Laser Energy in Oral Soft Tissue Applications
Lasers have been used for oral soft tissue dental procedures for more than 30 years, and have been researched since the middle 1960s. Their reported benefits over conventional treatment modalities include:
- Reducing numbers of appointments,
- Reducing stress,
- Improving visibility,
- Improving patient comfort, and
- Reducing complications.
Critics have commented that most of these advantages are anecdotal and need to be substantiated with further research. With scientific references, this paper will show:
- Fundamental laser-tissue interaction with oral soft tissue;
- Laser use can be minimally invasive compared to conventional modalities;
- Laser energy can aid in hemostasis, providing for improved visibility during a surgical procedure;
- Laser irradiation can reduce bacteria;
- Laser use can help in wound healing and can produce other photobiomodulation effects;
- Laser energy can reduce pain when compared to conventional methods.
Mechanism of Laser Interaction with Soft Tissue
The oral cavity contains a variety of soft tissue types including but not limited to dental pulp, mucosa, keratinized and non-keratinized gingiva. Furthermore, specific differences can exist for each tissue type, depending on location, tissue thickness, and degree of health. Depending on the wavelength of the laser device, the following interactions can be seen in varying degrees:
- Reflection – no interaction occurs as the beam reflects off the surface
- Transmission – no interaction occurs as the beam passes directly through the tissue
- Scattering – an interaction as the beam disperses in a non-uniform manner throughout the tissue
- Absorption – light radiation is absorbed by specific tissue elements. The predominant laser interactions within oral soft tissue are absorption and scattering. As will be explained further, tissue composition, laser emission mode, fluence, and thermal relaxation also affect tissue interaction.
Wavelength and Tissue Type
Laser wavelengths have been shown to be absorbed by different components such as hemoglobin, melanin, water, and hydroxyapatite. Currently available dental lasers operate in the visible or near-infrared region (532-1340 nm), near the boundary of the mid-infrared (2780 and 2940 nm), and far-infrared (10,600 nm) regions of the electromagnetic spectrum. With respect to the light radiation interacting at the tissue surface (incident beam), interaction is primarily determined by the laser irradiation affinity for specific chromophores comprising the tissue. A chromophore is a molecule or substance capable of absorbing specific laser wavelengths.13 Table 1 lists each available Class IV laser, wavelength, emission mode, delivery system, and primary chromophores. Whenever feasible it is best to match the appropriate wavelength to the main chromophore within the target tissue to maximize the absorption and achieve an enhanced treatment efficiency.
For example, inflamed tissue, which can contain dark pigment and hemoglobin chromophores, readily absorbs wavelengths in the visible and near-infrared regions. Furthermore, in situations of healthy or minimally pigmented tissue, wavelengths highly absorbed in water often will provide more efficient ablation.
The temporal emission mode of a laser is the propagation of a stream of photonic energy from the site of the beam origin, relative to time. Depending on how the laser active medium is energized, the laser photonic emission can occur – inherently – in a continuous-wave (CW) or free-running pulsed (FRP) emission mode. Typically, the energizing component of the laser is referred to as the pumping mechanism, which can be a flash lamp, electric current, or electric coils. The CW lasers can be further manipulated through device-specific mechanical, optical, or alternating current electro-optical interruption of the beam. This interruption of the CW laser beam can be termed ‘gated’ or ‘chopped,’ with each pulse identical in power and duration. Currently available CW dental lasers include KTP, all diodes, and CO2 lasers; and all have gated properties that vary by device. Some of these instruments have pulse durations as short as micro- and milliseconds, and some manufacturers have coined different terms, such as ‘superpulse’ and ‘ultraspeed,’ in describing their devices. With very short pulse durations, peak powers several times higher than CW powers can be produced. However, typical average powers for CW lasers can range from 0.5 to 5.0 W. If gating can be an optional operator choice in a continuous-wave laser, free-running pulsed emission is inherent to the device and the result of the pulsed excitation source. Currently, FRP is a characteristic seen in Nd:YAG, Nd:YAP, Er,Cr:YSGG, and Er:YAG lasers whose pulses have peak powers in the 1000 W range. Despite high peak powers, a FRP laser delivers low average power through extremely short pulse durations in the range of a few hundred microseconds.
The emission mode will have an effect on laser-tissue interaction through average power and peak power in relation to thermal relaxation factors of the target tissue. The pulse length, pause length, and penetration depth (the extent of the laser beam’s interaction within the tissue) also influence thermal relaxation of the target tissue. Thermal relaxation can be defined as the time required for the irradiated tissue to cool by 50% of its original temperature immediately after the laser pulse. The ability of the irradiated tissue to cool can be influenced directly by the laser operating parameters and the inherent thermal diffusivity (convection and conduction) of the tissue. Other factors are: area or volume of tissue exposure; technique and speed of movement of the laser beam over the target tissue; blood flow within the tissue; and the use of high-speed evacuation. Supplemental irrigation, application of ice, or a co-axial water spray can also be utilized to achieve cooling. Gating or chopping a continuous-wave device provides reduced risk of tissue damage due to less energy delivered to the tissue at a given time.
Energy Density (Fluence)
Energy density is defined as energy (Joules) per square centimetre of spot size (J/cm2). Through the use of various techniques and delivery systems, the laser beam spot size can be either defocused or focused. Depending on the degree of beam focus, the laser beam spot size can be altered and fluence will accordingly change. Decreasing the area of the laser spot size will increase the energy density and then (presuming optimal absorption characteristics in the tissue) the rate of ablation of the target tissue will increase up to a maximum ablation rate.