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By Roibeard O’hEineachain

BARCELONA - Current guidelines for the prevention of post-LASIK ectasia may not be sufficient for all eyes, as pre-operative pachymetry measurements can be misleading, and microkeratomes sometimes cut flaps that are thicker than planned, according to Roberto Pinelli, MD, Instituto Laser Microchirurgia Oculare, Brescia, who will be presenting his findings here at the 6th Winter Refractive Surgery Meeting of the ESCRS.

Asymmetric Irregular Astigmatism

“All authors think that the stromal bed should be at least 250 microns after LASIK, however we have had experience of a patient whose treatment fitted that criteria but developed post-LASIK ectasia anyway,” Dr. Pinelli told EuroTimes in an interview.

The female patient was the only one to develop ectasia from a series of 1,500 eyes undergoing LASIK with a follow-up of 36 months. She was 35 years old and had a pre-operative refraction of -4.0D and had undergone five measurements with two different pachymeters, which showed 520 microns of central pachymetry and did not reveal any corneal irregularities. She underwent LASIK with a 180 micron flap cut with the Hansatome and an ablation depth of 74 microns, leaving a stromal bed of 266 microns. At 18 months she developed ectasia, Dr. Pinelli explained, adding:

“Although every examination was performed correctly, ectasia developed. The patient had no pre-operative clinical characteristics that might have alerted us to her predisposition to this outcome. We feel therefore that contact pachymetry measurements are insufficient and unreliable in some eyes, because unlike non-contact pachymeters, such as the Orbscan, they cannot detect internal corneal irregularities. Ideally, I think what we need is some way of measuring not only the thickness but also the tensile strength and elasticity of the cornea.

“Furthermore, the <exact flap thickness is not always predictable with some microkeratomes,> although some of the newer models may be able to gauge the thickness more accurately. It is also important to remember that the flap does not contribute to post-operative corneal stability and that is because the collagen fibres have been cut and there is no suture to provide support. Therefore stromal bed thickness is the most important parameter of corneal stability.”

Strict Rules for Ectasia Prevention
Dr. Pinelli emphasised that in order to prevent ectasia, the planning of LASIK procedures must be based on the strictest possible rules using the most accurate types of measurements. These should include obtaining the mean of several contact pachymetry measurements done at different times of day, so that changes in pachymetry resulting from differing degrees of hydration may be taken into account.

Apparently Normal Corneal Map

Abnormal Internal Shape of Cornea at ORBSCAN

In addition, surgeons should obtain Orbscan measurements so that they can analyse the internal shape of the cornea. Moreover, when surgeons suspect that the flap thickness is greater than planned they should perform intra-operative pachymetry, he advised.
When performing LASIK, surgeons must first insure that the central pachymetry is more than 500 microns, otherwise they should perform PRK, he continued. The flap thickness should be from 130- 160 microns. In calculating ablation depth, surgeons should consider whether re-treatment might be necessary, in which case they should leave a stromal bed of more than the recommended minimum of 250 microns, he noted.

As an example, Dr. Pinelli said that if the cornea’s central thickness was 530 microns and the flap thickness was 160 microns, the maximum ablation would be 120 microns, leaving a stromal bed of 250 microns. However, if the patient is a likely candidate for enhancement the ablation should leave more than 250 microns, he said.

Retreatments Require Special Consideration
Special care is needed when performing retreatments due to changes in the cornea that occur after primary LASIK, he stressed, adding:

“Remember before re-treatment, that <epithelial thickening can occur after LASIK especially in eyes undergoing correction of high refractive errors, and this can make corneal thickness appear sufficient when it is not.> It is therefore important to determine the thickness of the stromal bed, basing the calculation on the initial treatment. If you are not the surgeon of the initial treatment then you should contact the surgeon who was.

“So for example, in a patient with a pre-operative pachymetry of 530 microns and a re-treatment flap of 160 microns, and an ablation depth of 90 microns in the initial surgery, the maximum ablation depth for retreatment would be 30 microns. Where the cornea is too thin at re-treatment for ablating to the necessary depth for optimal correction, my preferred strategy is to perform intra-epithelial LASIK, which involves performing the ablation on the back of the flap. I have good results with this approach."

Good Patient Selection Essential
Good patient selection is also vital in any strategy for avoiding ectasia, he said. There are several types of patients who should not undergo LASIK under any circumstances. They include those with irregular asymmetric astigmatism, subclinical keratoconus, pellucid marginal degeneration, and those with normal corneal topography but with an abnormal internal shape as detected by Orbscan, he said.

Dr. Pinelli noted that when ectasia does occur, insertion of intra-corneal ring segments can restore corneal stability and visual acuity, as was the case with the ectasia patient he encountered.

“The present situation is that while we have as yet only limited data on ectasia in the literature, the best advice appears to be to leave a stromal bed of 250 microns or more and use safe microkeratomes with flap thickness of 160 or 130 micron. After lamellar corneal surgery an increase of stress is a long-term condition and individual corneas may behave differently. Therefore, good patient selection for LASIK means choosing safe corneas and performing the necessary examinations for obtaining a post-operative stromal thickness sufficient to provide corneal stability.

“In the future, the incidence of ectasia may be reduced through the use of new microkeratomes which provide flaps of appropriate thickness with a less aggressive cut, and no-blade LASIK using the femtosecond laser or other new technology which will allow surgeons to fine-tune and customise flap thickness," Dr. Pinelli predicted.