Point/Counterpoint: Is It Safer to Create the LASIK Flap With Femtosecond Technology Than a Mechanical Microkeratome?

MITCHELL A. JACKSON, MD
Yes, femtosecond technology offers the benefits of customized flap creation, improved visual outcomes, and faster visual recovery.
Femtosecond lasers offer several advantages over mechanical microkeratomes for creating LASIK flaps. In addition to a superior safety profile, femotsecond technology offers the benefits of customized flap creation, improved visual outcomes, and faster visual recovery.

SAFETY
Femtosecond technology provides more consistent thin planar flaps than microkeratomes. Thinner flaps avoid weakening the cornea, which lessens the risk of corneal ectasia. They also allow a quicker return of corneal sensitivity and produce fewer neurogenically induced dry eye symptoms, which are common after LASIK. A recent Cleveland Clinic study found that eyes with femtosecond flaps had a lower incidence of LASIK-associated dry eye and required less dry eye treatment than eyes with flaps created using a mechanical microkeratome.¹

Because femtosecond technology does not cut through the deeper layers of the cornea, structural stability is improved. Flap-related complications are virtually eliminated with femtosecond technology. With microkeratomes, if the flap is incomplete, the procedure must be aborted. With femtosecond technology, the surgeon can continue the procedure and create a new flap.

Another advantage is that femtosecond technology provides full visualization of the eye at all times, and surgeons can operate on patients with any orbital configuration or anatomy much easier than with a microkeratome. I have found that, with femtosecond technology, an eyelid speculum is not required in the majority of cases; this instrument can take up valuable room, especially when the patient has a small orbit or tight eyelid anatomy.

The surgeon has increased control over the flap’s dimensions and architecture with femtosecond technology, specifically the IntraLase FS laser (Abbott Medical Optics Inc., Santa Ana, CA) compared with microkeratomes. For example, oval flaps have several advantages over the round flaps created with microkeratomes. The former preserve more corneal lamellar fibers, prevent dry eye symptomatology, and move the hinge to a more peripheral position that maximizes exposure of the stromal bed for full delivery of the excimer laser ablation. Oval flaps also allow for wider hinges, resulting in more stable flaps and greater preservation of the corneal nerves.

CUSTOMIZED FLAP
Microkeratomes are designed to create flaps that are limited to set sizes based on the ring selected (eg, 8.5 or 9.5 mm). With femtosecond technology, surgeons can customize the size, depth, shape, and side-cutting architecture of the flap. The Intralase FS allows surgeons to make an inverted side cut, which provides better adhesion and less slippage of the flap. The side-cut angles of the LASIK flap can range from 30° to 150°. In my experience, the adhesion is so strong that, even an hour after surgery, it looks like no surgery was performed. The flap fits similarly to a manhole cover back into its hole. This superior adhesion provides faster healing and reduces the incidence of epithelial downgrowth.

In the past, microkeratomes could create flaps faster than femtosecond technology. Newer lasers have a faster repetition rate, and the speed of the flap’s creation approaches 10 seconds for a 9-mm diameter flap. Less energy is delivered with the newer technology, a change that is associated with a lower postoperative incidence of diffuse lamellar keratitis (DLK) and a greater safety margin compared with a microkeratome- created interface. The lower energy and tighter spot separation with the IntraLase technology makes intraoperative lifting of the flap virtually effortless, approaching the ease of a microkeratome-assisted flap lift.

Although the cost of microkeratomes is less than that of a femtosecond laser, the former can only be used for LASIK. Surgeons can use femtosecond technology for various procedures such as femtosecond lenticule extraction, Descemet’s stripping endothelial keratoplasty, deep anterior lamellar keratoplasty, and other lamellar and rotational keratoplasties. Femtosecond technology is also FDA approved for creating an anterior capsulotomy in cataract surgery.

VISUAL OUTCOMES
A study conducted by Schallhorn and colleagues found that patients whose LASIK flaps were created with a femtosecond laser had faster visual recovery and better UCVA compared with those whose flaps were made with a mechanical microkeratome.²

The 2008 study included 2,000 eyes treated for low myopia and astigmatism. Half of the eyes had flaps created with femtosecond technology, and half of the eyes had flaps created with a mechanical microkeratome. All eyes underwent wavefront-guided LASIK with a Visx S4 IR Advanced CustomVue excimer laser (Abbott Medical Optics Inc.). At all time points measured (1 day, 1 week, 1 month, and 3 months), the percentage of eyes that achieved a UCVA of 20/20 or better was significantly higher in the femtosecond laser group compared with the mechanical microkeratome group. At 1 day, the percentage of eyes achieving 20/20 UCVA was 90% in the femtosecond technology group and 83% in the microkeratome group. By 3 months, 96% of the femtosecond eyes achieved 20/20 UCVA compared with 93% of the microkeratome eyes. The percentage of eyes achieving 20/20 UCVA by 3 months was similar between the two groups, but the patients who had flaps created with the mechanical microkeratome did not experience the “wow” factor on the day after surgery.²

CONCLUSION
Femtosecond technology offers several important advantages over mechanical microkeratomes for the LASIK flap’s creation. In addition to an improved safety profile and fewer flap complications, femtosecond technology allows surgeons to customize the flap for each patient.

