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Femtosecond lasers differ in their indications, energy delivery parameters, and hardware features. Understanding the implications of these factors will enable an objective assessment of the available technology.
Take-home message: Femtosecond lasers differ in their indications, energy delivery parameters, and hardware features. Understanding the implications of these factors will enable an objective assessment of the available technology.
By Cheryl Guttman Krader; Reviewed by Ronald R. Krueger, MD, MSE
Cleveland-Understanding how hardware design and technical specifications affect femtosecond laser performance provides surgeons with the fundamental knowledge they need to evaluate the available systems and choose one that will best fit their needs, according to Ronald R. Krueger, MD, MSE.
Dr. Krueger discussed the implications of different laser parameters and provided an overview of the features of commercially available femtosecond laser platforms. The latter are represented by systems used for cornea/refractive applications only (IntraLase, Abbott Medical Optics; FS200, Alcon Laboratories, Femto LDV Z4 and Z6; Ziemer; VisuMax, Carl Zeiss Meditec), cataract surgery only (LENSAR, LENSAR; Catalys, Abbott Medical Optics), and units that can be used for both cornea/refractive and cataract procedures (LenSx, Alcon; Femto LDV Z8, Ziemer; Victus, Technolas). (Note: approved indications for each laser vary by country.)
“At the present time, the best lasers for refractive procedures are those with refractive-only optics and design, while the best lasers for cataract surgery have cataract-only optics and design,” said Dr. Krueger, medical director, Department of Refractive Surgery, Cole Eye Institute, Cleveland Clinic, Cleveland, OH.
“This is important to consider, as providers who seek both a refractive and cataract laser might be tempted to get one for both indications,” he said.
Factors affecting energy delivery with femtosecond lasers include pulse duration, numerical aperture, and wavelength. When it comes to pulse duration and wavelength, shorter is better, as both require less energy density for photodisruption.
Currently, surgeons do not have options when it comes to laser wavelength since all femtosecond lasers use a near infrared wavelength.
However, ultraviolet units, which would significantly reduce the energy density needed for photodisruption, may be available in the future, Dr. Krueger said.
In terms of numerical aperture, larger is preferable, as it provides a tighter focus and thereby enables the use of lower energy.
Pulse frequency (repetition rate) and pulse separation are other important variables in the performance of femtosecond lasers. A higher pulse frequency may allow for a shorter procedure time and/or a shortening of pulse separation distance. The latter is important for achieving the desired tissue effect with lower energies, which helps to minimize collateral tissue damage and procedural side effects.
Of the femtosecond lasers used for refractive surgery, the Femto LDV stands out among the others by having the fastest pulse frequency (20 MHz versus 40 to 500 kHz) and the lowest pulse energy (>0.03 uJ versus 0.15 to 1 uJ).
Nevertheless, the Femto LDV does not have a benefit for shortening procedure length since it delivers pulses in an overlapping pattern, said Dr. Krueger, who is also professor of ophthalmology, Cleveland Clinic, Lerner College of Medicine, Case Western Reserve University, Cleveland, OH.
Femtosecond lasers also vary with respect to clinical interfacing. The corneal laser platforms have either a curved or flat interface, and there are pros and cons for each type. A flat interface may be technically easier to use than a curved interface, but causes more tissue deformation and IOP elevation. Although it has a flat interface, the FS200 features an enhanced suction ring design to minimize IOP elevation.
A curved interface is more vulnerable to suction loss than a flat interface, and makes the laser more sensitive to centration issues, as x-y adjustments after docking may be limited. The latter issue, however, is usually not a concern when the laser is being used for flap creation.
The cataract femtosecond lasers have either curved lens or fluid interfaces. Dr. Krueger said that fluid coupling, which is found on the LENSAR and Catalys lasers, is a desirable feature for cataract femtosecond lasers since it allows for better intraocular imaging and laser delivery without light scatter losses, thereby improving the accuracy of treatments involving the capsule and lens.
The laser pulse pattern, raster versus spiral, also varies between refractive lasers, and is usually determined by the type of coupling. Lasers with a flat interface will typically best deliver spots in a raster pattern, whereas curved interface lasers typically work best with a spiral delivery.
Corneal lasers can also be differentiated by whether or not they have adaptations for evacuating gas bubbles, which is important to limit opaque bubble layer (OBL) formation. The latter problem is partially controlled by an intrastromal pocket to sequester gas bubbles when using the IntraLase laser, and with an evacuation canal for externalizing bubbles when using the FS200.
Other lasers have not integrated specific features for gas evacuation, and while gas bubble build-up is less of a problem with the lower energy Femto LDV and VisuMax lasers, it is not avoidable and may still lead to undesirable OBL.
The cataract femtosecond lasers all have image guidance systems that are necessary for achieving precise delivery of laser energy into the eye to target tissues. Most of the lasers use OCT, although posterior imaging may be somewhat limited with that technology. The LENSAR uses a proprietary 3-D “augmented reality” imaging system that provides excellent detail. While all cataract lasers have software to compensate for astigmatism axis alignment, the compensation for lens tilt is better with the fluid interface lasers.
Highlighting specific features of the various lasers, Dr. Krueger said the IntraLase stands out as the first femtosecond laser and the current market leader in the refractive surgery marketplace. The FS200 shares many similarities with the IntraLase, although it is faster and causes less increase in IOP because it uses lower suction.
The Victus with curved coupling was the first to offer both refractive and cataract indications. The Femto LDV models have the lowest pulse energy and spacing with the highest pulse frequency and their own unique modified raster pattern along 1 mm tracks. The Z4 model does not offer a side cut.
The VisuMax causes the least corneal compression and is most technically sophisticated. It is also currently the only system that can be used for small incision lenticular extraction (SmILE), a femtosecond-only refractive surgical procedure with a small incision. Since SmILE is gaining popularity internationally, surgeons in the United States who have access to this laser will have an advantage when SmILE gains FDA approval, Dr. Krueger said.
Of the five femtosecond lasers that can be used for cataract surgery, the LenSx is the first in class and current market leader. The Catalys and LENSAR platforms offer fluid coupling, and the LENSAR also stands out for its unique sophisticated 3-D imaging system, “augmented reality,” Dr. Krueger noted.
The Victus is a curved coupling refractive laser that was modified to include the lens. The newest addition, the Femto LDV Z8, has very low energy with a high repetition rate and has both flat coupling for the cornea and fluid coupling for the lens. Despite these perceived benefits, there is little information available about its clinical performance, he noted.
Ronald R. Krueger, MD, MSE
This article was adapted from Dr. Krueger’s presentation during World Cornea Congress VII preceding the 2015 meeting of the American Society of Cataract and Refractive Surgery. Dr. Krueger is a consultant to Alcon Laboratories and LENSAR.