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Ultrasound in Wound Healing
Author: Joseph McCulloch, PT, PhD
LSU Medical Center, Director of the Physical Therapy Department

Topics Covered Here:

How does ultrasound benefit wound healing?

Frequency

Considerations in the use of ultrasound

Application of ultrasound to wounds

Application of ultrasound to periwound tissue


Ultrasound is a mechanical vibration delivered at a frequency above the range of human hearing. Clinical ultrasound units currently being manufactured typically deliver ultrasound at frequencies of 1 and 3 MHz with duty cycles ranging from 20 to 100 percent. Duty cycles less than 100% are usually termed pulsed ultrasound while a 100% duty cycle is referred to as continuous ultrasound.

How does ultrasound benefit wound healing?

  1. Inflammatory Phase - ultrasound causes a degranulation of mast cells resulting in the release of histamine. Histamine and other chemical mediators released from the mast cell are felt to play a role in attracting neutrophils and monocytes to the injured site. These and other events appear to accelerate the acute inflammatory phase and promote healing.
  2. Proliferative Phase - ultrasound has been noted to effect fibroblasts and stimulate them to secrete collagen 1. This can accelerate the process of wound contraction and increase tensile strength of the healing tissue 2. Connective tissue will elongate better if both heat and stretch are combined. Continuous ultrasound at higher therapeutic intensities provides and effective means of heating deeper tissue prior to stretch.

Frequency

As the frequency of ultrasound is increased, the penetration of the signal decreases. For most dermal wounds, it is preferable therefore, to utilize a frequency of 3 MHz. 1 MHz wound be more effective on deeper structures or periwound skin.

Considerations in the use of ultrasound

As with other medical devices, inappropriate use can result in serious complications. The basic precautions should be taken:

  1. Treat at the lowest intensity that will produce the desired result.
  2. Assure that the applicator is kept in constant motion throughout treatment and that the proper acoustic coupling medium is used.
  3. Reduce the intensity or terminate treatment if the patient complains of any increase in pain.

Application of ultrasound to wounds

  • Prior to ultrasound treatment, remove dressings and clean wound of foreign debris or dressing residue.
  • A hydrogel sheet should be placed in direct contact with te wound bed and wound margins, paying special attention to removing any air bubbles that might be present beneath the dressing. Remove any protective plastic covering on the hydrogel sheet.
  • In cases where a cavity type of wound exists that prevents complete contact between the hydrogel sheet and the wound base, as sterile aqueous hydrogel filler should be used. The cavity is filled with the aqueous gel and then covered wit the hydrogel sheet. Remove all underlying air bubbles between the aqueous hydrogel and hydrogel sheet.
  • Once the hydrogel sheet is in place, apply an ultrasonic coupling gel on top of the sheet. Select the appropriate sized applicator. (the area treated should be no larger than 1.5 to 2 times the size of the applicator. If a relatively large wound is being treated, it would need to be divided into sections.
  • Set the ultrasound machine to a 20 % duty cycle at a frequency of 3 MHz.
  • Applying light pressure, the sound head is placed in contact with the coupling medium on top of the hydrogel sheet and moved in a slow and deliberate manner using either a linear or circular technique.
  • Set the intensity to less than 0.5 watts per square centimeter (usually 0.3 watts/cm 2). Reduce the intensity if the patient experiences pain or heat.
  • Treatment duration is suggested to be about 1 to 2 minutes per zone.
  • Acute wounds are treated 1 to 2 times per day until acute symptoms (inflammation) subside. Treatments can then decrease to 2 to 3 times per week

Application of ultrasound to periwound tissue

When it is desired to provide mild heating to the periwound tissue in order to stimulate circulation, higher intensity ultrasound can be given. Extreme are should be used, however, to assure the tissue is capable of handling the thermal levels delivered.

To treat periwound tissue, 1 MHz, continuous ultrasound is usually employed. Again, an ultrasound applicator 1.5 to 2 times the size of the treatment area should be used. With an aqueous coupling medium in place, the sound head is placed lightly against the skin surface and moved in a slow and deliberate manner. The intensity is typically set to between 1 and 1.5 watts per square centimeter. This parameter is extremely variable and depends on the patient's circulatory, sensory and mental status. Thermal ultrasound should never be used in situations where impaired perception prevents the patient from sensing heat or pain. If at anytime during the treatment the patient should experience an increase in pain, the intensity should be turned down or the treatment discontinued.

Treatment duration is slightly longer than that of pulsed ultrasound since a mild thermal effect is desired. Initial treatment is about 2-3 minutes per zone and can be increased by 30 second increments to a maximum of 5 minutes per zone and delivered 3 times per week.


References:

  1. Harvey, W, et al: The in vitro stimulation of protein synthesis in human fibroblasts by therapeutic levels of ultrasound. Proceedings of Second Congress of Ultrasonic in Medicine. Excerpta Medica, Amsterdam, 1975, p 10.
  2. Dyson, M and Smalley, D: Effets of ultrasound on wound contraction. In Millner, R and Corket, U (eds): Ultrasound Interactions in Biology and Medicine. Plenum, New York, 1983, p 151
  3. Fyfe, MC and Chahl, LA: Mast Cell degranulation: A possible mechanism of action of therapeutic ultrasound. Ultrasound Med Biol 8 (Suppl 1);62, 1982
  4. Hashish, II: The effect of ultrasound therapy on post-operative inflammation, PhD Thesis, University of London, 1966.
  5. Yurt, RW: Role of the mast cell in trauma. In Dineen, Pan Hildick-Smith, G (eds): The Surgical Wound. Lea and Febiger, Philadelphia, 1981, p.37.

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