Mitchell A. Jackson, MD, is the founder and medical director of Jacksoneye. He is on the speakers’ bureaus of Abbott Medical Optics Inc. Dr. Jackson may be reached at (847) 356-0700; mjlaserdoc@msn.com.

1. Saolomao MQ,Ambrosio R Jr,Wilson SE.Dry eye associated with laser in situ keratomileusis:mechanical microkeratome versus femtosecond laser.J Cataract Refract Surg.2009;35(10):1756-1760.
2. Tanna M,Schallhorn SC,Hettinger KA.Femtosecond laser versus mechanical microkeratome:a retrospective comparison of visual outcomes at 3 months.J Refract Surg.2009;25(7 suppl):S668-S671.

JAMES S. LEWIS, MD
No, corneal lamellar surgery using metal-blade dissection has been around for over 100 years.
I believe that creating the LASIK flap with a femtosecond laser is not safer than doing so with a state-of-the-art mechanical microkeratome. In fact, I would contend that a modern mechanical microkeratome optimizes patients’ overall safety.

As surgeons, our job does not stop at the end of the case. Consequently, any discussion of patients’ safety must include their lifelong ophthalmic health. Patients’ tolerance for enhancements, their ability to have successful cataract surgery and to withstand trauma, and their need for a lifetime of corneal clarity all require consideration.

TRIED AND TRUE
Corneal lamellar surgery using metal-blade dissection has been in existence for more than 100 years. There are no recognized long-term adverse consequences of this technique. Separating the corneal lamella using a femtosecond laser is a relatively new procedure with a very short clinical track record, and photodisruption of the corneal stroma undoubtedly has some as-of-yet unrecognized consequences. One cannot say with clinical certainty that the patient’s overall ocular health will be preserved when this technology is used.

The manufacturers of femtosecond lasers assert that their LASIK flaps are planar. The geometric precision of this device cannot be denied (Figure 1). Modern mechanical microkeratomes create nearly planar flaps that are often described as “semiplanar” (Figure 2). Because the human cornea is itself semiplanar, it is not clear to me that absolute planarity is a virtue. Both the laser and microkeratome techniques preserve the mechanically important anterior peripheral stroma. I believe that a slightly thickened peripheral skirt to the corneal flap makes its repositioning after LASIK safer, easier, and less prone to associated striae.

Both femtosecond lasers and mechanical microkeratomes can easily fashion 100-µm central flaps. Femtosecond laser manufacturers recommend keeping the flap between 105 and 115 μm thick to avoid interface haze. Modern mechanical microkeratomes have not been associated with thin-flap corneal haze. The Moria OUP SBK (Moria, Antony, France) creates 99-µm flaps without scarring or haze.¹

FEMTOSECOND LASER FLAP COMPLICATIONS
Diffuse lamellar keratitis (DLK) has been the bane of LASIK surgeons for years. The development of single-use instrumentation has dramatically reduced the incidence of this sight-threatening complication. A new form of DLK has emerged that appears to be exclusively associated with femtosecond lasers,² and research suggests femtosecond laser energy is to blame.³

A new condition also attributed to femtosecond laser energy levels is transient light sensitivity syndrome.4 Suggested etiologies include an inflammatory response to necrotic cellular debris, activated keratoctyes, released cytokines, or gas bubble byproducts. There is no parallel condition associated with mechanical microkeratomes.

Femtosecond flaps do not exhibit the same level of homogeneity and smoothness of the lamellar surface as mechanical microkeratome flaps. The texture of the stromal bed is more polished in mechanical microkeratome produced flaps.5 Although some researchers suggest the superiority of femtosecond stromal beds, often these comparisons are of the 110-μm laser-created flaps versus 160-μm blade-created flaps.6 The collagen density and orientation at that level of the cornea do not provide for a valid comparison, which nullifies this conclusion.

GAS BUBBLES AND INFLAMMATION
Femtosecond laser flaps rely on intrastromal photodisruption with subsequent gas bubble formation. In most cases, this procedure appropriately cleaves the stroma along a natural collagen tissue plane. Unfortunately, any failure of corneal homogeneity such as previous scars, ophthalmic surgical incisions, or a lack of absolute corneal clarity can lead to unpredictable results. For example, an opaque layer of bubbles can form and delay or even require the cancellation of the planned LASIK surgery (Figure 3). Dissection along an old RK or arcuate keratotomy incision could lead to significant injury. The tendency of femtosecond gas bubbles to seek out corneal scars or surgical incisions eliminates their value in postkeratoplasty LASIK. Mechanical microkeratomes can be used in all of these scenarios without trepidation.

Femtosecond lasers produce more inflammation in the corneal flap than do modern mechanical microkeratomes.6 Increased inflammation may lead to greater keratocyte death and other unknown long-term sequellae. Some experts hypothesize that increased inflammation leads to greater adherence of the flap, but I am not convinced. To me, it sounds like a rationalization for the use of the laser.

