Chapter Ten Making it better
Depending on your musculoskeletal specialty, syllabus and personal preference, it may also be appropriate to ‘cherry pick’ certain modalities and ignore others, although, even if you are not intending to use them yourself, you should be aware of their effects as patients who have already undergone these therapies and part of their previous management may well present to you.
Over the last 50 years, many physical therapists have increasingly adopted the use of modalities into their clinical practice. They are useful in that they provide a range of treatment alternatives; however, whilst training in their indications, contraindications and usage is necessarily thorough, the physics behind their interaction with the patient and the machines themselves is often scanty or lacking altogether.
In this final chapter, we shall be taking a brief overview of the most common physical modalities, explaining the principles of how the machines operate and their physiological effects. We shall also briefly summarize their usage and the evidence for efficacy in different conditions.
We have already discussed ultrasonography as a tool for diagnostic imaging; however, it is also the oldest and most widely used of the physical modalities. The term ultrasound refers to cyclic sound pressure with any frequency greater than that of the upper limit of human hearing. Although this limit varies between individuals and genders and decreases with age, it is approximately 20 kHz (20 000 hertz) in healthy young adults and this figure serves as a useful lower limit in describing ultrasound (Fig. 10.1).
Although therapeutic ultrasound has been used for over 50 years in physical therapy, its application within the clinical environment has changed significantly over the last 20 years. Whereas, in the past, it was employed primarily for thermal effects, this is no longer the case; now it is much more widely used for its physiological effects, especially in relation to tissue repair and wound healing.
Whether used for diagnostic or therapeutic purposes, ultrasonography begins with the production of the sound waves. This takes place in the transducer, the component of the ultrasound equipment that is placed in direct contact with the patient’s body. It contains piezoelectric material, which converts the electrical impulses received from the pulse generator into sound waves. These can be continuous (therapeutic usage) or pulsed (therapeutic and diagnostic usage). In diagnostic imaging, the transducer also contains a second piezoelectric element that, when echo pulses return to the body surface, are converted back into electrical pulses that are then processed by the system and formed into an image.
As with all waves, the ultrasound wave has a wavelength and reciprocally related frequency. It also has amplitude and a velocity that will vary according to the density of the medium through which it is passing (Table 10.1).
The power and intensity of the beam are also considerations: you will recall from earlier chapters that power is the rate of energy transfer and is expressed in units of watts. Intensity is the rate at which power passes through a specified area and is expressed in units of watts per square centimetre. Intensity is the rate at which ultrasound energy is applied to a specific tissue location within the patient’s body. It is the quantity that must be considered with respect to producing biological effects and safety. The intensity of most diagnostic ultrasound beams at the transducer surface is on the order of a few milliwatts per square centimetre; this rises to as much as 2 Wcm−2 for therapeutic purposes.
In order to be of medical use, it is necessary to provide a medium through which the ultrasound can freely pass in order to reach the patient’s tissues. Unless this is done, ultrasound will be reflected at the metal/air interface found at the transducer head. This medium is referred to as a coupling medium, and several different types are used in practice including water, various oils, creams and gels. Ideally, the coupling medium should be sufficiently fluid to fill all available spaces, relatively viscous so that it stays in place and should allow the transmission of ultrasound waves with minimal absorption, attenuation or disturbance.
Once the beam enters the body, it can be absorbed, transmitted or reflected by the tissues therein. We have already dealt with this in regard to ultrasonographic imaging in Chapter 9; however, there are additional considerations for therapeutic usage.
In order to have a therapeutic effect, absorption of the applied energy is necessary; therefore the effectiveness of the modality will vary according to a tissue’s capacity to absorb the applied energy. The rate of absorption is related to protein content; tissues with a higher protein content (e.g. ligaments, tendons and scar tissue) will absorb ultrasound to a greater extent whilst tissues with high water and low protein content (e.g. blood and fat) absorb much less energy.
These absorption characteristics have also to be balanced against the wave reflection at the tissue surface. Although cartilage and bone also have a high-protein-low-water profile, a significant proportion of ultrasound energy striking the surface of structures composed of these tissues is likely to be reflected.
When ultrasound is absorbed, it generates heat within the absorbing tissue. The amount of heat will depend on the protein content of the tissue and the frequency of the applied ultrasound; the higher the frequency, the greater the absorption rate. A biologically significant thermal effect can be achieved if the tissue temperature is raised to 40–45°C for at least 5 minutes.
Although controlled heating can produce beneficial effects, including pain relief, decreased joint stiffness and increased local blood flow, ultrasound is relatively inefficient at generating sufficient thermal change to achieve a therapeutic effect at commonly applied clinical doses, and there are tissue heating methods available in clinical practice that are better at achieving the desired thermal changes.
