Indications and limitations of Er:YAG laser applications in dentistry

Keywords:medical, laser, fibers,  Time:30-03-2016
Since the early 1960s, lasers have been used in medicine and dentistry. Research and studies have prepared the pathway for cavity preparation without pain and discomfort.

Different wavelengths have been tested with variable results and several lasers have shown serious side-effects which could cause damage to dentin and enamel while insufficiently cutting dental hard tissues.1 Stimulated emission from Er3+ ions in crystals of yttrium, aluminum and garnet was presented in 1975, preparing the pathway to a new type of laser called Er:YAG.2 Its emitted wavelength of 2940 nm matches exactly the maximal absorption in water, being about 15 times higher than the absorption of a CO2 laser and 20,000 times that of a Nd:YAG laser.3 Also well absorbed in hydroxyapatite, this laser seems to have been made for effective removal of dentin and enamel with only minor side-effects such as thermal damage. The potential of Er:YAG lasers (ERL) for the ablation of hard tissue in dentistry was demonstrated already in 1989.4 Since its first introduction for dental use in 1992, Er:YAG lasers have been increasingly used in dental practice and are becoming more and more a comfortable method for caries removal for patients, as conventional cavity drilling may cause noise and pain.

An increasing number of manufacturers have marketed Er:YAG lasers (ERL) since 1997, when this type of laser received FDA approval for caries removal and cavity preparation in the United States.

Overview of Erbium:YAG lasers for dental use

The first available system on the market, the Key Laser 1, was introduced by KaVoa in 1992 and was further developed in Key Laser 2 and Key Laser 3. Nowadays many manufacturers are marketing Er:YAG lasers with important differences in their technical specifications (Table). The available maximum pulse energies range from 300 mJ (DELightd), over 600 mJ (Key Laser 3), 700 mJ (Smart 2940De), up to 1000 mJ (Fidelis Plus IIc and Opus Duob). The output power, which is the product of pulse energy times repetition rate, goes up to 12 W (Opus Duo) or even 15 W (Fidelis Plus II). For minimally invasive dentistry with an Er:YAG laser as an alternative to conventional mechanical drill a power of 10-12 W seems to be sufficient.5 Consequently, there seems to be no real need for the development of more powerful  Er:YAG  lasers, because  when speeding up treatment by increasing pulse energy and/or repetition rate, more side effects such as leaflets and cracks may appear, especially in enamel. Ablation is already sufficient at a power of around 6 W in dentin and a very fast ablation, especially in deeper dentin layers, is possible with a power of around 10 W. Recently, an increased effectiveness using the so-called very short pulse (VSP) is discussed, pretending that the typical debris cloud formation above the ablated surface negatively influences ablation speed by partially absorbing energy of the following laser pulses. According to Lukac et al,6 the most effective ablation is reached with pulse lengths less than 100 µsec, but this study used vertical irradiation of the tooth surface, which did not allow a flush of the debris cloud by the spray device.  The laser beam is transferred to the operation field by water-free glass fibers (KaVo, DELight), articulated arms with mirrors (Smart 2940D, Fidelis Plus II) or flexible hollow fibers (Opus Duo).

Although a glass fiber delivery system is very easy to handle, there is a limitation in maximal power transmission at about 6 W. Therefore, more powerful Er:YAG lasers need a hollow transmission system for their light. Among them, the flexible hollow fiber from Opus Duo is more suitable for the daily use because of its better handling than the articulated arms used for example by Fidelis Plus II or Smart 2940D. With the exception of Opus Duo, all systems deliver a focused beam and this specification may be of much importance for an even distribution of the power density on the working surface, especially during smoothing and conditioning of the enamel structure that is superficially destroyed during cavity access and preparation with high energy densities. The spatial beam profile after transmission through the flexible hollow fiber has not been reported in the literature.

A water-free glass fiber has a quasi-Gaussian shape, whereas articulated arms achieve a distribution of even higher orders with an important maximum around the central peak, or a ring-shaped intensity distribution.7 However, even if different beam transfer technologies may have different advantages and disadvantages, it is impossible to compare different laser systems based on this property or on their parameter settings. Many factors, such as pulse formation, pulse width, beam profile and others have to be brought in relation to each other to allow an accurate comparison of their clinical efficiency.

Most manufacturers propose sapphire contact tips for tooth preparation, with similarly looking handpieces. The range of tip diameters goes from 400-700 µm (DELight) up to 200 µm1300 µm proposed by Opus Duo. A non-contact focusing handpiece used for hard tissue preparation by the Key Laser 3 (the sapphire tips of Key Laser 3 are exclusively designed for periodontal applications) is also proposed by some other manufacturers, but a precise working, especially in the means of minimally invasive dentistry where aiming accurately with the beam is of high importance, is very difficult. As a complementary tool to the Er:YAG laser, two manufacturers have included in their system a second laser emitting another wavelength, as Nd:YAG (Fotona) or CO2(Opus Duo). The Nd:YAG may offer wider indications in endodontology, whereas Er:YAG in combination with CO2 may allow a complete coverage of almost all dental indications assisted by a laser, except bleaching.