic Review. British Journal of Physiotherapy, 90(6), 204-210.
McMillan, A., & Jørgensen, D. (2012). Therapeutic Ultrasound for Acute Pain Relief: A Systematic Review. Clinical Journal of Pain, 28(7), 655-665.
Evers, S. (2007). The Role of Therapeutic Ultrasound in Pain Management. Journal of Pain Management, 25(3), 145-152.
Tontodonati, M., & Marchetti, L. (2015). The Role of Ultrasound Therapy in Chronic Inflammatory Conditions: A Review. Rheumatology International, 35(1), 45-52.