Femtosecond laser refractive surgery first entered the clinic in 2001 with the FDA approval of the IntraLase system. That system is now in its fifth iteration, and has since been joined by three other femtosecond laser platforms. EuroTimes got an update on the features, performance, and applications of the individual lasers at a refractive surgery session at the WOC.
Comparing and contrasting
Holger Lubatschowski PhD, Germany, began the session by providing an overview of basic principles of femtosecond laser performance and a side-by-side comparison of the technical features of each of the four commercial platforms. All of the femtosecond lasers use photodisruption to cut through corneal tissue, and they all operate at a wavelength of about 1040 nm, but they differ in a number of technical respects.
The Femtec (20/10 Perfect Vision) and IntraLase have a high pulse energy, the LDV (Ziemer) has a very low pulse energy, and the VisuMax (Zeiss) lies somewhere in between. With the IntraLase, Femtec, and VisuMax, the cutting process is directly visible to the operator, whereas it is seen only on the monitor using the LDV. The Femtec and VisuMax have a spherical contact interface to the cornea, while the VisuMax has a contact glass for corneal fixation.
The LDV delivers its laser beam through a mirror arm that fits under all excimer lasers and consists of just an oscillator without an amplifier. Therefore, the LDV can be used without moving patients during the surgical process, and it is also the smallest of the devices.
The four femtosecond lasers also differ in pulse width, which ranges from 250 femtoseconds for the LDV to 500 femtoseconds for the IntraLase and Femtec. The repetition rate is in the MHz range for the Femtec and in the kHz range for the other platforms, while pulse energy is in the nanoJ range for the LDV and in the milliJ range for the others.
IntraLase
Perry Binder, MD, US, presented worldwide clinical experience with the IntraLase femtosecond laser. Since the introduction of the first generation system, the IntraLase platform has been used to perform more than two million procedures, including over 500 IntraLase-enabled keratoplasties. Other uses have included astigmatic keratotomy, wedge resection, lamellar keratoplasty, penetrating keratoplasty, creation of INTACS channels, and corneal biopsy.
The recently introduced 5th generation device provides a number of important features and benefits. This 150 KHz femtosecond laser allows reductions in time for flap creation and total energy delivery. It also permits unique methods of flap customisation, including a bevel-in sidecut angle, which has been shown in various studies to result in stronger flap healing.
The newest IntraLase laser also offers a high-resolution video microscope for increased depth of focus during surgery, a touchscreen user interface, and digital video output.
“We think the fifth-generation IntraLase laser enables most of the idealised goals of LASIK surgery better than any currently existing femtosecond laser while placing all the tools necessary for customising the flap within the hands of the surgeon,” Dr Binder said.
Femtec
The Femtec is a highly capable workstation that is also able to perform roles beyond LASIK flap creation, said Julian Stevens, MD, UK.
A key feature of this platform is that it has a curved patient interface that is important because it translates into less distortion of the cornea and less compression of the eye. Therefore, IOP is minimally increased.
The Femtec also features excellent diagnostic graphics on the video screen that are particularly useful for monitoring shape when cutting corneal grafts, a torque feature that detects patient head movement, and a direct diagnostic link with customised treatments.
In addition to having proven itself as a capable flapmaker, the Femtec laser has been used for astigmatic keratotomy, intrastromal astigmatic keratotomy, and cutting corneal grafts.
However, the capability that most distinguishes it from other existing platforms is its potential to be used to perform intrastromal refractive surgery with no flap.
“The Femtec laser is a very capable workstation with excellent software. It is one of the most sophisticated and advanced systems available today and may bring us to the Holy Grail of femtosecond lasers, which is to use this technology to put energy into the cornea in order to achieve a shape change without cutting a flap,” Dr Stevens said.
LDV
Theo Seiler MD, PhD, Switzerland, discussed the Ziemer LDV based on his experience over the last two years. His presentation delivered three messages relating to precision, safety, and platform dependability.
