Consider the evolution of refractive surgery: it has gone from surface to deep, and, in many cases, back to surface. PRK was the procedure of choice in early surface surgeries, and its known complications were a painful first night, followed by changes in the corneal transparency, or haze with varying persistence. Subsequent studies showed that controlling pain with self-administration of topical anesthetics over 10 hours did not significantly affect wound healing. Laboratory studies showed that haze resulted from biochemical cross-talk between proliferating epithelial cells and stromal keratocytes. The importance of this interaction was demonstrated if epithelium was debrided but no damage was inflicted on the stroma. In these instances, the corneas healed clear, however, if the epithelium and stroma were simultaneously damaged (ie, PRK), the corneas temporarily lost transparency. If the epithelium was preserved but the stroma damaged, (ie, LASIK), no haze resulted. All surface procedures commonly had pain and haze.
Deep procedures such as LASIK only damage one cell type (ie, keratocytes). These procedures are not associated with haze or pain because of preservation of surface epithelium. As a result of cutting through one-third of the thickness of the stroma, however, both laboratory and clinical evidence shows that induced refractions are not biomechanically stable. Fortunately, the postoperative refractive changes are relatively small; many surgeons undertake enhancement procedures to correct refractive drift. There is an urgent need for detailed long-term follow-up of LASIK patients to clearly define the magnitude of biomechanical effects as reflected in refractive change.
The stroma represents 90% of the cornea. It is made up of collagen fibers packaged in a precise array of tape-like lamellae. In the superficial third of the stroma, lamellae are relatively small in cross section (1 x 50 µm) and are somewhat larger (2.5 x 200 µm) in the deeper two-thirds. The anterior lamellae are more interwoven in alternate layers and have fibers that cross from one lamella to the other. In contrast, the lamellae of the deeper layers are much more regular with less crossing fibers. X-ray diffraction studies have shown that further differences exist in lamellar structure when compared with those in the central, or optical zone, of the cornea with those in the periphery. In the center, lamellae in alternate layers cross predominantly at approximate right angles. Outside the optical zone, the angle of the inset increases to approximately 120º, and at the extreme periphery, the fibers are almost circumferential in orientation. We now know that in cross section, the cornea is strongest biomechanically superficially over the first 150 µm beneath Bowman's flap and relatively weak in the deeper two-thirds. Similarly, the central cornea is much weaker in its structural integrity than the cornea outside the optical zone. This means that, unfortunately, LASIK surgery — as it was originally practiced — cut a flap separating the superficial strong and deep weak cornea and simultaneously cut the weak center and the peripheral strong elements. Biomechanically, this was a worst case situation, and conventional microkeratomes worsened this situation because mechanical blades often cut thicker sections in the periphery than in the center, thus further weakening the postoperative structure.
A clear advantage of the femtosecond laser to create flaps is that flaps can be cut thinner and with optimal precision. Protagonists of surface procedures would argue: Why cut a stromal flap at all?
Advocates of PRK — and more recently epi-LASIK — have argued that using surface techniques avoids any substantial weakening of the cornea. Although correct, there is risk of corneal haze, an original problem of PRK. Manufacturers of keratomes used for epi-LASIK, suggest that the epithelial flap is lifted with no damage to the epithelial cells. These statements are incorrect. The mere fact that the epithelial cells are separated from Bowman's layer means that the anchoring filaments have been broken and, in most cases, damage has been sustained by the basal membranes of the basal epithelial cells. When the epi-LASIK flap is replaced, the viable epithelial cells are free to release their cytokines and initiate precisely the same cross-talk as that seen in PRK. The magnitude of the biochemical stimulus is, however, reduced because epithelial cells cannot proliferate immediately to cover the area of surgery, as epithelial cells of the flap impede them. Holes in the flap or significant flap movement cause apparent early haze. In summary, surface techniques produce biomechanically stable corrections with studies up to 14.5 years after surgery, but have the attendant problems of transient pain and haze. Stromal procedures avoid pain and haze, but may exhibit instability as a result of disturbed corneal biomechanics.
The Intralase (Irvine, Calif) is computer controlled, and the surgeon is able to center the keratectomy perfectly, ensuring that the laser makes a flap at a preset depth of uniform thickness from edge to edge. This optimal centration and uniformly thin flap may reduce the damage to the anterior lamellae (ie, strong part of the cornea) even in conventional LASIK procedures. The nature of the intrastromal ablation may also be helpful in facilitating postoperative wound strength. Conventional keratomes, even excluding the effect of flap unevenness, cut such a smooth surface that when the flap is replaced, its frictional coefficients are low and it may move on the bed. This may delay wound healing and may also contribute to a loss of predictive accuracy in the induced correction. In contrast, the Intralase section can be considered as a pattern of plasmas interconnected by a process similar to hydrodissection. The slightly undulant surface may reduce slippage during flap replacement and further facilitate rapid wound healing.
Outcomes afforded by this Intralase product now offer the potential for the best of both worlds: surface and stromal ablation. The precision of Intralase allows flaps to be cut at a sub-Bowman's level, approximately 40 µm from the surface of Bowman's flap. Because such flaps do not involve a significant amount of the strong component of the stroma, they have the potential of being biomechanically stable. Furthermore, as they do not involve damage to the epithelium, they will not incur pain or haze.
With the ability to create a precise thin Bowman's flap, quicker visual recovery and better visual acuity results, coupled with the unique facility to ablate an ultrathin sub-Bowman's flap, we have become strong believers in the advantages of femtosecond LASIK flaps.
