Ultrasound (US) is a form of mechanical energy (not electrical), and therefore, strictly speaking, not really electrotherapy at all, but does fall into the Electro Physical Agents grouping. Mechanical vibration at increasing frequencies is known as Sound Energy. The normal human sound range is from 16 Hz to something approaching 15-20,000 Hz (in children and young adults). Beyond this upper limit, the mechanical vibration is known as ultrasound. The frequencies used in therapy are typically between 1.0 and 3.0 MHz

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. 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. 

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. 

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. If the temperature of the damaged tissues is raised to 40‑45°C, then a hyperemia 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.

USES:

• Accelerate healing
• Increase extensibility of collagen – stretch scars and breaks adhesions
• Pain reduction

 Non-Thermal

The non‑thermal effects of US are now attributed primarily to a combination of cavitation and acoustic streaming. 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. Unstable 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. 

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. 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’, 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. 

Micro massage is also an important mechanical effect. In essence, the sound wave travelling through the medium is claimed to cause molecules to vibrate, possibly enhancing tissue fluid interchange & affecting tissue mobility.

Ultrasound Application in relation to Tissue Repair

The process of tissue repair is a complex series of cascaded, chemically mediated events that lead to the production of scar tissue that constitutes an effective material to restore the continuity of the damaged tissue.

 Inflammation

During the inflammatory phase, US has a stimulating effect on the mast cells, platelets, white cells with phagocytic roles and the macrophages. For example, the application of ultrasound induces the degranulation of mast cells, causing the release of arachidonic acid which itself is a precursor for the synthesis of prostaglandins and leukotriene – which act as inflammatory mediators. By increasing the activity of these cells, the overall influence of therapeutic US is certainly pro-inflammatory rather than anti-inflammatory. The benefit of this mode of action is not to ‘increase’ the inflammatory response as such, but rather to act as an ‘inflammatory optimizer’ (Watson 2007). The inflammatory response is essential to the effective repair of tissue, and the more efficiently the process can complete, the more effectively the tissue can progress to the next phase (proliferation).

A further benefit is that the inflammatory chemically mediated events are associated with stimulation of the next (proliferative) phase, and hence the promotion of the inflammatory phase also acts as a promoter of the proliferative phase. Employed at an appropriate treatment dose, with optimal treatment parameters (intensity, pulsing and time), the benefit of US is to make as efficient as possible to earliest repair phase, and thus have a promotional effect on the whole healing cascade. For tissues in which there is an inflammatory reaction, but in which there is no ‘repair’ to be achieved, the benefit of ultrasound is to promote the normal resolution of the inflammatory events, and hence resolve the ‘problem’.

Proliferation

During the proliferative phase (scar production) US also has a stimulative effect (cellular up regulation), though the primary active targets are now the fibroblasts, endothelial cells and myofibroblasts (Watson 2007).

These are all cells that are normally active during scar production and US is therefore pro-proliferative in the same way that it is pro-inflammatory – it does not change the normal proliferative phase, but maximizes its efficiency – producing the required scar tissue in an optimal fashion. Harvey et al. demonstrated that low dose pulsed ultrasound increases protein synthesis and several research groups have demonstrated enhanced fibroplasia and collagen synthesis. Recent work has identified the critical role of numerous growth factors in relation to tissue repair, and some accumulating evidence has identified that therapeutic US has a positive role to play in this context and also with heat shock proteins. 

Remodeling

During the remodeling phase of repair, the somewhat generic scar that is produced in the initial stages is refined such that it adopts functional characteristics of the tissue that it is repairing. A scar in ligament will not ‘become’ ligament, but will behave more like a ligamentous tissue. This is achieved by a number of processes, but mainly related to the orientation of the collagen fibers in the developing scar and also to the change in collagen type, from predominantly Type III collagen to a more dominant Type I collagen. The remodeling process is certainly not a short duration phase – research has shown that it can last for a year or more – yet it is an essential component of quality repair. 

The application of therapeutic ultrasound can influence the remodeling of the scar tissue in that it appears to be capable of enhancing the appropriate orientation of the newly formed collagen fibers and also to the collagen profile change from mainly Type III to a more dominant Type I construction, thus increasing tensile strength and enhancing scar mobility . Ultrasound applied to tissues enhances the functional capacity of the scar tissues.

What is ultrasound used for?

1. Recent injuries and inflammation

• Soft tissue injuries due to mechanical effect which helps to remove traumatic exudate and decrease danger of adhesion formation.
• Analgesia produced by US makes the condition more tolerable.
• Accelerated protein synthesis stimulates the rate of repair of damaged tissues.

2. Scar tissue

• Makes scare tissue more pliable which allows for more effective stretching of scars.
• If the scare is attached to underlying tissue, US will assist in gaining its release.

3. Chronic oedema
4. Pain relief

• Mechanical action on larger nerve fibres closes the pain gate therefore leading to pain relief.

5. Phonophoresis

• Induction of chemical agents. For example: It can be used to drive hydrocortisone ointment into the tissue of patients with arthritis, joint or muscle pathology.