As a scientist and inventor, my approach to most problems is to search for a way to improve technology—and therefore outcomes. At times, this has caused skepticism. Twenty years ago, at the first excimer lasers in ophthalmology meeting in Berlin, many attendees laughed when I explained that we should be operating on the optical zone of the cornea. Most everyone knows how that turned out!
Since the first commercial excimer laser system for refractive surgery arrived, I have remained an advocate of PRK. This has been due, primarily, to my concern over corneal integrity. When it comes to the impact on the biomechanical structure of the cornea, PRK is a much less invasive procedure compared with LASIK. In the past 2 years, however, my strong position on this point has changed.
Why? The advent of the femtosecond laser.
With this technology, surgeons may now precisely dissect corneal tissue, while also protecting corneal integrity. Particularly, I have come to believe that the IntraLase Femtosecond (FS) Laser (IntraLase Corp., Irvine, California) (Figure 1) is the best tool for the job, from both refractive and corneal surgery standpoints. Below is my account on why this is so.
LONGEST TRACK RECORD
Since its commercial introduction in 2002, the IntraLase FS laser has quickly gained acceptance and marketshare. In November 2006, the company marked its 1 millionth procedure. At the end of last year, according to a Marketscope market analysis, the IntraLase laser held a 30% share of the procedural market in LASIK in the United States.1
The concept for what would become the IntraLase laser started in 1994, when Ronald M. Kurtz, MD, a then-resident at the University of Michigan, first hit upon the concept of an ultrafast laser for eye surgery. The advantage of an ultrafast femtosecond laser over other types of lasers is its ability to precisely cut tissue with minimal damage to surrounding tissue. Furthermore, the femtosecond laser can cut from within the corneal tissue using a process called photodisruption that cleanly dissects the tissue. A majority of other lasers work from the surface and move inward.
Initially available at a speed of 6 kHz and then 10 kHz, IntraLase Corp., has quickly improved its femtosecond laser to reduce procedural times and improve outcomes as well as ease of use. A 15-kHz model was introduced in 2003, followed by the 30 kHz in 2005, and the 60 kHz in 2006. With the introduction of the 60 kHz, flap creation time with the IntraLase is now roughly on par with mechanical microkeratomes (±20 seconds). Additionally, the stromal bed and flap quality are even better than with previous generations of the laser. In the latter part of 2006, the company furthered the usefulness of the laser by introducing IntraLase enabled keratoplasty (IEK)—a package of software and hardware tools enabling surgeons to undertake a variety of corneal procedures.
FEMTOSECOND MARKET OVERVIEW
At this time in Europe, a number of femtosecond laser systems are commercially available in addition to the IntraLase system, including the Femto LDV (formerly known as the Da Vinci femtosecond laser; Ziemer Ophthalmic Systems AG, Port, Switzerland), the VisuMax (Carl Zeiss Meditec AG, Jena, Germany), and the Femtec femtosecond laser (20/10 Perfect Vision, Heidelberg, Germany). The Ziemer and Carl Zeiss Meditec systems have also received US Federal Drug Administration (FDA) marketing clearance. The Femtec laser received FDA approval, however, nothing has been filed for marketing clearance.
One scientific criteria used to evaluate any technology is to review the results and published data. There is a great deal of marketing material available on these three systems, but little has been published on their clinical results. No clinical peer-reviewed results have been released yet on the VisuMax system, and results presented with the 20/10 Perfect Vision laser, to date, have focused on the laser's results in penetrating keratoplasty or in conjunction with implantation of intracorneal rings.
Of the systems mentioned, only the Femto LDV has initial LASIK results available. At the 11th European Society of Cataract and Refractive Surgeons (ESCRS) Winter Refractive Surgery Annual Meeting in Athens, Bojan Pajic, MD, of Switzerland, presented results from 10 European sites. Dr. Pajic reported that the mean flap thickness achieved with the Femto LDV femtosecond laser was 141 ±8.5 µm standard deviation (SD) and an intended flap thickness of 140 µm. He compared this result to his experience with a series of IntraLase laser eyes. Here, the achieved flap thickness was 119 ±4.6 µm SD, and the intended flap thickness was 120 µm.
