Laser: Class 3 vs. Class 4—Where Do I Begin
Janet B. Van Dyke, DVM, DACVSMR
Canine Rehabilitation Institute, Inc. Wellington, FL, USA
Laser therapy, also called photobiomodulation, uses electromagnetic energy to stimulate tissues in the body. Laser is an acronym for light amplification by stimulated emission of radiation, and by definition is collimated and monochromatic. The depth of penetration of laser energy is determined by the wavelength of the light energy. Lasers a classified as Class 1, Class 2, Class 3a, Class 3b, and Class 4. These classes are determined by the milliwatts (mW) of power and were created to describe levels of danger associated with the use of each. Class 1 lasers have less than 0.5 mW of power. A typical example would be a garage door opener or television remote control. Class 2 lasers are between 0.5 and 1.0 mW. Most laser pointers are in this class. Class 3 lasers have between 1 and 500 mW. This group is referred to as therapeutic ‘cold’ lasers and come with ‘eye caution to eye danger’. Class 4 lasers have more than 500 mW and are referred to as ‘hot’ lasers. Surgical and industrial lasers are in this class, which has the precautions of ‘fire hazard and fire danger’. Terminology related to laser usage can be quite confusing. “Low level laser therapy”, “LLLT”, “Cold Laser”, “Class 4 laser therapy”, and “High intensity laser therapy” are often used incorrectly and are generally misleading. It is safer to use the terms photobiomodulation or therapeutic laser or laser therapy.
The physiological effects of laser include accelerated cell division via mitochondrial stimulation, increased leukocyte phagocytosis, stimulation of fibroblasts and collagen stimulation, enhanced synthesis of ATP, and angiogenesis. These effects lead to vasodilation, decreased inflammation, decreased edema, slowed nerve conduction and inhibited peripheral nociception. Indications for laser therapy include wound healing, pain relief (both acute and chronic), muscle spasms (including, perhaps, delayed onset muscle soreness or ‘DOMS’), and edema reduction. Laser is used in pain management practice to speed wound healing and control inflammation. Contraindications include use near the eye, over neoplasia, over open growth plates, pregnant uterus or the thyroid.
Laser application requires attention to probe positioning. The probe must be held perpendicular to the target tissue. The use of contact vs. non-contact techniques is determined by the power of the laser. High powered lasers require a non-contact technique due to the potential for burning tissues. Patients with dark skin will experience discomfort from skin heating so power must be turned down, if possible, or treatment times per area must be shortened. Heat generating lasers can cause pain if staples are in the field of treatment. Use of lasers to treat intraarticular areas of joints requires that the joint be in the open packed position. Laser dosing is measured in units of power being transmitted over time. Energy is measured in Joules. One Joule is the equivalent of one watt delivered for one second. To deliver one Joule of energy from a 500 mW (Class 3) laser, the treatment must be continued for 2 seconds. Most therapeutic regimens used in veterinary practice and clinical trials call for 1 to 8 Joules per square centimeter. This equates to 2 to 16 seconds of therapy time per square centimeter of treatment area. Laser takes considerably less time to use than therapeutic ultrasound where treatment times are often 5 to 20 minutes. The key to dosing is remembering that the depth of penetration is determined by the wavelength of the laser being used. The power does not alter depth of penetration. It affects only the time required to deliver the desired dose. Safety measures for use of lasers are primarily aimed at eye protection. Goggles are laser-specific and have varying optical densities. Sunglasses do not block laser light. In fact, as they cause pupil dilation, sunglasses increase the risk of laser damage to the retina. Patients should wear ‘doggles’ as well. Laser therapy should be done in areas with minimal reflective services to minimize accidental eye injury. Keeping the laser probe in contact with the patient can lower the chances for accidental laser exposure, however, some lasers have the potential to generate heat and could burn tissues if kept in contact with the tissues. Any practice using laser therapy should have a laser safety officer (LSO). The LSO needs to be aware of the nominal hazard zone which is the space in which the level of direct or indirect exposure to laser light could be hazardous. This is 20 feet (6.5 meters) for Class 4 lasers. A “Laser in Use” warning sign must be placed on the door to the treatment area, and ideally, there will be an automatic locking mechanism on the treatment door that prevents accidental entry to the room whenever the laser is in use. Many studies have been published related to laser therapy. One such study looked at wound healing in a rat skin model. This study showed that laser therapy at 4 Joules/cm2 every other day caused decreased inflammation and increased collagen deposition and gave the best results in increasing the burst strength of skin incisions. Another mouse study showed that paw edema could be prevented and treated using laser therapy. A canine clinical study showed wound healing in 3 weeks in what had been a very delayed (7 months) wound closure in a canine limb. Laser therapy reduced the time to ambulation in dogs after hemilaminectomy in one preliminary study. Human studies show significant improvement in pain and flexion measurements in knee osteoarthritis patients. Meta-analyses regarding laser use in tissue healing concluded that “phototherapy is highly effective for tissue repair” and that there is ‘strong support from experimental animal studies’. Meta-analyses regarding laser use for pain relief concluded that “phototherapy relieves pain of various etiologies”.