As this year’s Binkhorst Medal lecturer at the European Society of Cataract and Refractive Surgeons meeting, Dr. Douglas Koch stressed the need to understand the biomechanical changes we induce in the cornea with refractive surgery in order to maximize visual results.
“By ablating the cornea, we not only remove tissue from the anterior stromal surface, but we alter the epithelial thickness profile and induce biomechanical changes that involve both the anterior and posterior corneal surface,” Koch said.
This understanding led to the use of wavefront-guided ablation, which improved quality of vision with refractive surgery. It also led to a better understanding of higher-order corneal aberrations and how they can be used to increase depth of focus in post-refractive surgery eyes.
“We have been able to create larger, more reliably uniform optical zone sizes to reduce the induction of unwanted aberrations, predictably lower some aberrations and predictably produce some aberrations,” he said. “There is an upside of increased HOAs. Can we take advantage of induced HOAs? Yes, higher-order corneal aberrations can increase depth of focus in eyes with a history of corneal refractive surgery.”
Dr. Koch said higher-order aberrations also offer an opportunity for IOL power selection in healthy eyes because they can be used to predict the quality of vision and depth of focus for each individual patient.
In addition to spherical aberration, another criticial element in improving visual outcomes, Dr. Koch said, is incorporating posterior corneal curvature measurements into IOL power calculations.
He and his fellow researchers at Baylor College of Medicine in Houston found that not measuring posterior corneal astigmatism could result in incorrect estimation of total corneal astigmatism, reducing the efficacy of toric IOLs by over correcting with-the-rule astigmatism and under correcting against-the-rule astigmatism.
Using data from a Galilei Scheimpflug, they found an average of about 0.5 D of posterior astigmatism when patients had with-the-rule on the front, and about 0.3 D when they had against-the-rule on the front.
Trouble is, measuring posterior corneal astigmatism is a challenge, Dr. Koch said. The Galilei Dual Scheimpflug Analyzer (Ziemer, Port, Switzerland) does a moderately good job.
Dr. Koch has constructed a nomogram that takes into account the average posterior corneal astigmatism in the with-the-rule and against-the-rule groups. It also takes into account the need to leave these patients with a little bit of with-the-rule astigmatism to compensate for the gradual against-the-rule shift caused by aging.
It’s even more difficult in eyes that have undergone refractive surgery because this relationship between the front and back of the eye is lost. A formula based on RTVue Fourier-domain optical coherence tomography (Optovue) that does not rely on previous keratometry readings or refraction shows great promise, Koch said.
Both the imaging devices and formulas are a work in progress, but Dr. Koch is optimistic.
“The future presents an exciting challenge,” Koch concluded. “There is much to be done, and we are making great progress. I’m very excited with the tremendous future ahead.”