Difference in Tissue Temperature After Therapeutic Ultrasound Using Biofreeze Mixture and Ultrasound Gel as Coupling Media

Biofreeze, a topical analgesic, is commonly used as a coupling medium during therapeutic ultrasound to add greater relief of pain, although it may block transmission of ultrasound energy. Our objective was to compare a mixture of 1-part Biofreeze and 3-parts ultrasound gel to 100% ultrasound gel to assess intramuscular heating and perceived sensation of heat. Fifteen participants received a continuous 3-MHz ultrasound treatment with the Biofreeze mixture or 100% ultrasound gel on 2 diff erent days. We measured intramuscular tissue temperature and visual analog scale scores for heat sensation. The Biofreeze mixture produced a tissue temperature increase similar to the 100% ultrasound gel (P = .32), while producing a greater perceived sensation of heat (P .0001). However, due to Biofreeze’s viscosity, more of the mixture (12.9 2.6 mL) was needed than the ultrasound gel (10 0 mL) to prevent overheating of superfi cial tissues. [Athletic Training & Sports Health Care. 2013;5(x):xxx-xxx.] Therapeutic ultrasound is inaudible, acoustic vibrations of high frequency, sending ultrasound energy into the tissues, thus increasing temperatures deep within the tissues. For ultrasound energy to be absorbed into the tissues, a coupling medium is required. The most common type of coupling medium is ultrasound gel. It is mainly composed of distilled water and inert, nonrefl ective material that increases the viscosity of the mixture. A thin layer of ultrasound gel between the skin and the sound head serves 2 purposes. First, it allows an effi cient transfer of ultrasound energy to enter the target tissues by minimizing the air between the sound head surface and tissues.1,2 Second, ultrasound gel acts as a lubricant during an ultrasound treatment.1 Topical analgesic gels and creams have been used as ultrasound coupling media. Topical analgesics are used for the same purpose as common ultrasound gel, while also providing an added benefi t of a counterirritant to the treatment area. Topical analgesics provide a counterirritant to the treatment area by using the active ingredients of menthol, methyl salicylate, or capsaicin.1 The active ingredients in topical analgesic gels and creams stimulate thermoreceptors, which is similar to the effect of when either cold or heat is applied, thus creating a sensation of cold or heat.3 These sensations, in turn, have been shown to reduce the amount of pain perceived by patients.4,5 Although topical analgesics are used as coupling media, they are not always successful at allowing the transmission of ultrasound energy from the device into the tissues. Many topical analgesics have been shown to have poor transmission of ultrasound energy,6,7 but some have qualities that may aid in the better transmission of ultrasound energy. First, aqueous gel–based analgesics have been shown to allow the transmission of ultrasound energy better than cream-based agents.7 Second, using a mixture of topical analgesic and ultraThe authors are from the Exercise Sciences Department, Brigham Young University, Provo, Utah. Received: January 7, 2013 Accepted: August 15, 2013 Posted Online: December 9, 2013 The authors thank The Hygenic Corporation (Akron, Ohio) for funding this