Aspheric IOLs: customising the selection process

It is possible to select aspheric intraocular lenses (IOLs) based on preoperative corneal spherical aberration (SA), according to Mark Packer and colleagues from the Oregon Eye Institute, USA.

Cataract surgery patients requesting monofocal distance correction were offered aspheric IOL implantation by the researchers based on a targeted postoperative total ocular SA of zero. “I chose zero SA based on the work of Pablo Artal and Patricia Piers which showed that zero SA gives the best contrast sensitivity,” explained Dr Packer.

The researchers measured the corneal SA of 30 eyes from 18 patients (nine men, nine women) with topography and wavefront analysis using the iTrace corneal topographer (Tracey Technologies). The average preoperative corneal SA was 0.26 µ. The best corrective acuity varied from 20/20 to 20/70, and the mean keratometry and axial length was very average, indicating an average group of patients.

Cataract surgery was then performed on each of the patients. The appropriate aspheric IOL was selected based on whichever was closest to the corneal SA to try to produce a value of zero. The IOLs used in the study were SofPort AO (Bausch & Lomb), SN60WF (Alcon) and Tecnis Z9000/2 (AMO). Dr Packer demonstrated that two-thirds of the study population would benefit from Tecnis, one-third from SN60WF and a smaller percent from SofPort AO implantation.

Postoperatively, 93%of the patients were within ±0.5 diopter (D) emmetropia. 100% had a best corrected visual acuity of 20/30 or better, 90% had 20/20 or better, and the mesopic pupil size was just over 3.5 mm.

The mean postoperative total SA for all eyes was very near to zero at -0.013 µ. Predictability (mean absolute error + the IOL) at the 6 mm zone versus the measured results showed a mean absolute error in all groups of 0.06 µ. Dr Packer stressed the importance of measuring the cornea at 6 mm because all the selected IOLs have had the SA determined at this zone.

“When we look at the predicted versus measured results, 87% came within ± 0.1 µ of our target and nearly half came within 0.05 µ of our target,” said Packer. “Our total accuracy of 0.05 µ is pretty close to what we.'d expect given that our surgery induces ±0.03 µ depending on which way it moves.”

“From these conclusions, we believe it is feasible to customize the selection of IOLs and get good objective optical results,” he said.“However, the planned expansion of this study needs to include the measurement of the corneal topography postoperatively as well as preoperatively to see if the changes can be explained by changes in the cornea. We also need to incorporate psychometric testing in the form of contrast sensitivity to ascertain whether our outcomes are better or the same as simply choosing the average SA correction for all covers.”