All commercially available devices use a near infrared femtosecond laser with a wavelength of approximately 1053 nm. Despite the fact that the neodymium-doped yttrium aluminum garnet and femtosecond lasers have very similar wavelengths (Table 1), the ultrashort duration of the pulses (10−9 vs 10−15) in the latter causes significantly less damage in the collateral tissue. Varying the duration of the laser pulses and energy applied can generate different effects on the tissue (Figure 1). The main technical specifications that play a role in the femtosecond laser are the following:
Effects of the relationship between laser interaction time and energy intensity.
Laser pulse repetition rate;
There is an inverse relationship between the laser pulse duration and the energy required in each pulse to generate the optical breakdown. A shorter pulse (200–500 fs) needs lower energy to achieve the threshold of photodisruption than a longer pulse (500–1300 fs). The numerical aperture (NA) of the lens influences the laser spot in terms of diameter and volume. A higher NA focuses the beam with less dispersion and is the reason why higher NA devices use lower energy. It is also suggested that a higher NA increases the depth accuracy and overall precision of the lamellar cut.
After the optical breakdown occurs, plasma is created and a cavitation bubble formed. This bubble expands and cleaves the tissue. If a high-energy photodisruption is used, the bubble is larger and the pulses do not need to be placed close together. Low-energy systems create a very small bubble, with a greater number of pulses in an overlapping pattern being mandatory, since there is almost no tissue cleaving induced by the bubbles.
The first devices operated with a low KHz repetition rate (15 KHz – first Intralase model) and needed a higher energy to photodissection. Newer devices (even the newer high-energy devices such as IntraLase 150 KHz) intend to increase the repetition rate, which makes the procedure duration shorter and uses lower energy with an intention of diminishing the inflammation. In addition, the spot size and separation can be lowered in higher repetition rates to produce smoother surface cuts without increasing the time of the procedure.
In summary, the ideal device would include a high repetition rate, small spot size and low energy per pulse.
The geometry of the cuts performed is theoretically limitless. Vertical, horizontal and every imaginable geometrical pattern can be applied. However, limitations in cutting placement vary among devices, with newer ones tending to offer a more customizable cutting. To perform precise incisions in cataract surgery, an imaging system (optical coherence tomography and 3D-confocal-structured imaging technology are under research) is required, since the position of intraocular structures change and must be accurately localized after docking.
Two main pulse patterns are used in commercially available corneal cutting devices: raster and spiral. The first involves pulses that are applied in a linear pattern, starting at the hinge area, passing through the center of the cornea and finally extending to the opposite edge. The spiral pattern is applied when the laser pulses begin centrally and expand centrifugally out to the periphery (centripetally can also be used). Most devices use the raster pattern, which was found to produce a smoother stromal bed in the Intralase machine. Visumax (Carl Zeiss Meditec AG, Jena, Germany) uses the spiral pattern.
Furthermore, the method for fixating the eye, including the suction ring and docking system, varies among the devices. The amount of induced pressure is higher in devices that applanate the cornea, such as the Abbott Medical Optics IntraLase, WaveLight Ultraflap and LDV (Ziemer Ophthalmic Systems, Port, Switzerland), and lower in devices with a curved applanation docking interface, as found in the Visumax and Perfect Vision Femtec 20/10 lasers (Technolas Perfect Vision, Heidelberg, Germany).
Expert Rev Ophthalmol. 2011;6(1):55-65. © 2011 Expert Reviews Ltd.
Cite this: Comparison of Commercially Available Femtosecond Lasers in Refractive Surgery - Medscape - Feb 01, 2011.