Thin femtosecond laser flaps, age linked with post-LASIK interface haze

There was a 10 times greater risk of developing haze with a 90-µm flap compared with a 100-µm flap, but visual acuity was not significantly affected.

Ronald R. Krueger

SAN FRANCISCO — Thinner LASIK flaps made with an IntraLase laser posed an increased risk of light scatter and interface haze, especially among younger patients, according to a study presented here.

“Interface haze is associated with light scatter, and light scatter is associated with ultra-thin flaps at a setting of 90 µm,” Ronald R. Krueger, MD, said at the American Society of Cataract and Refractive Surgery meeting. “The take-home message is consider setting 100 µm or more when doing your IntraLase flaps, especially in patients of younger age.”

Amid burgeoning interest in thin femtosecond laser flaps, clinicians have begun to question at what point those flaps are too thin, increasing the risk of postoperative complications, Dr. Krueger said.

“Recently, there’s been more of a focused effort among femto-LASIK surgeons in making thinner and thinner femtosecond laser flaps,” he said. “The ideal was to get right under Bowman’s layer, making very thin flaps to try to preserve more residual stromal tissue and reduce the risk for biomechanical weakening and ectasia. This has led us to question: ‘Can we go too thin?’”

Methods and measures

The prospective study included 199 eyes that underwent LASIK with flaps made by the IntraLase FS 60 kHz femtosecond laser (Abbott Medical Optics) and analyzed from January to April 2008; 106 eyes had 90-µm flaps, and 93 eyes had a flap setting of mostly 100 µm. Ablation was performed with the LADARVision 4000 excimer laser (Alcon).

Optical coherence tomography (Carl Zeiss Meditec) and the Pentacam Scheimpflug camera (Oculus) were used to gauge flap thickness and central corneal thickness. Outcome measures also included cycloplegic refraction, spherical equivalent, uncorrected visual acuity, best corrected visual acuity and ablation depth. The C-Quant (Oculus) was used to assess light scattering, Dr. Krueger said.

“The two groups, however, were not exactly matched preoperatively. There was a higher level of myopia in the 90-µm group, probably the reason why thinner flaps were attempted,” he said. “Pachymetry was also a little bit lower in that group as well. As a result, our postoperative data was statistically analyzed with multivariate analysis, so we could really view independent risk factors.”

Another risk of thin-flap IntraLASIK is a flap tear during dissection. Meticulous technique helps to prevent tearing of thin 90-µm flaps.

“Although very thin, we had no difficulty in lifting these flaps with good dissection techniques,” Dr. Krueger said. “Holding the tip of the dissector down, you could easily avoid flap tears. In those surgeons who had previously experienced a flap tear, it was likely due to them pointing the tip of the dissector up a little, so that the force of dissection led to the flap tear.”

Light scattering and haze

Results showed that 32 eyes had some level of haze, based on the Fantes haze scale. Haze was quantified as trace, 1+, 2+ and 3+.

Ninety-one percent of eyes that developed interface haze had 90-µm flaps.

“It’s seen right at the level of the interface, maybe within the posterior portion of the flap,” Dr. Krueger said. “With multivariate postoperative analysis, we were able to find that younger age and thinner flap thickness were associated with a greater incidence of haze.”

With each consecutively younger decade of age, eyes were two times more likely to get haze than not, and there was a 10 times greater risk of getting haze with a 90-µm flap compared with a 100-µm flap.

In the study, preoperative light scattering was seen in older patients and was due to nuclear sclerosis.

“But postoperatively, we found that those patients who developed haze had a higher score of light scattering on average,” Dr. Krueger said. “In fact, when we looked at the scatter plot of light scattering vs. age, the eyes with haze all seemed to be above a certain light-scattering level, and this was primarily among the younger patients, who were less than 50 years of age.

“We observed the haze to be at the level of the interface, perhaps the lower part of the flap or the anterior bed, but right along the line of the interface,” he said.

Haze was seen 1 week after surgery and at 3 months postop, even after treatment with steroids.

“With time, haze seems to diminish, but I doubt it would go away completely,” Dr. Krueger said, adding that haze did not significantly affect visual acuity.

“Our patients all had excellent visual acuity. However, the light-scattering measurements recorded by the C-Quant demonstrate that there was actually some stray light being formed from the haze, contributing to a theoretical loss of contrast sensitivity. Perhaps it was sub-clinical to the patient,” he said. – by Matt Hasson

• Ronald R. Krueger, MD, can be reached at the Cleveland Clinic Foundation, 9500 Euclid Ave., Room i32, Cleveland, OH 44195; 216-444-8158; fax: 216-445-8475; e-mail: krueger@ccf.org. Dr. Krueger is a consultant to Alcon.