The Science Behind Laser Therapy

How do lasers work to heal a wide variety of musculoskeletal injuries? It is all about the well-proven basic science. Let’s circle back to your high school science class. You remember those exciting terms like mitochondria, ATP, and photochemical. Think of laser like an energy drink for the cell. Laser light energy stimulates the mitochondria (the powerhouse of the cell) to make more ATP (energy) so the cell can be more active.

Over the last few years, research has been carried out to better understand and evaluate the mechanisms of tissue healing from laser therapy. The goal of this research is to understand the biological and therapeutic effects of laser treatment. This requires unlocking the cellular and molecular mechanisms that underlie the therapeutic effects observed at a clinical level. How does lasering our cells make us feel better?

Why is this research important? It gives us a deeper knowledge of cellular and molecular mechanisms. This allows us to improve protocols and adapt them to different diseases and different patient characteristics. We can also increase the safety of the treatments. And the most exciting part? This research lays the groundwork for new clinical applications—like COVID-19 trials!

Let’s explore what researchers have learned about how laser therapy triggers healing through three processes: photochemical, photothermal, and photomechanical. The “photo” part of all those terms refers to the light emitted by the laser. That light has an impact on three levels: chemical (molecular changes), thermal (heat), and mechanical (cell structure).

Photochemical Effects

The light emitted from lasers has the ability to evoke changes on a chemical, or molecular, level. Photochemical effect results in the activation of mitochondria and an increase in adenosine triphosphate (ATP) synthesis. This results in the acceleration of healing processes. 

Laser light energy stimulates the mitochondria (the powerhouse of the cell) to make more ATP (energy) so the cell can be more active.

There are proteins in our body that are sensitive to light. That comes as no surprise, given we need sunlight to exist on our planet. Different wavelengths result in photochemical effects for different proteins. For example, the 808 nm wavelength significantly increases the activity of the mitochondrial respiratory chain by stimulating the enzyme (protein) cytochrome oxidase. The 905 nm wavelength significantly increases the activity of mitochondrial respiratory proteins I, II, III, IV. The combination of these two wavelengths creates an amplified energy system boosting the metabolism of the cell. In short, laser provides energy for healing.

Photothermal Effects

Light emits heat. This is basic science, as anyone standing outside on a sunny day can confirm. When light (eg. laser beams) is absorbed by an object (eg. tissues), the object converts the short wavelength light into long wavelength heat. This causes the object to get warmer. 

From a therapeutic point of view, the moderate heating of our tissues can have significant effects. The trick with lasers is to have a controlled increase in tissue temperature, while remaining below the threshold of damage. No one wants to leave a laser therapy treatment with blisters, so the beneficial effects of laser-induced heat must be triggered safely.

The photothermal effect of laser therapy speeds healing in three ways:

1. It increases the biochemical reaction speed and tissue metabolism. (In other words, heat amplifies the photochemical effect outlined above.) 
2. It moderates vasodilation (increase of bloody supply) which in turn increases oxygen and nutrient supply and speeds up the removal of catabolites (the results of broken-down molecules). 
3. It decreases the viscosity of fluids, which lowers the rigidity of tissues and lengthens the connective components due to the release of cross-linked collagen fibers. The result? Muscle, capsular, and ligament relaxation. 

Studies show that moderate heating caused by lasers also produces a general relaxation in patients, which induces a general analgesic and soothing effect.

In research undertaken by ASA of their proprietary Class 4 pulsed MLS® laser system, they found that the epidermis is cooled by the ambient air as the laser handpiece is moved. The pulsed laser energy results in a gentle increase in temperature that never exceeds 3-4℃, well below any concerns for thermal tissue damage. This small temperature increase provides the desired photothermal effect discussed above while never putting the patient at risk for complications. This safe and effective technology can even be applied to patients with metal implants without concern for heat energy buildup. So yes, you can go ahead and laser that painful total knee replacement. 

Photomechanical Effects

Here’s where a nifty chain reaction comes into play. The photothermal effect outlined can induce photomechanical effects. Heat puts things in motion. The gentle increase in heat causes mechanical forces that can act on your cells and the stuff around your cells (the extracellular matrix, or ECM).

Here’s the benefits of the photomechanical effect:

1. Generation of indirect, non-destructive mechanical effects by reversible deformation of the ECM. In other words, gentle heat puts the ECM in motion.
2. Regulation of growth and cell differentiation, protein synthesis and ECM production. In other words, when the ECM moves in a coordinated fashion, your cells make good stuff.
3. Maintenance of homeostasis of tissues with structural function like connective tissue, bone, muscle and cartilage. In other words, when the ECM moves in a coordinated fashion your orthopedic tissues are happy.

From a physiological point of view, mechanical stress of suitable intensity has a key role in maintaining the homeostasis of tissues with structural function, such as connective, bone, muscle and cartilage. It does this by influencing cell growth and differentiation, protein synthesis, and ECM production—all of which have been proven in the lab. Therefore, photothermal, and photomechanical effects strongly contribute to the anti-inflammatory, anti-edema, and anti-pain action as well as to the stimulation of tissue repair processes. 

Think of the photomechanical effect of laser like a gentle massage to your cells and their surroundings. It creates a positive mechanical therapy that helps to stimulate healing. And who doesn’t want a massage?

Why Wavelength Matters

Here’s the thing: in order for all of that scientific magic to take place, the laser needs to penetrate deep enough into the body to actually trigger a reaction. You can’t use a keychain laser pointer and expect results. 

Wavelength is the primary determinant of penetration depth. Different wavelengths are absorbed by different chromophores—water, skin pigment, and hemoglobin are the chromophores that affect laser depth of penetration.  

Depth of penetration increases almost linearly from 800 nm to 1200 nm (except for the range between 970-990 nm where there is a peak in the absorption spectra of hemoglobin and water). This is quite different from the goal of surgical lasers, where we want all the energy to be absorbed by water at the skin level so that no energy passes below the incision site. In laser therapy, penetrating below the skin allows the healing energy of the laser to reduce pain and inflammation in muscles, bones, joints, tendons and nerves. 

In conclusion, positive results from laser therapy don’t happen overnight. The effects are cumulative. The cellular response triggered by laser therapy takes time to build. From my experience working with hundreds of patients, I find that six to ten treatments are required for optimal results. 

**What is your experience with laser therapy? What questions do you have?

Recommended reading:

Knight KL, Draper DO. Principles of heat thermotherapy. In: Therapeutic modalities. The art and science. Lippincott Williams & Wilkins, a Wolters Kluwer business, Baltimora, Philadelphia, 2008, pp 188-199

Rossi F., Pini R. and Monici M. Direct and indirect photomechanical effects in cells and tissues. Perspectives of application in biotechnology and medicine. In: Cell Mechanochemistry.