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Postoperative anterior optical coherence tomography may provide a more accurate measurement of LASIK flap thickness than intraoperative subtraction pachymetry.
The research also demonstrated that a femtosecond laser (IntraLase FS, Advanced Medical Optics) can make flaps of more precise thickness and with a tighter standard deviation than the flaps made by a mechanical microkeratome (Hansatome, Bausch & Lomb). Additionally, the study showed that flaps in eyes treated with the femtosecond laser had a planar architecture, compared with the meniscus architecture found in the flaps made by the mechanical microkeratome.
The study included 100 eyes of 50 consecutive patients who had been treated with custom wavefront-guided LASIK for myopia using a proprietary excimer laser (STAR S4 IR, Advanced Medical Optics). One eye was treated with the mechanical microkeratome, and the fellow eye was treated with the femtosecond laser. The researchers in this study used a 15-kHz femtosecond laser, with a 9.2-mm superior hinged, 110-μm setting and a mechanical microkeratome with 160-μm plate and a 9.5-mm suction ring.
Subtraction pachymetry was performed intraoperatively and postoperatively with a pachometer (Corneo-Gage Plus, Sonogage). At 1-year follow-up, imaging with OCT (Visante, Carl Zeiss Meditec) was performed. A single observer performed the pachymetry and OCT measurements in masked fashion on the 24 eyes in 12 patients available for analysis at 1 year.
Pachymetry measured a flap thickness of 119 ± 19 μm in the femtosecond laser group and 137 ± 22 μm in the mechanical microkeratome group. OCT imaging revealed an achieved flap thickness of 107 ± 8 μm in the femtosecond group and 141 ± 15 μm in the mechanical group. No significant complications existed in either group.
According to Dr. Manche, the correlation between OCT and pachymetry was poor in the femtosecond laser group; sometimes, flap thickness was overestimated and sometimes it was underestimated. The correlation between the two measurements also was poor in the mechanical microkeratome group, for which pachymetry tended to underestimate the actual thickness of the flaps, he added.
The range of results using subtraction pachymetry was wide for both groups: in the femtosecond laser it was 69 to 173 μm and 87 to 179 μm in the mechanical microkeratome group.
"Looking at the small set of eyes that we had for the follow-up, we see mean thickness of 116 μm with the femtosecond laser, with a standard deviation of 16 μm. With the mechanical microkeratome, we saw 128 μm and somewhat less of a range, but the standard deviation was 22 μm," Dr. Manche said.
The results from postoperative OCT imaging were somewhat different. The mean flap thickness of the femtosecond laser measured a little bit thinner at 111 μm. The range also was significantly tighter (104 to 127 μm) than what was seen using subtraction pachymetry. In the mechanical microkeratome group, the flaps actually were thicker, at a mean of 146 μm, but the range and the standard deviation both were significantly tighter.
The intraoperative subtraction pachymetry was 115.6 ± 15.8 μm in the femtosecond laser group and postoperative OCT was 110.8 ± 5.9 μm. In the mechanical microkeratome group, the intraoperative subtraction pachymetry was 127.5 ± 22 μm and the postoperative OCT was 145.8 ± 7.9 μm.
"The measurements were very close in the femtosecond laser group, but the pachymetry significantly underestimated the thickness of the flaps in the mechanical microkeratome group," Dr. Manche said.
Dr. Manche and his colleagues also examined the actual flap thickness and the difference between the thickest measurement and the thinnest measurement of flaps on the Y axis. In the mechanical microkeratome group, the "best case" difference between the thinnest measurement and the thickest measurement was 7 μm; the "worst case" was a 44-μm variation. The average was about 20 μm.
The femtosecond laser provided more consistent, uniform flaps, with a variation between measurements of as little as 1 μm up to as much as 10 μm, with an average of about 5 μm.
"Postoperative OCT measurements demonstrated in a small group that femtosecond laser flaps were more precise, with tighter deviations, taking into account flap thickness as well as geometry or architecture," Dr. Manche said. "We saw planar flaps in the femtosecond laser group and meniscus flaps in the mechanical microkeratome group."
He added that he is not sure why eyes measured with OCT at 1 year did not have slightly thicker flaps in both groups, given that previous studies have shown that there is epithelial hyperplasia in the central cornea of eyes treated with LASIK. He said he is continuing to gather data, however, and hopes to have a larger sample size to examine over time.