short wave diathermy

Ultrasound Therapy

 

 

 

 

Therapeutic Ultrasound Thermal and Non Thermal Effects Overview
 

One of the therapeutic effects for which ultrasound has been used is in relation to tissue healing. It is suggested that the application of US to injured tissues will, amongst other things, speed the rate of healing & enhance the quality of the repair (Watson 2006). The following information is intended to provide a summary of some of the essential research in this field together with some possible mechanisms through which US treatments may achieve these changes. It is not intended to be a complete explanation of these phenomena or a comprehensive review of the current literature.  It may, none the less, provide some useful basic information for clinical application. Some of the 'wider' applications for ultrasound therapy (e.g. drug delivery, chemotherapy potentiation) are usefully reviewed in Paliwal and Mitragotri (2008).

The therapeutic effects of US are generally divided into: THERMAL & NON‑THERMAL.

Thermal

In thermal mode, US will be most effective in heating the dense collagenous tissues and will require a relatively high intensity, preferably in continuous mode to achieve this effect.

Many papers have concentrated on the thermal effectiveness of ultrasound, and much as it can be used effectively in this way when an appropriate dose is selected (continuous mode >0.5 W cm-2), the focus of this paper will be on the non thermal effects. Both Nussbaum (1998) and ter Haar (1999) have provided some useful review material with regards the thermal effects of ultrasound. Comparative studies on the thermal effects of ultrasound have been reported by several authors (e.g. Draper et al 1993, 1995a,b, Leonard et al 2004) with some interesting, and potentially useful results. Further work continues in our research centre with a comparison of contact heating and longwave ultrasound (Meakins and Watson, 2006) and comparison of different US regimes combined with US (Aldridge and Watson – in preparation)

It is too simplistic to assume that with a particular treatment application there will either be thermal or non thermal effects. It is almost inevitable that both will occur, but it is furthermore reasonable to argue that the dominant effect will be influenced by treatment parameters, especially the mode of application i.e. pulsed or continuous. Baker et al (2001) have argued the scientific basis for this issue coherently.

Lehmann (1982) suggests that the desirable effects of therapeutic heat can be produced by US. It can be used to selectively raise the temperature of particular tissues due to its mode of action.  Among the more effectively heated tissues are periosteum, collagenous tissues (ligament, tendon & fascia) & fibrotic muscle (Dyson 1981).  If the temperature of the damaged tissues is raised to 40‑45°C, then a hyperaemia will result, the effect of which will be therapeutic. In addition, temperatures in this range are also thought to help in initiating the resolution of chronic inflammatory states (Dyson & Suckling 1978). Most authorities currently attribute a greater importance to the non-thermal effects of US as a result of several investigative trials in the last 15 years or so.

Non-Thermal

The non-thermal effects of US are now attributed primarily to a combination of CAVITATION and  ACOUSTIC STREAMING (ter Haar 1999, 2008 Baker et al 2001, Williams 1987). There appears to be little by way of convincing evidence to support the notion of MICROMASSAGE though it does sound rather appealing.

CAVITATION in its simplest sense relates to the formation of gas filled voids within the tissues & body fluids. There are 2 types of cavitation ‑ STABLE & UNSTABLE which have very different effects. STABLE CAVITATION does seem to occur at therapeutic doses of US.  This is the formation & growth of gas bubbles by accumulation of dissolved gas in the medium.  They take apx. 1000 cycles to reach their maximum size. The `cavity' acts to enhance the acoustic streaming phenomena (see below) & as such would appear to be beneficial. UNSTABLE (TRANSIENT) CAVITATION is the formation of bubbles at the low pressure part of the US cycle. These bubbles then collapse very quickly releasing a large amount of energy which is detrimental to tissue viability. There is no evidence at present to suggest that this phenomenon occurs at therapeutic levels if a good technique is used. There are applications of US that deliberately employ the unstable cavitation effect (High Intensity Focussed Ultrasound or HIFU) but it is beyond the remit of this summary.

ACOUSTIC STREAMING is described as a small scale eddying of fluids near a vibrating structure such as cell membranes & the surface of stable cavitation gas bubble (Dyson & Suckling 1978). This phenomenon is known to affect diffusion rates & membrane permeability. Sodium ion permeability is altered resulting in changes in the cell membrane potential. Calcium ion transport is modified which in turn leads to an alteration in the enzyme control mechanisms of various metabolic processes, especially concerning protein synthesis & cellular secretions.

The result of the combined effects of stable cavitation and acoustic streaming is that the cell membrane becomes ‘excited’ (up regulates), thus increasing the activity levels of the whole cell. The US energy acts as a trigger for this process, but it is the increased cellular activity which is in effect responsible for the therapeutic benefits of the modality (Watson 2000, 2008, Dinno et al 1989, Leung et al 2004).

MICROMASSAGE is a mechanical effect which appears to have been attributed less importance in recent years. In essence, the sound wave travelling through the medium is claimed to cause molecules to vibrate, possibly enhancing tissue fluid interchange & affecting tissue mobility. There is little, if any hard evidence for this often cited principle.