“Thin Flap LASIK,” also known as SBK, offers safety and accuracy advantages over thicker flaps in LASIK laser eye surgery. A patient from Camarillo in Ventura County recently asked me what “thin flap LASIK” was.
In PRK laser eye surgery, no flap is made. In LASIK laser vision correction, the surface layer of the cornea is lifted as a “flap.” The excimer laser is then applied to the underlying cornea to deliver the new optical shape and then the flap is re-positioned.
It turns out that the thickness of the flap is a very important issue and one that a tremendous amount of technological attention has been paid to over the years.
The trend in modern LASIK eye surgery has been toward thinner and thinner flaps. Ten or fifteen years ago, most flaps were approximately 160 – 180 microns thick (by comparison, a typical cornea is about 550 microns thick). Flaps were thicker in that era partly because flap making technology was less advanced and was not sufficiently up to the task of reliably producing uniform, thin flaps.
Today, most researchers feel that flaps ideally should be 100 – 120 microns thick. A thinner flap has several advantages. First, it leaves more tissue under the flap (a greater “residual stromal bed” or RSB), meaning that the same excimer laser optical treatment will end up going less deep into the underlying tissue. More tissue left undisturbed means a better safety maragin. Also, recent studies have shown that thin flaps leave the cornea biomechanically less altered — again because there is more cornea left to give structural support under the laser treatment. Finally, in a wavefront era, thinner flaps can drape more perfectly over the highly intricate wavefront laser ablation and more perfectly maintain the optical shape the wavefront laser has sculpted into the cornea. A thicker flap tends to “mute” some of the detail of the wavefront shape.
The only real downside to thinner flaps is that they can be technically a bit more challenging for the surgeon to handle. However, with experience, this is a small issue and one that is significantly outweighed by the benefits of thin flap LASIK.
By the way, another name you’ll hear for thin flap LASIK is “Sub-Bowman’s Keratomileusis” or “SBK.” The outer layer of the cornea is Bowman’s membrane, therefore the name SBK implies a flap just underneath Bowman’s layer in the cornea. The other characteristic of SBK flaps besides being thin is that SBK flaps are “planar,” which means they are of uniform thickness throughout the flap. Studies show that thin, planar SBK flaps can be equally accurately created with either the most modern mechanical microkeratomes or femtosecond laser flap making devices.
In my opinion, SBK is the state of the art for LASIK flap architecture and one that I prefer for all my patients.
See Also
A Prospective Study Comparing Thin Flap LASIK (Sub-Bowman’s Keratomileusis) with Photorefractive Keratectomy (PRK)
This study shows no differences in outcomes at 6 months between thin flap LASIK (SBK) and PRK
Thin-flap Laser Assisted Keratomileusis (LASIK)
This paper discusses how thin-flap LASIK (SBK) has similar biomechanical properties to flapless LASIK (PRK) but has the faster recovery typical of LASIK.
Thin-flap (Sub-Bowman’s Keratomileusis) versus Thick-flap laser in situ keratomileusis for moderate to high myopia.
This study showed equivalent visual outcomes in thin flap versus traditional thick flap LASIK.
Real-Time Optical Coherence Tomography-Guided Femtosecond Laser Sub-Bowman Keratomileusis on Human Donor Eyes
This study shows that creation of a thin flap SBK architecture is technically possible with the femtosecond laser (such as Intralase or Ziemer)
Intralase: The most versatile Femtosecond Laser Choice
An overview of the use of the Intralase femtosecond laser to make thin flaps in SBK LASIK
Differences in Corneal Biomechanical Effects of Surface Ablation Compared with Laser In Situ Keratomileusis (LASIK) Using a Microkeratome or Femtosecond Laser
This study compares corneal biomechanics after surface ablation (PRK) with LASIK using both a mechanical microkeratome and a femtosecond laser to make LASIK corneal flaps
State of the Art SBK with a Mechanical Microkeratome
This study shows that a mechanical microkeratome can be used with equal precision as a femtosecond laser to make SBK LASIK flaps
Laser Corneal Refractive Surgery in the Twenty-First Century: A Review of the Impact of Refractive Surgery on Higher Order Aberrations (and vice-versa)
This paper provides an overview of the impact of flap thickness in LASIK on higher order aberrations.
Advanced Techniques: Creating Sub-Bowman’s (SBK) Flaps Using the Bausch and Lomb Zyoptix XP
This study demostrates that the Bausch and Lomb Zyoptix XP Microkeratome creates SBK flaps with equal precision to the femtosecond laser for LASIK flaps