MODALITY REFRESHER LASER By Rebecca Day, MCSP

What it is Laser is an acronym for light amplification by stimulated emission of radiation. The simplest explanation is that the laser uses light therapy to affect tissues of the human body.

The laser beam is a result of an electron being stimulated to move from a higher to a lower energy state (1).

Photons (packets of

energy) cause the electron to excite. Therefore laser is a form of photochemical energy which is short-lived and is non-thermal. Application of the laser beam is via either a single visual red light beam probe or a cluster of high intensity infrared light emitting diodes surrounding one central laser.

Wavelength and penetration Light, once directed at the tissue may be reflected, transmitted, scattered and absorbed (2). There are many arguments in the lit- erature concerning the penetration depths in human tissue.

It

varies according to the wavelength, which affects the scattering and absorption of light, but levels of between as little as 1-4 mil- limetres are generally accepted (1). A wavelength of between 650- 1000 nanometres (nm) has a depth of penetration of between 2- 3mm. This varies between human subjects due to factors such as the condition of the skin, age, skin hydration, blood and fat con- tent (3). To give an anatomical example the biceps femoris tendon is accepted as being five millimetres below the surface of the skin.

The energy produced by laser light, measured in joules, is relative- ly low compared to other modalities such as ultrasound.

For the

wavelengths used by low level lasers almost ninety-nine percent of the radiation penetrating the skin will be absorbed in the first 3.6mm of tissue (2). Another theory is that only 20 percent of the energy penetrates the epidermis. However it is thought to be pos- sible that the chemical processes initiated at more superficial lev- els may mediate effects at a deeper level (4).

Clinical trials consist of invitro and invivo studies, which although extensive have concentrated on the physiological effects of laser on cellular wound healing. A 660nm wavelength laser with a power density of 120mW/cm2 had a statistically significant effect on cells by increasing the fibroblast proliferation compared to a sham-irra- diating laser (laser placebo) (5).

Laser has also been used at various wavelengths to investigate the different effects on cells (6). This study revealed that wavelengths of 660nm, 820nm and 870nm all had the effect of encouraging macrophages to release factors that stimulated fibroblast prolifera- tion above control levels. However a wavelength of 880nm either inhibited the release of these factors or encouraged the release of some inhibitory factors. Few other studies have investigated the effects of different wavelength dosages.

The spectrum most commonly used by physiotherapists includes wavelengths from 660nm to 850nm.

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SUMMARY ● What is it? A form of low energy light therapy ● How does it work? Affects cell activity by speeding up

the healing process at lower energy levels compared to other modalities ● Limitations? Laser at the therapeutic wavelengths used (650-850 nanometres) is thought to be unable to pene- trate further than about 2-3.5 millimetres ● When is it most effective? Superficial wounds and scars particularly to the skin, or tendons/ligaments that have little tissue covering them ie. at joints ● When is it least effective? On tissues deeper than 3.5 millimetres ie. many muscles and tendons ● Suggested dosages? Wavelengths of 660nm to 850nm and power densities of 4-12 joules per cm2 ● Advantages? Low energy means that it is non-thermal and therefore causes no heating within the tissues thereby limiting swelling.

It also has no contraindications and therefore is the safest modality

How it is thought to work Much of the research over the past twenty years has indicated the potential of laser but only in the facilitation of the wound heal- ing process (3). Wound healing is a complicated interactive, inte- grative process involving cellular and chemotactic activity.

As mentioned laser light is non-thermal and has a photochemical effect on cells causing changes in their environment and metab- olism. Literature studies reveal a lack of basic information on the precise biological mechanisms by which laser may stimulate the healing process of inflammation, proliferation and remodelling.

Findings include enhancement of leukocyte phagocytosis, wound healing acceleration by increased activation of collagen production, encouragement of macrophages to release factors that stimulate fibroblast proliferation, catalase activity increased in leukocytes, increased epithelial activity and increased capillary formation (7).

An overview of the literature supports that laser interacts with cells such as macrophages and mast cells optimising the inflam- matory process, potentially accelerating healing (5).

The energy is absorbed initially within the cell, by the mitochon- dria, triggering a series of intracellular events, which lead even- tually to a membrane permeability change. This permeability change works to promote a cellular environment that is conducive to healing.

Conditions benefiting from laser Out of the eight papers examined, seven had positive results with lasers on conditions ranging from rheumatoid arthritis in hands (2), osteoarthritis in knees (8) and shoulder tendonitis (9). Decreases in perceived pain have also been reported after treat- ing tennis elbow with laser (10).

Unfortunately the validity of many of the studies was compro- mised by flaws in their trial design. For example many of the sub- ject numbers were small (in the trials quoted the highest number was 45), in one study pain was estimated by an interviewer and