Nonthermal Effects

The non-thermal mechanisms by which ultrasound may act on tissue have been outlined in Chapter 12. As far as consideration of physiological effects is concerned, these mechanisms may be divided into two classes: cyclic and non-cyclic.

Cyclic effects arise from the periodic nature of the sound pressure field and have been referred to as 'micro-massage' (Summer & Patrick 1964). This oscillatory motion may help in loosening adhesions present in soft tissue injuries.

The main non-cyclic effect thought to produce therapeutic benefit is acoustic streaming (see Chapter 12, Section 12.3.2). This may be due to stable, oscillating cavities or to radiation forces in intra- or extracellular fluids. Acoustic streaming can modify the local environment of membranes, altering concentration gradients and changing the diffusion of ions and molecules across them. Mihran et al. (1990) attributed the change in excitability of myelinated sciatic nerve in frogs to radiation pressure effects. It has been shown that the potassium content of some cells may be reduced following irradiation with ultrasound in vitro (Chapman et al. 1979), although cavitation bubbles may be involved in this type of experiment. Changes in the calcium content of smooth muscle cells may account for the increase in uterine contractions in mice induced by ultrasound (ter Haar et al. 1978), and Al Karmi et al. (1994) demonstrated that the increase in ionic conductance in frog skin induced by ultrasound was influenced by the presence of calcium ions. Mortimer and Dyson (1988) showed that exposure of cultured fibroblasts in vitro to intensities of 0.5-1.0 Wcm~2 increased the amount of intracellular calcium but that 1.5 Wcm~2 did not. It is extremely difficult to identify positively the different non-thermal effects that may arise in tissues, and indeed to isolate such effects from any tissue heating that must necessarily occur when sound is absorbed in tissue. It is probably easiest to identify effects due to cavitation by the criterion that application of increased ambient pressure will suppress them.

Some of the non-thermal effects of ultrasound may be detrimental if care is not exercised in its application. If a reflecting surface is contained within the irradiated volume, standing waves may be set up. If a blood vessel lies within this volume, red blood cell banding may occur, and if this is maintained for a significant length of time, tissues downstream may be deprived of oxygen.

Where cavitation bubbles are formed in tissue, a variety of effects may be observed. As mentioned earlier, streaming motions are set up around a stably oscillating bubble.

13.2.2.1 Increase in Extensibility and Strength of CollagenousTssues

A major factor that often impedes the recovery of soft tissue to injury is contracture associated with the injury, which may lead to an inhibition of normal motion. Slight tissue heating may increase mobility. Lehmann et al. (1970), for example, have described how the application of heat during stretching exercises may increase the elastic properties of collagenous structures. Gersten (1955) has shown that ultrasonic heating can lead to an increase in tendon extensibility. Scar tissue may also be rendered more supple by the use of ultrasonic treatment. Specific applications in physiotherapy are discussed in Section 13.3.

Enwemeka et al. (1990) found that when 0.5 W cm~2 (1 MHz) ultrasound was used for 5 min daily for 9 days to treat severed rabbit tendons, the tensile strength and energy absorption capacity were significantly increased. This was in contrast to the earlier findings of the same group (Enwemeka 1989) in which 1 Wcm~2 was not effective in increasing these properties. This finding - that effects of ultrasound may be more beneficial at low intensities than at higher ones - has been found in other model systems, such as calcium transport across membranes (Mortimer & Dyson 1988; see Section 13.2.2) and in wound healing (Byl et al. 1992, 1993). This may indicate that non-thermal mechanisms of action may also be playing a therapeutic role. Da Cunha et al. (2002) found similar results, showing that pulsed ultrasound (1 MHz; 0.5 Wcm~2 SATA; pulsed 1:4) enhanced healing of the rat Achilles tendon, whereas continuous wave exposure at the same intensity retarded it. Takakura et al. (2002) exposed rat medial collateral ligaments to low-intensity pulsed ultrasound (1.5 MHz; r.f. 1kHz; 200 ms pulses; 30mWcm~2). They found that 12 days following injury the exposed ligaments were stronger than the controls, but that this difference had disappeared by 21 days. The mean diameter of the collagen fibres was greater in the exposed ligaments than in the controls.

13.2.2.2 Decrease in Joint Stiffness

The range of motion of stiff joints may be increased when the contractures around them are heated (Backlund & Tiselius 1967). Ultrasound may be the heating modality of choice when the joint has significant soft tissue cover because its penetration into muscle is better than that of other forms of diathermic energy (Lehmann et al. 1959; Hand & ter Haar 1981).

13.2.2.3 PainRelief

Many patients report pain relief following heat treatment of an affected area. This may be instantaneous and long lasting. Ultrasound appears to be particularly beneficial in producing pain relief in some patients. For example, Rubin and Kuitert (1955) found it useful in producing relief from pain arising from phantom limbs, scars and neuromas. Mechanisms for pain relief are poorly understood and, if this is a localised effect in the tissue, non-thermal effects may be involved.

13.2.2.4 Changes in Blood Flow

Vascular changes are often seen in response to localised tissue heating and may be observed at a distance from the heated volume. It has been shown that muscle blood flow may increase two- to threefold following ultrasonic heating to temperatures in the range 40-45°C (ter Haar & Hopewell 1983). Similar effects have been reported by Paul and Imig (1955). Imig et al. (1954) attributed blood flow changes to local vasodilation and found similar effects with ultrasound and electromagnetic heating. Abramson et al. (1960) have shown blood flow changes following pulsed irradiation. The changes persisted for about half an hour following treatment.

Local dilation increases the oxygen supply and thus improves the environment of cells. This may have therapeutic benefit. An increased inflammatory response also may be seen.

A study of the microvascular dynamics of rat cremaster muscle has shown that, for sufficiently high intensities (>5 W cm~2 in the case quoted), there may be a decrease both in vessel lumen and volume flow in some vessels. This was, however, thought not to be a thermal effect but more likely to be due to cavitation or some other mechanical cause (Hogan et al. 1982).

13.2.2.5 Decrease in Muscle Spasm

Heat may induce a reduction in muscle spasm. This is thought to be a sedative effect of increased temperature on peripheral nerve endings (Fountain et al. 1960). Ultrasound may be used to produce this effect.

The extent of physiological response to heating may depend on a number of factors, including temperature achieved, time of heating, heated volume and rate of temperature rise. Ultrasound provides a method of rapidly heating a well-defined volume. The highly collagenous regions of superficial cortical bone, periosteum, menisci, synovium and capsules of joints, myofascial interfaces, intermuscular scars, fibrotic muscle, tendon sheaths and major nerve trunks are among the anatomical structures that are heated selectively by ultrasound.

In some conditions ultrasound can be the most effective form of thermotherapy (compared with short-wave diathermy, wax baths and infrared) and may be the treatment method of choice (Middlemast & Chatterjee 1978).

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