Enhance precision

Intraoperative wavefront aberrometry can help enhance precision and accuracy of refractive outcomes in patients undergoing cataract surgery, as well as providing a useful tool in eyes that have previously undergone corneal refractive surgery. “The use of intraoperative aberrometry [IA] improved the spherical equivalent [SE] outcomes in our series of cataract patients. IA successfully reduced the mean absolute value of prediction error [MAVPE] for intraocular lens [IOL] power selection and the distribution of mean absolute error [MAE] also shifted to lower errors compared to preoperative power predictions, with more eyes falling within ± 0.50 D of target,” said Michael Breen OD. Planning for refractive cataract surgery demands precise preoperative measurements to achieve accurate IOL powers in order to obtain desired refractive outcomes, noted Dr Breen. Current systems typically employ optical biometry, advanced keratometry and topography together with latest-generation IOL power calculation formulas, yet refractive outcomes are still not always as precise as expected, he said. “Errors in biometry, the contribution of the posterior cornea and the limitations of some IOL power formulas can result in refractive misses. While refractive outcomes in cataract surgery are definitely improving, a recent study of refractive outcomes in Sweden showed that only 71% of eyes ended up within ±0.50D of predicted postoperative SE,” he said. Dr Breen’s prospective, masked comparative study included 162 eyes of 149 cataract patients who underwent surgery at eight different sites in the USA. The IA device used was the ORA System® with VerifEye™ (Alcon Laboratories), a microscope-mounted aberrometer which allows surgeons to evaluate refractive findings, refine IOL power, cylinder power, and IOL alignment in real time during the surgery. IOL POWERS Evaluators were masked to the use of ORA, IOL type (toric IOL, non-toric, presbyopic) and treatment type. The surgeons calculated IOL powers and predicted postoperative SE based on preoperative measurements. The lens power formula used for preoperative calculation was chosen by the surgeon. A total of 84 toric lenses were implanted in this series of patients, with a mean IOL power of 19.22 D (range 6.00 D - 30.00 D). The Holladay 2 formula was used in 94 eyes, SRK-T in 64 eyes, Holladay 1 in 3 eyes and the Haigis formula in 1 eye. In terms of results, the MAVPE pre-op power was 0.35D (±0.37) and MAVPE using IA was 0.29D (± 0.26). The use of IA increased the proportion of eyes with a postoperative SE within 0.50D of predicted postoperative SE compared to the outcomes that might have occurred had preoperative calculated IOL powers been implanted in both the overall group and in the most frequently implanted subset group, said Dr Breen. In the future, a randomised contralateral eye, observer masked study comparing newer IOL formulas to IA would be an appropriate next step, said Dr Breen. Those studies should be IOL mode specific and preoperative formula specific in order to reduce variation, he added. USEFUL TOOL Catia Azenha MD said that IA may prove to be a useful tool in cataract surgery patients who have undergone prior laser refractive surgery. “Conventional biometry methods are less predictable in eyes with prior refractive surgery, so these patients represent a particular challenge for accurate IOL power calculation. IA is intended to reduce residual refractive error through aphakic refraction, revise preoperative biometry and IOL power choice, optimise lens location and tailor arcuate corneal incisions in eyes with astigmatic needs,” she said. Dr Azenha’s retrospective study carried out at the University Hospital of Coimbra, Portugal, included nine patients who underwent cataract surgery after a prior myopic LASIK procedure. All patients underwent IOL power estimation for a monofocal lens using the mean preoperative value obtained using the ASCRS online calculator, and during the surgery using the ORA System®. The surgeon was free to select the IOL power based on either measurement method. Comparative effectiveness analysis was carried out to evaluate the accuracy of IOL power determination between both methods. The IOL power prediction error was obtained by taking the originally targeted refraction minus the eventual postoperative outcome. The IA achieved a median absolute error of 0.38 (range 0.03–0.83) and a mean absolute error of 0.40 (± 0.28). The ASCRS online calculator showed a median absolute error of 0.36 (range 0–1.25) and a mean absolute error of 0.39 (±0.34). “The IA and ASCRS online calculator did not reveal significantly different capability to determine the estimated IOL power, although the IA showed a minor range of median absolute error. However the patient numbers in our study were very small so we need further studies with greater numbers to try to establish statistical significance,” she said. Michael Breen: michael.breen@alcon.com Catia Azenha: c.azenha@hotmail.com