The Ever-Evolving Thin-Flap LASIK

Biomechanical studies show that this procedure combines the benefits of LASIK and PRK.

BY OPHTHALMOLOGY MANAGEMENT NEWS STAFF

Thin-flap LASIK, also known as sub-Bowman keratomileusis (SBK), is a trend that has taken hold in the last several years due to advances in technology and more research into the biomechanics of the eye. SBK treatment appears to possess the advantages of LASIK, such as quick visual recovery and minimal pain, but the biomechanical stability and fewer dry eye symptoms of PRK, according to Daniel S. Durrie, M.D., of Overland Park, KS.

In SBK, originally defined by Dr. Durrie and Steven Slade, M.D., the LASIK flap is 90 to 110 μm thick. Here's a look at some of the latest study results and new techniques for thin-flap LASIK.

Evolution of the Procedure

Over the last 10 years, the evolution of LASIK has been heading toward creating thinner flaps, explains Dr. Durrie, who notes that advances in keratome technology were necessary for this trend to happen. "The reason we were creating deeper flaps is because we wanted to get a good flap every time. The way early microkeratomes worked, the flaps had the tendency to be deeper in the periphery and thinner in the center. If they weren't deep enough, you could get a button-hole," he says.

In 1998, when the Hansatome (Bausch & Lomb [B&L], Rochester, N.Y.) was introduced, 160 μm and 180 μm flaps were deep enough to achieve a good flap almost every time, and that became the worldwide standard because of safety. The idea of thin-flap LASIK did not really evolve until the IntraLase (Advanced Medical Optics [AMO], Inc., Santa Ana, Calif.) was introduced 8 years ago. Then, Dr. Durrie and others began to question whether thin flaps could be made safely.

"Initially with the IntraLase, we made deeper flaps just like we did with blades," he explains. "Usually, the depth was 140 μm or 150 μm in the cornea. There was not a lot of reason for surgeons to change, other than we noticed that we were having some dry eye issues in some patients who had pre-existing dry eye, and we also started talking more about the biomechanical structure of the eye and the structural integrity of the cornea after it had a flap."

Studies by John Marshall, Ph.D., of King's College, University of London, showed that the corneal biomechanics of LASIK procedures done with thinner flaps (around 100 μm in depth) looked as good as those of PRK cases. His work demonstrated that a thinner flap would be better for the cornea in the long-term because it involves the cutting of fewer fibers and nerves (Figure 1).

Clinical studies of SBK surgery at a depth of 100 μm are ongoing. Although the IntraLase remains the preferred method of flap creation in SBK cases, new bladed keratomes expressly designed for SBK surgery are being used as well. Proponents of SBK are currently asssessing the advantages and disadvantages of laser technology vs. bladed keratomes for thin-flap LASIK.

Figure 1. Studies by John Marshall, Ph.D., demonstrated that a thinner flap would be better for the cornea in the long-term because it involves the cutting of fewer fibers and nerves.

How It's Done

Dr. Durrie primarily uses the IntraLase to make an 8.5 mm LASIK flap of approximately 100 μm to 110 μm. "I have seen a significant improvement in dry eyes in my practice. The incidence of post-LASIK keratoconus has gone down, and I am comfortable with where we are right now. I get good flaps every time, I am getting quick visual recovery and cutting less fibers and less nerves than before," he says. Dr. Durrie has conducted several recent studies that look at SBK in his practice and confirm the positive outcomes of the procedure.^1,2

David R. Shapiro, M.D., in practice in Ventura, Calif., uses B&L's Zyoptix XP Keratome for SBK. "I perform thinflap LASIK—more specifically, SBK flaps—on all my LASIK patients," explains Dr. Shapiro, who presented a poster on the technique at the 2008 meeting of the American Society of Cataract and Refractive Surgeons (ASCRS).³

The SBK flap should be planar in architecture, he explains. Both the IntraLase and the XP keratomes have been shown to produce planar flaps, while older mechanical microkeratomes make meniscus-shaped flaps that might be "thin" in the center but thicker peripherally, he says (Figure 2).

