Retrospective analysis highlights benefits of thinner LASIK flaps

October 15, 2008

A retrospective study including 800 eyes compared outcomes using the Zyoptix XP (Bausch & Lomb) to create 120 μ flaps and the Hansatome (Bausch & Lomb) to create 160 μ flaps. Safety was excellent overall, the 120 μ flaps created with the Zyoptix had better thickness predictability and were associated with faster re-epithelialization and less discomfort.

Key Points

The upgraded model, however, seems to be associated with better flap thickness predictability, and a 120 μm flap created with the upgraded microkeratome is associated with faster re-epithelialization and better patient comfort compared with a 160 μm flap created with its predecessor, said Andrey Kovalev, MD, PhD, who is with the AILAS Medical Center in Kviv, Ukraine.

"We recently started using the CLB blades for the [upgraded microkeratome] to create 90 μm flaps," he said. "Patient comfort is about the same if not better compared with after 120 μm flap creation, but this technique leaves more residual stromal thickness for better safety and stability.

Specifics of flap creation

To compare the two microkeratomes and differences in healing and rehabilitation of the different flap thicknesses, Dr. Kovalev conducted a retrospective analysis that included 400 eyes of 200 consecutive patients who underwent creation of a 160 μm flap with the earlier microkeratome, and 400 eyes of 200 patients undergoing creation of a 120 μm flap with the upgraded model.

The two groups were well matched with respect to age (mean age, 31 years), gender, and preoperative myopia (earlier model, –3.66 D; upgraded model, –3.86 D).

Myopic LASIK was performed in all patients by only one surgeon, who used a single platform. Flap thickness was calculated based on ultrasound pachymetry measurements obtained before flap formation and immediately after flap lifting. Mean flap thickness was 116 ± 8 μm (range, 108 to 124 μm) in the upgraded microkeratome group, and 148 ± 18 μm (range, 144 to 172 μm) in the predecessor microkeratome group.

"There was a tendency for creating thinner flaps with both devices, but overall the [upgraded microkeratome] offered greater predictability as shown by the smaller standard deviation," Dr. Kovalev said.

Analyses of factors affecting flap thickness showed no correlation between the initial thickness of the cornea and the deviation of the flap thickness.

"We were surprised by this finding considering that others have reported thicker flaps in thicker corneas," he said. "We did note a tendency for thinner flaps in eyes with steeper corneas using the 8.5-mm vacuum rings, although the relationship was not statistically significant."

Rate of re-epithelialization was determined based on slit-lamp examinations performed at 30 minutes, 2 hours, and 1 day after surgery, and the results showed healing occurred faster in eyes with the 120 μm flaps.

Re-epithelialization of the flap edge had already begun within 30 minutes after surgery in 26% of eyes with a 120-μm flap. At 2 hours, 92% of the 120 μm flaps showed evidence of re-epithelialization at the edge compared with just 26% of the 160 μm flaps. Re-epithelialization of the flap edge was documented in all eyes postoperatively at 24 hours.

Assessing patient comfort

The study also investigated postoperative discomfort by asking patients to rate the degree of their pain on a scale of 0 (none) to 10 (intolerable) and its duration. The results for both endpoints favored the group with the thinner flaps. Mean pain severity was about 3.5 in eyes that had the 160 μm flap, and about 2.75 among eyes in the 120 μm flap group. The mean duration of pain in the 160 μm and 120 μm flap groups was 6 hours and less than 1 hour, respectively.

"The 120 μm flaps are more fragile and so require more careful attention with handling. However, they are also more elastic, and we believe this feature explains the differences in re-epithelialization and discomfort between the two groups," Dr. Kovalev said.

Other studies have shown the mechanism for discomfort involves penetration of tears under the flap, he said. Because the 120 μm flaps are more elastic, and the central and peripheral edges are in closer contact, re-epithelialization occurs faster to establish a barrier against tear entry.

No free flaps or buttonholes were associated with the use of either device. The predecessor and upgraded microkeratomes were associated with similar rates of decentered flaps (6% and 5%, respectively).

"We believe the decentered flaps are not due to decentration of the vacuum rings but due to globe asymmetry in eyes with excessive astigmatism," Dr. Kovalev concluded.

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