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A prospective study of 50 patients undergoing bilateral laser-vision correction surgery randomly assigned one eye to PRK and the fellow eye to femtosecond thin-flap (110 Âµm) sub-Bowman's keratomileusis (SBK). The results show that during 6 months of follow-up, dry eye signs and symptoms are generally similar with the thin flap and surface ablation procedures.
The study was performed at two centers and included 50 patients undergoing bilateral surgery. Patients with preoperative significant dry eye symptoms were excluded. SBK flaps were created with the femtosecond laser (IntraLase FS, IntraLase/Advanced Medical Optics) with a targeted thickness of 110 µm. A proprietary excimer laser (LADAR4000, Alcon Laboratories) was used for all ablations.
Dry eye was evaluated with a patient questionnaire, the Schirmer's I test, lissamine green staining, and corneal sensitivity measurement with Cochet-Bonnet monofilament aesthesiometry. All outcomes were assessed preoperatively and at 1, 3, and 6 months after surgery, and the subjective questionnaire was also administered at days 1 and 3 postoperatively. Forty-nine patients were seen at 6 months.
"Previous studies have shown SBK offers the advantage of excellent visual acuity and biomechanics along with decreased healing time relative to PRK," said Dr. Stahl, resident, department of ophthalmology, University of Kansas Medical Center, Kansas City. "However, it has always been assumed there would be worse dry eye signs and symptoms after any procedure involving a flap versus surface ablation only. Our research shows the dry eye profile of SBK is generally similar to PRK."
Dr. Stahl also emphasized that in considering these data, ophthalmologists need to keep in mind that the study involved patients without dry eye.
"A key question is whether there is any difference between these procedures in the population of patients with dry eye," Dr. Stahl said. "That is an issue we would like to investigate in the future, but the results of the present study should not be generalized to the dry eye population."
The results from the study showed the mean Schirmer's I test measurements were nearly identical in the PRK and SBK eyes at all visits. In both groups, there was an initial decrease after surgery, but a return to baseline level at 3 months and a further increase at 6 months.
Corneal staining was minimal. Lissamine green staining was present in 8% of PRK eyes at 1 month and in only 2% of SBK eyes, but no eye had staining worse than grade 1. At 3 months, 10% of SBK eyes had grade 1 staining, and the percentage of eyes with that finding had decreased to 2% in both groups at 6 months.
The corneal sensitivity measurements showed an average loss of 15% in the SBK eyes and 5% in the PRK eyes at 1 month. In the PRK eyes, the average loss was 7% at 3 and 6 months, while it decreased to 13% in the SBK eyes at 6 months.
"These differences between eyes were statistically significant, but notably, the loss of corneal sensitivity after SBK is much less than what is seen early after microkeratome LASIK or IntraLASIK with thicker flaps," Dr. Stahl said. "In a previous study where LASIK was performed with 140-µm flaps created with [a proprietary] microkeratome [Hansatome, Bausch & Lomb] or the [femtosecond laser], corneal sensitivity was decreased 46% and 40%, respectively, at 1 month."
The studies were conducted at Durrie Vision Center, Kansas City, KS, and Slade & Baker Vision Center, Houston. Principal investigators included Daniel S. Durrie, MD, and Stephen G. Slade, MD.
On the subjective questionnaire patients were asked which eye had more dryness. In the first few days, about one-half of the patients said there was no difference or were not sure, but among those who did, significantly higher proportions complained about the dryness of the PRK eye. By 1 week, more than one-half of the patients found no difference between eyes or were not sure. The proportion who did not discriminate between eyes with respect to dryness rose thereafter, reaching≥82% at 3 and 6 months. Among the patients who reported a difference, the PRK was consistently considered drier, but the results were not statistically significant.