Corneal Flap Architecture

The stromal flap in LASIK can be created with either a mechanical microkeratome or a femtosecond laser. The architecture, or morphology, of femtosecond laser and microkeratome flaps differ, especially in the periphery, as a result of the different means of flap creation. The application of femtosecond laser technology to LASIK surgery has led to improvement in flap thickness reproducibility, reduction of induced higher-order aberrations (HOAs), increased stromal bed quality and biomechanical response,¹ better control over flap diameter, independence from concerns regarding corneal contour and diameter, and lower risk of free and buttonhole caps.² This article compares the morphology of LASIK flaps created by a femtosecond laser and mechanical microkeratome.

FLAP FORMATION
A femtosecond laser flap is formed by laser photodisruption parallel (main flap) and perpendicular (sidecut) to the corneal surface. Microkeratome flaps are created by a single continuous mechanical cut. The femtosecond laser flap depends little on the nature of the treated cornea, except for corneal scars and opacifications, whereas mechanical interaction takes place between the microkeratome and the cornea.³ With microkeratomes, the course of the lamellar cut depends on the opening gap of the blade, its oscillation speed, the consistency of compression during forward movement of the blade, and the steepness and stiffness of the cornea.³

FLAP SHAPE
To prevent keratectasia after LASIK, central flap thickness accuracy is crucial; residual stromal thickness must be at least 250 µm. Femtosecond lasers are designed to make thinner LASIK flaps with a tighter range. They also tend to be more uniform in thickness from the center to the periphery than microkeratome flaps.¹ With mechanical microkeratomes, flap shape is typically thicker in the periphery and thinner in the center, thus creating a meniscus-shaped flap. This shape increases the incidence of buttonhole perforation and introduces lower-order aberrations such as astigmatism and HOAs such as trefoil. 3 Flaps created with the a femtosecond laser are more uniform in thickness, producing a planar shape.¹ This more regular shape induces less visual disturbance than microkeratome flaps.³

COMPARING FLAPS
Four years ago, we performed our first cases with the PresbyLens (ReVision Optics, Inc., Lake Forest, California) corneal inlay, which is implanted under a LASIK flap in plano presbyopic patients.

For the first 2 years of the study, we created flaps using the Hansatome microkeratome (Bausch + Lomb, Rochester, New York). For the last 2 years, we created flaps with the 60-kHz IntraLase femtosecond laser (Abbott Medical Optics Inc., Santa Ana, California). We used anterior segment optical coherence tomography to evaluate all patients for the past 3 years.

The architecture of flaps created with the IntraLase was more regular and accurate compared with flaps created with the microkeratome (Figure 1).

CONCLUSION
Flap preparation is a crucial part of the LASIK procedure, and with the advent of corneal inlays it will become an important part of these procedures as well. Regularity and reproducibility of flap morphology improve the safety and visual outcomes of corneal refractive surgical procedures.³

Based on our results to date with the PresbyLens, we recommend flap creation with a femtosecond laser for implantation. Flap thickness should be targeted for 150 µm.

Enrique Barragan, MD, is Medical Director of Laser Ocular Hidalgo, Monterrey, Nuevo Leon, Mexico. Dr. Barragan states that he is a consultant to Revision Optics, Inc. He may be reached at e-mail: enrique_barragan2000@yahoo.com.

Sandra Gomez, OD, is an employee with Revision Optics, Inc. Dr. Gomez may be reached at e-mail: sgomez@revisionoptics.com.

1. 1.Salomao M,Wilson S.Femtosecond laser in laser in situ keratomileusis.J Cataract Refractive Surg.2010; 36(6):1024-1032.
2. Martins AR,Murta J,Quadrado M,et al.Femtosecond laser versus mechanical microkeratomes for flap creation in laser in situ keratomileusis and effect of postoperative measurement interval on estimated femtosecond flap thickness. J Cataract Refractive Surg.2009;35(5):833-838.
3. Von Jagow B,Kohnen T.Corneal architecture of femtosecond laser and microkeratome flaps imaged by anterior optical coherence tomography.J Cataract Refractive Surg.2009;35(1):35-41.