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By Roibeard O’hEineachain

Tel Aviv — The Nidek MK 2000 microkeratome appears to provide a more predictable flap than either the Hansatome or ACS models, says Isaac Lipshitz MD.

He conducted a study that compared corneal flap thickness in the right and left eyes of 66 patients intraoperatively using the three different microkeratomes. The thickness of flaps created with the NIDEK was closest to that intended with the least variation between eyes.

“The Nidek was more accurate than the Hansatome and ACS in creating a predetermined flap thickness for the first and second operated eye. The difference in flap thickness between the right and the left eye was statistically significant, for both the Hansatome and ACS eyes.

“Only the Nidek MK 2000 microkeratome showed a reproducible and accurate incision for the second eye so that the difference in flap thickness between the two eyes was not statistically significant,” Dr Lipshitz said.

Predictable and more consistent
Dr Lipshitz, of the Ophthalmic Health Centre, Tel Aviv, Israel, noted that the thickness of flaps cut using the Nidek microkeratome was only 3% lower than predicted in the right eye and 2% lower than predicted in the left.

By comparison, the mean corneal flap thickness was 19.8% lower than predicted in the right eye and 23.3 % in the left using the ACS microkeratome. The corneal flap thickness was lower than predicted in 18 % of the right eyes and 24.4% of the left eyes with the Hansatome microkeratome.

The Nidek microkeratome, with a 130 micron depth-plate, created a flap with a mean thickness of 127.25 microns in the right eye and 127.54 microns in the left eye. Mean corneal flap thickness created by the ACS microkeratome, with a 160 micron depth setting, was 128.30 ± 12.57 microns in the right eye and 122.96 ± 13.30 microns in the left eye. The Hansatome microkeratome, with a
160-µm depth plate, created a flap of mean thickness of 141.16 microns in the right eye and 120.95 microns in the left eye.

The patients in the study ranged in age from 21 to 57 years, with a mean of 31. The same surgeon performed all procedures on the right eye first and then on the left eye using the same blade and surgical technique.

Dr Lipshitz used the Nidek in 24 patients, the ACS in 19 patients and the Hansatome in 23 patients. He measured corneal thickness preoperatively and intraoperatively using ultrasonic pachymetry (Pachette-DGH model 500).

He emphasised that flap thickness is an important factor in determining the amount of myopia that LASIK can correct. The general recommendation is to leave at least
250 microns of residual stroma to prevent iatrogenic keratectasia. On the other hand, thinner flaps are torn more easily during surgery.

“This study underlines the importance of intraoperative pachymetry and corneal flap thickness calculation during LASIK. Surgeons should be aware of the differences between intended and actual flap thickness, which may vary between the first and second eye and between different microkeratomes. This is even more important in cases where there is a relatively thin cornea and/or high refractive error,” Dr Lipshitz said.

Details of the study appear in the Journal of Refractive Surgery, Volume 18, May/June (Suppl) 2002.

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