There have been numerous peer-reviewed journal articles comparing the results achieved with IntraLase FS laser LASIK versus mechanical microkeratome LASIK. Of these, the most recent study, reporting on 200 eyes, compared the IntraLase FS laser with a mechanical microkeratome. The investigators concluded that (1) the femtosecond laser is safe and effective, and (2) it yields better visual results, because fewer changes in higher-order aberrations occur.2
One key difference between these three femtosecond laser systems (ie, VisuMax, Femtec, Femto LDV) and the IntraLase laser is in each system's flexibility. With IntraLase, surgeons may program any cutting depth, as well as an infinite variety of cuts. Furthermore, IntraLase is the only femtosecond laser to have FDA clearance for full-thickness corneal procedures. The IEK system provides surgeons with a much greater degree of precision and control than with manual trephines.
With the Femto LDV femtosecond laser, for example, there are preset flap depths and diameters, similar to what is seen with mechanical microkeratomes.
THE BIOMECHANICS
What truly interests me about the IntraLase femtosecond laser is the role it can play in protecting the integrity of the cornea. As I mentioned, I have long been a proponent of PRK, because of its negligible impact on corneal integrity. With the arrival of the femtosecond laser, I became interested in what effect this new keratome would have on the delicate corneal structure.
To determine what type of laser refractive surgery had the most minimal impact on the cornea, my colleagues from the Rayne Institute at St. Thomas Hospital, in London, and I compared various cornea flap creation methods (ie, Epi-LASIK devices, mechanical microkeratomes, IntraLase, PRK) on wound healing and biomechanics. We then evaluated corneal wounds with confocal specular microscopy and shearing interferometry.
From these results, we found that the greatest strength of the cornea, across its depth, lies within the first 150 µm of the stroma. In this region, the corneal lamellae are small and packed the tightest, therefore providing the greatest strength. In the x- and y-plane, the periphery of the cornea is stronger than in the center. We concluded that corneal flaps of approximately 80 µm to 100 µm—essentially a sub-Bowman's flap—with a diameter of approximately 8.5 mm provide the greatest biomechanical stability. During our study, we found the IntraLase FS laser to be the best method to create a sub-Bowman's flap, because of its ability to precisely customize the depth and diameter of the flap with excellent predictability. An IntraLase flap also achieves a planar corneal flap (ie, the flap is consistent across the entire surface of the cornea). When we used mechanical microkeratomes, there was a tendency to cut deeper into the periphery and thinner into the center. This not only weakened the cornea, but it may also have impacted the visual results, due to the creation of an uneven stromal bed.
PRK provides similar corneal strength and biomechanics to the IntraLase laser. There remains, however, a fairly significant trade-off with PRK; transient haze causes initial pain and delay in visual recovery. Patients want to see very quickly following laser refractive surgery, as seen during a study conducted by Steven G. Slade, MD, FACS, of Houston; and Daniel S. Durrie, MD, of Overland Park, Kansas.3 In this contralateral study of 50 patients, Drs. Durrie and Slade performed PRK on one eye and a new technique, sub-Bowman's keratomileusis (SBK), in the fellow eye. The results, presented at the 11th ESCRS Winter Refractive Surgery Annual Meeting in Athens, demonstrated that the SBK eyes had quicker visual recovery in the early postoperative period (ie, 1 day to 3 months). Patients also overwhelmingly preferred their SBK eye in subjective evaluations.
The take-home message—for now—is that you need to be within the stroma to achieve patient comfort. From a biomechanical standpoint, you need to be as superficial as you can get on the stroma, without involving the epithelium. That is why, in my mind, the IntraLase system offers the best of both worlds as of now.
John Marshall, PhD, is the Frost Professor of Ophthalmology and Chairman of the Academic Department of Ophthalmology, at St. Thomas' Hospital, and was formerly Sembal Professor of Experimental Ophthalmology at the Institute of Ophthalmology from 1982 to 1991. Professor Marshall states that he is a paid consultant to IntraLase Corp. He may be reached at june.spacey@kcl.ac.uk.
Up Front | Apr 2007
IntraLase: The Most Versatile Femtosecond Laser Choice
For optimal patient comfort and biomechanical soundness, stay as superficial within the stroma as you can, without involving the endothelium.
John Marshall, PhD
