OCT enhances IOL power calculations in post-LASIK eyes

The method is fast, easy and similar to taking a preoperative topography, study author says.

Issue: August 25, 2012

IOL power calculations based on Fourier-domain optical coherence tomography measurements proved highly predictable in eyes with previous myopic laser vision correction, a study found.

The accuracy of OCT-based IOL calculations equaled or exceeded that of standard methods, the study authors said.

“In post-LASIK corneas, the IOL power calculator with OCT is definitely better than the older methods such as contact lens over-refraction and the clinical history method,” David Huang, MD, PhD, the corresponding author, said in an interview with Ocular Surgery News. “It also tends to be better than regression-based methods like the Haigis-L. … It works well in both myopia and hyperopia.”

David Huang

The study was published in the Journal of Cataract and Refractive Surgery.

Patients and methods

The prospective case series included 16 eyes of 16 patients with previous myopic LASIK enrolled at two sites. Mean patient age was 59.4 years. The mean magnitude of previous laser vision correction was –4.93 D.

Investigators used the RTVue Fourier-domain OCT system (Optovue) to measure anterior, posterior and net corneal powers, and central corneal thickness.

Net corneal power was calculated using a thick-lens formula based on anterior and posterior corneal powers and central corneal thickness. IOL power calculations included final corneal values based on the mean of three OCT corneal power outputs.

OCT net corneal power was transposed to an effective corneal power before being included in the IOL formula.

Mean absolute error of postoperative refraction was compared with that of the Haigis-L formula.

All patients underwent phacoemulsification and in-the-bag implantation of AcrySof SN60AT, SA60AT, SN60WF, SN6AT3 or SN6AT4 IOLs (Alcon) or Tecnis ZA9003 monofocal IOLs (Abbott Medical Optics).

OCT scanning increases measurement time by mere seconds, Huang said.

“We recommend taking two scans of the cornea,” he said. “If you’re sitting in one place, that will take just a few more seconds. Overall, it’s just a few minutes of the technician’s time to get the patient’s head up, sit down and take a scan. It’s similar to taking a topography.”

Outcomes and observations

Study results showed that mean absolute error was 0.5 D for OCT-based IOL calculation, 0.76 D for the Haigis-L formula, 1.46 D for contact lens over-refraction and 1.78 D for the clinical history method. The difference between mean absolute error for OCT-based calculations and the Haigis-L formula was not statistically significant.

OCT-based calculations yielded a significantly smaller mean absolute error than did contact lens over-refraction or the clinical history method (both P < .05).

“You don’t need the clinical history. That’s actually not very important, other than the fact that they have had previous LASIK. You don’t need a pre-LASIK keratometry and such,” Huang said.

The greatest difference between OCT-based power calculations and conventional methods was seen in patients with previous high myopic correction, Huang said.

“I think that’s because there you have the biggest difference between the anterior and posterior curvature, and OCT is able to directly measure the posterior curvature,” he said. “It provides the crucial information that takes away the errors that can come from estimating that.”

Incorporating axial length measurement capability and an IOL power calculator into an OCT instrument would improve efficiency in the future, but currently these steps must be taken separately, Huang said.

“For post-LASIK patients, it’s worth the trouble to obtain corneal power and axial length measurements on two separate machines, so that you get as much information as possible and don’t end up having refractive surprises,” he said. “These are patients that are very demanding of their refractive outcomes.”

Huang provides a post-LASIK IOL calculator, which can be downloaded free of charge at www.COOLLab.net, for use with RTVue OCT-derived corneal power measurements. – by Matt Hasson

Reference:

Tang M, Wang L, Koch DD, Li Y, Huang D. Intraocular lens power calculation after previous myopic laser vision correction based on corneal power measured by Fourier-domain optical coherence tomography. J Cataract Refract Surg. 2012;38(4):589-594.

For more information:

David Huang, MD, PhD, can be reached at Casey Eye Institute, 3375 SW Terwilliger Blvd., Portland, OR 97239; email: davidhuang@alum.mit.edu.
Disclosure: Huang has financial relationships with Carl Zeiss Meditec and Optovue Inc. Grants from the National Institutes of Health and Optovue Inc. supported the study.