Though some of my patients benefit from a femtosecond laser flaps, for the majority in my practice, the drawbacks outweigh the benefits.
|Steven E. Wilson, MD|
I AM ONE OF MANY SURGEONS who've been using the femtosecond laser and striving to find its role in the care of refractive surgery patients in my practice. For more than three months, I switched almost exclusively to using the femtosecond laser to prepare LASIK flaps. After identifying both clinical and histopathological treatment issues, along with a notable slowdown in my surgery schedule when more than a few femtosecond laser cases are included, I'm now recommending femtosecond laser flaps only for patients who have specific clinical indications for this technology. These include: eyes in which the cornea is too thin for the intended level of correction with a potentially thicker flap prepared with a microkeratome; corneas that have mean corneal curvature greater than 48 D or lower than 42 D, where flap complications are more likely with a microkeratome (buttonhole flaps, small diameter flaps, and so on); and corneas with anterior basement membrane dystrophy. I've found that these situations now account for approximately 25 to 30 percent of my cases.
Early Inflammation, Impact on Cells
The biggest reason for my reluctance to use the femtosecond laser for all LASIK flaps is the obviously greater inflammation noted in the early postoperative period, resulting in slower visual recovery, even if eyes are treated with a potent corticosteroid such as prednisolone acetate 1% every hour beginning immediately after use of the femtosecond laser.
In my experience, some eyes will develop significant central diffuse lamellar keratitis if this intensive corticosteroid regimen is not used. Based on corneal wound healing research performed in my laboratory (Netto MV, Wilson SE, Dupps WJ, Mohan RR, Krueger RR, American Society of Cataract and Refractive Surgery Annual Meeting, Washington D.C., April 2005), including comparisons of the cellular responses with the Intralase (Irvine, CA) femtosecond laser compared to the Hansatome (Bausch & Lomb) microkeratome, this is likely due to greater epithelial injury or cell death produced at the flap edge with the femtosecond laser than with the Hansatome.
The greater the side cut energy used, the greater the number of epithelial cells injured or killed by the femtosecond laser. There is a vast literature, including our own basic research, showing that the normal uninjured epithelium is filled of pro-inflammatory cytokines like interleukin-1 and tumor necrosis factor alpha that only get out of the epithelial cells in significant amounts when the cells are damaged.1-3 When a large number of these epithelial cells are damaged, the released cytokines attract inflammatory cells into the cornea—both directly and by stimulating keratocytes to markedly up-regulate their production of pro-inflammatory chemokines that also attract inflammatory cells into the stroma. These are the normal pathways for a cornea to respond to the perils of an epithelial injury (for example, the onset of a herpes virus or adenovirus infection) and are markedly greater after LASIK flap formation with the femtosecond laser than with the microkeratome. Thus, immunohistological staining for specific inflammatory cells shows conclusively more monocytes in the cornea after formation of flaps with a femtosecond laser than with a microkeratome.
When side cut energies are decreased, there is less epithelial injury and lower release of these proinflammatory cytokines and inflammation. The release of these pro-inflammatory modulators, however, and the tendency towards inflammation will always be greater with the femtosecond laser than microkeratomes with a sharp blade, unless the epithelial injury produced by the femtosecond laser can be markedly reduced.
This epithelial injury difference is best seen with the scanning electron microscope, but can be easily confirmed by any surgeon at the operating microscope. The surgeon must use a spatula or other tool to locate the edge of the flap after use of a microkeratome, but a groove of epithelial injury is obvious after use of the femtosecond laser.
In my experience, if the femtosecond laser side cut energy is lowered to less than 2.0 mJ, I begin having difficulty lifting the flap in some eyes. If it's lowered to less than 1.7 mJ, I frequently cannot lift the flap at all, although I still see the groove at the operating microscope. My colleagues and I have tried many different combinations of stromal cut energy and side cut energy with the femtosecond laser and we have not been able to reduce the parameters to a level at which inflammation is negligible without having significant flap lifting difficulties. While efforts to make the stromal cut smoother by increasing parameters such as spot density and laser speed will be helpful to create smoother stromal beds, efforts should also be made to reduce epithelial injury by altering the configuration of the side cut where it progresses through the epithelium.
