ChiroACCESS Article



Therapeutic Ultrasound: A Review of the Literature



This information is provided to you for use in conjunction with your clinical judgment and the specific needs of the patient.

Daniel A. Martinez, MA, DC, Research Scientist

  

ChiroACCESS



Published on

October 1, 2010

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Electrical Stimulation Machine with UltrasoundUltrasound (US) has been a widely used and accepted adjunct modality for the management of many musculoskeletal conditions.  It was first introduced as a therapeutic modality in the 1950s, when both animal and human studies demonstrated its ability to safely heat tissue several centimeters below the skin.  In the late 1960s and 1970s, reports on the non-thermal therapeutic effects of US, primarily in the area of enhanced tissue healing, further bolstered its popularity  (1).  Despite the years of clinical use, the lack of studies confirming its benefits has led scientists to question the traditional view of its therapeutic benefits  (2).
 
Several papers reviewing the available literature have been published concerning the biophysical effects, application, and efficacy of therapeutic ultrasound, as well as the safety and calibration of ultrasonic equipment (2-7).  The purpose of this paper is to present a general overview of these findings.

Uses



According to a survey of orthopedic certified specialists, the most common uses for US were to decrease soft tissue inflammation, increase tissue extensibility, enhance scar tissue remodeling, increase soft tissue healing, decrease pain, and decrease soft tissue swelling.  Other uses were to deliver medication for soft tissue inflammation, pain management and soft tissue swelling (1).  

Biophysical Effects



Therapeutic ultrasound is the use of alternating compression and rarefaction of sound waves for therapeutic benefit.   When ultrasonic energy is induced into an attenuating material such as tissue, the amplitude of the wave will decrease with distance.  This attenuation is due to either the absorption or the scattering of sound waves (8).  Ultrasound therapies can be divided into “high” power or “low” power therapies where high power applications include high intensity focused ultrasound (HIFU) and lithotripsy;  low power applications include sonophoresis, sonoporation, gene therapy, and bone healing (4).

Biophysical effects of ultrasound are traditionally separated into 2 types: a thermal effect from absorption and a non-thermal effect from scattering.  The absorption of the ultrasonic sound energy leads to tissue heating.  The scattering is thought to be that portion of ultrasonic energy that changes direction and leads to the non-thermal effects (9).

Thermal effect: When ultrasound travels through tissue a percentage of it is absorbed, leading to the generation of heat within that tissue.  The basis of HIFU is to raise the temperature of tissue to a therapeutic level.   The amount of absorption depends upon the nature of the tissue, its degree of vascularization, and the frequency and intensity of the applied ultrasound.  Tissues with high protein content absorb ultrasound more readily than those with high fat content.  Thus, tissue with high water content and low protein such as blood and fat content absorb little of the US energy while those with lower water content and higher protein content such as ligament and tendon will absorb US more efficiently. Although cartilage and bone have the highest protein content, their densities cause problems with wave reflection and a significant proportion of US energy striking their surfaces is more likely to be reflected.  The best absorbing tissues in terms of clinical practice are those with high collagen content: ligament, tendon, fascia, joint capsule, and scar tissue (4;10).  Another key point is that the higher the US frequency, the greater the absorption rate.  Raising the temperature above normal thermal levels by a few degrees may have a number of beneficial physiological effects (4).  Most common frequencies used are in the range from 0.7 to 3.3 MHz (4).  Maximum energy absorption depth in soft tissue is between 2 to 5 cm (3).

Non-thermal effect: Therapeutic ultrasound produces a combination of non-thermal effects that are difficult to isolate from the thermal (11).  Non-thermal effects have been divided by ter Haar into cavitations and other mechanical effects such as acoustic streaming and micro streaming (4).  Cavitations refer to the behavior of bubbles within an acoustic field.  They are defined as the physical forces of the sound waves on micro-environmental gases within a fluid.  As the sound waves propagate through the medium, the characteristic compression and rarefaction causes microscopic gas bubbles in the gas to contract and expand.  The thought is that the rapid changes in pressure in and around the cell may alter the function of the cell.  Acoustic streaming has been described by ter Haar as “localized liquid flow in the fluid around the vibrating bubbles (12).  This has been defined as the physical forces of the sound waves that provide a driving force capable of displacing ions and small molecules.  At the cellular level, organelles and molecules of different molecular weight exist.  Many of these are free floating and may be driven to move around more stationary structures (11).  Microstreaming is set up in the fluids around acoustically driven bubbles.  This purports to lead to shear stresses on cell membranes in the vicinity, which can create transient pores through which ions and molecules may be transported (4). 