Dr. Shapiro's technique safely and reproducibly produced an SBK flap; buttonholes or free caps were not seen. "The XP flap can be tailored for each individual eye in diameter, hinge position and centration on the cornea," he says, adding that the Zyoptix XP shows less variation in flap thickness than the Hansatome and is less affected by measurable preoperative variables, such as spherical equivalent.

Dr. Shapiro points out that studies of high-frequency ultrasound by Dan Z. Reinstein, M.D., show that the need for enhancement surgery results from this biomechanical effect and from epithelial hypertrophy on top of the flap. "Minimization of the biomechanical effect translates into a lower enhancement rate as the only corneal shape change after surgery will tend to be that sculpted by the excimer laser," he says, although epithelial hypertrophy will still, in some cases, result in the need for an enhancement.

Proceed With Caution

Thinner flaps also allow safer application of deeper ablations by sparing more corneal tissue in the bed.

Dr. Shapiro does point out one caveat regarding the residual corneal bed. "Although the use of the 250 μm bed as a lower acceptable limit has come into debate, it is still the most widely accepted standard for a minimal residual of tissue to be left after flap creation and laser ablation," he says. "Too often, I have heard of surgeons trying to ‘squeeze’ in an ablation by calculating a residual stromal bed based on an intended flap thickness of exactly 100 μm. It is important to remember that, at best, there is still some plus/minus variation in achieved flap thickness, even with the IntraLase or the XP microkeratome, so one should always leave a ‘buffer’ of two standard deviations when predicting final flap thickness just in case a slightly thicker than intended flap is achieved."

Figure 2. Both the IntraLase and the XP keratomes have been shown to produce planar flaps, while older mechanical microkeratomes make meniscus-shaped flaps that might be thin in the center, but thicker peripherally.

Dr. Shapiro also advises caution in interpreting the plus/minus variation data for intended vs. achieved flap thickness, as there are conflicting accounts of where two standard deviations lie for both the IntraLase and the XP. "If there is any doubt about there being enough room, then it is very reasonable to simply perform PRK instead," he says.

Researching Biomechanics

In recent years, more research has focused on the biomechanics of the eye and wound healing after refractive surgery. A colleague of Dr. Marshall's, Nathaniel E. Knox-Cartwright, M.A., MRCOphth, of St. Thomas Hospital at Kings College, presented his laboratory-based study on SBK vs. conventional LASIK at ASCRS. It found that SBK is mechanically superior to conventional LASIK.⁴

Dr. Knox-Cartwright's physiological wound healing study was designed to quantify the mechanical consquences of surgical incisions at different depths. The study used 60 kHz IntraLase LASIK flap incisions, using side-cuts or subsurface delamination in human corneas at 90 μm and 160 μm depths. Strain was measured before, immediately following and 1 week after surgery using a radial shearing speckle interferometer. "To combine the mechanical advantages of surface ablation with the more rapid recovery following intrastromal ablation, the shallowest possible intrastromal flap should be created," he explains.

He found that postoperative strain increase positively correlated with vertical incision depth and did not recover during wound healing. "Compared to 90 μm depth flaps, 160 μm flaps resulted in significantly less strain increase (10% vs. 33%). A similar magnitude of strain increase was seen following creation of side cuts alone, a procedure in which a similar number of collagen fibrils are severed. Minimal strain increase followed delamination alone," he explains. "The benefits of thin-flap LASIK are that it maintains the benefits of conventional LASIK — rapid recovery, no haze and no pain — while minimizing the biomechanical impact of surgery. This is because shallower flaps cut fewer collagen fibers and so weaken the cornea less."

In a recent study, Dawson et al.⁵ also reported on the biomechanical and wound-healing characteristics of corneas after excimer laser keratorefractive surgery. They concluded that ophthalmic pathology and basic science research show that SBK and advanced surface ablation (ASA) are improvements in excimer laser keratorefractive surgery compared to conventional LASIK or PRK, particularly with regard to maintaining corneal biomechanics and perhaps moderately reducing the risk of corneal haze. However, most of the disadvantages caused by wound-healing issues remain.