After proposing that a femtosecond laser has a role in refractive surgery for procedures where flap dimension precision truly matters, Dr Seiler reported that in a series of 92 eyes undergoing creation of flaps with an attempted thickness of 110 microns, the mean ± SD thickness was 109 ± 3.7 microns with a range from 101 to 116 microns. Flap diameter outcomes also showed sufficient precision and reproducibility, although Dr Seiler noted that the desired flap diameter is obtained only if the suction is complete.
Safety has been excellent. Complications encountered in the last 200 patients consisted of strong adhesion necessitating manual cut in two eyes, failure of the scan in one eye, too small flap diameter in one eye, and inability to perform two treatments because of a small eye and deep orbit. There were no air bubbles in the anterior chamber or eyes with transient light sensitivity, and eyes with Sands of Sahara have been very infrequent.
The LDV has also proven itself to be an easy-to-use and dependable workhorse, he noted.
“We have had to cancel only one surgery session over the last two years and consider this platform a Volkswagen, not a Ferrari. There is no need for a technician, no calibration, and no extra temperature and humidity controls. We know that with the LDV, we can just turn it on, and it goes,” Dr Seiler said.
VisuMax
As described by Marcus Blum MD, the VisuMax platform (Zeiss) is an integrated solution combining a femtosecond laser and excimer laser. It uses a spherical contact interface system to the cornea with low suction pressure so there is no vision loss during suction, and treatment positioning is precise as the patient fixates on a blinking light within the system.
A study using high-frequency digital ultrasound to measure flap thickness showed it performs reliably and precisely for flap creation. In a series of eyes with an attempted thickness of 110 microns, the mean achieved was 112 microns, 25 per cent of eyes were within two microns of intended, and 88 per cent were within 10 microns.
However, ongoing research using the VisuMax femtosecond laser for femtosecond lenticule extraction (FLEx) as a new method of refractive surgery is what is distinguishing this platform, said Dr Blum.
He presented data from six months of follow-up of more than 100 eyes showing good safety, refractive stability, and reasonable accuracy but with slight overcorrections in lower myopes.
“We have been very grateful to use this system because it works perfectly for LASIK flap creation. However, we have tried to take the VisuMax beyond the point where it is only a flap maker. Obviously there is a learning curve, but we expect the outcomes with FLEx will become more precise as the technique is refined, and this approach would have the advantage of eliminating the need for two lasers,” said Dr Blum.
Novel application
The various femtosecond lasers are being used for a number of different indications in corneal surgery. Diverging from that path, Ronald Krueger, MD, US, described research he has been conducting using a femtosecond laser to cause intralenticular photodisruption as a method for restoring accommodation.
The basic concept is to use an ultrashort pulse laser to cause photo-phaco-modulation in order to alter lens elasticity.
“The idea for this procedure is that creating small laser microperforations within the hardened lens nucleus would enhance the sliding of fibres within the lens and thereby increase lens flexure,” he explained.
An early study in cadaver eyes showed that treatment of a lens from a 54-year-old donor with 100 suprathreshold pulses in an annular pattern was associated with increased deformation when submitted to strain. Subsequent studies have focused on identifying the mechanism for that change, identifying the best pattern for the laser treatment, evaluating the potential to induce cataract, and determining what happens to the bubbles created.
So far, animal studies showed the treatment did not cause cataract, alter light scattering, or result in thermal damage into the peripheral lens tissue. Using a finite element model, it was determined that the procedure works because of sliding fibres within the lens and that layered shells are the best potential pattern so far. With the use of lens tissue cultures, it was seen that the bubbles disappear, and using an instrument to evaluate relative lens resistance to displacement with gradient steps of applied compression force, the results achieved were seen to be repeatable. These data are now being used to determine the parameters for achieving an accommodative effect.
“So far, our studies show accommodation restoration with intralenticular femtosecond lens treatment is theoretically possible and experimentally feasible. With sufficient clinical efficacy, laser lens modulation could become a new strategy in the quest for presbyopic correction,” Dr Krueger said.
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For more information on femtosecond lasers in the clinic, listen to a EuroTimes podcast with Michael Knorz MD.
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