Tissue Interactions and Biological Effects
Once a laser beam is produced it is aimed at tissue to perform a specific task. As the energy reaches the biological interface one of four interactions will occur: reflection, transmission, scattering, or absorption.
Absorption – Specific molecules in the tissue known as chromophores absorb the photons. The light energy is then converted into other heat to perform work.
Reflection – The laser beam bounces off the surface with no penetration or interaction at all. Reflection is usually an undesired effect, but a useful example of reflection is found when Erbium lasers reflect off titanium allowing for safe trimming of gingiva around implant abutments.
Transmission – The laser energy can pass through superficial tissues to interact with deeper areas. Retinal surgery is an example; the laser passes through the lens to treat the retina. The deeper penetration seen with Nd:YAG and diode lasers is an example of tissue transmission as well.
Scattering – Once the laser energy enters the target tissue it will scatter in various directions. This phenomenon is usually not helpful but can help with certain wavelengths biostimulative properties.
Figure 6. The four tissue interactions.
Absorption is the most important interaction. Each wavelength has specific chromophores that absorb their energy. This absorbed energy is converted into thermal and and/or mechanical energy that is used to perform the work desired. Near infrared lasers like diodes and Nd:YAGs are mostly absorbed by pigments such as hemoglobin and melanin. Erbium and CO2 lasers are predominantly absorbed by water and hydroxyapatite. The shorter, near infrared wavelengths of diodes and Nd:YAG lasers also penetrate tissue more deeply than the longer, mid infrared wavelengths of the erbium and CO2 lasers.
Figure 7. Chromophores.
The wavelengths of the four most common dental lasers are shown where they occur in the electromagnetic spectrum. All are in the non-ionizing infrared part of the spectrum. Absorption patterns of the chromophore water, melanin, and hemoglobin are superimposed on the graph. This absorption is what converts light energy into thermal and/ or mechanical energy to do work.
There are five important types of biological effects that can occur once the laser photons enter the tissue: fluorescence, photothermal, photodisruptive, photochemical, and photobiomodulation.
Fluorescence is when a second wavelength of light is emitted from tissue molecules after exposure to laser light. An example in dentistry is when actively carious tooth structure is exposed to the 655nm visible wavelength of the Diagnodent diagnostic device. The amount of fluorescence is related to the size of the lesion, and this information is useful in diagnosing and managing early carious lesions.
Photothermal effects occur when the chromophores absorb the laser energy and heat is generated. This heat directly vaporizes the tissue and is used to incise or remove tissues. Photothermal interactions predominate when most soft tissue procedures are performed with dental lasers. Photothermal ablation is also at work when CO2 lasers are used on teeth as hard tissue is vaporized during removal. Heat is generated during these procedures and great care must be taken to avoid thermal damage to the tissues.
Photodisruptive effects (or photoacoustic) can be a bit more difficult to understand. Hard tissues are removed through a process known as photodisruptive ablation. Short-pulsed bursts of laser light with extremely high power interact with water in the tissue causing rapid thermal expansion of the water molecules. This causes a thermo-mechanical acoustic shock wave that is capable of disrupting enamel and bony matrices quite efficiently. Erbium lasers' high ablation efficiency results from these micro-explosions of superheated tissue water in which their laser energy is predominantly absorbed. Thus, tooth and bone are not vaporized but pulverized instead through the photomechanical ablation process. This shock wave creates the distinct popping sound heard during erbium laser use. Thermal damage is very unlikely as almost no residual heat is created when used properly, particularly when the concept of thermal relaxation is considered.
Photochemical reactions occur when photon energy causes a chemical reaction. These reactions are implicated in some of the beneficial effects found in biostimulation discussed below.
Photobiomodulation refers to lasers ability to speed healing, increase circulation, reduce edema, and minimize pain. Many studies have exhibited effects such as increased collagen synthesis, fibroblast proliferation, increased osteogenesis, enhanced leukocyte phagocytosis, and the like with various wavelengths. The exact mechanism of these effects is not clear, but it is theorized they occur mostly through photochemical and photobiological interactions within the cellular matrix and mitochondria. Biostimulation is used dentally to reduce postoperative discomfort and to treat maladies such as recurrent herpes and aphthous stomatitis. Low Level Laser Therapy (LLLT) and Biostimulation are other terms used to describe this phenomenon.
When a dental laser is employed, it can be used in contact mode or non-contact mode. The laser tip directly touches the target tissue in contact mode. In non-contact mode the laser is pointed at a distance from the target tissue anywhere from a few millimeters, such as in operative dentistry, or up to several centimeters when performing biostimulation.
When a laser heats oral tissues certain reversible or irreversible changes can occur:
Hyperthermia – below 50 degrees C
Coagulation and Protein Denaturation – 60+ degrees C
Vaporization – 100+ degrees C
Carbonization – 200+ degrees C
Irreversible effects such as denaturation and carbonization result in thermal damage that cause inflammation, pain, and edema.