For the study, Dawson et al. performed histologic, ultra-structural and cohesive tensile strength evaluations on 25 normal human corneal specimens, 206 uncomplicated LASIK spec i mens, 17 uncomplicated SBK specimens, four uncomplicated PRK specimens, two uncomplicated ASA specimens, five keratoconus specimens, 12 postoperative LASIK ectasia specimens and one postoperative PRK ectasia specimen. The findings were then compared to previously published studies.

Histologic and ultrastructural studies of normal corneas showed significant differences in the direction of collagen fibrils and/or the degree of lamellar interweaving in Bowman's layer, the anterior third of the corneal stroma, the posterior two-thirds of the corneal stroma and Descemet's membrane. Cohesive tensile strength testing directly supported these morphologic findings as the stronger, more rigid regions of the cornea were located anteriorly and peripherally.

In a 2005 study,⁶ Cobo-Soriano et al. demonstrated that thin-flap LASIK (with flaps less than 100 μm) is a safe technique to correct myopic defects because it blends the advantages of surface and lamellar procedures (minimal debilitation of corneal biomechanical architecture with the rapid and comfortable visual recovery of lamellar approaches). Moreover, it achieves excellent refractive outcomes, a lower rate of enhancements and a good visual performance with better contrast sensitivity test results.

The retrospective study, conducted in Madrid, Spain, comprised 280 consecutive eyes that had undergone LASIK for myopia correction using the Moria LSK-One microkeratome (Doylestown, Pa.) and the Technolas 217C excimer laser (B&L). Efficacy, predictability and contrast sensitivity indicators were compared between three groups of flap thickness: thin (<100 μm), medium (100-129 μm) and thick (>130 μm).

"Refractive results were excellent and comparable between the three groups. However, such visual-outcomes measures as efficacy, postoperative evolution of uncorrected visual acuity and contrast sensitivity were significantly better in the thin-flap group," Dr. Cobo-Soriano reported.

Efficacy results were 92.9%, 91.0% and 81.0% in the thin-, medium- and thick-flap groups, respectively (P< 0.05), and the rate of enhancements was 0%, 2.3% and 5.6%, respectively. In regard to contrast sensitivity changes between preop and postop values at 3 months, the thin flap group achieved the preop levels at three spatial frequencies (three, six and 18 cycles per degree), while the thicker-flap groups maintained lower than preoperative levels at more than two spatial frequencies. When comparing contrast sensitivity values between the three groups, the thin-flap group also obtained the best results at lower spatial frequencies.

Complications

In a study to assess the safety and complication rate of SBK in 3,009 eyes, Chang found that the complication rate of SBK is low and vision loss is also rare.⁷ The flaps were created using the IntraLase femtosecond laser, and excimer ablation was performed with the B&L H.Eye.Tech laser system. The complications of SBK (grouped into intra- and postoperative flap-related) were evaluated retrospectively. Overall, the total complication rate was 0.63%. Intra- and postoperative flap-related complication rates were 0.33% and 0.30%, respectively.

Some refractive surgeons choose to steer clear of thin-flap LASIK due to the procedure's potential complications. "I do not perform thin-flap LASIK," explains Salomon Esquenazi, M.D., assistant professor of ophthalmology and neuroscience research, Louisiana State University Health Sciences Center, New Orleans, and corneal department, Rand Eye Institute, Pompano Beach, Fla. "A study performed by my group published in the Canadian Journal of Ophthalmology⁸ recently showed that the rate of early postoperative complications, including button-holes and flap wrinkling, was unacceptably high. Currently, in candidates for thin flaps I perform ad vanced surface procedures with great success."

In the 2007 study, Esquenazi et al. evaluated the safety and long-term refractive stability of LASIK for myopic correction performed under thin flaps (<110 μm) and compared them with those of conventional thicker flaps (>110 μm). This retrospective study included 120 myopic eyes of 68 patients who underwent LASIK performed with the Moria M2 microkeratome and the Technolas Keracor 117C excimer laser.

