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Shallow corneal flaps offer biomechanical stability
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London-Thin, shallow corneal flaps of approximately 80 to 90 ?m created with an IntraLase femtosecond laser (IntraLase Corp.) may produce the best results in refractive surgery, effectively eliminating pain and haze while producing long-term visual stability, said John Marshall, PhD, in a keynote address during a session on femtosecond lasers here at the XXIV Congress of the European Society of Cataract and Refractive Surgeons (ESCRS).
Dr. Marshall, Frost Professor of Ophthalmology, King's College, University of London, and Department of Ophthalmology, the Rayne Institute, St. Thomas' Hospital, London, reviewed decades of refractive surgery, citing the pros and cons of PRK and LASIK and describing studies of flap creation that point to an approach that offers the best aspects of both types of procedures.
"With surface ablations, we had biomechanical stability but problems with wound healing," Dr. Marshall said. "With conventional LASIK, we had no problems with wound healing but did have biomechanical instability. It seems that we should get the best of both worlds with a sub-Bowman's flap."
Type of flap matters
Dr. Marshall and colleagues evaluated the corneal biomechanics and wound healing involved in refractive surgery predictability to understand better the impact on outcomes. They found evidence that the type of flap used in the procedure can affect the visual outcome. Their presentation at the ESCRS meeting showed physical measurements of changes in the biomechanical elements of the cornea and the fact that they were minimized with an exceedingly thin sub-Bowman's flap created with the femtosecond laser.
From the outset, refractive surgery outcomes have had limitations, Dr. Marshall said. Although the past 20 years have seen enormous improvements in lasers, algorithms, and other components of refractive surgery, a potent obstacle remains.
"However good the hardware is, ultimately we're up against the biology," Dr. Marshall said. Early work with surface procedures, such as PRK, showed that the major problems were pain-which could be controlled to some extent with anesthetics-and haze, which resulted from simultaneously wounding the corneal epithelium and the keratocytes within the stroma of the cornea.
The good news about surface ablations was that the refractive outcomes were quite stable, as shown by follow-up data now available for 14 to 15 years, he said.
LASIK was introduced to overcome problems with pain and haze associated with PRK. LASIK skirted the pain issue by not involving the epithelium and got around haze by only damaging the corneal keratocytes.