This increased permeability of both individual cell membranes and the endothelium is thought to enhance therapeutic uptake, and can locally increase the activity of drugs by enhancing their transport across membrane (13).  
 

Acoustic Spectrum



Audible sound is what humans hear in the approximate frequency range between 20Hz and 20 kHz.  The ultrasound frequency range starts at a frequency of about 20 kHz.  Most medical equipment operates in the ultrasonic frequency range between 1 to 15 MHz. (8).  Therapeutic ultrasound has a frequency range of 0.75-3 MHz, with most machines set at a frequency of 1 or 3 MHz.  Low-frequency ultrasound waves have a greater depth of penetration but are less focused.  Ultrasound at a frequency of 1 MHz is absorbed by tissues at a depth of 3-5 cm and is recommended for deeper injuries and in patients with more subcutaneous fat.  A frequency of 3 MHz is recommended for more superficial lesions at depths of 1-2 cm (14).

Thermal Dose



Healthy cellular activity depends upon chemical reactions occurring at the proper location at the proper rate.  The rate of chemical reactions and thus of enzymatic activity are temperature dependent.  An immediate consequence of a temperature increase is an increase in biochemical reaction rates. When the temperature becomes sufficiently high (i.e., approximately = 45° C) enzymes denature (8).  With this in mind, therapeutic applications require that the exposed target tissue undergoes reversible or irreversible change, depending on the goal of the treatment (4).  Application in a clinical environment is a combination of the selectable machine parameters:  frequency, power density, duty cycle and treatment time.  Ultrasound dosages are also varied by alteration of wave amplitude and intensity (Watts/cm2) which can be pulsed or continuous.  Continuous ultrasound has a greater thermal effect but either form at low intensity will produce non-thermal effects (14).  Studies utilizing continuous and pulsed frequencies at 1MHz and 3MHz confirm that ultrasound results are time and dose dependent.  The 3 MHz frequencies increased tissue temperature at a faster rate than the 1MHz frequency (11).  For example, a 2004 in vivo study concluded that pulsed ultrasound (3MHz, 1.0 W/cm2, 50% duty cycle, for 10 minutes) produces similar intramuscular temperature increases as continuous ultrasound (3MHz, 0.5 W/cm2, for 10 minutes) at a 2-cm depth in human gastrocnemius muscles (15). 

Coupling Media



Sound waves are transmitted through a round-headed wand that is applied to the skin; however, the waves may be reflected at the metal/air interface found at the treatment head.  It is therefore necessary to provide a medium through which the ultrasound can pass freely to reach the tissue without absorbing or changing the direction of the waves.  This medium should be sufficiently fluid to fill all available spaces, relatively viscous so that it stays in place, have impedance appropriate to the media it connects, and should allow transmission of ultrasound energy with minimal absorption, attenuation or disturbance (3).  For therapeutic applications, a number of methods are used to couple the sound into the tissue.  Where the acoustic window is relatively flat, and the transducer’s emitting surface is flat, aqueous gel may be used between the source’s front face and the skin.  For irregular tissue surfaces and/or irregular transducers, water may provide a better coupling medium (4).  In 2004, Casarotto et al compared 4 coupling media: gel, mineral oil, white petrolatum, and degassed water for density and temperature variation.  The results showed that the water and gel presented the highest transmission coefficient, the lowest reflection, and an attenuation coefficient and acoustic impedance close to that of the skin. However, when using direct contact and thin layers of coupling agents, any product may be used, because the effect of the attenuation coefficient does not play a significant role when layers are very thin (16).

Calibration and Safety



The need to measure and calibrate physiotherapy ultrasound machines was indentified in the early 1960’s, and specification standards for carrying out such measurements were put in place by the International Electro-technical Commission (IEC) (7).  In 2003, Daniel and Rupert tested 45 ultrasound units at various chiropractic clinics and found that 44% of the units failed either calibration or electrical safety inspection (17).  The calibration standard for power output is monitored by the FDA code of federal regulation title 21, part 1050.10 which states that temporal-average ultrasonic power shall not exceed ± 20% for all emissions greater that 10 % of the maximum emission (18).  The IEC standard for physiological equipment includes two limits for the purpose of patient protection.  The first limits the temperature of the frontal face of the transducer to no more than 41° C when operated under water with initial temperature 25°C.  A test involving three, 3 minute cycles is specified.  The second limit applies to ultrasound intensity.  The effective intensity shall not exceed 3 Wcm2.  Extended exposure of tissue to this intensity causes temperature increases which can result in tissue damage, particularly at the surface of bone.  Protection of non-target tissue regions is achieved from appropriate placement of the beam  (6). 