They concluded that while thin-flap LASIK (<110 μm) produces more intra- and early postoperative complications compared with thick-flap LASIK (>110 μm), there does not appear to be any difference in the long-term refractive results if a thin flap is created and there are no complications, or if the complications are successfully managed.

Future Outlook

For success with thin-flap LASIK, Dr. Durrie notes that the proper technology is crucial. "This procedure can't be done with any microkeratome. You have to have a laser or microkeratome that is designed to create thinner flaps," he says.

Flap customization is the next step, says Dr. Durrie. "We now are starting to customize the flaps more, especially with femtosecond lasers. We are starting to do the flaps a little smaller. Maybe we are going to angle edges a little more for a better fit to provide stability of the flap. This is a great time for us to start thinking about how to make the best flap for the individual patient," he says.

"If you look at the refractive surgery development, from the early to mid 1990s, we really hadn't had any major developments in the flap design. We were working on wavefront-guided surgery, wavefront-optimized surgery, blend zones, getting rid of halo and glare at night, accuracy and algorithms. It may be the flap is even more important than we thought and can play a role in the best quality surgery that we can do both safely and accurately," he says.

He notes he is continuing to do research on edge design and whether oval flaps or round flaps are better. "There are a lot of things I think we can fine-tune a little bit more, but I don't think I will go back to doing thick flaps again," he says.

According to Dr. Durrie, one thing that gets overlooked in all the talk of "thin-flap" LASIK is smaller flaps for LASIK. "It is just as important as the thin [issue] because when you go from a 9.5 mm diameter down to 8.5 mm diameter there is a big difference. But if you look at the overall surface area and the number of fibers that are being cut, the stronger fibers of the cornea are in the periphery, so smaller flaps are better biomechanically. I think smaller flaps will be the next area of research," he says.

However, in order to create a smaller flap, surgeons have to make sure they center the flap properly every time. The method varies depending on the type of keratome used. "I go into operating room everyday and say I am going to perfectly center every flap. It doesn't make any difference if you are doing hyperopic ablation or myopic ablation. With all of the lasers, the major optical zones are 6.5 mm to 7 mm, so surgeons can easily get the refractive correction within the bounds of the flap," Dr. Durrie says. OM

References

1. Waring GO 4th, Durrie DS. Emerging trends for procedure selection in contemporary refractive surgery: consecutive review of 200 cases from a single center. J Refract Surg. 2008 Apr;24:S419-423.
2. Durrie DS, Slade SG, Marshall J. Wavefront-guided excimer laser ablation using photorefractive keratectomy and sub-Bowman's keratomileusis: a contralateral eye study. J Refract Surg. 2008 Jan;24:S77-84.
3. Shapiro, David R. New Technique for Making Precise, Predictable Sub-Bowman LASIK flaps using B&L's Zyoptix XP Keratome. Poster presented at: ASCRS 2008 annual meeting, Chicago April 4-9.
4. Knox Cartwright, Nathaniel. Sub-Bowman Keratomileusis Vs. Conventional LASIK. Poster presented at: ASCRS 2008 annual meeting, Chicago, April 4-9.
5. Dawson DG, Grossniklaus HE, McCarey BE, Edelhauser HF. Biomechanical and wound healing characteristics of corneas after excimer laser keratorefractive surgery: Is there a difference between advanced surface ablation and sub-Bowman's keratomileusis? J Refract Surg. 2008 Jan;24:S90-96.
6. Cobo-Soriano R, Calvo MA, Beltrán J, Llovet FL, Baviera J. Thin flap laser in situ keratomileusis: analysis of contrast sensitivity, visual, and refractive outcomes. J Cataract Refract Surg. 2005 Jul;31:1357-1365.
7. Chang JS. Complications of sub-Bowman's keratomileusis with a femtosecond laser n 3009 eyes. J Refract Surg. 2008 Jan; 24:597-601.
8. Esquenazi S, Bui V, Grunstein L, Esquenazi I. Safety and stability of laser in situ keratomileusis for myopic correction performed under thin flaps. Can J Ophthalmol. 2007 Aug; 42:592-599.