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Four femtosecond laser microkeratomes have received FDA approval. An ophthalmologist reviews differences between the devices, why they may be important, and considerations for building a next-generation laser microkeratome.
Baltimore-Recent FDA approvals have expanded the number of femtosecond laser microkeratomes available to corneal and refractive surgeons in the United States, and some distinct differences exist between the devices. Further study is needed, however, to determine the clinical relevance, if any, of the unique features offered by the various platforms and to find solutions to existing challenges, said Roy S. Chuck, MD, PhD, at Current Concepts in Ophthalmology.
"Currently, [one proprietary femtosecond laser] (Intralase FS, Advanced Medical Optics) enjoys the most widespread use in the United States, and at Wilmer Eye Institute, it has become the microkeratome of choice for LASIK procedures. Newer instruments with some different characteristics have some potential advantages that will be tested through research and experience. None of these devices is ideal, and likely there will be additional entries into this product category," said Dr. Chuck, who holds the Tom Clancy Professorship of Ophthalmology, Cornea, and External Diseases, and is director, refractive surgery, Wilmer Eye Institute, Johns Hopkins University, Baltimore.
Dr. Chuck discussed several newer femtosecond lasers (20/10 Perfect Vision, Femtec; Femto LDV, Ziemer; Visumax, Carl Zeiss Meditec). Comparing and contrasting them, he noted that very slight differences exist between the devices in wavelength, and the Zeiss instrument is also differentiated as a "fiber" laser. All four, however, essentially are neodymium solid-state lasers. In other features, differences are to be noted as well, he said. The applanation lens for both the IntraLase FS and Femto LDV is flat, whereas the 20/10 and Visumax feature a curved applanation lens. The 20/10 uses scleral suction for applanation, whereas the Visumax has a smaller diameter applanator and depends on corneal suction.
"Less suction also results in less discomfort, but whether the other differences associated with curved applanation are significant remains to be seen," he said.
Portability is another feature distinguishing the femtosecond lasers; all of the platforms are large, immobile devices with the exception of the Femto LDV. The Femto LDV is smaller and portable because it interfaces directly with the excimer laser to use some of its optics via an articulating arm. Portability is an obvious advantage, but time will tell how stable the femtosecond laser optics are when the device is moved, Dr. Chuck said.
"Historically, the alignment of femtosecond laser optics was highly sensitive to any movement, and so creating a device that can be rolled around is an engineering feat. However, we must wait to see how it performs in clinical practice," he said.
The lasers also differ in their pulse width. Both the IntraLase FS and 20/10 have a pulse width in the 500-to 800-femtosecond range. Information about the pulse width of the Visumax is proprietary, but it is thought to be about 400 femtoseconds, whereas the Femto LDV has a pulse width of 200 to 300 femtoseconds that is truly shorter than the other devices. This difference also is associated with a higher pulse rate for the Femto LDV relative to the other instruments and has a potential benefit for reducing the energy required to cut the cornea.
Dr. Chuck explained that the bulkiest and most environmentally sensitive component of femtosecond lasers is the amplification stage to build power, which also stretches the pulse time, but the Femto LDV lacks this component. As a result, the Femto LDV can be smaller, have a shorter pulse time, and have a substantially reduced energy output. To compensate, however, the pulse rate of the Femto LDV is 1,000-fold greater than that of the other devices.
"The Femto LDV is a low-energy laser with a very high pulse frequency. Since corneal plasma-mediated ablation threshold is proportional to the square root of the pulse width, a shorter pulse will use less energy for cutting. Whether this results in a reduced incidence of inflammatory problems, such as diffuse lamellar keratitis, remains to be seen," Dr. Chuck said.
Building the ideal system
A Wilmer research group is collaborating with the FDA and an industry partner to develop a next-generation, ultrafast laser microkeratome. That project has several established goals that also will depend on identifying answers to some existing questions and overcoming certain challenges. One subject for further study is whether 1,053 nm represents the optimal wavelength. In addition, although there are theoretical benefits for a shorter-pulse laser, such a device would require very high repetition rates, as well as other more sophisticated components, which may be susceptible to disruption in a portable system.
Other targets for development include creating a device that uses a reduced suction pressure and has a short flap creation time.
"Flap creation time for the IntraLase FS has fallen with each new generation and is currently about 19 seconds. That remains a bit slower than the 5 to 10 seconds associated with use of a mechanical microkeratome, but it may be hard to beat for a femtosecond laser because the applanation and centration tasks are the most time consuming portions of the procedure," he said.
The Current Concepts meeting was sponsored by Wilmer Eye Institute at Johns Hopkins and co-sponsored by Ophthalmology Times.