Today, corneal laser refractive surgery can be considered a tailored procedure because, in most cases, it is customized to the patient. We now have good enough diagnostic tools and surgical options to achieve the best possible result in every case. However, to obtain optimal results we must commit to deciding on a case-by-case basis whether treatment on the corneal surface or lamellar treatment is the best approach.
When talking with patients, I always highlight how we prepare the eye for surgery, and I follow that with a description of how laser treatment provides the desired refractive effect. We should consider then two aspects: how do we prepare the cornea, and how do we treat it with the excimer laser? As we all know, the final visual and refractive results depend on the laser technique, such as standard, optimized, wavefront-based, topography-guided, or aspheric. There are subtle differences between treatments when comparing changes in higher-order aberrations, contrast sensitivity, and point spread function (PSF) that favor use of surface approaches, due to absence of the corneal flap, and therefore optical interference.1 The same may be true for lamellar techniques. But, if we assume that the final visual result is similar clinically and from the patient's point of view, then we must base our decision on how each technique affects corneal biomechanics.
In surgical planning, I always take into account corneal ultrasound pachymetry. Every eye with a central corneal thickness of less than 500 μm is evaluated for a surface approach. If the patient's refraction is too high (superior to -6.00 D), I choose an intraocular technique, such as a phakic IOL. If the refraction is within the acceptable range, I proceed with surface ablation.
Before the femtosecond laser era, I took part in many controversial discussions regarding flap thickness. Some of my colleagues argued that thicker flaps were better; however, this completely changed when approaches such as sub- Bowman's keratomileusis (SBK) became popular. We have learned (at last!) that thinner flaps are better for LASIK because we produce less stress on the cornea.2
We now know that the deeper we cut, the more we affect corneal biomechanics.3 SBK is sometimes compared with surface techniques because it has little effect on corneal biomechanics. However, a cut is still a cut; in some eyes (such as those with thin corneas), a surface technique is the most conservative approach. We should also remember that, although ectasia has been reported after PRK, it is a complication more frequently associated with LASIK.4
I determine the ablation depth by calculating the preoperative refraction and looking at the scotopic pupil to determine the ideal optical zone. When considering a lamellar technique, I always try to keep a residual stromal bed of approximately 300 µm. I will not ablate more than the equivalent of 20% of total corneal thickness, and as a general rule, I avoid ablations greater than 110 μm. I choose a conservative approach because I have very good results with phakic lenses.
Taking these general rules into account, some cases are better approached at the surface because surface treatment is more conservative and preserves more corneal tissue. In this situation, I aim for a final total corneal thickness of 400 μm or greater; other surgeons will go as low as 350 µm. In primary cases, I do not like to ablate more than 80 µm, as I prefer not to use mitomycin-C. This means I rarely use surface treatment for eyes with myopia greater than -5.00 or -6.00 D. Many surgeons who treat myopia in this range add mitomycin-C to avoid the formation of haze.
When considering the patient's refraction, I have learned three important points from my practice. First, patients with low myopia (less than -3.00 D) are good candidates for surface ablation because their visual results are typically excellent and their resultant quality of vision in the sensitive benchmarks of higher-order aberrations and contrast sensitivity is better than with lamellar techniques. We presume this is due to the absence of the lamellar cut; however, the femtosecond laser has improved the quality of vision after lamellar procedures. Second, enhancement rates after surface ablation are one-fifth those after lamellar techniques. Third, I perform lamellar techniques in eyes with high astigmatism and hyperopia because the ablation pattern promotes haze. This is a result of the epithelium's response to characteristic changes in corneal curvature. When a surface approach is chosen in these cases, use of mitomycin-C is mandatory.
RULE OUT IRREGULAR CORNEAS
Perhaps the most important criterion when choosing between surface and lamellar approaches is to rule out irregular corneas, especially with sign of a steep inferior cornea. Patterns of keratoconus are relatively easy to identify on standard corneal topography, but subclinical situations can be difficult to identify. Certain patterns on the axial keratometric and tangential maps signify an irregular cornea. For example, the anterior and posterior elevation maps will be decentered or too high with a steep inferior cornea; also, the pachymetry map will appear altered when the thinnest point on the cornea is located far from the center.
Many devices have specific software or indices to classify the cornea as normal or abnormal. I always insist that a quick look at the axial and tangential keratometric maps should be enough to alert us to suspicious cases. New devices have been introduced to study and identify these patients— topographers that employ Scheimpflug technology; optical coherence tomography devices; high-frequency ultrasound imaging to map the epithelial layer and to identify the thinnest suspicious points; and the Ocular Response Analyzer (ORA; Reichert, Inc., Depew, New York), which allows us to study biomechanical properties of the cornea and, with the most recent software, classify it as normal or abnormal.
I avoid lamellar techniques in suspicious cases. I treat some regular irregularities with a surface approach because it is less aggressive and can diminish the effect on corneal biomechanics. Examples of a regular irregularity include traumatic scars or after adenovirus keratitis, contact-lens—related warpage, moderate steep superior corneas due to chronic blepharitis, and irregularities after pterygium surgery. I have not seen a single case of ectasia in more than 10 years by following these basic rules—or maybe these patients just went to see another colleague.
We must choose the best possible surgical technique. On the surface side, several techniques are not only popular but produce impressive results, including PRK, advanced surface ablation, LASEK, and epi-LASIK. I have tried them all; below is a summary of my experience. Whereas PRK is an easy technique to perform, LASEK requires more skill and patience to properly manage the epithelium. Epi-LASIK, which makes the surgical procedure easier and faster and avoids the use of alcohol, is more expensive and introduces the potential complication of intrastromal incursion. LASEK maintains the protective effects of the epithelium, but its use is controversial, especially when we limit it to myopic cases below -6.00 D in which haze is less probable. Lastly, LASEK can be easily converted into PRK if the epithelial flap is difficult to obtain.
Surface techniques have a different postoperative period from LASIK. Despite the subtle differences among PRK, LASEK, and epi-LASIK, patients should be informed that with any surface ablation the recovery is longer than with LASIK. I prefer LASEK (Figure 1) because it maintains the epithelial flap as a protective shield, but I convert many cases to PRK when I am not able to obtain a high-quality flap. I have almost abandoned epi-LASIK. At our institution, we perform 30% to 40% of our laser cases using a surface approach, and we follow the indications mentioned above. I believe that lamellar and surface techniques are complementary; together they give us the surgical tools we need to provide the best solution for every individual.
Francesc Duch, MD, practices at the Institute Catala de Cirugia Refractiva, Barcelona, Spain. Dr. Duch is a member of the CRST Europe Editorial Board. He states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +34 93 418 99 29; e-mail: firstname.lastname@example.org.
- Decide on a case-by-case basis whether treatment on the corneal surface or lamellar treatment is the best approach.
- Surface treatments preserve more corneal tissue.
- Rule out irregular corneas, and avoid lamellar techniques in suspicious cases.