Femtosecond LASIK patients require higher doses of topical steroids in the early postoperative period. The increased use of steroids can result in the reactivation of unsuspected herpes simplex virus keratitis. Steroid use can also be associated with bacterial, viral, and fungal infections as well as IOP spikes. Flaps created with a mechanical microkeratome require minimal steroid treatment and therefore pose less risk to the patient.

Recent claims by the manufacturers of femtosecond lasers highlight their ability to make a beveled peripheral incision that enhances the flap’s adherence and reduces the risk of epithelial ingrowth. In my opinion, these statements are conjecture that is not supported by clinical research. I studied the immediate postoperative appearance of femtosecond flaps created by a world-class femtosecond laser surgeon and found a gap of 10 to 100 μm in every case (Figure 4).

Knorz and Voss 7 reported that corneal flaps in rabbits require more grams of force to dehisce when cut by a femtosecond laser. This was not a human stud. It involved only four mechanical microkeratome-treated eyes, and had an unacceptably high standard deviation. Furthermore, clinically relevant assessments of the flap’s strength should occur after 180 days rather than 75 days, as done in this article. Finally, the number of dislocated flaps outside the immediate postoperative period reported in the world literature is extremely low. Usually, trauma is contrecoup or glancing rather than perpendicular, as done by Knorz.

Letko et al8 reported less epithelial ingrowth after retreatments in femtosecond flap cases than in mechanical microkeratome flap cases. This retrospective study does not control for a number of important variables, including degree of correction, time since initial surgery, or the patient’s age. Furthermore, many surgeons have stopped lifting flaps because epithelial ingrowth is a vexing issue and advanced surface ablation with mitomycin C over a LASIK flap is very effective.

This “femtosecond furrow” should be expected, because the side cut requires more energy than the lamellar cut, and some tissue ablation or shrinkage must occur. This gap does not enhance corneal biomechanics, stabilize the corneal flap, or prevent epithelial ingrowth (Figures 5 and 6).

CONCLUSION
When used by a highly skilled surgeon, a modern mechanical microkeratome like the Moria SBK OUP creates a safe, reproducible, thin, physiologic, smooth flap without late untoward sequellae, inflammation, energy-related DLK, transient light sensitivity syndrome, an excessive need for steroids, or tissue loss. The inflammatory response to the femtosecond laser is an order of magnitude greater than that produced by a single-use microkeratome. (Figures 7 and 8). I am fan of the femtosecond laser for the placement of Intacs (Addition Technology, Des Plaines, IL), full-thickness corneal transplantation, and even cataract surgery. For LASIK flaps, however, I prefer well-honed cold steel.

James S. Lewis, MD, is in private practice in Elkins Park, Pennsylvania. He acknowledged no financial interest in the products or companies mentioned herein. Dr. Lewis may be reached at jslewis@jameslewismd.com.

1. One-Use-Plus SBK.N°10 - Highlights from AAO 2009 - Part 2/2.Moria. http://www.moria-surgical.com/default.asp?cat_id=276.Accessed February 1,2010.
2. Gil-Cazorla R,Teus MA,Benito-Llopis L,Fuentes I.Incidence of diffuse lamellar keratitis after laser in situ keratomileusis associated with the IntraLase 15 kHz femtosecond laser and Moria M2 microkeratome.J Cataract Refract Surg.2008;34:28-31.
3. Javaloy J,Munoz G,Vidal MT,et al.Transient light sensitivity syndrome and diffuse lamellar keratitis are related to femtosecond use.Cataract & Refractive Surgery Today Europe.November 2007;2(8):61-65.
4. Stonecipher KG,Dishler JG,Ignacio TS,Binder PS.Transient light sensitivity after femtosecond laser flap creation:clinical findings and management.J Cataract Refract Surg.2006;32:91-94.
5. Jones YJ,Goins KM,Sutphin JE,et al.Comparison of the femtosecond laser (IntraLase) versus manual microkeratome (Moria ALTK) in dissection of the donor in endothelial keratoplasty:initial study in eye bank eyes.Cornea.2008;27(1):88-93.
6. Sarayba MA,Ignacio TS,Binder PS,Tran DB.Comparative study of stromal bed quality by using mechanical,IntraLase femtosecond laser 15- and 30-kHz microkeratomes.Cornea.2007;26(4):446-451.
7. Knorz MC,Vossmerbaeumer U.Comparison of flap adhesion strength using the Amadeus microkeratome and the IntraLase iFS femtosecond laser in rabbits.J Refract Surg.2008;24(9):875-878.
8. Letko E,Price MO,Price FW Jr.Influence of original flap creation method on incidence of epithelial ingrowth after LASIK retreatment.J Refract Surg. 2009;25(11):1039-1041.
9. Netto MV,Mohan RR,Medeiros FW,et al.Femtosecond laser and microkeratome corneal flaps:comparison of stromal wound healing and inflammation.J Refract Surg. 2007;23(7):667-676.