Discussion



In a 2001 review of the effectiveness of US, Robertson and Baker concluded that there is little evidence that US is more effective than placebo for treating people with musculoskeletal conditions (2).  Indeed, most of the studies for review could not draw a definitive conclusion because of in sufficient evidence.  Several reviews also reported disagreement and confusion about the most efficacious treatment parameters for US.  Studies concerning the biophysical effects have been performed in vitro and have not been shown to have a clinical effect under therapeutic conditions (2).  However, despite the confusion and disagreement among studies, reports from clinicians suggest that US remains a popular modality and many experienced and advance-practice clinicians continue to use US regularly for specific conditions (1).   This year there have already been two significant publications supporting the use of ultrasound for osteoarthritis of the hip and knee.  A Cochrane systematic review (January 2010) and another recent clinical trial (May 2010) both provided support for therapeutic ultrasound in the management of patients with osteoarthritis. 

Following are conditions reviewed by ChiroACCESS that utilize therapeutic ultrasound.  Full reviews of these conditions can be found under Clinical Care Topics.

Carpal tunnel Syndrome:
One randomized controlled trial (RCT) showed US was effective in the treatment of mild to moderate CTS. The protocol utilized was 0.5 W/cm2 for 10 minutes daily for 4 weeks (19)  At least one other study has been published that found US to be no better than placebo (20). US has been tested in conjunction with laser (21), exercise and splinting (22;23) and demonstrated significant improvement. Dinser et al compared splinting and continuous US at 3 MHz and 1.0 W/cm2 for 3 minutes daily for two weeks to low level laser (LLL) and splinting and splinting alone. In this study US plus splinting was statistically superior to splinting alone but inferior to LLL plus splinting (23). It should be noted two of the above studies mentioned above found a possible negative effect on motor nerve conduction which should be considered prior to treatment (20;22)
 Lateral Ankle Sprain: A 2002 Cochrane review located 5 clinical trials relating to ultrasound therapy in the treatment of ankle sprains. Of the 5 trials, 4 showed no difference in sham and regular ultrasound while one showed improvements in pain and swelling (24).

Myofascial Pain Syndrome: In a 2007 RCT of 44 subjects, low intensity ultrasound (US), 5 minutes, 0.1 W per square centimeter, continuous at 1MHz was compared to high intensity US, 1 W per square centimeter, continuous at 1MHz. There was a statistically significant improvement in pressure pain thresholds in the high intensity group (25). In another RCT published in 2004 (26), a novel application of ultrasound therapy, high power pain threshold ultrasound (HPPTU), was compared to conventional ultrasound. HPPTU is described as continuous mode without movement over the trigger point. Intensity is slowly increased to the level of maximum pain. It is kept at that level for 4 to 5 seconds and then reduced to half-intensity for another 15 seconds. This procedure is repeated 3 times. The authors reported HPPTU was statistically superior to conventional US and required less time. Another RCT of 102 patients found conventional US was as effective as trigger point injections in treating MPS (27). Yet another RCT compared an ultrasound, massage and exercise group to a sham ultrasound, massage and exercise group. Ultrasound conferred no additional benefit to the treatment regimen (28).

Lateral Epicondylitis (LE): Only 1 study was located that demonstrated effectiveness in the treatment of LE. This randomized controlled trial utilizing ultrasound, performed in 1985, showed statistically significant improvement at completion of treatment and at the 1 year follow-up (29). Ultrasound was given at the following dosages:

A. Pulsed @ 1 MHz

B. 1 to 2 w/cm2, increasing as treatment progressed

C. 5 to 10 minutes, increasing as treatment progressed

D. 2 to 3 times/week

E. 4 to 6 weeks


More recent studies have not been as favorable. A recent review found no difference between ultrasound and placebo, although additional studies were recommended (30). A RCT performed in 2006 found low intensity ultrasound was not different than placebo (31). There is no evidence available to suggest phonophoresis is beneficial in the treatment of LE.
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