The following article is by Dr. Stephen Slade:
Why a Customized, Sub Bowman’s Flap may be the Best Approach in LASIK
Which is better – flap or no flap? Does a thinner flap offer definite benefits over thicker LASIK flaps? The debate over the optimum thickness of a LASIK flap or whether an epi-LASIK/PRK approach really is best seems to go on and on. I think this is about to change. Why? There is a growing body of evidence that suggests that the best option for our refractive surgery patients is creation of a customized, sub Bowman’s flap, and, the most consistent and safest method for making this flap is with a femtosecond laser. (John Marshall, ESCRS)
The debate over the appropriate thickness of a LASIK flap has much to do with the growing fear over corneal ectasia. Increasingly, we’re all becoming aware that there is a limit to how much corneal tissue can be removed without the risk of a negative response. This fear has a lot to do with the move back to PRK or epi-LASIK. If you take a 550- micron cornea, remove 50 microns when you remove the epithelium, then another 100 microns with the laser ablation, the patient is still left with 400 microns of cornea. With a traditional LASIK flap of 150 microns and the same 100-micron ablation, the remaining cornea drops down to 300 microns. Now if we had a 70-micron flap, the bed is 380 microns that is only a 5% difference in residual stroma compared to the PRK eye. And the thin flap LASIK has some strength from the flap edge as well as the reduced wound healing advantages of LASIK. Indeed, except in cases of extreme thick flaps most ectasia patients appear to be keratoconus patients that were not diagnosable at the time of surgery.
Prof. John Marshall and his group at King’s College in London have compared the relative strength of the cornea following PRK, epi-LASIK and thin flap LASIK or SBK, Sub Bowman’s keratomileusis and have concluded that with a femtosecond LASIK flap of 80 or 90 microns, the biomechanics of the SBK cornea are indistinguishable from the surface ablation cornea. Thus SBK might offer the patient the best of laser vision correction, the comfort and recovery of LASIK with optimum safety.
SBK does require a very consistent flap thickness across the entire flap to avoid button holes or perforations. In our current practice, all flaps are designed to be 90 microns across the entire flap diameter. With the femtosecond laser we are able to achieve consistent flaps with very small flap thickness standard deviations. (Figure 1)
This point about consistent thickness is an important one to consider, not only because of the relative strength of the flap, but also because of visual outcomes. Because of the way mechanical microkeratomes work, the resulting flap is thicker in the periphery and thinner in the center. With the femtosecond laser, the cornea is applanated during the flap-creation process, creating a flap that has relatively the same thickness across the diameter. A small study presented at the Wavefront Congress earlier this year by Ronald Krueger, MD, of the Cleveland Eye Clinic showed that there is enough of a difference in how the flaps are created to impact the refractive outcomes.
One final point to consider about flap-thickness consistency is this – it is relatively difficult, if not impossible to create consistently thin (± 100 microns) with mechanical microkeratomes because the standard deviation, even with the latest generation of mechanical microkeratomes is still too high.
There are a number of other advantages in using SBK flaps and controlling diameter as well:
- Fewer nerves are cut;
- A trend toward improved visual outcomes, particularly with customized LASIK; (Durrie, Slade ESCRS 2006)
- Thinner, smaller diameter flaps take less time to create;
- There is a reduced occurrence of post-LASIK dry eye, and;
- There is less risk of loss of suction with smaller diameter flaps.
- Fewer corneal fibers are cut leaving strength in the cornea.
My goal is to create a flap that averages 80 microns, with a diameter that is between 1 to 1.5 mm outside of the ablation zone. I have three excimer lasers in the operating rooms – an Alcon Ladarvision 4000 (Alcon, Ft Worth, TX), an Alcon Ladarvision 6000 (Alcon, Ft. Worth, TX) and a Bausch & Lomb Technolas 217 Laser (Bausch & Lomb, Rochester, NY). If I’m using the Alcon laser, the maximum ablation zone is 6 mm, so the flap diameter is set for 7.5 mm. With the B&L Laser, the flap size will be between 7.5 and 8 mm for a 6 mm ablation. I do not believe that the flap needs to be any wider than that and my patients have not experienced any glare or halos since I adopted this approach.
The great advantage of the femtosecond laser is that you can program the laser to customize the flap according to the diameter, flap thickness and even the pre-existing astigmatism to create a flap that matches the pattern. You can also program the laser to create a flap edge that has notches or is beveled in order to create even more surface area at the edge helping to improve wound strength.
We are currently involved in a clinical study with Dan Durrie, MD, in Overland Park, Kansas, to compare the clinical results and visual outcomes of these thinner flaps versus PRK. The study is contralateral and involves 50 patients who will undergo PRK in one eye and all laser LASIK with the 60 kHz femtosecond laser (IntraLase FS Laser, IntraLase Corp., Irvine CA) in the contralateral eye. Our early results at one and three months show the SBK eyes to be ahead of the PRK eyes in all metrics, biomechanics and wavefront.
Another point we hope to answer in this study is exactly what is the optimum thickness for the flap. If thick flaps present a risk, so do flaps that are too thin. We will be doing subtraction pachymetry in order to really look at this and attempt to get it quantified.
The current work that is being done into optimum flap thickness and customized sub Bowman’s flaps with the femtosecond laser provide us with a unique opportunity to redefine LASIK. I agree with John Marshall; SBK may well be the best way to do corneal surgery. Today, we have significantly better options to provide our refractive surgery patients with ever improving results.
Dr. Slade is medical director of Slade & Baker Vision Center in Houston.