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Surgeon: Relation of ectasia and correction needs more study
A correlation between posterior corneal ectasia and ablated tissue has been established, but more work is needed.
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NICE, France — The degree of posterior corneal ectasia following excimer laser treatment correlates with the amount of tissue ablated, according to clinical investigators.
“The idea is when we treat the cornea with LASIK or PRK, the cornea after the treatment is fine. The cornea is thinner then. The possibility exists that after some years this thinner cornea makes an ectasia, a bombé, so we induce a keratoconus because we have ablated too much tissue,” said Filippo S. Simona, MD, of Locarno, Switzerland.
In a poster presentation here at the European Society of Cataract and Refractive Surgeons meeting, Dr. Simona said that even though new technologies have improved the potential to observe and quantify morphological changes, further studies would be required.
“The stability is not a given,” he said. “Many studies show that the increase in this bulging of the posterior cornea is dependent on the amount of the ablated tissue. Orbscan II (Bausch & Lomb) topography gives us the opportunity to analyze the curvature of the posterior cornea.”
Trial eyes
According to Dr. Simona, president of the Swiss Eye Surgery Academy, the purpose of his study was to determine if LASIK or laser epithelial keratomileusis procedures invoke iatrogenic posterior keratectasia. He also evaluated the importance of these changes and looked for a relationship with the amount of the treated refractive error or with the residual corneal bed thickness.
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“We examined some cases before and 3 and 6 months after LASIK and LASEK,” he said. “We analyzed the results in terms of ectasia of the posterior part of the cornea. In keratoconus, the first part of the cornea affected is the posterior face, not the anterior face. We made graphs to show the relationship between the number of diopters taken away, because more diopters corrected signifies more ablation of tissue. We found that the amount of the ablated tissue is effectively related to the corrected diopters.”
The prospective study included 112 eyes of 61 patients. The corneas were observed using the Orbscan II (Bausch & Lomb) corneal topographer. The amount of refractive correction ranged from 4 D to –12 D.
“We have made three hypotheses,” Dr. Simona said. “First, 3 months after the procedure, reliable results may not be possible with the Orbscan topographer because it is an optical device. After LASIK or LASEK procedures, the cornea isn’t as transparent as in the preoperative phase. Second, we can induce iatrogenic keratoconus, but clinically this is not important because it’s stable. Third, we can make the cornea thinner, but this cornea is unstable and within a few years we have an iatrogenic clinically serious keratoconus.”
According to Dr. Simona, the amount of the posterior keratectasia correlates to the amount of the residual bed thickness. Despite the presence of a greater residual corneal bed thickness in LASEK, the amount of keratectasia is no less important than in the LASIK cases, he said.
“If we compare the posterior ectasia, we also find if you correct a lot of diopters, the bulging is more significant than if you correct a little myopia,” he said. “Looking at the graphs for LASIK and LASEK, there isn’t a big difference between the two. The posterior ectasia is proportional to the refractive correction for LASIK and LASEK. It’s also proportional to the thickness of the residual cornea. Also, with LASEK we leave more tissue in place and we don’t have a flap. We concluded there are factors that are important for the postop corneal structure and there are important biomechanical factors. For example, we take away the Bowman’s membrane, which stabilizes the cornea.”
According to Dr. Simona, the effect of posterior corneal ectasia after excimer laser treatment on the quality of vision is still unknown, and further studies will be required.
“The most important lesson is that posterior ectasia is here,” he said. “We can photograph and calculate it, and it’s directly proportional to the refractive error correction. If we correct more than –5 D, we must analyze the thickness and the posterior face of the cornea with great attention.
“Now we are trying to collect the data after 1 year for two reasons. First, we have a more transparent cornea and the results of an optical device are more reliable. The second is to follow up whether this ectasia is progressive or not.”
For Your Information:
- Filippo S. Simona, MD, can be reached at Piazza Stazione 9, CH-6600 Locarno, Switzerland; (41) 91-743-26-26; fax: (41) 91-743-59-66.