Study poses questions on CCT as parameter for refractive surgery

Findings may confirm that thin corneas found in North Africa have higher biomechanical stability than equally thin corneas in Europe.

Issue: June 2012

A study on the biomechanics of a large sample of thin corneas in a North African population has opened interesting questions on the validity of current selection parameters for refractive surgery.

“In a study presented at [the 2010 French Society of Ophthalmology meeting], we found that a high percentage of North African inhabitants would be classified as having thin corneas, according to the classic 500 µm parameter,” Sihem Lazreg, MD, said at the 2011 SFO meeting.

The study involved 1,615 patients from the Maghreb countries and a control group of Caucasian European patients from Bordeaux, France. More than 90% of the corneas in the control group had corneas more than 500 µm, while more than 40% of the corneas in the Maghreb group were below 500 µm.

These findings raised the question: Should these thin corneas be excluded from refractive surgery because they are weak?

“This [time] we have studied the biomechanics of these thin corneas, selecting 250 eyes from North Africa (Morocco and Algeria) and comparing them to 150 eyes from Bordeaux. We excluded risk factors, such as age less than 18 years and keratometry more than 48 D, as well as pathologic corneas with opacities and immune-allergic conditions,” Dr. Lazreg said.

Study

Sihem Lazreg

Each patient had only the right eye examined. Pachymetry, keratometry and Ocular Response Analyzer (ORA, Reichert) measurements for corneal hysteresis (CH) and corneal resistance factor (CRF) were measured. Student’s t-test was used for statistical comparison.

“ORA measurements showed that elasticity (CH) and resistance (CRF) in the thin [less than] 500 µm corneas of the North African group are comparable to those of ‘normal’ European corneas and better than those of thin 550 µm corneas in the European group. Mean CH was 0.636 µm higher and mean CRF was also 0.621 µm higher. In both cases, the difference was statistically significant,” Dr. Lazreg said.

She surmised that thin corneas may be stiffer and biomechanically more stable in the North African population studied.

Additional questions

More questions, however, arise from these findings: Is central corneal thickness a valid parameter to select patients for refractive surgery? If ORA measurements are not proportional to central corneal thickness, is the ORA a sufficient and reliable method to measure corneal biomechanical stability?

Despite many years of discussing this issue, safety parameters for refractive surgery are unclear, and many aspects of corneal biomechanics are still unknown.

Dr. Lazreg referenced a study, presented as a poster at the 2011 Association for Research in Vision and Ophthalmology meeting, that suggested elasticity and resistance may be related to only the anterior part of the cornea, up to Bowman’s layer, rather than the entire cornea.

“A specific pattern of collagen bands in the first 150 µm of the cornea may determine its elastic properties. This might explain why ORA measurements are not proportional to the thickness,” Dr. Lazreg said.

“The anterior face of the cornea may perhaps be responsible for its stiffness, elasticity and biomechanical stability independent of its thickness. It would be interesting to concentrate our observations on the corneal layer anterior to the Bowman’s rather than on the entire corneal thickness,” she said. – by Michela Cimberle

Reference:

Mikula ER, Sun H, Jester J, Juhasz T. Measurement of varying elasticity between the anterior and posterior human cornea with acoustic radiation force elasticity microscope. Presented at: Association for Research in Vision and Ophthalmology meeting; May 4, 2011; Fort Lauderdale, Fla.

For more information:

Sihem Lazreg, MD, can be reached at 1 Rue Saadi, Blida, Algeria; +213-25417455; email: slazbkt@yahoo.fr.
Disclosure: Dr. Lazreg has no relevant financial disclosures.