Our studies also show large differences in the number of TUNEL-positive keratocytes along the anterior and posterior lamellar interface at four hours following Intralase laser flap formation compared with microkeratome flap formation. The TUNEL assay is designed to stain cells that are undergoing programmed cell death (apoptosis), but under certain circumstances can also detect cells undergoing necrosis. In any case, a lot more keratocytes die in the stroma above and below the interface after femtosecond laser surgery, and this leads to greater proliferation of residual keratocytes detected by immunohistological staining for mitosis markers. Correspondingly greater numbers of activated keratocytes can be seen at the slit lamp in many patient corneas for weeks or months after surgery. These prominent cells will likely disappear with longer follow-up, but they could be a source of mild glare during the first year or so after femtosecond LASIK.
In defense of the femtosecond laser, our histopathological studies demonstrate that the laser does indeed provide a more planar flap than the Hansatome microkeratome. Intralase Corp. hypothesizes that this flap configuration is better for custom corneal ablation. However, I'm aware of no peer-reviewed data to support this hypothesis, and I remain skeptical that a difference can be demonstrated in the clinical results of LASIK with custom corneal ablation obtained with the femtosecond laser flaps compared with microkeratome flaps.
In the past year and a half, I've had only a single Hansatome flap-cutting complication—an irregular flap—when using the patient guidelines I mentioned above. In contrast, I've had four Intralase flap complications—a superiorly decentered flap, two losses of suction in the middle of the stromal pass, and a very thin flap when attempting to cut a 110-µm flap. The only significant complication was in the eye with the thin flap. When I attempted to cut an even deeper flap (180 µm) three months later, again with the femtosecond laser, another very thin flap was produced and could not be lifted. This patient will likely need transepithelial PRK with mytomycin.
I've also had two patients with severe "good vision photosensitivity syndrome" (GAP) or "transient light sensitivity syndrome" (TLS). Both patients underwent a 10-day course of 1% prednisolone acetate beginning with one drop every hour. One patient was effectively treated, but the other noted disappearance of her symptoms for only about a month before she had a recurrence of the severe photosensitivity without slit-lamp signs of inflammation. I then treated her with a three-week 1% prednisolone acetate taper plus a one-week Medrol oral steroid pack and her symptoms have not recurred. I believe GAP or TLS is likely an inflammation in the ciliary body or other posterior structures related to formation of the pocket with the femtosecond laser since corticosteroids have been shown to have little, if any, effect on activated keratocytes or myofibroblasts.
The Bottom Line
The corneal biology of the situation is very clear and, unfortunately, the femtosecond laser is caught between a rock and a hard place. Increase the side cut energy and you get flaps that are easier to lift, but more inflammation from activation of the normal corneal systems involving epithelial-stromal-inflammatory cell interactions; decrease the energy and you get less inflammation, but increased difficulty raising the flaps.
I guess my approach to the femtosecond laser vs. the microkeratome in LASIK comes down to a simple question. Why would I want to have more patients than necessary that take longer for surgery and have a slower visual recovery with more inflammation (that could also rarely develop TLS)? Some patients, however, definitely benefit from femtosecond laser flap formation. Therefore, the instrument is a critical addition to the armamentarium of any refractive surgery practice. I believe my approach to integrating both the microkeratome and femtosecond laser into LASIK surgery is optimizing surgery flow, while at the same time providing the best procedure for each patient.
Dr. Wilson is director of corneal research, and staff refractive and corneal surgeon at the Cole Eye Institute, the Cleveland Clinic Foundation. He has no commercial or proprietary interest in any microkeratome or the femtosecond laser.
|Vance Thompson, MD, FACS|
Sioux Falls, S.D.
I ALSO USE BOTH THE FEMTOSECond laser and blade microkeratome technology to create flaps. I am comfortable with both technologies having used microkeratome technology for more than 12 years and femtosecond laser technology for three-and-a-half years.