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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.
- 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.
- 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.
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.
As with other medical devices, inappropriate use can result in
serious complications. The basic precautions should be taken:
- Treat at the lowest intensity that will produce the
desired result.
- Assure that the applicator is kept in constant motion
throughout treatment and that the proper acoustic
coupling medium is used.
- Reduce the intensity or terminate treatment if the
patient complains of any increase in pain.
- 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
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:
- 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.
- 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
- Fyfe, MC and Chahl, LA: Mast Cell degranulation: A
possible mechanism of action of therapeutic ultrasound.
Ultrasound Med Biol 8 (Suppl 1);62, 1982
- Hashish, II: The effect of ultrasound therapy on
post-operative inflammation, PhD Thesis, University of
London, 1966.
- 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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