John Marshall, MD, is the Frost Professor of ophthalmology and chairman of the academic department of ophthalmology at St. Thomas Hospital, in London. He states that he is a paid consultant for Intralase. He may be reached at june.space@kcl.ac.uk or +44 20 7188 4296.
Epi-LASIK: Advanced Surface Ablation
By Jorg H. Krumeich, MD
I began performing epi-LASIK 3 years ago because it appeared to be a safer procedure compared with LASIK and enabled me to carry out corrections of up to 6.00 D with no or negligible haze.
WHAT IS EPI-LASIK?
Epi-LASIK, or advanced surface ablation, was initiated to avoid the common problems associated with PRK and LASEK (eg, postoperative pain, delayed visual rehabilitation and the risk of haze). Furthermore, complications normally associated with LASIK such as dry eye; diffuse lamellar keratitis; irregular stromal flap cuts (ie, partial flaps, thin flaps, button holed flaps, decentred flaps and free caps); induced higher order aberrations; epithelial ingrowth; flap striae; interface debris; corneal ectasia; glare; halos; and night vision problems are avoided.
Ioannis Pallikaris, MD, PhD, conceived the idea of epi-LASIK. He recognized that a dull-edged plastic separator, which moves across the path of least resistance, has the potential to separate the epithelium successfully below the basement membrane and just above Bowman's layer. Several published studies have demonstrated the safety and efficacy of the technique for the correction of low myopia. In one recent study by Pallikaris, 44 eyes (31 patients) underwent epi-LASIK for the correction of low myopia. Mean epithelial healing time was 4.86 ±0.56 days (range 3 to 5 days). The mean logMAR uncorrected visual acuity on the day of reepithelisation was 0.19 ±0.09 (range 0.40 to 0.10). At 1 month, the mean was spherical equivalent of the treated eyes (n=44), -0.30 ±0.60 D (range -1.00 to 0.87 D), and at 3 months it was (n=37), -0.10 ±0.40 D (range -0.75 to 0.75 D); 97% of eyes had clear corneas or trace haze 3 months after treatment.
INDUCED HIGHER ORDER ABERRATIONS
The main advantage of epi-LASIK is the avoidance of flap-related problems (eg, induced higher order aberrations) because biomechanics of the cornea are not compromised. Creation of a LASIK flap leads to disruption of the structural stability and biomechanical function of the cornea. The severing of the lamellae results in a loss of tension, allowing the peripheral lamellar tissue to absorb water, thereby flattening the cornea at the center, and as the periphery is untouched, a relative steepening at the periphery may result. If not counteracted by Q value measures, higher order aberrations or an oblate a cornea may ensue.
Several studies have demonstrated a significant change in the induced postoperative higher order aberrations from flap creation alone. By not creating a flap, the problems of buttonholes, micro- or macrostriae, diffuse lamellar keratitis, epithelial ingrowth, free flaps, slipped flaps or dislocated flaps are eliminated. The procedure allows corrections for thinner corneas that could not be done with LASIK because the chance for ectasia is less.
Dry eye. Dry eye is the most common complication of LASIK surgery. LASIK often exacerbates a preexisting dry eye condition, however, the eye returns to baseline by 6 months postop. LASIK may also cause a temporary dry eye in patients with subclinical dry eye prior to surgery. It is thought that symptoms of dry eye are caused by the severing of corneal nerve fibers causing termination of the feedback loop between the brain and the lacrimal glands in the eye. This results in the transmission of information being inhibited and the eye receiving inadequate amounts of natural lubricating tears. Corneal nerve fibers regenerate slowly, which may result in dry eye symptoms lasting up to 6 to 12 months following LASIK. Epi-LASIK removes only the ends of the nerves found at the interface of Bowman's layer and basement membrane, and, therefore, dry eye syndrome may be significantly reduced or nonexistent.
Haze. Corneal haze is a significant problem with PRK. In a presentation at the XXII Congress of the European Society of Cataract and Refractive Surgeons, John Marshall, MD, explained that haze occurs because there is a communication between healing epithelial tissues and the healing keratocytes of the stroma. Epi-LASIK uses gentle mechanical separation, thereby avoiding damaging cells and inciting keratocyte activation.
New postop pain regimes. Pain has been an issue with surface ablations. If care is taken to match the size of the epithelial flap and denuded Bowman's bed, however, pain is minimized with epi-LASIK. Comprehensive postoperative pain regimes can further minimize any discomfort patients experience. Pain can be reduced to well tolerable amounts by systemic medication 2 x 50 mg diclofenac tablets and 100 mg of sistrane, as well as topical voltaren eye drops every 1 to 2 hours for the first day if needed.
Another advantage of epi-LASIK is that reattachment of the cut epithelium may be attained by using a cooling regime before the cut (thus avoiding stretching of the epithelial layers by the returning microkeratome) and using pressure on the replaced epithelial layers. Epi-LASIK avoids many of the complications that can occur with LASIK. The procedure allows for potentially faster epithelial healing, significantly less haze and less discomfort compared to other forms of surface ablation. It is safe, and ultimately, patients do extremely well.
Jorg H. Krumeich, MD, is head of the Laser Vision Centre & Clinic, in Bochum, Germany. Dr. Krumeich is the inventor of the Barraquer-Krumeich microkeratome (Cooper Vision, Fairport, NY) for epi-LASIK. The keratome uses a metal blade of defined dullness. He did not provide financial disclosure information. Dr. Krumeich may be reached at JK@krumeich.de or +0 23 27 82 00 2.