A number of trials and meta-analyses produced during the 1980s and 1990s increasingly challenged the routine use of therapeutic ultrasound, showing it to be no better or, in some cases worse, than placebo for the treatment of a number of acute musculoskeletal injuries, as well as more chronic problems such as osteoarthritis, most probably owing to its heating effects, which can increase local inflammation.
At the same time, interest started to become focused on the use of ultrasound as a rehabilitative tool to assist tissues that were healing. The production of scar tissue is a complex series of cascaded, chemically mediated events that results in the deposition of collagen at the site of tissue damage.
The effects of ultrasound during the repair process are thought to vary, according to the primary events that are occurring in the tissues. Once tissue bleeding has ceased (ultrasound can increase blood flow, presumably through heat effects, and thus perpetuate this phase of injury response), it is appropriate to start using ultrasound as soon as is feasible. During the inflammatory phase, ultrasound has a stimulating effect on the mast cells, platelets and white blood cells, which mediate the inflammatory response; however, this intervention is not intended to increase the inflammatory response as such (though if applied with too greater intensity at this stage, this can be a complication) but rather to ‘optimize’ the inflammatory response, which is essential for the effective repair of tissue and inhibition of which can inhibit the repair phases that follow.
Dosage, intensity, pulsation, timing and the preferential absorption of the damaged tissue are all factors that must be considered carefully to make the initial earliest repair phase as efficient as possible, and thus have a promotional effect on the healing process as a whole. Despite the tendency for osseous structures to reflect rather than absorb ultrasound waves, it appears that the modality can also be of use in reducing fracture healing time.
During the proliferative phase, in which scar tissue is produced, ultrasound can also have a stimulative effect; the targets now are the fibroblasts, endothelial cells and myofibroblasts, the cells that activate collagen production in wound healing. Pulsed ultrasound appears to be more effective in encouraging fibroplasia and collagen synthesis than a continuous dose application.
Ultrasound may also have a beneficial role during the remodelling phase of repair, which can last for up to a year after the original injury. The proliferative stage produces a ‘generic’ scar, which is then refined so that it adopts some of the functional characteristics of the tissue that it is repairing. Although a scar in tendon will not become tendon itself, it will behave more like tendinous tissue. This is achieved mainly by the orientation of the collagen fibres in the developing scar and also to the change in collagen type, from predominantly type III to type I collagen, which has greater mobility and higher tensile strength.
It is important to emphasize that therapeutic ultrasound does not appear to alter the body’s normal physiological response, rather it optimizes and enhances it, thus reducing healing time and compensating for inhibition to the normal healing response.
By contrast to ultrasound, the application of cooling agents to an area of injury aims to limit the inflammatory response and its vascular, mechanical and biochemical nociceptive consequences. Cryotherapy is usually applied by means of a gel pack, containing material that remains pliant at the temperatures generated by domestic freezers (−20° to −26°C) or by the application of cooling gel, a substance with a high latent heat of vaporization, which achieves a similar effect by carrying heat away from the injured area as it evaporates.
There is good evidence that this approach, when used in the acute phase of injury, can reduce pain and ischaemia and increase mobility and return to normal activity; this may be at the expense of slightly longer secondary healing phases. Dosage and application need to be carefully monitored, particularly when the procedure is being performed by the patient in an unsupervised manner: 5 minutes for superficial and 15 minutes for deep structures appears to represent the maximum beneficial period for application and a latent period of 30–90 minutes for the local physiology to normalize also appears optimal.
The most common complication of cryotherapy is soft tissue damage from over-cooling. This can produce epidermal damage (‘cold burns’) and local ischaemic effects – it is therefore important for the clinician to monitor the duration and frequency of application as well as ensuring that ice packs are not applied directly to the unprotected skin.
There are a number of techniques in physical therapy that involve the application of an electromagnetic field of varying strengths to a variety of tissues. Generally, these fields are induced by high-frequency alternating currents and the application is either via pads placed on the skin over the area to be treated or, in less portable machines, by a transducer head that can be positioned on or directly above the skin.
Diathermy is simply the process of producing heat in the body by the application of external energy, most usually using electromagnetic fields, although, technically, ultrasound is also a form of diathermy.
However, with short-wave diathermy, the energy levels are substantially higher than with ultrasound; typically up to 100 times more energy is applied and the technique is therefore used to apply ‘deep heat’ to areas where the thermal effects of ultrasound are considered ineffective either because of the size of the area involved or the depth of the tissue.
Most typically, two pads are applied to the area and a high frequency current generated to produce electromagnetic radiation in the 25–50 MHz range, that of shortwave radio (hence the name). This produces a heating effect; however, given the comments above regarding heating effect and the detrimental action it can have when used inappropriately, it is not perhaps surprising that use of diathermy as a therapeutic modality has declined in recent years.