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Most corneas are steeper than 35 diopters (D) in their flattest meridian,1 but corneal refractive surgery, especially for high corrections, can cause corneas to become flatter than 35 D.2
It is commonly believed that substantial flattening of the cornea during myopic refractive surgery may have a negative effect on postoperative visual acuity (VA).3 In a retrospective study of eyes undergoing LASIK between 1995 and 1999, a postoperative mean keratometry reading of less than 36 D was associated with statistically fewer eyes with an uncorrected distance visual acuity (UDVA) of 20/25 or better and statistically more eyes with a loss of two lines or more of corrected distance visual acuity (CDVA) (P.S. Binder, personal communication, February 12, 2011). Newer ablation profiles, which preserve the prolate nature of the cornea, 4 are believed to provide better optical outcomes than previous generation lasers, but we failed to find publications that support this assumption.5
Marked changes within the transition zone area between the steeper, untreated peripheral cornea and the flatter, treated central area are known to have the potential for producing spherical aberration, especially in patients with large pupils.6--8 However, reports in the literature do not provide clear-cut answers regarding the effect of substantial postoperative flattened corneas on UDVA and CDVA.
The aim of this study was to evaluate VA outcomes following flattening of corneas to less than 35 D following photorefractive keratectomy (PRK).
Patients and Methods
The study was approved by the Institutional Review Board of the Tel-Aviv Medical Center, Tel-Aviv, Israel. We retrospectively analyzed the electronic medical records of all patients undergoing PRK in a refractive surgery facility (Care Vision, Tel Aviv, Israel) between January 1, 2006, and November 1, 2010. The data were routinely collected and entered into the electronic medical records database by the facility staff. All procedures were performed by eight ophthalmic surgeons who used the Allegretto 200Hz excimer laser (Alcon Laboratories, Inc., Irvine, CA) with a planned optical-zone diameter of 6.5 mm in most cases. The following data were retrieved and analyzed: patient demographics (age and gender), preoperative and postoperative CDVA, and keratometry readings obtained by corneal topography (Orbscan II; Bausch & Lomb, Rochester, NY). Inclusion criteria were: patients undergoing myopic PRK treatment, age 18 years or...