Comparative study examines outcomes with proprietary femtosecond lasers

A prospective, randomized, contralateral eye study including an extensive list of outcome measures is under way comparing sub-Bowman's keratomileusis performed with two different femtosecond lasers (IntraLase FS60, Advanced Medical Optics; Femto LDV, Ziemer). Early data show that both lasers create good-quality, planar thin flaps, and their use is associated with excellent vision and safety outcomes.

Key Points

Chicago-Early results from a prospective, randomized, contralateral eye study show that two proprietary lasers (IntraLase FS60, Advanced Medical Optics; Femto LDV, Ziemer) create good-quality, planar thin flaps, and their use for sub-Bowman's keratomileusis is associated with excellent vision and safety outcomes, said Daniel S. Durrie, MD, at the annual meeting of the American Society of Cataract and Refractive Surgery (ASCRS).

"Longer follow-up and additional analyses are planned in this clinical trial. For the time being, I can conclude that femtosecond laser technology is here to stay, and while the IntraLase laser is my preferred platform because it is very mature and proven reliable based on several million procedures, the Ziemer laser is worth looking at in this developing area, said Dr. Durrie, clinical professor of ophthalmology, University of Kansas Medical Center, Kansas City, and president, Durrie Vision, Overland Park, KS.

The study, which is being conducted as a two-center trial, was undertaken because of Dr. Durrie's commitment to conduct well-designed clinical trials to assess the role of new technology as it becomes available, he said. Femtosecond laser assignment is randomized for the dominant eye, and the fellow eye is operated on with the alternate instrument. All flaps are created with a 9-mm diameter and 110-µm attempted depth. The ablation is being performed with the same laser (Allegretto Wave 400 Hz, Alcon Laboratories/WaveLight) in both eyes with a refractive target of plano. The procedure for flap creation with both instruments was standardized in the interest of making the procedures as identical as possible.

Outcomes include flap-related complications and postoperative evaluation in masked fashion of high-contrast uncorrected and best-corrected EDTRS visual acuity, low-contrast uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA), manifest and cycloplegic refraction, and wavefront analysis. In addition, eyes are being evaluated at the slit-lamp and with anterior segment optical coherence tomography ([OCT]; Visante, Carl Zeiss Meditec) for flap uniformity and confocal microscopy (Confoscan 4, Nidek) for interface appearance. The patients are completing a satisfaction questionnaire at 3 months, and in a parallel human eye-bank eye study, bed smoothness after flap creation is being assessed with scanning electron microscopy.

1-month data

At the ASCRS meeting, Dr. Durrie reported results for some of the outcome measures based on 1-month data for 20 eyes of 15 patients.

No significant flap complications were seen. One patient developed a slipped flap in the eye operated on with the Ziemer laser, although Dr. Durrie suggested that it was perhaps due to eye rubbing and noted that the flap could be smoothed out without any untoward sequelae.

The OCT evaluation of flap uniformity used a technique previously published by Dr. Durrie. Sixteen thickness measurements were obtained for analysis from each flap, comprising four measurements across the length of the flap at each of four meridians. The measurements were made by two independent observers and analyzed statistically.

With the data averaged for all measured flaps in each group, the results showed that both femtosecond lasers created flaps that were very planar and not significantly different from one other.

Analyses of variations of the thickness measurements for individual flaps confirmed their uniformity. For both the IntraLase and Ziemer flaps, the standard deviation of thickness within the individual flaps was 4 µm.

UCVA outcomes at 1 hour were compared in nine patients whose fellow eyes were both evaluated at that time point. Based on the ETDRS chart results, high-contrast visual acuity at 1 hour was better in the IntraLase eye in six (67%) patients and in the Ziemer eye in two (22%) patients. When patients were queried about which eye saw better, however, only one (11%) picked the IntraLase eye, four (44%) picked the Ziemer eye, and four (44%) said vision was the same in both eyes.

"There has been a suggestion that vision recovery is faster after the Ziemer procedure, and it was interesting in our study that the patients' subjective opinion, but not the objective evaluation, was consistent with that observation," Dr. Durrie said.

At 1 day in 14 evaluated patients, 64% of IntraLase eyes and 71% of Ziemer eyes had UCVA of 20/20 or better, but the difference between groups was not statistically significant. Vision improved in both groups of eyes by 1 week and improved further at 1 month, but with the small population, no significant differences in UCVA existed between the groups.

"Visual acuity outcomes at 1 month were excellent overall, with 100% of eyes in the Ziemer group and 90% of eyes in the IntraLase group achieving 20/20 or better," Dr. Durrie said. "Better levels of vision, 20/16 and 20/12.5, were achieved by a higher proportion of eyes in the IntraLase group compared with the Ziemer group. However, data from a larger population are needed to verify if that represents a real difference."

Accuracy of the manifest refraction spherical equivalent and cylinder outcomes was similar in the two groups, which he said was not unexpected considering that the same excimer laser was used for all procedures. Confocal imaging studies of the interface were performed by George O. Waring IV, who is working as a fellow with Dr. Durrie. Dr. Waring's conclusions were that, regardless of the femtosecond laser used, little keratocyte activation and minimal interface debris occurred.

"Even in the worst case, the appearance of the interfaces in these eyes with femtosecond laser-created flaps was clearly superior to that of eyes that underwent flap creation with a bladed microkeratome," Dr. Durrie said.

Stromal bed thickness

Stromal bed smoothness was evaluated in a separate study in which each of the two lasers was used to create flaps in five human eye-bank globes not suitable for transplant. After the flaps were lifted and amputated, a corneoscleral rim was excised, immersion-fixed, and sent to an independent laboratory for scanning electron microscopy imaging.

The stromal bed quality was analyzed both quantitatively using software to evaluate texture and qualitatively by five ophthalmologists masked who ranked magnified images of each of the samples for the stromal surface smoothness. The quantitative analysis showed a statistically significant difference favoring the IntraLase group for a smoother stromal bed. In the qualitative analysis, all five graders unanimously agreed on their rankings of the individual flaps. Although all beds were considered very smooth, a significant difference also favored the IntraLase group for better stromal bed smoothness.

"Whether or not stromal bed quality is clinically relevant is unclear, but this is an endpoint that is used when evaluating excimer laser performance," Dr. Durrie concluded.